Section V. EXPANSION AND CONTRACTION IN WELDING OPERATIONS
a. Most of the welding processes involve heat. High-temperature heat is responsible for much of the welding warpages and stresses that occur. When metal is heated, it expands in all direction. When metal cools, it contracts in all directions. Some distortions caused by weld shrinkage are shown in figure 6-42.
b. There is a direct relationship between the amount of temperature change and change in dimension. This is based on the coefficient of thermal expansion. Thermal expansion is a measure of the linear increase in unit length based on the change in temperature of the material. The coefficient of expansion for the various metals. Aluminum has one of the highest coefficient of expansion ratios, and changes in dimension almost twice as much as steel for the same temperature change.
c. A metal expands or contracts by the same amount when heated or cooled the same temperature if it is not restrained. If the expansion of the part being welded is restrained, buckling or warping may occur. If contraction is restrained, the parts may be cracked or distorted because of the shrinkage stresses.
d. When welding, the metals that are heated and cooled are not unrestrained since they are a part of a larger piece of metal which is not heated to the same temperature. Parts not heated or not heated as much tend to restrain that portion of the same piece of metal that is heated to a higher temperature. This non-uniform heating always occurs in welding. The restraint caused by the part being non-uniformly heated is the principal cause for the thermal distortion and warpages that occur in welding.
e. Residual stresses that occur when metal is subjected to non-uniform temperature change are called thermal stresses. These stresses in weldments have two major effects: they produce distortion, and may cause premature failure in weldments.
6-26. CONTROLLING CONTRACTION IN SHEET METAL
a. The welding procedure should be devised so that contraction stresses will be held to a minimum order to keep the desired shape and strength of the welded part. Some of the methods used for controlling contraction are described below.
b. The backstep method as shown in view A, figure 6-43, may be used. With the backstep method, each small weld increment has its own shrinkage pattern, which then becomes insignificant to the total pattern of the entire weldment.
c. In welding long seams, the contraction of the metal deposited at the joint will cause the edges being welded to draw together and possibly overlap. This action should be offset by wedging the edges apart as shown in view B, figure 6-43. The wedge should be moved forward as the weld progresses. The spacing of the wedge depends on the type of metal and its thickness. Spacing for metals more than 1/8 in. (3.2 mm) thick is approximately as follows:
Metal In. per ft
Steel 1/4 to 3/8
Brass and Bronze 3/16
d. Sheet metal under 1/16 in. (0.16 cm) thick may be welded by flanging the edges as shown in figure 6-20, and tacking at intervals along the seam before welding. A weld can be produced in this manner without the addition of filler metal.
e. Buckling and warping can be prevented by the use of quench plates as shown in figure 6-44. The quench plates are heavy pieces of metal clamped parallel to the seam being welded with sufficient space between to permit the welding operation. These quench plates absorb the heat of welding, thereby decreasing the stresses due to expansion and contraction.
f. Jigs and fixtures may be used to hold members in place for welding. These are usually heavy sections in the vicinity of the seam (fig. 6-45). The heavy sections cool the plate beyond the area of the weld.
g. In pipe welding, spacing as illustrated in figure 6-43, is not practical. Proper alignment of pipe can be best obtained by tack welding to hold the pieces in place. The pipes should be separated by a gap of 1/8 to 1/4 in. (0.32 to 0.64 cm), depending on the size of the pipe being welded.
6-27. CONTROLLING CONTRACTION AND EXPANSION IN CASTINGS
a. Prior to welding gray iron castings, expansion and contraction are provided for by preheating. Before welding, small castings can be preheated by means of a torch to a very dull red heat, visible in a darkened room. After welding, a reheating and controlled slow cooling or annealing will relieve internal stresses and assure a proper gray iron structure.
b. For larger castings, temporary charcoal-fired furnaces built of fire brick and covered with fire resistant material are often used. Only local preheating of parts adjacent to the weld is usually necessary (fig. 6-46). Such local preheating can be done with a gasoline, kerosene, or welding torch.
c. Before welding a crack that extends from the edge of a casting, it is advisable to drill a small hole 1/2 to 1 in. (1.27 to 2.54 cm) beyond the visible the crack. If the applied heat causes the crack to run, it will only extend drill hole.
d. If a crack does not extend to the end of a casting, it is advisable to drill a small hole 1/2 to 1 in. (1.27 to 2.54 cm) beyond each end of the visible crack.
e. The above procedures apply to gray iron castings, as well as bronze welded castings, except that less preheat is required for bronze welded castings.
6-28. WELDING DISTORTION AND WARPAGE
a. General. The high temperature heat involved in most welding processes is largely responsible for the distortion, warpage, and stresses that occur. When heated, metal expands in all directions and when it cools, it contracts in all directions. As described in paragraph 6-25, there is a direct relationship between the amount of temperature change and the change in dimension of the metal. A metal expands or contracts by the same amount when heated or cooled the same temperature, if it is not restrained. However, in welding, the metals that are heated and cooled are not unrestrained, because they are a part of a larger piece of metal which is not heated to the same temperature. This non-uniform heating and partial restraint is the main cause of thermal distortion and warpage that occur in welding. Figure 6-47 shows the effects of expansion on a cube of metal. When the cube of metal is exposed to a temperature increase, it will expand in the x, y, and z directions. When it cools, if unrestricted, it will contract by the same amount as it expanded.
b. A weld is usually made progressively, which causes the solidified portions of the weld to resist the shrinkage of later portions of the weld bead. The portions welded first are forced in tension down the length of the weld bead (longitudinal to the weld) as shown in figure 6-48. In the case of a butt weld, little motion of the weld is permitted in the direction across the material face (transverse direction) because of the weld joint preparation or stiffening effect of underlying passes. In these welds, as shown in figure 6-48, there will also be transverse residual stresses. For fillet welds, as shown in figure 6-49, the shrinkage stresses are rigid down the length of the weld and across its face.
c. At the point of solidification, the molten metal has little or no strength. As it cools, it acquires strength. It is also in its expanded form because of its high temperature. The weld metal is now fused to the base metal, and they work together. As the metal continues to cool, it acquires higher strength and is now contracting in three directions. The arc depositing molten metal is a moving source of heat and the cooling differential is also a moving factor, but tends to follow the travel of the arc. With the temperature still declining and each small increment of heated metal tending to contract, contracting stresses will occur, and there will be movement in the metal adjacent to the weld. The unheated metal tends to resist the cooling dimension changes of the previously molten metal. Temperature differential has an effect on this.
d. The temperate differential is determined by thermal conductivity. The higher the thermal conductivity of the metal, the less effect differential heating will have. For example, the thermal conductivity of copper is the highest, aluminum is half that amount, and steel about one-fifth that of copper. Heat would move more quickly through a copper bar than through a steel bar, and the temperature differential would not be so great. This physical property must be considered when welding, along with the fact that arc temperatures are very similar but the metal melting points are somewhat different.
e. Another factor is the travel speed of the heat source or arc. If the travel speed is relatively fast, the effect of the heat of the arc will cause expansion of the edges of the plates, and they will bow outward and open up the joint. This is the same as running a bead on the edge of the plate. In either case, it is a momentary situation which continues to change as the weld progresses. By adjusting the current and travel speed, the exact speed can be determined for a specific joint design so that the root will neither open up nor close together.
f. Residual stresses in weldments produce distortion and may be the cause of premature failure in weldments. Distortion is caused when the heated weld region contracts non-uniformly, causing shrinkage in one part of the weld to exert eccentric forces on the weld cross section. The weldment strains elastically in response to these stresses, and this non-uniform strain is seen in macroscopic distortion. The distortion may appear in butt joints as both longitudinal and transverse shrinkage or shrinks more plates along contraction and as angular change (rotation) when the face of the weld than the root. The angular change produces transverse bending in the the weld length. These effects are shown in figure 6-50.
g. Distortion in fillet welds is similar to that in butt welds. Transverse and longitudinal shrinkage as well as angular distortion result from the unbalanced nature of the stresses in these welds (fig. 6-51). Since fillet welds are often used in combination with other welds in a weldment, the distortion may be complex.
h. Residual stresses and distortion affect materials by contributing to buckling, curling, and fracturing at low applied stress levels. When residual stresses and their accompanying distortion are present, buckling may occur at liner compressive loads than would be predicted otherwise. In tension, residual stresses may lead to high local stresses in weld regions of low toughness and may result in running brittle cracks which can spread to low overall stress areas. Residual stresses may also contribute to fatigue or corrosion failures.
i. Control of distortion can be achieved by several methods. Commonly used methods include those which control the geometry of the weld joint, either before or during welding. These methods include prepositioning the workplaces before welding so that weld distortion leaves them in the desired final geometry, or restraining the workplaces so they cannot move and distort during welding. Designing the joint so that weld deposits are balanced on each side of the center line is another useful technique. Welding process selection and weld sequence also influence distortion and residual stress. Some distorted weldments can be straightened mechanically after welding, and thermal or flame straightening can also be applied.
j. Residual stresses may be eliminated by both thermal and mechanical means. During thermal stress relief, the weldment is heated to a temperature at which the yield point of the metal is low enough for plastic flow to occur and allow relaxation of stress. The mechanical properties of the weldment may also change, but not always toward a more uniform distribution across the joint. For example, the brittle fracture resistance of many steel weldments is improved by thermal stress relief not only because the residual stresses in the weld are reduced, but also because hard weld heat-affected zones are tempered and made tougher by this procedure. Mechanical stress relief treatments will also reduce residual stresses, but will not change the microstructure or hardness of the weld or heat-affected zone. Peening, proofstressing, and other techniques are applied to weldments to accomplish these ends.
k. The welder must consider not only reducing the effects of residual stresses and distortion, but also the reduction of cracks, porosity, and other discontinuities; material degradation due to thermal effects during welding; the extent of nondestructive testing; and fabrication cost. A process or procedure which produces less distortion may also produce more porosity and cracking in the weld zone. Warping and distortion can be minimized by several methods. General methods include:
(1) Reducing residual stresses and distortion prior to welding by selecting proper processes and procedures.
(2) Developing better means for stress relieving and removing distortion.
(3) Changing the structural design and the material so that the effects of residual stresses and distortion can be minimized.
The following factors should be taken into consideration when welding in order to reduce welding warpage:
(1) The location of the neutral axis and its relationship in both directions.
(2) The location of welds, size of welds, and distance from the neutral axis in both directions.
(3) The time factor for welding and cooling rates when making the various welds.
(4) The opportunity for balancing welding around the neutral axis.
(5) Repetitive identical structure and varying the welding techniques based on measurable warpage.
(6) The use or procedures and sequences to minimize weldment distortion.
When welding large structures and weldments, it is important to establish a procedure to minimize warpage. The order of joining plates in a deck or on a tank will affect stresses and distortion. As a general rule, transverse welds should be made before longitudinal welds. Figure 6-52 shows the order in which the joints should be welded.
Warpage can be minimized in smaller structures by different techniques, which include the following:
(1) The use of restraining fixtures, strong backs, or many tack welds.
(2) The use of heat sinks or the fast cooling of welds.
(3) The predistortion or prebending of parts prior to welding.
(4) Balancing welds about the weldment neutral axis or using wandering sequences or backstep welding.
(5) The use of intermittent welding to reduce the volume of weld metal.
(6) The use of proper joint design selection and minimum size.
(7) As a last resort, use preheat or peening.No tags for this post.