Morphology of Secondary Phase Particles and Stability of Subgrain Structure in Martensitic Steel 10Cr9Co3W2MoVNbB (ASTM A335 P92) Under Creep Conditions

Abstract

The creep behavior of high-chromium martensitic steel 10Cr9Co3W2MoVNbB (ASTM A335 P92) at a temperature of 650°C and stresses 100 – 180 MPa is studied. The effect of creep on the subgrain structure and the characteristics of secondary particles are determined using transmission electron microscopy. Thermodynamic calculations are performed to determine the volume fraction of precipitates after heat treatment and creep. Precipitation of (Fe, Cr)₂(W, Mo) Laves phase particles and of (Fe, Cr)₂₃(C, B)₆ and (Nb, V)(C, N) particles is observed after short-term creep for 243 h. It is shown that pronounced coarsening of the Laves phase particles leads to a decrease in dispersion strengthening under conditions of long-term creep. Creep is accompanied by a decrease in dislocation density and an increase in the subgrain size. It is concluded that the change in the creep power-law exponent upon transition to long-term tests at 650°C is associated with degradation of microstructure, namely, with a pronounced increase in the subgrain size due to a decrease in the density of secondary particles which inhibit the migration of subgrain boundaries.