Application of Hard Shot Peening to Automotive Transmission Gears 920760

Although shot peening is an old technology, it has been revived in the Japanese automotive industry as a means to enhance the fatigue durability of steel components. Particular emphasis is on the application of “hard shot peening”. “Hard shot peening” is a high intensity peening technology which results in a higher magnitude of compressive residual stress and, therefore, greater fatigue resistance than conventional shot peening.
The first area of development was in high performance carburizing steels suitable for hard shot peening. Desirable traits were enhanced by reducing the carburizing anomalies resulting from intergranular oxidation and by the enhancing case toughness. Further improvement of fatigue resistance has been accomplished by dual peening, first with hard shot followed by smaller diameter steel shot at a lower intensity.
This paper also describes the development of long life shot media for hard shot peening.
THE DEVELOPMENT of high fuel efficiency vehicles is an urgent need in the automotive industry, especially with CAFE regulations becoming stricter. One approach is to reduce the weight of automotive parts to achieve greater fuel economy. To achieve this, two methods are effective; the application of lighter materials and the downsizing the parts by strengthening of the steels used.
There are many techniques to obtain higher strength such as alloying, the application of clean steels, high quality heat treatment, and introducing residual stress by shot peening. Among these methods, shot peening is gaining increased use as one of the most cost effective ways to enhance fatigue durability.
Shot peening is a cold working process performed by projecting hard particles onto the surface of parts which results in residual stress[l], [2] and [3]*. Even though it is an old technology, new developments are occurring in “hard shot peening”.
“Hard shot peening” is defined as a high intensity peening with an arc height larger than O.6mmA and using hard shot media such as HV550 or higher. It is well known that hard shot peening provides high compressive residual stress in a thin surface layer and results in remarkably high fatigue strengths[4], [5] and [6].
In spite of its benefits, there remains several problems to be solved in hard shot peening. The surface of a workpiece becomes roughened[7] and the life of the shot media decreases. This paper will present a study on high performance carburizing gear steel suitable for hard peening, shot peening technologies to obtain higher fatigue strength and long life shot media.


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