An Investigation On Phase Transition And Spheroidizing Annealing Process Of50CrV4Steel

In this paper, the medium-carbon high strength spring steel of50CrV4was taken for the subject of the research. The austenite phase transformation and continuous cooling transformation of supercooling austenite about50CrV4steel were investigated respectively. In order to find out the best spheroidization annealing process of the50CrV4steel, the cyclic spheroidizing and continuous annealing process were studied. Results are as follows:Heating rate has a significant effect on the Acl temperature for the onset of austenization and the Ac3temperature for the end of austenization. The determined temperatures of Acl and Ac3increase with heating rate increasing. The relation between the peak of transformation rate curve and the heating rate accords with the formulation: During continuous heating, the isochronal autenitic transformation of the experimental steel was described well by a phase transformation model involving site saturation and diffusion controlled growth. The formation of a new phase nuclei is caused by the uneven distribution and dispersion of carbon atoms in the parent phase, and the new phase of the growth process is dominated by the diffusion of carbon in austenite.The product of austenite transformation in different continuous cooling rates include ferrite, pearlite, bainite and martensite. Ferrite, pearlite can be obtained at the cooling rate lower than5℃/s. When the cooling rate is5℃/s, the microstructure of the transformed steel is a mixture of ferrite, bainite and a little martensite. Martensite can be obtained at the cooling rate over than10℃/s. The critical cooling rate of Ms transformation of50CrV4steel is10℃/s.The strength and hardness of the steel is reduced and the toughness is enhanced by the spheroidization experiments. The spheroidization ratio of cementite and the average size of particles increase with the number of cyclic heat treatment, after5cyclic heat treatments, the spheroidization ratio of cementite is100%, the plasticity of spheroidized steel is sensitive to the larger cementite particles.The results of continuous annealing experiment showed that the best continuous annealing process parameters of50CrV4steel are as follows: the soaking temperature is860℃, the soaking time is200s, the slow cooling rate is5℃/s, the overaged temperature is700℃and the overaged time is300s. The50CrV4steel can get good spheroidized microstructure by this continuous annealing process.