Effect of Cold Rolling on the Creep Rupture Strength of 12Cr-5.4W Ferritic Steel with δ-ferrite

Abstract

This study examined cold rolling (20%) effects on the creep rupture strength of 12Cr-5.4W ferritic steel with martensite and δ-ferrite at 700 °C. Creep rupture strength of the as-received steel was equal to or greater than that of Gr.92 steel under 100-50 MPa, but cold rolling decreased the creep rupture strength by as much as 10%. Microstructure observations of crept steels revealed coarsening of the Laves phase by cold rolling in δ-ferrite but not in martensite. This finding suggests that the Laves phase coarsening in δ-ferrite is related to short-circuit diffusion because fine sub-grain structures were observed inside δ-ferrite grains in the cold-rolled steel after creep. Also, the martensite lath structure in the cold-rolled steel recovered quickly. Collectively, these cold-rolling-related phenomena of microstructural degradations are inferred as factors decreasing the creep rupture strength. In the as-received steel, some creep voids were observed in martensite: most were observed adjacent to coarse vanadium nitrides. By contrast, creep voids in the cold-rolled steel were most numerous near the martensite – δ-ferrite interface. It was suggested that nucleation of creep voids near the interface in the cold-rolled steel was attributed to mechanisms such as the Laves phase size and distribution at the interface and stress concentration effects of dislocations that were piled up toward the interface.