Development of Plastic Region Tightening 1.6-GPa Ultra-High Strength Bolt with High Delayed Fracture Resistance

A new variable compression turbo (VC-Turbo) engine, which has a multi-link system for controlling the compression ratio from 8:1 to 14:1, requires high axial force for fastening the multi-links because of high input loads and the downsizing requirement. Therefore, it was necessary to develop a 1.6-GPa tensile strength bolt with plastic region tightening. One of the biggest technical concerns is delayed fracture. In this study, quenched and tempered alloy steels were chosen for the 1.6-GPa tensile strength bolt. To improve delayed fracture resistance along with high strength, the chemical composition and heat treatment conditions were optimized as follows: 1) High strength: Increase of C and Si contents and higher austenitizing temperature before quenching. 2) Delayed fracture resistance equal to current bolts: Fine carbide spheroidization by high temperature tempering and formation of hydrogen traps by adding Mo. 3) Suppression of strength reduction due to high temperature tempering for compatibility of strength and delayed fracture resistance: Improvement of tempering softening resistance with the addition of Cr and high Mo content. Based on the results of tensile tests and delayed fracture resistance tests in which the chemical composition and tempering temperature were varied, optimum values was found for the new 1.6-GPa tensile strength bolt. By using the ratio of the amount of critical diffusible hydrogen, Hc, and the amount of diffusible hydrogen invading from the surrounding environment, He, it was confirmed that the newly developed steel has delayed fracture resistance equal to or better than that of the current high strength bolt material.