Effect of tempering temperature and carbide free bainite on the mechanical characteristics of a high strength low alloy steel

The effect of tempering temperatures and the role of carbide free bainite (CFB) on the mechanical properties and susceptibility to hydrogen embrittlement of a low carbon Mn–Si–Cr steel have experimentally been investigated. The air-cooled steel consists of a mixed microstructure of carbide free bainite–martensite (CFB/M), which exhibits a combination of strength and toughness superior to a water-quenched martensite microstructure, after intermediate temperature tempering. The susceptibility to hydrogen embrittlement of the steel with the CFB/M microstructure decreases as the tempering temperature increases from 280 to 370 °C. This is attributed to the CFB/M mixed microstructure which not only possesses high temper resistance but also delays the tempered martensite embrittlement onset temperature. Films of carbon enriched retained austenite in the CFB exhibit good mechanical stability which enhances the toughness of the steel.