Austenitic stainless steel UNS S31603 (SS316L) is widely used in the resources industry due to its excellent corrosion resistance, ductility, and weldability. Recently, laser-based powder bed fusion (LPBF) manufacturing has gained popularity for creating SS316L components with complex geometries and superior mechanical properties. However, the rapid melting and solidification of the deposited layers during the thermal cycle of LPBF produce residual stresses. Components manufactured through LPBF are frequently used under applied stress in corrosive environments. Thus, it is crucial to understand their susceptibility to stress corrosion cracking (SCC) and the impact of residual stresses. This study investigated the combined effects of applied stress and temperature on the SCC behavior of LPBF SS316L using custom-made C-ring test specimens. Cold-drawn wrought SS316L was included for comparison. Stress relief heat treatment, microhardness testing, partial immersion testing, and microanalysis techniques, such as light optical microscopy, scanning electron microscopy, and electron backsacttered diffraction were used to quantify the SCC behavior. The outcomes of this study showed that stressed and unstressed LPBF SS316L specimens were highly susceptible to cracking around their printed holes. The SCC susceptibility was attributed to the residual stresses introduced by the printed supports, as both polished and as-printed holes showed similar cracking behavior. This work provides valuable insights and lays a foundation for further research into the impact of using C ring samples to investigate SCC susceptibility and sheds light on the SCC susceptibility of as-printed components of complex geometry printed with supports due to the influence of residual stresses.

Additive manufacturing (AM) encompasses the technologies used to produce physical objects from digital data by adding layers upon layers of material.1-3  Compared to traditional manufacturing, AM has the potential to reduce complexity in the supply chain in terms of quality, impact, cost, speed, and innovation.4  AM technologies include binder jetting, direct energy deposition, material extrusion, material jetting, sheet lamination, vat photopolymerization, and powder bed fusion (PBF).2-3,5  Nowadays, it is common to use AM to produce engineering-grade metals such as copper,6-8  stainless steels,9-12  titanium-based,13-16  aluminum-based,17-19  and nickel-based alloys.20-22  Thus, AM technologies have gathered growing interest from different industries such as biomedical, transport, aerospace, and energy among others.4,23-27 

Stainless steel SS316L (UNS S31603(1)) is widely known for its ductility, weldability, and good corrosion resistance in certain oxidizing conditions.28-31  However, when exposed to halides, such as chloride ions, while being stressed under tension, SS316L can be susceptible to stress corrosion cracking (SCC).32-38  SCC is a form of environmentally assisted cracking that occurs when a susceptible material is exposed to a specific environment above a certain tensile stress level. Cracks often nucleate from localized corrosion; thus, SCC occurs when the service temperature exceeds a critical localized corrosion temperature.35,39-42  In SCC, cracks can initiate and propagate at much lower stress levels than those required to fracture the material in the absence of a corrodent, i.e., once the cracking initiates, it propagates until the applied stress exceeds the fracture strength of the remaining ligament.32-33  Therefore, using AM technologies, such as PBF, to produce SS316L components with improved properties is of great interest.43-46 

In PBF, a high-intensity energy source, such as a laser (LPBF) or an electron beam (EB) PBF, is used to melt layers of powdered feedstock that solidify at a calculated rate of approximately 103 K/s to 107 K/s47-50  into near net shape parts.3,5,51  LPBF-manufactured SS316L is known to be produced with fully austenitic microstructures,12,52-53  extremely low porosity content,43,53-54  and nanosize nonmetallic inclusions.53,55-56  Moreover, LPBF SS316L has shown excellent localized corrosion resistance56-58  and outstanding tensile properties.43,53-54  However, the cyclic process of melting and rapid cooling through the deposited layers leaves LPBF SS316L parts with a metastable microstructure containing a high degree of residual stresses.53,59-60  These stresses, which can be between 250 MPa (36 ksi) and 500 MPa (72 ksi),53,59,61-63  usually start as tension loads at the external surfaces of the component, and gradually turn into compressive loads at its core.53,60  The magnitude of these process-induced residual stresses in LPBF-manufactured SS316L can be high enough for SCC to start at an applied stress lower than the material’s yield strength or even in the absence of an externally applied load.42,59,64  In addition, residual porosity and a rough surface finish, which are inherent in the LPBF manufacturing process, are factors known to increase the SCC susceptibility,65-69  as they facilitate localized corrosion initiation.70-75  Yazdanpanah, et al.,76  observed that SCC in LPBF-manufactured SS316L with high levels of residual stress started from microstructural heterogeneities, such as melt pool boundaries and grain boundaries, and from machining marks and pore sites, whereas for annealed specimens, only pitting was observed. Therefore, it is possible to decrease the SCC susceptibility by improving the surface finish of the printed material throughout grinding or machining. However, the localized plastic deformation from these processes may also introduce additional residual stresses that could further increase their SCC susceptibility.35,38,69,76-78  A more practical route to improve the SCC resistance of LPBF-manufactured components is reducing their intrinsic residual stresses by optimizing the printing parameters such as laser power, scanning strategy, and printing orientation,60,79-82  or by adding postprocessing steps to the as-printed object, e.g., stress relief heat treatment53,83-86  or shock peening.63,87-89 

This study investigated how the residual stresses on LPBF-manufactured SS316L C rings printed with supports influenced their high SCC susceptibility. To assess this, stressed and unstressed test specimens were immersed in an acidified chloride solution at boiling temperature, and their time-to-crack was monitored. Furthermore, stress relief heat treatment and partial immersion tests were conducted to mitigate the impact of residual stresses on the SCC susceptibility around the printed holes. A detailed post-testing characterization was performed using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD) analysis. The cracking behavior of cold-drawn wrought SS316L C rings was used as a comparison.

Materials

The SS316L used in this investigation was produced with LPBF. The material was additively manufactured using fresh nitrogen-atomized prealloyed SS316L powder with an average particle size of 35.7 μm, and a particle size distribution of D50 33.1 μm and D90 55.2 μm. The SS316L powder was kept for at least 12 h in an oven at 50°C (122°F) to remove moisture prior to use. Before printing, the build plate was preheated at 100°C (212°F) to decrease thermal gradients, the oxygen content was reduced to less than 0.2%, and the chamber was filled with high-purity argon. All LPBF SS316L specimens were produced with a layer thickness of 50 μm, no fill contour, no down-skin layer, and using a zig-zag pattern without rotation between layers to achieve the bulk volume. The laser power (P) was 275 W, the scanning speed (V) was 700 mm/s, and the hatching space (h) was 120 μm. Commercially available cold-drawn (CD) wrought SS316L, which is known for its high-yield strength and residual stresses resulting from its plastic deformation process,90-92  was used for comparison.

The LPBF SS316L elemental composition was determined with inductively coupled plasma atomic emission spectroscopy (ICP-AES). Table 1 summarizes the elemental composition of the prealloyed SS316L powder used in the LPBF process and the CD wrought SS316L rod bar, as reported in their manufacturer certificates. Table 1 also shows the externally analyzed composition of the as-printed LPBF SS316L and the nominal composition of UNS S31603 for comparison.

Table 1.

Elemental Composition of the Prealloyed SS316L Powder, LPBF-Manufactured SS316L, CD Wrought SS316L, and Nominal Composition of UNS S31603

Elemental Composition of the Prealloyed SS316L Powder, LPBF-Manufactured SS316L, CD Wrought SS316L, and Nominal Composition of UNS S31603
Elemental Composition of the Prealloyed SS316L Powder, LPBF-Manufactured SS316L, CD Wrought SS316L, and Nominal Composition of UNS S31603

Microstructure Characterization

The phased composition of the LPBF and CD wrought SS316L were identified via x-ray diffraction (XRD) using a Cobalt K alpha powder diffractometer radiation source operating at 35 kV and 40 mA with a LynxEye detector (Bruker D8 Discover). The XRD data were collected over an angular range of 40° to 130°, a step size of 0.015°, and a time interval of 0.7 s. The microscopy analysis was conducted on tested and untested samples that were cut, mounted in cold epoxy resin, wet ground with SiC abrasive paper, and mechanically polished down to a 1 μm surface finish. Samples intended for LOM analysis were chemically etched with a solution containing 100 mL H2O, 10 mL HNO3, and 100 mL HCl. The concentration of nitric acid and hydrochloric acid was 70% and 32%, respectively. Samples for EBSD analysis were polished to a mirror surface finish with 0.02 μm colloidal silica and then ion-milled for 30 min using a beam voltage of 8 kV at a glancing angle of 4° with full cycle rotational movements (TECHNOORG Linda, SEMPrep2). All samples for EBSD analysis were surface coated with a 5 μm carbon film to prevent electrostatic charging.

The microstructures were imaged using secondary electron (SE) and backscatter (BS) detectors in a field emission-scanning electron microscope (FE-SEM) (TESCAN system, CLARA). The crystallographic orientations were mapped using an Oxford symmetry EBSD detector in the FE-SEM, on samples tilted 70°, with a working distance of 20 mm, a beam energy of 28 kV, and a beam current of 1 nA. A clean-up process was applied to the EBSD data to assimilate any non or misindexed points into the surrounding neighborhood grains, ensuring that less than 10% of the points were modified. Grain boundaries were detected with a threshold misorientation of 10°, a minimum of 8 pixels of fractional difference of misorientation variation, and a kernel size of 3 × 3. Grain size was measured as the maximum Feret diameter. The kernel average misorientation (KAM) maps were obtained using 3 × 3 square kernels and a maximum misorientation angle of 5°. All data acquisition and subsequent postprocessing were conducted using the software Aztec and Aztec Crystal, respectively.

Mechanical Properties

Mechanical properties, including yield strength (Sy), tensile strength (Su), elongation at fracture, elongation after fracture, reduction of area, and elastic modulus (E), were measured at room temperature. The tests were conducted in triplicate specimens. The LPBF SS316L, provided by IMT Mines Albi (France), was tested using rectangular specimens that were 55 mm (2.20 in) long, 8 mm (0.30 in) wide, and 2 mm (0.08 in) thick manufactured in the horizontal orientation. The displacement during the tensile test was measured using an axial extensometer with a 15 mm (0.60 in) gauge length and +5 mm (0.20 in) travel length (Epsilon TechCorp). The crosshead speed in the universal testing machine (UTM) was set to 0.225 mm/min within the elastic region and 0.750 mm/min within the plastic region. The CD wrought SS316L was tested according to ASTM(2) E894  using custom rectangular subsize tensile specimens that were 100 mm (4 in) long, 6 mm (0.24 in) wide, and 3 mm (0.12 in) thick, also as per ASTM E8. The specimens were machined from a 25 mm (1 in) diameter rod bar. The displacement during the tensile test was measured with an axial extensometer with a 25 mm (1 in) gauge length and +25 mm (1 in) travel length (Epsilon TechCorp). The UTM crosshead speed was set to 0.375 mm/min within the elastic region and 1.25 mm/min within the plastic region. The displacement rates were different in both materials as their gauge lengths were different, as defined in ASTM E8. Figure 1 shows the geometry and dimensions of the LPBF SS316L and CD wrought SS316L tensile specimens.
FIGURE 1.

Geometry of the rectangular specimens used for the tensile tests (a) LPBF-manufactured SS316L and (b) machined CD wrought SS316L. Units in millimeters.

FIGURE 1.

Geometry of the rectangular specimens used for the tensile tests (a) LPBF-manufactured SS316L and (b) machined CD wrought SS316L. Units in millimeters.

Close modal

All tensile tests were conducted using a 50 kN Shimadzu UTM equipped with 50 kN manual nonshift wedge grips. All data were collected at a rate of 10 Hz using the Trapezium X software. The actual yield strength (AYS) of the LPBF SS316L and the wrought SS316L were calculated by intersecting their corresponding engineering stress-strain curves with an 0.2% offset line running parallel to the elastic portion of their curves, per ASTM E8.94  The slope of the offset line, i.e., an approximation of the material’s elastic modulus (E), was calculated using the least-squared method per ASTM E111.95  The microhardness was measured according to ASTM E38496  using a microhardness tester (Duramin-4, Struers), an applied load of 2 Kg (HV2), and a dwell time of 15 s. The bulk density was calculated using the Archimedes principle per standard ASTM B96297  with a density kit coupled to an analytical balance with a readability of 0.001 g and a linearity of ±0.002 g (ME203, Mettler-Toledo).

Stress Corrosion Cracking

The SCC behavior was investigated by monitoring the time required by the materials to crack when subjected to a stress level corresponding to 60% and 90% of their AYS while immersed in a boiling solution, i.e., 106°C (223°F). Unstressed specimens, i.e., 0% AYS, were also tested for comparison. The test solution was 25 wt% NaCl acidified to pH 1.50 with phosphoric acid (H3PO4), per ASTM G123.98  The test specimens used in this investigation were C rings designed per ASTM G38.99  The LPBF SS316L C rings were manufactured, as shown in Figure 2(a), and their holes were re-bored with a slightly larger drill bit to remove the printed supports. All surfaces of the C rings were sequentially wet-ground with abrasive papers from 80- to 600-grit SiC. Although the dimensions of the C rings were not measured after surface finishing, the grinding process produced no excessive material removal. Notably, during the initial stages of this investigation, the printed holes were left in their as-printed condition. However, after preliminary results (not shown), it was decided to polish them in the same way as the rest of the C ring. All tests were performed at least in duplicate, as explained below.
FIGURE 2.

(a) Geometry of the custom C-ring specimen used to investigate the SCC susceptibility of LPBF and CD wrought SS316L and (b) schematics of the constant-strain assembly according to ASTM G38.

FIGURE 2.

(a) Geometry of the custom C-ring specimen used to investigate the SCC susceptibility of LPBF and CD wrought SS316L and (b) schematics of the constant-strain assembly according to ASTM G38.

Close modal

The C rings were stressed to their corresponding material’s AYS using a constant-strain setup, per ASTM G38.99  The assembly, shown in Figure 2(b), consisted of two PEEK washers, two M6 titanium flat washers, one M6 titanium socket cap bolt, one M6 titanium flanged lock nut, and a strip of clear PTFE heat shrinkable tube molded to the bolt. The stress levels of 60% and 90% AYS were obtained by attaching a 0.3 mm (1/64 in) circumferential strain gauge (FLAB-03-11-1LJC-F, Tokyo Measuring Instruments) to the middle of the uppermost curved surface of the C ring, as shown in Figure 2(b). Then, the bolt was tightened until the reading in the data logger (Almemo 2590, Ahlborn) indicated the strain value corresponding to the required stress level. This procedure was conducted according to the stress considerations outlined in ASTM G38,99  which states that the nominal stress exists only along a line that runs across the C ring at the middle of its arc. Therefore, the strain should be measured at that location, where the strain is maximum. However, the circumferential stress may vary across the width of the C ring, and the extent of the variation depends on the width-to-thickness and diameter-to-thickness ratios of the specimen. In general, the stress is greater at the edges than in the middle, but only finite element modeling (FEM) can determine the actual location of the maximum stress for a given C-ring’s configuration, which was outside the scope of our work. All traces of the strain gauges were manually removed with 600-grit SiC abrasive paper. The electrical insulation between the bolt and the C ring was verified with a digital multimeter. All of the C rings tested at 0% AYS were also prepared, as shown in Figure 2(b), although no stress was applied to the bolt. The CD-wrought SS316L C rings were similarly prepared and included as a control.

Each test condition consisted of a flask containing 750 mL of the test solution and two C rings, one from each material stressed to the same corresponding AYS. The ratio of the volume of solution per exposed surface area of specimens was 17 mL/cm2 (109 mL/in2), which is more than threefold the minimum required by ASTM G123.98  The SCC susceptibility of each material was assessed as the time required to observe the first cracks. Therefore, longer exposure times without cracking indicated a lower SCC susceptibility. All C rings were removed from their test solution weekly, cleaned, and inspected for cracks at a magnification of 20× using LOM. C rings showing SCC were cut and prepared for post-test microscopy analysis, while C rings with no cracks continued the test in freshly prepared solution until the next inspection for a maximum of 6 weeks, per ASTM G123.98  As removing the specimens for inspection is expected to disturb the local corrosion cells and may affect the results,98  and due to the aggressive nature of the test solution for most stainless steels, a 1-week inspection frequency was considered sufficient to determine the onset of cracking.

Two different methods were used to investigate how residual stresses affect the cracking behavior of LPBF-manufactured SS316L. The first method involved subjecting duplicate C rings to a stress relief heat treatment in a vertical tube furnace under vacuum at 650°C (1,200°F) for 2 h. After stress relieving, the C rings were manually wet ground down to 600-grit using SiC abrasive paper, left overnight to recover the passive film, and tested unstressed, i.e., 0% AYS, in boiling solution (approximately 106°C [223°F]). In the second method, duplicate C rings were manually wet-ground down to 600 grit, stressed to 90% of their AYS, and left to passivate overnight. The specimens were then partially immersed in boiling solution while hanging upside-down, with the uppermost curved surface immersed in the solution, while the printed holes were kept above the solution level, Figure 3. Weekly inspections were conducted on all C rings to determine the onset of cracking.
FIGURE 3.

Schematic of the partial immersion test applied to the as-printed LPBF SS316L C ring.

FIGURE 3.

Schematic of the partial immersion test applied to the as-printed LPBF SS316L C ring.

Close modal

Microstructure Characterization

Figure 4 shows representative XRD patterns of LPBF and CD wrought SS316L. Both materials consisted entirely of γ-austenite (fcc), i.e., δ-ferrite (bcc) was not detected within the resolution of the technique. Figure 5 shows EBSD maps perpendicular to the build direction and corresponding color-coded inversed pole figures (IPF) of the LPBF and the CD wrought SS316L microstructures obtained from untested C rings. The IPF showed strongly textured LPBF and wrought SS316L microstructures indicated by the high-intensity poles in the {111} pole figure, i.e., 2.32 and 2.27 times random, respectively. Figure 5 also indicates relatively large austenitic grains in the LPBF and the CD-wrought SS316L microstructures. Their average grain sizes were 57±52 μm in the LPBF SS316L and 44±34 μm in the CD wrought SS316L. The overall grain size distributions were D50 39 μm and D90 124 μm for the LPBF SS316L, and D50 34 μm and D90 90 μm for the CD wrought SS316L. The fitted ellipse aspect ratios were 3.0±1.8 for the LPBF SS316L and 3.9±3.6 for the CD wrought SS316L.
FIGURE 4.

XRD patterns of LPBF and CD wrought SS316L showing the presence of γ (fcc) austenite as the only phase in their microstructures.

FIGURE 4.

XRD patterns of LPBF and CD wrought SS316L showing the presence of γ (fcc) austenite as the only phase in their microstructures.

Close modal
FIGURE 5.

EBSD maps and color-coded inverse pole figures of untested C rings showing textured microstructures in the (a) as-printed LPBF SS316L perpendicular to its printing direction and (b) CD wrought SS316L.

FIGURE 5.

EBSD maps and color-coded inverse pole figures of untested C rings showing textured microstructures in the (a) as-printed LPBF SS316L perpendicular to its printing direction and (b) CD wrought SS316L.

Close modal

Mechanical Properties

Figure 6 shows representative engineering stress-strain curves of LPBF SS316L and its CD wrought counterpart in their elastic regions and corresponding AYS. The AYS of LPBF and CD wrought SS316L were 529 MPa (77 ksi) and 646 MPa (94 ksi), respectively. Although the displacement rates differed for each material due to their different gauge lengths, which may affect their tensile properties,94  the C rings were stressed to their corresponding material’s AYS, which allowed a proper evaluation of SCC susceptibility despite the difference in geometry and displacement rates.
FIGURE 6.

Engineering stress-strain curves within the elastic region of (a) LPBF SS316L and (b) CD wrought SS316L along with the location of their corresponding AYS.

FIGURE 6.

Engineering stress-strain curves within the elastic region of (a) LPBF SS316L and (b) CD wrought SS316L along with the location of their corresponding AYS.

Close modal

The bulk density of LPBF and CD wrought SS316L were 7.894±0.013 g/cm3 and 7.953±0.027 g/cm3, respectively. The bulk porosity of LPBF SS316L was less than 1%. Table 2 summarizes additional tensile properties, such as elastic modulus, tensile strength, elongation at fracture, elongation after fracture, and reduction of area. The average microhardness of the as-printed LBPF SS316L, the stress-relieved LBPF SS316L, and the CD wrought SS316L were 224±3 HV2, 195±2 HV2, and 282±3 HV2, respectively.

Table 2.

Mechanical Properties of As-Printed LPBF SS316L, CD-Wrought SS316L, and Standard Requirements for UNS S31603

Mechanical Properties of As-Printed LPBF SS316L, CD-Wrought SS316L, and Standard Requirements for UNS S31603
Mechanical Properties of As-Printed LPBF SS316L, CD-Wrought SS316L, and Standard Requirements for UNS S31603

Stress Corrosion Cracking

Evidence of SCC in the as-printed LPBF SS316L C rings tested in boiling solution while stressed to 0% and 90% AYS is shown in Figures 7(a) and (b), respectively. The figures show that regardless of the applied stress, the stressed and unstressed C rings cracked with similar morphology, initiating from corrosion pits near the edges of their printed holes. Figure 8(a) illustrates the results of one of the initial C ring tests with as-printed holes, whereas Figure 8(b) corresponds to one with the as-printed surface removed by grinding. Interestingly, both initiation sites were perpendicular to the hole/support interfaces, as depicted by the inset images in Figure 9. Highly branched SCC initiated near the printed holes from corrosion pits on unstressed and stressed LPBF SS316L C rings with the holes in as-printed and polished conditions, respectively (Figure 8). SCC is initiated from the polished surface of the C ring near the holes regardless of the stress state and surface condition. Figure 9 shows an EBSD map and corresponding band contrast, revealing transgranular SCC in an unstressed (0% AYS) LPBF SS316L C ring that cracked from the same location around the printed holes. The SCC behavior of fully immersed as-printed LPBF and CD wrought SS316L C rings under different applied stress levels is summarized in Table 3. As seen in Table 3, a trend was observed between the immersion condition and time to crack initiation. In this regard, fully immersed as-printed LPBF C rings took longer to crack when stressed to higher stress levels than partially immersed samples. Nevertheless, cracks always initiated near the same locations around their printed holes and perpendicular to the printed supports, regardless of the applied stress. The SCC resistance of the CD wrought SS316L decreased with applied load, in agreement with the literature33,41-42,103  and the cracks always initiated on the curved surface.
FIGURE 7.

Photographs of as-printed LPBF SS316L C rings fully immersed in boiling solution showing similar SCC morphologies under (a) unstressed (0% AYS) and (b) stressed (90% AYS) conditions. The red arrows indicate the location of the hole/support interfaces.

FIGURE 7.

Photographs of as-printed LPBF SS316L C rings fully immersed in boiling solution showing similar SCC morphologies under (a) unstressed (0% AYS) and (b) stressed (90% AYS) conditions. The red arrows indicate the location of the hole/support interfaces.

Close modal
FIGURE 8.

Photographs of LPBF SS316L C rings with (a) as-printed holes and (b) polished holes, showing similar SCC that initiated from pits near their printed holes in the unstressed (0% AYS) and stressed (90% AYS) conditions, respectively.

FIGURE 8.

Photographs of LPBF SS316L C rings with (a) as-printed holes and (b) polished holes, showing similar SCC that initiated from pits near their printed holes in the unstressed (0% AYS) and stressed (90% AYS) conditions, respectively.

Close modal
FIGURE 9.

Band contrast image and corresponding EBSD map of SCC in as-printed unstressed (0% AYS) LPBF SS316L specimens after full immersion in boiling solution showing transgranular cracking.

FIGURE 9.

Band contrast image and corresponding EBSD map of SCC in as-printed unstressed (0% AYS) LPBF SS316L specimens after full immersion in boiling solution showing transgranular cracking.

Close modal
Table 3.

SCC Behavior of LPBF SS316L and CD Wrought SS316L C Rings Tested at Different Conditions in Acidified Chloride Boiling Solution.(A)

SCC Behavior of LPBF SS316L and CD Wrought SS316L C Rings Tested at Different Conditions in Acidified Chloride Boiling Solution.(A)
SCC Behavior of LPBF SS316L and CD Wrought SS316L C Rings Tested at Different Conditions in Acidified Chloride Boiling Solution.(A)
The role of residual stresses on the unexpected SCC behavior in LPBF SS316L C ring samples was investigated by conducting a stress relief heat treatment on as-printed C rings. Figure 10 shows KAM maps perpendicular to the build direction of the as-printed and stress-relieved LPBF SS316L C rings, as indicated. The as-printed LPBF SS316L microstructure contained local strain around their grain boundaries caused by its processing history. This local strain, depicted by the green areas in Figure 10(a), indicates a high degree of residual stress.104-105  Inversely, the microstructure of the heat-treated LBPBF SS316L showed fewer areas with local strain, Figure 10(b). A quantitative representation of the reduction in residual stresses due to the heat treatment is shown in Figure 10(c). As seen in this figure, the stress relief effect of the heat treatment reduced the magnitude of the KAM angles. The overall KAM distributions went from D50 0.96°, D90 1.89° in the as-printed LPBF SS316L, to D50 0.64°, D90 1.43° in the heat-treated LPBF SS316L. A Kolmogorov-Smirnov (K-S) test using OriginLab’s statistical tools was conducted to confirm that both distributions were significantly different, and the results are presented in Figure 10(c).
FIGURE 10.

KAM maps of untested LPBF SS316L C rings in their (a) as-printed and (b) stress-relieved conditions. The KAM histogram in (c) shows the redistribution of local misorientations after the stress relief process.

FIGURE 10.

KAM maps of untested LPBF SS316L C rings in their (a) as-printed and (b) stress-relieved conditions. The KAM histogram in (c) shows the redistribution of local misorientations after the stress relief process.

Close modal
Figure 11 illustrates the SCC of a stress-relieved LPBF SS316L C ring when tested in boiling solution without applied stress, i.e., 0% AYS. As shown in Figure 11, in the absence of applied stress, cracking still occurred at the same locations near the printed holes and perpendicular to the printed supports. Moreover, the crack morphology was similar to its as-printed counterparts, although the initial main crack propagated longer before branching in the annealed specimen. As seen in Table 3, the heat-treated specimens cracked 1 week after the as-printed counterparts. These findings indicated that the SCC resistance of unstressed LPBF SS316L C rings slightly improved after stress relief.
FIGURE 11.

Photographs of an unstressed (0% AYS) stress-relieved LPBF SS316L C ring after full immersion in boiling solution showing identical SCC morphology and crack location as its as-printed counterparts.

FIGURE 11.

Photographs of an unstressed (0% AYS) stress-relieved LPBF SS316L C ring after full immersion in boiling solution showing identical SCC morphology and crack location as its as-printed counterparts.

Close modal
The as-printed LPBF SS316L C rings stressed to 90% AYS were also partially immersed in boiling 25% NaCl (pH 1.5), as shown in Figure 3. As the dissolved oxygen concentration is much lower at the boiling temperature (106°C [223°F]) than at room temperature,106  pitting and SCC were supported by the hydrogen evolution reaction rather than the oxygen reduction reaction, which allows a direct comparison between fully and partially immersed tests. After exposure, the partially immersed printed material cracked transgranularly from its uppermost curved surface, i.e., where the nominal applied stress is maximum,99,107  Figure 12. Figure 13 shows the SCC behavior of the fully immersed 90% AYS CD wrought samples. As seen in Figure 13, the CD-wrought SS316L samples cracked perpendicular to the direction of the applied stress from its curved surface. However, cracking initiated from the edges of the C ring rather than from its middle area. Cracking from the edges of the samples was attributed to the residual stresses introduced during the machining process.35,38,108  Nevertheless, both materials cracked along the line of maximum applied stress when stressed to 90% AYS. Notably, the percentage of the total pitted area in the fully immersed samples (approximately 0.22%) was, on average, 4 to 5 times higher than the percentage of pitted area in the partially immersed samples (approximately 0.05%). Therefore, considering that the immersed area of the fully immersed sample is about three times larger compared to the partially immersed sample, this suggests that the difference in the immersed surface of the samples had no impact on the distribution of pitting or the onset of cracking.
FIGURE 12.

As-printed LPBF SS316L C ring stressed at 90% AYS showing (a) its partial immersion setup, (b) SCC located at the middle of its uppermost curved surface, and (c) an etched imaged showing the resulting transgranular SCC morphology.

FIGURE 12.

As-printed LPBF SS316L C ring stressed at 90% AYS showing (a) its partial immersion setup, (b) SCC located at the middle of its uppermost curved surface, and (c) an etched imaged showing the resulting transgranular SCC morphology.

Close modal
FIGURE 13.

CD-wrought SS316L 90% C ring showing (a) full immersion setup, (b) photograph illustrating SCC that started from the edge of the uppermost curved surface, and (c) a micrograph showing the resulting transgranular SCC morphology.

FIGURE 13.

CD-wrought SS316L 90% C ring showing (a) full immersion setup, (b) photograph illustrating SCC that started from the edge of the uppermost curved surface, and (c) a micrograph showing the resulting transgranular SCC morphology.

Close modal

All LPBF SS316L C rings manufactured with printed supports were consistently susceptible to SCC. Regardless of whether the specimens were stressed (90% AYS) or unstressed (0% AYS), as-printed or heat-treated, they all developed cracks from the same location near the printed holes and perpendicular to the hole/support interface, as illustrated in Figures 7, 8, and 11. All cracks exhibited a highly branched morphology, indicating a similar fracture behavior across the different test conditions and, thus, suggesting the presence of highly localized residual stresses at the cracking sites. The stress relief heat treatment applied to the specimens reduced residual stresses, as evidenced by the KAM maps and distribution shown in Figure 10, as well as the 13% reduction in microhardness. Interestingly, the stress-relieved specimens exhibited a slightly different cracking behavior than their as-printed counterparts, as can be seen in Figures 7 and 11, in which the length of the main crack before branching was around three times longer than in the as-printed condition. Nevertheless, the cracking location remained consistent near the printed holes, indicating that the stress relief process was only partially effective at reducing SCC susceptibility. The surface finish of the printed holes did not play a role in SCC initiation, as evidenced by Figures 7 and 8, where all cracks occurred in the same location—i.e., from pits located on the polished surface of the C rings and near the printed holes—regardless of the hole’s surface roughness. These observations suggested that factors other than the hole’s surface roughness, particularly residual stresses, may be influencing the SCC behavior of the LPBF SS316L C rings.

The influence of residual stresses around the printed holes was further demonstrated through partial immersion tests conducted on stressed C rings (90% AYS). As the printed holes were positioned above the solution line, Figure 3, the specimens cracked along the axis of maximum applied stress, Figure 12. This SCC behavior, characterized by highly branched transgranular cracks, was comparable to that observed in CD wrought C rings, Figure 13, although the wrought specimens cracked from the edges instead of the center or the curved surface. The different crack initiation site was attributed to the residual stresses introduced during the machining process of the CD-wrought SS316L C rings.35,38,69,76-78  These results underscore the significant role of residual stresses in dictating the path of crack propagation, regardless of the manufacturing method used.

The results from the partial immersion tests summarized in Table 3 indicated that the LPBF C rings developed SCC 1 week after the CD wrought samples. Notably, no cracks were observed at the printed holes, confirming the absence of residual stress influencing the SCC of the material at those locations. This apparent marginal improvement in the SCC resistance of the printed material compared to its wrought counterpart could be attributed, for instance, to a delay in pit nucleation and the lower applied stress.109-110  However, to provide a comprehensive understanding of this phenomenon, further analysis would be necessary, for instance, by including testing LPBF C rings manufactured at different orientations and specimens with drilled holes instead of printed ones.

It is hypothesized that the hole/support interface, located at the 12 and 6 o’clock positions, as shown in Figure 14, experienced volumetric contraction during the rapid cooling from the melting temperatures during the production of the C rings, resulting in the observed residual stress effect. This phenomenon may have introduced compressive stresses through the supporting structure, forming two additional sites of maximum tensile residual stress at the 3 and 9 o’clock positions, as shown in Figure 14. This stress effect, resembling a “C ring within the C ring” scenario, is believed to be the primary cause of the systematic cracking of the C rings at the 3 and 9 o’clock positions.
FIGURE 14.

Hypothesized: (a) residual stresses produced in the hole/support interface of the C rings, (b) unstressed C ring with SCC initiating from corrosion pits at the 3 and 9 o’clock sites, and (c) “stress relieving” effect of the applied load over the residual stresses.

FIGURE 14.

Hypothesized: (a) residual stresses produced in the hole/support interface of the C rings, (b) unstressed C ring with SCC initiating from corrosion pits at the 3 and 9 o’clock sites, and (c) “stress relieving” effect of the applied load over the residual stresses.

Close modal

Finally, this study found that the SCC susceptibility of the LPBF C rings decreased as the applied stress increased, particularly in the unstressed LPBF C rings, which exhibited faster crack initiation than their stressed counterparts, as reported in Table 3. In the absence of rigorous FEM, which is beyond the scope of this investigation, these unexpected results can be explained using vector analysis. In this regard, the 3 and 9 o’clock positions are the locations of the highest tensile residual stress in unstrained C ring, Figures 14(a) and (b). In contrast, in stressed C rings, Figure 14(c), the stress acts perpendicular to the main component of the residual stress, reducing the magnitude and changing the direction of the stress. This unintentional “stress relieving” effect is hypothesized to have lowered the SCC propensity (measured as a longer crack initiation time) around the printed holes. Nevertheless, as all stressed LPBF C rings continued to crack at their printed holes suggests that the resulting “vector stress” in this area was above the externally applied stress.

This study investigated the SCC behavior of LPBF-manufactured SS316L using stressed and unstressed C rings immersed in boiling, 106°C (223°F), 25% NaCl (pH 1.5). The analysis included a stress relief heat treatment and a partial immersion test to elucidate the effect of residual stresses, as well as a comprehensive microstructure characterization to analyze the cracking behavior. The following conclusions were drawn based on the results presented above:

  • Localized residual stresses introduced by the printed supports reduced the SCC resistance of SS316L C rings manufactured by LPBF. This observation has important implications, as it suggests that LPBF-manufactured components with complex geometries that require printed supports may also contain residual stresses, leading to decreased SCC resistance.

  • Provided that the effect of residual stresses is mitigated or removed by a stress relief heat treatment, LPBF SS316L had a marginally better SCC resistance than CD wrought SS316L when stressed at 90% of their corresponding AYS values, as determined by a 1 week delay in crack initiation time.

  • A stress relief heat treatment improved the SCC resistance of LPBF SS316L. However, more research is needed to determine the degree of improvement that can be achieved and to identify the best temperature and duration of the heat-treatment process.

Trade name.

(1)

UNS numbers are listed in Metals & Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

(2)

ASTM International, 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.

The authors acknowledge the financial support of Woodside Energy, as well as the access to the instruments of the Microscopy and Microanalysis Facility (MMF) at Curtin University, and the LPBF SS316L manufactured C rings supplied by IMT Mines Albi, Institut Mines-Telecom in France.

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