Monday, 21 May 2012
Pre-setting of parts
Fig. 1 Pre-setting of parts to produce correct alignment after welding
- Pre-setting of fillet joint to prevent angular distortion
- Pre-setting of butt joint to prevent angular distortion
- Tapered gap to prevent closure
HELP TO CONTROL DISTORTION
Adopting the following assembly techniques will help to
control distortion:
- Pre-set parts so that welding distortion will achieve overall alignment and dimensional control with the minimum of residual stress
- Pre-bend joint edges to counteract distortion and achieve alignment and dimensional control with minimum residual stress.
- Apply restraint during welding by using jigs and fixtures, flexible clamps, strongbacks and tack welding but consider the risk of cracking which can be quite significant, especially for fully welded strongbacks.
- Use an approved procedure for welding and removal of welds for restraint techniques which may need preheat to avoid forming imperfections in the component surface.
Friday, 11 May 2012
What is Weld Distortion?
What is Weld Distortion?
Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all welding on one side of a part will cause much more distortion than if the welds are alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion, such as physical and mechanical properties that change as heat is applied. For example, as the temperature of the weld area increases, yield strength, elasticity, and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase (Fig. 3-1). These changes, in turn, affect heat flow and uniformity of heat distribution.
But if the steel bar is restrained -as in a vise - while it is heated, as shown in Fig. 3-2(b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction (in thickness) and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Fig. 3-2 (c). The bar is now shorter, but thicker. It has been permanently deformed, or distorted. (For simplification, the sketches show this distortion occurring in thickness only. But in actuality, length is similarly affected.)
In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. On cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but it is restrained from doing so by the adjacent base metal. Because of this, stresses develop within the weld and the adjacent base metal. At this point, the weld stretches (or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. But only those stresses that exceed the yield strength of the weld metal are relieved by this straining. By the time the weld reaches room temperature - assuming complete restraint of the base metal so that it cannot move - the weld will contain locked-in tensile stresses approximately equal to the yield strength of the metal. If the restraints (clamps that hold the workpiece, or an opposing shrinkage force) are removed, the residual stresses are partially relieved as they cause the base metal to move, thus distorting the weldment.
Shrinkage Control - What You Can Do to Minimize Distortion
To prevent or minimize weld distortion, methods must be used both in design and during welding to overcome the effects of the heating and cooling cycle. Shrinkage cannot be prevented, but it can be controlled. Several ways can be used to minimize distortion caused by shrinkage:
1. Do not overweld
The more metal placed in a joint, the greater the shrinkage forces. Correctly sizing a weld for the requirements of the joint not only minimizes distortion, but also saves weld metal and time. The amount of weld metal in a fillet weld can be minimized by the use of a flat or slightly convex bead, and in a butt joint by proper edge preparation and fitup. The excess weld metal in a highly convex bead does not increase the allowable strength in code work, but it does increase shrinkage forces.
When welding heavy plate (over 1 inch thick) bevelling or even double bevelling can save a substantial amount of weld metal which translates into much less distortion automatically.
In general, if distortion is not a problem, select the most economical joint. If distortion is a problem, select either a joint in which the weld stresses balance each other or a joint requiring the least amount of weld metal.
2. Use intermittent weldingAnother way to minimize weld metal is to use intermittent rather than continuous welds where possible, as in Fig. 3-7(c). For attaching stiffeners to plate, for example, intermittent welds can reduce the weld metal by as much as 75 percent yet provide the needed strength.
3. Use as few weld passes as possible
Fewer passes with large electrodes, Fig. 3-7(d), are preferable to a greater number of passes with small electrodes when transverse distortion could be a problem. Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used.
4. Place welds near the neutral axis
Distortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. Figure 3-7(e) illustrates this. Both design of the weldment and welding sequence can be used effectively to control distortion.
5. Balance welds around the neutral axis
This practice, shown in Fig. 3-7(f), offsets one shrinkage force with another to effectively minimize distortion of the weldment. Here, too, design of the assembly and proper sequence of welding are important factors.
6. Use backstep weldingIn the backstep technique, the general progression of welding may be, say, from left to right, but each bead segment is deposited from right to left as in Fig. 3-7(g). As each bead segment is placed, the heated edges expand, which temporarily separates the plates at B. But as the heat moves out across the plate to C, expansion along outer edges CD brings the plates back together. This separation is most pronounced as the first bead is laid. With successive beads, the plates expand less and less because of the restraint of prior welds. Backstepping may not be effective in all applications, and it cannot be used economically in automatic welding.
7. Anticipate the shrinkage forcesPresetting parts (at first glance, I thought that this was referring to overhead or vertical welding positions, which is not the case) before welding can make shrinkage perform constructive work. Several assemblies, preset in this manner, are shown in Fig. 3-7(h). The required amount of preset for shrinkage to pull the plates into alignment can be determined from a few trial welds.
Prebending, presetting or prespringing the parts to be welded, Fig. 3-7(i), is a simple example of the use of opposing mechanical forces to counteract distortion due to welding. The top of the weld groove - which will contain the bulk of the weld metal - is lengthened when the plates are preset. Thus the completed weld is slightly longer than it would be if it had been made on the flat plate. When the clamps are released after welding, the plates return to the flat shape, allowing the weld to relieve its longitudinal shrinkage stresses by shortening to a straight line. The two actions coincide, and the welded plates assume the desired flatness.
Another common practice for balancing shrinkage forces is to position identical weldments back to back, Fig. 3-7(j), clamping them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released. Prebending can be combined with this method by inserting wedges at suitable positions between the parts before clamping.
In heavy weldments, particularly, the rigidity of the members and their arrangement relative to each other may provide the balancing forces needed. If these natural balancing forces are not present, it is necessary to use other means to counteract the shrinkage forces in the weld metal. This can be accomplished by balancing one shrinkage force against another or by creating an opposing force through the fixturing. The opposing forces may be: other shrinkage forces; restraining forces imposed by clamps, jigs, or fixtures; restraining forces arising from the arrangement of members in the assembly; or the force from the sag in a member due to gravity.
8. Plan the welding sequence
A well-planned welding sequence involves placing weld metal at different points of the assembly so that, as the structure shrinks in one place, it counteracts the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a complete joint penetration groove weld in a butt joint, as in Fig. 3-7(k). Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Fig. 3-7(l). In these examples, the shrinkage in weld No. 1 is balanced by the shrinkage in weld No. 2.
Clamps, jigs, and fixtures that lock parts into a desired position and hold them until welding is finished are probably the most widely used means for controlling distortion in small assemblies or components. It was mentioned earlier in this section that the restraining force provided by clamps increases internal stresses in the weldment until the yield point of the weld metal is reached. For typical welds on low-carbon plate, this stress level would approximate 45,000 psi. One might expect this stress to cause considerable movement or distortion after the welded part is removed from the jig or clamps. This does not occur, however, since the strain (unit contraction) from this stress is very low compared to the amount of movement that would occur if no restraint were used during welding.
9. Remove shrinkage forces after welding
Peening is one way to counteract the shrinkage forces of a weld bead as it cools. Essentially, peening the bead stretches it and makes it thinner, thus relieving (by plastic deformation) the stresses induced by contraction as the metal cools. But this method must be used with care. For example, a root bead should never be peened, because of the danger of either concealing a crack or causing one. Generally, peening is not permitted on the final pass, because of the possibility of covering a crack and interfering with inspection, and because of the undesirable work-hardening effect. Thus, the utility of the technique is limited, even though there have been instances where between-pass peening proved to be the only solution for a distortion or cracking problem. Before peening is used on a job, engineering approval should be obtained.
Another method for removing shrinkage forces is by thermal stress relieving - controlled heating of the weldment to an elevated temperature, followed by controlled cooling. Sometimes two identical weldments are clamped back to back, welded, and then stress-relieved while being held in this straight condition. The residual stresses that would tend to distort the weldments are thus minimized.
10. Minimize welding time
Since complex cycles of heating and cooling take place during welding, and since time is required for heat transmission, the time factor affects distortion. In general, it is desirable to finish the weld quickly, before a large volume of surrounding metal heats up and expands. The welding process used, type and size of electrode, welding current, and speed of travel, thus, affect the degree of shrinkage and distortion of a weldment. The use of mechanized welding equipment reduces welding time and the amount of metal affected by heat and, consequently, distortion. For example, depositing a given-size weld on thick plate with a process operating at 175 amp, 25 volts, and 3 ipm requires 87,500 joules of energy per linear inch of weld (also known as heat input). A weld with approximately the same size produced with a process operating at 310 amp, 35 volts, and 8 ipm requires 81,400 joules per linear inch. The weld made with the higher heat input generally results in a greater amount of distortion. (note: I don't want to use the words "excessive" and "more than necessary" because the weld size is, in fact, tied to the heat input. In general, the fillet weld size (in inches) is equal to the square root of the quantity of the heat input (kJ/in) divided by 500. Thus these two welds are most likely not the same size.
Other Techniques for Distortion Control
Water-Cooled Jig Various techniques have been developed to control distortion on specific weldments. In sheet-metal welding, for example, a water-cooled jig (Fig. 3-33) is useful to carry heat away from the welded components. Copper tubes are brazed or soldered to copper holding clamps, and the water is circulated through the tubes during welding. The restraint of the clamps also helps minimize distortion.
Strongback
The "strongback" is another useful technique for distortion control during butt welding of plates, as in Fig. 3-34(a). Clips are welded to the edge of one plate and wedges are driven under the clips to force the edges into alignment and to hold them during welding.
Thermal Stress Relieving
Except in special situations, stress relief by heating is not used for correcting distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished.
Summary: A Checklist to Minimize DistortionFollow this checklist in order to minimize distortion in the design and fabrication of weldments:
Do not overweld
Control fitupUse intermittent welds where possible and consistent with design requirements
Use the smallest leg size permissible when fillet welding
For groove welds, use joints that will minimize the volume of weld metal. Consider double-sided joints instead of single-sided jointsWeld alternately on either side of the joint when possible with multiple-pass welds
Use minimal number of weld passes
Use low heat input procedures. This generally means high deposition rates and higher travel speedsUse welding positioners to achieve the maximum amount of flat-position welding. The flat position permits the use of large-diameter electrodes and high-
deposition-rate welding procedures
Balance welds about the neutral axis of the memberDistribute the welding heat as evenly as possible through a planned welding sequence and weldment positioningWeld toward the unrestrained part of the member
Use clamps, fixtures, and strongbacks to maintain fitup and alignment
Prebend the members or preset the joints to let shrinkage pull them back into alignment
Sequence subassemblies and final assemblies so that the welds being made continually balance each other around the neutral axis of the section
Following these techniques will help minimize the effects of distortion and residual stresses.
Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all welding on one side of a part will cause much more distortion than if the welds are alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion, such as physical and mechanical properties that change as heat is applied. For example, as the temperature of the weld area increases, yield strength, elasticity, and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase (Fig. 3-1). These changes, in turn, affect heat flow and uniformity of heat distribution.
Fig. 3-1 Changes in the properties of steel with
increases in temperature complicate analysis of what happens during the
welding cycle - and, thus, understanding of the factors contributing to
weldment distortion.
Reasons for Distortion
To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Fig. 3-2. As the bar is uniformly heated, it expands in all directions, as shown in Fig. 3-2(a). As the metal cools to room temperature it contracts uniformly to its original dimensions.
To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Fig. 3-2. As the bar is uniformly heated, it expands in all directions, as shown in Fig. 3-2(a). As the metal cools to room temperature it contracts uniformly to its original dimensions.
Fig. 3-2 If a steel bar is uniformly heated while
unrestrained, as in (a), it will expand in all directions and return to
its original dimentions on cooling. If restrained, as in (b), during
heating, it can expand only in the vertical direction - become thicker.
On cooling, the deformed bar contracts uniformly, as shown in (c), and,
thus, is permanently deformed. This is a simplified explanation of basic
cause of distortion in welding assemblies.
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But if the steel bar is restrained -as in a vise - while it is heated, as shown in Fig. 3-2(b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction (in thickness) and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Fig. 3-2 (c). The bar is now shorter, but thicker. It has been permanently deformed, or distorted. (For simplification, the sketches show this distortion occurring in thickness only. But in actuality, length is similarly affected.)
In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. On cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but it is restrained from doing so by the adjacent base metal. Because of this, stresses develop within the weld and the adjacent base metal. At this point, the weld stretches (or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. But only those stresses that exceed the yield strength of the weld metal are relieved by this straining. By the time the weld reaches room temperature - assuming complete restraint of the base metal so that it cannot move - the weld will contain locked-in tensile stresses approximately equal to the yield strength of the metal. If the restraints (clamps that hold the workpiece, or an opposing shrinkage force) are removed, the residual stresses are partially relieved as they cause the base metal to move, thus distorting the weldment.
Shrinkage Control - What You Can Do to Minimize Distortion
To prevent or minimize weld distortion, methods must be used both in design and during welding to overcome the effects of the heating and cooling cycle. Shrinkage cannot be prevented, but it can be controlled. Several ways can be used to minimize distortion caused by shrinkage:
1. Do not overweld
The more metal placed in a joint, the greater the shrinkage forces. Correctly sizing a weld for the requirements of the joint not only minimizes distortion, but also saves weld metal and time. The amount of weld metal in a fillet weld can be minimized by the use of a flat or slightly convex bead, and in a butt joint by proper edge preparation and fitup. The excess weld metal in a highly convex bead does not increase the allowable strength in code work, but it does increase shrinkage forces.
When welding heavy plate (over 1 inch thick) bevelling or even double bevelling can save a substantial amount of weld metal which translates into much less distortion automatically.
In general, if distortion is not a problem, select the most economical joint. If distortion is a problem, select either a joint in which the weld stresses balance each other or a joint requiring the least amount of weld metal.
2. Use intermittent weldingAnother way to minimize weld metal is to use intermittent rather than continuous welds where possible, as in Fig. 3-7(c). For attaching stiffeners to plate, for example, intermittent welds can reduce the weld metal by as much as 75 percent yet provide the needed strength.
Fig. 3-7 Distortion can be prevented or minimized by
techniques that defeat - or use constructively - the effects of the
heating and cooling cycle.
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3. Use as few weld passes as possible
Fewer passes with large electrodes, Fig. 3-7(d), are preferable to a greater number of passes with small electrodes when transverse distortion could be a problem. Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used.
4. Place welds near the neutral axis
Distortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. Figure 3-7(e) illustrates this. Both design of the weldment and welding sequence can be used effectively to control distortion.
Fig. 3-7 Distortion can be prevented or minimized by
techniques that defeat - or use constructively - the effects of the
heating and cooling cycle.
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5. Balance welds around the neutral axis
This practice, shown in Fig. 3-7(f), offsets one shrinkage force with another to effectively minimize distortion of the weldment. Here, too, design of the assembly and proper sequence of welding are important factors.
6. Use backstep weldingIn the backstep technique, the general progression of welding may be, say, from left to right, but each bead segment is deposited from right to left as in Fig. 3-7(g). As each bead segment is placed, the heated edges expand, which temporarily separates the plates at B. But as the heat moves out across the plate to C, expansion along outer edges CD brings the plates back together. This separation is most pronounced as the first bead is laid. With successive beads, the plates expand less and less because of the restraint of prior welds. Backstepping may not be effective in all applications, and it cannot be used economically in automatic welding.
Fig. 3-7 Distortion can be prevented or minimized by
techniques that defeat - or use constructively - the effects of the
heating and cooling cycle.
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7. Anticipate the shrinkage forcesPresetting parts (at first glance, I thought that this was referring to overhead or vertical welding positions, which is not the case) before welding can make shrinkage perform constructive work. Several assemblies, preset in this manner, are shown in Fig. 3-7(h). The required amount of preset for shrinkage to pull the plates into alignment can be determined from a few trial welds.
Prebending, presetting or prespringing the parts to be welded, Fig. 3-7(i), is a simple example of the use of opposing mechanical forces to counteract distortion due to welding. The top of the weld groove - which will contain the bulk of the weld metal - is lengthened when the plates are preset. Thus the completed weld is slightly longer than it would be if it had been made on the flat plate. When the clamps are released after welding, the plates return to the flat shape, allowing the weld to relieve its longitudinal shrinkage stresses by shortening to a straight line. The two actions coincide, and the welded plates assume the desired flatness.
Another common practice for balancing shrinkage forces is to position identical weldments back to back, Fig. 3-7(j), clamping them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released. Prebending can be combined with this method by inserting wedges at suitable positions between the parts before clamping.
In heavy weldments, particularly, the rigidity of the members and their arrangement relative to each other may provide the balancing forces needed. If these natural balancing forces are not present, it is necessary to use other means to counteract the shrinkage forces in the weld metal. This can be accomplished by balancing one shrinkage force against another or by creating an opposing force through the fixturing. The opposing forces may be: other shrinkage forces; restraining forces imposed by clamps, jigs, or fixtures; restraining forces arising from the arrangement of members in the assembly; or the force from the sag in a member due to gravity.
8. Plan the welding sequence
A well-planned welding sequence involves placing weld metal at different points of the assembly so that, as the structure shrinks in one place, it counteracts the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a complete joint penetration groove weld in a butt joint, as in Fig. 3-7(k). Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Fig. 3-7(l). In these examples, the shrinkage in weld No. 1 is balanced by the shrinkage in weld No. 2.
Fig. 3-7 Distortion can be prevented or minimized by
techniques that defeat - or use constructively - the effects of the
heating and cooling cycle.
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Clamps, jigs, and fixtures that lock parts into a desired position and hold them until welding is finished are probably the most widely used means for controlling distortion in small assemblies or components. It was mentioned earlier in this section that the restraining force provided by clamps increases internal stresses in the weldment until the yield point of the weld metal is reached. For typical welds on low-carbon plate, this stress level would approximate 45,000 psi. One might expect this stress to cause considerable movement or distortion after the welded part is removed from the jig or clamps. This does not occur, however, since the strain (unit contraction) from this stress is very low compared to the amount of movement that would occur if no restraint were used during welding.
9. Remove shrinkage forces after welding
Peening is one way to counteract the shrinkage forces of a weld bead as it cools. Essentially, peening the bead stretches it and makes it thinner, thus relieving (by plastic deformation) the stresses induced by contraction as the metal cools. But this method must be used with care. For example, a root bead should never be peened, because of the danger of either concealing a crack or causing one. Generally, peening is not permitted on the final pass, because of the possibility of covering a crack and interfering with inspection, and because of the undesirable work-hardening effect. Thus, the utility of the technique is limited, even though there have been instances where between-pass peening proved to be the only solution for a distortion or cracking problem. Before peening is used on a job, engineering approval should be obtained.
Another method for removing shrinkage forces is by thermal stress relieving - controlled heating of the weldment to an elevated temperature, followed by controlled cooling. Sometimes two identical weldments are clamped back to back, welded, and then stress-relieved while being held in this straight condition. The residual stresses that would tend to distort the weldments are thus minimized.
10. Minimize welding time
Since complex cycles of heating and cooling take place during welding, and since time is required for heat transmission, the time factor affects distortion. In general, it is desirable to finish the weld quickly, before a large volume of surrounding metal heats up and expands. The welding process used, type and size of electrode, welding current, and speed of travel, thus, affect the degree of shrinkage and distortion of a weldment. The use of mechanized welding equipment reduces welding time and the amount of metal affected by heat and, consequently, distortion. For example, depositing a given-size weld on thick plate with a process operating at 175 amp, 25 volts, and 3 ipm requires 87,500 joules of energy per linear inch of weld (also known as heat input). A weld with approximately the same size produced with a process operating at 310 amp, 35 volts, and 8 ipm requires 81,400 joules per linear inch. The weld made with the higher heat input generally results in a greater amount of distortion. (note: I don't want to use the words "excessive" and "more than necessary" because the weld size is, in fact, tied to the heat input. In general, the fillet weld size (in inches) is equal to the square root of the quantity of the heat input (kJ/in) divided by 500. Thus these two welds are most likely not the same size.
Other Techniques for Distortion Control
Water-Cooled Jig Various techniques have been developed to control distortion on specific weldments. In sheet-metal welding, for example, a water-cooled jig (Fig. 3-33) is useful to carry heat away from the welded components. Copper tubes are brazed or soldered to copper holding clamps, and the water is circulated through the tubes during welding. The restraint of the clamps also helps minimize distortion.
Fig. 3-33 A water-cooled jig for rapid removal of heat when welding sheet meta.
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Strongback
The "strongback" is another useful technique for distortion control during butt welding of plates, as in Fig. 3-34(a). Clips are welded to the edge of one plate and wedges are driven under the clips to force the edges into alignment and to hold them during welding.
Fig. 3-34 Various strongback arrangements to control distortion during butt-welding.
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Thermal Stress Relieving
Except in special situations, stress relief by heating is not used for correcting distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished.
Summary: A Checklist to Minimize DistortionFollow this checklist in order to minimize distortion in the design and fabrication of weldments:
Do not overweld
Control fitupUse intermittent welds where possible and consistent with design requirements
Use the smallest leg size permissible when fillet welding
For groove welds, use joints that will minimize the volume of weld metal. Consider double-sided joints instead of single-sided jointsWeld alternately on either side of the joint when possible with multiple-pass welds
Use minimal number of weld passes
Use low heat input procedures. This generally means high deposition rates and higher travel speedsUse welding positioners to achieve the maximum amount of flat-position welding. The flat position permits the use of large-diameter electrodes and high-
deposition-rate welding procedures
Balance welds about the neutral axis of the memberDistribute the welding heat as evenly as possible through a planned welding sequence and weldment positioningWeld toward the unrestrained part of the member
Use clamps, fixtures, and strongbacks to maintain fitup and alignment
Prebend the members or preset the joints to let shrinkage pull them back into alignment
Sequence subassemblies and final assemblies so that the welds being made continually balance each other around the neutral axis of the section
Following these techniques will help minimize the effects of distortion and residual stresses.
Welding Circuit Shock Hazards
Utilizing proper grounding in the welding environment is a good
practice, but it does not remove all possibility of electrical shock.
The welding circuit is energized by welding voltage. A person will
receive a shock if they become the electrical path across the welding
circuit. Precautions must be taken to insulate the welder from the
welding circuit. Use dry insulating gloves and other insulating means.
Also maintain insulation on weld cables, electrode holders, guns and
torches to provide protection.
Similarly, electric shock originating from the
electrical supply system can be prevented. Proper maintenance of
electrical equipment and extension cords will insulate the welder from
electrical sources.
Nondestructive Weld Examination
The ABC's of Nondestructive Weld ExaminationReprinted courtesy of Welding Journal magazine
An understanding of the benefits and drawbacks of each form of nondestructive examination can help you choose the best method for your application.
The philosophy that often guides the fabrication of welded assemblies and structures is "to assure weld quality." However, the term "weld quality" is relative. The application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The first step in assuring weld quality is to determine the degree required by the application. A standard should be established based on the service requirements.
Standards designed to impart weld quality may differ from job to job, but the use of appropriate weld techniques can provide assurance that the applicable standards are being met. Whatever the standard of quality, all welds should be inspected, even if the inspection involves nothing more than the welder looking after his own work after each weld pass. A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality.
Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic particle, ultra-sonic and radiographic (X-ray). The growing use of computerization with some methods provides added image enhancement, and allows real-time or near real-time viewing, comparative inspections and archival capabilities. A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.
An understanding of the benefits and drawbacks of each form of nondestructive examination can help you choose the best method for your application.
The philosophy that often guides the fabrication of welded assemblies and structures is "to assure weld quality." However, the term "weld quality" is relative. The application determines what is good or bad. Generally, any weld is of good quality if it meets appearance requirements and will continue indefinitely to do the job for which it is intended. The first step in assuring weld quality is to determine the degree required by the application. A standard should be established based on the service requirements.
Standards designed to impart weld quality may differ from job to job, but the use of appropriate weld techniques can provide assurance that the applicable standards are being met. Whatever the standard of quality, all welds should be inspected, even if the inspection involves nothing more than the welder looking after his own work after each weld pass. A good-looking weld surface appearance is many times considered indicative of high weld quality. However, surface appearance alone does not assure good workmanship or internal quality.
Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrant, magnetic particle, ultra-sonic and radiographic (X-ray). The growing use of computerization with some methods provides added image enhancement, and allows real-time or near real-time viewing, comparative inspections and archival capabilities. A review of each method will help in deciding which process or combination of processes to use for a specific job and in performing the examination most effectively.
Visual Inspection (VT)
Visual Inspection (VT)Visual inspection is often
the most cost-effective method, but it must take place prior to, during
and after welding. Many standards require its use before other methods,
because there is no point in submitting an obviously bad weld to
sophisticated inspection techniques. The ANSI/AWS D1.1, Structural
Welding Code - Steel, states, "Welds subject to nondestructive
examination shall have been found acceptable by visual inspection."
Visual inspection requires little equipment. Aside from good eyesight
and sufficient light, all it takes is a pocket rule, a weld size gauge, a
magnifying glass, and possibly a straight edge and square for checking
straightness, alignment and perpendicularity.
Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used.
During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld defects that can be recognized visually are cracking, surface slag inclusions, surface porosity and undercut.
On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of metal filler is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass is most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses.
Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress.
After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated.
Before checking for surface flaws, welds must be cleaned of slag. Shotblasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld."
Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability.
Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used.
During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld defects that can be recognized visually are cracking, surface slag inclusions, surface porosity and undercut.
On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of metal filler is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass is most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses.
Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress.
After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated.
Before checking for surface flaws, welds must be cleaned of slag. Shotblasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld."
Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability.
Radiographic Inspection
Radiographic InspectionRadiography (X-ray) is one
of the most important, versatile and widely accepted of all the
nondestructive examination methods - Fig. 1. X-ray is used to determine
internal soundness of the welds. The term "X-ray quality," widely used
to indicate high quality in welds, arises from this inspection method.
Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show discontinuities and inclusions within the opaque material. The permanent film record of the internal conditions will show the basic information by which weld soundness and be determined.
X-rays are produced by high-voltage generators. As the high voltage applied to an X-ray tube is increased, the wavelength of the emitted X-ray becomes shorter , providing more penetrating power. Gamma rays are produced by the atomic disintegration of radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt 60 and Iridium 192. Gamma rays emitted from these isotopes are similar to X-rays, except their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are considerably longer due to the longer intensity.
When X-rays or gamma rays are directed at a section of weldment , not all of the radiation passes are through the metal. Different materials, depending on their density, thickness and atomic number, will absorb different wavelengths of radiant energy.
The degree to which the different materials absorb these rays determines the intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. Thicker areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the radiograph will be lighter - Fig 2.
Whether in the shop or in the field, the reliability and interpretive value of radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with the background. To be sure that a radiographic exposure produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on the part so that its image will be produced on the radiograph.
IQI's used to determine radiographic quality are also called penetrameters. A standard hole-type penetrameter is a rectangular piece of metal with three drilled holes of set diameters. The thickness of the piece of metal is a percentage of the thickness of the specimen being radiographed. The diameter of each hole is different and is a given multiple of the penetrameter thickness. Wire-type penetrameters are also widely used, especially outside the United States. They consist of several pieces of wire, each of a different diameter. Sensitivity is determined by the smallest diameter of wire that can be clearly seen on the radiograph.
A penetrameter is not an indicator or gauge to measure the size of a discontinuity or the minimum detectable flaw size. It is an indicator of the quality of the radiographic technique.
Radiographic images are not always easy to interpret. Film handling marks and streaks, fog and spots caused by developing errors may make it difficult to identify defects. Such film artifacts may mask weld discontinuities.
Surface defects will show up on the film and must be recognized. Because the angle of exposure will also influence the radiograph, it is difficult or impossible to analyze fillet welds by this method. Because a radiograph compresses all the defects that occur throughout the thickness of the weld into one plane, it tends to give an exaggerated impression of scattered type defects such as porosity or inclusions.
An X-ray image of the interior of the weld may be viewed on a fluorescent screen, as well as on developed film. This makes it possible to inspect parts faster and at a lower cost, but the image definition is poorer. Computerization has made it possible to overcome many of the shortcomings of radiographic imaging by linking the fluorescent screen with a video camera. Instead of waiting for film to be developed, the images can be viewed in real time. This can improve quality and reduce costs on production applications such as pipe welding, where a problem can be identified and corrected quickly.
By digitizing the image and loading it into a computer, the image can be enhanced and analyzed to a degree never before possible. Multiple images can be superimposed. Pixel values can be adjusted to change shading and contrast, bringing out small flaws and discontinuities that would not show up on film. Colors can be assigned to the various shades of gray to further enhance the image and make flaws stand out better. The process of digitizing an image taken from the fluorescent screen - having that image computer enhanced and transferred to a viewing monitor - takes only a few seconds. However, because there is a time delay, we can no longer consider this "real time." It is called "radioscopy imagery."
Existing films can be digitized to achieve the same results and improve the analysis process. Another advantage is the ability to archive images on laser optical disks, which take up far less space than vaults of old films and are much easier to recall when needed.
Industrial radiography, then, is an inspection method using X-rays and gamma rays as a penetrating medium, and densitized film as a recording medium, to obtain a photographic record of internal quality. Generally, defects in welds consist either of a void in the weld metal itself or an inclusion that differs in density from the surrounding weld metal.
Radiographic equipment produces radiation that can be harmful to body tissue in excessive amounts, so all safety precautions should be followed closely. All instructions should be followed carefully to achieve satisfactory results. Only personnel who are trained in radiation safety and qualified as industrial radiographers should be permitted to do radiographic testing.
Radiography is based on the ability of X-rays and gamma rays to pass through metal and other materials opaque to ordinary light, and produce photographic records of the transmitted radiant energy. All materials will absorb known amounts of this radiant energy and, therefore, X-rays and gamma rays can be used to show discontinuities and inclusions within the opaque material. The permanent film record of the internal conditions will show the basic information by which weld soundness and be determined.
X-rays are produced by high-voltage generators. As the high voltage applied to an X-ray tube is increased, the wavelength of the emitted X-ray becomes shorter , providing more penetrating power. Gamma rays are produced by the atomic disintegration of radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt 60 and Iridium 192. Gamma rays emitted from these isotopes are similar to X-rays, except their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are considerably longer due to the longer intensity.
When X-rays or gamma rays are directed at a section of weldment , not all of the radiation passes are through the metal. Different materials, depending on their density, thickness and atomic number, will absorb different wavelengths of radiant energy.
The degree to which the different materials absorb these rays determines the intensity of the rays penetrating through the material. When variations of these rays are recorded, a means of seeing inside the material is available. The image on a developed photo-sensitized film is known as a radiograph. Thicker areas of the specimen or higher density material (tungsten inclusion), will absorb more radiation and their corresponding areas on the radiograph will be lighter - Fig 2.
Whether in the shop or in the field, the reliability and interpretive value of radiographic images are a function of their sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness of its image and its contrast with the background. To be sure that a radiographic exposure produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on the part so that its image will be produced on the radiograph.
IQI's used to determine radiographic quality are also called penetrameters. A standard hole-type penetrameter is a rectangular piece of metal with three drilled holes of set diameters. The thickness of the piece of metal is a percentage of the thickness of the specimen being radiographed. The diameter of each hole is different and is a given multiple of the penetrameter thickness. Wire-type penetrameters are also widely used, especially outside the United States. They consist of several pieces of wire, each of a different diameter. Sensitivity is determined by the smallest diameter of wire that can be clearly seen on the radiograph.
A penetrameter is not an indicator or gauge to measure the size of a discontinuity or the minimum detectable flaw size. It is an indicator of the quality of the radiographic technique.
Radiographic images are not always easy to interpret. Film handling marks and streaks, fog and spots caused by developing errors may make it difficult to identify defects. Such film artifacts may mask weld discontinuities.
Surface defects will show up on the film and must be recognized. Because the angle of exposure will also influence the radiograph, it is difficult or impossible to analyze fillet welds by this method. Because a radiograph compresses all the defects that occur throughout the thickness of the weld into one plane, it tends to give an exaggerated impression of scattered type defects such as porosity or inclusions.
An X-ray image of the interior of the weld may be viewed on a fluorescent screen, as well as on developed film. This makes it possible to inspect parts faster and at a lower cost, but the image definition is poorer. Computerization has made it possible to overcome many of the shortcomings of radiographic imaging by linking the fluorescent screen with a video camera. Instead of waiting for film to be developed, the images can be viewed in real time. This can improve quality and reduce costs on production applications such as pipe welding, where a problem can be identified and corrected quickly.
By digitizing the image and loading it into a computer, the image can be enhanced and analyzed to a degree never before possible. Multiple images can be superimposed. Pixel values can be adjusted to change shading and contrast, bringing out small flaws and discontinuities that would not show up on film. Colors can be assigned to the various shades of gray to further enhance the image and make flaws stand out better. The process of digitizing an image taken from the fluorescent screen - having that image computer enhanced and transferred to a viewing monitor - takes only a few seconds. However, because there is a time delay, we can no longer consider this "real time." It is called "radioscopy imagery."
Existing films can be digitized to achieve the same results and improve the analysis process. Another advantage is the ability to archive images on laser optical disks, which take up far less space than vaults of old films and are much easier to recall when needed.
Industrial radiography, then, is an inspection method using X-rays and gamma rays as a penetrating medium, and densitized film as a recording medium, to obtain a photographic record of internal quality. Generally, defects in welds consist either of a void in the weld metal itself or an inclusion that differs in density from the surrounding weld metal.
Radiographic equipment produces radiation that can be harmful to body tissue in excessive amounts, so all safety precautions should be followed closely. All instructions should be followed carefully to achieve satisfactory results. Only personnel who are trained in radiation safety and qualified as industrial radiographers should be permitted to do radiographic testing.
Magnetic Particle Inspection (MT)
Magnetic Particle Inspection (MT)Magnetic particle inspection is a method of locating and defining discontinuities in magnetic materials. It is excellent for detecting surface defects in welds, including discontinuities that are too small to be seen with the naked eye, and those that are slightly subsurface.
This method may be used to inspect plate edges prior to welding, in process inspection of each weld pass or layer, postweld evaluation and to inspect
repairs - Fig. 3.
It is a good method for detecting surface cracks of all sizes in both the weld and adjacent base metal, subsurface cracks, incomplete fusion, undercut and inadequate penetration in the weld, as well as defects on the repaired edges of the base metal. Although magnetic particle testing should not be a substitute for radiography or ultrasonics for subsurface evaluations, it may present an advantage over their methods in detecting tight cracks and surface discontinuities.
With this method, probes are usually placed on each side of the area to be inspected, and a high amperage is passed through the workplace between them. A magnetic flux is produced at right angles to the flow of current - Fig. 3. When these lines of force encounter a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more tenaciously than elsewhere, forming an indication of the discontinuity.
For this indication to develop, the discontinuity must be angled against the magnetic lines of force. Thus, when current is passed longitudinally through a workpiece, only longitudinal flaws will show. Putting the workpiece inside a solenoid coil will create longitudinal lines of force (Fig. 3) that cause transverse and angular cracks to become visible when the magnetic powder is applied.
Although much simpler to use than radiographic inspection, the magnetic particle method is limited to use with ferromagnetic materials and cannot be used with austenitic steels. A joint between a base metal and a weld of different magnetic characteristics will create magnetic discontinuities that may falsely be interpreted as unsound. On the other hand, a true defect can be obscured by the powder clinging over the harmless magnetic discontinuity. Sensitivity decreases with the size of the defect and is also less with round cracks such as gas pockets. It is best with elongated forms, such as cracks, and is limited to surface flaws and some subsurface flaws, mostly on thinner materials.
Because the field must be distorted sufficiently to create the external leakage required to identify flaws, the fine, elongated discontinuities, such as hairline cracks, seams or inclusions that are parallel to the magnetic field, will not show up. They can be developed by changing the direction of the field, and it is advisable to apply the field from two directions, preferably at right angles to each other.
Magnetic powders maybe applied dry or wet. The dry powder method is popular for inspecting heavy weldments, while the wet method is often used in inspecting aircraft components. Dry powder is dusted uniformly over the work with a spray gun, dusting bag or atomizer. The finely divided magnetic particles are coated to increase their mobility and are available in gray, black and red colors to improve visibility. In the wet method, very fine red or black particles are suspended in water or light petroleum distillate. This can be flowed or sprayed on, or the part may be dipped into the liquid. The wet method is more sensitive than the dry method, because it allows the use of finer particles that can detect exceedingly fine defects. Fluorescent powders may be used for further sensitivity and are especially useful for locating discontinuities in corners, keyways, splines and deep holes.
Liquid Penetrant Inspection (PT)
Liquid Penetrant Inspection (PT)Surface cracks
and pinholes that are not visible to the naked eye can be located by the
liquid penetrant inspection. It is widely used to locate leaks in welds
and can be applied with austentic steels and nonferrous materials where
magnetic particle inspection would be useless.
Liquid penetrant inspection is often referred to as an extension of the visual inspection method. Many standards, such as the AWS D.1. Code, say that "welds subject to liquid penetrant testing�shall be evaluated on the basis of the requirements for visual inspection."
Two types of penetrating liquids are used - fluorescent and visible dye. With fluorescent penetrant inspection, a highly fluorescent liquid with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the liquid into the surface openings, and the excess is then removed. A "developer" is used to draw the penetrant to the surface, and the resulting indication is viewed by ultraviolet (black) light. The high contrast between the fluorescent material and the object makes it possible to detect minute traces of penetrant that indicate surface defects.
Dye penetrant inspection is similar, except that vividly colored dyes visible under ordinary light are used - Fig. 4. Normally, a white developer is used with the dye penetrants that creates a sharply contrasting background to the vivid dye color. This allows greater portability by eliminating the need for ultraviolet light.
The part to be inspected must be clean and dry, because any foreign matter could close the cracks or pinholes and exclude the penetrant. Penetrants can be applied by dipping, spraying or brushing, but sufficient time must be allowed for the liquid to be fully absorbed into the discontinuities. This may take an hour or more in very exacting work.
Liquid penetrant inspection is widely used for leak detection. A common procedure is to apply fluorescent material to one side of a joint, wait an adequate time for capillary action to take place, and then view the other side with ultraviolet light. In thin-walled vessels, this technique will identify leaks that ordinarily would not be located by the usual air test with pressures of 5-20 lb/in.2 When wall thickness exceeds � in., however, sensitivity of the leak test decreases.
Liquid penetrant inspection is often referred to as an extension of the visual inspection method. Many standards, such as the AWS D.1. Code, say that "welds subject to liquid penetrant testing�shall be evaluated on the basis of the requirements for visual inspection."
Two types of penetrating liquids are used - fluorescent and visible dye. With fluorescent penetrant inspection, a highly fluorescent liquid with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the liquid into the surface openings, and the excess is then removed. A "developer" is used to draw the penetrant to the surface, and the resulting indication is viewed by ultraviolet (black) light. The high contrast between the fluorescent material and the object makes it possible to detect minute traces of penetrant that indicate surface defects.
Dye penetrant inspection is similar, except that vividly colored dyes visible under ordinary light are used - Fig. 4. Normally, a white developer is used with the dye penetrants that creates a sharply contrasting background to the vivid dye color. This allows greater portability by eliminating the need for ultraviolet light.
The part to be inspected must be clean and dry, because any foreign matter could close the cracks or pinholes and exclude the penetrant. Penetrants can be applied by dipping, spraying or brushing, but sufficient time must be allowed for the liquid to be fully absorbed into the discontinuities. This may take an hour or more in very exacting work.
Liquid penetrant inspection is widely used for leak detection. A common procedure is to apply fluorescent material to one side of a joint, wait an adequate time for capillary action to take place, and then view the other side with ultraviolet light. In thin-walled vessels, this technique will identify leaks that ordinarily would not be located by the usual air test with pressures of 5-20 lb/in.2 When wall thickness exceeds � in., however, sensitivity of the leak test decreases.
Ultrasonic Inspection (UT)
Ultrasonic Inspection (UT)Ultrasonic inspection is a
method of detecting discontinuities by directing a high-frequency sound
beam through the base plate and weld on a predictable path. When the
sound beam's plate path strikes an interruption in the material
continuity, some of the sound is reflected back. The sound is collected
by the instrument, amplified and displayed as a vertical trance on a
video screen - Fig. 5.
Both surface and subsurface detects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be detected by other methods.
The ultrasonic unit contains a crystal of quartz or other piezoelectric material encapsulated in a transducer or probe. When a voltage is applied, the crystal vibrates rapidly. As an ultrasonic transducer is held against the metal to be inspected, it imparts mechanical vibrations of the same frequency as the crystal through a couplet material into the base metal and weld. These vibrational waves are propagated through the material until they reach a discontinuity or change in density. At these points, some of the vibrational energy is reflected back. As the current that causes the vibration is shut off and on at 60-1000 times per second, the quartz crystal intermittently acts as a receiver to pick up the reflected vibrations. These cause pressure on the crystal and generate an electrical current. Fed to a video screen, this current produces vertical deflections on the horizontal base line. The resulting pattern on the face of the tube represents the reflected signal and the discontinuity. Compact portable ultrasonic equipment is available for field inspection and is commonly used on bridge and structural work.
Ultrasonic testing is less suitable than other NDE methods for determining porosity in welds, because round gas pores respond to ultrasonic tests as a series of single point reflectors. This results in low-amplitude responses that are easily confused with "base-line noise" inherent with testing parameters. However, it is the preferred test method for detecting plainer-type discontinuities and lamination.
Portable ultrasonic equipment is available with digital operation and microprocessor controls. These instruments may have built-in memory and can provide hard-copy printouts or video monitoring and recording. They can be interfaced with computers, which allows further analysis, documentation and archiving, much as with radiographic data. Ultrasonic examination requires expert interpretation from highly skilled and extensively trained personnel.
Both surface and subsurface detects in metals can be detected, located and measured by ultrasonic inspection, including flaws too small to be detected by other methods.
The ultrasonic unit contains a crystal of quartz or other piezoelectric material encapsulated in a transducer or probe. When a voltage is applied, the crystal vibrates rapidly. As an ultrasonic transducer is held against the metal to be inspected, it imparts mechanical vibrations of the same frequency as the crystal through a couplet material into the base metal and weld. These vibrational waves are propagated through the material until they reach a discontinuity or change in density. At these points, some of the vibrational energy is reflected back. As the current that causes the vibration is shut off and on at 60-1000 times per second, the quartz crystal intermittently acts as a receiver to pick up the reflected vibrations. These cause pressure on the crystal and generate an electrical current. Fed to a video screen, this current produces vertical deflections on the horizontal base line. The resulting pattern on the face of the tube represents the reflected signal and the discontinuity. Compact portable ultrasonic equipment is available for field inspection and is commonly used on bridge and structural work.
Ultrasonic testing is less suitable than other NDE methods for determining porosity in welds, because round gas pores respond to ultrasonic tests as a series of single point reflectors. This results in low-amplitude responses that are easily confused with "base-line noise" inherent with testing parameters. However, it is the preferred test method for detecting plainer-type discontinuities and lamination.
Portable ultrasonic equipment is available with digital operation and microprocessor controls. These instruments may have built-in memory and can provide hard-copy printouts or video monitoring and recording. They can be interfaced with computers, which allows further analysis, documentation and archiving, much as with radiographic data. Ultrasonic examination requires expert interpretation from highly skilled and extensively trained personnel.
Choices Control Quality
A good NDE inspection program must recognize the inherent limitations
of each process. For example, both radiography and ultrasound have
distinct orientation factors that may guide the choice of which process
to use for a particular job. Their strengths and weaknesses tend to
compliment each other. While radiography is unable to reliably detect
lamination-like defects, ultrasound is much better at it. On the other
hand, ultrasound is poorly suited to detecting scattered porosity, while
radiography is very good.
Whatever inspection techniques are used, paying attention to the "Five P's" of weld quality will help reduce subsequent inspection to a routine checking activity. Then, the proper use of NDE methods will serve as a check to keep variables in line and weld quality within standards.
The Five P's are:
1. Process Selection - the process must be right for the job.
2. Preparation - the joint configuration must be right and compatible with the welding process.
3. Procedures - the procedures must be spelled out in detail and followed religiously during welding.
4. Pretesting - full-scale mockups or simulated specimens should be used to prove that the process and procedures give the desired standard of quality.
5. Personnel - qualified people must be assigned to the job.
Whatever inspection techniques are used, paying attention to the "Five P's" of weld quality will help reduce subsequent inspection to a routine checking activity. Then, the proper use of NDE methods will serve as a check to keep variables in line and weld quality within standards.
The Five P's are:
1. Process Selection - the process must be right for the job.
2. Preparation - the joint configuration must be right and compatible with the welding process.
3. Procedures - the procedures must be spelled out in detail and followed religiously during welding.
4. Pretesting - full-scale mockups or simulated specimens should be used to prove that the process and procedures give the desired standard of quality.
5. Personnel - qualified people must be assigned to the job.
Why is preheat sometimes required before welding?
Preheating the steel to be welded slows the cooling rate in the weld
area. This may be necessary to avoid cracking of the weld metal or heat
affected zone. The need for preheat increases with steel thickness, weld
restraint, the carbon/alloy content of the steel, and the diffusible
hydrogen of the weld metal. Preheat is commonly applied with fuel gas
torches or electrical resistance heaters.
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