Development of Plastic Region Tightening 1.6-GPa Ultra-High Strength Bolt with High Delayed Fracture Resistance
In our new Variable compression Turbo (VC-Turbo) engine, which has multi-link system to control the compression ratio (from 8:1 to 14:1 with this system), the high axial force is required for fastening of multi-link system which has the high input and down-sizing requirement. Therefore, it was necessary to develop the fastening bolts of the 1600MPa tensile length. The biggest issue of high strength bolt is delayed fracture with hydrogen embrittlement. In this study, the quenched and tempered alloy steels were chosen for 1600MPa tensile strength bolt by taking into account mass production and low cost.