At present, with the development of rapid fire system, the requirements for high rate of fire, high temperature and high impact are increasing day by day. In addition to sufficient strength and ablative resistance, the requirements for toughness are also getting higher and higher. The researchers took 30CrNi2MoVNb steel as an example to heat the material, namely: Quenching + low temperature tempering (Q&T), isothermal quenching + low temperature tempering (A&T), quenching + carbon distribution + low temperature tempering (Q&P&T), by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), hardness test, tensile test, impact test, etc., The effect of heat treatment process on the microstructure and properties of test steel was studied to optimize the microstructure and properties, and to better tap the potential of its strength and toughness. The test results show that:
(1) After Q&P&T and Q&T treatment, the microstructure of the test steel is tempered martensite and residual austenite, in which the residual austenite is mainly distributed in the martensite lath as a thin film (10-40nm), and fine dispersed ε -carbide (about 100nm) precipitate in the tempered martensite lath. After Austempering treatment, the microstructure of the steel is martensite, lower bainite and residual austenite, and fine dispersed cementite precipitate in the matrix. Compared with Q&T process and A&T process, the residual austenite volume of Q&P&T process is up to 8.8%.
(2) After Q&P&T treatment, the yield strength, tensile strength and impact toughness of the test steel are 1011.0mpa, 1630.6mpa and 109.7j /cm2, respectively. The impact toughness is 19.8% higher than that after Q&T treatment and 9.7% higher than that after A&T treatment. Its yield ratio decreased from 0.82 (A&T) to 0.62. The film – like residual austenite is the main reason for the improvement of impact toughness.
(3) The impact toughness of test steel increases with the increase of volume fraction of residual austenite and lower bainite, and the contribution of residual austenite to impact toughness is greater than that of lower bainite. The strength is determined by martensite strength (tempered martensite strength), lower bainite strength, fine grain strengthening and other factors. After Q&T, A&T, Q&P&T processing, the fracture mode of the test steel is ductile fracture.
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