Influence of Thermal Exposure on Structural Changes of M23C6 Carbide in P91 Steel

Thermal aging effect on the structural changes of M23C6 carbide precipitates was investigated after a long-term exposure of P91 steel at 600, 650, and 700 A degrees C. The identification of alloy carbides and calculation of M23C6 lattice parameters was accomplished by x-ray diffraction analysis using the Topas program based on the Le Bail pattern decomposition method. This work revealed that M23C6 carbide crystal lattice parameter progressively increases as a function of time at high-temperature exposure due to enhanced diffusion of alloying elements from matrix into the carbide lattice. Kinetic parameters of M23C6 lattice transformation were described using Johnson-Mehl-Avrami equation. The activation energy E, rate constant k, and Avrami exponent n were established, which made it possible to develop a time-temperature relationship for P91 steel structural parameter transformation. The obtained dependence of M23C6 lattice parameter transformation could be used as an indicator for the assessment of the real temperature exposure time of heat-resistant P91 steel.