The mechanical properties of as-quenched and tempered steels are affected by austenitizing temperature. The present work has investigated the effect of austenitizing temperature on martensitic microstructure, carbide precipitates and mechanical properties of 30NiCrMoV12 alloy steel for the axle of high-speed train. The martensitic microstructure and carbide precipitates were studied using OM, FE-SEM, TEM, EBSD and EDS. Thermodynamic calculation of equilibrium precipitation were carried out by Thermo-Calc software. The results showed that the prior austenite grains, martensitic packets, blocks and laths were coarsening with increasing austenitizing temperature. Besides, with increasing austenitizing temperature, after tempering the amount of large size carbides precipitated at martensitic lath boundaries decreased while the amount of small size carbides precipitated in matrix increased. Meanwhile, phase transformation from M 23 C 6 to M 7 C 3 during tempering was enhanced with increasing austenitizing temperature. Coarse grains and wide martensitic laths were beneficial to reducing the amount of strip-like M 23 C 6 carbides precipitated at martensitic lath boundaries due to the reduction of boundary area and thereby obtaining more fine precipitates in matrix. The strength and impact toughness could be improved to a certain extent by refining carbides in tempered steel with higher austenitizing temperature. However, the degree of favorable influence on impact toughness resulting from refining carbides was lower than the negative effect from coarse martensitic structures. Therefore, the toughness is deteriorated and the strength is improved with increasing austenitizing temperature.

The hot tensile tests were conducted in this study to investigate the effects of Nb, B, Mo, and V on hot ductility of 25CrMo alloy steel in a temperature range of 650–850°C with strain rates of 0.005 and 0.5 s−1. Besides, the influences of ferrite transformation and precipitates on hot ductility were also investigated by the use of SEM and TEM. Thermo-Calc and J Mat Pro were used for calculating equilibrium precipitates and CCT curves, respectively. The results indicated that the hot ductility is deteriorated with the addition of 0.04% Nb due to Nb(C,N) particles and ferrite transformation. The addition of B inhibits ferrite transformation and improves hot ductility. The hot ductility is improved with increasing strain rate from 0.005 to 0.5 s−1 due to the nucleation and growth behavior of ferrite. The fast strain rate promotes nucleation of ferrite; however, the ferrite has no sufficient time to grow up. The addition of Mo inhibits ferrite transformation and improves hot ductility. The addition of 0.12% V has no obvious effect on ferrite transformation. The hot ductility has deteriorated a little with the addition of 0.12% V due to the solution V that increases stress during hot deformation.

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The effect of cooling rate on martensitic structure and carbides precipitation behavior was investigated based on Cr-Mo alloy steel with different quenching media of oil, water and 10% NaCl-water, respectively. The influence mechanism of martensite structure and carbide precipitates on mechanical properties was also studied. The results showed that martensite packets and blocks were refined with water quenching, however, they were coarse with oil or 10% NaCl-water quenching. Martensite laths were refined and dislocation density increased with increasing cooling rate. The carbides in tempered steel were coarse obviously with 10% NaCl-water quenching. The impact toughness deteriorated significantly with 10% NaCl-water quenching due to coarsening of martensite structure and carbides.