Advanced heat-resistant martensitic steels: Long-term creep deformation and fracture mechanisms

Abstract

Creep behavior at 650 °C of the 9% Cr martensitic steels with different W, N and B contents was investigated using the threshold stress concept. Creep strength breakdown and the change in the “apparent” stress exponent were revealed at the applied stresses ranging from 140 to 120 MPa in both steels. Moreover, the threshold stresses, which were estimated by extrapolating the two parts of curves “Minimum creep rate vs. Applied stress” corresponding to the high and low stresses at a creep rate of 10−11 s−1, decreased from 94 … 98 MPa for both steels to 30 and 65 MPa in the 9Cr3W and 9Cr1.5WBN steels, respectively, at the transition from the high stress region to the low stress region. The sources of these threshold stresses are considered to be dislocations, the M23C6 carbides and Laves phase particles located at the low-angle lath boundaries as well as MX carbonitrides. Analysis of true stress exponent revealed no evidence for the change in creep mechanism at the transition from the high stresses to low stresses. This was creep controlled by high temperature climb of dislocations due to lattice diffusion. On the other hand, the creep fracture mechanism in the 9Cr3W steel was changed from the transgranular fracture at the high stresses to the intergranular fracture at the low stresses. This was accompanied with the formation of the high fraction of the coarse particles of Laves phase.