Tensile and Fracture Properties Evaluation of Additively Manufactured Different Stainless Steels via Small Punch Testing

Abstract

Laser powder bed fusion (LPBF) can fabricate hydraulic components with significant weight reduction, and in this study, small punch tests (SPTs) evaluated the tensile and fracture properties of four stainless steels (30Cr13, 316L, 15-5PH, 17-4PH), alongside metallographic, scanning electron microscope (SEM), and Electron Backscatter Diffraction (EBSD) analyses which examined their fracture modes, grain orientation, phase distribution, and grain boundary distribution. The tensile property results showed ductility rankings as 316L > 17-4PH > 15-5PH > 30Cr13, with correlations between R_p0.2 and R_m from SPT and uniaxial tensile tests for all four, while high-magnification SEM fractographs revealed ductile dimples on 15-5PH, 17-4PH, and 316L SPT specimens versus distinct cleavage fracture in 30Cr13. EBSD analysis indicated austenite content order as 316L > 17-4PH > 30Cr13 > 15-5PH, grain size order as 316L > 17-4PH > 15-5PH > 30Cr13, and high-angle grain boundaries ranking as 15-5PH > 30Cr13 > 17-4PH > 316L; additionally, notched SPT specimens inspected per EN 10371 for fracture toughness showed J-integral (J_IC) values in the order 316L > 17-4PH > 15-5PH > 30Cr13, consistent with ductility and grain size results.

1. Introduction

Water-based hydraulic technology utilizes water-based fluids as the working medium, offering an environmentally friendly, safe, and resource-efficient alternative to traditional oil-based hydraulic systems in specific applications. Lightweight design serves as the primary strategy for water-based hydraulic systems to address weight restrictions, broaden application possibilities, and enhance performance. This approach not only reduces equipment weight and power demands for drive units but also enhances equipment durability, maneuverability, and load-bearing capacity while promoting energy efficiency and emission reduction. The integration of additive manufacturing (AM) technology, in conjunction with layout optimization and topological optimization design, enables hydraulic components and system structures to be more compact, smaller in volume, and lighter in weight [1]. The whole AM family is categorized with seven different sub-strategies according to their process variables and printing material types, including fused filament fabrication, vat-photopolymerization, material jetting techniques, binder jetting, laser powder bed fusion (LPBF), direct energy deposition, and sheet lamination methods. Among them, LPBF is primarily employed for the production of metal and ceramic objects, making it a prominent AM process for producing mechanical components [2]. This technique employs a high-power-density laser to melt metallic powders and join them together [3,4,5]. Many studies have focused on the material properties and mechanical behaviors of LPBF-manufactured materials using conventional standardized specimens, e.g., [6,7,8]. The conventional standardized specimens usually require a bulk of material, which is difficult or unapplicable to extract from in-service components. Hence, effective testing methods for remaining-life assessment of LPBF-manufactured components using a small amount of material are desirable.

Small punch tests (SPTs) only require a small volume of material for sampling without affecting the integrity and operational performance of in-service components. SPTs are widely used to estimate various material properties, such as tensile properties [9,10,11], fracture toughness [12,13], creep properties [14,15], and fatigue properties [16,17]. In recent years, some studies have been performed to evaluate the material properties of additive manufactured materials.

Lucon et al. [13] performed SPTs on additively manufactured (ALM) Ti-6Al-4V specimens fabricated under various processing parameters and heat-treatment conditions to assess their tensile and fracture toughness. Hurst et al. [18] conducted SPTs to evaluate the tensile and creep characteristics of ALM alloys, including IN718 and Ti-6Al-4V, utilizing small disc specimens obtained from a thin-walled ALM aerofoil blade. Lulu-Bitton et al. [19] studied the effects of gaseous hydrogen charging and electrochemical hydrogen charging on the microstructure and mechanical properties of wrought and electron beam melted (EBM) Ti-6Al-4V alloys. Wang et al. [20] performed uniaxial creep and small punch creep tests on deposited and ultrasonic micro-forging treated (UMFT)-assisted AM Hastelloy X to assess creep damage induced by AM defects. Lewis et al. [21] examined the fatigue behavior of LPBF nickel-based superalloy C263 and EBM titanium alloy Ti-6Al-4V in comparison with conventionally manufactured versions of these alloys. Wang et al. [17] assessed the fatigue performance of AM GH4169 related to AM defects and utilized various machine learning algorithms to predict small punch fatigue life. Mao and Takahashi [22] conducted pioneering work to correlate the F_e and F_m values from SPTs with tensile test-obtained R_p0.2 and R_m values. Song et al. [23] developed empirical correlations between the results of SPTs and uniaxial tensile tests for an anisotropic ASTM A350 alloy extracted from a failed forging flange. García et al. [24] discussed the effects of using different methods to obtain F_e from SPTs on the accuracy of the yield stress estimation. Janča et al. [25] summarized and compared different methods for obtaining F_e.

Previous studies have predominantly examined the material properties and mechanical behavior of LPBF-manufactured materials using conventional standard specimens, which necessitate a significant material volume and are challenging to extract from operational components. Consequently, researchers have adopted the small punch test (SPT) method to analyze the properties of additive manufacturing materials and traditional non-additive manufacturing materials. However, prior investigations have been restricted to individual SPT specimens. This study employed both standard and notched specimens to assess the tensile and fracture characteristics of LPBF-manufactured stainless steels including 30Cr13, 316L, 15-5PH, and 17-4PH. Force-displacement profiles were obtained for two SPT specimens for each of the four LPBF-manufactured materials. By refining the precision and focus of the analysis on tensile properties and fracture behavior, this study enhances the accuracy and comprehensiveness of small-sample testing for evaluating the mechanical properties of LPBF stainless steels. This research presents a precise experimental approach for appraising the remaining operational lifespan of components fabricated from such materials.

2. Small Punch Testing

2.1. Materials and Test Specimen

This study utilizes SPT testing on specimens composed of various materials to offer insights for material selection and lightweight structural design in components of water hydraulic systems. The medium in water hydraulic systems, a water-based fluid, can lead to corrosion of metal parts. Hence, this investigation concentrates on examining four stainless steel grades: 30Cr13, 316L, 15-5PH, and 17-4PH. Notably, 30Cr13, with elevated carbon content, exhibits superior strength, hardness, and wear resistance but comparatively weaker corrosion resistance. Next, 316L, an austenitic stainless steel containing molybdenum, delivers exceptional corrosion resistance, good weldability, and ductility, facilitating ease of processing and forming. Moreover, 15-5PH, a precipitation-hardening stainless steel, combines high strength with commendable corrosion resistance, achieving high hardness and toughness post heat treatment, maintaining stability at elevated temperatures, and demonstrating good machinability. Additionally, 17-4PH, another precipitation-hardening stainless steel, showcases high strength, high toughness, and excellent corrosion resistance, along with remarkable fatigue resistance, consistent mechanical properties at room temperature, and good machinability. These stainless steels are produced using the LPBF process, with production equipment employing high-speed laser cladding equipment (output power approximately 6000 W, spindle speed 1500 r/min, three-axis translation speed approximately 10 mm/s, powder feeding accuracy approximately 1.5%). Self-fusing iron-based alloy powders are used (powder production process: gas atomization and air cooling), with a particle size range of −300 to +500 M and a weld overlay reference hardness of 44-47HRC. The chemical composition of stainless steel is shown in

Table 1. Chemical compositions of stainless steels specimens (wt%).

Materials	C	P	Si	Mo	Cr	Mn	Fe	Al	Ni	Nb	S	Cu	Ta
30Cr13	0.32	0.02	0.83		13.58	0.89	Bal		0.42		0.013
316L	0.021	0.032	0.81		16.76	1.53	Bal		12.56		0.019
15-5PH	0.056	0.028	0.6		14.32	0.58	Bal		4.21	0.33	0.0028	3.15	0.0006
17-4PH	0.061	0.023	0.74		16.31	0.67	Bal		4.36	0.32	0.003	3.86	0.0005

. Uniaxial tensile tests were conducted on four types of stainless steel to determine their yield stress and ultimate strength. The tests were conducted in accordance with GB/T228.1-2021, using a computer-controlled electronic universal testing machine (WDW-300) with a test temperature of 23 °C and a humidity of 51% RH. One sample was tested for each type of stainless steel (manufactured using traditional processes), and the strength values are shown in

Table 2. The yielding stresses and ultimate strengths of stainless-steel specimens.

Materials	Yielding Stress (MPa)	Ultimate Strength (MPa)
30Cr13	753	1829
316L	373	620
15-5PH	806	1431
17-4PH	745	1162

.

The standard SPT specimen used in this study [11] has a diameter of 10 mm, a thickness of 0.5 mm, and an arithmetic mean roughness (R_a) of 0.4 µm. EN 10371 [26] outlines a method for determining fracture toughness, J-integral (J_IC), utilizing a notched SPT specimen with identical dimensional parameters to the standard SPT specimen. The notched SPT specimen features a lateral notch at its center, simulating a crack. To secure the specimen in the test rig, the effective crack length (a_eff) is approximately 4 mm. The correlations between the SPT data and uniaxial test results were determined following the guidelines outlined in CEN CWA 15672 [27] and EN ECISS/TC101/WG1 [28]. The wear mechanism of the standard SPT specimens was examined through scanning electron microscopy (SEM) analysis. Refer to Figure 1 for the dimensions of the notched SPT specimen.

2.2. Test Apparatus and Procedure

The test rig depicted in Figure 2 was utilized to determine the tensile properties and fracture toughness of stainless steel through SPTs; Figure 2b shows the various components of the SPT testing device and the positions of the test samples. The dimensions of the test rig align with those detailed in a prior investigation [11]. In the SPTs, a compression bar drives a spherical indenter to compress the clamped specimens at a constant rate of 0.5 mm/min under an external force. Simultaneously, a data acquisition system records the force on the compression bar and the deflection change at the center of the specimen’s lower surface to generate force-deflection curves. The axisymmetrically tested specimens primarily experience radial and circumferential tensile stresses. With increasing displacement applied to the specimen, its deformation escalates until eventual fracture. Four types of LPBF-manufactured stainless steel are used as test objects, with three samples prepared for each type. Three repeated tests are conducted for each type of stainless steel.

3. Results and Discussion

3.1. Metallographic Analyses

Metallographic analysis was conducted on samples of 15-5Ph, 17-4Ph, 3Cr13, and 316L, prepared through wire cutting, sandpaper grinding, polishing, and etching. The metallographic structures of the LPBF-manufactured stainless steels are depicted in Figure 3. Figure 3a,d illustrate tempered martensite in LPBF-manufactured 15-5PH and austenite structures in 316L, respectively. Conversely, Figure 3b,c exhibit the metallographic structure of 17-4PH and 30Cr13, likely composed of tempered troostite with notable carbide presence at the grain boundaries.

3.2. SPT Aanalysis

3.2.1. Method for Determining the Elastic–Plastic Transition Force

Numerous studies have examined the empirical relationships between the strength properties (R_p0.2 and R_m) and the characteristic parameters derived from SPT force-deflection outcomes. Equations (1)–(3), established in prior research [23,24,25], are commonly acknowledged correlations:

$$R_{p0.2}={\alpha }_1\left({\displaystyle \frac{F_e}{h_0^2}}\right)+{\alpha }_2$$

(1)

$$R_m={\beta }_1\left({\displaystyle \frac{F_m}{u_m{h}_0}}\right)+{\beta }_2$$

(2)

$$R_m={\beta }_1^{\prime }\left({\displaystyle \frac{F_m}{u_m{h}_0}}\right)+{\beta }_2^{\prime }$$

(3)

where ${\alpha }_1$, ${\alpha }_2$, ${\beta }_1$, ${\beta }_2$, ${\beta }_1^{\prime }$, and ${\beta }_2^{\prime }$ are the empirical correlation factors which depend on the geometry of the test rig.

In our previous study [11], we utilized two different methods to determine the elastic–plastic transition force, F_e, as recommended by CEN CWA 15672 and EN ECISS/TC101/WG1. In the current study, we also employed these two methods to determine F_e and plotted the results according to standard documents. Figure 4 illustrates a typical result for determining F_e in LPBF-manufactured 15-5PH using both methods.

Table 3. Determined $F_e\left(CWA\right)$ and $F_e\left(two secants\right)$ values for the four LPBF-manufactured stainless steels.

Materials	$F_e\left(CWA\right)$, N	$F_e\left(two secants\right)$, N
30Cr13	666.53	1137
	621.06	976
	720	1122
316L	162.37	165.66
	169.26	216.07
	621.06	994.63
15-5PH	479.06	775.2
	399	694
	602.48	781
17-4PH	339.93	462.59
	562.7	800.85
	601.06	788.23

provides a summary of the $F_e\left(CWA\right)$ and $F_e\left(two secants\right)$ values determined for the four LPBF-manufactured stainless steels. The empirical correlation factors in Equations (1)–(3) can be derived by correlating ${\displaystyle \frac{F_e}{h_0^2}}$, ${\displaystyle \frac{F_m}{u_m{h}_0}}$, and ${\displaystyle \frac{F_m}{h_0^2}}$ values calculated from the SPT test data with the uniaxial tensile test data for R_p0.2 and R_m in the four LPBF-manufactured stainless steels.

Table 4. The values of empirical correlations factors.

${\alpha }_{1\left(CWA\right)}$	${\alpha }_{2\left(CWA\right)}$	${\alpha }_{1\left(two secants\right)}$	${\alpha }_{2\left(two secants\right)}$	${\beta }_1$	${\beta }_2$	${\beta }_1^{\prime }$	${\beta }_2^{\prime }$
0.0477	550.62	0.1196	365.77	0.142	727.71	−0.098	1921.4

presents the empirical correlation factor values.

3.2.2. SPT Results and Estimation of Strength Properties

Figure 5 displays the force-deflection curves of four materials manufactured using LPBF. Among these materials, 30Cr13 exhibits the highest scatter in results compared to the other stainless steels. Notably, all materials, except for 316L, demonstrate a characteristic behavior in SPT curve: initially increasing with deflection, then experiencing a sudden decrease (pop-in), followed by a subsequent increase. Upon reaching maximum load, the load decreases with further deflection, indicating material cracking. The deflection at which cracking occurs is observed to be the smallest for 30Cr13, followed by 15-5PH and 17-4PH, with 316L exhibiting the highest. Consequently, the ductility ranking of the stainless steels is as follows: 316L > 17-4PH > 15-5PH > 30Cr13. Prior to cracking, 30Cr13 demonstrates the highest SPT force at the same deflection, followed by 17-4PH, 15-5PH, and 316L, suggesting that 30Cr13 possesses the highest tensile strength, followed by 17-4PH, 15-5PH, and 316L.

It can be seen in Figure 6 and Figure 7 that the offsets are distributed in the lower and upper bounds within a 2-factor and a 1.5-factor of the unity lines, respectively. This could give reasonably accurate predictions for R_p0.2 and R_m.

3.3. SEM Analyses on the Ruptured Surfaces of Standard SPT Specimens

SEM fractographs of the ruptured surfaces of SPT specimens for four LPBF-manufactured stainless steels are presented in Figure 8a–h. Figure 8a displays a low-magnification SEM micrograph of the 30Cr13 LPBF-manufactured SPT specimen, revealing cracks originating from the specimen’s bottom and extending radially towards it, with no circumferential cracking observed. In contrast, Figure 8c shows the SEM micrograph of the 316L LPBF-manufactured SPT specimen, where radial cracks are absent. For the LPBF-manufactured 15-5PH and 17-4PH SPT specimens, circumferential cracking is predominant, as depicted in Figure 8e,g, with some radial cracks also visible, particularly pronounced in the 15-5PH specimens compared to the 17-4PH specimens. This suggests slightly better ductility in 17-4PH, consistent with the force-deflection curve results in Figure 5. High-magnification images in Figure 8d,f,h reveal numerous ductile dimples on the fracture surfaces of the LPBF-manufactured 316L, 15-5PH, and 17-4PH SPT specimens. However, a cleavage fracture is evident at high magnification in the LPBF-manufactured 30Cr13 SPT specimen.

3.4. EBSD Analyses on the LPBF-Manufactured Four Stainless Steels

Samples of 30Cr13, 316L, 15-5PH, and 17-4PH prepared using the LPBF process were analyzed for grain orientation, phase distribution, and grain boundary distribution. The microstructural information of 30Cr13 is shown in Figure 9a–c. The inverse phase figure (IPF) of 30Cr13 (Figure 9a) indicates the absence of a distinct microstructure. The phase distribution diagram (Figure 9b) shows a combination of martensite and partial austenite phases. Similarly, 316L also lacks a distinct microstructure, primarily composed of austenite, as shown in Figure 10a,). The IPF of 15-5PH (Figure 11a) also indicates the absence of microstructural features, while the phase distribution diagram (Figure 11b) shows a primary martensite phase (BCC) with a small amount of austenite phase (FCC). For 17-4PH, Figure 12a,b show the absence of microstructural features, with the primary phases being martensite and austenite. EBDS analysis of the phase distribution of the four stainless steels further validates the results of the metallographic analysis. EBDS analysis of 17-4PH and 30Cr13 shows that both have martensite as the primary phase, and the presence of carbides may be related to precipitation strengthening of the martensite matrix, further supporting the observation of “grain boundary carbides” in the metallographic analysis. The austenite content decreases progressively in the order of 316L, 17-4PH, 30Cr13, and 15-5PH. The austenite phase exhibits excellent plasticity, and EBDS analysis shows that 316L has the highest austenite content, supporting the finding that 316L has the best ductility. Grain size is largest in 316L, followed by 17-4PH, 15-5PH, and 30Cr13. Grain size is a key factor influencing strength (fine-grain strengthening effect), so 30Cr13 exhibits the highest tensile strength due to fine-grain strengthening, while 316L has the lowest strength due to coarse grains. As shown in Figure 9c, Figure 10c, Figure 11c and Figure 12c, 15-5PH has the highest content of high-angle grain boundaries (the red portions in the grain boundary information diagram represent high-angle grain boundaries). Next, 30Cr13 has a lower content of high-angle grain boundaries, followed by 17-4PH, while 316L has the lowest content of high-angle grain boundaries. Moreover, 30Cr13 has a low austenite content (with brittle martensite dominating) and a low content of high-angle grain boundaries (weak resistance to crack propagation), resulting in typical cleavage brittle fracture during fracture, consistent with the SEM features of “no ductile pits and rapid radial crack propagation.” Furthermore, 316L, 17-4PH, and 15-5PH all contain austenite, whose plastic deformation capability can absorb fracture energy, resulting in numerous ductile dimples on the fracture surface (a characteristic of plastic fracture); among these, 17-4PH has a higher austenite content than 15-5PH and a lower content of high-angle grain boundaries than 15-5PH (though the influence of grain boundaries on toughness is dominated by the microstructural composition), resulting in fewer radial cracks and slightly better ductility in 17-4PH compared to 15-5PH, consistent with SEM observation results.

3.5. Estimations of Fracture Toughness, J_IC

The force-deflection curves of notched SPT specimens for the four AM stainless steels are depicted in Figure 13. The deflection at the maximum force, u_m, can be derived from Figure 13 and is presented in

Table 5. Deflection at the maximum force, u_m, of the tested notched SPT specimen.

Materials	um for Different Specimens, mm	Average Value of um, mm
30Cr13	0.213	0.256
	0.264
	0.290
316L	1.35	1.39
	1.42
	1.40
15-5PH	0.769	0.760
	0.729
	0.781
17-4PH	1.04	1.04
	1.04
	1.04

. In accordance with EN 10371 [26], the deflection at the maximum force, u_m, of the tested notched SPT specimen can be correlated with the notch opening displacement, δ_IC, corresponding to crack initiation, as illustrated in Figure 14. The linear relationship between fracture toughness, J_IC, and the notch opening displacement, δ_IC, is defined by Equation (4):

$$J_{IC}^{SPT}=R_{p0.2}\times {\delta }_{IC}$$

(4)

The fracture toughness, $J_{IC}^{SPT}$, estimated through notched opening displacement is detailed in

Table 6. The fracture toughness, $J_{IC}^{SPT}$, estimated by using the notched opening displacement.

Materials	${\delta }_{IC}$ Determined by um, mm	$J_{IC}^{SPT}$
30Cr13	0.067	50
316L	1.218	455
15-5PH	0.291	234
17-4PH	0.569	424

. It is evident from Table 6 that the highest $J_{IC}^{SPT}$ value is observed in LPBF-manufactured 316L, followed by the second highest $J_{IC}^{SPT}$ in LPBF-manufactured 17-4PH. In contrast, 30Cr13, among the other three LPBF-manufactured stainless steels, exhibits the lowest $J_{IC}^{SPT}$.

3.6. Summary

The study confirms the efficacy of the SPT in assessing mechanical properties of additive manufacturing materials and establishing correlations with traditional uniaxial test results. Previous research has mainly examined single SPT specimens, focusing on materials like titanium and nickel alloys. This study investigates four types of LPBF stainless steel (30Cr13, 316L, 15-5PH, 17-4PH) using both standard and notched SPT specimens. Through microstructural analysis, SEM, and EBSD analysis, it elucidates differences in tensile properties, fracture modes, and the relationship between microstructural characteristics (composition, grain size, grain boundary distribution) and macroscopic properties. This research addresses gaps in SPT studies on additively manufactured stainless steels, offering insights into their unique performance patterns and the impact of texture on performance, surpassing existing studies focused on macroscopic parameters.

4. Conclusions

In this study, the tensile and fracture properties of four LPBF-manufactured stainless steels were explored through SPTs using two types of small disc specimens. The following are the conclusions.

Metallographic analysis of stainless steels produced by LPBF reveals that 15-5Ph exhibits tempered martensite, 316L shows austenite, and both 17-4PH and 30Cr13 display tempered martensite with abundant carbides at grain boundaries. Tensile testing indicates that 316L possesses the highest ductility, followed by 17-4PH, 15-5PH, and 30Cr13, while 30Cr13 exhibits the highest tensile strength, followed by 17-4PH, 15-5PH, and 316L. Differences in metallographic structure significantly influence mechanical properties. For instance, 316L, characterized by an austenitic structure, exhibits superior ductility, whereas 30Cr13, with carbide inclusions, demonstrates the highest tensile strength but the lowest ductility. The correlation between R_p0.2 and R_m values obtained from SPT testing and those acquired from uniaxial tensile testing was established for the four stainless steel variants produced via LPBF.

High magnification SEM fracture images reveal that SPT specimens of 30Cr13 steel grade produced by LPBF display cleavage fracture, demonstrating distinct brittle fracture features. In contrast, SPT specimens of 15-5PH, 17-4PH, and 316L steel grades manufactured by LPBF exhibit numerous ductile pits on the fracture surface, indicating clear plastic fracture characteristics. Notably, 17-4PH shows fewer radial cracks compared to 15-5PH, suggesting slightly superior ductility, aligning with the SPT curve outcomes.

EBSD analysis revealed that none of the four stainless steel grades displayed significant texture. The order of austenite content was found to be 316L > 17-4PH > 30Cr13 > 15-5PH. The austenite phase demonstrated superior plasticity, with 316L exhibiting the highest austenite content (single austenite phase), correlating with its superior ductility. In terms of grain size, the sequence was 316L > 17-4PH > 15-5PH > 30Cr13. Grain size significantly impacts strength (fine-grain strengthening effect), leading to 30Cr13 displaying the highest tensile strength due to fine-grain strengthening, while 316L showed the lowest strength due to coarse grains. Regarding high-angle grain boundaries, the order was 15-5PH > 30Cr13 > 17-4PH > 316L.

Fracture toughness of notched SPT specimens was evaluated in accordance with the EN 10371 standard [26]. The $J_{IC}^{SPT}$ values were ranked as follows, consistent with the observed ductility and grain size trends: 316L > 17-4PH > 15-5PH > 30Cr13.