The non-uniform expansion and contraction of weld metal and adjacent base metal during the heating and cooling cycle of the welding process.
At the precise time the weld metal solidifies and fuses with the base metal, it is at its maximum expended state, actually occupying the greatest volume it can occupy as a solid. On cooling, it contracts to the volume it would normally occupy at lower temperatures if it were not restrained from doing so by the adjacent base metal.
Stresses develop within the weld, finally reaching the yield strength of the weld metal. At this point, the weld “stretches,” or yields and thins out, thus adjusting to the volume requirements of the joint being welded. But only those stresses that exceed the yield strength of the weld metal are relieved by this accommodation. At the time the weld reaches room temperature, (assuming complete restraint of the base metal so that it cannot move), the weld tends to have locked-in tensile stresses approximately equal to the yield strength. If one or more of the restraints are removed, the locked-in stresses find partial relief by causing the base metal to move, thus causing deformation or distortion.
Shrinkage which produces stresses that lead to distortion in the base metal adjacent to the weld further compounds the problem of shrinkage in the weld. During welding, the base metal near the arc is also heated to the melting point. A few inches away, the temperature of the base metal is substantially lower. This sharp temperature differential causes non-uniform expansion, followed by base metal movement, or metal displacement, if the parts being joined are restrained. As the arc passes further down the joint, thus relocating the source of heat, the base metal begins to cool and shrink along with the weld metal. If the surrounding metal restrains the heat-affected base metal from contracting normally, internal stresses build up; these combine with the stresses developed in the weld metal and increase the tendency to distort.
The volume of this adjacent base metal, which contributes to the distortion, can be controlled by changing the welding procedures. Higher welding speeds reduce the amount of adjacent material that is affected by the heat of the arc, and progressively decrease distortion. Higher welding speeds can be achieved by using powdered-iron manual electrodes, semi-automatic or automatic submerged-arc welding equipment, or automatic gas metal arc welding equipment.
Modes of Distortion
Knowledgeable consideration of the distortion phenomenon and the effects of shrinkage on various types of welded assemblies is invaluable when planning fabrication designs and setting up welding procedures to minimize distortion. See Figure D-7.
Shrinkage of the weld can cause various types of distortion and dimensional changes. A butt weld between two pieces of plate, by shrinking transversely, changes the width of the assembly as shown in Figure D-7 (A). It also causes angular distortion, as
in Figure D-7 (B). Here, the greater amount of weld metal at the top of the weld produces greater shrinkage at the upper surface, causing the ends of the plate to lift. Increasing either the included angle or the weld reinforcement will cause even greater distortion. Longitudinal shrinkage of the same weld would have a tendency to deform the joined plate, as shown in Figure D-7 (C).
Angular distortion, as in Figure D-7 (D), is a problem with fillet welds. If fillets in a T-assembly are above the neutral axis (center of gravity) of the assembly, the ends of the member tend to be bent upward, as in Figure D-7 (E); if the welds are below the neutral axis, the bending of the member is in the opposite direction, as in Figure D-7 (F).
Control of Shrinkage
Shrinkage from the effects of the heating and cooling cycles cannot be prevented, but can be controlled. There are various practical procedures and design strategies for minimizing the distortion caused by shrinkage.
- Keep the shrinkage forces as low as possible by using only the amount of weld metal required by the joint. The more metal placed in a joint, the greater the shrinkage forces will be. See Figure D-8 (A). Correctly sizing the weld for the service requirement of the joint helps control distortion. In a conventional fillet weld, only the effective throat is used in determining the strength of the weld. The amount of weld metal can be minimized in a fillet by using a flat or slightly convex bead. Excess weld metal in a highly convex bead does not increase the allowable strength
in code work, yet it adds to the development of shrinkage forces.
In a butt joint, proper edge preparation, fit-up and reinforcement are important to minimize the amount of weld metal required. When maximum economy is the objective, the plates should be spaced from .8 to 2 mm (1/32 to 1/16 in.) apart. A bevel not exceeding 30″ on each side will give proper fusion at the root of the weld, yet require minimum weld metal.
For thicker plates, the bevel may be decreased by increasing the root opening, or a J-or U-groove preparation adopted, to further reduce the amount of weld metal. A double-V joint requires about half of the weld metal of a single-V joint. See Figure D-8 (B).
Another way to minimize the amount of weld metal is to use intermittent welds where possible, rather than continuous welds. As an example, when attaching stiffeners to plate, intermittent welds will reduce the volume of weld metal by 75%, yet will provide all the strength needed. See Figure D-8 (C).
- Use as few weld passes as possible. When transverse distortion is a potential problem, a few passes with large electrodes are preferable to a large number of passes with small electrodes, because the shrinkage resulting from each pass tends to be cumulative. See Figure D-8 (D).
- Place welds near the neutral axis, as shown in Figure D-8 (E). This reduces distortion by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment.
- Balance welds around the center of gravity. This will balance one shrinkage force against another. This design and welding sequence will effectively control distortion. See Figure D-8 (F).
- Use backstep welding. With this technique, the general progression of welding may be, for example, from left to right, but each bead is deposited from right to left. As shown in Figure D-8 (G), as each bead is placed, the heat from the weld along the edges causes expansion, temporarily separating the plates at B. However, as the heat moves out across the plate to C, the expansion along the outer edges CD brings the plates back together. Expansion of a plate is most pronounced when the first bead is laid. With successive beads, the plates expand less and less because of the locking effect of prior welds. In some cases, backstepping may have less effect, and it cannot be economically used in fully automatic welding.
- Make shrinkage work in the desired direction. By locating parts out-of-position before welding, shrinkage can be utilized constructively to pull them back into alignment. See Figure D-8 (H). Pre-bending or pre-springing the parts to be welded, as shown in Figure D-8 (I), is a simple example of using mechanically-induced opposing forces to counteract weld shrinkage. The top of the weld groove, which will contain the bulk of the weld metal, is lengthened when the plates are sprung, since it becomes the convex side of a curve. Thus, the completed weld is slightly longer than it would be if it were made on a flat plate. When the clamps are released after welding, the plates tend to resume their flat shape, and the longitudinal shrinkage stresses of the weld can be relieved by shortening it to a straight line. The two actions coincide, and the welded plates assume the desired flatness.
- Balance shrinkage force with opposing forces. Opposing forces may be:
(a) other shrinkage forces
(b) restraining forces imposed by clamps, jigs, and fixtures
(c) restraining forces arising from the arrangement of members in the assembly
(d) the counter force produced by the force of gravity action on the sag in a member.
A common practice for balancing shrinkage forces in identical weldments is to position the workpieces back-to-back and then clamp them tightly together.
See Figure D-8 (J). The welds are completed on both assemblies and allowed to cool before the clamps are released. Pre-bending can be combined with this method by inserting wedges at suitable positions between the workpieces before clamping.
Locking the workpieces in the desired position in clamps, jigs or fixtures to hold them until welding is finished is probably the most widely used method of controlling distortion in small assemblies of components. The restraining forces provided by clamps cause the build-up of internal stresses in the weldment until the yield point of the weld metal is reached. For typical welds on low-carbon plate, this would probably be approximately 310 MPa (45 000 psi). After welding, one might expect this stress to cause considerable movement or distortion when the workpiece is removed from the jig or clamps. This does not occur, however, since the strain (unit contraction) can be calculated to be a very low value compared to the amount of movement that would have occurred if no restraint were used during welding.
(8) A well planned welding sequence is often helpful in balancing shrinkage forces against each other. The intent should be to place weld metal at different points on the structure so that as it shrinks in one place, it will counteract the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a butt weld, as shown in Figure D-8 (K). Another is making intermittent fillet welds, shown in Figure D-8 (L).
(9) One way to help control shrinkage forces occurring during or after welding is by peening, but peening is not a definitive practice. Peening is a mechanical method of applying force to the weld to make it thinner, thereby making it longer and relieving
residual stresses. 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 also because of a possible work-hardening effect, so the utility of this technique is limited.
In special cases, stress relief by controlled heating of the weldment to an elevated temperature, followed by controlled cooling is another way to remove shrinkage forces. Sometimes two identical weldments are placed back-to-back, clamped together, welded and then stress-relieved while held in this straight condition. The residual stresses that would tend to distort the weldment are thus removed.
(10) Reduce welding time. Since complex cycles of heating and cooling are in progress during welding, and time is required for heat transmission, the time factor affects distortion. In general, it is advantageous to finish the weld quickly before too great a volume of surrounding metal expands because of the heat. The amount of shrinkage and distortion is affected by the welding process used, the type and size of electrode, welding current, and travel speed. Using mechanized welding equipment reduces the time of welding and the amount of metal affected by heat, and consequently tends to reduce distortion.
To deposit a weld in thick plate with a process operating at 175 amps, 25 volts and 7.6 cm (3 in.) per minute, 34 400 joules of energy per linear centimeter are required. The same size weld produced with a process operating at 310 amps, 30 volts, and 20.3 cm (8 in.) per minute requires only 27 500 joules per linear centimeter of weld. The difference represents “excessive” heat available for transmission farther into surrounding metal, increasing its temperature, and producing added expansion and displacement of metal.