1. Introduction
17-4PH stainless steel is a precipitation-hardenable alloy widely used in aerospace and marine applications due to its excellent corrosion resistance, wear resistance, and high strength1,2). The alloy typically contains approximately 17% chromium and 4% nickel, along with copper (Cu) and niobium (Nb), which enable its mechanical properties to be enhanced through heat treatment3). The addition of Cu contributes to corrosion resistance and provides improved strength and resistance to hydrogen embrittlement through precipitation hardening4), while Nb enhances strength through the formation of Nb-rich phases and contributes to precipitation strengthening during aging treatment5). Upon solution treatment, the alloy transforms into a martensitic structure that imparts high strength and toughness, although its hardness remains relatively low6). Con- ventional manufacturing methods for 17-4PH include metal injection molding (MIM), investment casting, and other processes. Among them, MIM is a relatively cost-effective method that forms parts by heating or extruding material layer by layer without the use of lasers. However, typical characteristics of MIM, such as surface defects and internal porosity, even under optimized parameters, tend to degrade mechanical performance. Moreover, MIM parts often exhibit lower tensile and fatigue properties due to internal defects and poor surface finish7). Investment casting allows the fabrication of complex geometries by pouring molten metal into molds; however, the resulting parts have been reported to possess low hardness (<20 HRC), failing to meet required specifications due to the presence of retained austenite8). Although high-performance and complex-shaped 17-4PH parts are increasingly in demand, conventional processes face limitations in machinability and shape complexity. As such, additive manufacturing (AM) has emerged as a promising solution to overcome these limitations.
Additive manufacturing (AM) is a layer-by-layer fabrication technique capable of producing geometrically complex components directly from digital models9,10). Among various AM methods, Selective Laser Melting (SLM) fully melts metal powder using a high-energy laser beam, producing near-full-density parts with excellent mechanical properties and complex geometries. C.Y. Yap and I. Yadroitsev emphasized that material properties and application-specific performance depend heavily on the optimization of process parameters in SLM, including laser power and scan speed, which directly affect the final microstructure and properties of the built part11,12). During the fabrication of 17-4PH via SLM, rapid solidification tends to promote the formation of dendritic martensitic microstructures13). The repetitive and localized heating-cooling cycles between the laser beam and powder bed induce high thermal gradients, leading to significant residual stress and non-equilibrium microstructures. These factors can affect subsequent heat treatment response and influence the material’s mechanical properties14). Studies have shown that SLM-fabricated stainless steels often contain substantial amounts of retained austenite15).
Despite the growing adoption of SLM in the field of AM, studies focused on optimizing post-heat treatment processes to improve the mechanical performance-such as microstructure refinement, hardness, and tensile strength-remain limited. Post-heat treatment is essential not only for relieving residual stress in SLM-built 17-4PH but also for improving mechanical strength and corrosion resistance. Precipitation-hardenable (PH) stainless steels derive their mechanical properties from martensitic transformation followed by aging, where the aging time and temperature critically influence strength, toughness, and corrosion resistance. Through appropriate solution and aging treatments, 17-4PH can achieve high yield and tensile strength, making it particularly valuable for bearing applications16,17). Moreover, nanometer-scale Cu-rich precipitates formed during aging are known to further enhance mechanical properties relevant to bearing performance18). Luca Facchini et al. reported that residual stress in 17-4PH can be influenced by heat treatment conditions such as H1025 and H1150, and confirmed that martensitic transformation occurs from retained austenite during aging19). Guennouni et al. demonstrated that over-aging at 620 °C for 4 hours leads to a significant increase in retained austenite20,21). Tao Zhou showed that mechanical properties can be tailored by controlling microstructural characteristics during aging, including phase composition changes and Cu precipitate growth22). Zemin Wang further noted that the number density of Cu precipitates increases within the 350-450 °C range but decreases at higher temperatures due to coarsening effects23).
While extensive research has been conducted on stress relief via post-heat treatment, there remains a lack of comprehensive studies on how solidification structure, phase fraction, and residual stress evolve in SLM-processed 17-4PH. In particular, the correlation between microstructure, mechanical properties, and wear behavior is yet to be fully elucidated. To address this gap, it is necessary to design optimal heat treatment strategies that not only relieve residual stress but also improve mechanical and tribological performance through precipitation behavior associated with aging. Therefore, in this study, we investigated post-heat treatment methods specifically tailored for SLM-built 17-4PH stainless steel. By applying solution treatment and subsequent aging, we aimed to enhance mechanical properties in accordance with the characteristics of PH stainless steels. The relationship between heat treatment conditions, microstructural evolution, mechanical properties, and wear behavior was comprehensively examined.
2. Experimental Procedures
The chemical composition of the 17-4PH stainless steel powder used in this study is presented in Table 1. Gas-atomized powders with particle sizes ranging from 15 to 53 μm were utilized, and specimens for tensile and wear testing were fabricated using a Selective Laser Melting (SLM) system (Concept Laser M1, Concept Laser, GE Additive, Ohio, USA) under an argon atmosphere at Dong-Eui University Converging Materials Core Facility supported by the Korea Basic Science (NFEC-2018-09-246087). The process parameters employed in the SLM process included a laser power of 180 W, scanning speed of 1000 mm/s, hatch spacing of 0.105 mm, beam diameter of 0.05 mm, layer thickness of 0.035 mm, focal offset of -3 mm, and a rotation angle of 90° between adjacent layers.
Table 1
Chemical composition of 17-4PH precipitation hardening stainless steel
| 17-4PH | Fe | Cr | Ni | Cu | Si |
|---|---|---|---|---|---|
| Wt% | 15.0 | 15.0 | 3.0~5.0 | 3.0~5.0 | ≤1.0 |
For the post-processing heat treatment, the specimens underwent solution treatment at 1050 °C for 30 min, followed by furnace cooling. Aging was performed at various temperatures (420 °C, 480 °C, 540 °C, and 600 °C), selected within the precipitation-hardening range, for 4 hours, followed by air cooling. For clarity, the heat-treated specimens were designated as as-built, SA-420 °C, SA-480 °C, SA-540 °C, and SA-600 °C, respectively.
The microstructural and precipitate evolution of 17-4PH steel under different aging temperatures were examined using optical microscopy (OM; ECLIPSE LV150L, Nikon, Japan) and field-emission scanning electron microscopy (FE-SEM; Quanta 200 FEG, FEI Company, Amsterdam, The Netherlands). Samples were prepared by grinding with 400, 600, and 1000 grit SiC papers, followed by polishing with 3 μm and 0.04 μm alumina suspensions. For improved microstructural contrast, chemical etching was performed using Fry’s reagent (40 mL HCl, 5 g CuCl₂, 30 mL distilled water, and 25 mL ethanol).
X-ray diffraction (XRD) analysis was conducted using an X’Pert PRO MPD diffractometer with CuKα radiation, over a 2θ range of 30°-90°. Rietveld refinement was applied to quantify phase fractions. Vickers hardness was measured using a microhardness tester (HM-210, Mitutoyo, Tokyo, Japan) under a load of 0.1 kgf, with 12 indentations performed per sample.
Tensile tests were carried out at room temperature using a universal testing machine (KSU-20M, Kyoungsung, Seoul, Republic of Korea) at a strain rate of 10-3 s-1. Tribological properties were evaluated via a ball-on- disk wear test using a tribometer (RB-102PD, R&B Co., Ltd., Seoul, Republic of Korea) under dry conditions. The test parameters included a normal load of 30 N, rotation speed of 1000 RPM, and a sliding distance of 1000 m. A tungsten carbide (WC) ball was employed as the counter material.
3. Results and Discussion
3.1 Microstructural Analysis
Fig. 2 presents the phase analysis results of 17-4PH stainless steel specimens subjected to various heat treatment conditions, as determined by X-ray diffraction (XRD). Diffraction peaks corresponding to both BCC and FCC crystal structures were detected under all conditions, and similar diffraction behavior was observed in both the as-built and solution-treated (SA) specimens. It is generally known that the low carbon content in 17-4PH alloys results in minimal lattice distortion between ferrite and martensite phases, making it difficult to distinguish between BCT and BCC structures through XRD analysis24,25). In this study, based on previously reported literature, the α-phase observed in the SA specimen was interpreted as BCT martensite, while the FCC phase in the as-built specimen was identified as retained austenite. Due to rapid solidification conditions inherent in the SLM process, the as-built microstructure of 17-4PH primarily consists of δ-ferrite and austenite phases. These metastable phases progre- ssively transform into martensite upon aging heat treatment. During the aging process, Cu-rich precipitates were uniformly formed within the martensitic matrix. According to Sabooni et al.26), the Cu precipitates formed under these conditions are on the nanometer scale and effectively hinder dislocation motion, thereby contributing to improvements in mechanical strength and wear resistance. On the other hand, with increasing aging temperature, the intensity of the γ-Fe (111) diffraction peak gradually weakened, and nearly disappeared above 540 °C. This indicates that retained austenite was progressively transformed into martensite during aging, and that high-temperature aging promotes the formation of a stable martensitic phase27).
Fig. 3 shows the optical microscopy (OM) observations of 17-4PH stainless steel specimens subjected to various aging heat treatment conditions. In the Fig. 3 (a) as-built specimen fabricated via the SLM process, a non-dense and heterogeneously distributed phase structure was observed. This can be attributed to localized thermal cycling and variations in solidification rates resulting from repeated laser heating and melting. Due to the rapid solidification characteristics of the SLM process, the transformation to BCT martensite was insufficient, and relatively stable δ-ferrite and austenite phases remained in the microstructure. The martensite start temperature of 17-4PH stainless steel is approximately 105 °C, which enables the formation of martensitic structures relatively easily upon air cooling after high-temperature solution treatment. The Fig. 3 (a) as-built specimen exhibited a high level of residual stress, and its microstructure was primarily composed of δ-ferrite and austenite. These findings are consistent with the work of Alnajjar et al.28), who reported the coexistence of δ-ferrite and martensite in the as-built condition. In the heat-treated specimens (Fig. 3 (b) SA-420 °C, Fig. 3 (c) SA-480 °C, Fig. 3 (d) SA-540 °C, Fig. 3 (e) SA-600 °C), the microstructure predominantly consisted of lath martensite and prior austenite. The martensite appeared as a lamellar structure aligned along laths, while the austenite exhibited blocky grain boundaries. This supports the presence of reversed austenite formed through the aging process following solution treatment29). The presence of retained austenite is believed to result from the local enrichment of austenite-stabilizing elements such as Cu and Ni during the aging process, which lowers the austenitization temperature and leads to the partial retention of the austenite phase30). These microstructural features were further examined in detail through scanning electron microscopy (SEM), allowing clearer identification of precipitate distributions and phase boundary morphologies.
Fig. 3
OM micrographs of 17-4PH with different heat treatment, (a) as-built, (b) 420 °C, (c) 480 °C, (d) 540 °C, and (e) 600 °C
Fig. 4 presents the microstructural analysis of SLM- fabricated 17-4PH stainless steel specimens using scanning electron microscopy (SEM). The aging temperature range employed in this study (as-built to 600 °C) is below the regime in which static recrystallization predominates. Accordingly, the observed “refinement” is interpreted not as grain re-nucleation/growth but as a reduction in the effective grain size (EGS) arising from (i) the transformation of retained austenite (RA) to martensite during aging and (ii) the development of the lath-martensite substructure (packet/block/lath). This interpretation is consistent with reports on L-PBF 17-4PH showing a decrease in RA and an increase in martensite upon aging/heat treatment, as well as with studies indicating that the mechanical response of lath martensite is governed by EGS (i.e., the packet/block size)31). Fig. 4(a) shows the microstructure of the Fig. 4 (a) as-built specimen, which consists of austenite and δ-ferrite phases. Additionally, nanoscale precipitates were observed within the grains. Upon aging treatment, the austenite transformed into martensite, accompanied by internal shear deformation, and some regions exhibited twinning structures. As the aging temperature increased, the amount of precipitates also increased, which is expected to correlate directly with the observed increase in hardness32). In the heat-treated specimens, precipitates of various sizes were randomly distributed throughout the matrix, and they were primarily composed of stable phases such as NbC. These precipitates became more prominent after heat treatment compared to the Fig. 4 (a) as-built condition, indicating that the aging treatment promoted active precipitation reactions33). The precipitates were irregularly distributed along grain boundaries and within grains, suggesting that they may significantly influence not only the mechanical strength and hardness but also the wear resistance of the material.
3.2 Analysis of Precipitates and Mechanical Pro- perties
Fig. 5 shows the SEM analysis results of precipitates observed in the microstructure of 17-4PH stainless steel specimens after aging treatment. In the Fig. 5 (a) as-built specimen, numerous micropores were observed, which are presumed to originate from the uneven melting of metal powder and rapid solidification during the SLM process. In contrast, a significant reduction in porosity was observed in the heat-treated specimens. This is attributed to the relief of internal stress and partial recrystallization during the post-heat treatment, leading to an increase in microstructural density. SEM observations of the aged specimens revealed that Nb-rich phases and carbides were precipitated along grain interiors and lath boundaries. In particular, carbides distributed along the lath boundaries are considered to contribute to the improvement of mechanical strength and hardness34). The morphology of the precipitates varied depending on the aging temperature: rod-like precipitates were observed at relatively lower temperatures, whereas more spherical precipitates formed under higher aging conditions. Cu-rich precipitates were difficult to detect in low-magnification SEM images due to their extremely fine size. These findings are consistent with previous reports indicating that Cu precipitates exist in nanoscale sizes ranging from 3 to 6 nm4). Ni and Cu, known as austenite-stabilizing elements, were initially present within the austenite grains, but were redistributed during the martensitic transformation induced by aging. Nb, a key element in carbide formation, was found to be abundantly distributed at grain boundaries and within grains. This distribution trend aligns with the observations reported by Cheruvathur et al. regarding Nb precipitate behavior in 17-4PH stainless steel. These nanoscale precipitation phenomena are considered to be critical factors contributing positively to the enhancement of mechanical strength, hardness, and wear resistance of the alloy.
Fig. 5
17-4PH SEM micrographs precipitate of (a) as-built, (b) 420 °C, (c) 480 °C, (d) 540 °C and (e) 600 °C
Fig. 6 presents the Vickers hardness values of 17-4PH stainless steel specimens subjected to various aging temperatures. The results indicated that the as-built specimen exhibited the lowest hardness among all conditions, while hardness increased progressively with rising aging temperatures. This trend can be attributed to the phase transformation from austenite (FCC) to martensite (BCT) during the aging process, which leads to a denser and harder microstructure. The increase in both the number and size of precipitates, particularly Cu-rich phases, also contributed to the overall hardness improvement. At an optimal aging temperature, fine and uniformly distributed Cu-rich precipitates were effectively formed, thereby enhancing the material’s hardness. However, when the aging temperature exceeded the optimal range, over-aging phenomena were observed, leading to a slight decrease in hardness. This behavior is consistent with previous studies reporting that, in the range of 500-700 °C, coarsening of precipitates leads to a gradual reduction in hardness35). In this study, a slight difference in hardness was noted between the SA-540 °C and SA-600 °C specimens, with the SA-600 °C condition exhibiting marginally reduced hardness due to more pronounced spheroidization of precipitates. Furthermore, it has been reported that above 550 °C, Cu-rich precipitates tend to grow rapidly and coarsen, which results in a decrease in hardness and strength 0while simultaneously improving ductility and toughness36). Viswanathan et al. also reported that the reduction in hardness at 600 °C is closely related to microstructural changes, including the coarsening of precipitates and the reduction of the martensitic phase fraction37). These findings suggest that excessive aging temperatures may degrade hardness due to microstructural instability. Therefore, it is essential to establish a balanced aging condition that optimizes hardness while maintaining precipitate morphology and overall structural integrity.
Fig. 7 illustrates the variations in tensile strength and elongation of 17-4PH stainless steel specimens fabricated by SLM under different aging heat treatment conditions. The as-built specimen, which did not undergo any post-processing heat treatment, exhibited the lowest ultimate tensile strength (UTS) of 1012 MPa. Upon aging treatment, both the UTS and yield strength (YS) showed an increasing trend. Notably, the SA-420 °C specimen recorded the highest UTS of 1224 MPa, surpassing that of the SA-480 °C condition (1102 MPa). Similarly, the YS of the SA-420 °C specimen reached 746 MPa, which is 89 MPa higher than that of the SA-480 °C specimen (657 MPa). In terms of ductility, the SA-480 °C specimen demonstrated superior elongation, reaching 17.28%, while the highest overall elongation was observed in the as-built condition at 17.58%. However, aside from elongation, the as-built sample exhibited inferior mechanical properties compared to heat-treated specimens, which is attributed to its high internal residual stresses and incomplete phase transformation. The SA-600 °C specimen showed the lowest YS at 639 MPa among all heat-treated conditions. This decrease is considered to result from over-aging phenomena, such as coarsening of precipitates and the increased fraction of reverted austenite, which tend to enhance ductility but reduce strength. Overall, both tensile and yield strengths increased with aging temperature up to a certain point, beyond which over-aging led to a slight decline in hardness and strength while enhancing ductility. The difference in tensile and yield strengths across various aging conditions remained relatively modest, which may be due to the narrow aging temperature range and consistent holding time (4 hours), resulting in limited thermal energy variation. The mechanical behavior trend of UTS and YS appeared to be consistent across all conditions. After a temporary drop in YS at SA-480 °C, a recovery was observed at SA-540 °C. This is attributed to the morphological evolution of precipitates from a needle-like to a more spherical shape, which reduced the stress concentration factor (SCF), allowing more uniform stress distribution and thereby improving ductility and crack resistance.
Fig. 7
Tensile strength, yield strength, and elongation with variations at different aging temperatures
Fig. 8 presents the fracture surface morphology of 17-4PH stainless steel specimens subjected to various aging heat treatment conditions, examined using scanning electron microscopy (SEM). All specimens exhibited numerous dimple structures, which are characteristic of ductile fracture, formed by the coalescence of micro-voids during plastic deformation. In particular, the Fig. 8 (a) as-built specimen showed a larger number and size of dimples, indicating a higher degree of plastic deformation prior to fracture. This observation aligns with the high elongation value recorded for the as-built condition in Fig. 7, further confirming its ductile fracture behavior. However, in some specimens, cleavage features were also observed, suggesting the occurrence of mixed-mode fracture, combining both ductile and brittle characteristics38). It is well known that metals with a body-centered cubic (BCC) structure, such as 17-4PH, are prone to cleavage fracture at low temperatures or under specific microstructural conditions. Especially in the Fig. 8 (e) SA-600 °C specimen, coarse precipitation of carbides-primarily CuS-was identified along grain boundaries. These coarse precipitates act as stress concentrators, promoting localized cleavage fracture. The corresponding fracture surfaces exhibited relatively flat facets, correlating with the decrease in tensile and yield strengths observed in Fig. 6. This implies that over-aging at high temperatures induced the coarsening of precipitates, which in turn led to the formation of brittle fracture zones39). In addition, high- magnification SEM analysis revealed the presence of finely aligned lattice-like microstructures near the grain boundaries, which are believed to result from Cr carbide precipitation along the boundaries or the development of deformed slip bands during aging treatment. These microstructural features may act as potential sites for micro-crack initiation, contributing to brittle fracture at a very fine scale40). Overall, the fracture behavior varied significantly depending on the aging temperature from fully ductile fracture in the as-built and low-temperature aged specimens to mixed and localized brittle fracture modes in high-temperature aged samples. This variation is closely related to the morphology, composition, and distribution of precipitates, as well as the stability of the grain boundary microstructure under different thermal conditions.
3.3 Wear Behavior Analysis
Fig. 9 shows the scanning electron microscopy (SEM) images of the wear tracks formed on 17-4PH stainless steel specimens after dry sliding wear tests under various aging heat treatment conditions. In all specimens, a number of debris particles were observed around the wear tracks, which are considered to be the result of material detachment due to repeated contact and frictional loading. The average width of the wear track was widest in the as-built specimen, measured at approximately 3251 μm. As the aging temperature increased, the wear track width showed a decreasing trend in the order of Fig. 9 (e) SA-600 °C → Fig. 9 (b) SA-420 °C → Fig. 9 (c) SA-480 °C → Fig. 9 (d) SA-540 °C. Notably, the Fig. 9 (d) SA-540 °C specimen exhibited the most uniform and narrow wear track with a smooth surface morphology, indicating stable and improved wear resistance. This improvement is attributed to the optimized distribution of precipitates and microstructural stabilization achieved at this aging condition. Although the Vickers hardness generally increased with increasing aging temperature, a direct correlation between hardness and wear resistance was not observed. For instance, the Fig. 9 (e) SA-600 °C specimen exhibited relatively high hardness values, yet showed broader and more irregular wear tracks with unstable wear behavior. This anomaly is attributed to the formation of coarse Cr- and Nb-rich precipitates under over-aging conditions, which may act as abrasive particles during sliding, thereby promoting abrasive wear. These results suggest that wear resistance is not solely dependent on hardness, but is also strongly influenced by complex factors such as precipitate morphology and distribution, grain refinement, phase stability, and microstructural homogeneity. Therefore, enhancing wear performance requires a comprehensive approach that considers not only hardness but also controlled precipitation behavior and uniform microstructural evolution through well-optimized heat treatment conditions. Fig. 10 presents the coefficient-of-friction (CoF) results as a function of aging heat-treatment condition. The SA-540 °C specimen exhibits the most stable friction response-despite a higher average CoF than the other specimens-attributable to the formation of a continuous oxidative tribofilm and a homogeneous precipitation- hardened matrix.
Fig. 9
Wear surface analysis on the specimen with aging treatment temperature of (a) as-built, (b) 420 °C, (c) 480 °C, (d) 540 °C and (e) 600 °C
Fig. 11 presents the SEM observations of the worn surfaces of 17-4PH stainless steel specimens subjected to different aging treatments after the dry sliding wear tests. The wear mechanisms in metallic materials are generally classified into abrasive wear, adhesive wear, fatigue wear, and fretting wear. These mechanisms are strongly influenced by external factors such as applied load, relative sliding speed, surface hardness, roughness, lubrication conditions, and other environmental variables. Such parameters significantly affect the wear behavior and fracture modes during frictional contact, altering the relative contribution of each wear mechanism. In all specimens, fine particulate debris was observed along the worn surfaces, which can be attributed to the formation of metal oxides induced by frictional heating. This observation indicates that oxidative wear is the dominant wear mechanism under the applied test conditions. The oxide layers were distributed irregularly across the entire wear track width, suggesting continuous surface oxidation caused by repeated contact under dry conditions. Furthermore, adhesive wear was also commonly observed across most specimens. This type of wear typically occurs under high load and high-speed sliding conditions, where frictional heating leads to localized surface softening or partial melting. The subsequently detached material fragments may resolidify on the wear surface, resulting in a layered or rough morphology indicative of adhesive wear. Notably, the Fig. 11 (e)SA-600 °C specimen exhibited a complex wear mechanism, where a combination of oxides, fine scratches, adhesive layers, and surface cracks were observed, pointing to an unstable wear behavior. Although the Fig. 11 (c) SA-480 °C specimen showed the narrowest average wear track width, SEM analysis revealed the presence of surface cracks and irregular wear features, indicating a relatively unstable wear mechanism. In contrast, the Fig. 11 (d) SA-540 °C specimen displayed the most uniform wear surface, with minimal signs of adhesion or cracking apart from the oxidized regions. This suggests that the optimized precipitate distribution and stable microstructure at this aging condition significantly contributed to improved wear resistance. Overall, oxidative wear was identified as the primary wear mechanism in 17-4PH stainless steel, and the occurrence of adhesive wear and cracking varied depending on the aging temperature. Among the tested conditions, Fig. 11 (d) SA-540 °C exhibited the most stable and favorable wear behavior, while the over-aged Fig. 11 (e) SA-600 °C specimen demonstrated reduced wear resistance due to increased adhesive interactions and oxide formation.
4. Conclusion
In this study, optimal aging heat treatment conditions were investigated to improve the mechanical and wear properties of 17-4PH stainless steel specimens fabricated via selective laser melting (SLM). Five different heat treatment conditions were applied: as-built, SA- 420 °C, SA-480 °C, SA-540 °C, and SA-600 °C, with each specimen subjected to aging treatment for 4 hours. The effects of aging temperature on microstructure, precipitation behavior, mechanical strength, and wear resistance were systematically analyzed.
X-ray diffraction (XRD) analysis revealed that austenite in the as-built specimen gradually transformed into martensite with increasing aging temperature. Above 540 °C, the γ-Fe diffraction peaks disappeared and BCC phases became dominant, indicating that solution and aging treatment promoted Cu-rich precipitation and activated the precipitation strengthening mechanism.
Optical microscopy and scanning electron microscopy (SEM) analysis showed that the lath martensitic structure became more pronounced with increasing aging temperature, and Nb-rich precipitates were also observed. The SA-540 °C specimen exhibited a smooth and uniform microstructure with well-distributed precipitates both along the grain boundaries and within grains, indicating a highly stabilized microstructural condition. Wear test results demonstrated that the SA-540 °C specimen exhibited the most stable wear resistance, as evidenced by consistent wear track depth and width. Although the SA-480 °C specimen had the narrowest wear track, SEM observations revealed significant cracking, while the SA-600 °C specimen showed unstable wear behavior due to stress concentration caused by coarsened precipitates. Across all specimens, oxidative and adhesive wear mechanisms were identified, which were attributed to frictional heat and surface oxidation.
Overall, the findings confirm that variations in precipitate morphology and microstructure, driven by aging temperature, have a significant impact on the mechanical and tribological properties of SLM-fabricated 17-4PH stainless steel. Among the tested conditions, the SA-540 °C aging treatment was determined to be the most effective, achieving a balanced enhancement in precipitate refinement, structural stability, mechanical strength, and wear resistance. These results validate the potential of SLM-based manufacturing of 17-4PH for use in high-load, high-friction applications such as bearings and wear-resistant components. Further- more, this study provides valuable insight into heat treatment optimization for enhancing the performance and reliability of additively manufactured metallic components.
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