The effect of different powder mixtures used in the boriding process on the surface properties of AISI 304 stainless steel material

AISI 304 stainless steel, which is used in many areas such as chemistry, petrochemistry, storage tanks and food storage, attracts attention in terms of surface hardness and wear resistance, especially when its industrial applications are evaluated. In this study, it was aimed to improve the surface properties of the AISI 304 stainless steel material used as the substrate material. To develop the best surface properties, boriding layers of varying percentages were created. In order to create these layers, B4C, KBF4, SiC and graphite powders were compared using variable ratios. Microhardness and wear tests were performed on the borided samples and microstructure examinations were carried out using optical, SEM, XRD and EDX. It has been determined that the B4C used as boron source should not be less than 20% for the formation of the boriding layer and the double phase FeB/Fe2B. The powder mixture ratio with the highest thickness and hardness value of the boriding layer formed is the powder mixture with 20% B4C, 50% KBF4, 10% SiC and 20% graphite content. It was observed that the layer thickness increased by 63% and the hardness value increased by 11%. It was observed that this powder mixture gave the lowest wear rate compared to the other powder mixtures in the study. The difference between the highest and lowest wear rate is more than 3 times greater.


Introduction
Stainless steels is an engineering material used in applications where good cost, mechanical strength, wear, good plastic processability and corrosion resistance are required depending on operating conditions [1,2].AISI 304 stainless steel is one of the widely used chromium-nickel containing stainless steels due to its heat resistance, low temperature resistance, corrosion resistance and mechanical properties [3,4].AISI 304, which belongs to the class of austenitic stainless steels, is used in the energy, medicine, aviation, shipping, and nuclear industries because of the properties mentioned above [3,5,6].However, due to poor wear performance, different methods are applied to improve the wear properties of 304 stainless steel by hardening the surface with surface treatments [5].
Boriding is a thermochemical surface treatment generally used in the industry to improve the surface properties of materials and provide them with properties such as increased hardness, corrosion resistance, and wear resistance [7,8].This process has mostly been used to improve the tribological properties of ferrous, some non-ferrous and superalloy materials [7,[9][10][11].Industrial boriding can be applied to most steels such as structural steels (AISI 1014, 1045 and 4140) and austenitic stainless steels [12,13].The basic principle of protecting steels by forming boride and intermetallic coatings is the strong diffusion bonding of these coating materials to crystal structures and surfaces [14].By combining it with a substrate material, boron forms a single (Fe 2 B) or double (FeB+Fe 2 B) phase iron boride layer on the surface [15][16][17][18].Boron atoms can diffuse into iron alloys due to their relatively small size and mobile nature [7].Boridation of ferrous materials results in the formation of single (Fe 2 B) or double (FeB/Fe 2 B) layers with a well-defined composition [15,17,18].The boriding process is known for relatively rapid diffusion depending on the time and temperature and significantly higher mechanical properties than other thermochemical processes, and therefore it has technological importance [16,[19][20][21][22].The thickness of the boriding layer formed after the boriding process depends on the temperature, processing time, type of substrate material and boron potential conditions surrounding the material surface [23,24].For boron potential, patent-protected factors consisting mostly of 5% B 4 C, 5% KBF 4 and 90% SiC (commercial Ekabor ® ) are applied [25].
The main purpose of this study is to determine the optimum parameters that improve the mechanical properties of borided stainless steel, where different mixture powders are applied in different proportions instead of commercial boron powder mixtures used for boriding process.Naemchanthara al., Mu al. and Günen et al, used a commercial boronizing powder mixture in their studies.Günen et al performed boriding at 1000 °C for four hours.When a similar study was examined, the thickness obtained measured 33.53 μm and the hardness as 1.264 HV 0.05 , but the wear resistance was not examined.In the study conducted by Aichholz et al, wear resistance was examined, and the wear rate was found to be 1.2 [13,[26][27][28].In the experimental study, it was observed that the boriding layer thickness increased by approximately two times, the hardness value was more than 1800 HV 0.1 , and the powder mixture ratio decreased by approximately two times.Therefore, it is theorised that the boriding powder mixtures to be used for AISI 304 stainless steel in this study can be taken into consideration in determining the layer thickness, hardness and wear resistance on the material surface and can serve as a reference in obtaining the desired boriding surfaces.
The boriding process was applied to the AISI 304 stainless steel surface with six different powder mixtures consisting of B 4 C, KBF 4 , SiC, and graphite using boriding and box boriding technique.Optical microscope, scanning electron microscope (SEM), x-ray diffraction (XRD), boriding layer thickness, microhardness, and wear values of borided samples were examined.

Experımental study
The aim of this study was to improve the surface properties of AISI 304 stainless steel.The chemical components of AISI 304 used as a base material are given in table 1. Boriding layers of various thicknesses were created to develop the best surface properties.To determine the powder component that increases the boron potential in  the formation of boriding layers, B 4 C, KBF 4 , SiC, and graphite powder mixtures in different proportions (shown in table 1) were used.B 4 C was used as a boron source, KBF 4 as an activator, graphite as a reductant, and SiC as a deoxidizer.Figure 1 shows the schematic view of the test tube made of AISI 304 stainless steel.B 4 C, KBF 4 and graphite powders were put into the test tube using the ratios shown in table 2. AISI 304 stainless steel samples with rectangular geometry with dimensions of 15 mm × 10 mm were placed in the middle of the powder mixtures placed in the test tube.SiC powder used as deoxidizer was placed on the top of the tube and its lid was closed.The boriding process was applied at 1000 °C for four hours.
To determine the mechanical properties of samples borided using powder mixtures prepared in various proportions, microhardness measurements and wear tests were performed using pin-on-disk.Optical microscope, SEM, and energy dispersive x-ray spectroscopy (EDX) images were taken for microstructure examinations in all of the boriding processes.
In order to determine the mechanical properties of samples borided using powder mixtures prepared in various proportions; Microhardness measurements and wear tests were performed using pin-on-disk.Optical microscope, XRD, SEM and EDX images were taken for microstructure examinations in all boriding processes.
The formation of the boriding layer in the samples obtained as a result of the boriding process with the parameters laid out above were then analyzed.To examine the boriding layers and microstructure images, the surface was prepared metallographically by sanding with 60, 240, 400, 600, 800 and 1000 mesh sandpapers and polishing with 3 μm diamond solution.The prepared sample surfaces were etched with V2A etching reagent.In order to examine the effect between the thickness of the boriding layer and the stainless steel layer, microstructure images were taken with the Clemex program on a Nikon Eclipse L150 brand light microscope.A Jeol JSM 6060 scanning electron microscope (SEM) with energy dispersive x-ray spectroscopy (EDX) attachment was used to observe the resulting microstructure images.
A Rigaku SA-HF3 brand device was used to perform phase determination by XRD analysis.The Cu(Kα) rays formed within the scope of the analysis carried out with 40 kV and 20 mA have a wavelength of 1.5406 Å (λ = 1.5406Å); the measurements 20°-80°were chosen as the scanning area, and 1°/min chosen as the scanning speed.In order to examine the effect of borided samples on the hardness of boriding layers, micro hardness measurements were made at 10 μm from the surface of the sample sections towards the center on the Bulut Makina Microbul-1000D model device.
The obtained experimental samples were subjected to a dry environment wear test on a Nanovea MT/60/NI type pin-on-disk wear device under a 50 N load, at a sliding distance of 250 m.

Microstructural investigations
The FeB/Fe 2 B, Fe 2 B phases and the diffusion zone that form the boriding layer can be seen in figure 2. It has been observed that different powder mixtures cause differences in boriding layer thickness and morphology.When the images are examined, the formed FeB/Fe 2 B and Fe 2 B phases attract attention in the microstructure.
SEM images obtained in the back-scattered electron (BE) mode of borided samples using powder mixtures of different compositions are shown in figure 3. Figure 4 shows the SEM image and EDX analysis of the F sample, in which both phase structures, FeB/Fe 2 B and Fe 2 B, were obtained.E. Dokumaci et al emphasized in EDX analysis that the Fe 2 B phase, which is rich in iron (Fe), is formed and that during the formation of this phase, the  boron (B) element reacts with Fe and chromium (Cr) to form the boriding layer, and as the amount of boron increases, the FeB layer is formed [10].In the EDX analysis, Cr, nickel (Ni) and carbon (C) elements, which came from the matrix and formed compounds with B, were observed in addition to Fe.In the SEM images given in figure 3, when the morphology of the boriding layers obtained on 304 stainless steel samples boronized with A, B, C, D, E and F powder mixtures is examined, double-phase FeB/Fe 2 B layers are formed, while in the C powder mixture, only single-phase Fe 2 B is formed.The resulting layer thicknesses vary depending on the amount of boron in the powder mixture and its diffusion with other activating components.While the B 4 C ratio used as the boron source in the C powder mixture is 10%, it is 20% and above in other powder mixtures, and therefore it can be seen how the boron source is effective in phase formation.
The XRD graph of AISI 304 stainless steel samples, which are not boronized and boronized at 1000 °C for 4 h, is shown in figure 5

Layer thickness
Optical microscope examinations were carried out to determine the effect of powder mixtures on the thickness of the boriding layer.The thickness of the observed boron layers was measured separately for seven different FeB+Fe 2 B and Fe 2 B phases from the surface to the center, and the average value was taken.The average boriding layer thicknesses obtained for different parameters can be seen in figure 6.It has been observed that powder mixing ratios have a significant effect on the thickness of the boriding layer.
When the boriding layer thicknesses of the samples obtained with different powder mixtures were compared, it was observed that different layer thicknesses were achieved.Double-phase (FeB/Fe 2 B) and singlephase (Fe 2 B) layer thicknesses formed in boriding layers were compared.In this comparison, the highest layer thicknesses for both phases belonged to the F powder mixture (20% B 4 C, 50% KBF 4 , 10% SiC and 20% graphite), as seen in figure 5. Fe 2 B layer thickness was measured as 70.13 μm and FeB/Fe 2 B as 43.558 μm.
In the C powder mixture containing only 10% B 4 C, 40% KBF 4 , 30% SiC and 20% graphite, no FeB/Fe 2 B double phase was formed and the Fe 2 B layer thickness was the lowest, at 32,587 μm.The reason why the FeB/Fe 2 B double phase does not form in the C powder mixture is thought to be that the B 4 C powder ratio used as the boron source is not sufficient.
Compared to the literature, it was observed that the boriding layer thickness was measured as 41-43 μm at most in applications made using standard boriding mixture powder on 304 stainless steel [7,10,13,34,35], whereas in this study, it was observed that the layer thickness could be obtained at higher values.

Hardness
The changes in the hardness profiles obtained from the borided surface towards the center for different powder mixtures are given comparatively in figure 7. Measurements could not be taken at a distance of up to the first 10 μm due to the fragile structure of the surface.The hardness value of AISI 304 stainless steel before boriding was measured at approximately 210 HV 0.1 .When the hardness values of the samples were compared after boriding with A, B, C, D, E and F mixtures separately for 4 h at 1000 °C, the highest value was found in the F powder mixture (20% B 4 C, 50% KBF 4 , 10% SiC, and 20% graphite).In sample F, the boriding layer was calculated to be 1994 HV 0.1 in the FeB phase on the outermost surface.It has been stated in the literature that the austenitic stainless steel AISI 304, which is the base material, has a near-surface hardness of the boron layer higher than 1800 HV [13,34,35].Accordingly, it is seen that the surface hardness of the AISI 304 stainless steel used as a base material on the sample surfaces after the boriding process increases and the hardness value forms a decreasing curve from the surface to the center.Therefore, the boriding process not only increases the hardness value of the substrate material, but also the powder mixtures used are effective in increasing the hardness value.[34,36].
During the wear tests, friction coefficients were recorded.Figure 9 shows the comparison of the post-wear friction coefficients of borided samples obtained with different powder mixtures.When the friction coefficients are compared, the friction coefficient of sample A increased from 0.18 to 0.26.In sample B, while it was 0.18 at the beginning of the test, it increased at a constant rate to reach 0.29.The friction coefficient of sample C remained constant from 0.15 to 0.23.Sample D gave a curve that increased from 0.15 to 0.24, and E sample increased from 0.15 to 0.74, and the F sample increased from 0.14 to 0.24.When the wear rates and friction coefficients were examined, it was seen that the wear resistance of the boriding surface obtained with the F powder mixture was the highest compared to other powder mixtures.
As the movement of the abrasive tip on the sample surface becomes more difficult, the friction coefficient increases.It causes deformation and wear loss due to the impact of the abrasive tip on the sample surface.Lubricants are used to reduce friction between the two surfaces, but previous studies have shown that boride layers have self-lubricating properties [27].In another studies, the presence of B 4 C and SiC hard particles in powder mixtures increases the friction coefficients as the surfaces lose their natural slickness.It affected the friction coefficients of powders containing different proportions of B 4 C and SiC [37,38].Friction coefficients were consistent with wear volume losses.
The SEM images taken after the abrasion test as it shows in figure 10 applied with a pin-on-disk to the borided samples with different powder parameters (A, B, C, D, E and F).When the images were examined, wear marks on samples A, E and F could be seen.It was observed that the wear marks of the boronized samples obtained from F powder mixtures were less than the wear marks of the samples obtained with A and E powder mixtures.Wear residues formed on the wear surfaces of borided samples obtained from B, C and D powder mixtures.For this reason, the friction coefficient remained at a constant value for a while during the test.It is known that these residues are caused by the material breaking away from the surface during wear, and the same situation was encountered in similar studies [28].
The entire study was evaluated; Boriding was done on AISI 304 stainless steel using B 4 C, KBF 4 , SiC and graphite powders at 1000 °C during 4 h.Powder mixtures in 6 different ratios were prepared for the boriding process.Mechanical tests obtained as a result of experimental studies and comparisons of their results are show in table 3.

Conclusion
As a result of the study, the effects of boriding applied to AISI 304 stainless steel at 1000 °C for four hours on the morphology, hardness, and wear resistance of the boriding layer were examined.The main results can be summarized as follows: 1.The boriding layer on AISI 304 stainless steel is obtained with a smooth and compact morphology by using boriding powder mixtures containing B 4 C, KBF 4 , SiC, and graphite prepared in six different ratios.It was confirmed by EDX and XRD studies that the dominant phases were FeB and Fe 2 B. It has been shown that in addition to these phases, there are also CrB and Ni 3 B phases.
2. It has been determined that the powder mixture composition that provides the highest layer thickness and hardness values among the powder mixtures used in the boriding process is the F mixture with 20% B 4 C, 50% KBF 4 , 10% SiC, and 20% graphite content.In order for the double phase to form in the boriding layer, B 4 C used as boron source must be present in the mixture at least 20%.The highest layer thickness was 70.13 μm and the hardness value was 1994.1 HV 0.1 .
3. When the wear rates are compared, the highest wear rate was obtained as 3,38 in the E powder mixture (20% B 4 C, 40% KBF 4 , 30% SiC, and 10% graphite), while the lowest wear rate obtained was the F powder mixture (20% B 4 C, 50% KBF 4 , 10% SiC, and 20% graphite), which measured at 0.77.In addition, the highest friction coefficient was obtained in the E powder mixture, while the friction coefficient of the F mixture was obtained at the lowest value.
4. It has been observed that increasing the rate of KBF 4 in the boriding powder mixture, the B 4 C compound being at least 20% (wt), and the presence of the SiC compound used as a deoxidizer at the rate of 10% (wt) increases the coating thickness, hardness, and wear resistance.

Figure 1 .
Figure 1.Schematic view of the mechanism used in the experimental study.

Figure 4 .
Figure 4. (a) SEM image of the FeB/Fe 2 B and Fe 2 B phases formed in samples boronized using different powder mixtures, (b) EDX analysis of the FeB/Fe 2 B phase and (c) EDX analysis of the Fe 2 B phase.

Figure 5 .
Figure 5. XRD graph of AISI 304 stainless steel boronized with powder mixtures A, B, C, D, E and F.
. The XRD results, which are compatible with microstructural observations and EDX analyses, show that the prominent phases in the boride layer are FeB and Fe 2 B. It also shows that there are CrB and Ni 3 B phases due to the amount of chromium and nickel content in the alloy.It was stated by Khenifer et al that Cr and Ni alloy elements, which are found in high amounts in the AISI 304 substrate material, lead to the formation of significant chromium and nickel borides in boridized steels.Active boron reacts with Cr and Ni to form boride. Turkoglu et al stated that Cr and Ni atoms are directed towards the bottom of the boride layer due to their low solubility in the FeB and Fe 2 B phases.The results obtained were consistent with the literature [7, 13, 27, 29-33].

Figure 6 .
Figure 6.Thickness measurements of FeB/Fe 2 B and FeB layers formed in boronized samples using different powder mixtures (a) Mixture A, (b) Mixture B, (c) Mixture C, (d) Mixture D, (e) Mixture E, (f) Mixture F.

Figure 7 .
Figure 7.Comparison of hardness values from surface to center of boronized samples using different powder mixtures (a) Mixture A, (b) Mixture B, (c) Mixture C, (d) Mixture D, (e) Mixture E, (f) Mixture F.

Figure 8 .
Figure 8.Comparison of wear rates of 304 stainless steel samples subjected to boriding treatment at 1000 °C for 4 h using different powder mixtures.

Figure 9 .
Figure 9.Comparison of friction coefficients of 304 stainless steel samples subjected to boriding treatment at 1000 °C for 4 h using different powder mixtures.

Figure 10 .
Figure 10.Post-wear SEM images of 304 stainless steel samples subjected to boriding treatment at 1000 °C for 4 h using different powder mixtures (a) Powder mixture A, (b) Powder mixture B, (c) Powder mixture C, (d) Powder mixture D, (e) E powder mixture, (f) F powder mixture.

Table 1 .
Chemical components of AISI 304 stainless steel.

Table 2 .
Weight (%) ratios of mixture powders used in the boriding process.

Table 3 .
Powder mixture ratios and comparative results of the boriding process applied to AISI 304 Stainless steel.The weight loss data obtained after wear was used to calculate the relative wear rate.The wear rate results are show in figure 8.The powder mixing ratios and wear rates are, respectively; A, 2.1 mm −3 /m, B, 2.41 mm −3 /m, C, 2.1 mm −3 /m, D, 1.49 mm −3 /m m, E, 3.38 mm −3 /m, and F, 0.77 mm −3 /m.When the test results were analyzed, the lowest wear rate was found to be in powder mixture F (20% B 4 C, 50% KBF 4 , 10% SiC, and 20% graphite), and the highest wear rate was found in powder mixture B (20% B 4 C, 60% KBF 4 , 0% SiC, and 20% graphite).It was determined that the FeB phase has a more brittle structure than the Fe 2 B phase and reduces the wear resistance, while the wear resistance of the Fe 2 B phase is higher.Similar results were obtained in the studies conducted by Çetin et al, and Alias et al