Wear 263 (2007) 99–110
Microstructure and abrasive wear behaviour of shielded metal arc welding hardfacings used in the sugarcane industry V.E. Buchanan a,∗ , P.H. Shipway b , D.G. McCartney b a
b
School of Engineering, University of Technology, 237 Old Hope Road, Kingston 6, Jamaica, West Indies School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UK Received 11 August 2006; received in revised form 25 November 2006; accepted 9 December 2006 Available online 26 March 2007
Abstract The abrasive wear behaviour of hypereutectic and hypoeutectic based Fe-Cr-C hardfacings are reported and interpreted in terms of the microstructures. The coatings were deposited onto a grey cast iron substrate by shielded metal arc welding using two commercial hardfacing electrodes. The abrasive wear resistance was performed on a modified block-on-ring machine that simulated the wear conditions experienced in a sugar cane mill. Microstructural studies were carried out using optical and SEM techniques. It was found that the hardness of the hypereutectic coating was significantly higher than the hypoeutectic coating. In both cases, optimum hardness was achieved within the first deposited layer. The abrasion tests showed that there was no significance difference in the wear resistance of the hardfacings at the higher loads and there was contrasting wear behaviour in the dry and slurry conditions. The abrasive wear mechanisms were found to be predominantly microploughing and microcracking in the hypoeutectic and hypereutectic coatings, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Hardfacing; Welding; Microstructure
1. Introduction The deposition of overlay coatings by welding or thermal spray techniques is frequently employed (either during maintenance or in the manufacture of new components) in a wide range of industries [1,2] to improve the wear resistance of surfaces in contact. In the sugarcane industry hardfacing is carried out on grey cast iron mill rollers to reduce wear, to permit the use of higher extraction loads and to aid the movement of the crushed cane through the rollers [3]. Fig. 1 shows the schematic arrangement of a typical cane mill that is used to extract the juice from the sugarcane. The cane mill essentially consists of three large horizontal roller shells that are shrunk onto a steel shaft and arranged in the form of an isosceles triangle. The sugarcane, which is cut and shredded, is passed through the rotating rollers to extract the cane juice. The rollers are machined with a series of V-shaped grooves around the circumference to improve gripping (and thus transport) of the sugarcane through the mill.
∗
Corresponding author. Tel.: +1 876 349 0800; fax: +1 876 977 2267. E-mail address:
[email protected] (V.E. Buchanan).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.12.053
During service the rollers exhibit wear that causes loss of productivity, which has to be balanced with the high cost of roller replacement. Hugot [4] has documented several causes of wear in the mill, which include (i) corrosion of the rollers by the acidic cane juice, (ii) sliding contact of scrapers and trash plate against the surface of the rollers, and (iii) sliding of the bagasse (the fibrous part of the sugarcane) against the roller surface. Additionally, the reduction of programmed maintenance interruptions and the presence of impurities (sand, earth, etc.) from the increased use of unwashed, chopped cane have also lead to increased wear [5]. Occasionally, an untreated roller may lose up to 5% of its diameter in one season (120 days, 24 h/day). For a typical 800 mm diameter roll, this equates to a reduction of 40 mm. Mill engineers have introduced several measures to increase the lifespan of the rollers. The earliest method was the deposition of discrete, hardfacing weld droplets onto the crests and flanks of the roller teeth [6] which was later modified to the deposition a continuous bead on top of the tooth with discrete weld spots made on the flanks [7]. However, the process was considered inefficient as the arced globules separated from the substrate by shear failure through the heat-affected zone. Unwanted cracks
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V.E. Buchanan et al. / Wear 263 (2007) 99–110
standing of the mechanisms that dominate the degradation of these two classes of material. In this study, the wear rates and mechanisms of typical hypereutectic and hypoeutectic hardfacings are assessed under conditions representative of those observed in a sugarcane mill and the degradation mechanisms understood in terms of the hardfacing microstructures and the environmental conditions. 2. Experimental details Fig. 1. Schematic arrangement of sugarcane mill.
and porosity were also produced within the weld and heataffected zone where the trapped cane juice became aggressively corrosive leading to premature failure of the arced globule [8]. Although these methods improved both the life of the rollers and mill productivity it was observed that increased wear was observed in other elements of the mill, owing to occasional sliding contact of these elements with the hardfaced elements during the milling operation. Further modifications and improved welding processes [9–11] have since been made such that the rollers can undergo an entire season without the need for additional weld maintenance. Chandel [12] pointed out that when selecting an appropriate hardfacing alloy, the wear mode, the environmental resistance, the weldability and the costs of the process must all be considered. In addition, the metallurgical compatibility between the base metal and alloy also influences the choice of a hardfacing electrode. With these considerations in mind, manufacturers have formulated specialised welding electrodes for sugarcane mill rollers for deposition under the welding environments typically observed in this context. Iron-based hardfacing materials are the most popular type in the sugar industry due to their relatively low cost and ease of application. These alloys are either of a hypoeutectic or hypereutectic composition and their wear resistance is attributed to a microstructure in which hard carbides are dispersed in a relatively soft matrix [13,14]. The high Fe-Cr-C alloys are particularly attractive because the carbides can form relatively large micro-constituents, which provide enhanced abrasion resistance. Within the iron-based hardfacings there are a number of microstructures and wide differences in composition that provide varying degrees in abrasion resistance. Kotecki and Ogborn [15] performed a comprehensive study on the low-stress abrasion resistance of numerous iron-based hardfacing alloys as a function of microstructure and hardness and concluded that microstructure was the most important factor in determining wear resistance. Despite this, maintenance engineers continue to equate increasing hardness of the hardfacings with increasing abrasion resistance. Although increased hardness can result in increased wear resistance, particularly when comparing the increase in wear resistance of the hardfacings over mild steel substrate, different hardfacings with similar hardness values can have significant differences in wear resistance [15,16]. Both hypereutectic and hypoeutectic hardfacings are currently utilized on sugarcane mill rollers, but with little under-
2.1. Materials The base material for the deposition of the hardfacings was grey cast iron; the chemical composition of which is given in Table 1. Two commercially available shielded metal arc welding (SMAW) electrodes, commonly used in the sugar industry, were used to deposit the hardfacings. The electrodes are designated as A1 and A2 and are of diameters 3.2 and 4 mm, respectively. Analyses of the electrodes (Table 2) showed that the wires were largely iron and the coating contained, in addition to the fluxing agents, the alloying elements for the hardfacings. Bagasse board, which is made of compressed cane fibre, was used as the counterface material in the abrasion test programme; Fig. 2 shows the typical structure of the bagasse board. Table 1 Chemical composition (wt%) of grey cast iron disc C Si Mn P S Ni Cr Mo Cu Fe
3.64 1.90 0.66 0.09 0.13 0.081 0.14 0.028 0.022 Bal
Table 2 Chemical composition (wt%) of SMAW electrodes Element
C Si S Mn Cr P Ni Mo Cu Al Ca Mg Ti Fe
A1
A2
Metallic wire
Flux coating
– 0.02 0.05 0.30 0.06
