Rolling-contact-fatigue behavior in backup roll of 5% Cr forged steel

With a focus on the backup roll,a rolling-contact-fatigue experiment was performed on samples of 5% Cr forged steel. The P-S-N fatigue curves were determined and the fatigue strength was calculated. The emergence of cracks on the test-sample surfaces was observed at different fatigue cycles.A micro-hardness tester was used to measure the hardness of the subsurface fatigue layer. The microstructures were analyzed at various magnifications with an optical microscope,scanning electron microscope,and transmission electron microscope.Based on these tests,the rolling-contact-fatigue mechanism of the large forged steel backup roll was also considered.The results showed that the contact-fatigue strength of the tested backup roll steel was 1 249 MPa; the surface fatigue crack lengthened continuously as the number of cycles increased and followed an S-shaped curve;the subsurface fatigue hardness reached its highest value at about 90( HV) increment from the matrix hardness of 540( HV) in the backup roll; the subsurface martensite/bainite microstructure was crushed and the dislocation density was greatly increased.Under alternating contact stresses, the surface/subsurface material was damaged and exhibited many microdefects.At the least, the surface fatigue layer on backup rolls should be fully removed before the microcracks enter a period of rapid propagation.