Effect of Tempering on the Microstructure and Mechanical Properties of Heat-Resistant Steel 10Kh9K3B2MFBR Alloyed with Ta and B

The effect of air quenching and subsequent tempering on the structure, mechanical properties and fracture mechanism of steel 10Kh9K3B2MFBR alloyed with tantalum and boron is studied. Transmission electron microscopy is used to show that after the air quenching, the steel acquires a structure of lath martensite with nanosize particles of (Nb, Ta)(C, N) carbonitride and cementite. Residual film-like austenite morphology is located over the boundaries of laths and blocks. The steel in this state is characterized by a high strength (σ_0.2 = 1020 MPa) and an impact toughness of 24 J ∙ cm². During the low-temperature tempering, dispersion hardening and decomposition of retained austenite with precipitation of cementite chains along the lath boundaries lead to embrittlement. Increase of the tempering temperature in the range of 500 – 750°C is accompanied by a monotonic decrease in the strength characteristics. However, a significant increase in the impact toughness occurs only at tempering temperatures ≥ 780°C. It is concluded that the relatively high tempering temperature required to ensure a satisfactory impact toughness is explainable by enhanced precipitation of particles of a M23C6-type carbide and (Nb, Ta)(C, N) carbonitride, which retards the retrogression processes in the lath martensite structure.