CHEMICAL PRE-TREATMENT OF MECHANICALLY MILLED RECYCLED HARDMETAL POWDERS Zikin, A., Yung, D., Hussaionova, I. & Ilo, S.

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1 8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING April 2012, Tallinn, Estonia CHEMICAL PRE-TREATMENT OF MECHANICALLY MILLED RECYCLED HARDMETAL POWDERS Zikin, A., Yung, D., Hussaionova, I. & Ilo, S. Abstract: The present research concentrated on chemical pre-treatment of recycled powders bathed in acid media. Influential factors including concentration, submerging time, solution composition, cleaning conditions (magnetic or ultrasonic agitation) and temperature were studied in detail. Results showed the importance of chemical pre-treatment process. Sulphuric acid was selected as a suitable reagent as it is readily available, affordably priced and highly reactive. Furthermore, sulphuric acid did not dissolve the cobalt binder in WC-Co as compared to many other concentrated acids. Hardfacings produced from pretreated recycled powders showed an improved quality with almost no surface defects. Key words: chemical treatment, powder surface cleaning, WC-Co, pta hardfacing 1. INTRODUCTION Tungsten carbide hardmetal is one of the most renowned powder metallurgical products, mainly used for manufacturing machine tools operating under severe wear stress conditions. When alloyed with different elements including Fe, Co or Ni, tungsten carbide s superior hardphase properties provides high wear resistance for materials used in cutting, mining, drilling tools and other industrial equipment [1-3]. However, the high marked requirements lead to permanently increasing prices and wastage of cemented tungsten carbide. Therefore, research and development into alternative solutions to reuse or recycle hardmetal scrap containing tungsten carbide has become economically important in recent years. The technology to recycle hardmetal scraps from worn and damaged cermets requires application of different processes for the breakup, disjunction and purification of hardmetal particles. One effective mechanical method for recycling of cermets and hardmetal scrap is disintegrator milling [4-5] which yields powders of uniform particle size and shape. However, disintegrator milling hard and dense hardmetals also wears the mechanical grinding medium, contaminating the derived powder. Surface contamination, especially by oxides, can significantly influence quality, properties and performance (decrease weldability; increase porosity) of recycled hardmetal powder [6-7]. During the Plasma Transferred (PTA) hardfacing, contaminants can cause CO 2 or other gases to form resulting in pores. This hindrance factor affects the eventual surface quality of nickel-based compositions. To produce reliable pore-free coatings, the quality of recycled powders needs to be improved. The present study is focused on chemical pre-treatment of the powders produced from WC-Co scrap. The process of hardmetal scarp cleaning has been developed based on the commercially cheapest method of chemical pre-treatment and simplicity of experimental procedure that allows quick, efficient and effective treatment of the recycled hardmetal powders to be used for PTA hardfacing process. The main objectives of the present research are as follow: (1) to remove iron contamination from the surface of recycled 777

2 WC-Co powder particles; (2) to optimise the parameters for chemical treatment; (3) to improve quality of the weld transferred from the recycled WC-Co scrap. 2. EXPERIMENTAL 2.1 Powder preparation and characterisation Recycled WC-Co hardmetal powders were produced from hardmetal scrap by disintegrator milling as described elsewhere [4]. The milling blades as well as disintegrator device frame are constructed from steel. Therefore, as expected, some of the steel (namely iron and iron based particles) would contaminate the resulting surface as shown in Fig. 1. Chemical composition and grain size distribution of the recycled WC-Co powder are listed in Table 1. It should be noticed that coarse hardmetal particles represent fine (3-5 µm) tungsten carbide grains bonded together by Cobinder. Some trace amount of titanium nitrite (2-3µm layer thickness) was found in the grinded and milled powder mixture (see Fig. 1) as the WC-Co scarp was sourced from cutting tool elements. Fig. 1. Optical microscopy image of recycled carbide powder after disintegrator milling Material Grain size, µm Chemical composition, wt % Co, 6-10 Fe, 2-4 Cr, rest WC* WC-Co * Various due to wear of grinding media Table 1. Characterisation of recycled powder 2.2 Chemical pre-treatment procedure Standard laboratory reagents were used in all tests: H 2 SO 4 stock was 95% and the reagent was serially diluted to 50%, 40%, 30%, 20%, and 10% for the experiments using distilled water. The main parameters influencing the process of surface cleaning were studied: time, concentration (diluted acids), ultrasonic energy and temperature. Hardmetal powder (20 g) was placed into a pyrex flask with 3 ml of acid under consideration. Magnet stirrer was used to continuously and thoroughly mix the acid with the powder during the reaction time. The stirrer ensured a constant flow of acid between individual particles. Magnetic stirrer was used for every experiment with exception of ultrasonic agitation experiments. In some tests, ultrasonic energy was introduced into the acid bath instead of magnetic agitation. The water was kept at room temperature, however, vibrational energy from the ultrasonic bath quickly increase the temperature to ~35 C. Experiments were performed with H 2 SO 4 of 10%, 20%, and 30% based on the results from magnetic agitation research. For further optimization of the process, the temperature of solution treatment was varied in the range C while other variables were fixed. To stop acid reactions at the specified time, water was slowly poured to quench the reaction. Water washing was repeated until the mixture became clear. This step was followed by ethanol washing to remove impurities and remaining acids. Isopropyl alcohol was the next to sensitized any water and sulphuric acid products. Finally, benzene was added in the mixture and then poured out to help with drying. All samples were dried overnight in a 45 C oven to remove all traces of moisture. Dried, chemically treated powders were weighted using analytical balance. Percent weight lost was calculated when compared to an initial weight. Pycnometer method measured the density of powders. 778

3 Surface of the powders before and after cleaning process was studied by scanning electron microscopy (SEM). Chemical composition was analysed by energy dispersive X-ray spectroscopy (EDS) and X-ray fluorescence (XRF). 3. RESULTS AND DISCUSSION Time, min H 2 SO 4 95% 3.1 Time factor SEM images of recycled WC-Co powder are presented in Fig. 2 clearly showing powder surface contamination. EDS and XRF analysis revealed iron and iron oxide residues. To evaluate the effect of cleaning procedure duration, 95% concentrated H 2 SO 4 acid has been used for an adjusted time with 10 min intervals between 0 (initial powder without any treatment) and 60 min. Fig. 3 exhibits quantitative data for the weight percentage of some main elements found on the sample surface. The total weight lost due to the acid treatment is between % that means carbide dissociation rate even after 60 minutes of exposure in the acid bath stays constant. With increased time of treatment degradation of Co binder from surface takes place. Cobalt content decreases from its pre-treatment level of approximately 11 % to about 3 % post-treatment. a) b) Fig. 3. Surface analysis based on EDS measurements - constant sulphuric acid concentration of 95% at different time scales Furthermore, the use of strong sulphuric acid also left unfavourable sulphur residues on particle surface. The speculation is that sulphur presence on hardmetal grains could lead to problems (mainly the formation of pores) during the PTA process. Some iron contamination remains after 60 minutes exposure to concentrated H 2 SO 4 acid as indicated by backscattering SEM images (see Fig. 4). Based on in-experiment observations, the reaction of concentrated H 2 SO 4 significantly decreases contaminations after just 30 minutes; however, the process also removes Cobinder with increasing exposure time. 3.2 Diluted acids Fig. 5 corresponds to the surface analysis results based on the diluted acids with concentration decreasing from 50 % to 10 % at 15 minutes exposure time. contaminations contaminations Fig. 2. SEM micrographs of recycled WC- Co powder surface: a) BSE, magnitude 100x; b) BSE, magnitude 500x Fig. 4. SEM micrograph of recycled powders after pre-treatment 95 % H 2 SO 4, T 60 mins 779

4 H2S04 Concentration, % T 15 min H2SO4 Concentration % T 15 min, Ultrasonic Fig. 5. Surface analysis based on EDS measurements - different acid concentration at 15 min experiment With increasing acid concentration a trend of decreasing iron weight percentage is observed. This indicates a decrease in superficial iron and iron oxide contaminations on the hardmetal grains. Co wt% remains at a constant level of about 10% for all acid concentrations, except for experiments with 50% H 2 SO 4 acid where Co wt% slightly drops to 9%. Furthermore, probably due to lower acid concentration, no sulphur contamination is detected. SEM observations (see Fig. 6) confirm that certain amount of surface impurities remains on the material. The total weight loss is observed at between % for 15 min tests. 3.3 Ultrasonic cleaning Promising results (see Fig. 7) are achieved by using ultrasonic bath and vibration energy. Even 10% concentration of H 2 SO 4 is able to significantly remove Fe-based contaminants, without dissolving Co binder Fig. 7. Surface analysis based on EDS measurements - ultrasonic energy at different concentration with constant time metal and not adversely affect the surface of the grain and structure. After 15 minutes cleaning with 30% concentration H 2 SO 4 under ultrasonic bath iron content on the surface becomes negligible and no oxides can be detected (Fig. 8). The amount of free iron also appears to be reduced significantly compared to any other previous results. Co is maintained at 11%. 3.4 Temperature Increasing temperature during chemical cleaning does not seem to affect refinement quality of the grains until > 80 C (see Fig. 9). Reaction with 30% concentrated H 2 SO 4 at temperatures 23, 40 and 60 C are found to be similar at removing the contamination from the surface of hardmetal grains. Nevertheless, below 80 C, temperature treatment has a detrimental effect compared to ultrasonic cleaning. Fig. 6. SEM micrograph of recycled powders after pre-treatment; 30 % H 2 SO 4, T 15 mins Fig. 8. SEM micrograph of recycled powders after pre-treatment; T - 15 mins in ultrasonic bath 780

5 T 15 min, H 2 S0 4 30% Temperature 80 C 60 C 40 C 23 C Fig. 9. SEM micrograph of recycled powders after pre-treatment; different temperatures with constant time and constant acid concentration Between room temperature and 60 C, Fe concentration and Co concentration are constant when using 30% H 2 SO 4. Once the temperature hits 80 C both concentrations (Fe and Co) decrease significantly. Iron and iron oxides are fully removed from the grain surfaces (Fig. 10); however, Co binder metal is also removed. Therefore, changing temperature is counterproductive to maintaining elemental Co in the hardmetals. To protect the cobalt binder, treatment temperature should not be artificially raised above 60 C. 3.5 Pre-treatment effects SEM analysis of the treated particles with different acid concentrations indicates that surfaces of hardmetal grains after cleaning become finer with higher acid concentration (Fig. 11). It is assumed the acid starts to penetrate between phases of WC and Co-binder while dissolving both. This etching effect leads to increased Fig. 10. SEM micrograph of recycled powders after pre-treatment; T - 15 mins at 80 C Fig. 11. SEM backscattering images of recycled grains surface: a) before chemical treatment; b) after treatment with 30% H 2 SO 4 and 15 min; c) after treatment with 50% H 2 SO 4 and 15 mins weight loss and refined carbides. Similar effect was observed also for cleaning at temperature of 80 C. Based on experimental analyses, the recommendations for increasing quality of recycled powders using pre-treatment are as follow: 30 % H 2 SO 4, ultrasonic bath, 15 minutes cleaning time. For further evaluation, initial powder is welded with the same parameters as pre-treated powder. Cross-section image of both weld deposits and SEM micrograph of treated hardfacing are presented in Fig. 12. The results indicate significant improvement of weld deposit quality. No porosity is found in either cross or longitude sections for pretreated weld. This chemical pre-treatment method has demonstrated to significantly improve the quality of recycled powders, 781

6 after pores before 3. Application of chemically pretreated recycled hardmetal powders by PTA hardfacing has revealed high improvement of weld deposit quality and the main problem of welds porosity was solved. 6. REFERENCES Fig. 12. Cross-section of weld deposit and SEM image of hardfacing and should be employed for future processing of contaminated WC-Co powders. 4. ACKNOWLEDGEMENT This work was founded from the Austrian Comet-Program (governmental funding program for pre-competitive research) via the Austrian Research Promotion Agency (FFG) and the TecNet Capital GmbH (Province of Niederösterreich) and has been carried out within the Austrian Center of Competence for Tribology (AC2T research GmbH) and partially supported by graduate school Functional materials and processes, receiving funding from the European Social Fund under project in Estonia. 5. CONCLUSION Based on the study within this work, the following conclusions can be drawn: 1. The chemical pre-treatment method, used in the present study was successfully applied for improving the quality of recycled WC-Co powders minutes of reaction time and ultrasonic energy could be enough to remove all iron and iron oxide residues from powders surface. [1] Zum Gahr, K.-H. Microstructure and Wear of Materials. Elsevier, New York [2] Ndlovu, S. The Wear Properties of Tungsten Carbide-Cobalt Hardmetals from the Nanoscale up to the Macroscopic Scale. Der Technischen Fakultät der Universität Erlangen, PhD thesis, Nürnberg, [3] Mellor, B.G. Surface Coatings For Protection Against Wear. Woodhead Publishing, Cambridge, [4] Tümanok, A., Kulu, P., Mikli, V., Käerdi, H. Technology and equipment for production of hardmetal powders from used hardmetal, In Proc. 2nd International DAAAM Conference 2000, [5] Zimakov, S., Pihl, T., Kulu, P., Antonov, M., Mikli, V. Applications of recycled hardmetal powder, Proceedings of The Estonian Academy of Sciences. Engineering, 2003, 9/4, [6] Zikin, A., Hussainova, I., Winkelmann, H., Kulu, P., Badisch, E. accepted for publication, Int. J. Heat Treatment Surf. Eng. doi: / z [7] Yao, Z., Stiglich, J.J., Sudarshan, T.S. Nano-grained Tungsten Carbide-Cobalt (WC/Co). PM Special Features, Metal Powder report, 1998, 53/3, CORRESPONDING ADDRESS MSc. Arkadi Zikin AC2T Research GmbH, Viktor Kaplan- Strasse 2, 2700 Wiener Neustadt, Austria Phone: , Fax: , 782