Hydrogen transport and fracture toughness of case hardened steel

The purpose of this study is to assess the effects of effective case depth (ECD) and tempering temperature on hydrogen transport and fracture toughness of carburized AISI 8620 alloy steel. The material was machined into thin discs for permeation and into compact-tension specimens for fracture toughness measurements. The specimens were pack carburized at 930 °C and cooled to ambient temperature. The carburized specimens were austenitized at 840 °C in a high temperature salt bath, then oil quenched and tempered at various temperatures for one hour. Assessment of hydrogen transport was conducted by the electrochemical permeation technique. Both permeability and effective diffusivity decrease as ECD increases and tempering temperature decreases. Fracture toughness of pre-charged carburized 8620 steel increased with depth of carburization. Fracture toughness of hydrogen pre-charged carburized 8620 steel was minimum at tempering temperatures between about 150–400 °C. The results indicated good empirical correlation between fracture toughness and apparent solubility at higher ECD and tempering temperature. While this study shows that correlations exist, there are compositional and microstructural factors to be sorted out before any definitive relationship can be established.