Effect of Nb addition and Pre-strain on Hydrogen Embrittlement of Low-carbon Steels with Ferrite-pearlite Structure

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.