Studies on the hydrogen-assisted cracking processes in high-strength steels

Abstract

The equilibrium and propagation kinetics, which are characterized in terms of the threshold stress intensity and crack propagation rate, respectively, of hydrogen-assisted cracking in high-strength steels have been investigated in gaseous and aqueous environments. The threshold stress intensity and the crack propagation rate depended strongly on the hydrogen pressure, temperature, yield strength and impurity level, From both the fractographic analysis and micromechanistic consideration for the respective fracture modes, in particular, the concepts underlying fracture mode transition for hydrogen-assisted cracking in high-strength steel have been proposed. In chapter Ⅲ, fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from equilibrium aspects at room temperature with 8.6 mm thick double cantilever beam(DCB) specimens. The threshold stress intensity $K_{th}$ necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The $K_{th}$ necessary for the occurrence of HAC increased with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. As $K_{th}$ increased, the corresponding HAC fracture mode changed from intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension accompanied by the IG mode is higher than that for crack extension accompanied by the MVC mode. the fracture mode transition with varying hydrogen pressure and yield strength is discussed simultaneously in terms of the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies of the $k_{th}$ From the present experimental results, it seems that the HAC fracture mode of high-strength steels is primarily determined by the applied stress inten...