고강도 구상흑연주철의 피로한도에 미치는 2상 기지조직의 영향

Effects of 2-phase matrix structure on fatigue limit of high strength ductile iron

Rotary bending fatigue tests were performed to investigate the effects of 2-phase matrix structure on fatigue limit with prepared specimens in high strength ductile irons. Two types of the specimens with different microstructures have been used. Series A has sorbite and series B has bainite. Fatigue limits of both specimens are improved comparing with as cast specimen. The fatigue limit is higher in series B than in series A. The reason why the fatigue limit of series A shows inferiority to that of series B is due to the transition of micro fatigue cracks to mesocrack occurs very rapidly, so increased stress intensity factor drives the fatigue crack growth. The higher fatigue limit of series B which has bainite is caused by the γ layer contained in microstructure impede the rapid growth of micro fatigue crack to mesocrack and α layer around graphite has the higher capacity fro the absorption of plastic deformation energy than sorbite.