Plasma transferred arc hardfacings reinforced by ...

Plasma transferred arc hardfacings reinforced by chromium carbide based cermet particles A. Zikin*1,2, I. Hussainova2, H. Winkelmann1, P. Kulu2, E. Badisch1 In this work, multiphase metal matrix composite coatings consisting of chromium carbide and chromium carbide based cermets were produced using plasma transferred arc hardfacing technique. Cr3C2 and Cr3C2–Ni powders were mixed with Ni based metal alloys and deposited on austenitic plates in order to minimise oxidation of the substrate at elevated temperature. The effect of the Ni binder on the dissolution level of the primary carbides during processing was studied. Furthermore, chemical pretreatment of the hard phases was applied to improve the weldability of the precursor powders. The microstructures of the materials were analysed by both scanning electron microscopy and energy dispersive spectroscopy. Micromechanical properties, including hardness and elastic modulus of the constituents, were measured by nanoindentation. The materials under consideration were characterised like tribomaterials by subjecting them to abrasive and impact conditions at temperatures up to 700uC. Results indicated that Ni binder can significantly decrease dissolution of the primary carbides during processing and improve the microstructure of the coatings obtained. Tribological tests revealed high wear resistance at elevated temperatures, whereas chemically pretreated Cr3C2–Ni based composites exhibited the lowest wear rates. Keywords: Tribology, Surface treatment, High temperature wear, Chromium carbide, Hardfacing, PTA

coatings but only a few studies11,12 have been concentrated on the processing of Cr3C2 in combination with Ni based matrix by hardfacing technologies. Perhaps it is because the high temperature of the electron, plasma or laser beam results in extreme dissolution of the primary chromium carbides and changing microstructure of the coating produced. Dissolved Cr3C2 forms hard, but very brittle structures, which significantly decrease the wear resistance of the hardfacings. In the present research, the use of chromium carbide based cermet powder is suggested as a possible way to overcome the problem of carbide dissolution. For this purpose, Cr3C2 powder was mixed with a Ni binder material, then subsequently pressed and sintered. The resultant bulk materials are then refined to powder size. Cermet powders produced in this way are hardfaced in combination with a Ni based matrix and have been analysed in detail.

Introduction Plasma transferred arc (PTA) process is an efficient technology commonly used for the fabrication of wear resistant hardfacings. As a unique heat source for surface modification, PTA exhibits enormous potential because of its low cost (compared with other surface deposition processes), easy operation and no need for any special surface treatment.1–4 Furthermore, the PTA technique allows the production of high quality coatings (good metallurgical bonding and low levels of porosity) consisting of metal matrix and carbide hard phases. Most widely used industrial hardfacings consist of a Ni based matrix, reinforced by fused WC/W2C hard phases. The advantages and superior properties of such structures in abrasive and corrosive environments have been shown previously.5–8 However, commercially used WC/W2C hard phases are subjected to oxidation at temperatures .600uC. Therefore, the development of advanced hardfacings using oxidation resistant hard phases is of great interest. One of the promising candidates could be chromium carbide since it is a relatively cheap material and exhibits oxidation resistance at high temperatures.9,10 Cr3C2 is often used in the production of thermal sprayed

Experimental The chemical composition and additional information of initial powders are listed in Table 1. Figure 1 shows the main processing steps used to produce the wear resistant thick coatings. Cr3C2 powder was mixed with Ni metal powder (20 wt-%) using a low energy milling technique. After pressing and sintering, the bulk materials produced were crushed to powder using a disintegrator milling machine as described. The powder produced was then sieved with z310–150 mm mesh.

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AC2T Research GmbH, Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria Department of Materials Engineering, Tallinn University of Technology (TUT), Ehitajate 5, 19086 Tallinn, Estonia

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*Corresponding author, email [email protected]

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Plasma transferred arc hardfacings reinforced by chromium carbide based cermet particles

indenter and operating in linescan mode. For indentation, an applied normal load of 5 mN was used. In order to simulate field conditions in the laboratory as realistically as possible, wear tests were performed using the cycling impact abrasion test for combined impact–abrasion behaviour developed at the Austrian Centre of Competence for Tribology (AC2T) and described elsewhere.13

Results and discussion Chemical pretreatment

1 Processing steps for production of novel coatings

Purification of the powder was performed by chemical treatment in chloric acid and successive cleaning with water and isopropyl alcohol. These processing steps were done to clean the particle surface after milling and to improve the weldability and mechanical properties of the hardfacing. Plasma transferred arc hardfacing was performed using an EuTronic Gap 3001 DC apparatus. To prevent oxidation of the substrate, all materials were deposited on the austenitic stainless steel (316L). Parameters such as current, oscillation and welding speed, substrate, powder feedrate, nozzle distance to workpiece and process gas flowrates were optimised in relation to the hardfacing behaviour and practical hardfacing procedures. Hardfacing was carried out in a single layer without preheating or heat control of the substrate for reduced dilution. Test samples were cut, ground and finished with a 1 mm polish. Hardness measurements were carried out with a standard Vickers hardness machine. To determine the hardness of each phase in the microstructure, e.g. the hard phases and metallic matrix, microhardness HV0?1 tests were used. Characterisation of microstructure was performed by optical microscopy after etching with Murakami’s reagent as well as by SEM/energy dispersive spectroscopy (EDS). The nanoindentation measurements on the carbides and matrix were performed using a Hysitron TriboIndenter apparatus (Hysitron Inc., USA) equipped with a Berkovich

A certain quantity of impurities, oxides and waste material is found on the surface of chromium carbides after the disintegrator milling process (Fig. 2b). These impurities can influence the weldability of the powders and decrease the quality of the resultant hardfacing. A method of fast and cheap chemical treatment was developed for cleaning the surface of the milled powder, which significantly improves the properties of cermet particles. Figure 2c and d presents SEM/EDS analysis of the surface of the cermet powder particles after the chemical treatment where no additional material is seen on the particle surfaces. It was determined that 10–15 min of cleaning time is enough to purify the surface of milled particles without initiating the dissolution of the main elements – carbides and binder.

Microstructure and hardness In the present study, three different hardfacings produced with the same welding parameters were analysed and compared to reveal differences between the coatings obtained from standard chromium and cermet (treated/untreated) powders. For all materials, the content of hard phases was chosen to be 40 vol.-% (to reach a homogeneous distribution of hard phases in the matrix). Figure 3 illustrates the typical cross-sectional optical micrographs of the coatings. Metco 70C-NS Cr3C2 was used as reference material (Fig. 3a). The microstructure consists of reprecipitated carbides with a very low content of undissolved carbide particles. Those primary carbides appear close to the substrate region. The highest level of dissolved and precipitated chromium carbides is found in the upper layer of the coating, where reprecipitated carbides also substantially grow in size forming coarse, elongated precipitates in the matrix. Similar results were obtained in Ref. 14, where Cr3C2 was laser clad on a Ni based matrix. The compound macro hardness of Cr3C2 reinforced coating shows a hardness of 560 z/2 25 HV(500 gf). Figure 3b shows the microstructure of a coating produced from chemically untreated cermet particles in combination with the Ni based matrix. The microstructure of this coating can be divided into three phases: a matrix zone, dissolved and reprecipitated carbides, and a certain content of ‘grain from’ primary cermets

Table 1 Information about initial materials Material

Supplier

Composition/wt-%

Cr3C2 Ni Matrix

Sulzer Metco 70C-NS MMC Norilsk Nickel Castolin 16221

8.6 C; rest Cr 99.9 Ni 0.2 C; 4 Cr; 1 B; 2.5 Si; max. 2 Fe; 1 Al; rest Ni


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a SEM image of standard Cr3C2 powder; b SEM image of untreated Cr3C2–Ni powder; c SEM image of cleaned Cr3C2–Ni powder; d chemical composition of powder surface based on EDS analysis 2 Comparison of chromium carbide powders

hardness values. The hardness increases (compared with other coatings) due to the high content of uniformly distributed primary cermet particles.

homogeneously distributed throughout the matrix. It was assumed that the Ni binder in the cermet particles protected the primary carbides from dissolution and helped keep the particles in their original form. Unfortunately, a quantity of carbide still melts and coarse elongated pin form reprecipitated carbides can be observed. The overall macrohardness of the coating is 640¡30 HV(500 gf), whereas matrix hardness is 390¡ 28 HV0?1. Very promising results were obtained with chemically treated cermet particles (Fig. 3c). The microstructure indicates a large amount of primary carbides, uniformly distributed in matrix with a very low level of carbide dissolution. A closer look at this microstructure (Fig. 4) shows that between the primary carbides, there is Ni binder and dissolution occurring to some degree only at the border line between matrix and cermet phases. It is assumed that a low level of dissolution could be explained with the cleaned and chemically activated surface of cermet particles. However, the formation of stress cracks in these structures is also noted. The overall hardness of this coating is 730 z/2 110 HV(500 gf) (50 g). The increase in hardness of the Cr3C2-Ni reinforced chemically treated coating is due to a high content of uniformy distributed cermet particles. At the same time such structure leads to a high spread of

Nanoindentation Micromechanical properties of the phases (hardness and reduced Young’s modulus) were determined by nanoindentation measurements and statistically analysed using bubbles graphics. Figure 5 shows the hardness and reduced Young’s modulus of the Cr3C2 reinforced coating. The results for the hardfacing show a mean nanoindentation hardness of 4?4 GPa in the matrix phase with some variation in elastic modulus between 230 and 300 GPa. ‘Carbide A’ are reprecipitated hard phases, which show a low range of measured values (both hardness and elastic modulus). ‘Carbide B’ are primary Cr3C2 hard phases, which are found in the matrix. The large range of measured values for the primary carbides could be explained by the beginnings of the reprecipitation process. However, there is a visible difference between primary and reformed carbides. The main difference in microstructure between hardfacings with treated and untreated cermet particles is in the content of dissolved particles. Therefore, nanoindentation results obtained for the phases in those coatings are compared and presented in Fig. 6.

a reference – hard phases Cr3C2; b hard phases Cr3C2–Ni before treatment; c hard phases Cr3C2–Ni after treatment 3 Optical microscopy cross-sectional images of produced hardfacings

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6 Nanoindentation measurements for Cr3C2–Ni reinforced coatings

4 Cross-sectional image (SEM) of hardfacing reinforced with chemically treated cermet particles

For cermet reinforced coatings, matrix measurement values are similar with those for Cr3C2 reinforced coating. ‘Matrix in carbides’ is the identified phase between chromium carbides in cermet particle (Ni binder). Strong deviations in the values can be explained by some diffusion of carbides into the binder matrix, increasing its hardness and elastic modulus. Hardness values of primary chromium hard phases (carbide A) have a mean reduced Young’s modulus of 348 GPa and hardness values from 20 up to 28 GPa. Such difference in results could be attributed to the presence of M7C3 mixed phases, which appear during sintering of C3C2– Ni. Formation of these phases is described in details elsewhere.15 ‘Carbide B’ corresponds to apparent reprecipitated hard phases and the values are comparable to those found with Cr3C2 reinforced coatings.

Wear testing It is well known that abrasive wear behaviour is mostly influenced by material hardness and microstructural features. However, in conditions where impact wear is a dominant factor, increased plasticity and high content of fine grained hard phases uniformly distributed in matrix result in improved wear resistance of a material.13 Therefore, it is assumed that hardfacings, reinforced with chromium carbide based cermets, could be an alternative solution to commercially used tungsten carbide reinforced coatings, especially at temperature .600uC. Initial wear investigation results produced by testing materials at temperature up to 700uC are given in Fig. 7, where Cr3C2 Metco is a coating reinforced with standard Cr3C2 particles: Cr3C2–Ni (U) – coating reinforced with

5 Nanoindentation coating

measurements

for

Cr3C2

reinforced

chromium carbide based cermet particles (chemically untreated); Cr3C2–Ni (C) – coating reinforced with chromium carbide based cermet particles (chemically cleaned); and Ref. – commercially used standard coating (Ni based matrix z60 wt-% fused WC/W2C hard phases). Results indicate that at temperatures up to 550uC, the commercially used coating Ref., taken as the reference, has shown the highest wear resistance. However, starting from 300uC, the tungsten carbide reinforced hardfacing continuously increases volumetric wear by a factor of 2 at every subsequent test temperature, whereas chemically treated chromium carbide based cermet coatings show similar wear values at all temperatures. At a temperature of 700uC where WC/W2C undergoes oxidation, the newly developed hardfacings show even better wear resistance. Additionally, it should also be noted that content of hard phases in the Cr3C2–Ni based materials is smaller compared with the commercially used coating. Furthermore, it is observed that uniformly distributed primary cermet particles give better combined impact/ abrasion wear resistance, compared with coatings consisting mostly of reprecipitated hard phases (Cr3C2 Metco). The reason why cermet particles show improved wear resistance can be ascribed to the fine grain size of the primary carbides. This means that the material produced consists of the coarse cermet particles uniformly distributed in the matrix. The cermet particles themselves consist mostly of fine Cr3C2 grains that neither dissolve nor reprecipitate. Structures obtained still preserve relatively high hardness, but at the same time show increased ductility and good mechanical and tribological properties.

7 Cycling impact abrasion test wear values: reference material – Ref. (Ni based matrix z60 wt-% of fused WC/W2C hard phases)


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been partially supported by graduate school ‘Functional materials and processes’, receiving funding from the European Social Fund under project 1?2?0401?09-0079 in Estonia and also partially supported by ESF grant 8211. The authors are also grateful to BSc Der LiangYung for his support and powder treatment.

Conclusions Based on this study, the following conclusions can be drawn: 1. Chromium carbide based cermets can be successfully applied to the production of wear resistant hardfacings using PTA technology. 2. The coatings produced show a microstructure with a higher proportion of primary cermet hard phases compared with coatings using standard Cr3C2 powders. 3. Chemical treatment of the milled powders significantly improves the quality of the coating produced, resulting in minimum level of carbides dissolution. 4. Under combined impact/abrasion conditions, the hardfacings containing chemically treated cermet particles have shown promising results with high wear resistance. The wear rate at 700uC is lower than that of the commercially used WC/W2C reinforced Ni based hardfacings. Finally, it should be pointed out that the results of the present research require further detailed investigations including process optimisation.

References 1. B. G. Mellor: ‘Surface coatings for protection against wear’; 2006, Cambridge, Woodhead Publishing Limited. 2. R. Chattopadhyay: ‘Advanced thermally assisted surface engineering processes’; 2004, New York, Kluwer Academic Publishers. 3. R. L. Deuis, J. M. Yellup and C. Subramanian: Compos. Sci. Technol., 1998, 58, 299–309. 4. J. Wilden, J. P. Bergmann and H. Frank: J. Therm. Spray Technol., 2006, 15, (4), 779–785. 5. R. Colac¸o and R. Vilar: Wear, 2003, 254, 625–634. 6. Ch. Just, S. Ilo and E. Badisch: Surf. Coat. Technol., 2010, 205, 35–42. 7. C. Katisch and E. Badisch: Surf. Coat. Technol., 2001, 206, 1062– 1068. 8. M. Vagra, H. Winkelmann and E. Badisch: Proc. Eng., 2011, 10, 1291–1296. 9. S. Matthews, B. James and M. Hyland: Corros. Sci., 2009, 51, 1172–1180. 10. M. Antonov and I. Hussainova: Wear, 2009, 267, 1798–1803. 11. J. Nurminen, J. Na¨kki and P. Vuoristo: Int. J. Refract. Met. Hard Mater., 2009, 27, 472–478. 12. Z. Huang, Q. Hou and P. Wang: Surf. Coat. Technol., 2008, 202, 2993–2999. 13. H. Winkelmann, E. Badisch, M. Kirchgaßner and H. Danninger: Tribol. Lett., 2009, 34, 155–166. 14. G. L. Goswami, S. Kumar, R. Galun and B. L. Mordike: BARC, 2004, 249, 64–70. 15. I. Hussainova, J. Pirso, M. Antonov, K. Juhani and S. Letunovits: Wear, 2007, 263, 905–911.

Acknowledgements This work was funded from the ‘Austrian CometProgram’ (governmental funding programme for precompetitive research) via the Austrian Research Promotion Agency (FFG) and the TecNet Capital GmbH (Province of Niedero¨sterreich) and has been carried out within the ‘Austrian Center of Competence for Tribology’ (AC2T research GmbH) in cooperation with Tallinn University of Technology. This work has

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