Effects of DC plasma nitriding parameters on microstructure and properties of 304L stainless steel

Effects of DC plasma nitriding parameters on microstructure and properties of 304L stainless steel

M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 1 9 7 –2 0 3 Effects of DC plasma nitriding parameters on microstructure and properties of 304...

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M A TE RI A L S CH A RACT ER IZ A TI O N 60 ( 20 0 9 ) 1 9 7 –2 0 3

Effects of DC plasma nitriding parameters on microstructure and properties of 304L stainless steel Jun Wang a,b , Ji Xiong a,⁎, Qian Peng b , Hongyuan Fan a , Ying Wang b , Guijiang Li a , Baoluo Shen a a

School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China Nuclear Power Institute of China, Chengdu 610041, PR China

b

AR TIC LE D ATA

ABSTR ACT

Article history:

A wear-resistant nitrided layer was formed on a 304L austenitic stainless steel substrate by

Received 14 July 2008

DC plasma nitriding. Effects of DC plasma nitriding parameters on the structural phases,

Accepted 30 August 2008

micro-hardness and dry-sliding wear behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, scanning electron microscopy, micro-hardness testing

Keywords:

and ring-on-block wear testing. The results show that the highest surface hardness over a

304L stainless steel

case depth of about 10 µm is obtained after nitriding at 460 °C. XRD indicated a single

DC plasma nitriding

expanded austenite phase and a single CrN nitride phase were formed at 350 °C and 480 °C,

Microstructure

respectively. In addition, the S-phase layers formed on the samples provided the best dry-

Temperature

sliding wear resistance under the ring-on-block contact configuration test.

Wear

1.

© 2008 Elsevier Inc. All rights reserved.

Introduction

Austenitic stainless steels are ternary Fe–Cr–Ni alloys with excellent corrosion resistance, and for this reason are widely used in many industrial domains, such as in biomedical, food and chemical processing, petrochemical and especially in nuclear industries [1,2]. However, their tribological properties, such as hardness and wear resistance are poor [3,4]. An increase in hardness without losing corrosion resistance could significantly broaden their applications [5]. Austenitic stainless steel is known as a material difficult to nitride [6], and standard nitriding techniques such as gas nitriding or ion nitriding, which are widely used in industry, failed to improve its mechanical properties without loss of corrosion resistance. The problem is associated with the fact that these techniques are efficiently applied at temperatures higher than 450 °C. At these temperatures, the precipitation of CrN resulting in a depletion of Cr from the matrix takes place. This induces a strong decrease of the corrosion resistance that negates the beneficial effect of increased hardness. So the

traditional gaseous processes (applied above 480 °C) are not apt for surface hardening of stainless steel without loss of corrosion resistance. Fortunately, it has been demonstrated that plasma nitriding is an effective means of increasing the surface hardness and wear resistance of stainless steels [7]. In particular, lowtemperature plasma nitriding can improve the wear resistance of popular austenitic stainless steels such as Types 316 and 304 without loss of corrosion resistance [8,9] by producing a metastable phase consisting of supersaturated nitrogen atoms (nitrogen concentration is 20–30 at.%) in the austenite matrix. This phase is usually called expanded austenite [10,11] S-phase, or γN phase[12,13] and exhibits four to five times improved hardness while the wear resistance is enhanced by several orders of magnitude [14]. The understanding of the various processes taking place on the very surface of the material is of great importance, because they control the incorporation rate of nitrogen atoms that become available for the temperature-dependent inward diffusion into the lattice. Specific nitriding conditions and/or

⁎ Corresponding author. Tel.: +86 13880085119. E-mail address: [email protected] (J. Xiong). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2008.08.011

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Table 1 – Chemical composition of stainless steel used in the experiment (mass %) Cr

Ni

Mo Cu Nb Mn

304L 19.20 8.60 –





Si

S

P

C

Fe

1.95 0.96 0.03 0.043 0.03 Bal.

pre-treatment of the material surface could be optimized in such a way as to increase the nitrogen uptake or by increasing the nitrogen incorporation rate during the treatment. Moreover, understanding the role of the plasma nitriding parameters could contribute to optimizing the efficiency of nitriding in order to produce thicker modified layers for identical treatment times. However, not enough research work has been carried out on the effects of plasma nitriding parameters on the microstructure and dry-sliding wear behavior of 304L stainless steel to understand it [15]. In addition, new insights into the dry-sliding mechanism of the S-phase layers may be achieved by such testing. Therefore, the purpose of this paper was to investigate the effect of DC plasma nitriding parameters on the surface structure of 304L austenite stainless steel, and thereby to study the wear properties and the mechanisms of 304L stainless steel under dry-sliding wear conditions in a ringon-block contact configuration.

2.

Experimental

2.1.

Materials and Treatments

The material used in this work was a 304L austenite stainless steel with the chemical composition (in wt.%) shown in Table 1. The specimens were cut from a hot rolled bar and then machined into a size of 10 mm × 10 mm × 10 mm. The flat surfaces of the block sample were manually ground to 1200 grit to achieve a fine finish (Ra ~ 0.1 µm) and then ultrasonically cleaned with acetone, alcohol and distilled water in succession before plasma nitriding. Plasma nitriding was carried out using a Type CTI-7C400M DC plasma nitriding unit in a 20% N2 + 80% H2 atmosphere at a pressure of 6 × 10− 3 bar. A wide range of processing temperatures from 350 to 480 °C and times ranging from 4 to 8 h were used (Table 2). Sputtering pre-treatment was carried out

Fig. 1 – Optical micrograph and SEM micrograph of crosssectional microstructure of sample nitrided at (a) 400 °C. (b) 440 °C.

during the heating-up stage for a period of about 2 h using the same gas composition.

2.2.

Characterization

After DC plasma glow-discharge nitriding, the microstructure of the samples was characterized with a Type Dmax-1400 Xray diffraction analyzer (XRD) using Cu K α radiation (λ = 0.154056 nm) and the micro-hardness was measured

Table 2 – Different nitriding processes used for 304L SS Process 0 1 2 3 4 5 6 7 8 9 11 12 13

Temperature/°C

Time/h

Pressure/Pa

N2:H2

Voltage/V

Current density /mA cm− 2

0 350 400 420 460 480 420 400 350 400 420 400 350

4 4 4 4 4 4 4 4 4 8 8 8

600 600 600 600 600 400 400 400 200 600 600 600

62:240 58:196 47:201 45:180 49:199 42:172 40:161 45:186 42:168 46:191 60:246 53:220

520 520 520 520 520 520 520 520 520 520 520 520

0.89 0.90 0.80 0.90 0.80 0.90 0.90 0.90 0.90 0.80 0.85 0.80

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199

same nitriding process as the present study. Comparing the current results for Type 304L with those from Ref. [16] for AISI Type 316, the activation energy of the Type 304L sample is much lower, which indicates that a thicker diffusion layer can be formed in Type 316.

3.2.

Fig. 2 – Case depths of 304L SS plasma nitrided at various temperatures.

using a Type HV-1000 Vickers micro-hardness tester. The cross-sectional morphology was observed by using optical microscopy (OM). Elemental depth profiles were obtained with a Type JSM5910-LV scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS). Dry-sliding tribological tests were performed using a rotational speed of 200 rpm (equating to a linear sliding velocity of 9.59 cm/s) for durations up to 3600 s. A normal operating load of 30 N was used. For the counter body, 50-mm diameter GCr15 rings were selected. The wear loss was measured with a Type TG328A photoelectron analytical balance with an accuracy of 0.1 mg. The wear rates of the original substrate and all the nitrided layers were calculated using the equation K = W/S where W is the wear weight in mg, and S is the total sliding distance in km.

3.

Results and Discussion

3.1.

Microstructural Analysis

Fig. 1(a, b) shows the cross-sectional microstructure of nitrided samples. The modified layer appears in both of the samples after etching. It should be noted that there is no obvious diffuse zone on the austenitic stainless steel. A dense case approximately 6.5 µm thick (Fig. 1(a)) produced at 400 °C is seen to be thinner than that produced at 440 °C (Fig. 1(b)). The variation of treated layer thickness as a function of temperature in the plasma nitriding process is shown in Fig. 2. It can be seen that thickness of the treated layer is a maximum at 480 °C and the thickness is approximately linear with temperature. By analyzing the data of layer thickness against time, an Arrhenius-type plot was constructed (Fig. 3); the results of which indicate an activation energy of 42.7 kJ/mol. To set this in context, the activation energy obtained by Fewell et al. for the low-pressure RF-plasma nitriding of AISI 316 is 71.1 kJ/mol [16] and by Menthe and Rie for the pulsed-plasma nitriding of the same alloy is 129.6 kJ/ mol [17]. The first of these used the

XRD Analysis

Fig. 4 shows different X-ray diffraction patterns of nitrided and untreated samples for various nitriding temperatures. (The results in Fig. 4 are those shown in Table 2 for “processes” 0 through 5; for all cases, the nitriding time was 4 h and the pressure was 600 Pa.). A conventional diffraction pattern of unnitrided samples (labeled “0” in the figure) is shown for comparison. The untreated stainless steel contains deformation martensite (ferrite) in the surface region as a consequence of mechanical polishing. According to these patterns, it is obvious that the microstructures of the nitrided layer were significantly dependent on the nitriding temperature. With an increase in nitriding temperature, the expanded austenite peaks were weakened (labeled as “S”), while the CrN peaks were clearly detected at 460 °C and 480 °C (temperatures labeled 4 and 5). The S-phase peaks disappear at temperature ≥460 °C. This is probably due to the fact that the precipitation of CrN depletes the expanded phase of chromium at 460 °C, favoring the formation of a mixed layer of expanded martensite (or austenite) and CrN (S-phase → αN + CrN)[18]. In fact, the solubility limit of nitrogen in the austenitic structure is exceeded at 460 °C and 480 °C and then precipitation of Cr nitrides and ferrite occurs. Normally, the decomposition of expanded austenite (S-phase) yields ferrite and CrN leading to broadened ferrite peaks due to the presence of small precipitates of CrN. Moreover, in case of sample nitrided at 480 °C, a single CrN phase in addition to the austenite reflections from the substrate material is observed. The reason for this is most probably owing to the decomposition and chemical reaction of metastable S-phase with the incoming active nitrogen species during the plasma nitriding process. Also, a relatively stable CrN nitride is much easier to form at 480 °C because Cr nitride formation is favored due to

Fig. 3 – Plot of layer thickness [ln(d2)] versus inverse temperature (1/T).

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Fig. 4 – XRD patterns of nitrided and unnitrided 304L SS at different temperatures (600 Pa).

Fig. 5 – XRD patterns of nitrided and unnitrided 304L SS at different pressures (at 400 °C).

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Fig. 6 – XRD patterns of nitrided and unnitrided 304L SS at different times.

the high negative enthalpy and low Cr diffusivity in the matrix at temperatures higher than 450 °C [19]. The expanded austenite in the nitride layers is observed to exhibit a set of broad peaks. This broadening is probably due to the gradient of nitrogen, residual stresses, and possible defect structure of the nitride layers [20]. Fig. 5 shows X-ray diffraction patterns of nitrided and unnitrided samples for various nitriding pressures from 200 to 600 Pa; these specimens were nitrided at 400 °C. According to

these patterns, the compositions of the nitrided layers were slightly dependent on the nitriding pressure. With an increase in pressure, the expanded austenite and CrN peaks are more intense, which indicate that high gas pressure can boost the extent of the reaction. X-ray diffraction patterns of nitrided and unnitrided samples for various nitriding times are shown in Fig. 6; results

Fig. 7 – Micro-hardness of 304L SS plasma nitrided at various temperatures.

Fig. 8 – Dry-sliding wear rate of 304L SS as a function of nitriding temperature.

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are shown for nitriding temperatures of 400 and 420 °C. According to these results, the phase compositions of the nitrided layer did not change within the variation of the nitriding times considered. The influence of nitriding time on the microstructure was mainly in the nitrided layer depth and hardness, and not in the phase compositions.

3.3.

Hardness and Wear Behavior

Fig. 7 shows the micro-hardness of the nitrided layers as a function of temperature. As shown in Fig. 7, a steep microhardness increase was found on the sample surface and was observed to exhibit a continued gradual increase until the temperature exceeded 460 °C. The maximum values measured from the treated surface are observed to be approximately 1200 HV0.1 at 460 °C, which is about 4 times as hard as the untreated material (328 HV0.1). This dramatic increase in hardness is probably due to the formation of Cr nitrides. At very low temperatures (e.g. 350 °C), the resultant layer is so thin that significant substrate effects were observed during indentation hardness testing. The decrease in surface hardness at higher temperature (e.g. 480 °C) was accompanied by the formation of a significant volume of CrN [21]. Dry-sliding wear rate data for the unnitrided 304L steel and nitrided 304L steel layers is shown in Fig. 8; all nitriding results are for materials nitrided for 4 h for the various temperatures indicated. The dry-sliding wear rate of the nitrided layers is lower than that of the unnitrided substrate, by a factor of nearly 20 for a nitriding temperature of 350 °C. As the nitriding temperature increases from 350 to 480 °C, the wear rate is observed to increase slightly but remains much lower than that of the unnitrided material. A hard S-phase layer can probably be associated with the minimum value of wear rate at 350 °C. In addition, an increasing concentration of Cr nitrides in the reaction layer with an increase in temperature is probably responsible for the dry-sliding wear resistance decrease. For these tests, the dry-sliding wear rate of the nitrided steel reaches a maximum value at 480 °C (Fig. 8) where a single CrN nitride layer is produced.

4.

Conclusions

A wear-resistant nitride layer was produced on the surface of Type 304L stainless steel by a DC plasma process. Parametric variations of the nitriding process included the plasma treatment temperature, time and pressure. Microstructural analysis of the nitrided layers showed that a single expanded austenite phase (S-phase) was formed at temperatures between 350 and 400 °C, and a single CrN nitride layer was formed at 480 °C. The microstructures of the nitrided layer were strongly dependent on the nitriding temperature, but not on the nitriding pressure. The effect of nitriding time, over the range from 4 to 8 h, was insignificant. Surface properties in terms of hardness and drysliding wear resistance were improved by DC plasma nitriding. S-phase layers on the samples exhibited the best dry-sliding wear resistance under ring-on-block contact configuration testing.

Acknowledgments This work was carried out with the financial support from The Key Nuclear Fuel and Nuclear Materials Laboratory of China, PRC, under grant Contract No.W05-09.And the author (J.W) would like to thank Dr. Li Cong of China Guangdong Nuclear Power Corporation and Prof. Gao Shengji of Sichuan Univ. China for their invaluable discussions during the course of the research.

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