Many characteristics of a weld can be evaluated during welding inspection, some relating to the welds size, and others relating to the presence of weld discontinuities. The size of a weld can be extremely important, as it can often relate directly to the weld's strength and associated performance, undersized weld's may not withstand stresses applied during service. Weld discontinuities can also be important. These are imperfections within or adjacent to the weld, which may or may not, dependent on their size and/or location, prevent the weld from meeting its intended performance. Typically these discontinuities, when of unacceptable size or location, are referred to as welding defects, and can sometimes cause premature weld failure through reduction of the weld strength or through producing stress concentrations within the welded component.
The inspection of welds can be conducted for a number of reasons. Perhaps the most fundamental reason is to determine whether the weld is of suitable quality for its intended application. In order to evaluate a weld's quality, we must first have some form of measuring block with which to compare its characteristics. It is impractical to attempt to evaluate a weld's quality without some form of specified acceptance criteria.
Weld quality acceptance criteria can originate from a number of sources. The welding fabrication drawing/blue print will typically provide weld sizes and possibly other welding dimensional information, such as length and location of welds. These dimensional requirements will usually have been established through design calculations or taken from proven designs that are known to meet the performance requirements of the welded connection.
Acceptable and unacceptable levels or amounts of weld discontinuities for welding inspection are usually obtained from welding codes and standards. Welding codes and standards have been developed for many types of welding fabrication applications. It is important to choose a welding standard that is intended for use within the particular industry or application in which you are involved.
Welding inspection can often require a wide variety of knowledge on the part of the welding inspector: the understanding of welding drawings, welding symbols, weld joint design, welding procedures, code and standard requirements and inspection and testing techniques, to name a few. For this reason many welding codes and standards require that the welding inspector be formally qualified or have the necessary knowledge and experience to conduct the inspection services. There are a number of welding inspection training courses available and a number of welding inspector certification programs internationally. The most popular program used in the USA is administered by the American Welding Society (AWS). This is the Certified Welding Inspector (CWI) program. Certification as a welding inspector: will typically require demonstration of an individual's knowledge of welding inspection through passing examination.
In order to further appreciate the extent of welding inspection we will need to examine specific areas of inspection techniques and welding inspection applications. I have chosen the following topics to provide this welding inspection overview:
Inspection and Testing for Welding Procedure Qualification – Types of inspection used for these requirements and how they can be an essential part of the overall welding quality system.
Visual Inspection – Often the easiest, least expensive, and probably, if performed correctly, the most effective method of welding inspection for many applications.
Surface Crack Detection – Methods such as Liquid Penetrant Inspection and Magnetic Particle Inspection – How they are used and what they will find.
Radiographic and Ultrasonic Weld Inspection – Methods known as Non Destructive Testing (NDT) and used typically to examine the internal structure of the weld in order to establish the weld's integrity without destroying the welded component.
Destructive Weld Testing – Methods used to establish weld integrity or performance, typically through sectioning and/or breaking the welded component and evaluating various mechanical and or physical characteristics.
One of the main ingredients of a successful welding quality system is the establishment, introduction and control of a sound welding inspection program. Only after the full evaluation of the weld quality requirements/acceptance criteria, the full appreciation of the inspection and testing methods to be used, and the availability of suitably qualified and/or experienced welding inspectors can such a program be established.
By Tony Anderson
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Monday, April 19, 2010
Destructive Testing of Welds
Destructive weld testing, as the name suggests, involves the physical destruction of the completed weld in order to evaluate its characteristics. This method of testing is used frequently for a number of applications. Some of these applications include welding procedure qualification and welder performance qualification testing, sampling inspection of production welds, research inspection, and failure analysis work. A number of destructive weld testing methods are used to determine weld integrity or performance. Typically they involve sectioning and/or breaking the welded component and evaluating various mechanical and/or physical characteristics. We shall briefly examine some of the more common methods of this type of welding inspection. We shall consider the macro etch test, the fillet weld break test, the transverse tension test, and the guided bend test. We shall consider how they are used, and what types of weld characteristics they are designed to determine. We shall examine their advantages over other inspection methods and their limitations.
Macro Etch Testing – This method of testing typically involves the removal of small samples of the welded joint. These samples are polished across their cross-section and then etched using some type of mild acid mixture, dependent on the base material used. The acid etch provides a clear visual appearance of the internal structure of the weld. Particular interest is often shown at the fusion line, this being the transition between the weld and the base material. Such items as depth of penetration, lack of fusion, inadequate root penetration, internal porosity, cracking and inclusions can be detected during inspection of the etched sample. This type of inspection is obviously a snapshot of the overall weld length quality when used for sampling inspection of production welds. This type of testing is often used extremely successfully to pinpoint welding problems such as crack initiation, when used for failure analyses.
Fillet Weld Break Test – This type of testing involves breaking a sample fillet weld that is welded on one side only. The sample has load applied to its unwelded side, transverse to the weld and directed to its unwelded side (typically in a press). The load is increased until the weld has failed. The failed sample is then inspected to establish the presence and extent of any welding discontinuities. This test will provide a good indication as to the extent of discontinuities within the entire length of weld tested (normally 6 to 12 inches) rather that a cross-sectional snapspot like the macro etch test. This type of weld inspection can detect such items as lack of fusion, internal porosity and slag inclusions. This testing method is often used in conjunction with the macro etch test. These two testing methods complement each other by providing information on similar characteristics in different detail and in different ways.
Transverse Tension Test – Since a large proportion of design is based on tensile properties of the welded joint, it is important that the tensile properties of the base metal, the weld metal, the bond between the base and the weld, and the heat-affected zone conform to the design requirements. Tensile strength of the welded joint is obtained by pulling specimens to failure. Tensile strength is determined by dividing the maximum load required during testing by the cross-sectional area. The result will be in units of tension per cross-sectional area. This test is nearly always required as part of the mechanical testing when qualifying welding procedure specifications for groove welds.
Guided Bend Test – This is a test method in which a specimen is bent to a specified bend radius. Various types of bend tests are used to evaluate the ductility and soundness of welded joints. Guided bend tests are usually taken transverse to the weld axis and may be bent in plunger type test machines or in wrap-around bend test jigs. Face bend tests are made with the weld face in tension, and root bend tests are made with the weld root in tension. When bend testing thick plates, side bend test specimens are usually cut from the welded joint and bent with the weld cross section in tension. The guided bend test is most commonly used in welding procedure and welder performance qualification tests. This type of testing is particularly good at finding liner fusion defects, which will often open up in the plate surface during the testing procedure.
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http://www.esabna.com/us/en/education/knowledge/weldinginspection/Destructive-Testing-of-Welds.cfm
Macro Etch Testing – This method of testing typically involves the removal of small samples of the welded joint. These samples are polished across their cross-section and then etched using some type of mild acid mixture, dependent on the base material used. The acid etch provides a clear visual appearance of the internal structure of the weld. Particular interest is often shown at the fusion line, this being the transition between the weld and the base material. Such items as depth of penetration, lack of fusion, inadequate root penetration, internal porosity, cracking and inclusions can be detected during inspection of the etched sample. This type of inspection is obviously a snapshot of the overall weld length quality when used for sampling inspection of production welds. This type of testing is often used extremely successfully to pinpoint welding problems such as crack initiation, when used for failure analyses.
Fillet Weld Break Test – This type of testing involves breaking a sample fillet weld that is welded on one side only. The sample has load applied to its unwelded side, transverse to the weld and directed to its unwelded side (typically in a press). The load is increased until the weld has failed. The failed sample is then inspected to establish the presence and extent of any welding discontinuities. This test will provide a good indication as to the extent of discontinuities within the entire length of weld tested (normally 6 to 12 inches) rather that a cross-sectional snapspot like the macro etch test. This type of weld inspection can detect such items as lack of fusion, internal porosity and slag inclusions. This testing method is often used in conjunction with the macro etch test. These two testing methods complement each other by providing information on similar characteristics in different detail and in different ways.
Transverse Tension Test – Since a large proportion of design is based on tensile properties of the welded joint, it is important that the tensile properties of the base metal, the weld metal, the bond between the base and the weld, and the heat-affected zone conform to the design requirements. Tensile strength of the welded joint is obtained by pulling specimens to failure. Tensile strength is determined by dividing the maximum load required during testing by the cross-sectional area. The result will be in units of tension per cross-sectional area. This test is nearly always required as part of the mechanical testing when qualifying welding procedure specifications for groove welds.
Guided Bend Test – This is a test method in which a specimen is bent to a specified bend radius. Various types of bend tests are used to evaluate the ductility and soundness of welded joints. Guided bend tests are usually taken transverse to the weld axis and may be bent in plunger type test machines or in wrap-around bend test jigs. Face bend tests are made with the weld face in tension, and root bend tests are made with the weld root in tension. When bend testing thick plates, side bend test specimens are usually cut from the welded joint and bent with the weld cross section in tension. The guided bend test is most commonly used in welding procedure and welder performance qualification tests. This type of testing is particularly good at finding liner fusion defects, which will often open up in the plate surface during the testing procedure.
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Inspection and Testing for Welding Procedure Qualification
Welding Procedures are the guidelines used to perform a weld. They are designed to provide a record of the welding variables used and the inspection results obtained during the procedure qualification test. They can also provide the instructions for the welder to use in production in order to produce acceptable welds. Usually welding procedures are developed in accordance with a welding code or standard, and with few exceptions*, require that physical weld samples be produced, inspected, and tested to establish qualification. Welding procedures are usually divided into two categories, the Procedure Qualification Record (PQR) and the Welding Procedure Specification (WPS).
Procedure Qualification Records are the documented values used during the actual welding test and all the inspection and test results obtained from the actual test samples.
Welding Procedure Specifications are usually documented work instructions that can be used by the welder to conduct welding operations, and are based on, but not necessarily the same as, the parameters used for the Procedure Qualification Record.
We will consider the Procedure Qualification Record and the inspection and testing performed during its qualification.
Qualification testing of a welding procedure normally requires documentation to show all the variables used during the welding test and the documented inspection and test results. The variables required to be documented are typically such items as: welding process used, size, type and classification of filler alloy, type and thickness of base material welded, type and polarity of welding current, amps and volts recorded, travel speed during welding, welding position, type and dimensions of joint design, preheating temperature, interpass temperature, post weld heat treatment details, and others. In addition to the recording of all the welding variables used during the test, in order to qualify a welding procedure, details of the inspection and test results must also be recorded. These records must show that the inspection and testing has proven that the weld samples have met or exceeded the specified standard requirement. The typical types of inspection and testing for each sample for Welding Procedure Qualification are:
Inspection and Testing for Fillet Welds (Tee Joints) - This involves visual inspection of the completed weld, followed by two macro etches, and one fillet weld break test. The welded sample is first inspected for any visual discontinuities and then sectioned, and two small samples removed at predetermined locations. These small samples are polished across their cross-section and then etched using some type of mild acid mixture, dependent on the base material used. The remaining welded sample is used as the fillet weld break test and is broken against the weld to reveal the internal structure of the weld for inspection.
Inspection and Testing for Groove welds (Butt Joints) – This involves visual inspection, followed by two transverse tensile tests, two root bend test and two face bend tests. (These tests are typical but may differ dependent on material thickness, type and standard requirements. Different and/or additional testing, such as side bends, all weld tensile tests, impact testing or other testing may be required.) The completed weld coupon, after visual inspection, is divided into predetermined small sections. Each section is prepared, usually by machining, to specific dimensions as prescribed by the standard. Each small sample is then tested mechanically to determine its characteristics. These samples are then inspected to determine their acceptability, against specified acceptance criteria, as laid down by the applicable code or standard. Typically the standard will provide the maximum size and location of various weld discontinuities and/or, as relevant, values such as minimum tensile strengths or minimum desired impact properties.
Samples that are found not to have discontinuities that exceed these specified limits, and that meet or exceed the minimum values as specified in the standard, will be acceptable, and the welding procedure will be qualified.
The welding procedure is an important part of the overall welding quality system, as it provides documented evidence that inspection and testing has been performed to ensure that welding can be conducted to meet a recognized standard.
* One exception to welding procedure qualification is the D1.1 Structural Welding Code for Steel, which will, under some circumstances, allow the use of pre-qualified welding procedures, however these procedures are still required to be documented and meet all of the relevant code requirements.
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http://www.esabna.com/us/en/education/knowledge/weldinginspection/Inspection-and-Testing-for-Welding-Procedure-Qualification.cfm
Procedure Qualification Records are the documented values used during the actual welding test and all the inspection and test results obtained from the actual test samples.
Welding Procedure Specifications are usually documented work instructions that can be used by the welder to conduct welding operations, and are based on, but not necessarily the same as, the parameters used for the Procedure Qualification Record.
We will consider the Procedure Qualification Record and the inspection and testing performed during its qualification.
Qualification testing of a welding procedure normally requires documentation to show all the variables used during the welding test and the documented inspection and test results. The variables required to be documented are typically such items as: welding process used, size, type and classification of filler alloy, type and thickness of base material welded, type and polarity of welding current, amps and volts recorded, travel speed during welding, welding position, type and dimensions of joint design, preheating temperature, interpass temperature, post weld heat treatment details, and others. In addition to the recording of all the welding variables used during the test, in order to qualify a welding procedure, details of the inspection and test results must also be recorded. These records must show that the inspection and testing has proven that the weld samples have met or exceeded the specified standard requirement. The typical types of inspection and testing for each sample for Welding Procedure Qualification are:
Inspection and Testing for Fillet Welds (Tee Joints) - This involves visual inspection of the completed weld, followed by two macro etches, and one fillet weld break test. The welded sample is first inspected for any visual discontinuities and then sectioned, and two small samples removed at predetermined locations. These small samples are polished across their cross-section and then etched using some type of mild acid mixture, dependent on the base material used. The remaining welded sample is used as the fillet weld break test and is broken against the weld to reveal the internal structure of the weld for inspection.
Inspection and Testing for Groove welds (Butt Joints) – This involves visual inspection, followed by two transverse tensile tests, two root bend test and two face bend tests. (These tests are typical but may differ dependent on material thickness, type and standard requirements. Different and/or additional testing, such as side bends, all weld tensile tests, impact testing or other testing may be required.) The completed weld coupon, after visual inspection, is divided into predetermined small sections. Each section is prepared, usually by machining, to specific dimensions as prescribed by the standard. Each small sample is then tested mechanically to determine its characteristics. These samples are then inspected to determine their acceptability, against specified acceptance criteria, as laid down by the applicable code or standard. Typically the standard will provide the maximum size and location of various weld discontinuities and/or, as relevant, values such as minimum tensile strengths or minimum desired impact properties.
Samples that are found not to have discontinuities that exceed these specified limits, and that meet or exceed the minimum values as specified in the standard, will be acceptable, and the welding procedure will be qualified.
The welding procedure is an important part of the overall welding quality system, as it provides documented evidence that inspection and testing has been performed to ensure that welding can be conducted to meet a recognized standard.
* One exception to welding procedure qualification is the D1.1 Structural Welding Code for Steel, which will, under some circumstances, allow the use of pre-qualified welding procedures, however these procedures are still required to be documented and meet all of the relevant code requirements.
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http://www.esabna.com/us/en/education/knowledge/weldinginspection/Inspection-and-Testing-for-Welding-Procedure-Qualification.cfm
Radiographic and Ultrasonic Testing of Welds
Radiographic and Ultrasonic Testing of Welds Radiographic and ultrasonic weld inspection are the two most common methods of non-destructive testing (NDT) used to detect discontinuities within the internal structure of welds. The obvious advantage of both these methods of testing is their ability to help establish the weld’s internal integrity without destroying the welded component. We shall briefly examine these two methods of non-destructive testing (NDT). We shall consider how they are used and what types of welding discontinuities they can be expected to find. We shall examine their advantages over other inspection methods and their limitations.
Radiographic Testing (RT) – This method of weld testing makes use of X-rays, produced by an X-ray tube, or gamma rays, produced by a radioactive isotope. The basic principle of radiographic inspection of welds is the same as that for medical radiography. Penetrating radiation is passed through a solid object, in this case a weld rather that part of the human body, onto a photographic film, resulting in an image of the object's internal structure being deposited on the film. The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object will cause exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film while inclusions of high density, such as tungsten, will appear as light areas. All discontinuities are detected by viewing shape and variation in density of the processed film.
Radiographic testing can provide a permanent film record of weld quality that is relatively easy to interpret by trained personnel. This testing method is usually suited to having access to both sides of the welded joint (with the exception of double wall signal image techniques used on some pipe work). Although this is a slow and expensive method of nondestructive testing, it is a positive method for detecting porosity, inclusions, cracks, and voids in the interior of welds. It is essential that qualified personnel conduct radiographic interpretation since false interpretation of radiographs can be expensive and interfere seriously with productivity. There are obvious safety considerations when conducting radiographic testing. X-ray and gamma radiation is invisible to the naked eye and can have serious heath and safety implications. Only suitably trained and qualified personnel should practice this type of testing.
Ultrasonic Testing (UT) – This method of testing makes use of mechanical vibrations similar to sound waves but of higher frequency. A beam of ultrasonic energy is directed into the object to be tested. This beam travels through the object with insignificant loss, except when it is intercepted and reflected by a discontinuity. The ultrasonic contact pulse reflection technique is used. This system uses a transducer that changes electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage, which causes a crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration. These vibrations are transmitted into the test piece through a coupling fluid, usually a film of oil, called a couplant. When the pulse of ultrasonic waves strikes a discontinuity in the test piece, it is reflected back to its point of origin. Thus the energy returns to the transducer. The transducer now serves as a receiver for the reflected energy. The initial signal or main bang, the returned echoes from the discontinuities, and the echo of the rear surface of the test piece are all displayed by a trace on the screen of a cathode-ray oscilloscope. The detection, location, and evaluation of discontinuities become possible because the velocity of sound through a given material is nearly constant, making distance measurement possible, and the relative amplitude of a reflected pulse is more or less proportional to the size of the reflector.
One of the most useful characteristics of ultrasonic testing is its ability to determine the exact position of a discontinuity in a weld. This testing method requires a high level of operator training and competence and is dependant on the establishment and application of suitable testing procedures. This testing method can be used on ferrous and nonferrous materials, is often suited for testing thicker sections accessible from one side only, and can often detect finer lines or plainer defects which may not be as readily detected by radiographic testing.
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http://www.esabna.com/us/en/education/knowledge/weldinginspection/Radiographic-and-Ultrasonic-Testing-of-Welds.cfm
Radiographic Testing (RT) – This method of weld testing makes use of X-rays, produced by an X-ray tube, or gamma rays, produced by a radioactive isotope. The basic principle of radiographic inspection of welds is the same as that for medical radiography. Penetrating radiation is passed through a solid object, in this case a weld rather that part of the human body, onto a photographic film, resulting in an image of the object's internal structure being deposited on the film. The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object will cause exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film while inclusions of high density, such as tungsten, will appear as light areas. All discontinuities are detected by viewing shape and variation in density of the processed film.
Radiographic testing can provide a permanent film record of weld quality that is relatively easy to interpret by trained personnel. This testing method is usually suited to having access to both sides of the welded joint (with the exception of double wall signal image techniques used on some pipe work). Although this is a slow and expensive method of nondestructive testing, it is a positive method for detecting porosity, inclusions, cracks, and voids in the interior of welds. It is essential that qualified personnel conduct radiographic interpretation since false interpretation of radiographs can be expensive and interfere seriously with productivity. There are obvious safety considerations when conducting radiographic testing. X-ray and gamma radiation is invisible to the naked eye and can have serious heath and safety implications. Only suitably trained and qualified personnel should practice this type of testing.
Ultrasonic Testing (UT) – This method of testing makes use of mechanical vibrations similar to sound waves but of higher frequency. A beam of ultrasonic energy is directed into the object to be tested. This beam travels through the object with insignificant loss, except when it is intercepted and reflected by a discontinuity. The ultrasonic contact pulse reflection technique is used. This system uses a transducer that changes electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage, which causes a crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration. These vibrations are transmitted into the test piece through a coupling fluid, usually a film of oil, called a couplant. When the pulse of ultrasonic waves strikes a discontinuity in the test piece, it is reflected back to its point of origin. Thus the energy returns to the transducer. The transducer now serves as a receiver for the reflected energy. The initial signal or main bang, the returned echoes from the discontinuities, and the echo of the rear surface of the test piece are all displayed by a trace on the screen of a cathode-ray oscilloscope. The detection, location, and evaluation of discontinuities become possible because the velocity of sound through a given material is nearly constant, making distance measurement possible, and the relative amplitude of a reflected pulse is more or less proportional to the size of the reflector.
One of the most useful characteristics of ultrasonic testing is its ability to determine the exact position of a discontinuity in a weld. This testing method requires a high level of operator training and competence and is dependant on the establishment and application of suitable testing procedures. This testing method can be used on ferrous and nonferrous materials, is often suited for testing thicker sections accessible from one side only, and can often detect finer lines or plainer defects which may not be as readily detected by radiographic testing.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Radiographic-and-Ultrasonic-Testing-of-Welds.cfm
Surface Crack Detection of Welds
One of the many Non Destructive Testing (NDT) techniques used for the inspection of welding is surface crack detection. We shall briefly examine the two most common methods of surface crack detection, Liquid Penetrant Testing (PT), sometimes referred to as Dye Penetrant Inspection, and Magnetic Particle Testing (MT). We shall consider how they are used, and what types of welding discontinuities they can be expected to find. We shall examine their advantages over other inspection methods and their limitations.
Liquid Penetrant Testing (PT) – This is probably the most common method of surface crack detection used. We shall consider the two most common methods of this testing method, Color Contrast and Fluorescent Dye. Both these methods use the same fundamental procedure. First, the application of a penetrating liquid to the surface of the weld to be tested, followed by a predetermined soak period to allow for adequate penetration of the liquid penetrant into any surface breaking discontinuities. Second, the careful removal of any excess penetrant, usually with a solvent or sometimes with a water wash, dependent on the method used. Third, the application of a developer to withdraw penetrant left behind within any discontinuity. These three steps are followed by the interpretation and evaluation of the test results. This will involve the detection of penetrant bleed-out from within any surface discontinuity. The method of detection is different for the color contrast and the fluorescent dye. The color contrast method is dependant on the bright contrast between the red penetrant dye and the white developer background covering the surface of the weld being tested, and the evaluation of the test is conducted in ordinary light. The fluorescent dye method is assisted by the use of an ultraviolet light (black light) that is used to illuminate the fluorescent dye and assist in the interpretation of the test.
This type of testing is limited to the detection of surface breaking discontinuities, that is, discontinuities which are open to the surface to which the penetrant has been applied. It cannot detect discontinuities that are sealed within the body of the weld such as internal porosity, or fusion defects. It is not usually suitable for testing rough or porous materials because interpretation of the test results can be hindered by false indications.
When compared to unassisted visual inspection, this type of inspection can provide a more sensitive inspection method that is more likely to detect smaller and finer surface breaking discontinuities, such as hair line cracks and micro surface porosity. This type of inspection may be suitable for both ferrous and nonferrous materials.
Magnetic Particle Testing (MT) – This is an NDT method used for detecting cracks, porosity, seams, inclusions, lack of fusion, and other discontinuities in ferromagnetic materials. Surface discontinuities and shallow subsurface discontinuities can be detected by using this method. This testing method consists of establishing a magnetic field in the part to be tested, applying magnetic particles to the surface of the part, and examining the surface for accumulations of particles that indicate discontinuities. A magnet will attract magnetic particles to its ends or poles, as they are called. Magnetic lines of force or flux flow between the poles of a magnet. Magnets will attract magnetic materials only where the lines of force enter and leave the magnet at the poles. If a magnet is bent and the two poles are joined so as to form a closed loop, no external poles will exist and consequently it will have no attraction for magnetic material.. This is the basic principle of magnetic particle testing. As long as the part has no cracks or other discontinuities, magnetic particles will not be attracted. When a crack or other discontinuity is present in the part being tested, north and south magnetic poles are set up at the edge of the discontinuity.
Only ferromagnetic materials can be tested by this method. Ferromagnetic parts that have been magnetized during testing may retain a certain amount of residual magnetism. Certain parts may require demagnetization if they are to function properly in service.
When using surface crack detection, whether liquid penetrant testing or magnetic particle, you should always consult the relevant specification involved for levels of acceptability and qualifications for equipment and operators. These methods of inspection are specialized and should be carried out by suitably trained and qualified inspection personnel.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Surface-Crack-Detection-of-Welds.cfm
Liquid Penetrant Testing (PT) – This is probably the most common method of surface crack detection used. We shall consider the two most common methods of this testing method, Color Contrast and Fluorescent Dye. Both these methods use the same fundamental procedure. First, the application of a penetrating liquid to the surface of the weld to be tested, followed by a predetermined soak period to allow for adequate penetration of the liquid penetrant into any surface breaking discontinuities. Second, the careful removal of any excess penetrant, usually with a solvent or sometimes with a water wash, dependent on the method used. Third, the application of a developer to withdraw penetrant left behind within any discontinuity. These three steps are followed by the interpretation and evaluation of the test results. This will involve the detection of penetrant bleed-out from within any surface discontinuity. The method of detection is different for the color contrast and the fluorescent dye. The color contrast method is dependant on the bright contrast between the red penetrant dye and the white developer background covering the surface of the weld being tested, and the evaluation of the test is conducted in ordinary light. The fluorescent dye method is assisted by the use of an ultraviolet light (black light) that is used to illuminate the fluorescent dye and assist in the interpretation of the test.
This type of testing is limited to the detection of surface breaking discontinuities, that is, discontinuities which are open to the surface to which the penetrant has been applied. It cannot detect discontinuities that are sealed within the body of the weld such as internal porosity, or fusion defects. It is not usually suitable for testing rough or porous materials because interpretation of the test results can be hindered by false indications.
When compared to unassisted visual inspection, this type of inspection can provide a more sensitive inspection method that is more likely to detect smaller and finer surface breaking discontinuities, such as hair line cracks and micro surface porosity. This type of inspection may be suitable for both ferrous and nonferrous materials.
Magnetic Particle Testing (MT) – This is an NDT method used for detecting cracks, porosity, seams, inclusions, lack of fusion, and other discontinuities in ferromagnetic materials. Surface discontinuities and shallow subsurface discontinuities can be detected by using this method. This testing method consists of establishing a magnetic field in the part to be tested, applying magnetic particles to the surface of the part, and examining the surface for accumulations of particles that indicate discontinuities. A magnet will attract magnetic particles to its ends or poles, as they are called. Magnetic lines of force or flux flow between the poles of a magnet. Magnets will attract magnetic materials only where the lines of force enter and leave the magnet at the poles. If a magnet is bent and the two poles are joined so as to form a closed loop, no external poles will exist and consequently it will have no attraction for magnetic material.. This is the basic principle of magnetic particle testing. As long as the part has no cracks or other discontinuities, magnetic particles will not be attracted. When a crack or other discontinuity is present in the part being tested, north and south magnetic poles are set up at the edge of the discontinuity.
Only ferromagnetic materials can be tested by this method. Ferromagnetic parts that have been magnetized during testing may retain a certain amount of residual magnetism. Certain parts may require demagnetization if they are to function properly in service.
When using surface crack detection, whether liquid penetrant testing or magnetic particle, you should always consult the relevant specification involved for levels of acceptability and qualifications for equipment and operators. These methods of inspection are specialized and should be carried out by suitably trained and qualified inspection personnel.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Surface-Crack-Detection-of-Welds.cfm
Visual Inspection of Welded Connections
Visual inspection is probably the most underrated, and often misused, method of welding inspection. Because of its simplicity, and the absence of sophisticated equipment, the potential of this method of inspection is quite often underestimated. Visual inspection of welding can often be the easiest to perform and is usually the least expensive to conduct. If carried out correctly, this type of inspection can often be an extremely effective method of maintaining acceptable welding quality and preventing welding problems. There are many areas within the welding operation that can be verified and evaluated by this method of inspection.
When designing an inspection plan, we need to establish the most appropriate areas to apply our inspection. We need to consider the possibility of preventing welding related problems, rather than finding problems which may have already occurred. Non- destructive testing (NDT), which is typically used for the inspection of completed welds, is usually designed and conducted to find welding problems after the fact, when the weld is completed. Visual inspection can often be utilized to prevent welding problems from happening in the first place. The welding inspection function is often divided into three areas. First, and often the least utilized, is pre-weld inspection. This type of inspection can often provide us the opportunity to detect and correct unacceptable conditions before they develop into actual welding problems. Second, inspection during the welding operation can often prevent problems in the completed weld through verification of the welding conditions and procedural requirements. Third, post-weld visual inspection is a relatively easy method of conducting completed weld quality evaluation. We shall consider each of these inspection stages in more detail.
Pre-Weld Inspection – This inspection is conducted prior to the start of the welding operation. This type of inspection is typically associated with checking the preparation of the welding joint and verification of parameters that would be difficult or impossible to confirm during or after welding. This is the area of inspection where we can best introduce controls that may prevent defective welding. Some areas of pre-weld inspection are joint preparation inspection/pre-weld setup. This may involve the dimensional inspection of root openings. Root openings that are too tight can cause inadequate root penetration. Root openings that are too large can cause over- penetration. Groove weld bevel angles, if too small, may cause lack of fusion, and if too large, can result in distortion of the weld joint from overheating and excessive shrinkage stress. Joint alignment (misalignment of the weld joint) can result in difficulty in producing a sound weld and stress concentration at its location, resulting in a reduction of fatigue life. Plate surface condition and cleanliness, pre-cleaning prior to welding, can often be of extreme importance. Improper or inadequate cleaning can result in unacceptable levels of porosity in the completed weld. Other pre-weld inspections may include preheat verification, temperature and heating method, presence and location of heat treatment monitoring devices, and type and efficacy of gas purging, if applicable.
Pre-weld inspection may also include evaluation and verification of documentation, material certification, filler alloy certification, welder performance qualification, welding procedure qualification, and welder and weld identification, for traceability, if applicable.
Inspection During Welding –
This is the inspection that is carried out during the welding operation and is concerned mainly with the requirements of the welding procedure specification (WPS). This inspection includes such items as interpass cleaning methods, interpass temperature control, welding current settings, welding travel speed, shielding gas type, gas flow rate, and welding sequence, if applicable. Also, any environmental conditions that may affect the quality of the weld such as, rain, wind, and extreme temperatures.
Post-Weld Inspection – This inspection typically conducted to verify the integrity of the completed weld. Many non-destructive testing (NDT) methods are used for post-weld inspection. However, even if the weld is to be subjected to NDT, it is normally wise to conduct visual inspection first. One reason for this is that surface discontinuities, which may be detected by visual inspection, can sometimes cause misinterpretation of NDT results or disguise other discontinuities within the body of the weld. The most common welding discontinuities found during visual inspection are conditions such as undersized welds, undercut, overlap, surface cracking, surface porosity, under fill, incomplete root penetration, excessive root penetration, burn through, and excessive reinforcement.
Conclusion - A good pre-weld inspection plan may provide us with an excellent opportunity to prevent welding problems before they start, through the detection and correction of situations that may cause welding problems or welding discontinuities.
Inspection conducted during the welding operation can often detect problems before they escalate and also helps to provide confidence in the final welded product.
Post-weld inspection can often provide an economical method of determining a weld’s acceptability with regard to many surface discontinuities.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Visual-Inspection-of-Welded-Connections.cfm
When designing an inspection plan, we need to establish the most appropriate areas to apply our inspection. We need to consider the possibility of preventing welding related problems, rather than finding problems which may have already occurred. Non- destructive testing (NDT), which is typically used for the inspection of completed welds, is usually designed and conducted to find welding problems after the fact, when the weld is completed. Visual inspection can often be utilized to prevent welding problems from happening in the first place. The welding inspection function is often divided into three areas. First, and often the least utilized, is pre-weld inspection. This type of inspection can often provide us the opportunity to detect and correct unacceptable conditions before they develop into actual welding problems. Second, inspection during the welding operation can often prevent problems in the completed weld through verification of the welding conditions and procedural requirements. Third, post-weld visual inspection is a relatively easy method of conducting completed weld quality evaluation. We shall consider each of these inspection stages in more detail.
Pre-Weld Inspection – This inspection is conducted prior to the start of the welding operation. This type of inspection is typically associated with checking the preparation of the welding joint and verification of parameters that would be difficult or impossible to confirm during or after welding. This is the area of inspection where we can best introduce controls that may prevent defective welding. Some areas of pre-weld inspection are joint preparation inspection/pre-weld setup. This may involve the dimensional inspection of root openings. Root openings that are too tight can cause inadequate root penetration. Root openings that are too large can cause over- penetration. Groove weld bevel angles, if too small, may cause lack of fusion, and if too large, can result in distortion of the weld joint from overheating and excessive shrinkage stress. Joint alignment (misalignment of the weld joint) can result in difficulty in producing a sound weld and stress concentration at its location, resulting in a reduction of fatigue life. Plate surface condition and cleanliness, pre-cleaning prior to welding, can often be of extreme importance. Improper or inadequate cleaning can result in unacceptable levels of porosity in the completed weld. Other pre-weld inspections may include preheat verification, temperature and heating method, presence and location of heat treatment monitoring devices, and type and efficacy of gas purging, if applicable.
Pre-weld inspection may also include evaluation and verification of documentation, material certification, filler alloy certification, welder performance qualification, welding procedure qualification, and welder and weld identification, for traceability, if applicable.
Inspection During Welding –
This is the inspection that is carried out during the welding operation and is concerned mainly with the requirements of the welding procedure specification (WPS). This inspection includes such items as interpass cleaning methods, interpass temperature control, welding current settings, welding travel speed, shielding gas type, gas flow rate, and welding sequence, if applicable. Also, any environmental conditions that may affect the quality of the weld such as, rain, wind, and extreme temperatures.
Post-Weld Inspection – This inspection typically conducted to verify the integrity of the completed weld. Many non-destructive testing (NDT) methods are used for post-weld inspection. However, even if the weld is to be subjected to NDT, it is normally wise to conduct visual inspection first. One reason for this is that surface discontinuities, which may be detected by visual inspection, can sometimes cause misinterpretation of NDT results or disguise other discontinuities within the body of the weld. The most common welding discontinuities found during visual inspection are conditions such as undersized welds, undercut, overlap, surface cracking, surface porosity, under fill, incomplete root penetration, excessive root penetration, burn through, and excessive reinforcement.
Conclusion - A good pre-weld inspection plan may provide us with an excellent opportunity to prevent welding problems before they start, through the detection and correction of situations that may cause welding problems or welding discontinuities.
Inspection conducted during the welding operation can often detect problems before they escalate and also helps to provide confidence in the final welded product.
Post-weld inspection can often provide an economical method of determining a weld’s acceptability with regard to many surface discontinuities.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Visual-Inspection-of-Welded-Connections.cfm
Weld Discontinuities - Part 1 Porosity - Incomplete Fusion - Incomplete Penetration
As a welding inspector, in order for you to successfully conduct your profession, it is important to understand weld discontinuities. To fully understand weld discontinuities, we must first examine some welding terminology. The term discontinuity is defined as an interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a defect. A defect on the other hand is defined as a discontinuity, or discontinuities, that by nature or accumulated effect (for example, total crack length) render a part or product unable to meet minimum applicable acceptance standards or specifications. The term “defect” designates rejectability. Because in this article we are examining these phenomena outside the requirements of any specific welding code or standard, and we will not be discussing their limitations in terms of these documents, we will use the term discontinuities.
Porosity
Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Porosity is caused by gases that are present in the molten weld. These gases may be trapped and form bobbles or gas pockets as the weld solidifies. The main reason for the presence of gases that cause porosity are dirty base material, moisture on joint surface or electrode, insufficient or improper shielding during the welding process, or incorrect welding conditions or techniques. Base material that is contaminated with hydrocarbons such as oil, grease or paint, will be susceptible to porosity during the welding process. Moisture in the form of water or hydrated oxides on base material and/or welding electrodes, or water leaks from poorly maintained equipment cooling systems, can introduce hydrogen into the welding process and cause major porosity problems during welding. The use of shielding gas, which has an inadequate flow rate, is contaminated from its source or within its delivery system, or is prevented from adequately protecting the molten weld metal through its removal by wind or draft, can seriously affect porosity levels.
Porosity is often classified into types based on its shape and distribution within the weld. Such descriptions as uniformly or randomly scattered porosity, cluster porosity, and linear porosity, or similar terms, are used to describe its distribution. Each of these porosity distributions may provide for different levels of acceptance within a welding code or standard. The presence of linear porosity, for instance, will usually have greater restrictions than those applied to scattered porosity, as linear porosity is often associated with fusion problems within the weld. The most practical methods for controlling or eliminating porosity is to use clean base materials, suitably stored and non-contaminated welding consumables, adequately maintained welding equipment, acceptable environmental conditions, and proven welding procedures.
Incomplete Fusion and Incomplete Joint Penetration
Since these terms are sometimes misused, it is important to understand the difference between these two weld discontinuities.
Incomplete fusion is a weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads. This absence of fusion may occur at any location within the weld joint and may be present in fillet welds and/or groove welds. Incomplete fusion may be caused by the inability, during the welding process, to elevate the base material or previously deposited weld metal to its melting temperature. It is often found on one leg of a fillet weld and is caused by incorrect welding angle that allows for an imbalance of heat between both sides of the joint. It may also be caused by failure to remove oxides or other foreign material from the surface of the base material to which the deposited weld metal must fuse.
Incomplete joint penetration is described as a joint root condition in a groove weld in which weld metal does not extend through the joint thickness. It is the failure of filler metal or base metal to completely fill the root of the weld. Some common causes of incomplete joint penetration are related to groove weld design or set up not suitable for the welding conditions. These problems develop in situations where the root face dimensions are too large, the root opening is too small, or the included angle of a v-groove weld is too narrow. All of these joint design characteristics restrict the ability of the weld to penetrate through the joint thickness. To help prevent this discontinuity, care should be taken to ensure the use of correct joint design and joint fit-up in accordance with welding procedure requirements.
The full understanding of these weld discontinuities will assist the welding inspector to identify them and, more importantly, help to prevent them from occurring in production.
It is often possible to use the welding inspection function as a preventative tool within the quality system. This offers far more efficiency to the overall quality system than purely using welding inspection as an appraisal technique to sort bad from good.
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http://www.esabna.com/us/en/education/knowledge/weldinginspection/Weld-Discontinuities-Part-1-Porosity-Incomplete-Fusion-Incomplete-Penetration.cfm
Porosity
Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Porosity is caused by gases that are present in the molten weld. These gases may be trapped and form bobbles or gas pockets as the weld solidifies. The main reason for the presence of gases that cause porosity are dirty base material, moisture on joint surface or electrode, insufficient or improper shielding during the welding process, or incorrect welding conditions or techniques. Base material that is contaminated with hydrocarbons such as oil, grease or paint, will be susceptible to porosity during the welding process. Moisture in the form of water or hydrated oxides on base material and/or welding electrodes, or water leaks from poorly maintained equipment cooling systems, can introduce hydrogen into the welding process and cause major porosity problems during welding. The use of shielding gas, which has an inadequate flow rate, is contaminated from its source or within its delivery system, or is prevented from adequately protecting the molten weld metal through its removal by wind or draft, can seriously affect porosity levels.
Porosity is often classified into types based on its shape and distribution within the weld. Such descriptions as uniformly or randomly scattered porosity, cluster porosity, and linear porosity, or similar terms, are used to describe its distribution. Each of these porosity distributions may provide for different levels of acceptance within a welding code or standard. The presence of linear porosity, for instance, will usually have greater restrictions than those applied to scattered porosity, as linear porosity is often associated with fusion problems within the weld. The most practical methods for controlling or eliminating porosity is to use clean base materials, suitably stored and non-contaminated welding consumables, adequately maintained welding equipment, acceptable environmental conditions, and proven welding procedures.
Incomplete Fusion and Incomplete Joint Penetration
Since these terms are sometimes misused, it is important to understand the difference between these two weld discontinuities.
Incomplete fusion is a weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads. This absence of fusion may occur at any location within the weld joint and may be present in fillet welds and/or groove welds. Incomplete fusion may be caused by the inability, during the welding process, to elevate the base material or previously deposited weld metal to its melting temperature. It is often found on one leg of a fillet weld and is caused by incorrect welding angle that allows for an imbalance of heat between both sides of the joint. It may also be caused by failure to remove oxides or other foreign material from the surface of the base material to which the deposited weld metal must fuse.
Incomplete joint penetration is described as a joint root condition in a groove weld in which weld metal does not extend through the joint thickness. It is the failure of filler metal or base metal to completely fill the root of the weld. Some common causes of incomplete joint penetration are related to groove weld design or set up not suitable for the welding conditions. These problems develop in situations where the root face dimensions are too large, the root opening is too small, or the included angle of a v-groove weld is too narrow. All of these joint design characteristics restrict the ability of the weld to penetrate through the joint thickness. To help prevent this discontinuity, care should be taken to ensure the use of correct joint design and joint fit-up in accordance with welding procedure requirements.
The full understanding of these weld discontinuities will assist the welding inspector to identify them and, more importantly, help to prevent them from occurring in production.
It is often possible to use the welding inspection function as a preventative tool within the quality system. This offers far more efficiency to the overall quality system than purely using welding inspection as an appraisal technique to sort bad from good.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Weld-Discontinuities-Part-1-Porosity-Incomplete-Fusion-Incomplete-Penetration.cfm
Weld Discontinuities - Part 2 - Unacceptable Weld Profiles
The profile of a completed weld may have considerable affect on the performance of that weld in service. It is the welding inspector’s function to identify these discontinuities through visual inspection and often to evaluate their acceptance, or rejection, through the use of the applicable welding code or standard acceptance criteria. Unacceptable weld profiles can cause problems associated with a reduction in base material thickness, a reduction in the affective weld size, or provide stress concentrations on the weld or plate surface. These types of weld discontinuities can often seriously detract from the overall performance of a welded component in service. We will consider some of the discontinuities associated with weld profiles: Undercut, Overlap, Insufficient Throat, and Excessive Convexity.
Undercut – This discontinuity is defined as a groove melted into the base metal adjacent to the weld toe, or weld root, and left unfilled by weld metal. The term undercut is used to describe either of two conditions. The first is the melting away of the base material at the side wall of a groove weld at the edge of a bead, thereby producing a sharp recess in the side wall in the area where the next bead is to be deposited. This type of undercut can facilitate the entrapment of inclusions that the recess which may be covered by a subsequent weld bead. This condition, if necessary, can be corrected, usually by grinding the recess away prior to depositing the next bead. If the undercut is slight, however, an experienced welder, who knows how deep the arc will penetrate, may not need to remove the undercut. Undercut of the side wall of a groove weld will in no way affect the completed weld if the condition is corrected before the next bead is deposited. The second condition is the reduction in thickness of the base metal at the line where the weld bead on the final layer of weld metal ties into the surface of the base metal. This position is known as the toe of the weld. This condition can occur on a fillet weld or a butt joint. The amount of undercut permitted at the surface of the completed weld is usually specified within the welding code or standard being used. The maximum permissible undercut requirements for completed welds should be followed stringently because excessive undercut can seriously affect the performance of a weld, particularly in services subjected to fatigue loading. Both types of undercut are usually caused by the welding technique used during welding, incorrect electrode positioning and/or incorrect travel speed. High currents and a long arc length can increase the probability for undercut.
Overlap – This discontinuity is defined as the protrusion of weld metal beyond the weld toe or weld root. This condition can occur in fillet welds and butt joints and can produce notches at the toe of the weld that are undesirable due to their resultant stress concentration under load. This discontinuity can be caused by incorrect welding techniques or insufficient current settings.
Insufficient Throat – This condition describes a weld profile that is usually concave in shape, and due to its concavity, provides an inadequate throat thickness. Excess concavity, that can produce an unintentional reduction in throat thickness, can occur in fillet welds and butt joints. The problem associated with this discontinuity is its ability to considerably reduce that part of the weld that controls the weld’s strength, namely the throat thickness. This condition is usually caused by excessive welding current or arc lengths.
Excessive Convexity –This discontinuity can produce a notch effect in the welded area and, consequently, stress concentration under load. For this reason, some codes and standards will specify the maximum convexity of a weld profile. This condition is usually caused by insufficient current or incorrect welding techniques.
Conclusion – A number of discontinuities are associated with the exterior profile or shape of the completed weld. These discontinuities are generally detected through visual inspection of the weld, however, some are detectable through other inspection methods such as radiography, liquid penetrant, and magnetic particle inspection. The maximum acceptable limitations associated with these discontinuities is dependent on the performance requirements of the welded component and is usually specified in the appropriate welding code, standard or specification. The welding inspector is often required to determine the extent of these discontinuities and to establish their acceptance, or rejection, based on the relevant acceptance criteria.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Weld-Discontinuities-Part-2-Unacceptable-Weld-Profiles.cfm
Undercut – This discontinuity is defined as a groove melted into the base metal adjacent to the weld toe, or weld root, and left unfilled by weld metal. The term undercut is used to describe either of two conditions. The first is the melting away of the base material at the side wall of a groove weld at the edge of a bead, thereby producing a sharp recess in the side wall in the area where the next bead is to be deposited. This type of undercut can facilitate the entrapment of inclusions that the recess which may be covered by a subsequent weld bead. This condition, if necessary, can be corrected, usually by grinding the recess away prior to depositing the next bead. If the undercut is slight, however, an experienced welder, who knows how deep the arc will penetrate, may not need to remove the undercut. Undercut of the side wall of a groove weld will in no way affect the completed weld if the condition is corrected before the next bead is deposited. The second condition is the reduction in thickness of the base metal at the line where the weld bead on the final layer of weld metal ties into the surface of the base metal. This position is known as the toe of the weld. This condition can occur on a fillet weld or a butt joint. The amount of undercut permitted at the surface of the completed weld is usually specified within the welding code or standard being used. The maximum permissible undercut requirements for completed welds should be followed stringently because excessive undercut can seriously affect the performance of a weld, particularly in services subjected to fatigue loading. Both types of undercut are usually caused by the welding technique used during welding, incorrect electrode positioning and/or incorrect travel speed. High currents and a long arc length can increase the probability for undercut.
Overlap – This discontinuity is defined as the protrusion of weld metal beyond the weld toe or weld root. This condition can occur in fillet welds and butt joints and can produce notches at the toe of the weld that are undesirable due to their resultant stress concentration under load. This discontinuity can be caused by incorrect welding techniques or insufficient current settings.
Insufficient Throat – This condition describes a weld profile that is usually concave in shape, and due to its concavity, provides an inadequate throat thickness. Excess concavity, that can produce an unintentional reduction in throat thickness, can occur in fillet welds and butt joints. The problem associated with this discontinuity is its ability to considerably reduce that part of the weld that controls the weld’s strength, namely the throat thickness. This condition is usually caused by excessive welding current or arc lengths.
Excessive Convexity –This discontinuity can produce a notch effect in the welded area and, consequently, stress concentration under load. For this reason, some codes and standards will specify the maximum convexity of a weld profile. This condition is usually caused by insufficient current or incorrect welding techniques.
Conclusion – A number of discontinuities are associated with the exterior profile or shape of the completed weld. These discontinuities are generally detected through visual inspection of the weld, however, some are detectable through other inspection methods such as radiography, liquid penetrant, and magnetic particle inspection. The maximum acceptable limitations associated with these discontinuities is dependent on the performance requirements of the welded component and is usually specified in the appropriate welding code, standard or specification. The welding inspector is often required to determine the extent of these discontinuities and to establish their acceptance, or rejection, based on the relevant acceptance criteria.
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http://www.esabna.com/us/en/education/knowledge/weldinginspection/Weld-Discontinuities-Part-2-Unacceptable-Weld-Profiles.cfm
Weld Discontinuities - Part 3 Cracking
Cracks in a weldment are probably the most dreaded of all the weld discontinuities. Because of the wide range of applications and the many types of materials welded, cracking is an extremely complex subject. We will examine some basic theory and characteristics of different types of cracking in welded connections.
Cracks will occur in the weld metal when localized stresses exceed the ultimate strength of the metal. For this reason, we need to consider some important variables when designing a welding procedure to best resist cracking. The crack sensitivity of the base material may be associated with its chemistry and/or its susceptibility to the formation of elements which will reduce its ductility. The introduction of excessive stresses to the weld joint, particularly in conjunction with a material in a crack-sensitive condition, can cause cracking to occur. Stresses in and around the weld are characteristic of the welding operation, which often introduces extreme localized heating, together with expansion and contraction during the welding process. Cracking is often associated with stress concentration near discontinuities in welds and base metal, and near mechanical notches associated with the weldment design. Hydrogen embrittlement, which is a condition that causes a loss of ductility and exists in weld metal due to hydrogen absorption, can contribute to crack formation in some materials
Cracks are usually classified into one of two types: Hot Cracks and Cold Cracks.
Hot cracks develop at elevated temperatures, propagate between the grains of the material, and commonly form during solidification of the weld metal.
Cold cracks develop after solidification of the weld as a result of stresses and propagate both between grains and through grains. Cold cracks in steel are sometimes called delayed cracks and are often associated with hydrogen embrittlement.
We can divide cracks into an additional two types: Cracks in the base material and cracks in the weld metal.
Cracks in the Base Material
Heat-Affected-Zone (HAZ) cracking is most often associated with hardenable base material. High hardness and low ductility in the heat-affected zone is often a result of the metallurgical response to the welding thermal cycles. In ferritic steels, hardness increases and ductility decreases with an increase in carbon content and an increase in the cooling rate from the welding temperature. The heat-affected zone hardness is related to the hardenability of the base material, which in turn is dependent on the base material chemical composition. Carbon has a predominant effect on the hardenability of steel. Perhaps an extreme example of this hardenability and its effect on base metal cracking is when we consider the welding of cast iron. This material contains between 2% and 4.5% carbon, which gives the alloy great hardness and low ductility. If we attempt to weld this material without serious consideration to cooling rates and residual stress, we will invariably encounter base material cracking.
Cracks in the Weld Metal
We can divide weld metal cracks into three types: Transverse, longitudinal and crater cracks.
Transverse weld metal cracks are perpendicular to the direction of the weld. This type of crack is more common in welds that have a high degree of restraint.
Longitudinal weld cracks travel in the same direction as the weld and are often confined to the center of the weld. This type of crack may be an extension of a crack that originally initiated at the end of a weld.
Crater cracks can be formed by an abrupt weld termination if a crater is left unfilled with weld metal. These cracks are usually star shaped and initially only extend to the edge of the crater. However, these cracks can propagate into longitudinal weld cracks.
The Effect of Cracks on the Weld Integrity
Cracks in any form are usually unacceptable discontinuities and are considered most detrimental to the performance of the weld. A crack, by its nature, is sharp at its extremities and consequently acts as a stress concentration. The stress concentration effect of a crack is greater than that of most other discontinuities. Cracks have a tendency to propagate and can contribute to weld failure if subjected to stress in service. Cracks, regardless of size, are not normally permitted in weldments governed by most fabrication codes. They are required to be removed, usually by grinding or gouging, and the excavation filled with sound weld metal.
Conclusion
The successful welding procedure will incorporate in its requirements the controls necessary to overcome the tendency for crack formation. Such controls, dependent on material type, may be preheating temperature, interpass temperature, preparation of and type of welding consumables, and post-weld heat treatment. It is the responsibility of the welding inspector to evaluate these welding procedural controls during their inspections, thereby ensuring that welding is performed in accordance with welding procedures that have been designed to minimize the probability of weld cracking.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Weld-Discontinuities-Part-3.cfm
Cracks will occur in the weld metal when localized stresses exceed the ultimate strength of the metal. For this reason, we need to consider some important variables when designing a welding procedure to best resist cracking. The crack sensitivity of the base material may be associated with its chemistry and/or its susceptibility to the formation of elements which will reduce its ductility. The introduction of excessive stresses to the weld joint, particularly in conjunction with a material in a crack-sensitive condition, can cause cracking to occur. Stresses in and around the weld are characteristic of the welding operation, which often introduces extreme localized heating, together with expansion and contraction during the welding process. Cracking is often associated with stress concentration near discontinuities in welds and base metal, and near mechanical notches associated with the weldment design. Hydrogen embrittlement, which is a condition that causes a loss of ductility and exists in weld metal due to hydrogen absorption, can contribute to crack formation in some materials
Cracks are usually classified into one of two types: Hot Cracks and Cold Cracks.
Hot cracks develop at elevated temperatures, propagate between the grains of the material, and commonly form during solidification of the weld metal.
Cold cracks develop after solidification of the weld as a result of stresses and propagate both between grains and through grains. Cold cracks in steel are sometimes called delayed cracks and are often associated with hydrogen embrittlement.
We can divide cracks into an additional two types: Cracks in the base material and cracks in the weld metal.
Cracks in the Base Material
Heat-Affected-Zone (HAZ) cracking is most often associated with hardenable base material. High hardness and low ductility in the heat-affected zone is often a result of the metallurgical response to the welding thermal cycles. In ferritic steels, hardness increases and ductility decreases with an increase in carbon content and an increase in the cooling rate from the welding temperature. The heat-affected zone hardness is related to the hardenability of the base material, which in turn is dependent on the base material chemical composition. Carbon has a predominant effect on the hardenability of steel. Perhaps an extreme example of this hardenability and its effect on base metal cracking is when we consider the welding of cast iron. This material contains between 2% and 4.5% carbon, which gives the alloy great hardness and low ductility. If we attempt to weld this material without serious consideration to cooling rates and residual stress, we will invariably encounter base material cracking.
Cracks in the Weld Metal
We can divide weld metal cracks into three types: Transverse, longitudinal and crater cracks.
Transverse weld metal cracks are perpendicular to the direction of the weld. This type of crack is more common in welds that have a high degree of restraint.
Longitudinal weld cracks travel in the same direction as the weld and are often confined to the center of the weld. This type of crack may be an extension of a crack that originally initiated at the end of a weld.
Crater cracks can be formed by an abrupt weld termination if a crater is left unfilled with weld metal. These cracks are usually star shaped and initially only extend to the edge of the crater. However, these cracks can propagate into longitudinal weld cracks.
The Effect of Cracks on the Weld Integrity
Cracks in any form are usually unacceptable discontinuities and are considered most detrimental to the performance of the weld. A crack, by its nature, is sharp at its extremities and consequently acts as a stress concentration. The stress concentration effect of a crack is greater than that of most other discontinuities. Cracks have a tendency to propagate and can contribute to weld failure if subjected to stress in service. Cracks, regardless of size, are not normally permitted in weldments governed by most fabrication codes. They are required to be removed, usually by grinding or gouging, and the excavation filled with sound weld metal.
Conclusion
The successful welding procedure will incorporate in its requirements the controls necessary to overcome the tendency for crack formation. Such controls, dependent on material type, may be preheating temperature, interpass temperature, preparation of and type of welding consumables, and post-weld heat treatment. It is the responsibility of the welding inspector to evaluate these welding procedural controls during their inspections, thereby ensuring that welding is performed in accordance with welding procedures that have been designed to minimize the probability of weld cracking.
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What The Welding Inspector Should Know About Preheating And Postweld Heat Treatment
When welding some base materials and for some service conditions, preheating and/or postweld heat treatment may be a requirement. These types of thermal treatments are generally required in order to ensure suitable weld integrity and will typically prevent or remove undesirable characteristics in the completed weld. Any form of heat treatment is costly since it demands extra equipment, extra time, and extra handling. For these reasons, heat treatment should only be undertaken after careful consideration of the advantages it may offer. In certain cases heat treatment will be mandatory, as with heavy sections of low alloy steels, whereas in other cases, it will be a justifiable precaution against early failure in service.
There are a number of reasons for the incorporation of these thermal treatments within the welding procedure, and we will consider some of the most common.
Preheating
Preheat, as defined within the AWS Standard Welding Terms and Definition, is “the heat applied to the base metal or substrate to attain and maintain preheat temperature”. The preheat temperature is defined by the same document as “the temperature of the base metal in the volume surrounding the point of welding immediately before welding is started. In a multipass weld, it is also the temperature immediately before the second and subsequent passes are started” (Interpass Temperature).
Preheating may be performed by the use of gas burners, oxy-gas flames, electric blankets, induction heating, or by heating in a furnace. For good results, it is essential for the heating to be uniform around the joint area. Intense, non-uniform heating is of little use in retarding cooling and may be detrimental in causing higher residual stresses, distortion, or undesirable metallurgical changes in the base material. When preheating is specified, the entire weld joint should be heated evenly through the material thickness to the desired minimum temperature. To obtain a uniform temperature through the material thickness, it is desirable to apply the heating sources to one side of the material surface and to measure the material temperature on the opposite side. Whenever the heating and temperature measurement must be conducted from the same surface, the inspector must assure that more than just the surface of the material has been heated. It is important to ensure that the entire material thickness has been heated to a uniform temperature. In addition to establishing a preheat temperature, an interpass temperature limitation may need to be considered for some applications. This information should be shown in the welding procedure specification. When an interpass temperature is specified, the weld area must be inspected prior to depositing the next weld bead. Welding may not continue if the measured temperature exceeds the maximum interpass conditions specified in the welding procedure. The weldment must be permitted to cool down to the specified upper limit of the interpass temperature before continuing with the weld.
Dependent on the metallurgical properties of the material, and/or the desired mechanical properties of the welded component, preheat and interpass temperature may be evaluated for different reasons. For instance, a procedure for welding mild steel, which has a low carbon content, relatively low hardenability, and is used in an application with no special service requirements, may consider a minimum preheat and interpass temperature based on the material thickness. Welding procedures used for the heat-treatable low alloy steels and chromium-molybdenum steels with impact requirements will normally specify a minimum and maximum requirement for preheat and interpass temperatures. These low alloy materials can have high hardenability and are susceptible to hydrogen cracking. Allowing these materials to cool too quickly or overheating them can seriously affect their performance requirements. When welding the nickel alloys, we are concerned primarily with high heat input during the welding operation. The heat input of the welding process, and the preheat and interpass temperature can seriously affect these materials. High heat input can result in excessive constitutional liquation, carbide precipitation, and other harmful metallurgical phenomena. These metallurgical changes may promote cracking or loss of corrosion resistance. Procedures for welding some aluminum alloys such as the heat-treatable, 2xxx, 6xxx, and 7xxx series, are often concerned with overall heat input reduction. With these materials, the maximum preheat and interpass temperature is controlled in order to minimize its annealing and over-aging influence on the heat-affected zone (HAZ) and consequent loss in tensile strength.
On critical applications, the preheat temperature must be precisely controlled. In these situations, controllable heating systems are used, and thermocouples are attached to monitor the part being heated. These thermocouples provide a signal to the controlling unit that can regulate the power source required for heating. By using this type of equipment, the part being heated can be controlled to extremely close tolerances.
Some of the reasons for preheating are:
a) To drive away moisture from the weld area: Typically, this is performed by heating the surface of the material to a relatively low temperature, just above the boiling point of water. This will dry the plate surface and remove the undesirable contaminants that may otherwise cause porosity, hydrogen embrittlement, or cracking through the introduction of hydrogen during the welding process.
b) To lower the thermal gradient: All arc welding processes use a high temperature heat source. A steep temperature differential occurs between the localized heat source and the cool base material being welded. This temperature difference causes differential thermal expansion and contraction and high stresses around the welded area. Reducing the temperature differential by preheating the base material will minimize problems associated with distortion and excessive residual stress. If preheating is not carried out, a large differential in temperature can occur between the weld area and the parent material. This can cause rapid cooling, leading to the formation of martensite and probable cracking when welding some materials with high hardenability.
Postweld Heat Treatment
A number of different types of post-weld heat treatments are used for different reasons and for different materials.
a) Post-weld heat treatment is most generally used for stress relief. The purpose of stress relieving is to remove any internal or residual stresses that may be present from the welding operation. Stress relief after welding may be necessary in order to reduce the risk of brittle fracture, to avoid subsequent distortion on machining, or to eradicate the risk of stress corrosion.
b) For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure. This treatment is generally performed after the weld has cooled, but under certain circumstances, it may be necessary to perform this treatment before it has cooled to prevent cracking.
c) Extremely coarse weld structures in steel, such as those obtained with the electro-slag welding process, may require normalizing after welding. This treatment will refine the coarse grain structure, reduce stresses after welding, and remove any hard zones in the heat-affected zone.
d) The precipitation hardening alloys, such as the heat treatable aluminum alloys, are sometimes required to undergo post-weld heat treatment to regain their original properties. In some cases, only an aging treatment is used, although a full solution heat treat and artificial aging treatment will provide better recovery of properties after welding.
When the welding operations involve preheating and/or post-weld heat treatment, it is important that the welding inspector understand these requirements in order to ensure that they are being conducted correctly and in terms of the relevant welding procedure specifications and/or code requirements.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/What-The-Welding-Inspector-Should-Know-About-Preheating-And-Postweld-Heat-Treatment.cfm
There are a number of reasons for the incorporation of these thermal treatments within the welding procedure, and we will consider some of the most common.
Preheating
Preheat, as defined within the AWS Standard Welding Terms and Definition, is “the heat applied to the base metal or substrate to attain and maintain preheat temperature”. The preheat temperature is defined by the same document as “the temperature of the base metal in the volume surrounding the point of welding immediately before welding is started. In a multipass weld, it is also the temperature immediately before the second and subsequent passes are started” (Interpass Temperature).
Preheating may be performed by the use of gas burners, oxy-gas flames, electric blankets, induction heating, or by heating in a furnace. For good results, it is essential for the heating to be uniform around the joint area. Intense, non-uniform heating is of little use in retarding cooling and may be detrimental in causing higher residual stresses, distortion, or undesirable metallurgical changes in the base material. When preheating is specified, the entire weld joint should be heated evenly through the material thickness to the desired minimum temperature. To obtain a uniform temperature through the material thickness, it is desirable to apply the heating sources to one side of the material surface and to measure the material temperature on the opposite side. Whenever the heating and temperature measurement must be conducted from the same surface, the inspector must assure that more than just the surface of the material has been heated. It is important to ensure that the entire material thickness has been heated to a uniform temperature. In addition to establishing a preheat temperature, an interpass temperature limitation may need to be considered for some applications. This information should be shown in the welding procedure specification. When an interpass temperature is specified, the weld area must be inspected prior to depositing the next weld bead. Welding may not continue if the measured temperature exceeds the maximum interpass conditions specified in the welding procedure. The weldment must be permitted to cool down to the specified upper limit of the interpass temperature before continuing with the weld.
Dependent on the metallurgical properties of the material, and/or the desired mechanical properties of the welded component, preheat and interpass temperature may be evaluated for different reasons. For instance, a procedure for welding mild steel, which has a low carbon content, relatively low hardenability, and is used in an application with no special service requirements, may consider a minimum preheat and interpass temperature based on the material thickness. Welding procedures used for the heat-treatable low alloy steels and chromium-molybdenum steels with impact requirements will normally specify a minimum and maximum requirement for preheat and interpass temperatures. These low alloy materials can have high hardenability and are susceptible to hydrogen cracking. Allowing these materials to cool too quickly or overheating them can seriously affect their performance requirements. When welding the nickel alloys, we are concerned primarily with high heat input during the welding operation. The heat input of the welding process, and the preheat and interpass temperature can seriously affect these materials. High heat input can result in excessive constitutional liquation, carbide precipitation, and other harmful metallurgical phenomena. These metallurgical changes may promote cracking or loss of corrosion resistance. Procedures for welding some aluminum alloys such as the heat-treatable, 2xxx, 6xxx, and 7xxx series, are often concerned with overall heat input reduction. With these materials, the maximum preheat and interpass temperature is controlled in order to minimize its annealing and over-aging influence on the heat-affected zone (HAZ) and consequent loss in tensile strength.
On critical applications, the preheat temperature must be precisely controlled. In these situations, controllable heating systems are used, and thermocouples are attached to monitor the part being heated. These thermocouples provide a signal to the controlling unit that can regulate the power source required for heating. By using this type of equipment, the part being heated can be controlled to extremely close tolerances.
Some of the reasons for preheating are:
a) To drive away moisture from the weld area: Typically, this is performed by heating the surface of the material to a relatively low temperature, just above the boiling point of water. This will dry the plate surface and remove the undesirable contaminants that may otherwise cause porosity, hydrogen embrittlement, or cracking through the introduction of hydrogen during the welding process.
b) To lower the thermal gradient: All arc welding processes use a high temperature heat source. A steep temperature differential occurs between the localized heat source and the cool base material being welded. This temperature difference causes differential thermal expansion and contraction and high stresses around the welded area. Reducing the temperature differential by preheating the base material will minimize problems associated with distortion and excessive residual stress. If preheating is not carried out, a large differential in temperature can occur between the weld area and the parent material. This can cause rapid cooling, leading to the formation of martensite and probable cracking when welding some materials with high hardenability.
Postweld Heat Treatment
A number of different types of post-weld heat treatments are used for different reasons and for different materials.
a) Post-weld heat treatment is most generally used for stress relief. The purpose of stress relieving is to remove any internal or residual stresses that may be present from the welding operation. Stress relief after welding may be necessary in order to reduce the risk of brittle fracture, to avoid subsequent distortion on machining, or to eradicate the risk of stress corrosion.
b) For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure. This treatment is generally performed after the weld has cooled, but under certain circumstances, it may be necessary to perform this treatment before it has cooled to prevent cracking.
c) Extremely coarse weld structures in steel, such as those obtained with the electro-slag welding process, may require normalizing after welding. This treatment will refine the coarse grain structure, reduce stresses after welding, and remove any hard zones in the heat-affected zone.
d) The precipitation hardening alloys, such as the heat treatable aluminum alloys, are sometimes required to undergo post-weld heat treatment to regain their original properties. In some cases, only an aging treatment is used, although a full solution heat treat and artificial aging treatment will provide better recovery of properties after welding.
When the welding operations involve preheating and/or post-weld heat treatment, it is important that the welding inspector understand these requirements in order to ensure that they are being conducted correctly and in terms of the relevant welding procedure specifications and/or code requirements.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/What-The-Welding-Inspector-Should-Know-About-Preheating-And-Postweld-Heat-Treatment.cfm
What you should know about welding codes and standards
What Are Welding Codes and Standards, When Are They Used, and How Are They Developed
Many aspects of the design and fabrication of welded components are governed by documents known as codes and standards. Other names used for such documents include guides, recommended practices, regulations, rules, and specifications. These documents are often specified by an end user/purchaser as a contractual agreement in order to control the characteristics of the welded fabrication that may affect its service requirements. They are also used by the manufacturer to assist in the development and implementation of their welding quality system. Many end users of welded components have developed and issued specifications that have been compiled by them to address their specific requirements. Such specifications may be limited in application and related only to that customer’s situation and requirements. National interest in areas such as public safety and reliability has promoted the development of welding codes and standards that command broader recognition both on a national and industry-specific basis. Numerous committees have been developed over the years within national engineering and technical societies that continue to evaluate the needs of industry and develop new welding codes and standards. Such committees are comprised of members who are technical experts and represent all interested parties such as manufacturers, end users, inspection authorities, and government agencies. The membership of these committees is balanced in order to prevent any one interest group from controlling the committee. On completion of a new or revised document by the specific committee, it is usually then reviewed and approved by a review committee, and if accepted, then published in the name of the applicable engineering society.
Documents that have significant influence upon public health and safety are sometimes adopted by legislative bodies or by federal regulating agencies. In those jurisdictions, such documents become law and are often referred to as Codes or Regulations.
The welding inspector should be aware of what codes or standards are applicable within their jurisdiction, understand the requirements of the relevant documents and perform their inspection accordingly.
Sources Of Codes and Standards Of Interest To The Welding Industry
The following are some of the more popular sources of welding codes and standards found in the USA.
American Welding Society (AWS) – Probably the largest producer of welding codes and standards in the USA. The AWS publishes many documents addressing the use and quality control of welding. These documents include such general subjects as Welding Definitions and Symbols, Classification of Filler Metals, Qualification and Testing, Welding Processes, Welding Applications, and Safety.
American Society of Mechanical Engineers (ASME) – This society is responsible for the development of the Boiler and Pressure Vessel Code, which contains eleven sections and covers the design, construction, and inspection of boilers and pressure vessels. ASME also produces the Code for Pressure Piping, which consists of seven sections. Each section prescribes the minimum requirements for the design, materials, fabrication, erection, testing and inspection of a particular type of piping system. Both of these documents are American National Standards.
American Petroleum Institute (API) – This institute publishes many documents relating to petroleum production, a number of which include welding requirements. The most well known is possibly API Std 1104 – Standard for Welding Pipelines and Related Facilities.
What The Welding Code and Standard Generally Provides
The specific content and requirements of a welding code or standard can vary in detail, however, there are a number of elements within these types of documents which are common and which we will examine.
The Scope and General Requirements: This is found at the beginning of the document and is important as it will normally provide a description as to the type and extent of welding fabrication for which the document was developed and intended to be used. It may also provide information relating to the limitations for the use of the document. Care should be taken to use codes and standards that are applicable for your particular application.
Design: If the document provides a section for design, it may refer the user to a secondary source of information, or it may contain minimum requirements for the design of specific welded connections.
Qualification: This section of the document will typically outline the requirements for qualification testing of welding procedure specifications (WPS) and also those requirements for qualification of welding personnel. It may provide the essential variables, these being the change limitations that govern the extent of qualification. Such variables are typically the welding process, type and thickness of base metal, filler metal type, electrical parameters, joint design, welding position, and others.
This section of the document may also provide the qualification testing requirements. Usually this is divided into welding procedure and welder performance testing requirements. Typically, it will provide the types and sizes of test samples to be welded and prepared for testing, the testing methods to be used, and the minimum acceptance criteria to be used for the evaluation of test samples.
Fabrication: This section, when included in the document, will typically provide information associated with the fabrication methods and/or workmanship standards. It may contain information and requirements on such items as base materials, welding consumable classification requirements, shielding gas quality, heat treatment requirements, preparation and care of base material, and other welding fabrication requirements.
Inspection: This section of the document will typically address the welding inspector’s qualification requirements and responsibilities, acceptance criteria for discontinuities, and requirements relating to procedures for nondestructive testing.
Opportunities For The Welding Fabricator To Improve Weld Quality And Reliability
With the move by more manufacturing organizations toward the implementation of quality management systems, such as ISO 9000, and the requirement of such systems for process control, we must consider welding as a special process and, consequently, its formal control. Welding codes and standards are often used by the welding fabricator to assist with the development of their process control system. If we consider the major elements of process control, as specified by such standards for quality systems, we will recognize those same elements as being addressed within the welding code or standard. The first requirement for process control is documented procedures defining the manner of production. For welding, this is the welding procedure specification (WPS). A second requirement is criteria for workmanship, which shall be stipulated in the clearest practical manner. For welding, this may be the code or standard acceptance criteria. A third requirement is qualification of personnel. This may be addressed by the welder performance qualification. Regardless of the manufacturer’s overall quality system, there may be opportunities available through the selection and use of an appropriate welding code or standard for welding quality and reliability improvements.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/What-you-should-know-about-welding-codes-and-standards.cfm
Many aspects of the design and fabrication of welded components are governed by documents known as codes and standards. Other names used for such documents include guides, recommended practices, regulations, rules, and specifications. These documents are often specified by an end user/purchaser as a contractual agreement in order to control the characteristics of the welded fabrication that may affect its service requirements. They are also used by the manufacturer to assist in the development and implementation of their welding quality system. Many end users of welded components have developed and issued specifications that have been compiled by them to address their specific requirements. Such specifications may be limited in application and related only to that customer’s situation and requirements. National interest in areas such as public safety and reliability has promoted the development of welding codes and standards that command broader recognition both on a national and industry-specific basis. Numerous committees have been developed over the years within national engineering and technical societies that continue to evaluate the needs of industry and develop new welding codes and standards. Such committees are comprised of members who are technical experts and represent all interested parties such as manufacturers, end users, inspection authorities, and government agencies. The membership of these committees is balanced in order to prevent any one interest group from controlling the committee. On completion of a new or revised document by the specific committee, it is usually then reviewed and approved by a review committee, and if accepted, then published in the name of the applicable engineering society.
Documents that have significant influence upon public health and safety are sometimes adopted by legislative bodies or by federal regulating agencies. In those jurisdictions, such documents become law and are often referred to as Codes or Regulations.
The welding inspector should be aware of what codes or standards are applicable within their jurisdiction, understand the requirements of the relevant documents and perform their inspection accordingly.
Sources Of Codes and Standards Of Interest To The Welding Industry
The following are some of the more popular sources of welding codes and standards found in the USA.
American Welding Society (AWS) – Probably the largest producer of welding codes and standards in the USA. The AWS publishes many documents addressing the use and quality control of welding. These documents include such general subjects as Welding Definitions and Symbols, Classification of Filler Metals, Qualification and Testing, Welding Processes, Welding Applications, and Safety.
American Society of Mechanical Engineers (ASME) – This society is responsible for the development of the Boiler and Pressure Vessel Code, which contains eleven sections and covers the design, construction, and inspection of boilers and pressure vessels. ASME also produces the Code for Pressure Piping, which consists of seven sections. Each section prescribes the minimum requirements for the design, materials, fabrication, erection, testing and inspection of a particular type of piping system. Both of these documents are American National Standards.
American Petroleum Institute (API) – This institute publishes many documents relating to petroleum production, a number of which include welding requirements. The most well known is possibly API Std 1104 – Standard for Welding Pipelines and Related Facilities.
What The Welding Code and Standard Generally Provides
The specific content and requirements of a welding code or standard can vary in detail, however, there are a number of elements within these types of documents which are common and which we will examine.
The Scope and General Requirements: This is found at the beginning of the document and is important as it will normally provide a description as to the type and extent of welding fabrication for which the document was developed and intended to be used. It may also provide information relating to the limitations for the use of the document. Care should be taken to use codes and standards that are applicable for your particular application.
Design: If the document provides a section for design, it may refer the user to a secondary source of information, or it may contain minimum requirements for the design of specific welded connections.
Qualification: This section of the document will typically outline the requirements for qualification testing of welding procedure specifications (WPS) and also those requirements for qualification of welding personnel. It may provide the essential variables, these being the change limitations that govern the extent of qualification. Such variables are typically the welding process, type and thickness of base metal, filler metal type, electrical parameters, joint design, welding position, and others.
This section of the document may also provide the qualification testing requirements. Usually this is divided into welding procedure and welder performance testing requirements. Typically, it will provide the types and sizes of test samples to be welded and prepared for testing, the testing methods to be used, and the minimum acceptance criteria to be used for the evaluation of test samples.
Fabrication: This section, when included in the document, will typically provide information associated with the fabrication methods and/or workmanship standards. It may contain information and requirements on such items as base materials, welding consumable classification requirements, shielding gas quality, heat treatment requirements, preparation and care of base material, and other welding fabrication requirements.
Inspection: This section of the document will typically address the welding inspector’s qualification requirements and responsibilities, acceptance criteria for discontinuities, and requirements relating to procedures for nondestructive testing.
Opportunities For The Welding Fabricator To Improve Weld Quality And Reliability
With the move by more manufacturing organizations toward the implementation of quality management systems, such as ISO 9000, and the requirement of such systems for process control, we must consider welding as a special process and, consequently, its formal control. Welding codes and standards are often used by the welding fabricator to assist with the development of their process control system. If we consider the major elements of process control, as specified by such standards for quality systems, we will recognize those same elements as being addressed within the welding code or standard. The first requirement for process control is documented procedures defining the manner of production. For welding, this is the welding procedure specification (WPS). A second requirement is criteria for workmanship, which shall be stipulated in the clearest practical manner. For welding, this may be the code or standard acceptance criteria. A third requirement is qualification of personnel. This may be addressed by the welder performance qualification. Regardless of the manufacturer’s overall quality system, there may be opportunities available through the selection and use of an appropriate welding code or standard for welding quality and reliability improvements.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/What-you-should-know-about-welding-codes-and-standards.cfm
Inspection and Testing for Welding Procedure Qualification
Welding Procedures are the guidelines used to perform a weld. They are designed to provide a record of the welding variables used and the inspection results obtained during the procedure qualification test. They can also provide the instructions for the welder to use in production in order to produce acceptable welds. Usually welding procedures are developed in accordance with a welding code or standard, and with few exceptions*, require that physical weld samples be produced, inspected, and tested to establish qualification. Welding procedures are usually divided into two categories, the Procedure Qualification Record (PQR) and the Welding Procedure Specification (WPS).
Procedure Qualification Records are the documented values used during the actual welding test and all the inspection and test results obtained from the actual test samples.
Welding Procedure Specifications are usually documented work instructions that can be used by the welder to conduct welding operations, and are based on, but not necessarily the same as, the parameters used for the Procedure Qualification Record.
We will consider the Procedure Qualification Record and the inspection and testing performed during its qualification.
Qualification testing of a welding procedure normally requires documentation to show all the variables used during the welding test and the documented inspection and test results. The variables required to be documented are typically such items as: welding process used, size, type and classification of filler alloy, type and thickness of base material welded, type and polarity of welding current, amps and volts recorded, travel speed during welding, welding position, type and dimensions of joint design, preheating temperature, interpass temperature, post weld heat treatment details, and others. In addition to the recording of all the welding variables used during the test, in order to qualify a welding procedure, details of the inspection and test results must also be recorded. These records must show that the inspection and testing has proven that the weld samples have met or exceeded the specified standard requirement. The typical types of inspection and testing for each sample for Welding Procedure Qualification are:
Inspection and Testing for Fillet Welds (Tee Joints) - This involves visual inspection of the completed weld, followed by two macro etches, and one fillet weld break test. The welded sample is first inspected for any visual discontinuities and then sectioned, and two small samples removed at predetermined locations. These small samples are polished across their cross-section and then etched using some type of mild acid mixture, dependent on the base material used. The remaining welded sample is used as the fillet weld break test and is broken against the weld to reveal the internal structure of the weld for inspection.
Inspection and Testing for Groove welds (Butt Joints) – This involves visual inspection, followed by two transverse tensile tests, two root bend test and two face bend tests. (These tests are typical but may differ dependent on material thickness, type and standard requirements. Different and/or additional testing, such as side bends, all weld tensile tests, impact testing or other testing may be required.) The completed weld coupon, after visual inspection, is divided into predetermined small sections. Each section is prepared, usually by machining, to specific dimensions as prescribed by the standard. Each small sample is then tested mechanically to determine its characteristics. These samples are then inspected to determine their acceptability, against specified acceptance criteria, as laid down by the applicable code or standard. Typically the standard will provide the maximum size and location of various weld discontinuities and/or, as relevant, values such as minimum tensile strengths or minimum desired impact properties.
Samples that are found not to have discontinuities that exceed these specified limits, and that meet or exceed the minimum values as specified in the standard, will be acceptable, and the welding procedure will be qualified.
The welding procedure is an important part of the overall welding quality system, as it provides documented evidence that inspection and testing has been performed to ensure that welding can be conducted to meet a recognized standard.
* One exception to welding procedure qualification is the D1.1 Structural Welding Code for Steel, which will, under some circumstances, allow the use of pre-qualified welding procedures, however these procedures are still required to be documented and meet all of the relevant code requirements.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Inspection-and-Testing-for-Welding-Procedure-Qualification.cfm
Procedure Qualification Records are the documented values used during the actual welding test and all the inspection and test results obtained from the actual test samples.
Welding Procedure Specifications are usually documented work instructions that can be used by the welder to conduct welding operations, and are based on, but not necessarily the same as, the parameters used for the Procedure Qualification Record.
We will consider the Procedure Qualification Record and the inspection and testing performed during its qualification.
Qualification testing of a welding procedure normally requires documentation to show all the variables used during the welding test and the documented inspection and test results. The variables required to be documented are typically such items as: welding process used, size, type and classification of filler alloy, type and thickness of base material welded, type and polarity of welding current, amps and volts recorded, travel speed during welding, welding position, type and dimensions of joint design, preheating temperature, interpass temperature, post weld heat treatment details, and others. In addition to the recording of all the welding variables used during the test, in order to qualify a welding procedure, details of the inspection and test results must also be recorded. These records must show that the inspection and testing has proven that the weld samples have met or exceeded the specified standard requirement. The typical types of inspection and testing for each sample for Welding Procedure Qualification are:
Inspection and Testing for Fillet Welds (Tee Joints) - This involves visual inspection of the completed weld, followed by two macro etches, and one fillet weld break test. The welded sample is first inspected for any visual discontinuities and then sectioned, and two small samples removed at predetermined locations. These small samples are polished across their cross-section and then etched using some type of mild acid mixture, dependent on the base material used. The remaining welded sample is used as the fillet weld break test and is broken against the weld to reveal the internal structure of the weld for inspection.
Inspection and Testing for Groove welds (Butt Joints) – This involves visual inspection, followed by two transverse tensile tests, two root bend test and two face bend tests. (These tests are typical but may differ dependent on material thickness, type and standard requirements. Different and/or additional testing, such as side bends, all weld tensile tests, impact testing or other testing may be required.) The completed weld coupon, after visual inspection, is divided into predetermined small sections. Each section is prepared, usually by machining, to specific dimensions as prescribed by the standard. Each small sample is then tested mechanically to determine its characteristics. These samples are then inspected to determine their acceptability, against specified acceptance criteria, as laid down by the applicable code or standard. Typically the standard will provide the maximum size and location of various weld discontinuities and/or, as relevant, values such as minimum tensile strengths or minimum desired impact properties.
Samples that are found not to have discontinuities that exceed these specified limits, and that meet or exceed the minimum values as specified in the standard, will be acceptable, and the welding procedure will be qualified.
The welding procedure is an important part of the overall welding quality system, as it provides documented evidence that inspection and testing has been performed to ensure that welding can be conducted to meet a recognized standard.
* One exception to welding procedure qualification is the D1.1 Structural Welding Code for Steel, which will, under some circumstances, allow the use of pre-qualified welding procedures, however these procedures are still required to be documented and meet all of the relevant code requirements.
source from
http://www.esabna.com/us/en/education/knowledge/weldinginspection/Inspection-and-Testing-for-Welding-Procedure-Qualification.cfm
Quality inspections
To ensure quality welds, it is important to have a quality weld inspection program in place. In order to do so, a company must understand how to evaluate weld characteristics, determine weld quality, and have a welding inspector capable of performing a number of different testing methods.
Ensuring that welders follow specific procedures is a crucial step in the overall welding quality system.
There are a number of reasons to inspect a weld, the most fundamental of which is to determine whether its quality is good enough for its intended application. To evaluate the quality of a weld, it is necessary to have a form of measurement to compare its characteristics and a qualified individual to perform the evaluation. It is not practical to evaluate quality without some form of specified acceptance criteria. It's also not practical for a person who is not well-versed in the necessary procedures to perform this task.
Evaluation of weld characteristics includes the size of the weld and the presence of discontinuities. The size of a weld can be extremely important, as it often correlates directly to strength and associated performance. Undersized welds may not withstand stresses applied during service, and oversized welds can produce stress concentrations or contribute to the potential for distortion of a welded component.
Uncovering weld discontinuities also is important because imperfections within or adjacent the weld, depending on their size or location, may prevent the weld from meeting its intended function. When discontinuities are an unacceptable size or in an unacceptable location, they are called welding defects, and they can cause premature weld failure by reducing strength or producing stress concentrations within the welded component.
Determining
Weld Quality
Weld quality acceptance criteria can originate from a number of sources. The welding fabrication drawing or blueprint provides weld sizes and other welding dimensional requirements such as length and location. These dimensional requirements are established through design calculations or are taken from proven designs that meet the performance requirements of the welded connection.
The number of acceptable and unacceptable weld discontinuities for welding inspection usually is obtained from welding codes and standards. Welding codes and standards have been developed for many types of welding fabrication applications. It is important to choose a welding standard intended for use within the particular industry or application in which you are involved.
Weld Inspector Responsibilities
Imperfections within or adjacent the weld may prevent the weld from meeting its intended function.
Welding inspection requires a knowledge of weld drawings, symbols, joint design, procedures, code and standard requirements, and inspection and testing techniques. For this reason, many welding codes and standards require that the welding inspector be formally qualified, or have the necessary knowledge and experience to conduct the inspection.
Weld inspection is only as good as the person running the tests. Here are a few things that a welding inspector should know and tasks he should be able to perform:
1.Welder Performance and Welding Procedure Qualification. Specific procedures must be followed to qualify welders and welding procedures. The qualification process is an integral part of the overall welding quality system, and the welding inspector often is required to coordinate and verify these types of qualification tests.
These qualifications typically involve producing welded samples representative of the welds that will be used in production welding. These welded samples usually are required to be tested after completion. Radiographic, microetching, guided bends, transverse tension, and nick-break fracture are some of the tests that are used. The test results must meet or exceed the minimum requirements as stipulated in the welding code or standard before the procedure can be qualified.
2.Visual Inspection. This is often the easiest, least expensive, and most effective method of welding inspection for many applications if performed correctly. The welding inspector must be capable of identifying all of the different welding discontinuities during visual inspection. He also must be able to evaluate, in terms of the relevant welding code or standard, the significance of identified discontinuities to determine whether to accept or reject them during testing and production.
A welding inspector with good eyesight can be trained relatively quickly by a competent instructor and can prove to be a major asset to the welding quality system (good vision is obviously essential for visual inspection).
3.Surface Crack Detection. A welding inspector sometimes is required to conduct weld testing by surface crack detection methods. He also may have to evaluate the test results of these testing methods. The inspector should understand testing methods, such as liquid penetrant and magnetic particle inspection. Additionally, he must know how the tests are used and what they will find.
4.Radiographic and Ultrasonic Weld Inspection. These two inspection methods are in a group known as nondestructive testing (NDT). These inspection methods are used to examine the internal structure of the weld to establish the weld's integrity, without destroying the welded component. The welding inspector may be required to understand this type of testing and be competent in the interpretation of the results. Radio-graphic and ultrasonic weld inspection are the two most common methods of NDT used to detect discontinuities within the internal structure of welds. The obvious advantage of both methods is their ability to help establish the weld's internal integrity without destroying the welded component.
Radiographic testing makes use of X-rays produced by an X-ray tube or gamma rays produced by a radioactive isotope. The basic principle of radiographic inspection is the same as that for medical radiography. Penetrating radiation is passed through a solid object, in this case a weld, onto a photographic film, resulting in an image of the object's internal structure. The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object will cause exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter.
Therefore, areas of the weld where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film, while inclusions of high density, such as tungsten, will appear as light areas. All discontinuities are detected by viewing shape and variation in density of the processed film.
Ultrasonic testing makes use of mechanical vibrations similar to sound waves but of higher frequency. A beam of ultrasonic energy is directed into the weld to be tested. This beam travels through the weld with insignificant loss, except when it is intercepted and reflected by a discontinuity. The ultrasonic contact pulse reflection technique is used. This system uses a transducer, which changes electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage, which causes a crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration.
These vibrations are transmitted into the test piece through a coupling fluid, usually a film of oil, called a couplant. When the pulse of ultrasonic waves strikes a discontinuity in the test piece, it is reflected back to its point of origin. Thus, the energy returns to the transducer. The transducer now serves as a receiver for the reflected energy. The initial signal or main bang, the returned echoes from the discontinuities, and the echo of the rear surface of the test piece are all displayed by a trace on the screen of a cathode-ray oscilloscope.
5.Destructive Weld Testing. Destructive methods to establish weld integrity or performance include sectioning, bending, or breaking the welded component and evaluating various mechanical or physical characteristics. Some of these tests are the guided bend test, macroetch test, reduced section tensile test, fracture test, and Charpy V-notch impact test. These tests are used during welding procedure or welder performance qualification testing. The welding inspector often is required to conduct, supervise, or evaluate these testing methods.
6.Interpretation of Welding Details and Weld Symbols. The welding inspector should be competent in the ability to read engineering and manufacturing drawings, and be able to interpret all details and symbols that provide information about the welding requirements.
By looking at only some of the welding inspector's functions, it is easy to see that the welding inspector can have many responsibilities. These duties generally change from one engineering or manufacturing environment to another. However, the welding inspector's primary job is to help coordinate the welding quality control operations within the organization.
One of the main ingredients of a successful welding quality control system is the establishment, introduction, and control of a sound welding inspection program. A program can be established only after completing an evaluation of the weld quality requirements or acceptance criteria, acquiring knowledge of the inspection and testing methods, and using properly qualified and experienced welding inspectors.
By Tony Anderson
July 10, 2007
http://www.thefabricator.com/article/weldinginspection/quality-inspections
Ensuring that welders follow specific procedures is a crucial step in the overall welding quality system.
There are a number of reasons to inspect a weld, the most fundamental of which is to determine whether its quality is good enough for its intended application. To evaluate the quality of a weld, it is necessary to have a form of measurement to compare its characteristics and a qualified individual to perform the evaluation. It is not practical to evaluate quality without some form of specified acceptance criteria. It's also not practical for a person who is not well-versed in the necessary procedures to perform this task.
Evaluation of weld characteristics includes the size of the weld and the presence of discontinuities. The size of a weld can be extremely important, as it often correlates directly to strength and associated performance. Undersized welds may not withstand stresses applied during service, and oversized welds can produce stress concentrations or contribute to the potential for distortion of a welded component.
Uncovering weld discontinuities also is important because imperfections within or adjacent the weld, depending on their size or location, may prevent the weld from meeting its intended function. When discontinuities are an unacceptable size or in an unacceptable location, they are called welding defects, and they can cause premature weld failure by reducing strength or producing stress concentrations within the welded component.
Determining
Weld Quality
Weld quality acceptance criteria can originate from a number of sources. The welding fabrication drawing or blueprint provides weld sizes and other welding dimensional requirements such as length and location. These dimensional requirements are established through design calculations or are taken from proven designs that meet the performance requirements of the welded connection.
The number of acceptable and unacceptable weld discontinuities for welding inspection usually is obtained from welding codes and standards. Welding codes and standards have been developed for many types of welding fabrication applications. It is important to choose a welding standard intended for use within the particular industry or application in which you are involved.
Weld Inspector Responsibilities
Imperfections within or adjacent the weld may prevent the weld from meeting its intended function.
Welding inspection requires a knowledge of weld drawings, symbols, joint design, procedures, code and standard requirements, and inspection and testing techniques. For this reason, many welding codes and standards require that the welding inspector be formally qualified, or have the necessary knowledge and experience to conduct the inspection.
Weld inspection is only as good as the person running the tests. Here are a few things that a welding inspector should know and tasks he should be able to perform:
1.Welder Performance and Welding Procedure Qualification. Specific procedures must be followed to qualify welders and welding procedures. The qualification process is an integral part of the overall welding quality system, and the welding inspector often is required to coordinate and verify these types of qualification tests.
These qualifications typically involve producing welded samples representative of the welds that will be used in production welding. These welded samples usually are required to be tested after completion. Radiographic, microetching, guided bends, transverse tension, and nick-break fracture are some of the tests that are used. The test results must meet or exceed the minimum requirements as stipulated in the welding code or standard before the procedure can be qualified.
2.Visual Inspection. This is often the easiest, least expensive, and most effective method of welding inspection for many applications if performed correctly. The welding inspector must be capable of identifying all of the different welding discontinuities during visual inspection. He also must be able to evaluate, in terms of the relevant welding code or standard, the significance of identified discontinuities to determine whether to accept or reject them during testing and production.
A welding inspector with good eyesight can be trained relatively quickly by a competent instructor and can prove to be a major asset to the welding quality system (good vision is obviously essential for visual inspection).
3.Surface Crack Detection. A welding inspector sometimes is required to conduct weld testing by surface crack detection methods. He also may have to evaluate the test results of these testing methods. The inspector should understand testing methods, such as liquid penetrant and magnetic particle inspection. Additionally, he must know how the tests are used and what they will find.
4.Radiographic and Ultrasonic Weld Inspection. These two inspection methods are in a group known as nondestructive testing (NDT). These inspection methods are used to examine the internal structure of the weld to establish the weld's integrity, without destroying the welded component. The welding inspector may be required to understand this type of testing and be competent in the interpretation of the results. Radio-graphic and ultrasonic weld inspection are the two most common methods of NDT used to detect discontinuities within the internal structure of welds. The obvious advantage of both methods is their ability to help establish the weld's internal integrity without destroying the welded component.
Radiographic testing makes use of X-rays produced by an X-ray tube or gamma rays produced by a radioactive isotope. The basic principle of radiographic inspection is the same as that for medical radiography. Penetrating radiation is passed through a solid object, in this case a weld, onto a photographic film, resulting in an image of the object's internal structure. The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object will cause exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter.
Therefore, areas of the weld where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film, while inclusions of high density, such as tungsten, will appear as light areas. All discontinuities are detected by viewing shape and variation in density of the processed film.
Ultrasonic testing makes use of mechanical vibrations similar to sound waves but of higher frequency. A beam of ultrasonic energy is directed into the weld to be tested. This beam travels through the weld with insignificant loss, except when it is intercepted and reflected by a discontinuity. The ultrasonic contact pulse reflection technique is used. This system uses a transducer, which changes electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage, which causes a crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration.
These vibrations are transmitted into the test piece through a coupling fluid, usually a film of oil, called a couplant. When the pulse of ultrasonic waves strikes a discontinuity in the test piece, it is reflected back to its point of origin. Thus, the energy returns to the transducer. The transducer now serves as a receiver for the reflected energy. The initial signal or main bang, the returned echoes from the discontinuities, and the echo of the rear surface of the test piece are all displayed by a trace on the screen of a cathode-ray oscilloscope.
5.Destructive Weld Testing. Destructive methods to establish weld integrity or performance include sectioning, bending, or breaking the welded component and evaluating various mechanical or physical characteristics. Some of these tests are the guided bend test, macroetch test, reduced section tensile test, fracture test, and Charpy V-notch impact test. These tests are used during welding procedure or welder performance qualification testing. The welding inspector often is required to conduct, supervise, or evaluate these testing methods.
6.Interpretation of Welding Details and Weld Symbols. The welding inspector should be competent in the ability to read engineering and manufacturing drawings, and be able to interpret all details and symbols that provide information about the welding requirements.
By looking at only some of the welding inspector's functions, it is easy to see that the welding inspector can have many responsibilities. These duties generally change from one engineering or manufacturing environment to another. However, the welding inspector's primary job is to help coordinate the welding quality control operations within the organization.
One of the main ingredients of a successful welding quality control system is the establishment, introduction, and control of a sound welding inspection program. A program can be established only after completing an evaluation of the weld quality requirements or acceptance criteria, acquiring knowledge of the inspection and testing methods, and using properly qualified and experienced welding inspectors.
By Tony Anderson
July 10, 2007
http://www.thefabricator.com/article/weldinginspection/quality-inspections
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