Effect of Tungsten on a Dispersion of M(C,N) Carbonitrides in 9 % Cr Steels Under Creep Conditions

Abstract

Two 9Cr–3Co–0.5Mo–VNb steels with 2 and 3 wt% W subjected to normalizing at 1050 °C and subsequent tempering at 750 °C for 3 h were crept at 650 °C under applied stresses ranging from 100 to 220 MPa. Both steels exhibit the creep strength breakdown at a rupture time of ~2000 h. The two-phase separation of M(C,N) carbonitrides to Nb- and V-rich particles occurs during tempering. No additional strain-induced coarsening of M(C,N) carbonitrides takes place under creep conditions. Long-term ageing leads to coarsening of both types of carbonitrides in the 9Cr–3Co–2W–VNb steel. 1 wt% W additives completely cease coarsening of Nb-rich M(C,N) particles at rupture times ≤2000 h and hinder their coarsening at higher times. Dimensions of V-rich M(C,N) particles in the 9Cr–3Co–3W–VNb steel are less than in the 9Cr–3Co–2W–VNb one. Moreover, in the 9Cr–3Co–2W–VNb steel the transformation of V-rich M(C,N) carbonitrides into Z-phase (CrVN) starts to occur at a rupture time of ~5000 h; particles of Z-phase with an average dimension of ~50 nm appear. At a rupture time of ~11,000 h, Z-phase particles with an average size of ~270 nm completely replace nanoscale V-rich carbonitrides. The increase in W content to 3 wt% slows down this process. Only separated particles of Z-phase with an average size of ~50 nm were found in the 9Cr–3Co–3W–VNb steel at a rupture time of ~16,000 h. No consuming of Nb-rich M(C,N) particles for the formation of Z-phase were found in both steels.