Hydrogen embrittlement behavior of medium-Mn steel with precipitate

Abstract

ABSTRACT

Hydrogen embrittlement behavior of medium-Mn steel with precipitate

Tak Min Park Dept. of Materials Science and Engineering The Graduate School Hanyang University

Recently, medium-Mn steel, which can be defined as steel with a Mn concentration between 3 wt% and 12 wt%, is widely investigated as a third-generation advanced high-strength steel with excellent mechanical properties and reduced material cost. While medium-Mn steels exhibit an almost single phase of martensite after hot and cold rolling, they display a mixed microstructure of ferrite (α) and retained austenite (γR) after annealing. The excellent mechanical properties of medium-Mn steels are attributed to the transformation-induced plasticity (TRIP) phenomenon that occurs in the γR during deformation. Furthermore, recent advancements have focused on increasing the stacking fault energy (SFE) to activate both TRIP and twin-induced plasticity (TWIP), leading to further enhancements in mechanical properties. Despite their excellent mechanical properties, medium-Mn steels are still susceptible to hydrogen embrittlement (HE). It is reported that HE of medium Mn steel causes premature fracture through the aggregation of diffusible hydrogen into strain-induced martensite (α) when the TRIP phenomenon occurs during deformation. Previous studies have primarily focused on methods to suppress TRIP by enhancing γR stability in order to reduce HE. It has been reported that the presence of precipitates can improve HE resistance in steels with a body-centered cubic (BCC) structure, such as ferritic steel and tempered martensitic steel. However, studies on HE resistance of medium-Mn steel with precipitate have not yet been performed. This dissertation systematically investigates the improvement of HE resistance in medium-Mn steel through the formation of precipitates. First, mechanical properties and HE resistance were confirmed through carbides mainly generated inside α phase. Medium-Mn steel containing carbides exhibited a high YS and high tensile strength without a reduction in elongation, with the carbides mainly observed in the α phase. The YS improvement was attributed to grain refinement and the precipitation hardening effect. The higher tensile strength without a decrease in elongation was due to the activation of TWIP effect and the enhanced of dynamic strain aging (DSA) in the later stages of deformation. These various dynamic strengthening mechanisms were activated due to the low carbon concentration in γR and the high carbon concentration in α resulting from carbide precipitation. However, it was observed that the presence of carbides had no effect on improving HE after H-charged. The elongation showed a significant decrease, similar to that of the medium-Mn steel without carbide. This is primarily because diffusible H primarily dissolves in the γR phase, which has a high solid solubility, while carbides predominantly precipitate in the α phase. As a result, the carbides do not act as trapping sites for diffusible H, leading to HE regardless of the with or without of carbides. Second, a systematically investigation was conducted to determine whether the formation of precipitates within the γR phase could enhance the HE phenomenon. Intercritical annealing and aging were performed on medium-Mn steel with the addition of Cu, leading to the formation of Cu-rich particles within the γR phase. The amount of Cu-rich particles increased after aging. It was confirmed that the HE phenomenon was improved in medium-Mn steel with Cu compared to the steel without Cu. Particularly, the HE resistance increased despite the appearance of more TRIP and similar TWIP phenomena after aging. The improved resistance confirmed that diffusible H primarily dissolved in the γR and acted as more trapping sites as the amount of Cu-rich particles increased after aging.