Crack Initiation and Propagation Mechanism of Rapidly Tempered High-Si Steel during Hydrogen Embrittlement

In this study, we focused on fine dispersion of Fe2-3C(ε) by combining the addition of Si and rapid tempering to improve the hydrogen embrittlement resistance of high-strength spring steels. The aim of this study is to clarify the fracture mechanism of rapidly tempered high-Si steels: the JISSUP7 (2.0Si) and SAE9254 (1.4Si) spring steels were tempered at different tempering rates by induction (IH) and furnace heating (FH) methods. Bending test were carried out during the cathodic hydrogen charging to observe the fracture origin and morphology of the steels. The size and volume of carbides were quantified using small-angle X-ray scattering method (SAXS) and synchrotron radiation XRD. The distribution of carbides was observed with the replica method: facets were observed at the fracture origin of the 2.0Si-IH steel and 1.4Si-IH steel, which contained retained γ in the microstructure. It was considered that the facets formed because retained γ at the grain boundaries transformed into martensite during hydrogen embrittlement, promoting intergranular cracking; the 2.0Si-IH steel contained the largest amount of retained γ, but also contained fine Fe2-3C(ε) in the lath. This suggests that dislocations and hydrogen are less likely to accumulate at the grain boundaries, resulting in the longest fracture life. In other words, in rapidly tempered high Si steels (2.0Si-IH steels), the fine dispersion of Fe2-3C(ε) has more influence on the suppression of crack initiation and propagation than the increase in the amount of retained γ, and this contributes significantly to the fracture life.