A dissimilar metal weld (DMW) in this article refers to a weld joining two materials from different alloy systems. A common power plant application is joining a ferritic low alloy steel to an austenitic stainless steel. A schematic of a DMW is shown in Figure 1.
A buttering layer is often used to provide a transition between the considerably different physical and mechanical properties of the parent materials. A power plant application of DMW is given in Table 1.
Table 1 An example of DMW joint materials and their properties
|C.T.E. (1/K) at 300°C
|Yield strength (MPa) at 300°C
|Tensile strength (MPa) at 300°C
|19.0 x 10 -6
|19.0 x 10 -6
|19.0 x 10 -6
|14.0 x 10 -6
The most significant feature of dissimilar metal welds (DMWs) with respect to residual stresses is differences in coefficient of thermal expansion between the parent and weld metals. There may also be differences in yield strength, which limit the magnitudes of the residual stresses which can exist in the component materials (as is also the case in similar metal welds with non-matching yield strengths), but do not change the general characteristics of the residual stress field.
The residual stresses in similar and dissimilar metal welds are generated by the thermal contraction of the weld metal and the adjacent heated parent metal, and hence the residual stress distribution in an as-welded DMW is broadly similar to that in a similar metal weld. Although information on the magnitude and distribution of welding residual stresses is available in several codes and standards, these are not validated extensively for DMWs. It is recommended that residual stresses in DMWs should be measured physically or calculated numerically by computational welding simulation.
If the structure containing the DMW is subject to post-weld heat treatment (PWHT), then its residual stresses will be completely different from those at similar metal welds. Most of the original as-welded residual stresses will be relieved during the heat-up and hold period of the PWHT procedure. During cool down, a new set of residual stresses will be generated because of the differential contractions of the different regions. After PWHT:
- The longitudinal residual stresses (parallel to the welding direction) will tend to be tensile in the material with the higher coefficient of expansion, and compressive in the material with the lower coefficient of expansion, with a discontinuity in the stress field at the interface.
- Shear stresses will occur at the interface, with peak values at the intersection with the surface. These may contribute to the initiation or propagation of cracking at the interface.
- Localised transverse residual stresses may be found on the surface near the interface. Longer range transverse stresses will depend on the restraint acting across the joint.
A simple prediction of the residual stress field at a heat-treated DMW may be obtained using a finite element model by assuming that the weldment is stress-free at the end of the temperature hold period, and then calculating the stresses generated due to differential contraction during cooling. In practice however, there may be some additional residual stresses resulting from the original welding operation, and not fully relieved during PWHT.
It should be remembered that additional thermal stresses will be generated when the temperature of the DMW changes. A change of temperature from ambient to a higher temperature will partially relieve the residual stresses generated during cool-down from PWHT.
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