Laser Surface Alloying of Sintered Stainless Steel

A characteristic feature of sintered stainless steel (SSS) is its porosity. Porosity results in a lower density of steel, making attractive components for producing lightweight structures and materials used in industry (e.g., the automotive industry or aerospace). Scientists also observe that porosity adversely affects steel’s properties, especially its strength properties. One of the proposals for improving the discussed properties is the use of surface treatment of sintered stainless steels, e.g., with the use of concentrated energy sources such as plasma beams or laser beams. However, this proposal is an incidental subject of research, which is not justified from the point of view of the obtained research results presented by a few research groups. In this study, the surface modification (surface treatment) of sintered stainless steel was presented. The authors proposed the use of two surface treatments in order to compare them and obtain the best results. The first treatment was the deposit of Cr3C2–NiCr coatings on SSS surfaces using the atmospheric plasma spraying (APS) method. The second treatment was to create surface layers on SSSs by laser alloying the surface with a CO2 laser. Due to high precision and ease of automation, the most common methods in surface alloying treatment are laser technologies. This research’s main aim was to analyze the microstructure and strength properties of the SSS surface layer. The research confirms that applying the Cr3C2–NiCr coating and modifying the surface layer through the laser alloying method improves the mechanical properties of SSSs.


Introduction
Currently, the most dynamically developing group of engineering materials are sintered stainless steels produced using powder metallurgy technology (PM technology) [1][2][3][4]. The possibility of obtaining a two-phase structure with different proportions of austenite and ferrite, being the two basic structural components, is a valuable advantage of sintered stainless steels [5][6][7]. Lailatul P. H. and Maleque M. A. subjected [8] duplex stainless steel to an appropriate modification to make this material suitable for tribological and hightemperature applications. For this purpose, surface modification using fine SiC powder preplacement by the TIG torch technique has been performed. In turn, we showed in [9][10][11] the advantages of modifying sintered duplex steels by alloying the surface of the steel with Cr 3 C 2 powder. Oke S. et al. reviewed [12] the impact of spark plasma sintering process parameters on microstructure, phase evolution, and mechanical properties of reinforced stainless steels. Attention was paid to the influence of alloying elements and ceramic reinforcements, their dispersion into the stainless steel matrix, and the importance of the matrixreinforcement interface. A combination of properties (e.g., very good mechanical strength was found that the created coatings and layers had a positive effect on the improvement of the mechanical properties of sintered stainless steels. The geometric structure of the resulting modified surface was subjected to a detailed analysis because they are considered to largely influence several phenomena that take place within the analyzed layer [44][45][46].

Materials
The water-atomized powders were used to obtain the multiphase stainless sinters. The powders were manufactured by Höganäs (Höganäs Municipality, Sweden) from AISI 316L steel and AISI 409L steel. The chemical composition of the steel powders is shown in Table 1. For both types of powder, the nominal particle size was 150 µm. In the investigation, the powders were mixed in three different proportions of ferritic and austenitic steel powders. Details of the proportion are presented in Table 2. The sintering of samples was carried out at the temperature of 1250 • C for 30 min. Then they were cooled down at a cooling rate of 0.5 • C/s. The powders used were compacted uniaxially with the addition of 1% Acrawax C lubricant at 720 MPa. The entire process was carried out in a reducing atmosphere with the use of 100% hydrogen. This approach significantly reduces the oxidation of the batch and prevents an undesirable reduction in chromium content.

Surface Treatment Techniques
In order to improve the functional properties, the surfaces of multiphase sintered stainless steels were treated with the use of concentrated energy sources: plasma and laser. Plasma spraying (APS) was used to apply a coating modifying the surface layer. Chromium carbide, previously mechanically mixed in a proportion of 75% wt. Cr 3 C 2 and 25% NiCr, was selected as the coating material. This coating was prepared at the following parameters: Voltage: Ur = 50 V, Current intensity: Ir = 45 A, Distance of the plasmatron from the surface: 120 mm, Plasma forming gases flow: argon~45 L/min and hydrogen~12 L/min. The resulting coating had a thickness of around 60 µm.
The laser alloying (LSA) of sintered stainless steel was carried out at the Laser Center of the Kielce University of Technology Laboratory. To produce surface layers, pulsed laser alloying was carried out by a TRUMPF Lasercell 1005 (Ditzingen, Germany). The CO 2 laser generator has a power of 3800 W. The stand has a modern gas installation with a mixer for shielding gases. The control system is based on a Siemens Sinumerik 840D postprocessor (München, Germany). Table 3 presents the parameters chosen experimentally for the laser alloying process. The laser alloying parameters were established in accordance with the guidelines of the manufacturer of the laser device and the previous experience of the authors. The laser treatment was performed in an atmosphere of argon as a protective gas. The tests used a focusing head. The TEM 00 beam defined the radiation energy distribution.

Macro Nad Microstructure Analysis
After applying Cr 3 C 2 -NiCr powder and the surface alloying process, sintered stainless steels were subjected to macroscopic evaluation using the Olympus SZ61 stereomicroscope (Shinjuku, Japan). The microstructure was analyzed using the light optical microscope Olympus GX41 and scanning electron microscope Jeol JSM-6610LV (Akishima, Japan).

Chemical and Phase Composition Analysis
The chemical composition analysis was carried out by scanning electron microscope Jeol JSM-6610LV.

Surface Geometric Structure and Microhardness Analysis
Surface geometric structure (SGS) and roughness were tested at the Kielce University of Technology and, more specifically, in its Laboratory of Computer Measurements of Geometric Quantities. The optical profiler Talysurf CCI was used for the measurements. It applies the patented algorithm of coherence correlation by Taylor Hobson. Thus, 10 pm axial resolution is attained. The area undergoing testing was 1.65 × 1.65 mm 2 , while the horizontal resolution was 1.65 × 1.65 µm 2 . TalyMap Platinium (version 6.2) software was applied to analyze the surface 3-D images so that the SGS of the layers could be determined. The measurement of SSS microhardness was carried out on a Shimadzu HMV-G Series tester using the Vickers methodology (with a load of 980.7 mN).

Statistical Analysis
From a statistical point of view, the process is described by two explanatory variables, one of which gives the alloying speed and the second of which gives the proportions of the sinter composition. The sinter is a two-component mixture of AISI316L and AISI409L steels, the percentages of which must, in each case, sum up to 100%. For statistical analysis purposes, the share of AISI316L steel was adopted as the related lambda explanatory variable, and thus the share of AISI409L steel is described as (1-lambda). The alloying speed was adopted as the rate explanatory variable. Due to the nature of the available dataset, it is possible to assess the impact of the explanatory variables using a two-way analysis of variance (ANOVA) using a linear model [29,47]. The MINITAB 20.1.3 program was used to analyze and create the related graphs.  horizontal resolution was 1.65 × 1.65 μm 2 . TalyMap Platinium (version 6.2) software was applied to analyze the surface 3-D images so that the SGS of the layers could be determined. The measurement of SSS microhardness was carried out on a Shimadzu HMV-G Series tester using the Vickers methodology (with a load of 980.7 mN).

Statistical Analysis
From a statistical point of view, the process is described by two explanatory variables, one of which gives the alloying speed and the second of which gives the proportions of the sinter composition. The sinter is a two-component mixture of AISI316L and AISI409L steels, the percentages of which must, in each case, sum up to 100%. For statistical analysis purposes, the share of AISI316L steel was adopted as the related lambda explanatory variable, and thus the share of AISI409L steel is described as (1-lambda). The alloying speed was adopted as the rate explanatory variable. Due to the nature of the available dataset, it is possible to assess the impact of the explanatory variables using a two-way analysis of variance (ANOVA) using a linear model [29,47]. The MINITAB 20.1.3 program was used to analyze and create the related graphs.  The surface of the Cr3C2-NiCr coating (obtained by scanning electron microscopy) before the laser alloying process is shown in Figure 2. The surface of the Cr 3 C 2 -NiCr coating (obtained by scanning electron microscopy) before the laser alloying process is shown in Figure 2.  Figure 3 shows a cross-section of the microstructure of the Cr3C2-NiCr coating (the image from Jeol JSM-6610LV scanning microscope). The thickness of the resulting chromium carbide coating was approx. 60 μm.     Structural elements typical of the chosen deposition method (APS) can be observed in the results obtained for this coating: porous lamellar structures with cracks and nonmelted particles.

Microstructure of Powder, Coating (Cr3C2-NiCr) and Laser Alloyed Tracks
The impact of laser alloying on the quality of the steel surface was assessed by macroscopic tests. The main criteria for evaluation were the continuity of the band, comparable width, and relatively smooth surface without craters. Figures 4-6 present the surface morphology of the laser-alloyed tracks after the laser alloying process for SSSs with Cr3C2-NiCr coating obtained by an Olympus SZ61 stereo microscope. Structural elements typical of the chosen deposition method (APS) can be observed in the results obtained for this coating: porous lamellar structures with cracks and nonmelted particles.
The impact of laser alloying on the quality of the steel surface was assessed by macroscopic tests. The main criteria for evaluation were the continuity of the band, comparable width, and relatively smooth surface without craters. Figures 4-6 present the surface morphology of the laser-alloyed tracks after the laser alloying process for SSSs with Cr 3 C 2 -NiCr coating obtained by an Olympus SZ61 stereo microscope.      The metallographic section was prepared, which was etched with copper sulfate solution. Figures 7-9 show a fragment of the microstructure of the laser stitch obtained after the laser alloying treatment of the Cr3C2-NiCr coating (the image was obtained using the Olympus GX41 optical microscope).    The metallographic section was prepared, which was etched with copper sulfate solution. Figures 7-9 show a fragment of the microstructure of the laser stitch obtained after the laser alloying treatment of the Cr3C2-NiCr coating (the image was obtained using the Olympus GX41 optical microscope). The metallographic section was prepared, which was etched with copper sulfate solution. Figures 7-9 show a fragment of the microstructure of the laser stitch obtained after the laser alloying treatment of the Cr 3 C 2 -NiCr coating (the image was obtained using the Olympus GX41 optical microscope).  In the surface layers, the microstructural examinations of the cross-section showed a homogeneous cellular-dendritic structure. This structure was formed after alloying treatment. Columnar crystals, oriented in the direction of heat transfer, were also observed in the structure. As a result of contact of the alloyed layer with the basic material, a transient zone was created, in which nucleation and an increase in the primary structure crystals occurred. Table 4 presents the results of the laser alloying process considering the width and depth of the cross-section.          An example of surface microstructures after the laser alloying process (laser power: 3 kW and speed laser: 1 m/min) for SSS with Cr 3 C 2 -NiCr coating obtained by the Jeol JSM-6610LV scanning microscope is presented in Figure 10. An example of surface microstructures after the laser alloying process (laser power: 3 kW and speed laser: 1 m/min) for SSS with Cr3C2-NiCr coating obtained by the Jeol JSM-6610LV scanning microscope is presented in Figure 10.

Chemical and Phase Composition Analysis
Tables 5-7 present an analysis of the chemical composition of the surface layers in SSSs with Cr 3 C 2 -NiCr coating after laser alloying. The obtained results made it possible to evaluate three aspects: comparing the surface after treatment, determining the migration of alloying elements during the crystallization process, and determining the chemical composition homogeneity in the surface layers. Table 5. EDX-analysis after laser alloying (AZ-alloying zone, HAZ-heat-affected zone, NM-native material) for sample 1.  Table 7. EDX-analysis after laser alloying (AZ-alloying zone, HAZ-heat-affected zone, NM-native material) for sample 3. The line mapping analysis in Figures 11-13 illustrated the elemental migration across the cross-section after the laser alloying process (laser power: 3 kW and speed laser: 1 m/min) for SSS with Cr 3 C 2 -NiCr coating.

Parameters of Laser
To determine the phase composition on the laser-alloyed surface layer, an X-ray phase analysis was performed. Results of the identification of the phase composition for samples 1, 2, and 3 are presented in Figure 14. The phase analysis for individual specimens showed the austenitic and ferritic phases with the content proportional to the powders used. The line mapping analysis in Figures 11-13 illustrated the elemental migration across the cross-section after the laser alloying process (laser power: 3 kW and speed laser: 1 m/min) for SSS with Cr3C2-NiCr coating. Figure 11. Line mapping analysis EDS on cross-section after laser alloying process for sample 1. Figure 11. Line mapping analysis EDS on cross-section after laser alloying process for sample 1. The line mapping analysis in Figures 11-13 illustrated the elemental migration across the cross-section after the laser alloying process (laser power: 3 kW and speed laser: 1 m/min) for SSS with Cr3C2-NiCr coating. Figure 11. Line mapping analysis EDS on cross-section after laser alloying process for sample 1.    To determine the phase composition on the laser-alloyed surface layer, an X-ray phase analysis was performed. Results of the identification of the phase composition for samples 1, 2, and 3 are presented in Figure 14. The phase analysis for individual specimens showed the austenitic and ferritic phases with the content proportional to the powders used.

Surface Geometric Analysis
The exemplary Figure 15a-c shows the topography images taken on samples that underwent laser alloying, while Figure 15d presents the topography of the Cr3C2-NiCr surface layer.

Surface Geometric Analysis
The exemplary Figure 15a-c shows the topography images taken on samples that underwent laser alloying, while Figure 15d presents the topography of the Cr 3 C 2 -NiCr surface layer.

Surface Geometric Analysis
The exemplary Figure 15a-c shows the topography images taken on samples that underwent laser alloying, while Figure 15d presents the topography of the Cr3C2-NiCr surface layer. Tables 8-11 summarize the most crucial SGS parameters obtained for the studied specimens.  Tables 8-11 summarize the most crucial SGS parameters obtained for the studied specimens. Sa (µm) 6.01 Table 9. Parameters of the SGS after laser alloying (sample 1). The sintered stainless steel samples on which the coatings were applied had the value of the Sa parameter in the range of 4.42-17.72 µm. It is a basic amplitude parameter used in the quantitative assessment to determine the considered elements' physical condition. Treatment by laser led to the reduction of the Sa parameter of the Cr 3 C 2 -NiCr layer after laser alloying (sample 1), which proves that we were dealing with laser smoothing (reduction of the height of the peaks). On the other hand, laser treatment of the Cr 3 C 2 -NiCr layers (samples 2 and 3) elevated the arithmetic mean values of the surface height Sa by 1.5 to 3 times. A tendency of the same nature could be seen in the case of the mean square surface height Sq as well as of the other parameters, such as the maximal values of peak height (Sp), pit height (Sv), and height (Sz). Supplementary information on the shape of the surface of the tested samples before and after laser treatment can be obtained from the amplitude parameters of skewness coefficient (asymmetry, Sku) as well as concentration coefficient (kurtosis, Ssk). The parameters mentioned above enable the detection of the local elevations and depressions on the surface, as well as any defects (e.g., scratches, delamination). The parameter of the Cr 3 C 2 -NiCr coating after laser alloying (sample 3) had negative values of the Ssk parameter, proving that numerous peaks and depressions are present on the layer. Positive asymmetry Ssk (skewness) values for the coating after laser treatment provide information that we are dealing with a smooth surface devoid of elevations in the shape of a plateau.

SGS Parameters Speed of Movement (m/min)
The surface inclination values proved to be close to one another. They were all within 2.75÷8.67 for Sku. Thus, the ordinates' distribution was close to the normal distribution function for all the specimens.
Before the treatment by laser, the Cr 3 C 2 -NiCr coating was characterized by a random isotropic structure (Iz = 92.61%). Laser alloying led to the creation of periodic structures in an area between anisotropic and isotropic structures (Iz = 42.30÷75.69%).

Microhardness
The assessment of mechanical properties was made based on hardness measurements by the Vickers method. Table 12 presents the hardness of the Cr 3 C 2 -NiCr coating, alloying zone, heat-affected zone, and native materials obtained after laser alloying. The results represent the mean of three measurements.  Figure 16 shows a summary of the results of hardness measurements to all samples after laser alloying with Cr 3 C 2 -NiCr coating obtained by the Vickers method.    Two-way ANOVA with a linear model for two explanatory variables, lambda (share of AISI316L) and rate (rate of laser alloying process), was applied to the measured hardness of AZ (Table 13) and hardness of HAZ values (Table 14). The plots of the marginal means are provided in Figure 17 (AZ) and Figure 18 (HAZ), respectively.

Parameters of Laser
The analysis clearly showed that the effect of alloying speed is negligible (statistically insignificant), and the composition of the sinter explains the variability of hardness in over 95% (AZ) and over 99% (HAZ). Other uncontrolled factors impact less than 5% (AZ) and less than 1% (HAZ).  Two-way ANOVA with a linear model for two explanatory variables, lambda (share of AISI316L) and rate (rate of laser alloying process), was applied to the measured hardness of AZ (Table 13) and hardness of HAZ values (Table 14). The plots of the marginal means are provided in Figure 17 (AZ) and Figure 18 (HAZ), respectively. The analysis clearly showed that the effect of alloying speed is negligible (statistically insignificant), and the composition of the sinter explains the variability of hardness in over 95% (AZ) and over 99% (HAZ). Other uncontrolled factors impact less than 5% (AZ) and less than 1% (HAZ).
The application of the Cr 3 C 2 -NiCr coating and the laser alloying method resulted in the homogenization of the microstructure. This has been confirmed by both microstructural and hardness testing. It has been observed that the preparation of Cr 3 C 2 -NiCr coating on sintered stainless steels led to over 5 times higher hardness and consequently improved the mechanical properties of SSSs. In addition, the use of the laser alloying method improved the properties, as shown by the higher hardness values measured in the alloying zone. The highest hardness values for the alloying zone were obtained for sample 1 (Table 12 and  The application of the Cr3C2-NiCr coating and the laser alloying method resulted in the homogenization of the microstructure. This has been confirmed by both microstructural and hardness testing. It has been observed that the preparation of Cr3C2-NiCr coating on sintered stainless steels led to over 5 times higher hardness and consequently improved the mechanical properties of SSSs. In addition, the use of the laser alloying method  The application of the Cr3C2-NiCr coating and the laser alloying method resulted in the homogenization of the microstructure. This has been confirmed by both microstructural and hardness testing. It has been observed that the preparation of Cr3C2-NiCr coating on sintered stainless steels led to over 5 times higher hardness and consequently im-

Conclusions
It is essential and challenging to select the appropriate coating and associated laser control parameters so that the alloying of the surface layer improves the functional properties of sintered stainless steels. This article presents the strength properties of SSSs and the characteristics of their structure and surface roughness after laser alloying. The research confirms that by applying the Cr 3 C 2 -NiCr coating and modifying the surface layer through the laser alloying method, the mechanical properties of SSSs are improved. The following conclusions were made: Appropriate laser modification of surface SSSs allows for the homogenization of the structure (homogeneous cellular-dendritic structure), which improves the strength properties (reduction of steel porosity).
The laser treatment of SSSs smoothed the surface of the sintered stainless steel, which is confirmed by the positive values of the surface asymmetry Ssk (skewness) for the coating.
The results obtained in the examination showed that using a Cr 3 C 2 -NiCr coating and laser alloying method causes a hardness increase, resulting in increased wear resistance.
The best hardness values for the alloying zone were obtained for sample 1, in which the contribution of the ferritic phase was the highest (20% 316L + 80% 409L). The laser alloying method used by the authors is a promising proposal for hardening a surface layer of SSSs.
The surface geometric structure parameters of the surface of the Cr 3 C 2 -NiCr coatings after laser alloying are 1.5 to 3 times greater than that of the coatings without this treatment.
The modification proposed in the article allows for broader use of these modern materials. Further tests should include measurements of internal stresses and tests of resistance to erosion of plasma-sprayed coatings before and after laser alloying.