Studies on the Fatigue Fracture of Tufftrided Steel

Abstract

The authors previously reported on the behaviour of residual stress and change in microstructure due to cyclic stressing by rotary bending of tufftrided carbon steel specimens. They also pointed out that no change was observed in residual stress and microstructure on the surface layer of the specimen during the fatigue process. The strength of tufftrided steel was influenced by the amount of precipitation of nitriding materials in the matrix.In order to clarify the improved mechanism of fatigue-resistance of tufftrided steel, however, it is required that more detailed investigation will be made. It is especially important to make inquiry of the effects of mean stress superposed to the cyclic stresses and the contribution of residual stress. Investigation was therefore made of the fatigue limit and the fatigue life on the alternate bending with mean stress, and of the change in residual stress during the fatigue process.Plate type 0.07% carbon steel specimens were used as the experimental material, and these specimens, after being machined and annealed, were tufftrided at 570°C for 90min. The alternate stress and mean stress were applied to the specimen by Schenck type reversed bending fatigue testing machine. The residual stress in the specimen surface on the way of fatigue process were measured by X-rays. By taking the residual stress as a mean stress and by calculating the equivalent stress amplitude in reversed bending, the contributions of residual stress to the fatigue limit and the fatigue life were studied. An ideal S-N curve, which did not include the influence of residual stress, was introduced.The results of the experiment and discussions thereanent are summarized as follows:(1) Both the fatigue limit and the fatigue life decrease as the mean stress increases. The fatigue limit in the case of supeprosed mean stress coincides with the initiation of plastic deformation.(2) The compressive residual stress in the specimen surface, where the tensile mean stress is loaded with cyclic stress, almost disappears in the initial stage of fatigue life. On the other hand, the residual stress on the compression side slightly decreases at about-20kg/mm2 in the initial stage, but this value remains to the fracture. It is considered that there is no contribution of compressive residual stress of the fatigue strength.(3) Based on the ideal S-N curve, the contribution of residual stress to fatigue limit which is improved by tufftriding is about 14∼30% in case the fatigue is caused by reverse bending.On the other hand, it is evident that 50∼90% of the whole fatigue life is influenced by the compressive residual stress.