Effect of retained austenite on the hydrogen content and effective diffusivity of martensitic structure

In this work, the effects of retained austenite on the hydrogen content and the effective hydrogen diffusivity of an otherwise fully martensitic structure have been studied. In the electrochemical permeation experiment, the results on the first permeation transient indicate that high-carbon as-quenched specimens have a lower effective diffusivity than those with an additional subzero treatment. This was due to the presence of retained austenite in the former specimens, which afforded more sites for hydrogen trapping throughout the specimen membrane, hence lowering the hydrogen diffusion during the first transient. As the hydrogen traps were filled up, however, the second permeation transients gave similar effective diffusivity for both as-quenched and quenched + subzero-treated specimens. After hydrogen charging in hydrogen sulfide solution, the hydrogen contents of the specimens were determined using the vacuum hot extraction method. The results show that the hydrogen contents of as-quenched specimens were higher than those of the specimens subjected to quenched + subzero treatment. This again was due to the existence of retained austenite in as-quenched martensitic matrix, where the interfaces between the retained austenite and martensitic plates provided extra sites for hydrogen trapping. The hydrogen content of the presenting retained austenite in the martensite was independent of the carbon content of the specimen, but only depended on the hydrogen-charging period.