Consideration of the Role of Nb, Al and Trace Elements in Creep Resistance and Embrittlement Susceptibility of 9–12% Cr Steel

Abstract

The creep resistance of advanced chromium steels can be significantly increased due to precipitation of very small particles of vanadium nitride (VN). The volume fraction of VN is controlled by content of nitrogen which is available for the formation of VN–NAV. The solubility and precipitation of VN, Nb(C,N) and AIN in austenite and ferrite was analysed using relevant solubility products. The dependence of NAV on the Nb and A1 contents was determined. Based on results obtained over many heats of chromium modified steels, the calculated values of nitrogen in solid solution — Nss — were used to assess the creep rupture strength of chromium bolting steel (mean considered chemical composition in wt%: 0.18C; 11 Cr; 0.8Mo; 0.3V; 0.4Nb; 0.06N; 0.02A1). Increasing Nb content from 0.1 to 0.5% leads to decreasing creep rupture strength over a period of 100 000 h at 600°C of ca. 30%. Lowering A1 content from 0.03 to 0.003 wt% produces higher creep rupture strength of ca. 15%.