PREDICTION OF FATIGUE-STRENGTH IN HSS-CO, S55C AND THE FRICTION WELDED MATERIALS BASED ON STATISTICAL EVALUATION OF INCLUSION SIZE

In this paper, the effects of nonmetallic inclusions on the fatigue st rength of medium and high strength steels are quantitatively investiga ted by considering the relationship between the Vickers hardness, Hv a nd the maximum size of nonmetallic inclusion, root(area)max. The maxim um size of nonmetallic inclusion defined by the square root of the pro jected area in an inclusion can be estimated by the statistics of extr eme values with a definite number of specimens or structural component s. While most of other studies using the parameter root(area)max have performed the experiments with the specimens with small artificial def ects or notches, this study investigates the possibility of prediction on fatigue strength in the unnotched smooth specimens. The results sh ow that strength of the friction welded joints of HSS-Co to S55C carbo n steel is almost equal to that of the weaker material in the optimum welding conditions. Under the limiting condition for the nonpropagatin g cracks emanating from defects or inclusions, the threshold stress in tensity factor range Delta K-th and the lower limit of fatigue strengt h sigma(wi) were successfully estimated from the largest inclusion siz e root(area)max. From the analysis of SEM fractographs, it can be conc luded that the fatigue fractures of the specimens are associated prima rily with the inclusions located at the outer periphery of the specime ns.