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.
The developed bolts have been used as connecting rod bolts in mass-produced motorcycles since 1990. They contributed to improvement of engine performance with high axial tension fastening and compactness.