RESIDUAL-STRESS RELAXATION IN AN AISI-4140 STEEL DUE TO QUASI-STATIC AND CYCLIC LOADING AT HIGHER TEMPERATURES

Residual stresses can be relaxed by supplying sufficiently high amount s of thermal and/or mechanical energy, which converts the residual ela stic strains to microplastic strains. In order to better understand th is relaxation behavior, shot peening induced residual stresses in norm alized condition and in quenched and tempered condition of the steel A ISI 4140 (German grade 42 CrMo 4) were investigated in annealing exper iments, quasistatic loading experiments and bending fatigue experiment s at 25, 250 and 400 degrees C. The residual stress relaxation during alternating bending occurs in different regimes. First, thermal relaxa tion reduces the residual stresses during specimen heating. The relaxa tion during the first cycle can be discussed on the basis of the effec ts due to quasistatic loading, if the inhomogeneous distribution of th e loading stress is taken into account. Differences in the behavior af ter the two heat treatments result from the Bauschinger-effect and eff ects of dynamic strain ageing. Owing to cyclic creep effects, the inte rval between the first cycle (N = 1) and the number of cycles to crack initiation Ni is characterized by residual stresses which decrease li nearly with the logarithm of N. Finally for N > N-i the reduction of r esidual stresses with the logarithm of N is stronger than linear. (C) 1998 Elsevier Science S.A. All rights reserved.