HYDROGEN TRANSPORT AND FRACTURE-TOUGHNESS OF CASE-HARDENED STEEL

The purpose of this study is to assess the effects of effective case d epth (ECD) and tempering temperature on hydrogen transport and fractur e toughness of carburized AISI 8620 alloy steel. The material was mach ined into thin discs for permeation and into compact-tension specimens for fracture toughness measurements. The specimens were pack carburiz ed at 930-degrees-C and cooled to ambient temperature. The carburized specimens were austenitized at 840-degrees-C in a high temperature sal t bath, then oil quenched and tempered at various temperatures for one hour. Assessment of hydrogen transport was conducted by the electroch emical permeation technique. Both permeability and effective diffusivi ty decrease as ECD increases and tempering temperature decreases. Frac ture toughness of pre-charged carburized 8620 steel increased with dep th of carburization. Fracture toughness of hydrogen pre-charged carbur ized 8620 steel was minimum at tempering temperatures between about 15 0-400-degrees-C. The results indicated good empirical correlation betw een fracture toughness and apparent solubility at higher ECD and tempe ring temperature. While this study shows that correlations exist, ther e are compositional and microstructural factors to be sorted out befor e any definitive relationship can be established.