Abstract
300M grade steel is widely used in aerospace industries for variety of applications typically for landing gears. These steels are quenched and tempered to a high hardness of 50HRC (1675 MPa). The tempered martensite microstructure obtained by this process has very high strength and hence the susceptibility to hydrogen embrittlement is also high. Fundamentally, these bulk mechanical properties depend on the material microstructure itself. Therefore, it is may be possible to improve the hydrogen embrittlement resistance of the material by modifying the microstructuctural features without compromising the hardness. Bainite is considered to be a different microstructure compared to quenched and tempered martensite, although the constituents are essentially the same, i.e. non-equiaxed ferrite and carbides. However, features such as dislocation density, lath dimensions, interphase area, volume fraction of the retained austenite, precipitate thickness and its orientation are quantitatively different. Moreover, unlike tempered martensite, the bainite forms at a higher temperature by isothermally transforming the austenite. Generally, bainite has better toughness and ductility compared to tempered martensite at the same hardness. In this study, in 300M steel, the HE characteristics of bainite are studied by determining the hydrogen diffusion characteristics by electrochemical permeation, and its hydrogen embrittlement susceptibility using Incremental Stepped Loading (ISL) technique. Thermal Desorption Spectrometer was used to quantify the hydrogen uptake in the steel.