| Then Hypo-eutectoid carbon steel, containing 0.3% of carbon, was treated by repeated thermal cycling after hot work-quenching. The results of simple repeated thermal cycling treatment and that of repeated thermal cycling treatment after hot work-quenching were compared and discussed mainly in point of the spheroidization rate (sph) of pearlite and number of fine spherical cementite (NO/100u²) deposited in ferrite particles. Consequently the shape of pearlite, the cooling velocity and the effect of working degree and steady heating were examined, and also the mechanical properties were surveyed. The results are as follows (1) Repeated thermal cycling treatment after hot work-quenching accelerated the spherical tendency than steady heating treatment after hot work-quenching and the effect of hot work-quenching on the repeated thermal cycling. (2) The thermal peak of the simple thermal cycling treatment was 200 cycles and the cycles less than this peak deserves the nucleation process and that more than this peak comes under the growth process of nucleus. The thermal peak of thermal cycling treatment after hot work-quenching, however, was 100 cycles and both treatment showed that the nucleation takes place, until 300 cycles, even it is a very small amount. (3) When the repeated thermal cycling treated after hot work-quenching spheroidization rate was promoted in proportion to the increase of working degree at high temperature, the number of cycles, and the cooling velocity. (4) In the case of repeated thermal cycling treatment the structure of sample accelerated the spherical tendency in order of coarse pearlite-fine pearlite-martensite. (5) On the whole, it could be understood that the toughness was increased in contrast with decrease of tensile strength and hardness of the material, according to the number of cycling in the case of repeated thermal cycling. Tensile strength and hardness in the case of repeated thermal cycling treatment after hot work-quenching were decreased than those of simple repeated thermal cycling treatment. Consequently, it is anticipated that the toughness would be increased. |
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