Dissolution of cementite into austenite during ultra rapid heating

Recently, ultra rapid heating (greater than or equal to 1000 degrees C/sec) has been applied to improve mechanical properties of induction hardening steels. In such a case, it is quite important to control the dissolution of spheroidized cementite during heating, which affect the properties severely after quenching. In the present study, we applied the ultra rapid heating to a S45C steel to investigate the dissolution behavior of the spheroidized cementite. The dissoluble maximum cementite particle size dependence on the heating rate and maximum heating temperature was examined. The shrinking rate of cementite during heating is low in the early stage and becomes high in the final stage because of temperature and carbon concentration gradient. An exact diffusion equation was derived to describe the dissolution of spherical cementite to austenite during ultra rapid heating by using Green's function considering a moving boundary. The maximum radius of cementite that can completely dissolve by heating can be well estimated from the derived equation, and agrees well with the experimental results. The observed shrinkage rate of cementite was somehow smaller than calculated. This may be due to the overlapping of carbon diffusion zone from the nearest cementite.