PSI - Issue 43

Jiří Man et al. / Procedia Structural Integrity 43 (2023) 203 – 208

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Author name / Structural Integrity Procedia 00 (2022) 000 – 000

complex shape parts with an internal architecture from numerous metallic materials (Yadroitsev et al. (2021)). At present, austenitic stainless 316L steel represents one of the most intensively studied SLM materials due to its perspective utilization in demanding sectors as nuclear, hydrogen and biomedical industry (Kong et al. (2021)). The SLMed 316L steel possesses the unique complex but out-of-equilibrium 1 hierarchical structure which results in an outstanding combination of considerably higher strength than their conventionally produced counterparts (doubled yield stress is typical and especially noticeable) without reduced ductility. Destabilization of austenitic structure of SLMed 316L steel and the formation of DIM (deformation induced martensite) at cryogenic temperatures and its effect on the tensile behavior and impact toughness has been demonstrated only recently (Hong et al. (2019), Wang et al. (2021), Tang et al. (2022)). Contrary to these low temperature studies the results of Kaiserslautern’ fatigue group (see Blinn et al. (2018, 2019a, 2019b, 2020)) repeatedly indicated the possible destabilization of austenitic structure of SLMed 316L steel during fatigue loading even at room temperature. The ferritescope measurements performed by Blinn et al. on the surfaces of testing specimens fatigued under stress control to the end of fatigue life showed a limited formation of DIM the amount of which increased with decreasing applied stress amplitude. Any microstructural investigations supporting the results of these magnetic measurements have not been, however, performed so far. The present work thus represents the first attempt in this field. The paper reports the results on microstructural changes in two SLMed 316L steels manufactured using two different SLM manufacturing systems and powders which were in as-built state cyclically loaded at room temperature in two fatigue laboratories (IPM Brno and TU Kaiserslautern). Several microscopic techniques were adopted to reveal the distribution and morphology of DIM in the volume of fatigued steels. The results of microstructural changes due to fatigue are confronted with ferritescope measurements performed both on the surface and in the bulk of fatigued specimens. 2. Experimental Two batches of AISI 316L steel of similar chemical composition (denoted as 316L-IPM and 316L-TUK – see Table 1) were fabricated from different recycled gas-atomized powders using two different manufacturing SLM systems. Cylindrical net-shape fatigue specimens were built vertically using a SLM 280 HL machine (SLM Solutions, Lübeck, Germany) under argon protective atmosphere with building platform preheated to 100°C. The hatching strategy was stripe/meander with contouring and rotation of 67° after each layer. Another methodology was adopted at TU Kaiserslautern. In this case cylindrical bars were built using a ProX DMP 320 device (3D Systems, Leuven, Belgium) horizontally under nitrogen protective atmosphere without any building platform preheating. The scanning strategy was chessboard pattern with unidirectional contouring and rotation of 245° after each layer. Then the fatigue specimens were manufactured by turning the SLMed bars and polishing the gauge length. Relative density of both SLMed steels evaluated using Archimedes’ method (Sartorius precision balance ) was found to be higher than 99.65%. Other important details including SLM processing parameters and fatigue specimen geometries can be found for both 316L steels elsewhere (Blinn et al. (2019b) and Man et al. (2022)). Cylindrical specimens without any post-heat treatment were fatigued at room temperature until failure under two different loading modes: total strain-control mode was adopted to vertical net-shape 316L-IPM specimens at IPM Table 1. Chemical composition (wt.%), characteristic threshold temperatures M S and M d30 (Pickering (1978)) and stacking fault energy (SFE, de Bellefon et al. (2017)) of SLMed 316L steels manufactured using two different AM systems. Steel sign. AM system C Si Mn Cr Ni Mo N M S (°C) M d30 (°C) SFE (mJ/m 2 ) 316L-IPM SLM Solutions 0.02 0.67 0.99 16.7 12.6 2.41 0.08 – 321 – 90 27 316L-TUK 3D Systems 0.02 0.40 0.36 17.7 12.9 2.47 0.06 – 307 – 94 28

1 Some authors instead of ‘out -of- equilibrium’ structure use the term ‘metastable’ structure (see e.g. Kong et al. (2021)). However, this should not be confused with the stability/metastability of austenitic structure against martensite formation in the case of austenitic stainless steels.