Effects of Microstructural Characteristics on Hydrogen Embrittlement Properties of High-strength Martensitic Steels

Abstract

In the backgrounds of increasing demands for advanced mechanical properties in structural materials, researches on the advanced high-strength steels (AHSS) have been actively conducted by modifying and controlling various alloying elements, microstructures, and deformation behaviors. The hot-stamping process is introduced to guarantee the ultra-high-strength over 1.5 GPa by inducing full martensitic transformation during stamping and subsequent quenching processes and overcome the limits of high-strength steels with sufficient formability. However, their full martensitic microstructure and high-strength characteristic degrade the mechanical properties required for actual using environment, especially hydrogen embrittlement (HE) susceptibility. HE is a cracking phenomenon initiated by the penetration and accumulation of diffusible H atoms inside the metals. As a result, the mechanical properties such as strength and elongation are deteriorated. The microstructural characteristic including crystal structure, defects, inclusion, precipitate, etc., primarily affects the HE properties of the steels. This is because these various microstructure factors can locally trap H and affect H-diffusion mechanism. The purpose of this research is to understand and clarify the microstructural factors affecting HE in high-strength martensitic steels. The understanding on HE ultimately leads to the development of ultra-high strength steels with excellent resistance to HE. Two main concepts are designed to investigate the effect of each microstructural feature: (1) effects of grain size and complex carbide precipitates, (2) effects of solid solution and grain boundary segregation of solute atom. First, an alloying element of Ti is utilized in the (Nb+Mo) multi-alloyed system of 1.8-2.0 GPa-grade martensitic steels. The alloying effects of Ti on resistance to HE were investigated via controlling Ti content and conducting slow-strain-rate tensile (SSRT) tests and thermal desorption analyses (TDA) after the H charging. The complex addition of Nb, Mo, and Ti promotes the formation of nano-sized (Nb,Ti)C and (Nb,Mo,Ti)C complex precipitates. The increased Ti content to 0.03 wt.% increases the volume fraction of the precipitates, which effectively refines the prior austenite grain size and increases interfacial incoherency of precipitates. The grain refinement increases the density of potential trapping site, thereby dispersing diffusible H atoms and suppressing their accumulation. In addition, the increased volume density and interfacial incoherency of precipitates raises their activation energy for H desorption (Ea) from 83 to 106 kJ/mol, which provides more stable H trapping sites. Therefore, the higher Ti content (up to 0.03 wt.%) significantly improves the resistance to HE of the (Nb+Mo)-alloyed system. This is because the maximized effects of grain refinement and nano-sized complex precipitates as H trapping sites. Second, the alloying element of Mo is added in 2.0 GPa-grade martensitic steels to investigate effect of solid solution and grain boundary segregation of solute atom. The effects of 0.15 wt.% Mo on microstructural evolution and HE susceptibility are investigated through atom probe tomography (APT) and SSRT tests after the H charging and H-permeation test, respectively. Most of Mo are dissolved in the matrix as a solute during the austenitization, due to the low precipitate temperature of Mo enriched carbides. Some of the solute Mo tends to segregate at prior austenite grain boundaries (PAGBs) with C and B. The alloying of Mo significantly reduces the elongation loss caused by H-induced degradation with sufficient post-elongation. This is because solute Mo effectively decreases H-diffusivity, resulted from the high H affinity and the repulsive strain field by a large atomic size of Mo. Furthermore, the segregation of Mo at PAGBs enhances the grain-boundary cohesion, thereby leading to the sufficient post elongation by preventing a transition of ductile to brittle intergranular fracture mode even in the H-charged condition. This tendency of fracture mode coincides well with the microcrack propagation path. The strengthened PAGBs alters the H-induced crack path from PAGBs to grain interiors of H-enhanced slip planes, which improves the resistance to HE in the Mo steel.