Resistance of Quench and Partitioned Steels Against Hydrogen Embrittlement

Multiphase ultra-high strength steels (UHSS) containing retained austenite (RA) appear to be among the most interesting steels for the automotive industry. Developments in the last decades have allowed obtaining a very good combination of mechanical strength and ductility. Quenching and partitioning (Q&P) steels have been proposed as third-generation UHSS, reaching ultimate tensile strength up to 1300 MPa along with excellent fracture elongations of more than 15%. However, the use of Q&P steels is mainly limited by their susceptibility to hydrogen embrittlement (HE). The present work investigates the influence of the Q&P heat treatment parameters on the mechanical properties and on the HE resistivity of 20Mn-Si wire rod steel. The HE resistivity was measured using incremental step load testing with in situ electrochemical hydrogen charging according to ASTM F1624-12 standard. A comprehensive microstructure characterization was performed to examine volume fraction, nucleation sites and morphologies of RA. Although the mechanical properties were similar after Q&P heat treatment, an increase in the partitioning time revealed a significant increase in the HE threshold stress of more than 200 MPa.