Abstract: Effect of quenching and tempering process on microstructure, mechanical properties and impact fracture morphology of the core zone in U-shaped test block of ZG34CrNiMo steel was studied by means of optical microscope, scanning electron microscope and mechanical property test. The results indicate that compared to conventional quenching and tempering processes, when the ZG34CrNiMo steel istreated by optimized quenching and tempering process (high-temperature normalizing, higher cooling rate of quenching liquid and high-temperature tempering with fast cooling), the martensitic transformation can be fully completed in the core of the U-shaped specimen during quenching, the grain size is refined from grade 7.6 to 8.3, a large number of elliptical dimples appear in the impact fracture, and the average low-temperature impact absorbed energy at -35 ℃ is increased from 7.23 J to 43.00 J. By using trace amounts of vanadium or rare earth elements to improve the composition of the steel, the comprehensive mechanical properties of the core zone specimen are further improved significantly, especially with average low-temperature impact absorbed energy of 77.00 J at -35 ℃.

Key words: ZG34CrNiMo steel, quenching and tempering treatment, microstructure, low-temperature impact property