Papers by Keyword: Z-Phase

Abstract: A 10%Cr martensitic steel with 3%Co and 0.008%B tempered at 770°C exhibits no creep strength breakdown at a temperature of 650°C up to an extremely high rupture time of ∼4×10⁴ h under an applied stress of 120 MPa. The minimum creep rate was ∼3×10^-11 s^-1. Microstructural characterization showed that superior creep resistance associated with a high stability of tempered martensite lath structure. Boundary M₂₃(B⋅C)₆ phase particles are highly stable against coarsening under long-term aging and creep conditions. These particles retain their orientation relationship with ferritic matrix unchanged under creep at a temperature of 650°C. As a result, no migration of lath boundaries and their transformation to subboundaries diminishing the long-range elastic stress fields take place. The role of M(C,N) carbonitrides in achieving extraordinary high creep strength consists in hindering the knitting reaction between mobile lattice dislocations and lath boundaries.

Abstract: The precipitation site, main metallic composition and number density of Z phase have been investigated in T91 in order to clarify the influence of Z phase formation on recovery of martensitic structure and creep strength degradation. The Z phase particles were mainly present around prior austenite grain boundaries and/or packet boundaries in the steels crept at 550oC and 600oC. The Z phase particles were found in specimens crept at 550oC to 650oC. There was no indication of Z phase formation up to about 62475.0 h at 500oC and 14106.5 h at 700oC. The Nb content of Z phase observed at 550oC was lower than that at 600oC. The number density of Z phase measured at 550oC was lower that that at 600oC, indicating that the preferential recovery of martensitic lath structure around prior austenite grain boundary is not remarkable at 550oC in contrast with 600oC.