Effect of retained austenite on the hydrogen content and effective diffusivity of martensitic structure
- PDF / 1,010,852 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 96 Downloads / 217 Views
I.
INTRODUCTION
IN order
to form martensite in steel, quenching of the steel from the austenitic field is required. Since the martensitic transformation finishing temperature of highcarbon steel is normally below the room temperature, untransformed austenite is retained in the martensitic matrix. The amount of retained austenite in steel increases with carbon content. ~1,2] When the specimen is subjected to subzero treatment in liquid nitrogen after quenching, the retained austenite may be transformed to martensite. The effects of a martensitic structure on the hydrogen content and diffusivity have been studied by a number of investigators, t3-71 For example, our work t3j indicated that the high-carbon (0.93 pct C) fully martensitic steel occludes the maximum hydrogen content. It is followed by low-carbon (0.23 pct C) martensitic steel, and the medium-carbon (0.44 pct C) martensitic steel occludes the minimum hydrogen content. However, the effective hydrogen diffusivity (Dell) decreases with increasing carbon content. The variations in hydrogen content and diffusivity depend not only on the carbon contents, but also on the different martensitic morphologies. Sakamoto and Mantani t51 pointed out that minimum diffusivity and maximum solubility of hydrogen are obtained when the steel has an as-quenched martensitic structure, as compared with the tempered martensitic ones. This result can be explained in terms of the hydrogen trapping process, involving lattice imperfections, such as dislocations, lat-
tice vacancies, and microvoids produced by the martensitic transformation. Newman and Shreir t71 also indicated that the martensitic structure has the lowest hydrogen solubility, but it increased with tempering, reaching a maximum value when tempered at 300 ~ and decreased with further increase in tempering temperature. The hydrogen diffusivity acts in the converse behavior of that observed for solubility. However, the effects of retained austenite have scarcely been mentioned. It is possibly due to the limited use of high-carbon martensitic steel in service. On academic grounds, however, it is interesting to study the effect of retained austenite on the behavior of hydrogen in steels which will provide the basic knowledge for the study of tempering effects. In this work, both as-quenched and quenched + subzero treatments were applied on steels with different carbon contents. The main structural difference between the as-quenched and quenched + subzero-treated specimens is the amount of retained austenite in the structure. The electrochemical permeation technique was used to obtain the effective hydrogen diffusivity of these specimens. The hydrogen sulfide charging method, with the vacuum hot extraction method for hydrogen content determination, was performed to measure the hydrogen uptake in steel. The results clearly indicate the behavior of hydrogen in martensitic steel. In particular, the relationships between hydrogen contents and diffusivity for different carbon contents and morphologies of martensite can be es
Data Loading...
