Hydrogen embrittlement properties of rapid tempered high-Si steel and effects of the carbide and retained austenite on its properties

Abstract

Automotive suspension springs are required to be high-strength and lightweight, and currently have a maximum strength of 2000 MPa. In addition, they must have high resistance to hydrogen embrittlement in the service environment. From previous research, Si addition or rapid tempering improves the hydrogen embrittlement resistance of low alloy steel. In this study, we investigated the hydrogen embrittlement properties of steel samples with different Si contents and tempering rates and the effects of the fine iron carbides and retained austenite on its properties for 2000 MPa suspension spring steel. JISSUP7 (2.0Si) and SAE9254 (1.4Si) spring steels were tempered at different tempering rates by induction (IH) and furnace heating (FH) methods. Four-point bending tests under corrosion cycles were performed on these steels, and the time to failure was measured. The results show that the 2.0Si-IH steel with higher Si content and higher tempering rate has the longest fracture life and highest resistance to hydrogen embrittlement, even with relatively high diffusible hydrogen content. The size and volume fraction of iron carbides and retained austenite were evaluated by TEM, EBSD, and synchrotron XRD, and the 2.0Si-IH steels were found to have the smallest size and the highest volume fraction of fine iron carbides Fe_2-3C(ε) and the highest amount of retained austenite. It is considered that the fine iron carbides of Fe_2-3C(ε) work as hydrogen trap sites and that their high dispersion suppresses dislocation movement. They suppress hydrogen accumulation in stress concentrated areas and are expected to improve resistance to hydrogen embrittlement.