Microstructural Evolution in a 9%Cr-3%Co-3%W-VNb Steel during Creep

Abstract

Microstructural evolution in a 9Cr-3Co-3W-0.2V-0.06Nb-0.05N-0.005B steel crept at T=650°C under an applied stress of 140 MPa up to strains of 1, 3, 5.75 and 12%, which represent primary, secondary and tertiary creep stages and rupture, respectively, was studied. The steel was initially normalized from 1050°C, and finally tempered at 750°C for 3h. After tempering the boundaries of tempered martensite lath structure (TMLS) were decorated by M23C6 carbides, M6C carbides and Laves phase particles. The 3% W additives provide the narrow size distribution of the boundary particles excepting M6C carbides. The depletion of thermodynamically none-equilibrium content of W from the solid solution during creep leads to following events. (i) Continuous precipitation of small Laves phase particles occurs during all creep stages and results in the formation of bimodal size distribution. As a result, the average size of Laves phase particles remains unchanged during creep. (ii) Coarsening of M23C6 carbides starts to occur only at the transition to tertiary creep. (iii) Transformation of laths to subgrains followed by their growth is observed during all stages of creep. The density of particle located at lath/subgrain boundaries decreases from 5.6 to 2.6 μm-1 during creep up to rupture. However, no full transformation of TMLS into subgrain structure has been revealed.