Correlation between structural changes of M23C6 carbide and mechanical behaviour of P91 steel after thermal aging

The results of thermal aging effect on the P91 steel after a long-term exposure at 600, 650 and 700 °C are presented. Mechanical behaviour of the aged material was assessed by performing tensile, fracture toughness and short term creep tests along with microstructure examination. Sample aging duration and temperatures were chosen considering the structural changes of M23C6 carbide, which are described by the exponential equation as a function of time and temperature. The time-temperature dependence was previously established from M23C6 lattice expansion as a result of alloying elements' diffusion into the carbide. The main focus is to determine whether the same relationship could be used for the assessment of the mechanical properties of P91 steel. Tensile, fracture toughness and creep test data at different aging duration and temperature indicated that predicted data correlate well with experimental data and gives a possibility to forecast changes caused by steel thermal exposure. Experimental results demonstrate that the used method provides a sufficiently good technique to predict the mechanical behaviour of P91 steel during aging.