Local fracture criterion for quasi-cleavage hydrogen-assisted cracking of tempered martensitic steels

Electrochemical permeation cell built on a Instron tensile testing machine allows fracturing notched specimens under hydrogen flux while monitoring simultaneously the flow stress and the permeation anodic current. Analysis of the fracture surfaces reveals that cracking initiates at the hydrogen-entry surfaces as quasi-cleavage regions followed by ductile propagation. Quasi-cleavage zones were fully developed at the maximum engineering stress and often initiate around inclusions for unnotched specimens. The observation of multiple cracks at the hydrogen-entry surfaces and correlations between quasi-cleavage features and martensitic boundaries suggest that cracking is related to decohesion of cropping out and inner boundaries. The local mechanical states and diffusible hydrogen concentration and flux at quasi-cleavage failure were calculated by finite element method (FEM). The results predict local failure criteria in terms of decrease of local maximum principal stress and equivalent plastic strain with local diffusible hydrogen concentration, revealing that the inclusion of hydrogen-mechanical-structural interactions at the charging surfaces is necessary to complete fracture analysis.