Precipitation hardening of tempered martensite lath structure in a 10%Cr-3%Co-3%W-0.2%Re steel upon creep at 650 °C

Abstract

9-12% Cr martensitic steels are used as materials for fossil power plants with steam temperature more than 600 °C. Superior creep strength of these steels is attributed to a dispersion of nanoscale boundary M23C6 carbides and Laves phase particles. The aim of present work is to reveal the mechanisms of grain boundary pinning by precipitates and their effect on the thermal stability of lath martensitic structure of a Re-containing (in wt %) 10Cr, 3Co, 3W steel. The structural evolution in a Re-containing (in wt %) 10Cr, 3Co, 3W steel with a low N content (0.002 wt %) and high B content (0.01 wt%) after creep at temperature of 650 °C under the stresses of 200 - 140 MPa was investigated. The time to rupture after creep test at 650 °C/140 MPa was about 11,000 h. Tis steel was solution treated at 1050 °C and tempered at 770 °C, formerly. The structure in the grip portion of the crept specimen changed scarcely after creep exposure for ~11,000 h. In contrast, the structural changes in the gauge portion were characterized by transformation of the tempered martensite lath structure into relatively coarse subgrain structure. The formation of a well-defined subgrain structure in the gauge section was accompanied by the coarsening of M23C6 carbides and precipitations of Laves phase during creep.