EFFECTS OF NONMETALLIC INCLUSIONS AND SMALL SURFACE PITS ON THE FATIGUE-STRENGTH OF A HIGH-STRENGTH STEEL AT 2 HARDNESS LEVELS

The effects of nonmetallic inclusions and artificial corrosion pits on the fatigue strength of backup roll steel at two hardness levels are investigated. The crucial factors which control the fatigue strength of hard steels are the Vickers hardness (HV) and the square root of the projection area of inclusions or pits (square-root-area). The fatigue strength prediction equation was proposed using HV and square-root-area. The lower limit of large scatter in the fatigue strength was predicted by this equation using the maximum inclusion size, which is determined by the statistics of extreme values, and HV. The quantitative perspective to the reduction of maximum inclusion size in the steel making process was suggested on the basis of the prediction equation and the extreme value data. The proposed method was applied to a new backup roll steel with high fatigue resistance.