Investigation In Ductility Dip Cracking Of The Ni-based Alloy Welding Joint

Ductility dip cracking(DDC) is a solid-state, intergranular form of cracking. It is often found in nickel base alloy, austenitic stainless steel and other materials with single phase structure. Because of its small size, it is difficult to detect and possesses great harm.In this paper, nickel based alloy’s weld joints of the safety end from nuclear pressure vessel is under studied. High temperature mechanical properties of FM52 M is characterized through high temperature tensile test and spot varestraint test is used to characterize the hot cracking susceptibility. Moreover, strain-to-fracture(STF) test has been improved in order to get more specific local strain measurement. The value of grain boundary sliding is used as a new criterion to evaluate the susceptibility of DDC under various temperature and deformation. The results show that the threshold strain of the test is 6% which is larger than 3% obtained by previous STF method. Heat treatment, peak temperature, strain rate all have a great influence on DDC susceptibility.In order to study the mechanism of DDC, microhardness was used to mark interested micro-regions, from which SEM images have been made before and after the test to explain the roles of grain boundary sliding and precipitates in crack initiation and propagation.In this paper ABAQUS is used to simulate the pipe butt welding joint of the safety end from pressure vessel. The plastic strain distribution of weld bead and heat affected zone in axial and circumferential direction is under discussed. In addition, the maximum principal plastic strain accumulation during DDC sensitive temperature from four element which has the largest plastic strain is figured out. It can be used to evaluate the ductility dip cracking susceptibility of welding joint.