Diffusible hydrogen in steels can cause embrittlement. Hydrogen has much lower diffusivity in austenite than in ferrite. Therefore, austenite is relatively immune to embrittlement. However, the poor stability of austenite can lead to martensitic transformation. And martensite cannot dissolve all the hydrogen inherited from austenite due to its lower solubility. So, what will happen to this excess hydrogen? This is the topic explored in this project.

Three types of steel with different austenite contents and austenite stability were investigated: duplex steel, austenitic steel and bainitic steel. Their deformation induced phase transformation of austenite and the corresponding hydrogen desorption behaviour were studied using microscopy, diffraction, thermal desorption analysis and microprinting.

This work has shown that the change in hydrogen desorption behaviour after compression strongly depends on the dissolved hydrogen concentration and the extent of phase transformation. For duplex steel, containing a large hydrogen concentration (up to 40 ppmw) and no phase transformation after compression, there is no noticeable change in hydrogen behaviour according to room-temperature desorption and constant-heating-rate thermal desorption analysis. On the other hand, the austenitic steel, after compression, revealed a significant burst in the hydrogen desorption rate, indicating an overall increase in hydrogen diffusivity. In the case of the bainitic steel, with very low saturated hydrogen concentration (less than 0.5 ppmw) and a little phase transformation (less than 10%) after compression, a significant reduction in the hydrogen desorption rate was detected since the newly strain-induced defects were able to trap almost all the diffusive hydrogen.