1 Introduction

The austenitic stainless-steel class 316L is known for its favorable characteristics, including high ductility, superior corrosion resistance due to its low carbon content preventing sensitization, and excellent performance in cryogenic environments (Ref 1,2,3). In recent years, the metal injection molding (MIM) process has gained prominence, enabling the production of components with intricate geometries, small dimensions, and large production volumes. This powder metallurgy technology employs an injection method to process a mixture of metallic powder and polymeric binders, resulting in green parts with relatively low mechanical strength (Ref 4, 5).

These green parts produced through the MIM process undergo debinding processes to remove the binding polymers. Subsequently, the pieces are sintered either in vacuum furnaces or in a controlled atmosphere. During sintering, the parts can experience a volumetric contraction of approximately 20-30%, achieving densities ranging from 99 to 100% of the final steel density (Ref 6, 7). The industrial application of MIM can benefit different stainless steels, such as Catamold 316L. In this regard, the application of Catamold 316L steel finds utility in various fields, such as medicine and electronic cryogenics (Ref 6,7,8). However, austenitic stainless steels like Catamold 316L face limitations in terms of their mechanical strength and tribological properties, as their hardening mechanisms rely solely on plastic deformation (Ref 9). Previous studies exploring the application of the plasma nitriding process have demonstrated positive effects on surface microhardness, fatigue life through the generation of compressive stresses, and corrosion resistance by forming expanded austenite (Ref 8, 9).

In addition to the challenges posed by equipment costs and operational issues, such as the presence of a hollow cathode and variations in heating conditions, complex geometries of parts can further complicate the control of compound layers and diffusion. To overcome these challenges, this research proposes ionic oxynitrocarburizing in salt baths as an alternative to plasma processes. The method involved the introduction of approximately 3% cyanide into the liquid bath, which, upon dissociation, facilitates the diffusion of nitrogen and carbon (Ref 10, 11). The degradation of the passivated Cr2O3 layer, caused by the oxygen source, promotes the diffusion process. During the nitriding process, a single-phase compound layer of Fe2-3N (ε) is formed, reducing the risk of embrittlement in polyphase layers (Ref 13, 14). Subsequently, the oxidation process contributes to the sealing of the compound layer, resulting in magnetite (Fe3O4) formation that significantly enhances corrosion resistance. It is assumed, therefore, that adopting this ionic oxynitrocarburizing method can better address the challenges associated with complex geometries.

The research conducted in this study involved a specific cycle comprising nitriding, oxidation, ceramic microchip polishing, and subsequent oxidation. Specifically, the primary objective of this study is to address knowledge gaps on the diffusion process in Catamold 316L MIM steels, which exhibit higher porosity levels than conventional steels. Another important aspect of the method developed is the investigation of the behavior of the composite layer, diffusion, corrosion resistance, and residual stresses that arise from the application of the oxynitrocarburizing process in complex geometries.

This research establishes a scientific contribution to the field by mapping the behavior of the substrate, particularly concerning the presence of precipitates that may form along the austenitic grain boundaries. The understanding of diffusion kinetics and their correlation with residual surface stresses, as determined through XRD tests, can open up new possibilities for applying this thermochemical cycle in steels with tribological limitations. Through our research, valuable empirical insights can be gained to enhance the performance and applicability of these steels in the industry. This study also contributes to complementing the previous literature (Ref 15,16,17,18,19,20,21).

2 Methods

The raw material utilized in this research was Catamold 316L Basf. To obtain the samples, nine cylindrical specimens with a diameter of 12 mm and a height of 5 mm were produced using an Arbor injector at a temperature of 120 °C, referred to as green samples. The polymer binder (debinding) was removed in a Cremer oven, employing nitric acid vapor with a purity of 99.99%.

Following this, the samples underwent sintering in a controlled atmosphere furnace utilizing 100% hydrogen. The chemical composition of the samples was analyzed using a Foutrin optical emission spectrometer. Finally, the sintered samples were subjected to the thermochemical oxynitrocarburizing treatment.

The experimental cycle commenced by preheating the samples in an atmosphere-free oven at 300 °C for 30 min. Subsequently, the specimens were immersed in the TF1 nitriding bath containing aerated cyanate at temperatures of 570 °C for 30, 60, and 120 min. Following the nitriding stage, the samples underwent oxidation in the AB1 bath for 15 min, followed by rapid cooling in water at room temperature. The oxidized samples were then subjected to glass microsphere blasting, submerged once again in the AB1 bath for 15 min, and cooled in water at room temperature.

Subsequently, the oxynitrocarburized samples were characterized using different methods. Optical microscopy analysis was conducted using an Olympus GX 51S microscope, while scanning electron microscopy (SEM) was performed using a Jeol JSM-6610LV instrument. Energy-dispersive spectroscopy (EDS) microprobe analysis was also carried out. Vickers microhardness measurements were obtained using a Shimadzu HMV 2 T equipment.

The electrochemical tests were conducted on an Autolab PGSTAT302N potentiostat using a three-electrode electrochemical cell, using a saturated calomel electrode as a reference and a platinum rod as a counter electrode. The electrolyte used was a 0.1 mol L−1 NaCl solution, with PH 5.7 at a temperature of 22 °C. The OCP open circuit potential was monitored for 3600 seconds, followed by cyclic potentiodynamic polarization, starting from 300 mV cathodic with respect to the OCP to 3000 mV anodic with respect to the OCP under a scan rate of 1 mV S−1. The assay was set to reverse the direction of the scan when the run reached 0.01 B C−2.

The x-ray diffraction (XRD) analyses were conducted directly on the sample surface without prior preparation. XRD analysis was performed using a GE Seifert Charon XRD M apparatus with monochromatic CrKα radiation. The apparatus operated at a voltage of 40 kV, a current density of 30 mA, and a step size of 0.02° (2θ) per second. The following section shows the results obtained.

3 Results and Discussion

Close to the surface, the analysis revealed the formation of a layer of compounds exhibiting darkening on the external side of the cross section. It was verified directly by optical microscopy and scanning electron microscopy. A compound layer (white) and a diffusion layer were observed. The austenitic substrate did not exhibit any microstructural evidence of sensitization. Figure 1 illustrates the darkened compound layer near the surface, the compound layer (white), and the diffusion layer, indicating the rupture of the Cr2O3 film and the occurrence of diffusion. The oxidation behavior of the composite layer was similar to that observed in previous studies on AISI 304L austenitic stainless steel following the degradation of the passivated film (Ref 5, 6, 11, 12).

Fig. 1
figure 1

Layer of compounds in optical microscopy after oxynitrocarburizing for 30 min (a), 60 min (b), 120 min (c) and morphology in the 120-min cycle in SEM (d)

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In steels produced using MIM technology, the degradation of the Cr2O3 layer is intensified due to the increased porosity, resulting in micro-tensions between the pores and the passivated surface film (Ref 5, 6, 11, 12). During the experiments, no signs of erosion or rupture were observed in the composite layer, which exhibits a brittle behavior (Ref 5, 6). The surface condition of the studied samples, being raw from sintering, also contributes to the initial degradation of the Cr2O3 layer. This phenomenon hinders the formation of oxide films with reduced and nonuniform thickness (Ref 5, 6). Moreover, SEM/EDS analyses confirmed the presence of a composite layer with a single-phase morphology, as depicted in Fig. 1(d). Furthermore, all samples subjected to different oxy-nitrocarburizing times exhibited a uniform diffusion layer without significant variations.

The EDS microprobe analysis of the diffusion zone revealed that the main mechanism involved the solid solution of nitrogen in austenite and a reduced presence of CrN precipitates (Ref 3, 4). Figure 2 illustrates the compound layer and the diffusion layer, visible in (a) and (b) of the EDS microprobe peaks, within the compound layer and beneath the diffusion bed.

Fig. 2
figure 2

Identification of the compound, diffusion, and substrate layers in (a). The qualitative analysis obtained in the EDS test is presented in (b)

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The formation of the diffusion layer following the composite layer is intensified by the presence of porosities and the permeability of magnetite to nitrogen (Ref 6). Additionally, the presence of the ε-Fe3N monophasic compound layer facilitates nitrogen diffusion (Ref 6). However, the presence of expanded austenite was not detected, which can be attributed to the long cycles applied and the surface irregularity of the samples in the raw sintered condition, without passivation, leading to the heterogeneous behavior of Cr2O3 (Ref 6).

Table 1 presents the depths of the compound layers and shows the corresponding average microhardness values observed after oxynitrocarburizing for durations of 30, 45, and 60 min. The outcomes indicated an increase in the depths of the compound layers as the nitrocarburizing time increased, although no direct correlation was observed. This suggests that the kinetic effect of the process is reduced with longer durations due to barriers formed by nitrogen supersaturation, the presence of magnetite, and other barriers within the austenite (Ref 4, 5).

Table 1 Values of composite layer and superficial microhardness of the samples
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Figure 3 illustrates the x-ray diffraction (XRD) profiles of the base material and the oxynitrocarburized samples obtained at 30, 60, and 120 min. The XRD analysis revealed the presence of prominent peaks corresponding to CrN, Fe3O4, Fe2-3N, and Gamma Iron. Nonetheless, no evidence of the gamma line or expanded austenite layer was detected. This suggests that the formed compound layer is predominantly monophasic, which contributes to its enhanced stability and reduced fragility (Ref 5,6,7,8, 18). The presence of Fe3O4 peaks potentially enhances the corrosion resistance of stainless steel. Additionally, the low carbon content in Catamold 316L steel minimizes the formation of carbon-based precipitates (Ref 5).

Fig. 3
figure 3

Results of the x-ray diffraction of the oxynitrocarburized samples

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The Vickers microhardness profile (HV 0.05) results revealed that the effective diffusion layer thickness was 0.1 mm for the sample subjected to a 30-min thermochemical treatment, 0.15 mm for the 60-min treatment, and 0.22 mm for the 120-min treatment. These findings showed that the microhardness was significantly higher near the surface, gradually decreasing toward the substrate. This behavior can be attributed to the austenitic nature of the studied steel, which relies on plastic deformation as its primary hardening mechanism. However, the absence of a distinct transition zone in the oxynitrocarbonized layer may make it susceptible to chipping.

Figure 4 displays the anodic potentiodynamic polarization curves of the samples in the initial sintered condition, considered as the reference, and after the application of oxynitrocarbonation for 30, 60, and 120 min.

Fig. 4
figure 4

Anodic potentiodynamic polarization curves of the samples

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In all the experiments performed, the oxynitrocarburized samples exhibited superior corrosion resistance compared to the sintered base metal. Among the tested samples, the ones nitrided for 30 min, followed by a 120-min thermochemical treatment, demonstrated the best resistance to corrosion and passive film formation. However, it is noteworthy that the behavior of the oxynitrocarburized sample treated for 60 min differed from the others. The results indicated that this particular sample exhibited subsurface chromium segregation, which seemingly contributed to the heterogeneity of the composite layer and the sealing of magnetite formation. The effectiveness of the applied cycles is evident in the formation of a protective film that enhances the material’s performance in critical environments, even in sintered condition (Ref 7).

The analysis of residual stresses resulting from the oxy-nitrocarburizing cycle is crucial for steels produced through the MIM process. Austenitic stainless steels like Catamold 316 L primarily rely on plastic deformation for hardening, making it challenging to generate consistent compressive stresses. XRD analysis revealed surface stress values of − 451 MPa, − 2691 MPa, and − 2769 MPa in the transverse and longitudinal directions after oxynitrocarburizing for 30, 60, and 120 min, respectively.

Figure 5 displays the average stress values in both directions. Notably, the results indicated that oxynitrocarburizing for over 60 min significantly increased compressive stresses, attributed to the presence of the Epsilon single-phase layer on the surface.

Fig. 5
figure 5

Residual stresses in the longitudinal direction in (a) and Hoop in (b)

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This study found that the generation of residual stresses in the oxynitrocarburizing process is influenced by the interaction of Cr nitrides and carbides with final oxidation, resulting in the formation of Fe3O4. Additionally, it was verified that changes in residual stress states occur due to nitrogen supersaturation, leading to a sharp transition in microhardness between the composite, diffusion, and substrate layers.

These findings align with previous research conducted on different materials using processes other than MIM (Ref 8, 19, 21). The effect of time variation was significant on the depth of the compound and diffusion layer. However, in microhardness and electrochemical tests, the data showed randomness. This condition is related to the preliminary condition of the substrate. However, the metallurgical and mechanical properties of Catamold 316L steel showed significant improvements. It is noteworthy that the effect of interstitial diffusion of nitrogen and carbon in the oxynitrocarburization process must be optimized since, in regions of reduced thickness, embrittlement may occur due to the encounter of layers (Ref 5, 6).

The paper’s findings show that the inclusion of a glass microsphere blasting step between the first and second oxidations contributes to the development of a thin layer of plastic deformation, further increasing residual compressive stresses along the surface. This blasting also contributes to reducing the final roughness of the components (Ref 10, 11). The increase in layer depth directly correlates with the intensification of residual stresses, a phenomenon observed in samples subjected to nitrocarburizing for 60 and 120 min.

4 Conclusions

Based on the results and discussion of the oxynitrocarburization of Catamold 316L steel, the conclusions regarding metallurgical and mechanical properties are indicated as follows:

  • The oxynitrocarburizing process resulted in the formation of a monophasic Epsilon layer of compounds in the austenitic stainless steel Catamold 316L samples. The compound layer was monophase Epsilon, which reduced the possibility of its embrittlement.

  • The presence of magnetite was observed on the outer part of the composite layer. The presence of magnetite represents a factor that contributed to corrosion resistance, even with the presence of CrN in stainless steel.

  • Surface microhardness values exceeded 1000 HV in all samples.

  • The samples exhibited enhanced corrosion resistance in the sintered condition with subsequent oxynitrocarburizing compared to the sintered base material.

  • Factors associated with diffusion kinetics related to the porosity level were not observed in the samples.

  • Compressive residual stresses were detected in different regions of all samples, with values exceeding − 2000 MPa for 60 and 120 min cycles. The sample oxynitrocarburized for 60 min showed higher values of compressive residual stresses.

  • The high compressive residual stress values and the surface microhardness are highly beneficial for improving the tribological properties of Catamold 316L steel produced by the MIM process.

  • Using ionic oxynitrocarburizing in salt baths presents an alternative to plasma processes for treating Catamold 316L steel, providing improved diffusion of nitrogen and carbon.