A multi-method approach to the study of hydrogen trapping in a maraging stainless steel: the impact of B2-NiAl precipitates and austenite

Abstract

The trapping behavior of hydrogen in a Fe-Cr-Ni-Al-Mo alloy (PH13–8Mo martensitic stainless steel) was investigated. Three different metallurgical states of the same alloy were obtained by heat treatment in an attempt to separate the impact of different microstructural features on the hydrogen trapping behavior. Microstructure analysis including dislocation density measurements and characterization of B2-NiAl precipitates and reverted austenite was conducted, using mainly X-ray Diffraction and Transmission Electron Microscopy. From electrochemical permeation conducted under varying hydrogen fugacity, the trapping characteristics (binding energy, trap density) of the three metallurgical states studied were first determined using the analytical one trap model of Kumnick and Johnson. Another more sophisticated numerical model was then developed in order to introduce two different types of traps. This model was used to simulate the permeation curves and the trapping characteristics were identified using an inverse approach. In a separate approach, Thermal Desorption Spectroscopy was also used to determine the trapping energies and the amount of hydrogen stored in different traps. Combination of these different experimental and modelling approaches have produced consistent results. It is shown that low-misfit coherent B2-NiAl precipitates have a very limited trapping capability. The high dislocation density, including dislocation walls (martensite lath boundaries), significantly traps hydrogen, at an intermediate energy of about 35 kJ/mol. Filmy reverted austenite has a double impact: hydrogen is trapped both at the martensite-austenite interfaces (∼35 kJ/mol) and in the austenite bulk (∼20 kJ/mol).