Microstructural evolution and creep behaviour of bainitic, martensitic, and martensite–ferrite dual phase Cr–2W steels

Abstract

The microstructural evolution and creep behaviour of Cr–2W–0·lC (wt-%) steels containing from 2 to 15%Cr have been investigated after creep rupture testing at 873 K for up to about 72×106 s (20 000 h). The creep curves consisted mainly of a transition creep region, where creep rate decreased with time, and an accelerated creep region, where creep rate increased with time. For the bainitic or martensitic steels, which contain from 2 to 9%Cr, the minimum creep rate decreased and the time to rupture increased with increasing Cr concentration. However, for the dual phase martensite and δ ferrite steels (12–15%Cr), the minimum creep rate increased and the time to rupture decreased with increasing Cr concentration, i.e. with increasing δ ferrite volume fraction. The microstructural evolution during creep was greater in the bainitic than in the martensitic steels. Evolution was not observed in the δ ferrite. The resistance to creep deformation is considered to depend on pinning by carbides preferentially arranged along the lath subgrain boundaries in the bainite and martensite. The effect of microstructural evolution during creep on the creep rate and time to rupture of the steels is comprehensively discussed.MST/1617