Application of Hard Shot Peening to Automotive Transmission Gears

Abstract

Although shot 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 shot Hard shot peening is a high intensity 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 is gaining increased use as one of the most cost effective ways to enhance fatigue durability. Shot is a cold working process performed by projecting hard particles onto the surface of parts which results in residual stress[l]-[3]*. Even though it is an old technology, new developments are occurring in shot peening . Hard shot peening is defined as a high intensity 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 provides high compressive residual stress in a thin surface layer and results in remarkably high fatigue strengths[4]-161. 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 technologies to obtain higher fatigue strength and long life shot media. APPLICATION OF HARD SHOT PEENING NEW STEELS FOR HARD SHOT PEENING-Although shot is beneficial, it is important to reconsider the materials being peened to obtain a higher fatigue strength. The surface of a part becomes roughened by the high energy projection of hard particles and the surface roughness is influenced by the material being peened. This roughening is accelerated by the existence of carburizing anomalies. Intergranular oxidation is one of these defects which occurs at prior austenite grain boundaries during the carburizing process with an endothermic atmosphere. It depletes oxide forming alloying elements adjacent to the grain boundary, which results in a local decrease in the hardenability and promotes the forming of soft transformation products on quenching. Because they are oxide formers, silicon, manganese and chromium are also hardenability increasing elements. Another disadvantage of hard shot peening, the decrease in toughness of peened parts, is caused by the decomposition of retained austen*Numbers in parentheses designate references at end of paper. ite to martensite and work-hardening. It is inevitable as long as conventional carburizing steels are used. It is, therefore, important to select the optimum carburizing steel in the application of hard shot peening. High performance gear steel, DSG1, is one of the candidates. The fundamental properties of DSGl have already been published[8][9]. The chemical composition of DSGl is shown in Table 1. This steel is characterized by decreased intergranular oxidation and soft transformation products by reducing its silicon content to less than 0.15%. Furthermore, case toughness is improved by reducing the grain boundary embrittling element phosphorus and by increasing the molybdenum content up to 0.4%. Table 1 Chemical composition of DSGl steel fwt .X') MECHANICAL PROPERTIES OF DSGl Steel