Behaviour of Z phase in 9–12%Cr steels

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

“…[3] Thermodynamic calculations also show Cr to be the most important element, and this is the main difference between Cr(V,Nb)N Z-phase and (V,Nb)N MX. [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.…”

“…As the Z-phase precipitation is very slow, they will appear as few coarse particles, which do not contribute significantly to creep strengthening as opposed to the MX nitrides, resulting in a net loss of creep strength. [1][2][3] Thus, the long-term creep strengths of these steels are dependent on the Z-phase being slow to precipitate and not appearing in significant numbers within the lifetime of the power plants. It is now known that this precipitation process is significantly faster in 12 pct Cr steels, which have been favored for their high oxidation resistance needed in planned future power plants operating at higher temperatures.…”

Assessment of microstructure stability and mechanical properties of X10CrWMoVNb9-2 (P92) steel after long-term thermal ageing for high-temperature applications

“…The precipitation of Z-phase results in the disappearance of finedispersion MX precipitates, which causes a sudden decrease in creep strength in 12%Cr martensitic steels. However, in contrast to these steels, the impact of Zphase precipitates on creep strength in 9%Cr steel is slight [21,22].…”

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

“…[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.…”

“…The values of Q for time to rupture of many kinds of heat-resistant steels do not exceed 500 kJ/mol, [16][17][18] although Q increases to some extent at high temperatures in some cases. [16] In a high-temperature region higher than 973 K (700°C), neither Laves phase nor Z phase precipitates in high-Cr martensitic steel containing Mo and/or W, [25,29] and therefore, major strengthening factors are considered to be both W and/or Mo, most of which are soluble, and fine MX particles that precipitate stably in the martensitic matrix. Kadoya and Shimizu [13] studied the creep behavior at 873 K (600°C) of iron-containing soluble W and Mo and finely dispersed MX particles.…”