Relaxation of residual stresses induced by turning and shot peening on steels

Experiments on the relaxation of residual stresses in steels by fatigue loading are described. This question is of interest because it is well known that compressive residual stresses are often induced by special surface treatments (such as shot peening) to improve the fatigue life of metal parts; however, if cyclic relaxation occurs, the beneficial effects can, in part, vanish during service. Two hardened and tempered steels of grade C45 and 39NiCrMo3 were used in the tests. For both materials, different specimens were given two surface treatments: simple turning without successive surface treatment, inducing on the surface a moderate tensile residual stress state, and shot peening, inducing high residual compressive stresses. The specimens were submitted to constant-amplitude tension-compression fatigue loading, and the surface residual stresses were measured after 0, 1, 10^5 cycles and more. Results show that relaxation occurs from the very first cycle; the amount of residual stress relaxation depends on many parameters and on the type of steel. The results are in agreement with data obtained by other researchers.