R. B. Waterhouse

An analysis of shot peening

Abstract It is shown that the residual stresses obtained during shot-peening are directly proportional to the treated materials hardness, and that the depth of material influenced depends on the velocity of approaching shot. The relationship between the shot size and depth hardened is developed, and experimental evidence is provided to verify the major points of the analysis. Useful curves, enabling the variation of residual stress with depth to be estimated, are included, and this enables the shot-peening treatment to be matched to alleviate any subsequent contact-pressure loading that the material may experience during service.

The effect of shot-peening on the fretting-fatigue strength of an age-hardened aluminium alloy (2014A) and an austenitic stainless steel (En 58A)

Abstract Both surface hardening and residual compressive stresses in the surface have beneficial effects in combating fretting-fatigue damage: Shot peening produces both these effects. By applying carefully controlled permanent strains to shot-peened specimens it has been possible to remove the residual compressive stress without affecting the surface work-hardening. The two alloys used in this work have very different work-hardening characteristics, but in both cases it has been shown that removal of the compressive surface stress completely nullifies the effect of shot-peening. It is therefore concluded that surface work-hardening has only a very minor role in improving fretting-fatigue performance.

The metallography of a cobalt-based implant alloy after solution treatment and ageing

The ageing characteristics of a commercial Co-Cr-Mo-C alloy after solution treatment were investigated using optical and electron microscopy. An M23C6-type carbide was identified by X-ray and electron diffraction after ageing treatments between 650 and 1150° C. Nucleation and growth of this carbide took place on intrinsic stacking faults by Suzuki segregation in the cobalt matrix. High stacking-fault densities gave rise to intragranular striations which were visible after etching once precipitation had occurred. Ageing temperatures of 925° C and above increased the stability of the f c c cobalt matrix and led to precipitation on undissociated dislocations. Grain-boundary carbides were evident at all ageing temperatures.

THE FATIGUE OF AN ALUMINIUM ALLOY PRODUCED BY FRETTING ON A SHOT PEENED SURFACE

ABSTRACT An Al-4.2%Cu-0.7%Mg-0.7%Si age hardening alloy (2014A) has been shot peened and subjected to plain and fretting fatigue. The shot peening improves both plain and fretting fatigue strengths, and in particular the fretting fatigue strength of the alloy in the fully aged condition is increased by 130%. The improvement in fretting fatigue properties is largely due to the residual compressive stresses induced into the surface by the shot peening process. Surface roughening also has a small beneficial effect but work hardening of the surface does not influence the fretting fatigue properties.

Fatigue-crack initiation in IMI 829 caused by high-temperature fretting

Fretting fatigue tests have been carried out on a recently developed near-α creep-resistant alloy, IMI 829. The fretting fatigue characteristics have been determined at temperatures of 400 and 600° C under conditions of high and low fatigue stresses. In each case fatigue-crack initiation has been examined and related to the fretting damage on the surface of the specimen, and the microstructure of the alloy.

The metallographic characteristics of fatigue cracks in the titanium alloy IMI829 at 400°C and 600°C

High temperature fatigue tests under fluctuating axial tension were carried out on the alloy in the fully heat treated condition, the microstructure consisting of a β matrix containing colonies of aligned α‐platelets. Fatigue cracks were initiated by fretting at temperatures of 400°C and 600°C. A detailed metallographic examination has been made of the fretted region and the resulting fatigue crack and these characteristics have been related to the microstructure of the alloy. At test temperatures of 400°C the fatigue cracks initially propagated at right angles to the α‐platelets but where their orientation was unfavourable, propagation also occurred parallel to the α‐platelets. At test temperatures of 600°C the initial direction of crack propagation was independent of the microstructure.