Combination of plasma nitriding and nitrocarburizing treatments of AISI 630 martensitic precipitation hardening stainless steel

Combination of plasma nitriding and nitrocarburizing treatments of AISI 630 martensitic precipitation hardening stainless steel

Accepted Manuscript Combination of plasma nitriding and nitrocarburizing treatments of AISI 630 martensitic precipitation hardening stainless steel I...

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Accepted Manuscript Combination of plasma nitriding and nitrocarburizing treatments of AISI 630 martensitic precipitation hardening stainless steel

Insup Lee PII: DOI: Reference:

S0257-8972(18)31403-8 https://doi.org/10.1016/j.surfcoat.2018.12.078 SCT 24149

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

28 November 2017 7 December 2018 17 December 2018

Please cite this article as: Insup Lee , Combination of plasma nitriding and nitrocarburizing treatments of AISI 630 martensitic precipitation hardening stainless steel. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.12.078

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ACCEPTED MANUSCRIPT Combination of Plasma Nitriding and Nitrocarburizing Treatments of AISI 630 Martensitic Precipitation Hardening Stainless Steel Insup Lee* Department of Advanced Materials Engineering, Dongeui University, Busan 614-714, Republic of Korea *Corresponding author, Email: [email protected], Phone: 011-1754-3989, Fax: 82-505-182-6896

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Low temperature plasma nitriding and nitrocarburizing were experimented to study the effect of these treatments on the surface hardness and the expanded martensite layer (α'N layer) thickness of the AISI 630 martensitic precipitation hardening stainless steel. Plasma nitrided samples show thicker α'N layer thickness (about 16 µm) but their corrosion resistances are worse than the untreated sample. On the other hand, plasma nitrocarburized samples shows better corrosion resistance, whereas their α'N layer thickness values (around 10 µm) are very low. But the surface hardness values of both the nitrided and nitrocarburized samples are quite similar (approximately 1250 HV0.05). The purpose of the present study is to combine the beneficial effects of both plasma nitriding and plasma nitrocarburizing. Therefore, in order to get a higher surface hardness and thicker α'N layer thickness with good corrosion behavior, a series of combination of simultaneous nitriding and nitrocarburizing were investigated. This process is termed here as “Multiple Nitriding-Nitrocarburizing process”. The highest surface hardness (around 1350 HV0.05), highest α'N layer thickness (around 16 µm) and best corrosion resistance are achieved when treated with this noble process, compared to single plasma nitriding or single plasma nitrocarburizing process. Keywords: Plasma nitriding, Plasma nitrocarburizing, nitrocarburizing, Corrosion resistance, S-phase.

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1. Introduction Martensitic and precipitation hardening stainless steel has good mechanical properties and moderate corrosion resistance than other stainless steel grades, and for these reasons, these types of steels are attractive for many industrial sectors. For variety of applications including oil field valve parts, chemical process equipment, aircraft fittings, pump, nuclear reactor components, missile fitting and rotors of centrifugal compressors, this type of stainless steel has been used extensively. But for some applications it is necessary to further improve the steel surface properties, for instance, improvement in surface hardness is needed for fasteners, springs, ball bearings chains, valves and gears and betterment in corrosion resistance is needed for surgical and dental instruments, pressure vessels, steam and gas turbines. In order to broader applications of martensitic precipitation hardening stainless steel, the development of advanced surface engineering technologies were applied to tackle the problem [1–3].

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Plasma nitriding is one of the techniques that has been used successfully to modify a material surface. This is a plasma assisted thermochemical process in which the workpiece acts as a cathode. The ion bombardment heats the workpiece, cleans the surface and provides active species. Subsequently, higher hardness and improved load bearing capacity of the treated parts can be achieved. In the mid1980s, nitrogen supersaturated expanded austenite phase (γN or S phase) layers were obtained on the surface of austenitic stainless steel by low temperature nitriding. It is well known that low temperature nitriding prohibits the formation of Cr2N precipitates, which will deteriorate the corrosion resistance, in the S phase of austenitic stainless steel. [4,5]. Since then, low temperature thermochemical processes such as plasma nitriding, plasma immersion ion implantation (PIII), carburizing, nitrocarburizing which is a hybrid processes of nitriding and carburizing have all been investigated [6–8]. These treatments can be successfully used to produce nitrogen and/or carbon supersaturated expanded austenite phase on the surfaces of various austenitic stainless steels, and then *Corresponding author Address: Department of Advanced Materials Engineering, Dongeui University, Busan 614-714, Republic of Korea. E-mail: [email protected], Tel.: 010-7154-3989, Fax: +82-505-182-6896

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ACCEPTED MANUSCRIPT achieves combined improvements in surface hardness, wear resistance and corrosion resistance due to the superior properties of the “S-phase”. Furthermore, it has been shown that low temperature nitrocarburizing is advantageous over low temperature nitriding and carburizing for austenitic stainless steel based on the increased S-phase layer thickness, improved load bearing capacity and reduced treatment temperature [6,7,9].

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In contrast to the very hot research on austenitic stainless steel, much less attention was given to low temperature thermochemical processing of martensitic precipitation-hardening stainless steel. Very few research have been carried out on the effect of nitriding on martensitic precipitation hardening stainless steel [10–18]. The formed layer is called “α'N layer” (nitrogen supersaturated expanded martensite layer) in the case of martensitic stainless steels as like the “S-phase” in austenitic stainless steel. Some good results were found regarding surface hardness and α'N layer thickness by applying plasma nitriding [10–12], carburizing [13–16] and also nitrocarburizing [17,18]. But in all those cases poor corrosion behavior were reported because of the formation of chromium nitrides during nitriding. The Cr2N precipitates in the nitrogen enriched layer will cause chrominum depletion in their vicinity and eventually prevent the generation of a thin Cr2O3 surface film, termed a passive film. Therefore Cr2N precipitation is the limiting factor to maintain corrosion resistance in plasma nitriding of stainless steel. The processing temperature is the key factor in order to prevent the formation of Cr2N precipitates. The increase of the processing temperature more than 400°C for plasma nitriding of 630 stainless steel is insignificant, since it deteriorate the corrosion resistance due to the formation of Cr2N precipitates in the α'N layer (nitrogen supersaturated expanded martensite layer) [10–12].

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This research is based on experiments to develop know-how that can be applied directly in the industrial field. Therefore, it is a kind of technology development process through trial and error methods without being based on a profound theory. The main objective of this paper is not only to increase the α'N layer thickness and surface hardness but also to improve the corrosion behavior of the AISI 630 martensitic stainless steel. In order to do that, several nitriding and nitrocarburizing processes were conducted. But both of the desired properties cannot be achieved through either of these two processes. In general, plasma nitriding (NT) increases the α'N layer thickness of the treated samples whereas it gives poor corrosion resistance. On the other hand, plasma nitrocarburizing (NC) is helpful to improve the corrosion behavior of the treated samples but it reduces the α'N layer thickness. The originality of this work is to combine these two beneficial properties of nitriding and nitrocarburizing processes, which results in the increment of the α'N layer thickness, surface hardness, and improvement of corrosion behavior of the AISI 630 martensitic stainless steel. Repetition of a combination of nitriding and nitrocarburizing processes is made for the first time in my laboratory. Eventually, desired results are achieved through these processes. This series of nitriding and nitrocarburizing is termed here as the “Multiple Nitriding-Nitrocarburizing” or “Multiple NT-NC” process.

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2. Experimental Procedure: Circular coupons of solution treated AISI 630 martensitic precipitation-hardening stainless steel having a diameter of 16 mm and a thickness of 3 mm were prepared for processing. Typically, solution treatment is carried out at 1050°C for 30 mins/inch and then quenched in water. The composition of AISI 630 martensitic precipitation-hardening stainless steel is shown in the Table I. The solution-treated samples were labeled as “bare” in this paper. The samples were mechanically ground with SiC emery papers having a grit size of 280, 400, 800 and 1200 and then polished with 0.05 µm alumina paste to obtain the final mirror surface. Both plasma nitriding (NT) and plasma nitrocarburizing (NC) processes were performed in a plasma nitriding facility. Before placing the specimens on a sample holder in a vacuum chamber, they were rinsed with alcohol in an ultrasonic cleaner. The sample holder acts as the cathode and the vacuum chamber wall acts as the anode and is grounded. Initially, the vacuum chamber should be vented less than 50 mTorr (6.67 Pa) using a mechanical rotary pump. Pre-sputtering was performed for at least 40 minutes at 400°C to clean the sample surface using Ar (10 SCCM(Standard Cubic Centimeters per Minute)) and H2 (90 SCCM) before plasma treatment. The gas flow to the chamber is controlled by MFC (Mass Flow Controller) and heating of the substrates is performed by a heating element installed in the vacuum chamber. For plasma nitriding, a gas mixture composed of H2 (150-160 SCCM) and N2 (40-50 SCCM) was used. A gas mixture (3% CH4) of H2 (154 SCCM), N2 (40 SCCM) and CH4 (6 SCCM) was employed for plasma nitrocarburizing. Both processes maintained a constant pressure of 4 Torr (532 Pa) and a 2

ACCEPTED MANUSCRIPT constant bias voltage of 400 V. During these two plasma processes, nitrogen, hydrogen and methane gas are ionized and injected into the specimen under the influence of voltage bias. The duration of treatment remained constant for 15hours in both processes. After this processes, the bias voltage was reduced to zero and the specimens were cooled down to 200° C to avoid oxidation before removing them from the vacuum chamber.

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Plasma treated samples were sectioned for metallographic examination and surface hardness measurement. The microstructures of polished cross-sectional surfaces were observed by an Olympus BX51M optical microscope using Vilella’s reagent (1g picric acid + 100 ml Ethyl alcohol + 5 ml HCl) [19]. The different phases formed on the treated surfaces were analyzed by X-ray diffraction in a RigakuD/Max-200 diffractometer using Cu-Kα radiation (λ= 1.544 Å). Micro hardness measurements were performed using a digital micro Vickers hardness tester Matsuzawa MMT-X7B with an indentation load of 0.98N (100g) and a loading time 10 seconds. The distribution of nitrogen and carbon in the α'N layer of the treated samples was analyzed by Glow discharge optical emission spectrometry (GDOES) analysis. The corrosion characteristics of the plasma nitrocarburized layer was evaluated by a potentiodynamic polarization technique using a Princeton Applied Research VersaSTAT 3 potentiostat in a 3.5% NaCl solution. The Ph value of the solution is 6.4, temperature is around 24°C, and deaeration process is carried out for 30minutes by injection clean N2 (99.999%) gas through plastic tube into the solutuion. 3.5% KCl Ag/AgCl was used for the reference electrode and platinum (Pt) was selected for the counter electrode. The corroded surface of the samples after the corrosion was observed by an Olympus SZ61TRC zoom stereo microscope.

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3. Results and Discussion: Several plasma nitriding (NT) process on AISI 630 samples were conducted in order to seek out the suitable condition. The best optimum condition till now, was found when the samples were nitrided with 25% N2 gas at 400°C, 400V discharge voltage and 4.0 Torr (532 Pa) vacuum pressure for 15 hr [20]. The X-ray diffraction patterns from Figure 1, shows that the two theta values of α' phase of bare sample moves to lower angle (from about 45 to 43 degree) in nitrided sample. This shifting of α' phase confirms the formation of expanded martensite (α'N phase) in surface of nitrided sample. Also the presence of Cr2N peak at around 45 degree was found. Figure 2 represents the differences in optical micrographs of the nitrided, nitrocarburized and the multiple nitriding-nitrocarburized AISI 630 samples and Figure 3 shows that the differences in the thickness of α'N layer (which were measured from the optical micrographs in Figure 2) and surface hardness of the treated samples. From Figure 2(a) and Figure 3, it can be seen that the obtained α'N layer thickness from nitriding, is about 16 µm and the surface hardness is around 1250 HV0.05, although the corrosion resistance is worse than the bare one as shown in potentiodynamic polarization curves in Figure 5. This poor behavior of corrosion resistance is due to the formation of Cr2N precipitates as confirmed by the X-ray diffraction patterns (Figure 1). The summary of all the nitriding experiments is that after nitriding, higher thickness of α'N layer can be obtained but the corrosion resistance have to be sacrificed.

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On the other hand, various plasma nitrocarburizing (NC) on AISI 630 samples were investigated to observe the behavior of the α'N layer and the corrosion resistance. Nitrocarburizing has opposite effect on the AISI 630 samples than nitriding. Nitrocarburizing improves the corrosion behavior of the treated samples but decreases their α'N layer thickness, though their surface hardness values are almost same as the nitrided one. The best corrosion behavior till now, was found when the samples were nitrocarburized with 25% N2 and 3-4% CH4 at 400°C temperature, 400V discharge voltage and 4.0 Torr (532 Pa) vacuum pressure for 15 hr [20]. The X-ray diffraction patterns from Figure 1, also confirms the presence of α'N phase in surface of nitrocarburized sample. The intensity of XRD peak at around 45 degree is weaker in single nitrocarburized sample than that of the single nitrided sample. Moreover, there is an overlapping of Cr2N phase and ε-Fe2-3(N,C) phase in XRD patterns in single nitrocarburized sample that indicates that the XRD peak at around 45 degree is not solely Cr2N phase, instead of the only Cr2N phase for single nitrided sample. The XRD peak around 38 degree confirms the formation of ε-Fe2-3(N,C) phase which is generally represented as a secondary peak for ε-Fe2-3(N,C) phase formation. The primary peak for ε-Fe2-3(N,C) phase forms near 45 degree. Thus, it can be assumed that the presence of the peak at around 45 degree in XRD patterns, is mainly due to the higher amount of ε-Fe2-3(N,C) phase formation. In addition, the good corrosion behavior of the nitrocarburized sample as shown in Figure 5, indicates that the formation of Cr2N precipitates is very low. Figure 2(b) and Figure 3 depict 3

ACCEPTED MANUSCRIPT that, around 10 µm of α'N layer thickness, and 1300 HV0.05 of the surface hardness are obtained when sample was nitrocarburized with 4% CH4, where their corrosion resistance is much higher than the bare sample (Figure 5). Thus nitrocarburizing improves the corrosion resistance, but decreases the α'N layer thickness, compared to nitriding.

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Therefore, in order to unite the benefits of plasma nitriding (NT) process (which is higher α'N layer thickness) and benefits of plasma nitrocarburizing (NC) process (which is good corrosion resistance), multiple combinations of simultaneous nitriding and nitrocarburizing were investigated on the AISI 630 samples. In this paper, this assembled processes are termed as Multiple NitridingNitrocarburizing process. This process has several steps. Nitriding and nitrocarburizing were organized in alternate steps, even their gas ratios and processing time were altered in different steps. Briefly it can be described that in this process, both nitriding and nitrocarburizing are done simultaneously and periodically for multiple times in a single 15 hr process. Some of the process condition are shown in Table II. The main purpose of this process is to increase the α'N layer thickness as well as obtaining a good corrosion behavior. From all processes shown in Table II, two best process parameters BC 1 (Multiple NT-NC 2) & BC 2 (Multiple NT-NC 5), were chosen to compare with the single nitriding (NT) and single nitrocarburizing (NC) process. BC 1 is a 6 step process, where NT were done with 25% N2 gas and NC were done with 20% N2 & 3% CH4 gas. There were total three NT steps with total nitriding time of 3 hour and three NC steps with total nitrocarburizing time of 12 hour. On the other hand BC 2 is a 4 step process, where NT were done with 20% N2 gas and NC were done with 20% N2 & 3% CH4 gas. 4 hour nitriding time was divided into two step NT process and 11 hour nitrocarburizing time was divided into two step NC process.

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From the micrographs in Figure 2(c & d), it can be seen that the α'N layer from multiple NTNC processes, are much cleaner than the other processes, which represents the Cr2N precipitation-free surface layer. Moreover, as shown in Figure 3, BC 2 shows maximum surface hardness and α'N layer thickness, which are around 1350 HV0.05 and 16 µm respectively. The layer thickness values are much close to single-nitriding and higher than the single-nitrocarburizing process, but the surface hardness values are a bit higher. Even BC 1 has higher α'N layer thickness (around 14µm) than the single nitrocarburized one (around 10 µm), though their surface hardness values are quite same.

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The distribution of nitrogen and carbon in the α'N layer of the nitrided, nitrocarburized and Multiple NT-NC processes (BC) are observed by the GDOES analysis (Figure 4). It can be seen that, in nitrided sample, nitrogen content (wt%) is very higher in the near surface and decreases gradually when moving towards the bulk. The diffusion of nitrogen in the nitrocarburized sample is lower than that of the nitrided sample. On the other hand, in multiple NT-NC processes (BC 1 and BC 2), the depth profile of nitrogen content (wt%) is in-between of nitrided and nitrocarburized samples. Moreover, in the nitrocarburized and multiple NT-NC processed (BC) samples, carbon diffuses further and penetrates deeper from the surface than the nitrogen. It can be assumed that nitrogen pushes carbon to go deeper into the bulk. This phenomena happens due to the higher diffusivity of carbon than that of nitrogen in BCC iron. Since it is more dense than the b.c.c structure, nitrogen in the martensitic structure is halted by the carbon atoms and thus restricts the further formation of α'N layer which represents the nitrogen supersaturated martensite layer [21]. Moreover, from Figure 4 it is clear that the thickness of α'N layer, largely depends on the nitrogen content. It also shows that the total depth possessed by nitrogen and carbon, in multiple NT-NC processed (BC) treated samples are higher than that in single NC sample and also higher than the depth possessed by nitrogen in single NT sample. Additionally, for the same reason, the multiple NT-NC processed (BC) samples shows a small increase in surface hardness values. Thus according to GDOES data, documented in Figure 4, multiple NT-NC process (BC) represents higher total layer thickness and higher surface hardness than the single nitriding and single nitrocarburizing process. The anodic potentiodynamic polarization curves of the treated samples are depicted in Figure 5. The anodic polarization curves were recorded with a sweep speed of 1 mV/sec. The test involves polarizing the working electrode (metal) away from its equilibrium (open circuit potential - OCP) by imposing a steadily changing DC potential difference between the metal and the counter electrode (Pt) through a potentiostat, while recording the current response. The higher corrosion potential (Ecorr) and lower current density (icorr) indicate higher corrosion resistance. In Figure 5 and Table II, both the multiple nitriding-nitrocarburizing processes shows better corrosion resistance than the single nitriding 4

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and single nitrocarburizing process. The corrosion potential of BC 2 is 133 mV, which is much higher than the bare one and the single processes and the corrosion current density is 0.02 μAcm-2, which is the lowest. Also the pitting potential of the BC 2 sample is much higher than the other samples. Due to the higher corrosion potential and higher pitting potential and lower corrosion current density BC 2 sample shows the best corrosion behavior. Thus, the surface after corrosion test of BC 2 is shows better result than the single processes (Figure 6). There is almost no trace of pitting or crevice corrosion on the tested surface of BC 2. The suppression of Cr2N in the multiple NT-NC process is mainly due to carbon in the NC step. In addition, Chromium has a strong affinity to nitrogen than carbon. It is experimentally proved in my laboratory that increasing NT process time more than three hours in the single NT process deteriorates the corrosion resistance due to the formation of chromium nitride in the α'N layer associated with the buildup of higher nitrogen contents, resulting from longer NT process time. Thus NT process time was limited to a maximum of 2 hours at each NT step in the multiple NT-NC process. 4. Conclusions 1. The surface hardness of the AISI 630 precipitation hardening stainless steel increased from around 358 HV0.05 (Bare) to more than 1350 HV0.05 in multiple NT-NC process. Also the α'N layer thickness of Multiple NT-NC treated AISI 630 sample, increased to around 16 µm which is thicker than the single nitrocarburizing process.

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The corrosion resistance of AISI 630 sample, treated with BC improved significantly. There is almost no pitting or crevice corrosion on the surface of the tested sample.

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After several trial and errors, the optimized condition for thicker α'N layer thickness, higher surface hardness and good corrosion resistance are finalized as, periodically nitriding (NT) and nitrocarburizing (NC) [4 hr NT in two steps + 11 hr NC in two steps] alternatively in a 15 hr process (BC 2), when nitriding with 20% N2 content and nitrocarburizing with 20% N2 and 3% CH4 content at 400°C temperature, 400 V discharge voltage and 4.0 Torr vacuum pressure for 15 hr.

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5. References: [1] F. Alonso, a Arizaga, a Garcia, J. Onate, Surf. Coatings Technol. 66 (1994) 291–295. B. Tesi, T. Bacci, G. Poli, Vacuum. 35 (1985) 307–314.

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C.J. Scheuer, R.P. Cardoso, M. Mafra, S.F. Brunatto, Surf. Coatings Technol. 214 (2013) 30–37.

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[21] J.R.G. da Silva, R.B. McLellan, Mater. Sci. Eng. 26 (1976) 83–87.

List of Table Captions:

Table I: Chemical compositions of AISI 630 martensitic precipitation hardening stainless steel. Table II: All the Parameters and Results of Multiple Nitriding-Nitrocarburizing Process. List of Figure Captions:

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Figure 1: X-ray diffraction patterns (Cu-Kα radiation) generated from the Bare, Nitrided and Nitrocarburized surface of AISI 630 samples Figure 2: Optical micrographs of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples

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Figure 3: Surface hardness and α'N layer of nitrided (NT), nitrocarburized (NC) and multiple nitridingnitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples

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Figure 4: GDOES analysis of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples

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Figure 5: Anodic Potentiodynamic Polarization curves of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples

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Figure 6: Microscopic View after potentiodynamic polarization test of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples

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ACCEPTED MANUSCRIPT Table I: Chemical compositions of AISI 630 martensitic precipitation hardening stainless steel.

Fe

AISI 630

Bal.

C

Si

0.035 0.386

Mn

P

S

Ni

0.716

0.027

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4.131

Cr

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Cu

15.349 0.094 3.282

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Materials

ACCEPTED MANUSCRIPT Table II: All the Parameters and Results of Multiple Nitriding-Nitrocarburizing Process Steps

Processing Conditions

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4 5 6 1 2

Multiple NT-NC 2

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Total NT Time: 3 hr in 3 Steps & Total NC Time: 12 hr in 3 Steps

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Multiple NT-NC 4

Total NT Time: 3 hr in 3 Steps & Total NC Time: 12 hr in 3 Steps

Total NT Time: 4 hr in 2 Steps & Total NC Time: 11 hr in 2 Steps

Corrosion Potential

HV (0.05)

(μm)

(mV)

Corrosion Current Density (μA/cm2)

1309.38

16.18

-197.568

0.933

14.12

53.458

0.203

1301.161 4

15.73

-66.537

0.148

1270.68

14.51

-8.25

0.128

1349.63

16.41

132.645

0.021

1277.46

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Total NT Time: 3 hr in 3 Steps & Total NC Time: 12 hr in 3 Steps

α'N Layer Thickness

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Total NT Time: 6 hr in 3 Steps & Total NC Time: 9 hr in 3 Steps

Nitriding (15% N2) Nitriding (20% N2) Nitriding (25% N2) Nitrocarburizing (20% N2, 1% CH4) Nitrocarburizing (20% N2, 2% CH4) Nitrocarburizing (20% N2, 3% CH4) Nitriding (25% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (25% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (25% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (25% N2) Nitrocarburizing (25% N2, 4% CH4) Nitriding (25% N2) Nitrocarburizing (25% N2, 4% CH4) Nitriding (25% N2) Nitrocarburizing (25% N2, 4% CH4) Nitriding (20% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (20% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (20% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (20% N2) Nitrocarburizing (20% N2, 3% CH4) Nitriding (20% N2) Nitrocarburizing (20% N2, 3% CH4)

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Multiple NT-NC 1

1 2 3

Surface Hardness

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Time

3 4 5 6 1 2 3 4

*Treatment Temperature: 400°C; Discharge Voltage: 400V; Vacuum Pressure: 4.0 Torr (532 Pa); Total Processing Time: 15 hr. ** BC = Best Condition.

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 α'

 

 α' N

 Cr2N

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Figure 1: X-ray diffraction patterns (Cu-Kα radiation) generated from the Bare, Nitrided and Nitrocarburized surface of AISI 630 samples.

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(c) BC 1 (d) BC 2 Figure 2: Optical micrographs of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples.

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Surface Hardness(Hv0.1) 'N Layer Thickness( m )

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1200

Layer Thicknes s (

S u rface Hardn es s (Hv 0.1 )

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12 800

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BC 2

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Surface Hardness ((HV0.05))

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Figure 3: Surface hardness and α'N layer thickness of nitrided (NT), nitrocarburized (NC) and multiple nitridingnitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples.

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5



AC

2



0

0

1

  

Carbon Content (wt%)

4

 N- NT 25% N2  N- NC 25% N2; 4% CH4  N- BC 1  N- BC 2 3  C- NT 25% N2  C- NC 25% N2; 4% CH4  C- Multiple Method 1  C- Multiple Method 2

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10

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Nitrogen Content (wt%)

15

0 10

20

30

Depth (µm)

Figure 4: GDOES analysis of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples.

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ACCEPTED MANUSCRIPT  Bare  NT 25% N2  NC 25% N2; 4% CH4

1.5

 BC 1  BC 2

1.0



0.5

 

0.0

 

-0.5 -1.0 1E-9

1E-8

1E-7

1E-6

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Potential E(V) vs Ag/AgCl

2.0

1E-5

1E-4

2

1E-3

0.01

0.1

Current Density (A/cm )

Crevice Corrosion

Corrosion Scar Pitting Corrosion

(a) Bare

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CE

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NU

Figure 5: Anodic Potentiodynamic Polarization curves of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples.

(b) NT 25% N2

(c) NC 25% N2, 4% CH4

(d) BC 1 (e) BC 2 Figure 6: Microscopic View after potentiodynamic polarization test of nitrided (NT), nitrocarburized (NC) and multiple nitriding-nitrocarburizing [Best Conditions (BC 1 & BC 2)] processed AISI 630 samples.

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ACCEPTED MANUSCRIPT

Highlights (For Review)

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The changes have been highlighted in blue in the manuscript. The main changes are as follows:

1. All the comments and required changes were taken care of “step-by-step and point-bypoint” in the final manuscript which is presented in the following sections. 2. The originality of this work is to combine these two beneficial properties of nitriding and

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nitrocarburizing processes, which results in the increment of the α'N layer thickness,

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surface hardness, and improvement of corrosion behavior of the AISI 630 martensitic stainless steel. Repetition of a combination of nitriding and nitrocarburizing processes is

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made for the first time in my laboratory.

3. This research is based on experiments to develop know-how that can be applied directly

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in the industrial field. Therefore, it is a kind of technology development process through

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trial and error methods without being based on a profound theory. 4. The suppression of Cr2N in the multiple NT-NC process is mainly due to carbon in the

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NC step. In addition, Chromium has a strong affinity to nitrogen than carbon. It is experimentally proved in my laboratory that increasing NT process time more than three hours in the single NT process deteriorates the corrosion resistance due to the formation of chromium nitride in the α'N layer associated with the buildup of higher nitrogen contents, resulting from longer NT process time. Thus NT process time was limited to a maximum of 2 hours at each NT step in the multiple NT-NC process.

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