Increased resistance to hydrogen embrittlement in high-strength steels composed of granular bainite

This paper presents two high-strength steels composed of granular bainite microstructure that have higher extrinsic resistance to hydrogen embrittlement (HE) than conventional tempered-martensite steel, because the granular bainite steel traps less hydrogen than tempered-martensite steel. However, the granular bainite steels had lower intrinsic HE resistance than tempered-martensite steel because hydrogen in granular bainite steels becomes concentrated at microstructures composed of martensite islands and retained austenite. The granular bainite steels with less sulfur showed better HE properties, because S degrades grain-boundary strength. Since the extrinsic properties correspond to the HE resistance that is relevant in industry, granular bainite steels are suitable for applications in hydrogen environment.