Mechanical relaxation of residual stresses in induction hardened bars

The relaxation of compressive residual stresses in the induction hardened outer layer of bar products subjected to cyclic loading can greatly influence their fatigue behavior. A new test sample geometry was recently designed to aid in the study of stress relaxation in the outer surface region of round bars exposed to cyclic loading conditions. An overall C-shape with a T-shaped cross sectional geometry allows the outer surface of the specimen to experience higher stress levels than its inner surface under given loading conditions. These C-shaped specimens were machined out of 66.7 mm (2 5/8 inch) diameter bars of 1530 and 1530 vanadium modified type steels and induction hardened. Initial compressive residual stress levels of approximately 450 MPa (65 ksi) were found to decrease significantly after the first loading cycle and further decrease linearly as the number of applied loading cycles increased logarithmically. The magnitude of the stress relaxation was found to vary monotonically with the maximum cyclic stress over a range of applied stresses from 0.4 to 0.8 of the yield strength. Differences observed on the stress relaxation behavior of the microalloyed and non-microalloyed steels are discussed.