The Effect of Second Tempering on Hydrogen Embrittlement of Ultra-High-Strength Steel

Abstract

The objective of the present study is to enhance the resistance of hydrogen embrittlement (HE) via second tempering at 250 °C for 30, 60 and 120 minutes. Although second tempering results in a higher saturated hydrogen content for the second tempering specimens during a slow strain rate test (SSRT), it effectively reduces HE susceptibility. As the second tempering time increases, dislocation density decreases. In contrast, the size of the cementite and Mo_yC_x precipitates increase slightly. The density of Mo_yC_x precipitates increases, whereas the density of cementite remains approximately the same as the second tempering time increases. Regarding second tempering specimens, the volume fraction’s increase in Mo_yC_x precipitates, which acts as a hydrogen trap with high binding energy, plays an important role in reducing the HE susceptibility, and the decrease in dislocation density can also improve HE resistance. In addition, the growth of the interface of the cementite and matrix disperses more hydrogen, which could enhance HE resistance. The result also reveals that the cementite and matrix interface is a type of low-binding-energy hydrogen trap without plastic deformation, whereas the strain interface with interfacial dislocations is a type of high binding energy hydrogen trap under plastic deformation.