Liquid metal embrittlement susceptibility of zinc-coated martensitic sheet steels

Abstract

This paper investigates the liquid metal embrittlement (LME) behavior of three martensitic steels (MS-1, MS-2, and MS-3), each of 1500 MPa tensile strength. Their LME response was measured via hot ductility testing at a fixed strain rate over a range of temperatures (700–950 °C). Despite having the same room temperature strength and general lath martensitic microstructure, the three MS steels exhibited different LME responses. When LME occurred, it proceeded in an intergranular manner within a limited temperature range and with the most severe response at 850 °C. The lath martensitic microstructure was investigated in detail to correlate it to the differences in the LME response. It was found that the three MS steels had different microstructural morphologies and prior austenite grain sizes (PAGS). The LME sensitivity appears to correlate with these microstructural parameters. The MS-2 steel exhibited higher LME susceptibility and possessed the smallest PAGS and a high fraction of low-angle grain boundaries. In contrast, MS-1 was immune to LME at all tested temperatures and had a large PAGS and a high fraction of large-angle grain boundaries. To better compare hot ductility results between steels, a “LME index” was formulated based upon the measured LME severity over the tested temperature range, with a large LME index indicating a severe LME response. Among the three MS steels, MS-2 had the highest LME index while MS-1 had the lowest.