Modeling hydrogen embrittlement and fracture in advanced high strength steels

Abstract

Advanced high strength steels (AHSS) have seen rapid development in recent decades. This thesis investigated some of the most pressing issues in two different novel AHSS -- hydrogen embrittlement (HE) in press-hardened steels (PHS), and delamination toughening in the recently discovered deformed and partitioned (D\&P) and intercritically-annealed (IR) steel.

For HE in PHS, we approach the problem with a coupled model between the phase-field model for martensitic transformation and a non-equilibrium hydrogen diffusion model. We revealed the microscopic phase, stress, and hydrogen concentration distribution within a polycrystal representative volume element (RVE) of PHS1500. The model is able to predict the experimentally observed HE susceptibility of prior austenite grain boundaries (PAGB) under high hydrostatic stress. Furthermore, we proposed a microscopic fracture criterion based on the local hydrogen concentration to predict the possible locations for HE crack initiation.

Delamination toughening is the fundamental toughening mechanism in the D\&P and IR steels, yet the underlying physics is not well-understood. Here, we present both simulation and theoretical investigation of delamination toughening in a "crack divider" configuration. Both simulation and theoretical results revealed two possible mechanisms for which the toughness can be increased by means of delamination: the decrease in stress triaxiality and the increase in the size of the plastic zone within the delamination zone in front of the crack tip. The interplay between plasticity and delamination suggests that the beneficial effects of delamination toughening can be optimized by increasing the amount of plastic work before fracture.