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Article

Effects of Initial Microstructure on the Low-Temperature Plasma Nitriding of Ferritic Stainless Steel

by
Lingze Li
,
Ruiliang Liu
*,
Quanli Liu
,
Zhaojie Wu
,
Xianglong Meng
and
Yulan Fang
Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(10), 1404; https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12101404
Submission received: 31 August 2022 / Revised: 18 September 2022 / Accepted: 22 September 2022 / Published: 27 September 2022
(This article belongs to the Special Issue Surface Thermal Diffusion Treatment on Metallic Materials)
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Abstract

:
AISI 430 ferritic stainless steel with different initial microstructures was low-temperature plasma nitrided to improve its hardness and wear resistance in the present investigation. The microstructure and properties of the low-temperature nitrided layers on stainless steel with different initial microstructures were studied by an optical microscope, X-ray diffractometer, scanning electron microscope, microhardness tester, pin-on-disk tribometer, and electrochemical workstation. The results show that the low-temperature nitrided layer characteristics of ferritic stainless steel are highly initial-microstructure dependent. For the ferritic stainless steel with a solid solution and annealing treatment, it had the best performance after low-temperature plasma nitriding when compared with the stainless steel with other initial microstructures. The nitrided layer thickness reached 34 μm after nitriding at 450 °C for 8 h. The phase composition of the low-temperature-nitrided layer consisted mainly of a nitrogen “expanded” α phase (αN) and iron nitrides (Fe4N and Fe2–3N). The hardness of the nitrided layer could reach up to 1832 HV0.1. Moreover, the wear and corrosion resistance of the nitrided layer on the solution and annealing treated ferritic stainless steel could be improved at the same time.

1. Introduction

Ferritic stainless steel is a kind of stainless steel with a ferrite (α-Fe phase) microstructure, where the chromium content is 15~30 wt.%, with a body-centered cubic crystal structure. This kind of stainless steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, and Nb; therefore, it is an inexpensive and eco-friendly material. Ferritic stainless steel possesses the characteristics of high thermal conductivity, a small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance and so on [1,2]. AISI 430 ferritic stainless steel is preferred in many applications, including in the automotive, chemical and food industries, such as for fuel burners, screw nuts, household appliances and appliance parts [3]. However, the low hardness and wear resistance of ferritic stainless steel has limited its usage for wear-related applications. In order to improve the performance of ferrite stainless steel, it is often necessary to regulate the overall microstructure or conduct surface modifications.
Hot rolling [4] and heat treatments [5,6] are common methods for controlling the microstructure and properties of ferritic stainless steel. Hot rolling can improve the processing performance of ferritic stainless steel, and reduce or eliminate casting defects. The heat treatment of ferritic stainless steel mainly includes a solid solution treatment and an annealing treatment. A solid solution treatment can eliminate the residual secondary phase in ferritic stainless steel and eliminate brittleness at 475 °C. An annealing treatment can make the microstructure of ferritic stainless steel uniform and eliminate its high temperature embrittlement and intergranular corrosion after a high temperature treatment. Lu et al. studied the dissolution behavior of the σ phase of ferritic stainless steel after a solution treatment at 1000–1100 °C. It was found that the σ phase completely dissolved after the solution treatment, which improved the properties of the ferritic stainless steel [5]. Li et al. studied the effect of an annealing treatment on the microstructure, mechanical properties and corrosion resistance of X2CrNi12 ferritic stainless steel and found that the comprehensive performance was the best after annealing at 850 °C [6].
There are many methods that can be used for the surface modification of ferritic stainless steel, including laser melting [7], physical vapor deposition [8], plasma spraying [9] and a thermochemical heat treatment [10,11,12]. Additionally, carburizing [10,11] or nitriding treatments [12] can significantly improve the surface properties of steel. Among them, gas nitriding [13,14] and plasma nitriding [15,16,17,18,19] are widely used for the surface modification of ferritic stainless steel based on their comprehensive advantages. Compared with conventional nitriding, plasma nitriding does not rely on a chemical reaction but uses ionized nitrogen-containing gas for the nitriding, which has a wider temperature range and a higher nitriding rate [20,21]. However, common plasma nitriding (at 520–550 °C) can generally improve the wear resistance of stainless steel, but its corrosion resistance will be deteriorated.
Excitingly, studies have found that the plasma nitriding of stainless steel at lower temperatures (<460 °C) results in a high hardness and wear resistance. More importantly, the corrosion resistance of stainless steel will not be affected or it may even be improved [15,16,17,18,19]. However, the above rules for the low-temperature-nitriding of ferritic stainless steel do not always prove to be true. Some research has found that when ferritic stainless steel is plasma-nitrided below 400 °C, the wear and corrosion resistance of the ferritic stainless steel can be improved simultaneously. For example, Bruna et al. [15] investigated the low-temperature-nitriding of super ferritic stainless steel by plasma immersion ion nitriding. It was found that the corrosion resistance and hardness of the samples were improved; however, the thickness of the low-temperature-nitrided layer was only 2.2 μm. Some studies found that there were many cracks formed on a low-temperature nitrided layer of ferritic stainless steel [16,17,18,19]. For example, Elieser et al. [16] conducted a low-temperature plasma nitriding study on AISI 470 super-ferritic stainless steel. They found that the low-temperature plasma nitriding treatment could improve the wear resistance of the steels, but a large number of cracks appeared in the nitrided layer, which apparently influenced the properties of the low-temperature nitrided layer. Additionally, some studies have shown that the corrosion resistance of ferritic stainless steel will be deteriorated even when the nitriding treatments are conducted under a low-temperature zone (<460 °C). For example, David et al. [17] conducted low-temperature nitriding of X12Cr13 ferritic stainless steel. They found that after low-temperature nitriding at 400 °C, the hardness of the sample was significantly improved, but there were many cracks existing in the nitrided layer, and the corrosion resistance of the samples was reduced. Alphonsa et al. [18] found that the plasma nitriding of AISI 430F ferritic stainless steel at 450 °C showed an obvious existence of CrN phases in the nitrided layer. The corrosion resistance of the stainless steel plasma-nitrided at 450 °C deteriorated even when was in the low-temperature zone. Nii et al. [19] found that after low-temperature nitriding at 450 °C, the wear resistance of SUS 430 ferritic stainless steel apparently improved, but the nitrided layer was non-uniform, and the pitting corrosion resistance was decreased. Why were deteriorated corrosion resistances of the low-temperature nitrided layers on ferritic stainless steels frequently found by researchers? Does this mean that low-temperature nitriding is not suitable for ferritic stainless steel?
As can be found, some studies have proved that the original microstructure of the steel has a big influence on the nitrided layer [22,23]. Kochmansk et al. [22] treated 17-4PH stainless steel, firstly, with a heat treatment and then a nitriding treatment. They found that the heat treatment had an evident effect on the thickness and hardness of the nitrided layer. For the sample heat treated at higher temperatures, the nitrided layer became thicker, and the hardness increased up to 1800 HV0.1. Jeong et al. [23] found that the preheating of pure titanium before nitriding can reduce the nitriding hardening time of the sample, enhance the surface hardness of the sample, and improve the wear resistance of the sample.
Based on above background, the present work aims to solve the engineering technical problems of the cracks’ existence and corrosion resistance deterioration of a low-temperature nitrided layer on ferritic stainless steel. The microstructure and phase composition of a low-temperature nitrided layer on ferritic stainless steel can be affected by its initial microstructure. Then, the wear resistance and corrosion resistance of ferritic stainless steel can be improved at the same time by an innovative technology using a preheating treatment and low-temperature plasma nitriding. Consequently, the initial microstructure effects of AISI 430 ferritic stainless steel on the microstructure, hardness, wear resistance, and corrosion resistance of a low-temperature plasma nitrided layer were studied systematically.

2. Materials and Methods

2.1. Materials

The material used in this investigation was a AISI 430 ferritic stainless steel, of which the chemical composition tested by EDS is shown in Table 1.

2.2. Pre-Treatment

Before the low-temperature plasma nitriding, the delivered AISI 430 ferritic stainless steel was pre-treated by solid solution treatments and annealing treatments. The solid solution treatments were conducted at 1080, 1120, 1160, 1200 and 1240 °C for 1 h, with oil cooling, and the corresponding samples were named as S1080, S1120, S1160, S1200 and S1240, respectively. The optimal solution treated temperature was first chosen. Then, using the optimal solution treated sample (S1160), the annealing treatments were performed under 750, 800, 850, 900 and 950 °C for 1 h, with air cooling, which were then marked as A750, A800, A850, A900 and A950, respectively.
After the heat treatments, all the stainless steel samples were cut into a dimension of Ф25 mm × 5 mm. Then, the samples were ground up to 800# SiC paper with Ra 0.05~0.1 μm, and cleaned by an ultrasonic wave with acetone, ethanol, and deionized water in turn, and then blow-dried for use.

2.3. Low-Temperature Plasma Nitriding Treatment

The as-delivered, solid solution-treated, and solid solution-treated + annealed stainless steel samples were plasma nitrided at 450 °C for 8 h with a plasma nitriding furnace (PN-IV30A-15F/A), with an auxiliary heat source. The as-delivered sample, solid solution-treated at 1160 °C sample (S1160), and solid solution-treated at 1160 °C + annealed at 850 °C sample (A850) after low-temperature plasma nitriding were marked as DN450-8, SN450-8, and AN450-8, respectively. Before turning the power on, the air pressure in the furnace was pumped below 5 Pa. The detailed process parameters are shown in Table 2.

2.4. Microstructure Characterization

The microstructure of the stainless steel after the solution treatment and annealing heat treatment were observed by a metallographic microscope (OM). The metallographic sample was prepared according to the standard procedure. Before the OM observation, the samples were etched by a Marble’s reagent (1 g of CuSO4 + 10 mL of HCl + 10 mL of C2H5OH).
After the low-temperature plasma nitriding, the surface morphology of the sample was observed by a scanning electron microscope (SEM, JSM-6480A). The test voltage and current used for the EDS tests were 20 kV and 0.1 nA, respectively. The cross-section metallographic images of the nitrided layers were prepared according to the standard procedure. Before the SEM observation, the samples were also etched by the Marble’s reagent.
The phase composition of the samples before and after the low-temperature plasma nitriding were analyzed by X-ray diffractometry (XRD, Rigaku TTR-III). The Cu-Kα target was used as the radiation source, the working voltage was 40 kV, the current was 30 mA, the scanning mode was θ–2θ, and the scanning rate was 3°/min. The scan ranges were from 25° to 90°.

2.5. Property Test

The surface microhardness and microhardness profiles of the samples before and after the plasma nitriding were measured by a microhardness tester (DHT, HV-1000) at the load of 100 g for 15 s. For the surface hardness, six different positions were tested and the average was chosen. For the hardness profile, 2–3 points were tested at each position.
The friction and wear behaviors of the samples before and after the plasma nitriding were tested by a ball-on-disk tribometer. A Si3N4 ball with a diameter of 5 mm was used as the anti-wear part, the samples before and after the low-temperature plasma nitriding was set as a disk. All the samples were tested at a load of 10 N for 1800 s with a speed of 201 r/min. The radius of the worn track was 3 mm. The analytical balance (using a Secura224-1CN with an accuracy up to 0.0001 mg) was used to measure the quality before and after the wear tests. For ensuring the correctness of the measurements, three samples treated under the same conditions were ground and the qualities were tested, with the quality of each sample measured 3 times.
The corrosion resistances of the stainless steel samples before and after the plasma nitriding were examined by an electrochemical workstation (Germany Zahner). The tests were carried out in a three-electrode system, in which a calomel electrode was the reference electrode, a platinum sheet was an auxiliary electrode and the samples were the working electrodes, only leaving a 1 cm2 surface exposed to the electrolyte. The open circuit potential was measured with a scanning time of 1800 s in a 3.5% NaCl solution at 25 °C. Then, both electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curve tests were carried out. The EIS was obtained in the frequency range of 105 to 10−2 Hz. The potentiodynamic polarization tests were performed at a scan rate of 1.6 mV/s. The electrode potential was set between −500 mV and 1500 mV, the scanning rate was set to 1.6 mV/s, and the corrosion rate was calculated by the Tafel extrapolation method.

3. Results and Discussion

3.1. Microstructure of the Stainless Steel after Heat Treatment

Figure 1 shows the microstructure of AISI 430 ferritic stainless steel after a solid solution treatment at different temperatures. The microstructures of the as-delivered, S1080, S1160, S1200, and S1240 samples are shown in Figure 1a–f. It can be seen from Figure 1a that there was dispersed precipitation and ferrite with an evident rolling orientation. Figure 1b–f show the samples after the solid solution treatment, where the microstructure of the stainless steel apparently changed. There was mainly low carbon martensite and ferrite in the solid solution treated samples. The precipitation phases were dissolved, and the rolling orientation of the ferrite disappeared gradually. The ferrite microstructure was uniform, and the grain size was the same as that without an abnormal grain size. Moreover, with the increase in the solid solution treatment temperature, the rolling orientation of the ferrite disappeared gradually with the size of the ferrite increase. In general, there is no desired grain size for ferrite, the desired microstructure is a uniform one, with the grain size of the ferrite being almost the same without an abnormally large grain size. Based on these criteria, the optimal microstructure should be as the S1160 sample in the present investigation; therefore, the sample for the solid solution treated at 1160 °C was chosen for the next step’s annealing treatment.
Figure 2 shows the microstructure of AISI 430 ferritic stainless steel, solid solution-treated at 1160 °C with annealing at different temperatures. The microstructures of the A750, A800, A850, A900 and A950 samples are displayed in Figure 2a–e. It can be seen from the figures that after the annealing treatment, the microstructure was regulated and refined. There was no obvious rolling orientation of the ferrite, and the size of the ferrite decreased gradually. When the annealing temperature was higher than 850 °C, there was no obvious change in the size and orientation of the ferrite, but some significant precipitation appeared. Therefore, the optimal annealing temperature was 850 °C in the present investigation. For the next stage’s low-temperature plasma nitriding, the sample annealed at 850 °C was used.

3.2. Microstructure of Low-Temperature Plasma Nitrided Layer

3.2.1. Surface Morphology of the Nitrided Layer

The surface morphologies of the AISI 430 ferritic stainless steel samples with different initial microstructures after low-temperature plasma nitriding were observed, as shown in Figure 3. Moreover, we used the EDS to verify the presence of nitrogen on the top surface and the change trend of the nitrogen content with a different initial microstructure; however, the EDS method is not an accurate method to obtain the nitrogen content quantitatively. The contents of the typical elements are also shown in Table 3.
It can be known from the EDS that the sample surface mainly contained Fe, Cr and N. The top surfaces of the low-temperature nitrided layers were uniform and evenly covered by significant amounts of dispersive particles (e.g., the arrows in the pictures). The EDS was used to qualitatively analyze the elements in the A, B and C regions of the figures. As detected by the EDS, these particles should have been iron nitrides. For the as-delivered sample, there some dark holes appeared, as shown in the red circle in Figure 3a. For the solution-treated sample, some large-sized iron nitride was found. Obviously, the iron nitride distribution on the surface of the annealed stainless steel was more uniform and the surface microstructure of the nitrided layer was denser than those of the as-delivered and the solid solution-treated stainless steel after the low-temperature plasma nitriding.
Table 3 shows the content of the typical elements on the surface of the AISI430 ferritic stainless steel after nitriding. It can be seen from the table that the mass fraction of the N element on the surface of the DN450-8, SN450-8 and AN50-8 after the nitriding treatment was 12.29%, 12.73% and 13.48%, respectively, indicating that the microstructure had an effect on the nitriding.

3.2.2. Cross-Section Microstructures of the Nitrided Layer

Figure 4 shows the cross-section microstructures of the low-temperature plasma nitrided-AISI 430 ferritic stainless steel with a different initial microstructure. Figure 4a–c are, respectively, the microstructure of the DN450-8, SN450-8, and AN450-8 samples. There were obvious nitrided layers formed on the stainless steel with a different initial microstructure after the low-temperature plasma nitriding. The layer depths were determined by choosing five positions along the cross-section of the nitrided layer, then, the average of the layer thicknesses at five positions were chosen as the layer thickness, using the white dashed lines to divide the nitrided layer and the matrix. The low-temperature compound layer thicknesses of the as-delivered, solid solution-treated, and the annealed AISI 430 stainless steels were 21 μm, 23 μm and 34 μm, respectively, as marked by the white dashed line in Figure 4a–c, that is, the thickness of the compound layer on the annealed sample was obviously thicker than those of the as-delivered and solid solution-treated stainless steels. For the solution-treated and annealed sample, there were no obvious cracks shown in the compound layer; however, there appeared to be an evident crack (blue circle) that was parallel to the surface in the compound layer of the as-delivered stainless steel. The main reason for this is likely the uneven microstructure (including the precipitation phases and the rolling orientation) of the as-delivered stainless steel, which would also result in a poor performance of the compound layer [22]. Therefore, a heat treatment could eliminate microstructure defects in the low-temperature compound layer of ferritic stainless steel, which should have positive effects on its properties.

3.3. Phase Composition of a Low-Temperature Nitrided Layer

Figure 5 shows the XRD patterns of the low-temperature plasma nitrided layer on AISI 430 ferritic stainless steel with a different initial microstructure. Obviously, there was only α-Fe phase for the annealed sample. After the low-temperature plasma nitriding, the surfaces of the samples were mainly composed of a nitrogen “expanded” α phase (αN) [24] and iron nitrides (i.e., Fe2–3N and Fe4N). The existence of the αN phase can be clearly detected in Figure 5b. There were obvious peak shifts and a peak intensity weakness of the α-Fe phase for the samples after the low-temperature plasma nitriding. The peaks’ shifts to low diffraction angles for the α-Fe phase (110) was likely due to nitrogen diffusing into an iron crystal lattice [25]. The weakness of the peak intensity after the low-temperature nitriding was mainly due to the high stress of the nitrided layer after the nitrogen supersaturation [26].
In addition, compared with the as-delivered and the solid solution-treated samples, for the annealed sample, the degree of the peak shift to a low angle of the α-Fe phase was the biggest with the lowest peak intensity of a (110) plane for the α-Fe phase, which means that the degree of the nitrogen supersaturation in the α-Fe phase was the biggest. Therefore, the content of the αN phase should have been the highest in the nitrided layer of the annealed sample. In addition, the peak intensity of the Fe2–3N and Fe4N phase increased gradually after the solid solution and annealing treatments, meaning that the content of these phases in the nitrided layer increased after the heat treatment. Consequently, the content change of these phases in the nitrided layer on ferritic stainless steel with different initial microstructures will, apparently, affect the properties of a low-temperature nitrided layer [27,28].

3.4. Hardness of Low-Temperature Plasma Nitrided Layer

The hardness profiles of the AISI 430 stainless steel before and after low-temperature plasma nitriding are shown in Figure 6. It can be seen that the surface hardness of the stainless steel increased significantly after the plasma nitriding. The hardness of the as-delivered, solution-treated, and annealed stainless steel were 350 HV0.1, 470 HV0.1 and 415 HV0.1, respectively. After plasma nitriding at 450 °C for 8 h, the surface hardness of the as-delivered, solution-treated, and annealed stainless steels reached up to 1313 HV0.1, 1597 HV0.1 and 1832 HV0.1, respectively. These were three times higher than those of the samples before the low-temperature plasma nitriding. The high surface hardness of the samples after the low-temperature plasma nitriding was likely due to significant αN phase and iron nitride formations on the surface of the stainless steel, which apparently increased the plastic deformation resistance of the surface [29]. The dense microstructure with no cracks or phase composition change should account for the higher hardness of the annealed sample after the low-temperature plasma nitriding.
The effective hardening layer can be normally calculated with the position where the hardness is higher by 50 HV than that of the substate [30]. Based on this standard, the effective hardening layer thicknesses for the as-delivered, solution-treated, and annealed stainless steel after plasma nitriding at 450 °C were 38, 40 and 43 μm, respectively, which were higher than those of the values obtained from the cross-section SEM images. The microstructure shown in Figure 4 depicts the compound layer clearly but is not very uniform and clear for the diffusion layer boundary, which can only be given a rough value of the layer thickness. Moreover, the hardness of the nitrided layer decreased gradually with an increase in the layer depth. The gradient hardness profiles of the low-temperature plasma nitrided layers on the samples after the heat treatment would be better for obtaining the anti-impact properties [31].

3.5. Wear Resistance of Low-Temperature Plasma Nitrided Layer

3.5.1. Friction Coefficient of the Nitrided Layer

Figure 7 shows the friction coefficient curves of the stainless steel with a different initial microstructure before and after low-temperature plasma nitriding. It can be seen from the figure that low-temperature plasma nitriding can significantly reduce the friction coefficient of the AISI 430 ferritic stainless steel. The stable friction coefficients of the as-delivered, solution-treated, and annealed stainless steels before and after plasma nitriding at 450 °C for 8 h were about 0.75, 0.63, 0.70, 0.57, 0.54 and 0.46, respectively. Obviously, the stable friction coefficient of each sample after the low-temperature plasma nitriding decreased by 24.0%, 16.7% and 34.3%, respectively. Among them, the annealed stainless steel after the plasma nitriding at 450 °C for 8 h possessed the lowest friction coefficient, possessing the best wear conditions. The decreased friction coefficients were mainly due to the dense microstructure present after the low-temperature plasma nitriding [32].

3.5.2. Wear Resistance of the Nitrided Layer

In the present investigation, the wear resistance of the sample was characterized by the mass wear ratio, which is calculated by using Formula (1):
Wm = Δm/(FL)
where Δm is the weight loss with a unit of kg, F is the load with a unit of N, and L is the sliding distance with a unit of m.
Table 4 shows the weight loss and wear ratios of the AISI 430 ferritic stainless steel with different initial microstructures before and after plasma nitriding. The wear weight loss of the as-delivered, solution-treated, and annealed stainless steels before and after plasma nitriding at 450 °C for 8 h were about 0.0298, 0.0265, 0.0272, 0.0005, 0.0004 and 0.0001 g, respectively. The minimum wear weight loss was 0.0001 g for the AN450-8 sample. The value of the wear ratio adhered to a similar rule; therefore, the sample that was plasma nitrided at 450 °C for 8 h possessed the best wear resistance. The wear resistance of the samples after low-temperature plasma nitriding were apparently, improved, especially for the sample after the annealing treatment and the higher the hardness of the material, the higher wear resistance of the material. In the present investigation, the surface hardness of the ferrite stainless steel was apparently improved after the low-temperature plasma nitriding, then, the plastic deformation resistance of the stainless steel was improved along with the wear resistance of the steel [33,34].

3.6. Corrosion Resistance of Low-Temperature Plasma Nitrided Layer

3.6.1. Potentiodynamic Polarization Curves of the Nitrided Layer

The potentiodynamic polarization curves of the AISI 430 stainless steel samples before and after low-temperature plasma nitriding are shown in Figure 8. The corresponding corrosion parameters obtained from the polarization curve are shown in Table 5. It can be seen from the figure that the anodic polarization curves of the as-delivered and solid solution-treated AISI 430 stainless steels showed passivation zones, where pitting corrosion occurred; however, the annealed sample did not show a passivation zone, where general corrosion happened. We can observe from Table 4 that after the solid solution and annealing treatments, the corrosion potential of the AISI 430 ferritic stainless steel increased, along with current density decreases. Therefore, the corrosion resistance of a nitrided layer on ferritic stainless steel could be increased after a heat treatment.
For the as-delivered sample after low-temperature plasma nitriding, there was no obvious passivation zone. The corrosion parameters showed that the corrosion resistance of the as-delivered stainless steel after the low-temperature plasma nitriding apparently decreased. Similarly, for the solid solution-treated sample after the low-temperature plasma nitriding, there was no passivation zone. The corrosion parameters showed that the corrosion potential increased, but that the corrosion current density also apparently increased; therefore, the corrosion resistance of the solid solution-treated stainless steel still showed a decrease after the low-temperature plasma nitriding. For the annealed sample, there was no obvious passivation zone before and after the low-temperature plasma nitriding. The corrosion parameters in Table 5 show that the corrosion potential increased significantly, but that the corrosion current density decreased significantly; therefore, the corrosion resistance of the annealed stainless steel could be improved significantly by low-temperature plasma nitriding.
The reasons for the samples with a different initial microstructure having a different corrosion resistance after the same plasma nitriding treatment were likely due to the different microstructures and quantities of iron nitrides in the nitrided layers on the ferritic stainless steel with different initial microstructures [28].

3.6.2. EIS Test of the Nitrided Layer

To reveal the electrochemical mechanism taking place during corrosion, EIS measurements for the samples with different initial microstructures before and after the low-temperature plasma nitriding were conducted. Typical Nyquist and Bode plots of the samples are presented in Figure 9. The best equivalent electrical circuits for modeling and simulating the impedance spectra are shown in Figure 10. The parameter values obtained by the fitting process using the Zview software are displayed in Table 5. In Figure 10 and Table 5, RS is the solution resistance, Q is the constant phase element related to the double layer capacitance, Rct is the charge transfer resistance, Rct1 is the passive film resistance, Q1 is the constant phase element related to the passive film capacitance, Rct2 is the charge transfer resistance and Q2 is the constant phase element related to the double layer capacitance. The value of the exponent n indicates the deviation from the ideal capacitive behavior (when n = 1) in the case of a nonideal capacitance (when n < 1), and Q is used instead of the double-layer capacitance to account for the surface heterogeneity.
It can be seen from the Figure 9a,b that the Nyquist plots of all the samples exhibited depressed capacitive loops, which were related to the electrochemical reaction of the samples that the charge transfer was dominated by during the corrosion process. The phase angles of the solid solution-treated, and the annealed samples obviously increased after the low-temperature plasma nitriding. The phase angle of the as-delivered sample was the smallest, and the phase angle of the annealed sample was the largest. In addition, the as-delivered, solution-treated, and annealed samples of the phase angle values of the large semicircles remaining very close to 80° represent the formation and growth of a passive film [35].
Moreover, according to the Bode plots in Figure 9c, the impedance modulus of the solid solution-treated and the annealed samples were significantly higher than that of the as-delivered stainless steel. The impedance moduli of the as-delivered and solution-treated samples decreased after low-temperature plasma nitriding, while the impedance modulus of the annealed sample increased after the low-temperature plasma nitriding. It can be seen from the Bode plots in Figure 9d, that all the samples before nitriding had only one time constant, and the maximum time constant was about 10 Hz in the Bode-phase plots, that may be attributed to the charge transfer resistance and the double layer/space charge capacitance of the corrosion process [36]. Two time constants appeared after nitriding, indicating two different electrochemical processes at the interface [37], but with overlapping time constants in the AN450-8 graphical analysis [38].
It can be seen from Table 6 that after the solid solution and annealing treatments, the R of the AISI 430 ferritic stainless steel increased, the Q value was smaller, and the n index was closer to 1, i.e., close to the ideal capacitance. The polarization resistance (RP = Rct1 + Rct2) is defined as the capacity of the passive film to resist corrosive effects. As such, it is the value that represents the barrier property of a passive film [39]. It can be seen from the table that the value of RP for the AN450-8 sample was greater than that for the DN450-8 and SN450-8 samples. On the other hand, the Q of the AN450-8 sample was smaller and the n index was closer to 1, indicating that it was closer to the ideal capacitance. This may have been because the AN450-8 sample should have had a thicker and denser nitrided layer. The reasons for the corrosion resistance improvement in the annealed sample was likely due to the following reasons: firstly, the microstructure of the nitrided layer was denser with no cracks and could prevent the penetration of the corrosion solution; secondly, the phases in the nitrided layer possessed a high corrosion resistance [15,27].

4. Conclusions

In the present investigation, AISI 430 ferritic stainless steel was firstly pre-treated by a solid solution and annealing, and then was plasma nitrided at 450 °C for 8 h. The microstructure, phase structure, microhardness, wear resistance and corrosion resistance of the nitrided layer were characterized and analyzed. The main results were as following:
(1) The dense and uniform nitrided layers either with few cracks or being crackless, were obtained on AISI 430 ferritic stainless steel. The initial microstructure of the ferritic stainless steel was different, and the thickness of the nitrided layer was also different after low-temperature plasma nitriding. Among the samples, the annealed ferritic stainless steel after the low-temperature plasma nitriding had the thickest layer up to 35 μm.
(2) After the plasma nitriding of AISI 430 ferritic stainless steel with different initial microstructures, there were mainly nitrogen “expanded” α phase (αN) and nitrides (Fe2–3N and Fe4N) in the nitrided layer. The nitrided layer on the annealed sample contained more nitrogen “expanded” α phase (αN) and nitrides (Fe2–3N and Fe4N).
(3) The properties of the AISI 430 ferritic stainless steel with different initial microstructures after low-temperature ion nitriding were obviously different. The hardness of the DN450-8 sample with the lowest hardness was only 1313 HV0.1, while that of the AN450-8 sample had the highest hardness of 1832 HV0.1, and the hardness changed in gradient.
(4) The stable friction coefficient of each sample after the low-temperature plasma nitriding decreased by 24.0%, 16.7% and 34.3%, respectively, and the mass wear rate was 0.00367 of the sample before the plasma nitriding, indicating that the wear resistance was significantly improved.
(5) The corrosion resistances of the DN450-8 and SN450-8 samples were worse than that of those before the low-temperature nitriding; however, the polarization potential of the AN450-8 sample reached 0.027 V, and the corrosion current density was as low as 6.73 × 10−8 A/cm2, while its corrosion resistance was improved.

Author Contributions

L.L.: Investigation, Writing—original draft, Writing—review & editing. R.L.: Investigation, Supervision, Writing—review. Q.L.: Investigation, Writing—review. Z.W.: Investigation. X.M.: Investigation. Y.F.: Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (No. 51871071) and Natural Science Foundation of Heilongjiang Province (No. LH2019E029).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 12 01404 g001 550
Figure 1. Microstructure of AISI 430 ferritic stainless steel after a solid solution treatment at different temperatures (the red arrow refers to α-Fe). (a) Delivered, (b) S1080, (c) S1120, (d) S1160, (e) S1200, and (f) S1240.
Figure 1. Microstructure of AISI 430 ferritic stainless steel after a solid solution treatment at different temperatures (the red arrow refers to α-Fe). (a) Delivered, (b) S1080, (c) S1120, (d) S1160, (e) S1200, and (f) S1240.
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Figure 2. Microstructure of AISI 430 ferritic stainless steel, solid solution-treated at 1160 °C with annealing at different temperatures of (the red arrow refers to α-Fe). (a) A750, (b) A800, (c) A850, (d) A900, and (e) A950.
Figure 2. Microstructure of AISI 430 ferritic stainless steel, solid solution-treated at 1160 °C with annealing at different temperatures of (the red arrow refers to α-Fe). (a) A750, (b) A800, (c) A850, (d) A900, and (e) A950.
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Figure 3. Surface morphology of the nitrided layer on AISI 430 ferritic stainless steel with a different initial microstructure. (a) DN450-8, (b) EDS result at A, (c) SN450-8, (d) EDS result at B, (e) AN450, and (f) EDS result at C.
Figure 3. Surface morphology of the nitrided layer on AISI 430 ferritic stainless steel with a different initial microstructure. (a) DN450-8, (b) EDS result at A, (c) SN450-8, (d) EDS result at B, (e) AN450, and (f) EDS result at C.
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Figure 4. Microstructure of plasma nitrided layer on AISI 430 ferritic stainless steel with a different initial microstructure. (a) DN450-8, (b) SN450-8, and (c) AN450-8.
Figure 4. Microstructure of plasma nitrided layer on AISI 430 ferritic stainless steel with a different initial microstructure. (a) DN450-8, (b) SN450-8, and (c) AN450-8.
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Figure 5. XRD patterns of AISI 430 ferritic stainless steel with a different initial microstructure before and after low-temperature plasma nitriding. (a) Different processes, and (b) diffraction angle range 35–50°.
Figure 5. XRD patterns of AISI 430 ferritic stainless steel with a different initial microstructure before and after low-temperature plasma nitriding. (a) Different processes, and (b) diffraction angle range 35–50°.
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Figure 6. Hardness profiles of AISI 430 ferritic stainless steel with a different initial microstructure after low-temperature plasma nitriding.
Figure 6. Hardness profiles of AISI 430 ferritic stainless steel with a different initial microstructure after low-temperature plasma nitriding.
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Figure 7. Friction coefficient curves of AISI 430 ferritic stainless steel with a different initial microstructure before and after low-temperature plasma nitriding.
Figure 7. Friction coefficient curves of AISI 430 ferritic stainless steel with a different initial microstructure before and after low-temperature plasma nitriding.
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Figure 8. Corrosion polarization curves of AISI 430 ferritic stainless steel before and after low-temperature plasma nitriding.
Figure 8. Corrosion polarization curves of AISI 430 ferritic stainless steel before and after low-temperature plasma nitriding.
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Figure 9. Nyquist and Bode spots of AISI 430 stainless steel before and after plasma nitriding in a 3.5% NaCl solution. (a,b) Nyquist spectra, (c) Bode-impedance moduli, and (d) Bode-phase angle.
Figure 9. Nyquist and Bode spots of AISI 430 stainless steel before and after plasma nitriding in a 3.5% NaCl solution. (a,b) Nyquist spectra, (c) Bode-impedance moduli, and (d) Bode-phase angle.
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Figure 10. Best equivalent circuits used for modeling the EIS spectrum. (a) Equivalent circuits for the delivered, S1160 and A850 samples, and (b) equivalent circuits for DN450-8, SN450-8 and AN450-8 samples.
Figure 10. Best equivalent circuits used for modeling the EIS spectrum. (a) Equivalent circuits for the delivered, S1160 and A850 samples, and (b) equivalent circuits for DN450-8, SN450-8 and AN450-8 samples.
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Table
Table 1. Chemical composition of AISI 430 ferritic stainless steel (wt.%).
Table 1. Chemical composition of AISI 430 ferritic stainless steel (wt.%).
ElementsCMnPSSiCrNiFe
Content0.130.930.120.0020.6417.330.57Bal.
Table
Table 2. Process parameters of low-temperature plasma nitriding.
Table 2. Process parameters of low-temperature plasma nitriding.
Sample
Number		Temperature
(°C)		Time
(h)		N2:H2Pressure
(Pa)		Voltage
(V)		Current
(A)		Auxiliary Heat
Temperature (°C)
DN450-845081:32006505350
SN450-8
AN450-8
Table
Table 3. Typical element content of an AISI430 ferritic stainless steel surface after nitriding.
Table 3. Typical element content of an AISI430 ferritic stainless steel surface after nitriding.
ElementDN450-8SN450-8AN450-8
wt.%at.%wt.%at.%wt.%at.%
N7.3024.877.5125.097.6825.2
Cr17.8315.3016.9214.5217.3314.87
Fe74.8759.8375.5760.3974.9959.93
Table
Table 4. Weight loss and wear ratio of AISI 430 stainless steel with a different initial microstructure before and after low-temperature plasma nitriding.
Table 4. Weight loss and wear ratio of AISI 430 stainless steel with a different initial microstructure before and after low-temperature plasma nitriding.
ProcessQuality before Grinding (g)Quality after Grinding (g)Wear Loss (g)Wear Ratio
(kg/Nm)
Delivered19.789319.75950.02982636.24 × 10−11
S116019.789119.76260.02652344.31 × 10−11
A85019.789019.76180.02722406.23 × 10−11
SN450-819.926919.92650.000435.3857 × 10−11
AN450-819.925819.92570.00018.84643 × 10−11
Table
Table 5. Corrosion potential and corrosion current density of AISI 430 stainless steel before and after plasma nitriding.
Table 5. Corrosion potential and corrosion current density of AISI 430 stainless steel before and after plasma nitriding.
ProcessEcorr, V (vs. SCE)Ip, A/cm2
Delivered−0.17831.8 × 10−8
S1160−0.13816.2 × 10−8
A850−0.05910.2 × 10−8
DN450-8−0.16767.8 × 10−8
SN450-8−0.044168 × 10−8
AN450-80.0276.73 × 10−8
Table
Table 6. EIS fitting parameters of AISI 430 stainless steel with a different initial microstructure before and after plasma nitriding in a 3.5% NaCl solution.
Table 6. EIS fitting parameters of AISI 430 stainless steel with a different initial microstructure before and after plasma nitriding in a 3.5% NaCl solution.
SpecimensRs
(Ω·cm2)		Rct1
(kΩ·cm2)		R and Rct2
(kΩ·cm2)		Y-Q1
(Ω−1·cm−2·sn)		n-Q1Y-Q and Y-Q2
(Ω−1·cm−2·sn)		n-Q and n-Q2
Delivered29.32-20.32--1.045 × 10−40.7818
S116027.86-24.78--3.58 × 10−50.8001
A85029.29-36.87--3.03 × 10−50.8997
DN450-831.911.4984.9831.475 × 10−60.79411.075 × 10−50.8042
SN450-829.091.3275.2212.368 × 10−50.75392.831 × 10−40.4749
AN450-831.0132.7829.454.833 × 10−50.85521.795 × 10−40.8087
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Li, L.; Liu, R.; Liu, Q.; Wu, Z.; Meng, X.; Fang, Y. Effects of Initial Microstructure on the Low-Temperature Plasma Nitriding of Ferritic Stainless Steel. Coatings 2022, 12, 1404. https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12101404

AMA Style

Li L, Liu R, Liu Q, Wu Z, Meng X, Fang Y. Effects of Initial Microstructure on the Low-Temperature Plasma Nitriding of Ferritic Stainless Steel. Coatings. 2022; 12(10):1404. https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12101404

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Li, Lingze, Ruiliang Liu, Quanli Liu, Zhaojie Wu, Xianglong Meng, and Yulan Fang. 2022. "Effects of Initial Microstructure on the Low-Temperature Plasma Nitriding of Ferritic Stainless Steel" Coatings 12, no. 10: 1404. https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12101404

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