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)(2)B borides, formed along the grain boundaries in a 304 austenitic stainless steel containing boron of 33 ppm after solution treatment at 1100 degrees C for 1 h followed by isothermal ageing for 0.5 h at temperatures ranging from 750 to 1050 degrees 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 degrees C decreases, while it increases above 900 degrees C with increasing ageing temperature. The dissolution of (Cr,Fe)(23)(C,B)(6) carbides and the precipitation of (Cr,Fe)(2)B borides are associated with the changes of grain boundary hardening in this B-bearing 304 austenitic stainless steel between 750 and 1100 degrees C. The non-equilibrium boron segregation enhances the grain boundary hardening when the ageing temperature is above 900 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.