Work-Hardening During Peening

Author:  Dr. David Kirk | Coventry University
Source:  The Shot Peener magazine, Vol 39, Issue 2, Spring 2025
Doc ID:  2025012
Year of Publication:  2025
Abstract:  
INTRODUCTION Shot peening is normally applied in order to improve the fatigue properties of components. This improvement is due to two factors: (1) Work-hardening of the surface layer and (2) Compressive residual stress in the surface layer. This article is about work-hardening and fatigue improvement and is aimed at shot peeners rather than scientists. The key to understanding work-hardening is a crystal defect called a “dislocation.” In the early 1900s, scientists were baffled as to why metals started to plastically deform at much smaller stresses than their predicted theoretical strength. About 1934, various scientists proposed that the puzzle could be explained if the metals contained “dislocations”. Many metallurgists remained skeptical of this dislocation theory until the development of the transmission electron microscope in the late 1950s. With further research, based on transmission electron microscopy, we can now understand how work-hardening progresses during plastic deformation. For most components, fatigue life depends upon the applied levels of both static stress and alternating stress. Consider, as an example, a simple railway wagon as illustrated schematically in fig.1. If the wagon was stationary, then a certain level of force, F, would be exerted on the axles inducing a corresponding stress level. The magnitude of F would vary according to the amount of cargo put into the wagon. If now the wagon is being pulled along the track, with a force P, an alternating stress is superimposed on the static stress being applied to the axles. The fatigue life of the axles depends upon the combination of these two stresses. Any increase of either stress will shorten the fatigue life.