Hydrogen embrittlement of ultra-high-strength press-hardened steels

Abstract

Advanced high-strength steels (AHSS) are greatly in the pursuit of building lightweight and fuel-efficient automobiles without compromising crashworthiness. Among various AHSS, the ultra-high-strength press-hardened steel (PHS) is an excellent candidate for intrusion-resistant body-in-white components thanks to its good strength-toughness-formability combination. Nevertheless, its martensitic microstructure indicates severe hydrogen embrittlement (HE) susceptibility, which causes unpredicted fracture and damages the overall structural integrity. To mitigate or eliminate HE in PHS, one must first understand the underlying mechanisms that determine the hydrogen diffusion and hydrogen-related fracture process, which constructs the aim of this thesis. The topic is comprehended with a systematic review and a few experimental and numerical studies in this thesis. To initiate, an overview of the current research progress and state-of-the-art equipment to study HE in PHS is presented. Three major challenges in PHS’s HE evaluation were addressed: (i) diffusible hydrogen characterization; (ii) correlation between accelerated experiments and service conditions; and (iii) HE risks prediction models. The overview lays the foundation of this research and provides useful evaluation techniques and theoretical models for study. Fundamental questions regarding the hydrogen diffusion and trapping behavior in PHS are addressed in the subsequent study. Microstructures of PHS are tuned through the stamping process to obtain different hydrogen diffusion coefficients. A multi-trap diffusion model is proposed to quantitatively unify the diffusivity and microstructural features, including different defect species, defect densities, and their activation energies for hydrogen desorption. The work signifies the predominant role of grain boundaries in hydrogen trapping and further governing diffusion. We then focus on the hydrogen influence on PHS’s fracture process, namely the initiation and propagation of hydrogen-induced cracks. It is found that inclusions are the primary crack initiation sites in PHS. In particular, hard oxides and irregular-shaped inclusions with sharp edges are most likely to suffer from quasi-cleavage cracking upon hydrogen pre-charged tensile test. A mild control on the inclusion’s size (~10 to 5 μm) and density illustrates little improvement in HE, suggesting a higher level and more targeted cleanliness control is required. Next, the HE susceptibility of 1.5 and 2GPa-grade PHS are compared to study the strength vs. hydrogen resistance trade-off. The 1.5GPa-grade PHS presents a consistently higher hydrogen resistance than 2GPa-PHS evaluated by slow strain rate tensile and constant load tests. The better HE performance of the lower strength grade is attributed to its superior fracture toughness, which is assumed to be less affected even under high hydrogen concentration. Hence, the matrix martensite design is crucial to develop high-strength and high HE-resistant PHS. Finally, the HE performances of 2GPa-grade PHS under simulated crash conditions are investigated through hydrogen pre-charged high strain rate tests. A hydrogen diffusion coupling rate-dependent deformation model with detailed microstructure features is developed. It is concluded that a sufficient diffusion time and stress-assisted hydrogen redistribution to grain boundaries are the main factors that contribute to the final boundary decohesion in PHS. The results also propose applying high strain rate tests for PHS’s HE assessment.