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

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

“…This inference agrees with the observation of Z phase on PAGB in a creep-ruptured specimen of this steel after 3475 hours at 873 K (600°C). [27] Moreover, the heterogeneous recovery model proposed by Kimura et al, [35] where glide resistance for mobile dislocations is estimated to be small, grain-boundary assisted diffusion can be expected, and creep deformation is localized, agrees well with the observed results of small values of Q and V in Gr. III shown in Figures 8 and 9.…”

“…[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.…”

“…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). [27] Therefore, even though there might be local recovery due to Z-phase formation at 923 K (650°C), the influence on degradation at 923 K (650°C) should be understood to be limited, compared with degradation at 873 K (600°C).…”

Kinetics of Z-Phase Precipitation in 9 to 12 pct Cr Steels

“…[4] The results show that the Z-phase precipitation speed is fastest at 923 K (650°C) for the 12CrVNbN alloy, which is consistent with time-temperature-precipitation (TTP) maps for Z-phase formation in 9 to 12 pct Cr commercial steels. [14] The Z-phase precipitation is much slower in the 9CrVNbN alloy at all temperatures, but it occurs fastest at 873 K (600°C) compared to the other temperatures. The ''nose'' of the TTP diagram for 9 pct Cr steels is thus at a somewhat lower temperature than for 12 pct Cr steels.…”