Shot Peening Plus Subsequent Short-Time Annealing - A Way to Increase the Residual Stress Stability and Alternating Bending Strength of AISI 4140

The positive effects of shot peening are well known: the increase of fatigue strength and life, corrosion fatigue as well as stress corrosion resistance, and the capacity to diminish tensile loading stresses through inducing compressive residual stresses into the surface regions. Varying the shot velocity, mass flow, etc. to improve shot peening effects has also been exhaustedly studied and reported. Because the stability of the compressive residual stresses is of great importance, modifications of the shot peening process have been developed and investigated. The most important are stress peening and peening at elevated temperatures. The latter, so-called warm peening, has led to increases in alternating bending strengths up to 30 % compared to conventional shot peening for an AISI 4140. This results from both, static and dynamic strain aging effects, which stabilize the induced dislocation structure and residual stress distribution. In this paper the benefits of static strain aging will be discussed simply by conducting conventional shot peening with a subsequent annealing process. It will be shown that thermal treatment on conventionally shot peened samples can lead to increases of up to 22 % in fatigue limit for the investigated AISI 4140. This is accomplished without the technical and financial expenses associated with warm peening. The detrimental effect of thermal residual stress relaxation during the annealing treatment on the fatigue strength was successfully separated from the positive effect of static strain aging. It was succeeded to minimize the thermal residual stress relaxation, which was measured by X-ray diffraction method, while maximizing the static strain aging effects.