Revealing hydrogen-induced delayed fracture in ferrite-containing quenching and partitioning steels

Abstract

Hydrogen-induced delayed fracture (HIDF) of two ferrite-containing quenching and partitioning (QP) steels, QP980 and QP1180, has been studied by deep-drawn cup tests and slow strain rate tests (SSRT) in the present work. It is found that QP980 has better delayed fracture resistance than QP1180 due to both volume fraction and distribution of ferrite. Other factors, such as residual stress, plastic strain, retained austenite and strain-induced fresh martensite, are also carefully considered, but cannot account for the difference. In the QP steels, hydrogen-induced cracks nucleate in sensitive regions such as retained austenite or strain-induced fresh martensite, and prefer to propagate along prior austenite grain boundaries (PAGBs) and martensite packet boundaries. We also found that ferrite in QP steels can be used to blunt hydrogen-induced cracks, and interrupt the continuity of PAGBs and packet boundaries. QP980 steel has a higher volume fraction of interconnected ferrite, and discontinuous PAGBs and martensite packet boundaries, which are the main mechanisms responsible for the better HIDF resistance of QP980. Therefore, it is proposed that an appropriate volume fraction and distribution of ferrite phase should be present in QP steels for better HIDF resistance.