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This paper describes a newly developed steel composition and surface modification methods for improving the rolling contact fatigue strength of parts used in transmission systems, especially continuously variable transmissions (CVTs) to increase their torque capacity. The mechanisms of two types of typical rolling contact fatigue phenomenon in case hardening steel were examined with the aim of improving rolling contact fatigue strength. One concerned white etching constituents (WEC) and the other one concerned peculiar microstructural changes caused by hydrogen originating from decomposition of the lubrication oil as a result of repeated rolling contact stress cycles. The rolling contact fatigue strength limit due to WEC has been improved markedly by dispersing fine M23C6 alloy carbides in the martensite matrix at the subsurface layer of parts.

The application of both high strength gear steels and shot peening technology has succeeded in strengthening automotive transmission gears. This technology, though, improves mainly the fatigue strength at the tooth root, but not the pitting property at the tooth face. Therefore, demand has moved to the development of new gear steels with good pitting resistance. In order to improve pitting resistance, the authors studied super carburizing which is characterized by carbide dispersion in the case, especially processed with a plasma carburizing furnace. Firstly, the influence of the carburizing temperature and carburizing period on the carbide morphology was investigated and the optimum carburizing conditions were determined. Secondly, the fatigue strength and pitting resistance was evaluated using carbide dispersed specimens.

In recent years, efforts for development of high-performance and compact automobile engine are being made more actively than ever before. The connecting rod bolt is one of those parts which are very much required to be compact and light, since its size and weight affect the performance of the engine. However, if it is used under high stress, delayed fracture can be caused. Therefore, it is a common practice to keep the stress below 1177MPa. The developed alloy (HB149) is a precipitation hardening alloy steel of low carbon martensite, composed mainly of 0.35C-1.2Cr-1.0Mo-0.3V. With lowered intergranular inclusions by suppressing impurity elements such as phosphorus and sulfur, and with reduced grain boundary embrittlement by tempering it at a high temperature, this steel is excellent in resistance to delayed fracture. Using this steel, we developed a 1372MPa class (Class 14.9*) high strength connecting rod bolt.