The role of ultrasonic nanocrystalline surface modification at elevated temperature on the hydrogen charging behavior of high-Mn steels

Abstract

Hydrogen embrittlement, which originates from hydrogen localization in internal defects (i.e., grain boundaries, twins, and interface), induces both decohesion and strain localization in high- strength steels. Such embrittlement is a critical issue when attempting to prolong the material life in an actual environment. Therefore, microstructural design for the prevention of hydrogen invasion is essential to develop hydrogen-embrittlement resistant high-strength steels. Because the hydrogen embrittlement starts from the surface, in this work, ultrasonic nanocrystalline surface modification (UNSM) treatment at elevated temperature was applied to high-Mn steel. Microstructural analysis reveals that the UNSM treated high-Mn steel has a nanocrystalline layer and compressive residual stress at the surface region, while the martensitic transformation is suppressed by the processing at elevated temperature. This microstructural feature provides additional strength, prevents hydrogen invasion on the surface, and reduces localized hydrogen concentration in the internal defects. These beneficial effects improve the mechanical property in a hydrogen-containing environment of the UNSM-treated high-Mn steels simultaneously. This result sheds light on designing heterogeneous microstructure in high- strength steels to achieve the combination of high strength-ductility and prolonged lifetime in a hydrogen-rich environment.