高濃度真空浸炭と高周波焼入れによる複合熱処理を施した肌焼鋼の衝撃特性

抄録

Charpy impact value of the case hardening steel subjected to combined heat treatment with excess vacuum carburizing and subsequent induction hardening was evaluated. The purpose of this study is to clarify the effect of retained austenite and cementite on the impact property. The characteristic of combined heat treatment is that the initial microstructure can be designed easily. The initial microstructure is designed by carburizing and annealing at the hyper-eutectoid composition of 1.3 mass% C and subsequent induction heating temperature is chosen between A_cm and A₁ to obtain different amounts of retained austenite and cementite. The impact value improves by the induction hardening with increasing heating temperature and the tempering. The steel treated at the low heating temperature shows intra-granular fracture irrespective of the presence of pro-eutectoid cementite. As the heating temperature increases, the formation of C solid solution progresses by the decomposition of cementite and increased retained austenite transforms into the deformation-induced martensite by the impact energy, thereby increases intra-granular strength. Hence critical fracture strength transits to grain boundary strength and showed inter-granular fracture at the interface of cementite and matrix in this study. The impact value showed the correlation with the amounts of retained austenite before the test and the decrement in retained austenite before and after the test. The effect of retained austenite is due to the plastic deformation of austenite, the increase of the compressive residual stress generated by deformation-induced martensite transformation, and the consumption of the impact energy as the driving force for deformation-induced martensite transformation.