Abstract
The changes in the microstructure of a hypo-eutectoid carbon steel by thermal cycling below Ac1 was investigated by the point counting method. The maximum heating temperature of thermal cycling was 690°C and the holding time was 240 seconds. In order to find out the characteristic feature of the changes in the microstructure by thermal cycling, the changes in the microstructure by steady (isothermal) heating were compared with those by thermal cycling.
The lamellar pearlite of the original structure was spherodized and the hardness was decreased by thermal cycling. The finer lamellar pearlite, the quicker this spherodizing process was proceeded. The rate of the spherodization by thermal cycling was more than twice as fast as in the case of steady heating. The characteristic features of spherodization by thermal cycling are as follows. The spherodized pearlite cementite was fine and uniform. The shape of pearlite colony was not changed so much by the spherodization. And the spherodizing process was proceeded very quickly in the colony. It was considered that the rapid spherodization by thermal cycling was due to the decomposition of the pearlite cementite at many positions and the precipitation of the fine spherical cementite in the proeutectoid ferrite by thermal cycling.
