STUDIES ON WEAR AND CORROSION RESISTANCE OF A36 STEEL COATED WITH TUNGSTEN CARBIDE AND COBALT

Transcription

1 STUDIES ON WEAR AND CORROSION RESISTANCE OF A36 STEEL COATED WITH TUNGSTEN CARBIDE AND COBALT A.R. Suresh 1 arsuresh2005@gmail.com P.Prathap 2 prathapeswaran@gmail.com Eldhose Mathew 3 eldhosetm1989@gmail.com 1,2,3 Kathir College of Engineering, Coimbatore, India Abstract Thermal spray coating is commonly used to improve hardness, wear resistance and corrosion resistance of the surface of engineering materials. Numerous researchers work on finding suitability of the coating for industrial applications. It is interesting to note that the most of the researchers opted WC-Co powder for coating the substrate using thermal spray process. Meanwhile, few researchers expressed about the impact of the decarburization on mechanical properties of the coating due to processing at high temperature in thermal spray process which reduces the efficiency of the coating process. Due to the nature of the high velocity oxy-fuel thermal spray process, residual stress build up in thick deposits is a significant and a limiting problem. The residual stress-state that evolves in a deposit is largely dependent on the thermal conditions to which the system has been subjected and the combination of quenching stresses, which arise during deposition and cooling stresses, post-deposition. It follows that precise control of these phenomena is essential, if a thick deposit is thermally sprayed. To overcome the above stated limitation, plasma spray technique is employed to coat tungsten carbide (WC) with cobalt (Co) on the substrate of material to improve the abrasive wear resistance. In this research work, coating of powder mix containing 88 % WC and 12% Co by weight on ASTM A36 steel substrate using plasma spray process was carried and the effect of the carburization process on wear properties of the resultant coating are studied. It was observed that coating of WC-Co on carburized A36 steel resulted in improvement in wear resistance and corrosion resistance comparing to uncoated / non-carburized steel. Keywords Thermal spray coating ; tungsten carbide (WC); cobalt (Co); ASTM A36 I.INTRODUCTION The plasma spray process is basically the spraying of molten or heat softened material on to a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The plasma spray gun comprises a copper anode and tungsten cathode, both of which are water cooled. Plasma gas flows around the cathode and through the anode which is shaped as a constricting nozzle. Powder is fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can be in the order of mm [1]. From the detailed literature survey, the existence of a research gap is observed related to addressing of the effects of decarburization in thermal spray process coating applied on the A 36 steel substrate [2]. The research work is conducted under this background with an aim to improve the wear properties of the coating through the introduction of carburization process before coating [3]. The thickness of layers produced by plasma based ion implantation ranges from nano to micrometers. Before plasma spray coating, carburization is introduced on the substrate to compensate the loss of carbon during spraying which will increase wear resistance of the steel. The surface of WC-Co coated A36 steel is examined with SEM (Scanning Electron Microscope) and X-ray diffraction. The characteristics and mechanical properties like hardness, wear and electro chemical corrosion of WC-Co coated A36 steel and plain A36 steel are compared. 1209

2 A. Tungsten Carbide Cobalt Powder In this research work, powder mix containing 88 % WC and 12% Co by weight is used as coating material due to its wide applications [4]. From the previous literatures, coatings of these powders provide excellent resistance against most forms of abrasive wear at lower service temperatures [5]. It has been observed that the fine-grained carbides particles of the coating material have excellent resistance against abrasion, erosion and fretting wear [6]. The powder mix containing 88 % WC and 12% Co by weight was crushed using ball milling process to get required grain size to produce very dense, well bonded coatings. The average particle size of WC-Co powder mix is 37μm. Table 1.1: Plasma spray coating process specifications Chemistry Manufacture Morphology Purpose 88% WC - 12% Co Sintered and Crushed Angular and Blocky Abrasive wear resistance Service temp. < 5000ºC Apparent density Process 4.3gm/cc APS, HVOF From the earlier published works, it was identified that the decarburization occurred on the surface of the steels coated using thermal spray process [7]. In order to overcome the above, carburization was done on the surface of the steel to compensate the loss of carbon in the steel during thermal spray process [8]. The chemical composition test indicated the increase in carbon content in the steel substrate after carburization. Table 1.2 Operating parameters of the plasma spray process Description Values Current (A) 500 Voltage (V) Powder feed (g/min) Spray distance (mm) Pressure (l/min) Flow rate ( m 3 /s) Argon Hydrogen 50 Argon Hydrogen B. Coating Procedure Specimens are ultrasonically cleaned free of dirt and oil. The operating parameters of the plasma spray process are given in Table 1.2. Powder mix was poured into the hopper and coated on the substrate to the 250 micron level in single pass [9]. Coated substrate was cooled in open atmosphere for an hour. Fig.1.1: Coated samples 1210

3 A. Scanning Electron Microscope Analysis II. RESULTS AND DISCUSSION A microstructure of the coating of 88 % WC - 12% Co applied on ASTM A36 steel substrate was examined using SEM. The SEM image of the WC-12%Co powder coating is shown in Figure 2.1. The dark regions in the SEM image shows dissolved tungsten carbide. The grey shades in the SEM image represent cobalt rich binder matrix [10]. Figure (a) shows the non-melted WC particle and dissolved WC particle. In figure (b) grey shades represent cobalt which strongly bonded with WC particle. The WC particles distribute uniformly in the coating. The sharp irregular shapes of the small WC particles indicate that these WC particles are not molten during spraying. Therefore, it can be assumed that the metal binder is partly or fully melted, where some of the WC particles remained in the solid state during spraying [11]. Fig 2.1 : SEM Images of Coated WC-12% Co Steel (a) 200X, (b) 500X B. X-Ray Diffraction Analysis The XRD pattern of the coated ASTM A36 steel (Figure a) shows sharp and intense peaks indicating perfect arrangement in specific orientations [12]. The diffraction peak at θ o confirmed the presence of carbon (C) along with iron (Fe) in ASTM A36 steel. The XRD reflections at Bragg s angle 2θ value of o, o and o representing (110), (200) and (211) planes for Fe (JCPDS ). X-Ray Diffraction pattern for coated material (Figure b) shows peaks for both Tungsten Carbide (WC) and Cobalt (Co) only, which indicate these two materials are well coated on the surface of the steel. The less intensity of WC and Co peaks are due to the size (10-45μm) of the coating material. The X-Ray Diffraction confirms the presence of WC-Co in the steel substrate. Fig 2.2: XRD result for (a) mild steel and (b) 88 % WC- 12%Co coated steel 1211

4 C. Wear test Wear test was done to know the wear rate of coated and un-coated material by using pin on disc apparatus. Fig 2.3 : Measurement of Wear of specimens with 0.5kg and 1kg loads Table 2.1: Wear rates of specimens with 0.5kg and 1kg loads Materials Wear rate in μm 0.5kg Load Bare ASTM A36 steel Carburized ASTM A36 steel Coated ASTM A36 steel without carburization Coated ASTM A36 steel with carburization before coating The specimens were tested using pin-on-disc apparatus as shown in the Figure 5.5 to know its ability to resist wear with abrasive grade of 120 rotating at a speed of 716 rpm under the loads of 0.5kg and 1.0kg. The loss of coating was measured in micro meters with 6 minutes cycle duration [13]. The wear test was conducted on bare ASTM A36 steel before and after carburization, plasma coated ASTM A36 steel substrate before and after carburization process [14]. The graphs generated by the testing machine between time in seconds and wear in micrometer are presented in Figure 5.5 for loads 0.5 kg and 1.0 kg. The test obviously revealed that wear is more in bare ASTM A36 steel substrate even after carburization process. The wear rate of the coating was considerably reduced after introduction of the carburisation process [15]. D.Corrosion Test Fig 2.4(a) Tafel curve of plain A36 sample 1kg 1212

5 Fig 2.4(b) Figure 3.5: Tafel curve of low carbon steel sample Fig 2.4(c) Tafel curve of coated and carburized steel sample Tafel curves of coated and uncoated samples were compared. The corrosion rates are mpy, mpy and mpy for WC-Co coated sample, carburized but uncoated sample and plain low carbon steel respectively. It was observed that the coating of WC-Co improves corrosion resistance of steel significantly III.CONCLUSION In this research work, the following conclusions could be made from the wear studies conducted on the specimens coated with powder mix containing 88 % WC -12% Co on ASTM A36 steel through plasma spray process. Homogeneous distribution and binding of WC-Co coating on the steel substrate was confirmed with XRD. Significant improvement in wear resistance of the WC-Co coated steel is observed comparing to uncoated / noncarburized steel. Coating of WC-Co on the A36 steel increased the corrosion resistance significantly comparing to uncoated steel and carburized steel. 1213

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