Development of Mo-Free Ultra-High Strength 1.6-GPa Bolt with Delayed Fracture Resistance for Multi-Link Type Engine 2024-01-2070

Mo-free 1.6-GPa bolt was developed for a Variable Compression Turbo (VC-Turbo) engine, which is environment friendly and improves fuel efficiency and output. Mo contributes to the improvement of delayed fracture resistance; therefore, the main objective is to achieve both high strength and delayed fracture resistance. Therefore, Si is added to the developed steel to achieve high strength and delayed fracture resistance. The delayed fracture tests were performed employing the Hc/He method. Hc is the limit of the diffusible hydrogen content without causing a delayed fracture under tightening, and He is the diffusible hydrogen content entering under a hydrogen-charging condition equivalent to the actual environment. The delayed fracture resistance is compared between the developed steel and the SCM440 utilized for 1.2-GPa class bolt as a representative of the current high-strength bolts. Owing to delayed fracture test, Hc/He of developed steel is 49.1 (Hc:3.39 ppm, He:0.069 ppm), and that of SCM440 is 4.14 (Hc:1.16 ppm, He:0.028 ppm). In the microstructure of the developed steel, cementite is dispersed by the addition of Si, and the film-like carbides that precipitated at the grain boundaries, causing delayed fracture, are suppressed. Therefore, Hc of the developed steel is higher than that of SCM440. Moreover, adding Si reduces the hydrogen diffusion rate; hence, the He of the developed steel is less than that of 1.2-GPa class bolt. Thus, we developed a novel 1.6-GPa tensile strength bolt that achieved higher hydrogen embrittlement resistance than the current high-strength bolt.