The effect of the transformation expansion of high-strength weld metals on the residual tensile stress in y-groove cold clacking test welds was discussed. A series of numerical simulation considering the effect of phase transformation such as volumetric change and difference in mechanical properties was performed to evaluate the residual stresses. First, the effect of transformation start temperature on the residual stress in the weld metal of y-groove cracking test specimen was investigated. The lower the martensitic transformation start temperature becomes, the lower the restraint stress σw becomes. However, the local stress σlocal at the weld root where cold cracks occur was much higher due to stress concentration. In addition, the local stress does not always decrease as decreasing martensitic transformation start temperature. Second, the restraint condition of numerical simulation model of y-groove cracking test was varied to decrease restraint intensity. As a result, the local stress σlocal increased with decreasing martensitic transformation temperature under low restraint condition. This means that the low transformation temperature welding wire do not necessarily effective for the prevention of cold cracking when the restraint strength varied.
Recently, in order to make actual use of 980 MPa class high-strength steels, a method for the prevention of cold cracking in welds using weld wires with low transformation temperatures are proposed (Nakamura, 2003). The mechanism for the prevention of cold cracking with the welding wire is summarized into the following two major points:
reduction in tensile stress in weld metals by transformation expansion,
entrapment of diffusible hydrogen to retained austenite.
In this study, the former point, the effect of the transformation expansion in weld metals on the reduction of tensile stress is discussed. Yamamoto (2008) et al. shows that cold cracking in HT780 steels is reduced when the low-temperature-transformation welding wire was applied.
