Effect of Ni and Mn on the Creep Behaviour of 9–10 %Cr Steels with Low N and High B

Abstract

Creep behavior and microstructure of two 9–10 %Cr–3 %Co martensitic steels with high B and low N was examined. A 10 %Cr steel with low Ni and Mn additives exhibits superior creep resistance and no creep strength breakdown appears at 650 °C. In this steel a dispersion of boundary M23C6 carbides and Laves phase remains nearly unchanged under creep condition and no significant coarsening of lath structure takes place. In contrast, a 9 %Cr steel containing 0.4 wt% Mn and 0.24 %Ni exhibits the creep strength breakdown and relatively low long-term creep strength. In this steel the boundary particles are susceptible to significant coarsening under long-term creep. As a result, the tempered martensite lath structure (TMLS) tends to transform to subgrain structure; significant subgrain growth is observed. No distinct difference in effect of creep and long-term aging on a dispersion of M(C,N) carbonitrides was revealed between two steels.