Influence of Heat Treatment Method on Grain-Refining during Warm Deformation of Medium Carbon Steel

The microstructure evolution and grain-refining mechanism in the process of ultrafine grain preparation with combination of different heat treatments and warm deformation were studied. 5140 steel，medium carbon low alloy steel, was used in this experimental and divided into three groups, which is one time quenching state, three times cyclic quenching state and original state without any heart treatment as the control group. All the samples of above three groups hold at 600°C for 5 min, then warm deformation was carried out immediately in YJ-450 Four-Pillar Hydraulic Machine. The deformation amount of above samples is controlled beyond 50%. The result shows that microstructure of the medium carbon steel samples after once quenching treatment and three times cyclic quenching treatments was significantly refined after warm deformation. The grain-refining mechanism of supersaturated single-phase ferrite in warm deformation is that new dislocation boundaries are produced continuously through dislocation cross-slip, which segments and refines the original grains. The grain size of three times quenching state after warm deformation is obviously finer than the grain size of one time quenching state after warm deformation under the same condition. And quenching times is a very significant factor to the extent of fining grain after warm deformation. Through three times cyclic quenching treatments and warm deformation, the size of grains has reached nano-scale.