WEB Hydrogen diffusion and trapping in maraging stainless steels: the role of specific microstructural featuresWednesday (23.09.2020) 09:45 - 10:00 S: Structural Materials 1 Part of:
The strength of maraging stainless steels (precipitation-hardened martensitic stainless steels) has been extensively increased over the past decades. However, this improvement has resulted in a possible increase of sensitivity to hydrogen embrittlement.
To understand the mechanisms for hydrogen embrittlement of maraging steels it is important to question the mechanisms of hydrogen diffusion and trapping in their microstructure. Hydrogen concentration in the material is known to be an embrittlement cause, but there is evidences that hydrogen mobility is responsible for ductility loss as well . The effect of hydrogen trapping is also essential, as traps can act as hydrogen sources during material straining  or can delay the time required to obtain a critical hydrogen concentration for interface decohesion .
One of the main issues to understand hydrogen diffusion and trapping in maraging stainless steels is the complexity of their microstructure mainly composed to a fine lath structure with high dislocation density, small precipitates, and a secondary austenitic phase.
PH13-8Mo is the reference material for this study. After annealing and quenching to form the martensitic matrix, an ageing treatment is used to develop NiAl hardening precipitates and reversed austenite. In this study we used several ageing treatments to create different precipitation states and austenite contents.
Microstructural analyses using EBSD, TEM and XRD was used to track some information about lath size, dislocation density, precipitation state and austenite content for each ageing treatment.
Electrochemical Permeation tests (EP) was used to quantify diffusible and trapped hydrogen at room temperature using the analysing method initially developed by Zackroczymski et all  and extended by Frappart et al . Thermal Desorption Spectroscopy (TDS) was then used to identify the hydrogen trapping energies.
Results highlighted that austenite content has a low impact on reversibly trapped hydrogen, but increases the concentration of hydrogen in “lattice”. Increase in trapped hydrogen concentration is associated with precipitates growth, resulting in a decreased
coherency of the precipitate/matrix interface as already shown by Wei et all . Finally, dislocation density has been measured, and a model for hydrogen trapping by dislocations have been used to take their deep trapping effect into account.
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