Study on the softening mechanism of P91 steel

Different tempering temperatures were designed to study on the softening mechanism of P91 steel. The variations in the microstructures and mechanical properties of the steel tempered at different temperatures were investigated. After tempered at the temperature below 790 °C, the microstructure of P91 steel mainly consisted of tempered lath martensite. M23C6 carbides were present not only at boundaries of various types, but also in laths or subgrains interiors. M23C6 carbides within laths or subgrains exhibited three kinds of morphologies, namely ellipsoid-like, irregular block-like and plate-like. MX phases were primarily distributed in laths or subgrains interiors and were comprised of spheroidal Nb-rich particles, V-rich particles with various morphologies and “winged” complexes. With increasing tempering temperature, dislocation density in lath or subgrains interiors decreased, causing a loss of dislocation hardening and M23C6 carbides were coarsened, which not only reduced their effect of preventing the migration of lath boundaries and hence accelerated the widening of lath, resulting in a loss of sub-boundary hardening, but also led to a loss of precipitation hardening. This made the yield and ultimate tensile strengths as well as the hardness of P91 steel decrease. In other words, the softening of P91 steel took place. After tempered at 810 °C, the microstructure of P91 steel mainly consisted of equiaxed subgrains or grains and the yield and ultimate tensile strengths of this steel at room temperature were already lower than the minimum values specified by ASTM A335 standard.