Z-phase Formation during Creep and Aging in 9–12% Cr Heat Resistant Steels

Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

“…The nose temperature of Z-phase formation is 923 K (650°C) for this steel, [28] and so, an unexpected drop in strength could occur at 923 K (650°C) rather than 873 K (600°C), at which the strength drop was observed most obviously as shown in Figures 2 and 7. However, the number density of Z-phase at 923 K (650°C) does not exceed approximately one-third of that of at 873 K (600°C).…”

“…[25] A creep duration of about 1000 hours at 873 K (600°C) is enough for the recovery of the lath-martensite structure including a decrease in dislocation density, but is not enough for the precipitation of Z phase for this steel. [26,27] Therefore, MX particles showing a powerful strengthening effect [27][28][29] have not yet decomposed, and it is thus reasonable to infer that the hardening observed in Figure 7 is not caused by something concerning the redistribution of constituent elements of MX particles, but certainly by the precipitation of Laves phase during creep. Thermal input of 873 K (600°C) for 1000 hours is enough for this steel to precipitate Laves phase.…”

“…[10,11] The degradation is summarized as follows. Z phase forms in ferritic/martensitic steels containing large amounts of Cr and N, V, and Nb after long thermal exposure similar to the case for austenitic steel; [26,30] the nose temperature of Z-phase precipitation (i.e., the temperature at which precipitation occurs earliest) is 923 K (650°C); [27] the composition of Z phase is Cr(Nb,V)N, and small amounts of Fe and Si are dissolved; [27,28] a thermodynamic calculation system showed that Z phase is more stable than MX in high-Cr ferritic/martensitic steel; [30][31][32] the nucleation mechanism is not well understood, but Z phase easily nucleates on VN and Z phase is further stabilized by dissolving Nb; [32][33][34] Nb precipitates forming MX on PAGB during normalization, and Z phase is found on PAGB near Nb containing MX; [28,35,36] Z-phase particles grow and/or become coarse by collecting nearby Cr, and dissolving and consuming finely dispersed MX particles in grains [35,36] ; and a region several microns in width along and adjacent to PAGB on which coarse M 23 C 6 and Z-phase particles grow is preferentially recovered, the timing of which corresponds to an unexpected and sharp drop in strength. [10,[35][36][37] According to the previous work shown above, the unexpected drop in strength in the times to rupture and specified strain observed at 873 K (600°C) after several thousands of hours for 11Cr-2W-0.4Mo-1Cu-Nb-V steel (shown in Figures 2, 7, and 11) is considered to be mainly because of the formation of Z phase on PAGB and the resulting local and heterogeneous recovery along PAGB.…”

TTP Diagrams of Z Phase in 9–12% Cr Heat-Resistant Steels

“…6) We reported previously that Z phase forms mainly around the PAGB and packet boundary. 7) Therefore, the deformation resistance around the PAGB will be very low due to the disappearance of MX carbonitrides, and this can promote preferential recovery around the PAGB. The coarsening rate of the Z phase is much higher than that of MX carbonitrides.…”

Microstructural Evolution and the Effect on Mechanical Properties of S30432 Heat-resistant Steel during Aging at 650°C

“…NbCrN) was formed by the consumption of Nb(C, N) during aging and creep. 10,11) However, it is difficult to find Z phase even though the specimen aged for 10 000 h at 650°C. It indicates that Nb(C, N) is relatively stable in S30432, and the transformation from Nb(C, N) to NbCrN does not occur in spite of aging for 10 000 h.…”