The effects of plasma nitriding process parameters on ... - Springer Link

Tribology Letters 8 (2000) 223–228

223

The effects of plasma nitriding process parameters on the wear characteristics of AISI M2 tool steel M.A. Pessin a , M.D. Tier b , T.R. Strohaecker a , A. Bloyce c , Y. Sun c and T. Bell c a Federal b

University of Rio Grande do Sul – UFRGS, Osvaldo Aranha, 99/610, Porto Alegre, RS, 90035-190, Brazil ˆ Universidade Regional Integrada – URI, Universidade das Missões, 505/210, Santo Angelo, RS, 98802-470, Brazil c The University of Birmingham, Edgbaston B15 2TT, Birmingham, UK

Received 6 December 1999; accepted 20 June 2000

The main aim of this work is to evaluate the effects of the plasma nitriding process on AISI M2 tool steel. In previous work, treatment time and temperature were varied to identify the treatment conditions for good wear behaviour. In the present work, the treatment time was fixed while temperature and gas pressure were varied. Samples were characterised by glow discharge optical spectroscopy, scanning electron microscopy, X-ray diffraction, surface microhardness and wear test. The specimens nitrided at 400 and 900 Pa showed the best wear performance, which is possibly due to reduction of the friction coefficient and the low adhesive wear observed. Samples processed at 200 Pa showed spalling during the wear test, indicating a brittle surface. Keywords: plasma nitriding, wear, tool steel

1. Introduction Plasma nitriding is a plasma-activated thermochemical diffusion process for the surface hardening of metallic materials such as steels, titanium [1] and aluminium alloys [2]. Improvements in friction coefficient, wear and fatigue resistance are produced on materials due to the high surface hardness of nitrided layers. The plasma nitriding process is accomplished in a vacuum chamber where the specimen is connected to a cathode [3]. A high voltage (about 300–1000 V) is applied between the cathode and the vessel, which works as an anode, and the gas pressure varies from about 100 to 1300 Pa (1–13 mbar). Under these conditions, an abnormal glow discharge that covers the specimen is obtained. The ion nitriding process normally is preceded by a cleaning and pre-heating stage that is carried out under an atmosphere of hydrogen. Then, the addition of the nitrogen initiates and sustains the nitriding action [4]. The positive nitrogen ions in the glow discharge are attracted towards the negatively charged work pieces. They impinge upon these surfaces and heat up the pieces to the required diffusion temperature. Hardening is accomplished due to the formation of diffusion and compound layers. The diffusion layer consists of very fine nitride particles [4], where the nitrogen content decreases towards the core. The compound layer, which is also called “white layer”, consists of iron nitrides, the epsilon phase (ε-Fe2–3 N) and/or the gamma prime phase (γ 0 -Fe4 N), whose thickness is generally less than that of the epsilon phase. Compared with conventional nitriding processes, plasma nitriding offers additional advantages, such as reduced treatment time, reduced distortion, demonstrates improved white layer control, as the properties of the nitrided layer can be  J.C. Baltzer AG, Science Publishers

adjusted by controlling the process parameters [4,5], and it is environmently friendly. Plasma nitriding is carried out on high-speed steel (HSS) tools to increase the wear resistance of the cutting edge and to reduce the adhesion of the work material to the tool. The nitrided surface enriched in nitrogen has better slip properties that facilitate easier cutting, resulting in operation at lower temperatures [6]. Moreover, the mechanical properties improvements on the specimen surface, such as fatigue and wear resistance, friction coefficient reduction and high hardness, do not result in bulk toughness loss. The purpose of the present work is to enhance the wear resistance of AISI M2 tool steel. This study was carried out in two stages. In the first one, reported by Tier et al. [6], different times and temperatures were analysed. The best wear results were observed for samples that were plasma nitrided at 450 and 500 ◦C for 60 min. In such cases, the nitrided layer was adequate to avoid the adhesion and material loss from the counterpart, with sufficient toughness to avoid spalling [6]. In the second stage, presented in this paper, the treatment time was fixed while temperature and gas pressure were varied. The gas pressure, which has already been studied by some authors [7–10], is related to important factors, such as ionisation and nitriding efficiency, plasma distribution, hole penetration, general coverage uniformity and effects on surface layer growth. According to Russet [8], the application of plasma nitriding technology for a certain component involves, besides other considerations, the establishment of the optimum working pressure. However, there is insufficient information in the literature to explain the precise mechanism by which gas pressure affects the plasma nitriding process.

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M.A. Pessin et al. / Plasma nitriding and wear of AISI M2 tool steel

2. Experimental The material used in this investigation was the AISI M2 tool steel heat treated (quenched and tempered) to a hardness of 835 HV (65.5 HRC). The chemical composition of the high-speed steel AISI M2 is shown in table 1. Cylindrical samples of 16 mm in diameter and 12 mm thickness were prepared as follows: For examination of cross section, samples were ground to 1200 grade SiC paper and polished to 1 µm diamond paste, and those submitted to the wear tests were only ground to 400 grade SiC paper. The samples were then pre-cleaned using chemical solvents and the final cleaning step was performed in the chamber by sputtering with hydrogen at 200 Pa for as long as was needed to reach the required temperature. The plasma nitriding process was carried out in a 40 kW Klöckner plasma nitriding unit under a discharge containing 25% nitrogen and 75% hydrogen. The voltage applied during the experiment was between 330 and 620 V. In previous work, reported by Tier et al. [6], samples were plasma nitrided at 400 Pa (4 mbar), at temperatures of 450 and 500 ◦ C for 18, 30 and 60 min. Since the best wear resistance was observed for samples processed for 60 min, in this investigation plasma nitriding was performed at 450 and 500 ◦ C and at 200, 400 and 900 Pa for 60 min. Glow discharge optical spectrometry (GDOS) was used to determine the nitrogen concentration profile through the Table 1 Chemical composition of AISI M2 tool steel. %C

%Cr

%Mo

%W

%V

%Mn

%Si

%Ni

%Fe

0.83

4.31

5.11

6.69

1.77

0.31

0.32

0.20

bal

(a)

nitrided layer. The analysis was carried out in a Leco GDS750QPD equipment. The structure of the nitrided compound layer was investigated using X-ray diffraction analysis in a Philips PW 1050 diffractometer. Specimens were scanned through 2θ values ranging from 50◦ to 160◦ in 0.05◦ steps using Cr Kα radiation. Wear tests were performed employing an Amsler machine using a disc-against-flat-surface-type test, without lubrication, and the load was set at 80 kg for 300 revolutions and 20 m/min speed. The nitrided steel was the flat surface in the test and the disc (counterpart – 50 mm diameter ×10 mm wide) was hardened and tempered to 393 HV (40 HRC) hardness. During the test the torque was measured and the friction coefficient obtained by applying the equation µ = T /(rN ), (1) where µ is the friction coefficient, T is the torque measured, N is the applied load, and r is the wheel radius. The Table 2 Iron nitrides identified (X-ray analysis), layer depth (GDOS test), surface hardness, friction coefficient, scar length (SEM) and wear depth. Temperature 450 ◦ C 500 ◦ C 200 Pa 400 Pa 900 Pa 200 Pa 400 Pa 900 Pa Compound layer composition Layers depth (µm) Surface hardness (HV 0.2) Friction coefficient Scar length (mm) Wear depth (µm)

ε and γ 0 ε and γ 0

–

ε

ε

–

22 1178

36 1274

31 1263

31 1260

57 1286

32 1272

0.80 2.9 42.1

0.25 1.3 8.5

0.25 1.8 16.2

0.80 2.5 31.3

0.25 1.3 8.5

0.25 1.3 8.5

(b)

Figure 1. X-ray diffraction patterns showing (a) the absence of compound layer and (b) the characteristic peaks of ε (1) and γ 0 (2) nitrides.


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Figure 2. Photomicrographs of samples processed at 400 Pa: (a) 450 and (b) 500 ◦ C. The upper layer in the photomicrographs is a nickel-plated one added to the plasma nitrided layer to protect it against damages during the metallography procedures.

wear scar was measured afterwards on the flat surface of the samples, which presented, approximately, a rectangular form. Its length is assumed to be the scar size. Scanning electron microscopy (SEM) was used to investigate the wear mechanisms at the surface of tested specimens, such as adhesion and spalling, and also to measure the wear-scar length. In order to analyse the specimens’ morphology, samples were prepared by cross-sectioning the nitride layer with a low speed saw, which was nickel plated, mounted, ground to 1000 grade SiC paper, polished to 1 µm diamond paste and etched with 4% Nital. Surface microhardness tests were conducted using a Vickers indenter and 200 gf.

3. Results and discussion Table 2 summarises the results of metallographic analysis, microhardness, X-ray diffraction and wear test. The untreated sample exhibited a scar length of 2.7 mm, a wear depth of 36.5 µm and friction coefficient µ = 0.95. The X-ray diffraction patterns for two of the conditions used are shown in figure 1. Although the microstructural analysis of all samples, shown in figure 2, did not reveal a compound layer, it can be seen from table 2 and figure 1 that the γ 0 phase was formed at 500 ◦C and the ε phase was observed for both temperatures, which indicates compound layer formation. Previous

226


(a)

(b) Figure 3. GDOS graphs showing the gradient in concentration of nitrogen and carbon with depth of samples nitrided at 450 ◦ C: (a) 400 and (b) 900 Pa.

investigations have indicated that the nitride formation ratio ε : γ 0 decreases when temperature is increased [11], in agreement with the iron nitride formation observed in this investigation. Also, it can be observed that plasma nitrided samples at 200 Pa did not show the presence of iron nitrides on its surface. However, γ 0 (Fe4 N) and ε (Fe2–3 N) phases were formed when the pressure was increased further to 400 and 900 Pa at 500 ◦ C, although only ε nitrides were observed at 450 ◦C. Furthermore, as already observed by Tier at al. [12], the formation of the γ 0 layer at higher temperatures can possibly be explained by the depletion of carbon from the specimen surface, that takes place during plasma nitriding at such temperatures. The nitrogen and carbon depth profiles obtained by GDOS analysis are presented in figure 3. It was observed that samples nitrided at 400 Pa exhibited deeper case depths than those nitrided at 200 and 900 Pa which showed rather similar case depths, see ta-

ble 2. Larger case depths could possibly be due to better conditions for plasma nitriding at 400 Pa pressure, due to better plasma distribution and nitriding efficiency. Similar surface hardnesses were measured on all samples, except for the sample plasma nitrided at 450 ◦ C and 200 Pa, which showed the thinnest nitrided layer. Figure 4 shows SEM analysis on the surface of the samples to characterise spalling, adhesion and the homogeneous wear behaviour of wear tested samples. As shown in figure 4(a), there was significant adhesion on the surface of the untreated specimen, which is possibly caused by the high friction coefficient (µ = 0.95). The improvements in wear resistance for 400 and 900 Pa at both temperatures are mainly due to the reduced friction coefficient (µ = 0.25). Investigations have indicated that the existence of a thin compound layer significantly reduces the adhesive wear [12], as has been observed with these samples (figure 4(b)).


227

(a)

(c)

(b)

(d)

Figure 4. SEM analysis: (a) adhesion on untreated sample; (b) the best wear behaviour (homogeneous wear) at 450 ◦ C and 400 Pa; (c) and (d) spalling at 500 ◦ C and 200 Pa.

As shown in figure 4 (c) and (d), samples plasma nitrided at 200 Pa showed spalling during the wear test, which is believed to be due to the lack of a compound layer. The spalling phenomena can be explained by a combination of factors. The high friction coefficient measured on the diffusion zone (µ = 0.80) results in high shear stresses on the sample surface, which combined with its low toughness produces a significant material loss. Due to the hardness increase from nitride precipitation in the diffusion zone, the plastic deformation capability of the material is reduced, resulting in a brittle fracture mechanism [12].

4. Conclusions This study has demonstrated that plasma nitriding can be successfully used to improve the wear resistance of AISI M2 tool steel as long as the proper nitriding parameters are applied. Additionally, it has been shown that careful control of the plasma conditions, such as gas pressure and temperature, will play an important role in the for-

mation of the compound layer thickness, structure and wear resistance. The samples nitrided at 200 Pa pressure showed spalling, resulting in poor wear behaviour. This is probably caused by the absence of compound layer, which has been shown to be efficient in preventing excessive adhesion. Due to the presence of iron nitrides, the friction coefficient was reduced and consequently the lower shear stresses present avoid the spalling mechanism. It was observed that samples plasma nitrided at pressure of 400 and 900 Pa showed homogeneous wear behaviour and had similar surface hardnesses. Also, it should be noted that the deepest layers were achieved when 400 Pa gas pressure was used, thus providing good diffusion conditions for nitrogen atoms. Further, it was observed that the iron nitride formation is related to the temperature and gas pressure of the process. These observations suggest that optimum nitriding performance is likely to be achieved by applying temperatures of 450– 500 ◦C and 400 Pa pressure. When the pressure was increased to 900 Pa, the properties achieved with the plasma nitriding process were slightly inferior to those at 400 Pa, although they were not as poor as at 200 Pa.

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Acknowledgement The authors thank CNPq, CAPES and FAPERGS for financial support. References [1] H.J. Brading, P.H. Morton and T. Bell, Surf. Eng. 3 (1992) 206. [2] H.Y. Chen, H.R. Stock and P. Mayr, Surf. Coat. Technol. 64 (1964) 139. [3] J. Lanagan, Transactions of the Institute of Metal Finishing 62(2) (1991) 50. [4] G. Krauss, Steels: Heat Treatment and Processing Principles (ASM International, 1990).

[5] B. Edenhofer, Heat Treat. Metals 2 (1974) 59. [6] M. Tier, A. Bloyce, T. Bell and T.R. Strohaecker, in: Surface Modification Technologies XI, eds. T.S. Sudarshan, M. Jeandin and K.A. Khor (IOM Communications Ltd., London, 1998) p. 887. [7] B. Edenhofer, Heat Treat. Metals 1 (1974) 23. [8] C. Ruset, Heat Treat. Metals 3 (1991) 81. [9] A. Leyland, K.F. Fancey, A.S. James and A. Matthews, Surf. Coat. Technol. (1990) 41. [10] M.A. Pessin, A.S. Rocha, M.D. Tier and T.R. Strohaecker, in: 2nd Int. Congr. Metallurgy and Materials Technology (CD-ROM annals), São Paulo, Brazil, October 1997. [11] Y. Sun and T. Bell, Mater. Sci. Eng. A 140 (1991) 419. [12] M.D. Tier, T.R. Strohaecker, T. Bell and M.A. Pessin, IV Seminário de Desgaste ABM (annals), São Paulo, Brazil, July 1998, p. 443.