On the nucleation and dissolution process of Z-phase Cr(V,Nb)N in martensitic 12%Cr steels

Precipitation of large Z-phase particles, Cr(V,Nb)N, replacing fine MX nitrides, (V,Nb)N, has recently been identified as a major cause of premature breakdown in long-term creep strength of a number of new 9-12%Cr martensitic steels. The Z-phase precipitates slowly during long-term exposure at around 650 °C accelerated by high Cr content in the steels. It appears that the nucleation process controls the precipitation rate of Z-phase. A 12%Cr steel, which had precipitated Z-phase during long-term operation at 660 °C/12,000 h, was further heat treated in order to investigate the dissolution and reappearance processes for the Z-phase. In both cases it appears that Z-phase and MX phase are in physical contact and have a preferred crystallographic orientation relationship. The proposed nucleation mechanism is a chromium diffusion controlled transformation of MX into Z-phase, which explains the rather low precipitation rate of Z-phase.