3-2(b), lateral expansion cannot take place. If restrained, as in (b), during heating, it can expand only in the vertical direction - become thicker. On cooling, the weld and the surrounding area contracts and shortens the workpiece as a result. It has been permanently deformed, or distorted. Welding distortion can often be minimized or eliminated through proper joint design, the use of appropriate welding techniques, and careful control of welding parameters. The cube would then only expand in y-direction and z-direction during heating. As the bar is uniformly heated, it expands in all directions, as shown in Fig. Solid and flux-cored wire for high deposition rates with the ability to weld over mild surface contamination. Shrinkage cannot be prevented, but it can be controlled. High speed, low spatter and controlled heat input solutions that ensure repeatability and high productivity. Several ways can be used to minimize distortion caused by shrinkage: Distortion can pose a major problem, since the final product is not the desired shape. It is almost impossible to predict accurately the amount of shrinking. On cooling, the deformed bar contracts uniformly, as shown in (c), and, thus, is permanently deformed. Figure 3-7(e) illustrates this. Control of Distortion in Weldments29 Welding Residual stresses and Welding Distortion behave in a contrary way Least root gap: As small as possible, but sufficient for good penetration Excessive gaps should be avoided Included angle should not exceed 60 For heavy sections, double-V preparation should be preferred. In such welds, the distortion or warpage can occur once the restraints have been removedaftercompletion of the welding. Place welds near the neutral axisDistortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. As each bead segment is placed, the heated edges expand, which temporarily separates the plates at B. This generally means high deposition rates and higher travel speedsUse welding positioners to achieve the maximum amount of flat-position welding. Welding distortion in a simple butt joint for a rectangular plate with single side weld, while allowing for the free expansion of the joint on both sides, can be divided into four types [21]: 1 . 3-7 Distortion can be prevented or minimized by techniques that defeat - or use constructively - the effects of the heating and cooling cycle. But in actuality, length is similarly affected.). Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds (see main photograph). As explained above, shrinkage stresses occur in fillet welds as well, in a manner similar to that in butt welds. It has been permanently deformed, or distorted. 8. The temperate differential is determined by thermal conductivity. 7. 3-7(f), offsets one shrinkage force with another to effectively minimize distortion of the weldment. This is the most common type of distortion that we come across. 3-7(k). 3-34(a). Subscribe to our newsletter to receive the latest news and events from TWI: This article covers several key issues on distortion in arc welded fabrications, especially basic types of and factors affecting the degree of distortion. This can be accomplished by balancing one shrinkage force against another or by creating an opposing force through the fixturing. Warpage definition in the dictionary says that it is the result of being bent or twisted out of shape, typically as a result of the effect of heat. What should be done to prevent warpage or distortion in welding of small parts such as sheet metal? They are: Reduce the volume of the weld Minimize the heat input Preheat Stress-relieving Reduce the number of passes needed for the joint Fixture the part Anticipate the distortion Remove the heat in the process Use symmetry about the neutral axis Mirror Identical Parts 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. Distortion is troublesome for a number of reasons, but one of the most critical is the potential creation of a weld that is not structurally sound. 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). The extent of distortion depends on various factors which we shall see later in this article. Let us see how distortion would happen in a butt weld between two plates. The flat position permits the use of large-diameter electrodes and high-, Balance welds about the neutral axis of the member, Distribute the welding heat as evenly as possible through a planned welding sequence and weldment positioning, Weld 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. These methods do not affect the hard heat affected zone of the weld. One might expect this stress to cause considerable movement or distortion after the welded part is removed from the jig or clamps. Water-Cooled Jig Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used. Initially, compressive stresses are created in the surrounding cold parent metal when the weld pool is formed due to the thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. 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. 3-2. On cooling, the deformed bar contracts uniformly, as shown in (c), and, thus, is permanently deformed. Distortion occurs in six main forms: Longitudinal shrinkage Transverse shrinkage Angular distortion Bowing and dishing Buckling Twisting The principal features of the more common forms of distortion for butt and fillet welds are shown below: The heat treatment also affects the mechanical properties of the weldment. Angular distortion causes bending in transverse direction, as illustrated in the figure below. Copyright 2023 TWI Ltd. All rights reserved. Figure 3-7(e) illustrates this. Likewise, when it cools it wants to contract by the same amount in all directions. The shrinkage forces increase with the amount of weld metal placed in the joint. Distortion due to elastic buckling is unstable: if you attempt to flatten a buckled plate, it will probably 'snap' through and dish out in the opposite direction. For example, as stainless steel has a higher coefficient of expansion than plain carbon steel, it is more likely to suffer from distortion. 3-7(c). 6. 3-7(d), are preferable to a greater number of passes with small electrodes when transverse distortion could be a problem. residual stresses are the result. If the base metal is thick, and has been prevented from distorting by use of heavy restraints, clamps and stiffeners, the shrinking weld metal still imposes stresses on the metal, which cannot now be relieved by deforming. As a result, the metal distorts because cool it must, and therefore contract it must. Distortion is troublesome for a number of reasons, but one of the most critical is the potential creation of a weld that is not structurally sound. When contra heating the surrounding material acts as the vice in the first example and prevents the material from moving. (4) The influences of gap correction and welding sequence on the final welding distortion can also be simulated. 3. When welding steel, the following allowances should be made to cover shrinkage at the assembly stage. Because of this, stresses develop within the weld and the adjacent base metal. The bigger the weld, the greater the shrinkage that will occur. Presetting involves the assembly of the components in such a way that they would compensate for the distortion and ensure that the fabrication is dimensionally correct after finishing. Prebending can be combined with this method by inserting wedges at suitable positions between the parts before clamping. In sheet-metal welding, for example, a water-cooled jig (Fig. It can be minimised in butt joints by adopting a joint type which balances the thermal stresses through the plate thickness. Residual stresses are not good for a weld. Jigs and welding fixtures help you to maintain accuracy and reduce distortion of metals when a joint is subjected to heat shrinkage forces after cooling. This practice, shown in Fig. 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. Place welds near the neutral axis If the stress that is generated from the expansion and contraction exceeds the parent metal yield strength, the joint may experience a localised plastic deformation. 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. tensile stress occurs when the rest of the metal resists contraction of the heated area and weld metal. Consider double-sided joints instead of single-sided joints, Weld alternately on either side of the joint when possible with multiple-pass welds, Use low heat input procedures. Fabrication of metal components and assemblies involving bending, forming, cutting and welding. Consider double-sided joints instead of single-sided joints, Weld alternately on either side of the joint when possible with multiple-pass welds, Use low heat input procedures. 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. Warping is formed when parts of the weld dish inside between several stiffened points. 9. For example, a double-sided in preference to a single-sided weld. 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. Brings the productivity of wire welding to outdoor applications, with no shielding gas required. from publication: Prediction of Welding Distortion of Large Structures | During welding, deformation is produced as an unavoidable . 1 is balanced by the shrinkage in weld No. Thermal Stress RelievingExcept in special situations, stress relief by heating is not used for correcting distortion. 3-5). 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. 3-7(c). In tension, residual stress may lead to running brittle cracks that can spread to low stress areas. Many small spots are substantially more effective than only a few . But as the heat moves out across the plate to C, expansion along outer edges CD brings the plates back together. This study identified the effect of welding electrode spacing on the distortion of AA5052 aluminum plates and different mechanical . Fig. Shrinkage Control - What You Can Do to Minimize DistortionTo 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. 8: Illustration of good joint design. 9. 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. Remove shrinkage forces after weldingPeening is one way to counteract the shrinkage forces of a weld bead as it cools. 3-34 Various strongback arrangements to control distortion during butt-welding. But if the steel bar is restrained -as in a vise - while it is heated, as shown in Fig. Within the contact tip of the modern approach, the wires are electrically insulated from one another. Several techniques, individually or in combination with others, can be applied for controlling distortion in welding of smaller structures. This distortion comes from residual stress. But, since volume expansion must occur during the heating, the bar expands in a vertical direction (in thickness) and becomes thicker. Do not overweld 7: weld thermal cycle location A,B and C. A greater the total heat input the greater the distortion. Correctly sizing a weld for the requirements of the joint not only minimizes distortion, but also saves weld metal and time. If these natural balancing forces are not present, it is necessary to use other means to counteract the shrinkage forces in the weld metal. Also, warpage and distortion in welding mean almost the same thing. 5. Advanced processes and automated solutions for critical and unique shipbuilding requirements. Source: Haynes International - Welding and Joining 5. Plastic deformation causes a permanent reduction in the component dimensions and distorts the structure. In these examples, the shrinkage in weld No. This ensures that the distortion leaves the joint in the desired shape after the welding. If the base metal is not restrained at its outer edges by restraints, it would respond to this pull by getting distorted in the angular direction, because that is the easiest movement available. What are welding defects? Filler metals made from the highest quality steel to maximize consistency, feedability and arc performance. Residual stresses can be minimized by both thermal methods and mechanical methods. Restating the case in welding, the weld metal is not unrestrained. This separation is most pronounced as the first bead is laid. 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. Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Fig. This is the property of metals. For example, in a thick double welded butt joint, welding alternatively at front and back can minimize the angular distortion. 3-33 A water-cooled jig for rapid removal of heat when welding sheet meta. During heating primarily compressive residual stress is developed in the region of base metal which is being heated for melting due to thermal expansion and the same (thermal expansion) is restricted by the low temperature surrounding base metal. The flat position permits the use of large-diameter electrodes and high-, Balance welds about the neutral axis of the member, Distribute the welding heat as evenly as possible through a planned welding sequence and weldment positioning, Weld 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, Software Version: WebSetup2.0.09015_5_2_2023_8_08_AM. The top of the weld groove - which will contain the bulk of the weld metal - is lengthened when the plates are preset. The thermal methods means post weld heat treatment (PWHT) of the weld. Fillet Welds 0.8mm per weld where the leg length does not exceed 3/4 plate thickness, Butt weld 1.5 to 3mm per weld for 60 V joint, depending on number of runs. This pre-positioning should be done in a direction opposite to the direction in which the distortion is expected to occur. In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. There are several factors that affect the extent of distortion or warpage that occurs in welding. This is in simple terms the reason behind distortion, warpage, and residual stresses in welding. 2. But as the heat moves out across the plate to C, expansion along outer edges CD brings the plates back together. The more metal placed in a joint, the greater the shrinkage forces. 1 Types of welding distortion Longitudinal distortion Longitudinal shrinkage happens along the length of the weld. 1. High deposition manual and automated solutions for heavy fabrication equipment and component manufacturers. As welding procedure is usually selected for reasons of quality and productivity, the welder has limited scope for reducing distortion. In such cases, other ways of controlling distortion should be adopted. Fig. This type of distortion is greatest when the workpiece is not properly secured. This distortion happens when the metal contracts after welding and pulls its edges towards each other. This paper demonstrates how the type of welding process influences the amount of distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished. The joint can be welded in accordance with a sequence that distributes and spreads the heat. In sheet-metal welding, for example, a water-cooled jig (Fig. Many shipyards currently use submerged arc welding (SAW) as their welding process of choice. But this method must be used with care. Minimize welding time 3-2(a). The change in dimensions of the metal depends on a physical property of the metal, which is denoted by its thermal coefficient (). Flux and wire combinations for single- and multiple-pass welding in automatic and semi-automatic applications. The result of this can be development of a crack, or warpage of the plate. In order to prevent this, physical restraints, such as stiffeners at the back side of a butt joint, clamps on the sides, etc., are generally used to prevent distortion from occurring. Anticipate the shrinkage forces Or, straightening can be achieved by thermal or application of a flame too. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding; this produces a dished shape. 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. The welds are completed on both assemblies and allowed to cool before the clamps are released. Shrinkage stresses in a fillet weld occur similarly as explained above, as shown in the figure below. Compared to manual arc welding, semi-automatic and automatic welding typically produce less distortion. Another common practice for balancing shrinkage forces is to position identical weldments back to back, Fig. Commentdocument.getElementById("comment").setAttribute( "id", "a9d857d0e0facf0c1e8dbdc9a7d4972c" );document.getElementById("ffc7842d79").setAttribute( "id", "comment" ); Save my name, email, and website in this browser for the next time I comment. 3-2(b), lateral expansion cannot take place. This solidified bead then resists the shrinkage of the later portions of the bead. Do not overweldThe more metal placed in a joint, the greater the shrinkage forces. A higher root gap would entail greater volume of weld metal, and therefore more shrinkage of metal, and thus higher chances of distortion. If the stresses are higher than the elastic limit it leads to plastic deformation and distortion of components occurs. To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Fig. Likewise, when it cools, if it is unrestrained it contracts equally in all directions. Fit-up should be uniform to produce predictable and consistent shrinkage. The welds are completed on both assemblies and allowed to cool before the clamps are released. Thus these two welds are most likely not the same size. 2. Balance welds around the neutral axisThis practice, shown in Fig. After attaining a peak value compressive residual stress gradually decreases due to softening of metal being heated. While welding, the following factors should be considered by the welder in order to reduce welding warping: Let us see now how to keep metal from warping when welding in large structures. The use of mechanized welding equipment reduces welding time and the amount of metal affected by heat and, consequently, distortion. As a general rule, weld volume should be kept to a minimum. 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. The required amount of preset for shrinkage to pull the plates into alignment can be determined from a few trial welds. If we can place the weld right at the neutral axis we can effectively control distortion. Angular distortion is caused by non-uniform transverse shrinkage of the solidified weld. As the bar is uniformly heated, it expands in all directions, as shown in Fig. These methods include peening, proof dressing, etc. When the material cools, the edges contracts and because of the initial plastic deformation that caused thickening, the final length is less than the original, leading to distortion in the opposite direction. 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. What is warpage in welding? 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. 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. Similarly, in a single sided fillet weld, non-uniform contraction will produce angular distortion of the upstanding leg. The following figure illustrates typical distortions found in a fillet welds. Longitudinal distortion affects the length of the weld assembly, although not by much. 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. In welding, the warpage meaning is same. Thus the completed weld is slightly longer than it would be if it had been made on the flat plate. 3-2(a). At this point, the weld stretches (or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. Nevertheless, a 'rule of thumb' has been composed based on the size of the weld deposit. 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. Beginning welders and even those that are more experienced commonly struggle with the problem of weld distortion, (warping of the base plate caused by heat from the welding arc). As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. The weld zone area receives intense heat during welding, while the remaining portion of the base metal is cooler. The local expansion and contraction takes place within the cold parent metal close to the weld area, which restricts movement from these forces, by this action residual stress builds up. But only those stresses that exceed the yield strength of the weld metal are relieved by this straining. The two actions coincide, and the welded plates assume the desired flatness. So, a metal having high can be expected to have higher shrinkage stresses, and thus higher amounts of warping in welding. 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. 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. The principal features of the more common forms of distortion for butt and fillet welds are shown below: Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage. Shrinkage cannot be prevented, but it can be controlled. If restrained, as in (b), during heating, it can expand only in the vertical direction - become thicker. Both butt and fillet joints are prone to distortion. This is because it is part of a larger piece of metal that is not heated to the same temperature as a whole. Another method for removing shrinkage forces is by thermal stress relieving - controlled heating of the weldment to an elevated temperature, followed by controlled cooling. In welded joints, the weld metal that is cooling down from its molten state is not allowed to contract in all directions since it is integral with the base metal. 3-1). This is because it typically has a higher deposition rate, has high the ability to produce a more continuous bead that lead to a more uniform thermal distribution pattern. The "strongback" is another useful technique for distortion control during butt welding of plates, as in Fig. Well-designed joints take minimum weld volumes and do not suffer from over-welding. Correctly sizing a weld for the requirements of the joint not only minimizes distortion, but also saves weld metal and time. Excessive joint gap can also increase the degree of distortion by increasing the amount of weld metal needed to fill the joint. Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Fig. Any strategy that addresses the question how to avoid warpage in welding must take these factors into consideration. For a better understanding of how this happens, the following figure with a bar of steel is used to illustrate an example. What is Welding Distortion? However, tensile stresses occur on cooling when the contraction of the weld metal and the immediate heat affected zone is resisted by the bulk of the cold parent metal. The understanding can then be stretched to all other kinds of welds. 4. This balances contraction stress (gets them working against each other) and limits distortion. The part is prevented physically from getting distorted. (For simplification, the sketches show this distortion occurring in thickness only. 3-33) is useful to carry heat away from the welded components. Four types of distortion are identified and considered seperately: transverse shrinkage, angular distortion, longitudinal shrinkage and bowing distortion. This is a simplified explanation of basic cause of distortion in welding assemblies. On the other hand, elastic buckling causes the metal sheet to be curved along the width. 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. So, in short, the more a material grows per 1C in temperature the more it will distort. Neutral axis of the part should be located, and welder should seek to distribute the heat evenly around the neutral axis. 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. This deformation is called plastic deformation (as opposed to elastic deformation), because the distortion is permanent. This non-uniform heating, differential cooling, and partial restraint is the main cause of thermal distortion and warpage that occur in welding. In circumferential seam joints where concentricity of the two shells being welded is a critical requirement, special weld sequence should be adopted in the welding of the cir-seam joint. If thebase plate is heavily restrainedfrom all ends, the deformation is prevented at the time of welding. 3-2 (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. In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. 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. This heat creates expansion and contraction. The figure below illustrates a suggested sequence for the part shown. The residual stresses that would tend to distort the weldments are thus minimized. depending on the studied process and always considering the four types of distortion explained by Masubuchi [14]. Fig. This produces warpingin the plate, and is generally observed in thin base metals. In general, it is desirable to finish the weld quickly, before a large volume of surrounding metal heats up and expands. Plan the welding sequenceA 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. But, since volume expansion must occur during the heating, the bar expands in a vertical direction (in thickness) and becomes thicker. Another way to minimize weld metal is to use intermittent rather than continuous welds where possible, as in Fig. Another method is peening. Heat would move more quickly through a copper bar than through a steel bar, and the temperature differential would not be so great. Here, too, design of the assembly and proper sequence of welding are important factors. Products to meet base material and process requirements such as corrosion and high temperature applications. Distortion, as we saw in the above paragraphs, results due to shrinkage of weld metal. Compressive residual stress reduces to zero as soon as melting starts and a reverse trend is observed during the cooling stage of welding. 3-33) is useful to carry heat away from the welded components. 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. These changes, in turn, affect heat flow and uniformity of heat distribution. The selection of process also influences the distortion. Use 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. 3-7(h). In heavy weldments, particularly, the rigidity of the members and their arrangement relative to each other may provide the balancing forces needed. Different metals have different properties, hence the value of is different for every metal. The residual stresses can be reduced by mechanical methods as well. 3-4) and gas metal arc welding (GMAW) (fig. Here are some tips to minimize welding distortion. Extend the life of metal parts that are subject to impact, abrasion or corrosive wear. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Fig. How do you fix a weld distortion? We see that a permanent reduction in length along the x-axis is the result. The thermal load
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