A comparative study of hydrogen trapping kinetics and embrittlement susceptibility of additively manufactured and wrought 316L austenitic stainless steel: influence of post-processing

Selective laser melting (SLM) is one of the common methods of additive manufacturing technology. SLM can be employed to customize mechanical components with complex geometry such as elbows and valves, which are expected to work in hydrogen atmospheres under high-pressure hydrogen gas. However, more studies are still needed to characterize the behavior of the printed mechanical components intended to work in contact with gaseous hydrogen. It is known that additively manufactured 316L boasts superior mechanical properties than those of conventional state. However, greater absorption of hydrogen can be induced in the SLMed 316L because of its higher density of dislocations, induced due to the rapid cooling rates during solidification. Accordingly, new production techniques, especially additive manufacturing, require re-evaluation of hydrogen susceptibility of austenitic stainless steels.

In this work, microstructural and mechanical behavior, in the presence of internal hydrogen, of the 316L wrought series and the as-built and post-heat-treated additively manufactured 316L samples, is studied. Experimentally, thermal desorption analysis (TDA) and electrochemical hydrogen permeation (EP) experiments are both used to analyze hydrogen trapping and diffusion kinetics, in the different series. In this regard, the hydrogen concentration, the apparent hydrogen diffusion coefficient (Dapp), the density of traps (NT) and hydrogen activation energies (Ea) attributed to the desorption of hydrogen from various microstructural singularities, are compared among the analyzed series. TDA analysis was carried out on hydrogen precharged samples at high temperature. Besides, mechanical behavior is analyzed at room temperatura by Slow Strain Rate Tests (SSRT) on smooth tensile samples, precharged in a high-pressure hydrogen reactor at 195 and 300ºC for 24h. Fracture mechanisms (HELP -Hydrogen Enhanced Localized Plasticity- and HEDE -Hydrogen Enhanced Decohesion-) and micromechanims are deeply analyzed.

Hydrogen damage was especially noted in the 316L wrought series (i.e. hot-rolled and annealed grade), where fracture micromechanism changed from ductile in the absence of hydrogen to quasi-brittle in the presence of internal hydrogen. In this case, the presence of HELP (Hydrogen Enhanced Localized Plasticity) mechanism was also confirmed. On the other hand, HE resistance enhanced in the printed samples, especially in the post-heat-treated SLM 316L series, where the fracture micromechanism was ductile. To explain the impact of the internal hydrogen on the mechanical properties of the studied series, the role of the strain-induced martensite, dislocation glide, deformation twinning, and cellular structures is discussed.