WEB Effect of porosity on H.E. of an low-alloy cast steelWednesday (23.09.2020) 10:10 - 10:25 S: Structural Materials 1 Part of:
The steel foundry is a very important industry and its products are used almost everywhere: in cars, trucks, engines, heavy equipment for construction, for energy… Nevertheless, due to the increasing demand for better mechanical properties, an extra effort has to be done to treat certain issues, for example hydrogen embrittlement (H.E.).
In fact, the hydrogen solubility in steel is very high at liquid state, so that hydrogen can be trapped inside the material during solidification. Moreover, under the current manufacturing conditions, cast steels usually present cavities, which can trap hydrogen in the gaseous state. This hydrogen can lead to cavity pressurization and enhance H.E.
The purpose of this research is to study the effect of porosity on hydrogen diffusion and trapping mechanisms in order to have a better understanding of hydrogen behavior in cast steels, which can lead to improve H.E. resistance. To highlight the porosity effect in the H.E. process, a comparison between two states of the same material was performed: the first is as cast state (containing cavities) and the second is a forged state (without cavities). The absence of porosity in the forged specimens was confirmed using X-ray tomography.
First, electrochemical permeation tests at ambient temperature were carried on different samples and the delay effect of porosity on the diffusion process has been proved. Then, the thermal desorption spectroscopy was used as a complementary technique of permeation. In this case, samples were charged using two methods. The first one is a chemical charging method. It consists of immersing a sample in an ammonium thiocyanate (NH4SCN) solution. Hydrogen content was varied by adjusting the concentration of NH4SCN. The second method is electrolytic charging using a 0.1M NaOH aqueous solution. Results have shown a significant difference between the forged samples and the as-cast samples.
The experimental study was complemented by a numerical model using finite elements method in order to study hydrogen diffusion and to predict the pressure inside the cavities. This model is based on data from National Institute of Standards and Technology for hydrogen under high pressure and take into consideration the concept of hydrogen fugacity.