Hydrogen embrittlement of AISI 4130 steel with an alternate ferrite/pearlite banded structure

The effects of anisotropy on the hydrogen occlusivity, diffusivity and stepwise cracking of AISI 4130 steel with a banded ferrite/pearlite structure have been studied. To determine the hydrogen occlusivity with respect to the direction of banding, hydrogen charging in a H₂S saturated saline solution was performed. After the specimens had been charged for 2 h, when they were sampled in the longitudinal direction they occluded more hydrogen than when sampled from other directions. When the specimens were sampled in the through-face direction (i.e. with the banding perpendicular to the direction of hydrogen entry), they occluded significantly less hydrogen than in the longitudinal and transverse directions. However, their hydrogen contents reached the same saturation level when the charging period was extended to 8 h. Electrochemical permeation experiments indicate that the effective hydrogen diffusivity along the through-face direction was nearly an order less than that along the other two directions. The effective hydrogen diffusivities of the specimens sectioned in the longitudinal and transverse directions were similar, but the hydrogen permeabilities were different. The results show that the permeation flux of hydrogen along the longitudinal direction was higher than that along the transverse direction. With regard to the susceptibility to hydrogen-induced stepwise cracking, the transverse-sectioned specimens suffered the largest amount of cracking, and the through-face specimens were the most resistant to hydrogen-induced cracking.