The effect of retained austenite upon the H content and the effective H diffusivity of a mainly martensitic structure, in steels which contained about 0.47wt%Mn and between 0.23 and 0.93wt%C, was studied by using electrochemical permeation techniques. The parameters of the first permeation transient indicated that as-quenched high-C specimens exhibited a lower effective diffusivity than did those which had been given an additional sub-zero treatment. This was attributed to the presence of retained austenite in the former specimens. The austenite provided additional sites for H trapping throughout the specimen membrane, and thereby decreased the H diffusivity during the first transient. As the H traps filled up, a second permeation transient indicated the existence of a similar effective diffusivity for as-quenched specimens and for specimens which had been quenched and heat-treated at sub-zero temperatures. After H-charging in H2S solution, the H content was determined by using the vacuum hot-extraction method. It was found that the H contents of as-quenched specimens were higher than those of specimens which had been subjected to quenching and sub-zero heat treatments. This behavior was also attributed to the presence of retained austenite in the as-quenched matrix. That is, the interfaces between retained austenite and martensitic plates were expected to provide extra sites for H trapping. The H content of the retained austenite in the martensite was independent of the C content of the specimen, and depended only upon the H-charging period.

S.L.I.Chan, H.L.Lee, J.R.Yang: Metallurgical Transactions A, 1991, 22[11], 2579-86