Effect of δ ferrite on fatigue strength of SUS630 stainless steel

To study the influence of δ ferrite on the fatigue mechanism of SUS630 stainless steel, rotating-bending fatigue tests were carried out in air at room temperature. The observation of the fatigue process at specimen surface by plastic replica method clarified that the crack initiation sites are soft δ ferrite grains and the size of δ ferrite is a crucial influencing factor for the fatigue strength. At the stress of the fatigue limit, many cracks initiated from the slip bands in δ ferrites, grew into the martensitic matrix and stopped. The aspect ratio b/a of the crack that was initiated from slip band in a δ ferrite was b/a ≅ 0.5. If δ ferrites are considered equivalent to defects, the fatigue limit can be estimated accurately by the √area parameter model. To estimate the lower limit of the fatigue limit, the maximum width of δ ferrite was predicted by statistics of extremes. In fact, fatigue crack did not always initiate in maximum δ ferrite grain because crack initiation is strongly influenced by the crystallographic orientations. However, from the viewpoint of the quality control of large number of machine parts, statistics of extremes is useful to estimate the lower limit of the fatigue limit.