The use of controlled post heat treatment cleaning and peening to enhance the fatigue strength and reliability of case carburized gears

Shot peening techniques are a well established method for improving the strength of engineering components subjected to bending fatigue through the introduction of compressive residual stresses. Recent work carried out as part of the British Gear Association (BGA) materials research programme has shown that the control of the post heat treatment method of gear cleaning can also introduce compressive residual stress levels, which also have a beneficial effect on strength. This paper presents results from the research work, which shows that the strength improvements are dependant on the type of cleaning method used and that they are critically dependant on the careful control of the technique, in a similar way to that used for shot peening. A variety of techniques of cleaning treatment have been examined, ranging from light alumina vapour blasting, to glass beads, grit and shot. In each case, the effect of the treatment on the residual stress depth profile in the material has been assessed. The changes in fatigue strength observed show a strong relationship with the measured change in the residual stress profile induced from the peening and/or blasting process. The site of fatigue crack initiation has also been characterised and was found to be dependant on the residual stress profile in the surface layers of the material. The change from surface to sub-surface crack initiation identified after some treatments partly helps to explain the large increases in fatigue strength observed. The results clearly show that by optimising the cleaning/peening process, gear reliability and strength can be greatly enhanced.