High temperature intergranular fracture of 304H austenitic stainless steel: contribution of tests on UHP Fe-Ni-Cr alloys selectively doped with C, P and S

304H stainless steel and a series of Ultra High Purity alloys selectively doped with C, C+P, C+S, C+P+S were used to investigate the conditions of intergranular cracking during simulated stress-relief heat treatment at 610°C. It was demonstrated that this cracking is caused by a synergy between sensitisation (intergranular carbide precipitation and formation of chromium depleted zone) and sulphur segregation, both enhancing grain boundary sliding. Auger spectroscopy measurements indicated (i) intergranular phosphorus segregation after heat treatment as opposed to (ii) sulphur segregation at internal cavity surfaces after high temperature slow strain-rate interrupted tests. Sulphur segregation to free surfaces was confirmed by “in situ” measurement at 600°C within the main Auger chamber. These results support the McMahon’s “dynamic embrittlement model” of intergranular cracking, where dynamic sulphur segregation plays a major role. Healing heat treatment at 800°C is proposed to decrease the stress-relief heat treatment susceptibility of high carbon austenitic stainless steels