Effect of the amount of CH4 gas on the characteristics of surface layers of low temperature plasma nitrided martensitic precipitation-hardening stainless steel

Effect of the amount of CH4 gas on the characteristics of surface layers of low temperature plasma nitrided martensitic precipitation-hardening stainless steel

SCT-21363; No of Pages 7 Surface & Coatings Technology xxx (2016) xxx–xxx Contents lists available at ScienceDirect Surface & Coatings Technology jo...

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SCT-21363; No of Pages 7 Surface & Coatings Technology xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Effect of the amount of CH4 gas on the characteristics of surface layers of low temperature plasma nitrided martensitic precipitation-hardening stainless steel Abhik Barua, Hojun Kim, Insup Lee ⁎ Department of Advanced Materials Engineering, Dongeui University, Busan 614-714, Republic of Korea

a r t i c l e

i n f o

Article history: Received 31 October 2015 Revised 10 March 2016 Accepted in revised form 11 July 2016 Available online xxxx Keywords: Plasma nitriding AISI 630 stainless steel Corrosion resistance Hardness Expanded martensite layer

a b s t r a c t Plasma nitriding was performed on AISI 630 martensitic precipitation hardening stainless steel samples at 400 V with a gas mixture of H2 and N2 for 19 h with changing N2 percentage, temperature and adding various percentages of CH4. After treatment the behavior of the surface layer was investigated by optical microscopy, X-ray diffraction, GDOES analysis and micro-hardness testing. A potentiodynamic polarization test was also used to evaluate the corrosion resistance of the samples. With increasing N2 percentage from 15% to 35% at 380 °C, the thickness of the expanded martensite (α′N) layer increases to about 20 μm and surface hardness also increases to about 1280 HV0.05 but the corrosion behavior is worse than the untreated sample. When nitrided with 25% N2, increasing nitriding temperature from 380 °C to 460 °C also increases the α′N layer thickness to around 38 μm and surface hardness to around 1270 HV0.05 (at 460 °C), but decreases the corrosion resistance. Thus in order to improve the corrosion resistance, various amounts of CH4 (0% to 6%) were introduced in the nitriding atmosphere at 400 °C with 25% N2 percentage. Increasing CH4 percentage slightly decreases the α′N layer thickness but greatly improves corrosion behavior. Moreover, with 4% CH4 percentage, the best corrosion behavior is obtained which also has around 11 μm α′N layer thickness and about 1290 HV0.05 surface hardness. Therefore, the conclusion can be drawn that, the highly suitable condition for plasma nitriding for a thicker expanded martensite layer, higher surface hardness and good corrosion resistance is plasma nitriding at 400 °C with 25% N2 and 4% CH4 for 19 h. © 2016 Elsevier B.V. All rights reserved.

1. Introduction 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 mid-1980s, nitrogen supersaturated expanded austenite phase (γN or S phase) layers were obtained on the surface of austenitic stainless steel by low temperature nitriding [1,2]. Since then, low temperature thermochemical processes such as plasma nitriding, plasma immersion ion implantation (PIII), carburizing, and nitrocarburizing which is a hybrid processes of nitriding and carburizing have all been investigated [3–5]. These treatments can be successfully used to produce a nitrogen and/or carbon supersaturated expanded austenite phase on the surfaces of various austenitic stainless steels, and then achieve combined improvements in surface

⁎ Corresponding author. E-mail address: [email protected] (I. Lee).

hardness, wear resistance and corrosion resistance due to the superior properties of the “S-phase”. Martensitic precipitation-hardening stainless steel has been used for a variety of applications including oil field valve parts, gears, fasteners, pump, nuclear reactor components, missile fitting and rotors of centrifugal compressors. However due to their poor tribological properties, their wider applications are restricted. The thermochemical processing of martensitic stainless steel got less attention compared to that of austenitic stainless steel. Very few research has been carried out on the effect of nitriding on martensitic precipitation hardening stainless steel [6–15]. The formed layer is called “α′N layer” (nitrogen supersaturated expanded martensite layer) in the case of martensitic stainless steels just 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 [6–8], carburizing [10,13], and also nitrocarburizing [14,15]. But in all those cases poor corrosion behavior was reported because of the formation of chromium nitrides during nitriding. Thus the main concern of the present study is to increase the corrosion resistance as well as the surface hardness and the thickness of the expanded martensite layer of low temperature (b 450 °C) nitrided AISI

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towards zero. It is easy to determine. However corrosion current density (icorr) is difficult to measure directly. It is generally measured by “Tafel regions”. Intersecting the extrapolated linear segment lines (Tafel line) of both anodic and cathodic curves with a horizontal line drawn from Ecorr, parallel to the current density axis will yield the icorr. Higher corrosion potential (Ecorr) and lower current density (icorr) indicate higher corrosion resistance. After corrosion, the corroded surface of the samples was observed by using an Olympus SZ61TRC zoom stereo microscope.

630 martensitic precipitation hardening stainless steel. Several investigations were done to determine the suitable nitrogen percentage and nitriding temperature for better corrosion resistance without compromising the layer thickness and surface hardness. Then, various amounts of CH4 gas were also introduced in the nitriding atmosphere and their effects were investigated. 2. Experimental procedure Circular coupons of solution treated AISI 630 martensitic precipitation-hardening stainless steel of 16 mm diameter and 3 mm thickness were prepared for the treatment. It is generally known that the solution treatment was done at 1050 °C for 30 min/in. and then quenched in water. The composition of AISI 630 martensitic precipitation-hardening stainless steel is given in the Table 1. The solution treated samples are denoted as “bare” in this paper. The surfaces of the circular coupons were mirror polished by means of an automatic polishing machine and then cleaned before starting the plasma treatment. Then, the samples were placed on the cathode table in the plasma ion nitriding system. The next step is a pre-sputtering operation. The plasma chamber has to be evacuated below 50 mTorr before starting this operation. During this operation Ar and H2 ion sputtering was performed for at least 40 min for further surface cleaning. After sputtering, a plasma nitriding process was immediately carried out at various N2 percentages (15%–35%), different temperatures (380 °C to 460 °C) and with the introduction of various amounts of CH4 (0% to 6%) for 19 h in the glow discharge environment in a gas mixture of N2 and H2 and at 4.0 Torr vacuum pressure and 400 V discharge voltage. After treatment, the samples were cooled in the vacuum chamber up to room temperature. The plasma nitrided samples were sectioned for metallographic examination and surface hardness determination. The samples were cut, mounted, polished and finally the cross-sectional surfaces were etched with Vilella's reagent (1 g picric acid + 100 ml ethyl alcohol + 5 ml HCl) [16]. The microstructures of the surface of all the samples were observed by using an Olympus BX51M optical microscope. A Rigaku D/Max-200 X-ray diffractometer was employed to analyze different phases formed on the treated surfaces by using Cu-Kα radiation (λ = 1.544 Å). Microhardness measurements were carried out with the digital Micro Vickers hardness tester Matsuzawa MMT-X7B using an indentation load of 0.49 N (50 g) and a loading time of 10 s. Glow discharge optical emission spectrometry (GDOES) analysis of the treated samples was also performed to analyze the distribution of nitrogen and carbon in the crystal lattice. A potentiodynamic polarization technique was applied to estimate the corrosion characteristics of the plasma nitrocarburized layer in a 3.5% NaCl solution by using a Princeton Applied Research VersaSTAT 3 potentiostat. For the reference electrode 3.5% KCl Ag/AgCl was selected and platinum (Pt) was used for the counter electrode. The anodic polarization curves were recorded with a sweep speed of 1 mV/s. 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 horizontal axis (X-axis) represents log current density, which measures the corrosion rate, while the vertical axis (Y-axis) represents applied potential. The corrosion potential (OCP) is illustrated by a horizontal line in each polarization curve. Ecorr is the intersection where cathodic current density and anodic current density approach

3. Result and discussion 3.1. XRD analysis Fig. 1 shows the XRD analysis of AISI 630 samples nitrided at 400 V for 19 h with (a) changing nitrogen percentage at 380 °C, (b) changing nitriding temperature at 25% N2 and (c) effect of adding CH4 with fixed 25% N2 percentage at 400 °C. It shows that the bare sample has a single diffraction peak of the martensite phase (α′–Fe, α–Fe with carbon). Elsewhere nitrogen expended martensite (α′N), ε-Fe2–3(N,C)-iron carbonitride and some Cr2N phases can be observed in the plasma nitrided samples. The peak of α′N is visible in the range of 40–45° which is relatively well-defined than the other peaks. This peak does not match with any possible phases including iron, iron nitride and chromium nitride in the treated surface. This peak can be related to the expanded martensite, as suggested by Kim et al. [6]. From Fig. 1(a) it is clear that, the increment in nitrogen percentage increases the possibility of chromium nitride formation. Thus nitrogen percentage should be low enough so that no chromium nitride can form. Also high temperature favors the formation of chromium nitride, as can be seen from Fig. 1(b). At 460 °C, the Cr2N peak represents the dominant peak, which is not good for corrosion behavior. Adding CH4 in the nitriding atmosphere decreases the formation of Cr2N (Fig. 1(c)). But this reducing effect of Cr2N happens only up to a certain percentage of CH4 which is 4% CH4. Addition of CH4 beyond 4% slightly increases the formation of Cr2N. In Fig. 1(c) the broad and low intensity peaks of the nitrocarburized samples at around 38° is ε-Fe2–3(N,C)iron carbonitride. It is quite difficult to identify the Cr2N peak separately, as it is overlapped with the ε-Fe2–3(N,C) peak. Also it is the reason the overlapped-Cr2N peak is higher in 2% CH4 than the 0% CH4. 3.2. GDOES analysis for effect of adding CH4 at 400 °C Fig. 2 shows the glow discharge optical emission spectrometry (GDOES) analysis of the nitrided samples treated with different amounts of CH4. It shows the nitrogen and carbon content (at.%) of the treated samples. It can be seen that the addition of CH4 decreases the nitrogen content (at.%) in the nitrided surface of the samples. From these analyses it is clear that the thickness of the α′N layer found on the microstructures (Fig. 3(c)) mainly depend on nitrogen. Higher nitrogen diffusion through the lattice means higher layer thickness. Thus, nitrogen content (at.%) determines the layer thickness which appears as a white layer in the microstructure. For this reason, the increment of CH4 decreases the nitrogen content (at.%) on the surfaces of 3% and 4% CH4 treated samples and thus decreases the layer thickness. 3.3. Effect of nitrogen percentage Optical micrographs of AISI 630 samples plasma nitrided at 400 V for 19 h with (a) changing nitrogen percentage at 380 °C, (b) changing

Table 1 Chemical compositions of AISI 630 martensitic precipitation hardening stainless steel. Materials

Fe

C

Si

Mn

P

S

Ni

Cr

Mo

Cu

AISI 630

Bal.

0.035

0.386

0.716

0.027

0.001

4.131

15.349

0.094

3.282

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Fig. 2. GDOES analysis for the effect of adding CH4 at 400 °C.

nitriding temperature at 25% N2 and (c) effect of adding CH4 with fixed 25% N2 percentage at 400 °C are shown in Fig. 3. In Fig. 4 surface hardness and α′N layer thickness of plasma nitrided AISI 630 samples are depicted. Moreover Fig. 5 shows the corrosion behavior of plasma nitrided AISI 630 samples by a potentiodynamic polarization test. By increasing the nitrogen percentage from 15% to 35% at 380 °C, the α′N layer thickness increased up to 20 μm (Fig. 3(a)). The surface hardness value also increased by about 1280 HV0.05. The effect of nitrogen percentage with layer thickness and surface hardness is shown in Fig. 4(a). Increasing nitrogen percentage, increases the active species present in the nitriding atmosphere, which means more dissociation of nitrogen molecules to ionic species for subsequent reactions and adsorption on the steel surface. More atomic reacted nitrogen atoms can then diffuse into interstitial sites of steel, resulting in a higher thickness of the α′N layer. Nevertheless, increasing the nitrogen percentage had an opposite effect on corrosion behavior. A higher nitrogen percentage deteriorates the corrosion resistance of the treated samples, even worse than the bare sample as depicted in the potentiodynamic polarization curve in Fig. 5(a). These nitrided samples show no passivation due to an unstable Cr2O3 film on the surface of the samples. The reason behind this poor corrosion behavior is Cr2N formation, confirmed earlier in the XRD analysis (Fig. 1(a)). The bare sample has a small passivation height. The current density for the bare specimen is low at the beginning, but it increases very rapidly when the potential exceeds 90 mV (vs SCE). The well-known cause of the sudden increase in current density is the break-down of the passive film which causes pitting on stainless steel surface, which leads to localized corrosion. Considering the layer thickness, surface hardness and corrosion behavior, 25% N2 was selected as a moderate nitrogen percentage for further experiments. 3.4. Effect of nitriding temperature

Fig. 1. XRD patterns of AISI 630 samples nitrided at 400 V for 19 h. (a) Changing nitrogen percentage at 380 °C. (b) Changing nitriding temperature at 25% N2. (c) Effect of adding CH4 with fixed 25% N2 percentage at 400 °C.

Increasing the nitriding temperature, sharply affected the layer thickness and the surface hardness which can be seen from Fig. 3(b). Changing the temperature from 380 °C to 460 °C when treated with 25% nitrogen, increased the layer thickness from around 13 μm to 38 μm and the surface hardness values from around 1204 HV0.05 to 1266 HV0.05. These changes are featured in Fig. 4(b). Higher nitriding temperature increases the diffusivity of nitrogen atoms through the lattice which increases the layer thickness due to the diffusion-controlled growth [17]. Nitrogen atoms diffuse only along the grain boundaries at low temperature. At higher temperature, the mobility of nitrogen atoms increases and the diffusion through the lattice becomes easier than the grain boundary diffusion. Thus a more uniform and thicker layer is formed due to the easy penetration of these nitrogen atoms through the grains. But the possibility of chromium nitride (Cr2N)

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Fig. 3. Optical micrographs of AISI 630 plasma nitrided samples at 400 V for 19 h. (a) Changing nitrogen percentage at 380 °C. (b) Changing nitriding temperature at 25% N2. (c) Effect of adding CH4 with fixed 25% N2 percentage at 400 °C.

formation also increases with increasing temperature. Cr2N precipitates along the grain boundaries and forms dark lines on the white expanded martensite layer (Fig. 3(b)). The XRD analysis of Fig. 1(b) also confirms the formation of these nitrides. The formation of these chromium

nitrides can also be explained by the diffusion mechanism. At low treatment temperature, the diffusion of an interstitial element (nitrogen) to the iron lattice is favorable, while the diffusion of a substitutional element (chromium) is restricted. Elsewhere, higher temperature favors

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Fig. 4. Surface hardness and the α′N layer of plasma nitrided AISI 630 samples. (a) Changing nitrogen percentage at 380 °C. (b) Changing nitriding temperature at 25% N2. (c) Effect of adding CH4 with fixed 25% N2 percentage at 400 °C.

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Fig. 5. Anodic potentiodynamic polarization curves of plasma nitrided AISI 630 samples. (a) Changing nitrogen percentage at 380 °C. (b) Changing nitriding temperature at 25% N2. (c) Effect of adding CH4 with fixed 25% N2 percentage at 400 °C.

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the diffusion of substitutional elements as their mobility increases and thus helps to precipitate chromium nitrides. Also Cr2N precipitates drop the corrosion resistance of the treated samples, as shown in Fig. 5(b). The corrosion behavior is still very poor compared to the bare specimen. Yet, considering the chromium nitride precipitation and layer thickness values, 400 °C was chosen as the effective temperature for the following analysis. The layer thickness and surface hardness values obtained at 400 °C were around 16 μm and 1235 HV0.05 respectively. 3.5. Effect of adding CH4 Addition of carbon to the plasma atmosphere during low-temperature plasma nitriding significantly alters the structural development and characteristics of the α′N layer in martensitic stainless steel. Methane was used as a carbon containing gas, as in the plasma chamber, methane dissociated to carbon and hydrogen. With moderate nitrogen percentage (25%) and at effective temperature (400 °C) various experiments were conducted by changing the amount of methane from 0% to 6% in the nitriding atmosphere. Adding methane decreased the layer thickness a little but it also decreased the dark lines (Fig. 3(c)). As from Fig. 4(c), the surface hardness values are quite average but the layer thickness decreased from around 16 μm to 10 μm when methane is introduced from 0% to 6%. The great breakthrough was achieved in their corrosion resistance. As can be seen from the anodic polarization curves from Fig. 5(c), corrosion resistance increased with increasing

CH4 percentage. A treated sample with 4% CH4 shows the best corrosion behavior. Corrosion potential increases from around −217 mV (bare) to − 63 mV (4% CH4) and corrosion current density decreases from around 1.031 μA/cm2 (bare) to 0.309 μA/cm2 (4% CH4). Also pitting potential is very high (around 1.3 V) compared to the bare sample (0.05 V). Although corrosion behavior decreased very little when CH4 percentage is further increased, like with the 6% CH4 treated sample shown in Fig. 5(c), it is still better than the bare one. Further research is needed to investigate the reason behind this behavior. Fig. 6 shows the macroscopic view after the potentiodynamic polarization test. It shows that the bare sample was severely attacked by crevice corrosion and pitting corrosion during the polarization test. There were clearly visible large corrosion pits inside the corrosion scar (the comparatively dark region in the center) and also a wide and deep crevice corrosion trench towards the edge of the corrosion scar on the bare sample. With increasing CH4 percentage the severity of pitting and crevice corrosion decreases. There is almost no corrosion that occurred in the 4% CH4 treated sample. The 6% CH4 treated sample has some pitting corrosion in the center. The fact that, the Cr2N peak of the 4% CH4 treated sample is lower than the other samples (Fig. 1(c)) and there are also less dark lines in the microstructure compared to the other samples (Fig. 3(c)), again confirms the best corrosion behavior of the 4% CH4 treated sample. The layer thickness and surface hardness values of the 4% CH4 treated sample at 400 °C and with 25% N2, are also relatively higher which are about 11 μm and 1294 HV0.05 respectively. Although the α′N layer thickness of these samples is decreased, the dark lines

Fig. 6. Microscopic view after potentiodynamic polarization test.

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on the layer are also reduced, which means less precipitations of chromium nitride, leading to better corrosion resistance. Explanations of this kind of behavior of carbon can be interpreted from the acquired results. However, there is almost no explanation in all the works done on the effect of CH4 or effect of carbon on martensitic stainless steel in the literature. Further research is needed to explain the exact reason for the effect of CH4. According to Fig. 4(c), it can be seen that the α′N layer thickness decreases with increasing CH4 percentage. As the α′N layer represents the nitrogen supersaturated martensite layer, it can be assumed that the nitrogen incorporation from the surface to the bulk of the sample is decreasing with an increasing amount of CH4 percent. GDOES analysis in Fig. 2 confirms this assumption. Fig. 2 also shows that the depth possessed by nitrogen in 0% CH4 and that possessed by both nitrogen and carbon in 3% & 4% CH4 are the same. So, the decrease in α′N layer thickness is caused only by a decrease in nitrogen incorporation. Also, for the same reason, the surface hardness remains the same and does not decrease though the α′N layer thickness decreases with an increasing amount of CH4. Moreover the diffusivity of carbon is higher than that of nitrogen in BCC iron. According to J.R.G. da Silva et al. [18] the energy required for transporting a C-atom from an octahedral site in BCC iron in a neighboring tetrahedral site would be less than the corresponding energy for an N-atom. Thus, carbon diffuses to a further depth from the surface than nitrogen. Being more closely packed than the BCC structure, in martensitic structure, nitrogen is halted by the carbon atoms and thus restricts the further formation of the α′N layer. 4. Conclusions Based on the experimental results obtained in this work, several conclusions can be drawn as follows: 1. An increase of nitrogen percentage from 15% to 35% in the plasma atmosphere during nitriding helped to increase the thickness of the expanded-martensite layer (α′N layer) up to 20 μm and surface hardness by about 1280 HV0.05 (at 35% N2). However it also increased the Cr2N precipitation which was the reason for their poor corrosion behavior. 2. Changing the temperature from 380 °C to 460 °C also increased the thickness of the α′N layer and surface hardness by around 38 μm and 1266 HV0.05 (at 460 °C) respectively. But again their corrosion resistance is poor at this high temperature due to the formation of Cr2N. 3. The addition of carbon leads to the decrease of layer thickness. It also decreases the dark lines on the α′N layer, which are the major cause of poor corrosion resistance and helps to improve corrosion behavior. Increasing CH4 percent from 0% to 6%, decreased the α′N layer thickness from around 16 μm (0%) to 10 μm (6%) approximately, though their hardness is quite the same. However, the introduction of CH4 helps significantly on their corrosion resistance. The best corrosion behavior was found when a sample was nitrided with 4% CH4. Thus from the above experiments the conclusion can be drawn that, the highly suitable condition for plasma nitriding for a thicker expanded

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martensite layer, higher surface hardness and good corrosion resistance is plasma nitriding at 400 °C with 25% N2 and 4% CH4 for 19 h.

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