POWDER CONSUMABLES FOR WEAR RESISTANCE AND REDUCED COSTS OF HARDFACED MINING COMPONENTS

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1 POWDER CONSUMABLES FOR WEAR RESISTANCE AND REDUCED COSTS OF HARDFACED MINING COMPONENTS S. Bengtsson and S. Dizdar ABSTRACT Höganäs AB, Höganäs, Sweden Hardfacing of components is widely used worldwide as a means of improving life of components and reducing operating costs. Numerous hardfacing techniques currently exist which utilize filler materials in wire/strip or powder form. One of the most commonly used techniques is the one referred to as GMAW/MIG (Gas Metal Arc Welding/ Metal Inert Gas) and it uses wire. It is highly developed and it is used in a very wide range of applications. Modern powder utilizing techniques however, such as Laser Cladding or PTA, are rapidly gaining ground in the industry due to their technical but most importantly economical merits. In this study, a direct comparison will be conducted between the two techniques both in technical and economical terms in order to investigate the viability of the new method. Furthermore, advanced powder materials for wear prone applications will be tested in an effort to identify possible gains from the use of modern techniques. 1. INTRODUCTION Mining components are heavily exposed to abrasive wear when engaging ground (GET). Abrasive wear rate is very high and the majority of the mining components are made from relatively inexpensive materials and designed to be easily replaceable. Materials such as alloy white iron, austenitic and wrought Mn steels are widely used 1. The total cost of the mining components when demands on a high mining productivity are present can be unacceptably high. The reasons can be found in production losses due to the worn component s replacement, the replacement cost and low material utilization since the worn components are discarded to scrap yards for re melting in steel and iron works. Maintenance costs in mining are said to be in the order of 30 to 50% of the operating costs. Equally significant is the cost of lost production when a machine is down. In the order of 1% improvement in equipment availability or productivity could improve the company profits by up to 3.5%. Thermal surfacing i.e. hardfacing is a widely used technology to improve useful service life of mining components 2,3. The components achieve a hard face, a hard coating metallurgically bonded to the component metallic body, capable of withstanding abrasive action of hard minerals from the ground. The total cost of the mining components can then be reduced. The purchase cost of a hardfaced component is higher than that of a basic wrought part. However worn component replacement occurs less frequently and these components can be reconditioned to as new. Modern hardfacing welding technologies, such as PTA (plasma transferred arc) and laser cladding, allow automated processing that gives high consistency and repeatability 5. Powder consumables are commonly used with these techniques while wire is used in conventional MIG welding. The precision required to ensure good and consistent weld beads is high so manual operation is not an option. On the other hand the fact that the process is automated will give higher repeatability, high productivity and better quality 5 in series production. Wear of mining components Mining components are exposed to abrasion from ore in the ground. The severity of abrasion depends on the characteristics of the ore to be exploited and the type of handling in ore preparation. The most important ore characteristics are hardness and sharpness of the sand/gravel/stone fractions together with the presence of water and salt. Ore preparation can include operations such as ore digging, haulage, crushing, screening, grinding, pumping, classifying and separation. Depending on pressure and transporting/travel speed to which the ore is subjected, the mining components are exposed to mainly four abrasive wear modes; high stress abrasion, low stress abrasion, gouging and erosion. High stress abrasion occurs in operations such as ore grinding and crushing when the loading is high, but under minor impact. This crushing of ore particles removes material from the mining component resulting in fine, grinding like scratches. Low stress abrasion occurs in more or less all ore processing operations. It is a major wear mode in screening, conveying, pumping, classifying and separation, when both the loading and ore transporting i.e. sliding velocity are moderate. The component surface exhibits a scratched appearance. Gouging abrasion occurs when loading is high and impact present such as in crushing, digging and loading when dealing with large chunks of ore or stones. The component surface is both scratched and plastically deformed by this. Erosion occurs mainly when ore has entered screening and particle refinement operations. The ore particles are small in size and are transported at relatively high speeds. In ore processing

2 which includes water, erosion occurs followed by corrosion. highest ranked can then be forwarded for field excavation testing. As shown, ore character and preparation handling determine the major and minor forms of abrasive wear. In summary, abrasive wear in mining, shows low stress abrasion as likely to occur most often in comparison to other abrasion forms. For the scope of this general paper on hardfacing materials for mining, low stress abrasion testing was therefore chosen for ranking of the test materials. The current work aims to compare conventional MIG technique with a high-end automated welding technique such as PTA. The comparison is done both in technical and economical terms in order to show the viability of this technique in mining applications. Welding coupons prepared with both techniques were machined and wear tested in order to compare the wear resistance of the overlays. The microstructure was also investigated with optical microscopy. The materials under investigation are tungsten carbide with a NiBCrSi matrix (Powder EN P Ni20 and Cored wire EN T Ni20) which are considered as state of the art and are extensively used in the mining industry. Finally an estimate of the process is attempted in order to compare the production costs of the two techniques. 2. EXPERIMENTAL Abrasive wear testing Low stress abrasion response of a material, i.e. its abrasive wear resistance can be evaluated in a number of different wear tests from model to field level. Figure 1 provides an illustration of the differences in approach to model testing in a laboratory and field wear testing for mining components. Model wear testing, such as dry sand rubber wheel testing according to ASTM G65 Figure 1, is fast, accelerated and effective since the component material is the only testing variable. The soil is modelled as a particular silica sand and kept as a constant. However, the testing demands skilled and experienced personnel to perform testing, ensure testing repeatability and analyse test results. The field testing in contrast deals with e.g. real teeth and bucket of an excavator. This includes, besides material testing variables, also component design, field excavating parameters, soil texture and humidity, impact load during digging and others Figure 1. The test results are relatively easily observed and quantified but the effect of single variables and their interactions cannot be resolved. This type of testing is relatively slow and expensive. Neither model testing in the laboratory nor field excavating tests can identify the most wear resistant material for the mining components. However, by starting with model testing in a laboratory, a number of material and cladding technique candidates can be ranked and those Sand feed (Ottawa silica AFS 50/70) 200 RPM CIIR-rubber lined wheel (228,6 x 12.7 mm) Figure 1. Abrasive wear testing levels in mining field excavating level and ASTM G65 dry sand rubber wheel testing. Test materials ASTM G65 - Procedure A, DSRW testing Load arm Weights 6.75 kg (130 N) Total 6000 rev. (4308 m sliding disance) Specimen 25x58x10 mm MIG welding or PTA cladding Ground, Ra < 1 µm The materials examined in this investigation are listed in Table 1. The W45Ni2.4C1.8B material (Cored wire EN T Ni20) was selected as a hardfacing wire used for MIG. The MIG claddings were produced by manual overlay welding on a EN235JR substrate using a 1,6 mm diameter W45Ni2,4C1,8B flux cored wire. The wire is a thin walled tube of a Ni-alloy filled with tungsten carbide and alloy particles. The tungsten carbide is crushed and sieved to a size range of µm. The carbide is in itself a mix of WC and W 2 C carbide. In order to improve on the anticipated problem of dilution of the coating material a second layer (A2) was performed. The PTA hardfacing alloy %4750 (Powder EN P Ni20) was selected as an equivalent material to the wire. The PTA deposition was performed using Höganäs Ni-base powder containing 50% (wt.%) tungsten carbide powder (4750) (B). The tung-

3 sten carbide powder is a melted WC and W 2 C carbide with a size range of µm. Abrasive wear ranking was done according to ASTM G65 dry-sand-rubber-wheel (DSRW) testing. Figure 1 illustrates the test. A plate test specimen is pressed against a rubber lined counter wheel which rotates with 200 RPM and abrades the specimen with the test sand brought into contact by gravity flow. The clad specimens were sectioned perpendicular or parallel to the welding direction. The samples were mounted in bakelite, ground and polished with the final step comprising of 1 µm diamond polish. It was not necessary to etch the samples in order to reveal the solidification structure and the carbides. The metallographic samples were investigated by light optical microscopy using a Leica DM4000M microscope. Code Coating material Substrate Technology A 1.6 mm flux cored EN 235 JR MIG-wire wire: W 45Ni 2.4C 1.8B B Powder: % 4570 EN 235 JR PTA Table 1 - Investigated materials and method of cladding produced by MIG welding (A1 and A2) as shown in Figure 3 and Figure 4. The structure for the PTA sample is shown in Figure 5. Here a smaller variation in carbide amount is evident. In the single layer MIG sample (A1 Figure 3) it is evident that a significant portion of the carbide has been dissolved into the matrix. This is also found in the double layer MIG sample (A2), but not uniformly (Figure 4). The PTA sample B (Figure 5) exhibits a higher amount of carbide and less variation in distribution compared to the MIG samples. By using even higher magnification it can be seen that the tungsten carbide particles are surrounded by a reaction layer as seen in Figure 6 and Figure 7 for the A1 and A2 samples respectively. A difference is seen between the two layers in the A2 sample were Figure 7 is the first and Figure 8 is the second layer. The second layer exhibits a thinner reaction zone around the carbide particles. The dissolution of carbide particles is lower in the top layer of the double overlay weld (A2) compared to the single layer overlay weld (A1). Also the PTA sample exhibits a reaction layer between the original carbide and the matrix as shown in Figure RESULTS AND DISCUSSION Microstructure An overview low magnification of the cases is found in Figure 2. All the samples are sectioned parallel to the welding direction and parallel to the rotation axis of the abrasive wear test. The wear track can be seen as an uneven border to each specimen starting from the upper right corner. The left corner is as-ground. The clad layer can easily be seen in all cases. The thicknesses of the layers are mainly controlled by the amount of grinding performed and not so much by the original thickness of the deposit. In all cases a number of defects can be observed; some porosity is found in the clad as well as small oxides. Cracks were not observed. Figure 2. Light optical micrograph of MIG and PTA specimens cut parallel welding direction. As-polished condition. The amount of carbides present at different locations in the clad varies significantly, especially for the overlays Figure 3.Light optical micrograph of MIG overlay welding A1. As-polished condition. The apparent hardness of the three materials is found in Table 2. The indents of a HV 30 test are quite large compared to the size of tungsten carbide particles and compared to the distance between particles. This means that the hardness value measured is a measure of the combined hardness of all parts of the microstructure. The single layer MIG overlay weld (A1) exhibits a hardness of 497 HV 30, while the A2 and B materials reach 556 and 762 HV 30 respectively. The major difference between the microstructure in these cases is the amount of tungsten carbide present. The A1 has the lowest amount of carbide retained followed by A2 with B having the highest. The carbide dissolution in the B material is very low due to the lower heat input process. Although the hardness of the matrix increases with an increasing amount of tungsten carbide dissolution, the net effect is

4 a decrease in hardness with the decreasing amount of carbides. Figure 4.Light optical micrograph of MIG overlay welding A2. Left part is area of the first layer, right part is in the second layer. As-polished condition. Figure 7. Light optical micrograph of MIG specimen A2 cut parallel welding direction. Picture is in the first layer. Aspolished condition. Figure 5. Light optical micrograph of PTA sample B. Aspolished condition. Figure 8. Light optical micrograph of MIG specimen A2 cut parallel welding direction. Picture is in the second layer. Aspolished condition. Figure 6. Light optical micrograph of MIG specimen A1 cut parallel welding direction. As-polished condition. Figure 9. Light optical micrograph of PTA specimen B cut parallel welding direction. As-polished condition. Code Coating material HV 30 A1 W 45Ni 2.4C 1.8B single layer. 497 A2 W 45Ni 2.4C 1.8B double layer 556 B % Table 2 - Typical hardness (HV 30 ) of test samples.

5 Abrasive wear testing The results of the ASTM G65 dry sand rubber wheel wear testing can be found in Table 3 expressed as average volume loss (AVL). The lowest wear of 9 mm 3 achieved PTA cladded samples (B). Single layer MIG samples (A1) achieved 2.5 times higher wear, 23 mm 3, while double layer MIG samples (A2) 3.5 times higher wear, 31 mm 3, what was the highest wear in the investigation. Explanations for such a large difference in wear resistance of the test samples coated with principally equal clad material can be found in Figure 3 to Figure 5. These show large carbides evenly distributed through the cladd for PTA samples (B) in comparison to MIG samples (A1 and A2). Hardness of the test samples (Table 2) show the highest overall hardness of 762 HV 30 for PTA cladded samples (B) and is directly proportional to the wear rate. Code Coating material AVL* P 50 (mm 3 ) A1 W 45Ni 2.4C 1.8B single layer. A2 W 45Ni 2.4C 1.8B double layer Scatter P 10 /P ,6 23 1,4 B % ,6 Table 3 - Average volume loss (AVL) and scatter of dry sand rubber wheel testing according to ASTM G65 Procedure A (6000 revolutions, 130 N) *) Five specimens were tested for each material and AVL data statistically analysed by log normal probability plot technique, to obtain 50, 10 and 90% probability levels of AVL. Costing A cost comparison between two methods should be systematic and include all relevant costs encountered 6,7. In the present case the necessity for automation of the PTA increases the investment cost for this method compared to the manual MIG welding. Labor cost may be increased if the geometry of the part to be overlay welded must be programmed into the system. In series production this cost can be divided between all the parts produced, while in a repair shop it may be needed to programme for each individual part. On the other hand the actual welding time is often shorter for the automated system as the process is continuous and less physically demanding for the operator. It is also possible to use higher deposition rates as welders are less exposed to heat radiation and fumes. The deposition rates in this case are based on actual measured deposition rates used. These are well below the maximum deposition rates for the techniques which in the case of PTA can be 12 kg/hour or more. In this comparison loss of filler material e.g. spatter, powder overspray are ignored. The test case used was a coupon of 100x70 mm used for fabrication of wear test samples. The PTA was operated at a deposition rate of 2.1 kg/h while the MIG was operated at 2.8 kg/h, see Table 4. The PTA covered the coupon in 5 passes using a travel speed of 70 mm/min and a weaving of 16 mm which resulted in a bead width of 22 mm. The MIG process was operated without weaving giving a bead width of 10 mm and it required 10 passes to completely cover the coupon. During the welding it became evident that the dissolution of the MIG deposited material was quite high and the thickness of the overlay was low with a somewhat uneven surface. Therefore a double layer deposit (A2) was produced, thus increasing layer thickness and decreasing the dilution. The welding live time is found in Table 4 as well as the estimated cost for the filler material. The energy consumption for the different methods has not been estimated. The cost for electric power is probably higher for the PTA method since more equipment is involved. The gas consumption for PTA can be slightly higher compared to MIG welding. However, these costs are strongly dependent on the size and model of equipment used. The efficiency of handling of the parts is a further factor that can influence cost. MIG A1 MIG A2 PTA B Deposition rate (kg/h) Welding speed (mm/min) Weaving (mm) No of passes per layer No of layers Overlay thickness (mm) Overlay Mass (kg) Welding live time (h) Filler material cost ( /kg) Weld material cost ( ) Table 4 Parameters and results noted for this comparison. Also labor costs are difficult to assess in a general case. The labor costs can be broken down into preparation time, weld time and handling time. In the test case the preparation, including programming, is exceptionally easy; it is a flat coupon of 100x70x8 mm. However, real cases are more complex as shown in Figure 10. Here preparation; gridning and sand blasting can require more time. The programming may take time depending on the complexity of geometry. In the case of repair welding the geometry must be measured or scanned.

6 5. REFERENCES Figure 10. Selected applications where hardfacing is used for durability. 4. CONCLUSIONS Cladding of test components exposed to abrasive wear have been compared in respect to wear performance. PTA cladded powder and MIG cladded wire with equivalent composition - Ni-based matrix with around 50 wt.% carbide. The following conclusion have been drawn: Lower heat input with PTA results in higher tungsten carbide retention in the clad. 3,5 times lower wear with PTA clad compared to single layer MIG clad. 2,5 times lower wear with PTA single clad compared to double layer MIG clad. Consistancy, repeatability and work environment can be improved by PTA techniques without increasing costs. 1. Bhushan, B (ed.), Modern tribology handbook, CRC Press, Boca Raton, Olson D L and Cross C E, Friction and Wear in the Mining and Mineral Industries, Chapter: Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, ASM International, 1992, p Tian, H H, Wear-resistant materials for engineering and mining applications, Engineering and Mining Journal; Jun 2002; 203, 6; p24 4. Dizdar S and Maroli B, Abrasive wear resistance of thermal surfacing materials for soil tillage applications, Proceedings of the International Thermal Spray Conference ITSC 2013: Innovative Coating Solutions for the Global Economy, May 13 15, 2013, Busan, Republic of Korea 5. Hauer I and Kampanis N. Cladding of submerged propeller shafts: a comparison between conventional and high end techniques and materials World Maritime Technology Conference, 29 th May 1 st June 2012, Saint-Petersburg, Russia. 6. Weman K, Welding Processes Handbook, Boca Raton : CRC Press ; Cambridge : Woodhead Publishing, 2003, ISBN , vii, Scotti, A; Silva, C R; Ferraresi, V A, A Quality and Cost Approach for Welding Process Selection, Journal of the Brazilian Society for Mechanical Sciences (Brazil), ISSN , 2000, Volym 22, Nummer 3, pp