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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:
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.