Development of Plastic Region Tightening 1.6-GPa Ultra-High Strength Bolt with High Delayed Fracture Resistance
To be published on April 2, 2019 by SAE International in United States
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. To improve the delayed fracture resistance, we proceeded the optimization of chemical composition and heat treatment condition as follows, 1) high-C and high-Si steel as a base to obtain sufficient strength even at high tempering temperature (around 600 degree) to make the fine and spherical carbide microstructure, 2) combined addition of Si, Cr and Mo as alloying elements for increasing temper softening resistance, 1.5% or more of precipitation hardening type element Mo was required to achieve high temperature tempering. We also expected a hydrogen trapping effect due to precipitation of Mo carbide in the grain, 3) The amount of carbon content of the bolt surface after heat treatment was optimized. After the mechanical test and delayed fractured test, we had developed the 1600MPa tensile strength bolt as a 0.58C-1.9Si-1.2Cr-1.7Mo steel. Aboutt the delayed fracutere tes, we set as the indicator of delayed fracture resistance by using the ratio of “Critical diffusible hydrogen amount [Hc]” and “Intrusion diffusible hydrogen amount of from the environment [He] “. We confirmed that this developed steel had safety on the current hight strength bolt material level. Finally, this bolt was mass-produced after confirming the quality in mass production.