Abstract
Additive manufacturing processes, e.g. laser powder bed fusion, offer promising opportunities to build custom metallic parts, but one major issue remains disregarded, i.e. the durability of additively manufactured (AM) parts. Indeed, extensive literature has shown that additive manufacturing processes lead to specific microstructures, which suggest potential differences regarding the durability of AM parts. In the current study, we are interested in the behaviour in the presence of hydrogen of 17-4PH martensitic stainless steel (MSS) manufactured by laser powder bed fusion, focusing on the effect of hydrogen on the stress-relaxation properties of the MSS.
The study was performed for both the AM MSS and its conventional counterpart (CM). They were both studied in the H900 metallurgical state, corresponding to a solution annealing at 1040°C followed by a 1h-aging at 480°C. First, their microstructures were analysed in such a way as to identify the major differences at the relevant scales. For both the AM and CM MSSs, a martensitic microstructure was evidenced, but results showed significant differences in the reversed austenite amount, equal to 0.8 and 12.6 % for the CM and AM MSSs, respectively.
Then, cylindrical specimens of AM and CM MSSs were H-precharged in a 10-3 M H2SO4 solution for 24h, 72h, 144h and 288h. Tensile tests were performed in air, at room temperature, and 10-3 s-1, on non-charged AM and CM samples to determine the offset yield strength (0.2%). The values were used as input parameters in the incremental stress-relaxation tests; these tests were performed with the same loading conditions than tensile tests on both non-charged and H-precharged samples, with initial incremental stress levels ranging from 80 to 110 % of YS0.2. Results showed that the stress relaxation amplitude significantly increased with H-charging duration for AM samples, whereas no change was observed for the CM samples, which was analyzed by referring to an effect of hydrogen on the mobile dislocations in austenite. Additional interrupted stress-relaxation tests were also conducted and allowed to evidence the influence of hydrogen on the static ageing of martensite for the CM sample insofar as this phenomenon can occur.