Modified Z-phase formation in a 12% Cr tempered martensite ferritic steel during long-term creep

Abstract

The formation of modified Z-phase in a 12Cr1MoV (German grade: X20) tempered martensite ferritic (TMF) steel subjected to interrupted long-term creep testing at 550°C and 120 MPa was investigated. Quantitative volumetric measurements collected from thin-foil and extraction replica samples showed that modified Z-phase precipitated in both the uniformly-elongated gauge (fv: 0.23 ± 0.02 %) and thread regions (fv: 0.06 ± 0.01 %) of the sample that ruptured after 139 kh. The formation of modified Z-phase was accompanied by a progressive dissolution of MX precipitates, which decreased from (fv: 0.16 ± 0.02%) for the initial state to (fv: 0.03 ± 0.01%) in the uniformly-elongated gauge section of the sample tested to failure. The interparticle spacing of the creep strengthening MX particles increased from (λ3D: 0.55 ± 0.05 μm) in the initial state to (λ3D: 1.01 ± 0.10 μm) for the uniformly-elongated gauge section of the ruptured sample, while the thread region had an interparticle spacing of (λ3D: 0.60 ± 0.05 μm). The locally deformed fracture region had an increased phase fraction of modified Z-phase (fv: 0.40 ± 0.20%), which implies that localised creep-strain strongly promotes the formation of modified Z-phase.