On the grain boundary hardening in a B-bearing 304 austenitic stainless steel

The precipitates, (Cr,Fe)23(C,B)6 carbides and (Cr,Fe)2B borides, formed along the grain boundaries in a 304 austenitic stainless steel containing boron of 33 ppm after solution treatment at 1100°C for 1 h followed by isothermal ageing for 0.5 h at temperatures ranging from 750 to 1050°C have been identified. The influence of these precipitates on the grain boundary hardening has been investigated by means of micro-Vickers hardness measurements. It is found that the degree of grain boundary hardening below 900°C decreases, while it increases above 900°C with increasing ageing temperature. The dissolution of (Cr,Fe)23(C,B)6 carbides and the precipitation of (Cr,Fe)2B borides are associated with the changes of grain boundary hardening in this B-bearing 304 austenitic stainless steel between 750 and 1100°C. The non-equilibrium boron segregation enhances the grain boundary hardening when the ageing temperature is above 900°C.