Evolution of microstructure and deformation resistance in creep of tempered martensitic 9–12%Cr–2%W–5%Co steels

Abstract

The microstructural evolution during creep at 923 K of four tempered martensite 9–12%Cr-steels modified with 2%W and 5%Co was quantified by electron microscopy. Coarsening of subgrains with strain towards the stress dependent steady state was confirmed. The evolution of the precipitate structure is similar with regard to M 23C 6 (M: metallic element, X: N, C) and Laves phase (Fe,Cr) 2(W,Mo) but differs strongly with regard to V-bearing precipitates. Low temper temperatures promote precipitation of V as M 2X instead of VX. This appears to be critical in causing anomalously fast breakdown of initially high creep resistance. Dissolution of small V-bearing precipitates and corresponding loss of precipitation hardening in the subgrain interior is proposed to be the reason for this breakdown. The dissolution is caused by fast coarsening of M 2X precipitates and precipitation of Z-phase at subgrain boundaries. Large M 2X precipitates at subgrain boundaries are proposed to be nucleation sites of Z-phase.