Effect of grain boundary behaviour on ductility dip cracking mechanism

Alloy 690 is a typical Ni-Cr-Fe alloy widely used in nuclear power application. However, the companion welding filler metal 52M (FM-52M) is susceptible to an intergranular hot cracking referred to as ductility dip cracking (DDC). To investigate the effect of grain boundary (GB) characteristics on cracking mechanism of FM-52M, the extent of GB sliding and cracking was recorded based on strain-to-fracture test. By the results, it was concluded that the strength of GBs mainly affected the cracking resistance, which could be improved by increase of intergranular precipitates. During hot tensile, GB sliding was observed inducing DDC in mainly two ways: initiate cracking where GB sliding was inhibited by inflexion or triple point, and assist in developing microcrack by skipping hard propagation part of GB. Moreover, cracking was found to have a clear relation to GB misorientation. The results gave an understanding of GB sliding to DDC mechanism.