The Effect of Heat Treatment on the Microstructure and Mechanical Properties of P91 Steel for High-Temperature...

The effect of high-temperature heat treatment on the microstructure and mechanical properties of post-operation P91 steel (X10CrWMoVNb9-1) was investigated. The tensile test results confirmed the significant effects of heat treatment type and duration on the properties of P91 steel. The absence of tempering resulted in high strength, reduced ductility, and limited microstructural stability. The tensile strength after normalization was nearly twice as high (1330 MPa) than the UTS value of the as-received sample (692 MPa). The decrease in elongation (9.9%) was related to the presence of supersaturated martensite in the microstructure. Tempering at 780 °C for 4 h with air cooling decreased the strength to UTS = 642 MPa and simultaneously increased the ductility (EL = 11.2%) at a hardness of 240 HV0.1. These changes were associated with microstructural evolution, including the formation and dispersion of precipitates. The most uniform microstructure, characterized by a relatively even distribution of 0.30 µm precipitates and reduced martensitic morphology, was observed after double tempering for 2 h + 2 h at 780 °C with air cooling. High-temperature tempering significantly changed the fracture characteristics and fracture morphology, eliminating martensite brittleness while gradually restoring the ductile nature of the fracture. The results indicate that controlled tempering allows for the formation of a stable microstructure with balanced mechanical properties suitable for high-temperature operation.