Role of Nb in modifying the microstructure of heat-resistant cast HP steel

Abstract

Heat-resistant cast stainless steels have widespread uses in the petrochemical industry in pyrolysis and reformer furnaces; 25%Cr-20% Ni or 25% CR-33% NI high-carbon steels, designated HK and HP types, respectively, have traditionally been used, but requirements for higher productivity have raised the demand for improved performance. In the more recently developed steels, the Cr carbides normally present are replaced by carbides of Nb or Ti, or both, and the resulting alloys are termed modified HK or modified HP steels. The additions of Nb or Ti provide improved structural stability at high temperature in that their carbides are more stable than the normal secondary carbide precipitates of Cr, thus contributing to restraint of dislocation movement. Perhaps more important still is the eutectic modification caused by Nb additions that could restrict grain boundary sliding. This article reports on an investigation into the role of Nb in modifying the microstructure of HP steel in the as-cast and artificially aged conditions, for a series of alloys utilizing different levels of Nb.