The Effect of MnS Precipitation on Sulfur Segregation and Intergranular Cracking

Abstract

IG (intergranular) cracking phenomena have been reassessed in heat resistant austenitic stainless steel (347H). After welding simulation, it is confirmed that IG cracking happened during tensile test. The segregated sulfur is observed on grain boundary of the fracture surface. It is assumed that during welding, MnS precipitates are dissolved and then soluble sulfur segregates to grain boundary for lower surface free energy. It is not clear that how much MnS precipitates are dissolved in the matrix, how much soluble sulfur segregates to grain boundary which causes IG cracking, and how much soluble sulfur remains in the matrix finally. This thesis shows hypothesis and explanations about IG cracking phenomena related to sulfur segregation in heat resistant austenitic stainless steel. It is confirmed that grain size effects to cause different fracture mode in stress rupture test. The different fracture mode depending on grain size is associated with deformation rate and sulfur segregation kinetics to grain boundary. It is confirmed that the different time to IG cracking depending on MnS precipitation heat treatment (1100°C 1hr) and different sulfur contents. This phenomenon is also associated with sulfur segregation kinetics related to MnS precipitates.