Hydrogen Isotope Permeation and Trapping in Additively Manufactured Steels

Additively manufactured (AM) austenitic stainless steels are intriguing candidates for the storage of gaseous hydrogen isotopes. Complex vessel geometries can be built more easily than by using conventional machining options. Parts built with AM steel tend to have excellent mechanical properties (with tensile strength, ductility, fatigue crack growth, and fracture toughness comparable to or exceeding that of wrought austenitic stainless steel). However, the solidification microstructures produced by AM processing differ substantially from the microstructures of wrought material. Some features may increase permeability, including both some amount of porosity and a greater amount of ferrite. Because the diffusivity of hydrogen in ferrite is greater than in austenite (six orders of magnitude at ambient temperature), care must be taken to retain the performance that is taken for granted due to the base alloy chemistry. Furthermore, AM parts tend to have greater dislocation densities and greater amounts of carbon, nitrogen, and oxygen. These features, along with the austenite/ferrite interfaces, may contribute to greater hydrogen trapping. We report the results of our studies of deuterium transport in various austenitic (304L, 316, and 316L) steels produced by AM (via either powder bed fusion or blown powder methods). The hydrogen permeability (an equilibrium property) changes negligibly (less than a factor of 2), regardless of chemistry and processing method, when tested between 150 and 500 °C. This is despite increases in ferrite content up to FN=2.7. However, AM materials exhibit greater hydrogen istotope trapping, as measured by permeation transients, thermal desorption spectra, and inert gas fusion measurement. The trapping energies are likely modest (<10 kJ/mol), but may indicate a larger population of trap sites than in conventional 300-series steels. INTRODUCTION Additively-manufactured (AM) metals, particularly those made from austenitic stainless steel powder that is melted with a laser, have been proposed for both building new parts [1] and for repairing damaged parts [2–4] for hydrogen service. While both of these use-cases are proposed to take advantage of AM’s unique abilities to build to complex (and even conformal) geometries, the material selected for the powder is not novel to the process. Conventionally-manufactured austenitic stainless steel is resistant to hydrogen-assisted fracture [5–7] and has a reasonably modest permeability for hydrogen isotopes at typical operating temperatures and pressures [8]. However, the solidification microstructures of AM austenitic steels differ from conventionally-proceesed microstructures. The grain size is fine (d~10-100 μm [9–11]). Although wrought or worked (e.g. rolled) material may also have d<100 μm [12,13], the grain structure of AM materials tends to be different. AM materials tend to have elongated grains (aspect ratios of 2-3:1) and are more highly textured. Due to their rapid solidification, AM austenitic stainless steels tend to retain more δ-ferrite than conventional materials (FN<6, though some still routes lead to levels below detection limits [14]). The initial dislocation density tends to be very high (ρ~2.4x10 14 m -2 [15] vs. ρ~10 12 m -2 for wrought [12]). The chemistry of the AM parts depends not only on the composition of the starting powder (which tends to have greater amounts of N and O) and the build environment (inert Ar, typically), but also on interactions with the laser, which may change the chemistry of the bulk part from the powder due to preferential vaporizing. For example, the level of N in parts built from virgin powder is typically about 0.04 wt. % [9], though building in inert N drives this to about 0.5 wt. % [16]). The distribution of elements within a built part is expected to be different because of the high solidification rate, though limited high resolution studies have been done to date. Finally, the porosity of AM built parts tends to be slightly higher due to closed pores in the starting powder and lack-of-fusion defects. These microstructural differences between AM and conventional materials lead to differences in properties. The literature typically reports greater yield strengths in AM steels, for example [1,9,10,14,15]. The different microstructure may also lead to a difference in how hydrogen isotope transport through AM materials. Porosity and ferrite, for example, may