Determination of the Critical Plastic Strain-Induced Stress of X80 Steel through an Electrochemical Hydrogen Permeation Method

The determination of critical plastic strain-induced stress (σcr) is significant to clarify the influence of stress level on an environmental fracture. The σcr of high strength pipeline steel is difficult to define precisely based on conventional stress-strain curve. Hydrogen permeation behavior is sensitive to change in dislocations induced by plastic strain. Hence, a method of determining the σcr of X80 steel is introduced by conducting electrochemical permeation experiments under slow-strain-rate tension. Hydrogen permeation current density (iH) increases and then decreases with increased stress level within Rt0.5 (0.5% total extension). The stress at iH,max corresponds to σcr. Prior to σcr, the effective diffusion coefficient of atomic hydrogen Deff is reversible. Once σcr is exceeded, Deff becomes irreversible. Binding energy EB of new hydrogen traps emerging during tension from 60% Rt0.5 to 90% Rt0.5 is close to that of dislocations in steel, which indicates that plastic deformation occurs in this tension stage. For X80 steel, the 95% confidence interval of σcr ranges from 62% to 65% Rt0.5.