Determination of the Critical Plastic Strain Induced Stress of X80 Steel through an Electrochemical Hydrogen Permeation Method
Abstract The determination of critical plastic strain induced stress s cr is significant to clarify the influence of stress level on an environmental fracture. The s cr 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 s cr of X80 steel is introduced by conducting electrochemical permeation experiments under slow strain rate tension. Hydrogen permeation current density i H increases and then decreases with increased stress level within R t0.5 (0.5% total extension) The stress at i H,max corresponds to s cr Prior to s cr the effective diffusion coefficient of atomic hydrogen D eff is reversible. Once s cr is exceeded, D eff becomes irreversible. Binding energy E B of new hydrogen traps emerging during tension from 60% R t0.5 to 90% R t0.5 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 s cr ranges from 62% to 65% R t0.5