• Goosen, William

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 ($f_v$: 0.230.02 %) and thread regions ($f_v$: 0.060.01 %) 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 ($f_v$: 0.160.02 %) for the initial state to ($f_v$: 0.030.01 %) in the uniformly-elongated gauge section of the sample tested to failure. The interparticle spacing of the creep-strengthening MX particles increased from ($λ_{3D}$: 0.550.05 $μm$) in the initial state to ($λ_{3D}$: 1.010.10 $μm$) for the uniformly-elongated gauge section of the ruptured sample, while the thread region had an interparticle spacing of ($λ_{3D}$: 0.600.05 $μm$). The locally deformed fracture region had an increased phase fraction of modified Z-phase ($f_v$: 0.400.20 %), which implies that localised creep-strain strongly promotes the formation of modified Z-phase. The modified Z-phase precipitates did not form only on prior-austenite grain boundaries and formed throughout the tempered martensite ferritic grain structure.

Topics

• grain
• phase
• extraction
• steel
• precipitate
• creep