Stress Corrosion Cracking Probability of Selective Laser Melted 316L Austenitic Stainless Steel under the Effect of Grinding Induced Residual Stresses

Abstract

Surface quality and dimensional tolerances of the selective laser melting (SLM) process are not good enough for many industrial applications and grinding as a common finishing process introduces many surface modifications. Investigation on the effect of grinding induced surface residual stress (RS) on early stages of stress corrosion cracking (SCC) of SLM manufactured 316L austenitic stainless steel was conducted. Potentiodynamic and galvanostatic tests in a 3.5% NaCl aqueous solution, XRD, SEM and energy-dispersive X-ray spectroscopy (EDX) analysis were performed. For annealed and specimens with a low RS magnitude, the dominant observation was pit initiation from existing pores and growth in the build direction. For specimens with medium RS level, SCC initiation from pore sites and propagation along melt pool boundaries and for specimens with the highest detected RS, crack initiation from melt pool boundaries, grains, machining marks, and pore sites were observed. Cracks propagated in different directions, i.e., along melt pool boundaries, near-surface transgranular, and transgranular through columnar microstructure. Galvanostatic tests showed three distinctive regions that corresponded to crack and pit initiation and growth. The synergistic effect of high dislocation density along melt pool boundaries, stress concentration in pore sites, molybdenum segregation, and surface RS was the cause of SCC susceptibility of specimens with high RS magnitude.

Revealing the stress corrosion cracking initiation mechanism of alloy 718 prepared by laser powder bed fusion assessed by microcapillary method

“…Therefore, it can be stated that the enhanced localized corrosion resistance of L-PBF 718 specimens under tensile loading, which is the case in almost all industrial applications, could be attributed to submicron structure and higher integrity of passive layer compared to the conventional material. These findings are also in complete agreement with the authors' previous investigations on L-PBF 316 L austenitic stainless steels, identifying the impact of columnar subgrain width on localized corrosion susceptibility [51,52]. Moreover, in a recent study on SCC behaviour of L-PBF 316 L by Cruz et al [74], they indicated higher SCC resistance of L-PBF specimens compared to wrought counterparts as a result of significantly higher resistance of L-PBF specimens to pitting corrosion, leading to less possible active sites for crack initiation.…”

“…6) for both L-PBF and conventional samples, while after tensile loading, conventional specimens exhibit lower E corr compared to L-PBF. Potentiodynamic results suggest a higher sensitivity of E break and breakdown current density (i break ) to tensile stress state than E corr and i corr highlighting a significant correlation between breakdown electrochemical parameters with the stress state of both groups, which is consistent with authors' previous investigations on 316 L austenitic stainless steel [50][51][52] using standard test methods, highlighting the similarity of potentiodynamic polarization behaviour between standard and microcapillary method, confirming the stability and high precision of the measurements in the current study. The potentiodynamic polarization results are summarized in Table 3.…”

“…12) confirms the selective dissolution of the subgrains matrix with unaffected boundaries. The same behaviour was observed in the authors' previous investigations on L-PBF processed 316 L austenitic stainless steel [51,52], revealing the selective dissolution of subgrains and subsequent mechanical rupture of the intact subgrain boundaries as the mechanism responsible for SCC propagation that was different from wrought counterparts [50]. Moreover, a recent investigation on SCC characteristics of L-PBF 316 L by Cruz et al [74] confirms the selective dissolution of subgrain observed in the current investigation, which is confirming the extent of such a phenomenon on austenitic microstructure of stainless steel alloys.…”

“…Thus, the presence of compressive RS could potentially improve the corrosion behaviour. For the case of L-PBF 718, RS magnitude, contrary to austenitic stainless steels [51,52,55], was tensile at the centre of the specimens, which could well be responsible for the lower corrosion resistance compared to conventional specimens with a compressive stress state. The difference in RS state between L-PBF stainless steels and alloy 718, could be attributed to the difference in the thermodynamical characteristics of Ni-Fe superalloys [41,53,54].…”

Stress corrosion cracking of AISI 304 under chromium variation within the standard limits: Failure analysis implementing microcapillary method

“…SCC is believed to be the most critical type of localized environmentally assisted damage, taking place owing to the synergistic effect of susceptible material, aggressive environment (unique to the material), and tensile stresses [9,10]. Stainless steels are widely considered susceptible to SCC in chloride-containing environments under applied or residual tensile stresses [11][12][13][14][15][16]. While international standards (AISI (UNS), ASTM) specify an acceptable range for the Cr content for each alloy designation (For instance, ASTM A276/A276M-17 allows a range between 18 and 20 wt% of Cr for AISI 304), the question that emerges is whether the permitted range could affect the corrosion characteristics and eventually higher SCC susceptibility for the lower limit.…”