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Ferritic-martensitic heat-resistant steels are used for high-temperature service applications in power plants and the petrochemical industry due to its high strength and creep properties at high temperatures. After the long-term operation under, structural changes are taking place, such as coarsening of precipitates and migration of alloying elements within carbides, leading to M23C6 carbide lattice expansion. The thermal aging effect on the structural changes of M23C6 carbide for P91 (9Cr, 1Mo) and P5 (5Cr, 0.5 Mo) steel was investigated after exposure up to 234 days at 700 °C. The electrochemical extraction method was used to extract carbide precipitates from the steel. The identification of alloy carbides and calculation of the M23C6 lattice parameter changes have been accomplished by XRD analysis using crystal structure parameters evaluation and Rietveld refinement method. Microstructure evolution and elemental composition changes were studied by SEM-EDX microanalysis. The chromium and molybdenum content in the M23C6 carbide, determined by SEM/EDX steadily, changes and approaches equilibrium as aging time increases. X-ray diffraction measurements show that the M23C6 carbide crystal lattice parameter increases at high-temperature exposure due to enhanced diffusion of alloying elements from the matrix into a carbide lattice. When the equilibrium for diffusion of the alloying elements is reached, the lattice parameter stops growing. The study shows a significant difference in aging behavior. The obtained knowledge of alloying elements diffusion and M23C6 lattice parameter transformation changes could be used as an indicator for the assessment of heat resistant steel after the long term service.

EFFECT OF THERMAL AGING ON STRUCTURAL CHANGES IN P91 AND P5 HEAT RESISTANT STEELS