Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel

Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel

ARTICLE IN PRESS Vacuum 80 (2006) 1032–1037 www.elsevier.com/locate/vacuum Effects of various nitriding parameters on active screen plasma nitriding...

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ARTICLE IN PRESS

Vacuum 80 (2006) 1032–1037 www.elsevier.com/locate/vacuum

Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel Sh. Ahangarania, F. Mahboubib,, A.R. Saboura a

Department of Materials Engineering, Tarbiat Modarres University, Tehran, Iran Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran

b

Received 18 September 2005; received in revised form 11 January 2006; accepted 11 January 2006

Abstract Active screen plasma nitriding (ASPN) is a novel surface modification technique that has many capabilities over the conventional DC plasma nitriding (CPN). In this study, 30CrNiMo8 low-alloy steel was active screen plasma-nitrided under various nitriding parameters such as active screen set-up parameters (different screen hole sizes, mesh sheet and plate top lids) and treatment temperature (520, 550 and 580 1C), in the gas mixture of 75% N2+25% H2 and chamber pressure of 500 Pa for 5 h. The properties of the nitrided specimens have been assessed by evaluating composition of phases, surface hardness, compound layer thickness and case depth using X-ray diffraction (XRD), microhardness measurements and scanning electron microscopy (SEM). It was found that the screen hole size and top lid type (mesh or plate) play an important role in transition of active species (nitrogen ions and neutrals) toward the sample surface, which in turn can affect the nitrided layer hardness and thickness. Treatment at higher temperature with bigger screen hole size resulted in a thicker compound layer and higher layer hardness. The compound layers developed on the samples treated under different conditions were dual phase consisting of g0 -Fe4N and e-Fe2–3N phases. r 2006 Elsevier Ltd. All rights reserved. Keyword: ASPN; Active screen plasma nitriding; Low-alloy steel

1. Introduction Plasma nitriding is an advanced surface modification technology which has experienced substantial industrial development over the past 30 yr [1]. This method is one of the most effective techniques for increasing wear resistance, fatigue strength, surface hardness and corrosion resistance of industrial components [2]. The basic mechanism of plasma diffusion treatment is a reaction between the plasma and the surface of the metal. In addition, the plasma mass transfer has an effect on the formation of nitride layer [3]. By means of a glow discharge in a gas mixture of H2 and N2, with the cathode (the steel samples) at a special range of temperature, atomic nitrogen can form and penetrate into the surface. Depending on the type and concentration of alloying elements and the process parameters (such as Corresponding author. Tel. +98 21 64542967; fax: +98 21 6405864.

E-mail address: [email protected] (F. Mahboubi). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.013

time and temperature), a diffusion zone is formed, with nitrogen penetrating into the steel. Frequently, a surface compound layer is also formed on top of the diffusion zone, with thickness in the micron range. In the diffusion zone of iron-base materials, the nitrogen exists as single atoms in solid solution at lattice sites or interstitial positions, until the limit of nitrogen solubility in iron is exceeded. As the nitrogen concentration increases toward the surface, very fine, coherent precipitates are formed when the solubility limit of nitrogen is exceeded. The hardness is only slightly changed by the nitrogen in solid solution, while it increases substantially when the nitride precipitates form, depending on the nitride-forming alloying elements. In the compound zone, g0 -Fe4N and e-Fe2–3N phases as well as nitrides with alloying elements are formed. These compound layers are called white layers because they appear white on polished and etched surfaces [4]. The improvement of conventional plasma nitriding (CPN) technology has been hindered in recent years. This

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is due to some deficiencies of CPN technique, such as arc damages to the components, heterogeneous temperature of parts with different dimensions and geometries, overheating of some parts by the ‘‘hollow cathode effect’’ and non-uniform appearance of work pieces by the ‘‘edging effect’’ [5,6]. To conquer the problems of CPN, a new method of plasma nitriding, known as active screen plasma nitriding (ASPN) or through cage (TC) plasma nitriding, has been recently developed [5,7–10]. In this method, the components are enclosed in a metal screen or cage and a negative high voltage is applied to the screen, instead of the components (which are placed in a floating potential), so that the term ‘‘active screen’’ is employed. Thus, the plasma generates on the screen rather than on the component surface, and heats it up. Radiation from the heated screen supplies the heat that brings components to the required temperature for treatment. Moreover, active nitriding species such as nitrogen ions (N+ 2 ) and neutrals (N2) [11], necessary for nitride layer formation, are provided by the plasma which is formed on the screen, and pass through it and reach the surface of components. Hence, uniform nitrided layers on all parts of different geometry and size can be produced. Since plasma is not generated on the component surface, the arcing damage and the ‘‘edge effect’’ can be avoided by this technique [5,7]. ASPN using similar parameters as CPN has resulted in similar microstructure, identical case hardness and identical case depth as CPN [5]. The aims of the present work were to investigate the ASPN behavior of 30CrNiMo8 low-alloy steel, as well as to understand the effect of different treatment parameters such as temperature and screen set-up.

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2. Experimental The alloy studied here (30CrNiMo8) possesses the following composition (wt%): 0.26–0.34% C; 0.3–0.6% Mn; 1.8–2.2% Cr; 0.3–0.5% Mo; 0.4% Si; 1.8–2% Ni; 0.035% P; 0.030% S; Fe balance. The samples to be treated were machined in the form of 20 mm diameter and 10 mm thick discs. The surfaces of the substrates were mechanically polished sequentially by 120, 400, 600, 800, 1200 and 3000 grit wet SiC emery paper, followed by fine polishing with alumina slurry to yield a mirror finish before being placed in the chamber. The treatments were carried out in a 5 kW conventional DC plasma nitriding unit. It contains a cylindrical stainless-steel vacuum chamber with dimensions of 50 cm diameter and 70 cm length (Fig. 1). The samples were placed in an active screen set-up (Fig. 2) which was put directly on the electrically insulated stage (cathode) of the plasma nitriding unit (the chamber wall served as anode). The active screen was a mesh cylinder of 70 mm diameter and 50 mm height, made of 0.8 mm thick perforated st37 steel sheet, which had equally distributed round holes of 6 mm (and 8 mm) diameter, with a detachable top lid. The top lid was made of the same mesh sheet as cylinder (and of st37 steel plate 0.8 mm thick). The mesh cylinder was placed on the DC nitriding worktable and connected to the cathodic potential. The distance between the sample surface and the top lid was 12 mm. The nearest distance from the cylinder to the sample edge was about 10 mm. Specimens were heated through radiation from the active screen and the top lid.

Fig. 1. Schematic diagram of the plasma nitriding system.

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Various techniques were used to characterize the microstructures and properties of the treated samples. Microstructure analysis was carried out using both optical microscopy (OM: Olympus—BX60) and scanning electron microscopy (SEM: Philips-XL30). X-ray diffraction (XRD) analysis (Xpert Philips X-ray differactometer using Cu Ka with PC-APD, Diffraction software) was applied for phase identification, and also indentation tests were carried out for microhardness measurements. 3. Results

Fig. 2. Schematic diagram of active screen set-up.

Fig. 3 shows microhardness profiles of ASPN samples treated at 520, 550 and 580 1C for 5 h. Microhardness profiles obtained from cross-sections of treated specimens show the presence of a slope interface between the case (nitrided layer) and the core. All samples show high surface microhardness values that drop decreasingly at the case/ core interface to substrate microhardness values. It can be observed in Fig. 3 that higher surface hardness values are obtained for nitriding temperatures of 580 1C. The steel top lid type (screen or plate) has little effect on microhardness of samples. This microhardness values at screen top lids are slightly higher than that of iron plate top lid. Fig. 4 shows optical micrograph of ASPN samples treated for 5 h at 520 and 550 1C with screen top lid. It can be observed in this figure that for screen top lid conditions, the thicknesses of the compound layer and diffusion zone

Fig. 3. Microhardness profiles of the samples active screen plasma nitrided at 520, 550 and 580 1C for 5 h, in 75% N2+25% H2; 6 mm (a, b) and 8 mm (c, d) hole size of screen.

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Fig. 4. Optical microscopy images of 30CrNiMo8 low-alloy steel active screen plasma nitrided at 520 and 550 1C for 5 h, in 75% N2+25% H2 with screen top lid, AS6 mm (a, b) and AS8 mm (c, d).

Fig. 5. Optical microscopy images of 30CrNiMo8 low-alloy steel AS- plasma-nitrided at 550 1C for 5 h, with iron plate top lid, AS6 mm (a) and AS8 mm (b).

increase with increasing treatment temperature, but increasing of screen hole size at the same temperature have no considerable effect on the layer thickness. On the other hand, for iron plate conditions, thickness of the layer increases with increasing screen hole size at the same temperature (Fig. 5). Fig. 6 shows SEM cross-sections of ASPN-treated samples for 5 h at 550 1C with screen top lid. It can be observed in this figure that hole size (6 or 8 mm) of the screen set-up has no or little effect on the thickness the layers. The compound layer thickness of the active screen plasma-nitrided 30CrNiMo8 low-alloy steel specimens as a function of the temperature and screen set-up parameters are given in Table 1.

ASPN at 520, 550 and 580 1C for 5 h produced different nitrided layers in terms of morphology, thickness and phase structure. Typical X-ray patterns of the AS plasmanitrided samples are shown in Fig. 7, indicating that the compound layers developed on samples are dual phase that consisted of g0 -Fe4N and e-Fe2–3N iron nitrides. While the hole size of the screen set-up increased, the intensity of the e-phase in the compound layer increased for both screen and iron plate top lid (Figs. 7 and 8). In this case, relative intensity of e at 550 1C as expressed by Ie (screen top lid)/ Ie(iron plate top lid) is 2.7 for screen set-up with 6 mm hole diameter, and is 2.5 for 8 mm hole diameter. This indicates that e-phase in the compound layer for screen set-up with

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Fig. 6. SEM cross-section images of 30CrNiMo8 low-alloy steel AS- plasma-nitrided at 550 1C for 5 h, with screen top lid; AS6 mm (a) and AS8 mm (b).

Table 1 Compound layer thickness as a function of temperature and screen set-up parameters Temperature (1C)

520

Screen set-up parameters Top lid

Screen hole size (mm)

Iron plate

6 8 6 8

3.3 3.6 4 4

6 8 6 8

6 13 10 10

6 8 6 8

13 13.6 14 14.4

Screen 550

Iron plate Screen

580

Compound layer (mm)

Iron plate Screen

screen top lid is higher than for screen set-up with iron plate top lid.

ions from plasma environment outside of the screen set-up towards the sample surface inside of the screen set-up. The minimum compound layer thickness were obtained at the screen set-ups with 6 mm hole size and iron plate top lid at 520 1C for 5 h (Table 1), because the smallest transition of active species (ions and neutrals) takes place through the top lid towards the surface of the specimens. It was observed that the highest surface hardness can be obtained with an increase in the treatment temperature and using screen top lid (Fig. 3 and Table 1). The XRD patterns shown in Figs. 7 and 8 indicate that ASPN-treated samples with both screen and iron plate top lid produce compound layers which consist of a mixed structure of g0 -Fe4N and e-Fe2–3N phases. However, the relative peak intensities of the two phases are different in samples treated under different conditions. Fig. 7 shows, while the temperature and/or hole size of screen set-up is increased, the intensity of e in the compound layer increased. In comparison, Fig. 8 shows that the peak of both e- and g0 -phases was reduced for the sample treated in screen set-up with a lower hole size (6 mm) and iron plate top lid. This again draws attention to the role of the screen hole size on ions and neutrals transition through them, which in turn affect the hardness and layer thickness of the treated samples.

4. Discussion 5. Conclusions The nitrided layer formed on treated specimens by ASPN method presented microstructures and surface properties dependent on the process temperature and screen set-up parameters. The hardness profiles of the active screen plasma-nitrided specimens exhibited high surface microhardness values that drop decreasingly at the case/core interface to substrate for 75% N2+25% H2 gas mixture at 520, 550 and 580 1C for 5 h with both 6 and 8 mm hole size of screen set-up, and both screen and iron plate top lid due to a decrease in the concentration of alloy nitrides towards the core. These microhardness values at screen top lids are slightly higher than that of iron plate top lid (Fig. 3). The higher hardness can be attributed to the role of holes of the top lid in facilitating the transition of

The aim of this investigation was to realize the effects of treatment conditions such as temperature and screen setup, on the active screen plasma-nitriding behavior (ASPN) of 30CrNiMo8 low-alloy steel. It is clearly shown that ASPN is a promising technique to optimize the nitriding process and to avoid the problems associated with the conventional plasma nitriding (CPN) method such as ‘‘edge effect’’ and ‘‘arc damage’’ to the surface of the specimens. The results suggest that the screen hole size and the type of top lid (mesh or plate sheet) have an effect on the compound layer thickness. Large screen hole size and mesh plate top lid, produce thicker compound layer. This can be

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Fig. 7. XRD patterns of active screen plasma-nitrided 30CrNiMo8 low-alloy steel at 520 and 550 1C for 5 h, with screen top lid, AS6 mm (a, b) and AS8 mm (c, d).

Fig. 8. XRD patterns of active screen plasma-nitrided 30CrNiMo8 low-alloy steel at 550 1C for 5 h, with iron plate top lid, AS6 mm (a) and AS8 mm (b).

attributed to the ease of transition of active species (nitrogen ions and neutrals formed on the screen), through the screen holes toward the surface of specimens, leading to the formation of thicker compound layer. The compound layer of the active screen plasma-nitrided specimens treated under various conditions of temperature and screen set-up consists of e-Fe2–3N and g0 -Fe4N phases. Increasing treatment temperature increases the thickness of the compound layer and the diffusion zone. Maximum surface hardnesses were found on the samples treated at 580 1C. References [1] Alves Jr C, da Silva EF, Martinelli AE. Effect of workpiece geometry on the uniformity of nitrided layers. Surf Coat Technol 2001;139:1–5. [2] Karakan M, Alsaran A, C - elik A. Effects of various gas mixtures on plasma nitriding behavior of AISI 5140 steel. Mater Char 2003; 49:241–6.

[3] Jeong B-Y, Kim M-H. Effectts of the process parameters on the layer formation behavior of plasma nitrided steels. Surf Coat Technol 2001;141:182–6. [4] Berg M, Budtz-Jørgensen CV, Reitz H, Schweitz KO, Chevallier J, Kringhøj P, et al. On plasma nitriding of steels. Surf Coat Technol 2000;124:25–31. [5] Li CX, Bell T, Dong H. A study of active screen plasma nitriding. Surf Eng 2002;18(3):174–81. [6] Li CX, Bell T. Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel. Corros Sci 2004. [7] Li CX, Bell T. Sliding wear properties of active screen plasma nitrided 316 austenitic stainless steel. Wear 2003. [8] Li CX, Georges J, Li XY. Active screen plasma nitriding of austenitic stainless steel. Surf Eng 2002;18(6):453–8. [9] Georges J, Cleugh D. Active screen plasma nitriding. In: Bell T, Akamatsu K, editors. Stainless steel 2000—thermochemical surface engineering of stainless steel. UK: The Institute of Materials; 2001. p. 377–87. [10] Li CX, Bell T. Principles, mechanisms and applications of active screen plasma nitriding. Heat Treat Met (UK) 2003;30(1):1–7. [11] Cleugh D. Plasma species analysis for in situ assessment of surface treatments. Surf Eng 2002;18(2):133–9.