Ratcheting short crack behavior in medium carbon bainitic back-up roll steel under mild tractive rolling contact

In order to improve the current grinding procedure of the back-up roll of CVC hot rolling mills, the ratcheting short crack propagation behavior of medium carbon bainitic back-up roll steel was experimentally investigated under its actual work conditions, and the mechanism was theoretically analyzed based on contact mechanics and shakedown theory. After nucleation, the ratcheting short cracks propagate by the shear growth mechanism driven by the plastic strain accumulation resulting from the process of ratcheting induced by repetitive asperity contacts. They arrest on reaching the maximum depth ranging from 1.6 to 4.5 μm due to the presence of a large “quiescent zone” for crack propagation under the depth at which the maximum orthogonal shear stress is equal to the shear yield strength. At about 70–80% of the surface distress life, the cracks resume propagating by turning parallel to contact surfaces because of the greatly enhanced effect of the lubricant fluid trapped due to the crack geometry change and the residual tensile stress in vertical direction occurring upon unloading due to plastic deformation in the thin surface layer induced by the high cyclic asperity contact stresses. According to the ratcheting short crack propagation behavior and its mechanism, the probable grinding interval and grinding depth were proposed based on the preventive grinding strategy.