FATIGUE FRACTURE BEHAVIOR OF LOW CARBON MARTENSITE AND MARTENSITE/BAINITE DUPLEX STRUCTURE IN STEEL 20CrMnMo

Fatigue crack propagation rate,da/dN,and threshold stress intensity range,ΔK_(th),of steel20CrMnMo containing low carbon martensite or low carbon martensite/bainite(LCM/B)duplex structure,obtained by oil quenching and austempered at 360℃,have been measuredusing specimens under four-point bending.Observation was also made of the crack path andfracture morphology with relation to microstructure.The formation of LCM/B duplexstructure,which caused by 20% lower bainite after short-time isothermal treatment,may re-markably increase ΔK_(th)and considerably decrease da/dN.The effect of microstructure onda/dN and ΔK_(th)was discussed with the emphasis on the crack propagation path.