Effects of Combined Diffusion Treatments on the Wear Behaviour of Hardox 400 Steel

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Procedia Engineering 68 ( 2013 ) 710 – 715

Malaysian International Tribology Conference 2013 (MITC2013) Effects of Combined Diffusion Treatments on the Wear Behaviour of Hardox 400 Steel

H. Mindivana

aAtaturk University, Engineering Faculty, Metallurgical and Materials Engineering Department, Erzurum and 25240, Turkey

Abstract

Despite its excellent tribological property, the service lifetime of Hardox400 steel in some metallurgical and mechanical engineering applications is insufficient. This study describes the development of a duplex treatment to hardox400 steel, which consists of pulsed plasma nitriding, followed by pack boriding. Structural analysis was performed using Optical Microscopy (OM), X-Ray Diffraction (XRD) analysis and Scanning Electron Microscopy (SEM). Microhardness measurements and reciprocating wear tests were determined in order to describe the mechanical properties. It is shown that duplex treatment combines the advantages of both separate process steps. A significant increase was observed in hardness and wear resistance after boronizing the nitrided layer.

© 2013 TheThe Authors. Author. PublishedPublished by by Elsevier Elsevier Ltd. Ltd. Selection and and/or peer-review peer-review under responsibilityunder responsibility of The Malaysian of the Malaysian Tribology TribolSocietyogy (MYTRIBOS), Society, Department Department of of MechanicalMechanical Engineering, Universiti Universiti Malaya, Malaya, 50603 50603 Kuala Kuala Lumpur, Lumpur, Malaysia Malaysia. Keywords: combined diffusion treatments; hardox steel; pack boriding; pulsed plasma nitriding; wear. ______

1. Introduction

The need to overcome wear and corrosion is an old and well recognized problem that presents a unique challenge to the designer and the developer of steel componentsfor many engineering applications, for example in crushing mills, sieves, shaft pins, skip hoist elements, conveyors, blades, gear and sprocket wheels, self-dumping cars elements, loading machines, trucks, front casting bulldozers, buckets and worm transporters [1]. Accordingly, efforts are being made to improve both the anti-wear and the anti-corrosion properties in service. One plausible solution is to construct components entirely from specialty wear and corrosion-resistant materials, but this can be very expensive. Since wear and corrosion are responsible for causing surface material loss, an appropriate surface modification process can be effective for minimizing costs and prolonging the service life of steels with a low cost and capability of mass production. In general, diffusion processes used to enhance the surface properties and the strength of metallic components include nitriding and boriding. The nitrided or borided steel surfaces with a high hardness have outstanding wear and corrosion resistance. Therefore, nitriding and boriding are common thermochemical surface treatments applied to improve the surface properties of machine parts [2–4]. Recently, however, there is a limit with single treatment (nitriding or boriding) to achieve a satisfactory tribological behavior because of both the porous compound structure of nitrided layers and the embrittlement of borided layers [5-7]. The advantages of two conventional surface treatment methods are combined and tribological performance can be greatly improved compared to a single treatment method. Hence, adopting the nitriding plus

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia doi: 10.1016/j.proeng.2013.12.243 H. Mindivan / Procedia Engineering 68 ( 2013 ) 710 – 715 711 boronizing duplex treatment to obtain a composite surface layer of nitride and boride on the steel parts may be an effective and alternative adequate way to improve the microstructure and the properties of the modified layer. In recent years, duplex treatments have already been successfully applied to various industries [5-9]. However, up to now, no report has been found on the researches in the field of nitriding plus boronizing duplex treatment. In this study, duplex treatment involving a combination of nitriding and boronizing was completed by a two-step method for Hardox 400 steel. The characteristics of microstructure formed by the duplex treatment were investigated using an Optic Microscopy (OM) and X-Ray Diffraction (XRD) analysis. Also, microhardness measurements and wear behaviours of pulsed plasma nitrided, borided and nitrided-borided samples were evaluated. The results are hopefully useful to propose an effective treatment to further enhance the performance of the single nitrided and borided steel.

2. Experimental

Before surface treatment, Hardox 400 steel substrates were cut to the dimensions of 20 mm×20 mm×4 mm, ground up to 1200 grid emery paper, and mechanically polished to an average surface roughness Ra=0.02 μm. The samples were then pulsed plasma nitrided at a constant pressure of 250 Pa and at temperature of 520°C for 12 h in a gas mixture of 75% N2 and 25% H2. The samples were then polished to obtain good adhesion prior to boronizing treatment. They were subsequently borided in a cylindrical stainless steel box filled with commercial Ekabor II powders (commercial boronizing agents containing B4C, KBF4 and SiC) for 6 h at 900 °C and cooled in furnace down to room temperature. XRD analyses of the surface layers were performed under CuKĮ radiation in thin film mode with angle of 3° (2θ). The samples were then cross sectioned and ground and polished using emery papers and alumina powder with 1 ȝm grain size. They were then etched with 3% Nital, for metallographic examinations. The thickness of coatings and their morphological examinations were examined using Optical Microscopy (OM). A HV-1000 Vickers microhardness tester was used to measure the hardness along the depth in cross-sections of the modified samples. Microhardness was measured at an indentation load of 10 g for 15 s. Reciprocating wear tests were performed under sliding distance of 50 m, sliding speed of 0.0128 m sí1, at room temperature (25 °C) and relative humidity of about 30% under dry sliding conditions. Wear tests were carried out by rubbing a Si3N4 ball with a diameter of 10 mm to the untreated and coated surfaces under normal loads of 5 N and 20 N, respectively. During wear testing, friction coefficient was continuously recorded. After the wear tests, the wear tracks developed on the surface of the samples were analysed by a 2D- profilometer and a Scanning Electron Microscopes (SEM). The wear volume was calculated by integrating the area across the wear scar profile measured by a stylus profilometer (MarSurf PS1), and then multiplying the length of the tracks. The wear test results were quantified in terms of wear rate (mm3/Nm) by considering the volume of the wear tracks (in the unit of mm3), test load (in the unit of N) and total sliding distance (in the unit of m).

3. Results and discussion

The microstructure of the untreated sample exhibits a tempered martensite structure, which is depicted in Fig. 1 a. Fig. 1 also shows cross-sectional micrographs of the Hardox 400 steel samples after pulsed plasma nitriding, boronizing and duplex treatment. In Fig. 1 a b, a single surface layer is seen for the nitrided and borided samples. However, in the post borided sample, the thickness of the duplex treated layers has increased and two distinct layers are noticed on its surface (Fig. 1 c.). The nitrided layer (the white surface layer), with a thickness of 7±1 ȝm, is ,evident in Fig. 1 a. The boronizing process produced thin FeB and Fe2B (׽80 ȝm thickness) layers on the steel which is further confirmed by XRD analysis (Fig. 2). The cross-section morphology of duplex treated sample consisted of an outer compound zone and an inner diffusion zone has a smooth and flat morphology when compared to a single boride layer formed on the surface of the Hardox 400 steel. While the compound layer has a thickness of approximately 20 ȝm, the diffusion layer has a thickness of approximately 70 ȝm and it is characterised by very good adherence due to its tooth-shape morphology. However, boronizing and duplex treatments changed the microstructure to ferrite and pearlite phases from martensite (Fig. 1 b, c). The hardness depth profiles of samples are shown in Fig. 3. The maximum surface hardness of the duplex treated sample was measured to be approximately 712 H. Mindivan / Procedia Engineering 68 ( 2013 ) 710 – 715

1005 HV0.01. The hardness profile of the duplex treated sample shows a two-step decrease. It appears that during post boronizing, boron has only diffused partly in the nitrided layer. It is possible to claim that these are a consequence of the presence of boride nitride and iron boride, which was verified by means of XRD (Fig. 2). Similar microhardness distribution is observed in the borided sample, but the surface microhardness is measured to be approximately 880 HV0.01, which is slightly lower than that measured in the duplex treated sample. Also, the hardness of the duplex treated sample is higher than that of the single borided sample in the diffusion zone (Fig. 3).

(a) (b) (c) Fig. 1. Cross-section OM micrographs of the (a) pulsed plasma nitrided, (b) borided and (c) duplex treated samples.

Fig. 2. XRD results of the pulsed plasma nitrided, borided and duplex treated samples.

Fig. 3. Microhardness depth profiles of the pulsed plasma nitrided, borided and duplex treated samples.

Variation of the wear rates of the untreated, pulsed plasma nitrided, borided and duplex treated samples with surface hardness is shown in Fig. 4. Generally, higher hardness corresponds to better wear resistance. It is obvious H. Mindivan / Procedia Engineering 68 ( 2013 ) 710 – 715 713 that the lowest wear rate belongs to the sample that has been nitrided plus borided which has the highest surface hardness. While the surface hardness value of the untreated sample was 217 HV0.01, this increased to 395 HV0.01, 884 HV0.01 and 1005 HV0.01 after pulsed plasma nitriding, boriding and duplex treatment, respectively. As seen in Fig. 4, the wear rate of the untreated sample was 18.8×10Ѹ6 mm3/Nm, but the wear rates of the pulsed plasma nitrided, borided and duplex treated samples were 4.0×10Ѹ6, 1.94×10Ѹ6 and 0.88×10Ѹ6 mm3/Nm, respectively. In the duplex treated sample, wear rate was measured to be approximately 21, 4.5 and 2.2 times smaller than those from the untreated, pulsed plasma nitrided and borided samples, respectively. When the wear surfaces were investigated (Fig. 5), it was observed that the worn surface of the borided and duplex treated samples had a basically smooth appearance while the obvious roughening occurs on the worn surface of the pulsed plasma nitrided sample under the applied load of 20 N. Whereas, the worn surface of the untreated sample primarily appears with grooves at the applied load of 5 N, which indicates typical abrasion wear. The absence of such grooves in the borided and duplex treated samples is probably due to the higher load bearing capability of the diffusion layer, as demonstrated by the microhardness profiles in Fig. 3. Whereas, a large amount of material transfer to the worn surface of the pulsed plasma nitrided sample from the Si3N4 ball due to adhesion, Fig.5, indicates the presence of adhesive wear. Comparing samples treated by both boriding and duplex processes with ones by a pulsed plasma nitriding process, the tendency to form adhesive wear in the wear track decreased extensively in the case of the borided and duplex treated samples due to the low welding tendency of the boride layer [10] and also the friction coefficient, as shown Fig. 6, was lower than that of the pulsed plasma nitrided sample. High hardness of the transition layer induced by a combination of nitriding and boriding could be attributed to the much improved wear resistance, but a normal load combined with friction force facilitated to adhesive wear on the hardened layer formed by a pulsed plasma nitriding process, and caused an increase in wear (Fig. 4) and friction coefficient (Fig. 6). The results show that the duplex treated sample has better adhesion of the modified layer to the steel substrate, it also has a more stable and lower friction coefficient, longer endurance life, and higher load-bearing ability.

Fig. 4. Variation of the wear rates and surface hardness of the untreated, nitrided, borided and duplex treated samples.

714 H. Mindivan / Procedia Engineering 68 ( 2013 ) 710 – 715

Untreated Pulsed Plasma Nitriding Boriding Duplex Treatment

Wear Track A A

Fig. 5. SEM micrographs of worn surfaces and EDX spectra of illustrated point with letter A on the SEM photo for the pulsed plasma nitrided sample.

Fig. 6. Friction test results of the untreated, pulsed plasma nitrided, borided and duplex treated samples. H. Mindivan / Procedia Engineering 68 ( 2013 ) 710 – 715 715

4. Conclusions

Boron nitride coatings can be produced by duplex surface treatment via post boriding of the pulsed plasma nitridedHardox400 steel. Duplex treated layers were thicker and much harder than the pulsed plasma nitrided and borided layers. Duplex treatment significantly increases the hardness and reduces the plastic deformation of the substrate. The reciprocating wear test results show that boriding process after pulsed plasma nitriding provides a beneficial effect in reducing the friction coefficient and wear rate.

Acknowledgement

The author would like to thank to Dr. E. E. Korkmaz, Er-Mir Textile and Machine Company, Turkey, for the support given in nitriding processes.

References

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