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 (sigma(cr)) is significant to clarify the influence of stress level on an environmental fracture. The sigma(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 sigma(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 sigma(cr). Prior to sigma(cr), the effective diffusion coefficient of atomic hydrogen D-eff is reversible. Once sigma(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 sigma(cr) ranges from 62% to 65% R-t0.5. (C) 2016 Elsevier Ltd. All rights reserved.