Dependence of Elevated Temperature Intergranular Cracking on Grain Size and Bulk Sulfur Content in TP347H Austenitic Stainless Steels

Abstract

Regardless of the bulk sulfur content, heat resistant TP347H austenitic stainless steels of the smaller grain size show a ductile fracture after rupture test, while the steels of the larger grain size show a typical intergranular cracking. The intergranular cracking is mainly due to the sulfur segregated at grain boundaries, and it is not inhibited even by the extremely low bulk sulfur content of 7 ppm. In steels showing the intergranular cracking, the time to failure is rather longer in the steel of the higher bulk sulfur content. This is due to MnS precipitates incoherently formed within the matrix, the interface of which acts as a strong sink of free sulfur segregating to grain boundaries. The fracture mode and the time to failure are determined by the combination of bulk sulfur content and grain size influencing the creep resistance and the equilibrium grain boundary segregation concentration of sulfur.