Mo系低合金焼結鋼の靱性および疲労強度向上のための焼結冷間鍛造工法と浸炭熱処理条件の最適化

抄録

The aim of this study is to obtain the high fatigue strength of 0.35%C-1%Mo sintered steel by the combination process of primary-sintering（PS）, cold-forging（CF）, carburizing and carbonitriding, quenching and tempering. The impact energy of the PS specimen increases exponentially at high PS temperatures. It becomes very high in case of the conditions of PS density of 7.4 Mg/m3 and PS temperature ranging from 1075℃ to 1100℃. Also, the impact energy depends on the volume diffusion in the sintered material and it was found that the impact energy is directly proportional to logarithm of the diffusion length of Fe atoms in γ-Fe. From investigations of both the impact load profile and the microstructure of PS-CF specimens, it was found that micro-cracks occur on the surface layer of specimens by CF load, irrespective of PS conditions: PS temperature, PS time and the density. These micro-cracks weaken the heated PS-CF specimens. Since the heat treatment leads the diffusion bonding of micro-cracks in the surface layer, the cracks can be recovered. When the PS-CF specimens were heated by the suitable second sintering and vacuum carbonitriding, both the superior impact energy and the superior fatigue limit of bending strength were obtained compared to those of wrought steel SCr420H. The results would not be obtained only by the suitable PS-CF processes, and show that the combination of heat treatments have the synergistic effect of micro-cracks recovery and pore spheroidizing, along with microstructure modification.