The structure of Fe-Cr-B coatings obtained using selected methods of thermal spraying

Transcription

1 IOP Conference Series: Materials Science and Engineering The structure of Fe-Cr-B coatings obtained using selected methods of thermal spraying To cite this article: K Szymaski and M Szpak 2012 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. Related content - Process of high temperature synthesis in producing composite carbide powders for thermally sprayed coatings K Szymaski and B Formanek - Thermal Arc Spray Overview Muhamad Hafiz Abd Malek, Nor Hayati Saad, Sunhaji Kiyai Abas et al. - The structure of coatings obtained in the Zn-31Al-3Mg bath by the batch hot dip method H Kania This content was downloaded from IP address on 01/07/2018 at 01:17

2 The structure of Fe-Cr-B coatings obtained using selected methods of thermal spraying K Szymański 1, M Szpak 2 1 Silesian University of Technology, Faculty of Materials Science and Metallurgy, Krasińskiego 8, Katowice, Poland 2 Energo-Coating, Klejowa 24, Radom, Poland krzysztof.szymanski@polsl.pl Abstract. This paper presents issues connected with the production of protective coatings via thermal spraying. Information about wire coating materials which contain phases from the Fe- Cr-B system is presented. Arc thermal spraying is characterised and its possible applications are determined. The results of the examinations of the structure of coatings obtained by means of various arc spraying systems are discussed. Coatings of this type are used in the machine building and power engineering industries. 1. Introduction Thermally sprayed coatings are widely used to protect surfaces against abrasive and corrosion wear. They are used, for example, to protect components of power engineering devices and boilers against abrasive wear induced by hard ash particles hitting at different angles and against high operating temperature up to ca. 700 o C, as well as the corrosion caused by the fuel combustion products. Therefore, the coatings that work in such conditions must have high hardness and required thickness, good adhesion to the substrate, low porosity and a complex phase composition which will guarantee a high corrosive resistance to compounds containing sulphur, chlorine and water vapour in oxygen depletion [1-5]. Among a number of methods of thermal spraying, the ones that are practically used for protecting boiler components are the flame method, arc spraying, plasma spraying and high velocity oxygen fuel spraying (HVOF). The flame methods and plasma spraying are relatively rarely used due to the complexity of the process (flame methods with fusing) or relatively high costs (plasma spraying) [6-8]. The most frequently used methods of thermal spraying used for protecting boilers are arc and high velocity methods. The HVOF thermal spraying allows obtaining highest quality coatings of carbide materials. They are characterised by: high hardness (up to 1600 HV), tightness (porosity <2 %) and good adhesion to the substrate (>80 MPa). However, they are relatively expensive. The characteristics of HVOF methods is a broad issue which has been addressed in many papers [9-13]. On the other hand, the biggest advantage of the arc method is the possibility to effectively cover huge areas, which translates into relatively low production costs of the coating. The development of the method and good functional properties of coatings result from the application of advanced 1 To whom any correspondence should be addressed. Published under licence by Ltd 1

3 composite materials with a pre-designed phase composition, with a metallic matrix resistant to corrosion and hard reinforcement phases. Properly produced coatings have a banded structure of low porosity (ca. 2 %), a high level of thickness (0.5-1 mm) and varied hardness, from ca. 350 (IN635) to HV (e.g. W-685 and CE-595). The different properties of coatings may be obtained by using materials with diverse chemical and phase compositions, and optimized parameters of coating production. Examples of used wire materials are presented in Table1. Table 1. Examples of wire materials which contain phases from the Fe-Cr-B system. No. Material name Approximate chemical composition of the wires 1. FMI - 3 Fe-Al-Cr-B 2. W-685 Fe-Cr-B 3. M 5 Fe-Cr- C 4. SF 27/ Fe-Ni-Cr-Si-B 5 FMI - 2 Fe-Al- Cr-B 6. CE- 595 Fe-Cr-Si-B 2. Aim and scope of the research The arc-sprayed coating materials presented in Table 1 are distinguished by a number of advantageous functional properties. However, the best coatings are obtained by using very good thermal spraying devices and correctly selecting the parameters of coating production. Therefore, the essential purpose of the research presented in this paper was to examine the influence of the type of the device on the structure of coatings obtained from the coating material which had been adopted as a model. Chemical composition of the sprayed material is presented in Table 2. Table 2. Approximate chemical composition of the material used in the tests. Percentage content of chemical elements in the wire Fe C Si Mn Cr Mo W B balance Coatings were sprayed onto plates made of the P235 steel with surface roughness R a >10 m, previously cleaned by abrasive blasting to the SA3 class. Three devices were used in the process of thermal spraying: Smart ARC (Sulzer Metco), AT 400 (Flame Spray Technologies) and OSU 300A LD/U-2 (OSU Hessler). The coatings were sprayed at varied process parameters which were selected individually for each device - their range is presented in Table 3. Table 3. Devices and the range of spraying parameters. Device Spraying parameters Voltage (V) Amperage (A) Distance (cm) Other Smart ARC HV nozzle AT standard nozzle OSU open system After the process of spraying, the macrostructure of the coatings was examined researchers looked for exfoliations, discontinuities and other superficial faults. Positively evaluated samples with the required coating thickness were submitted for metallographic examination. Microsections were 2

4 observed on an Olympus GX51 light microscope and a Hitachi S3400N scanning microscope with an analysis of chemical composition. Porosity was calculated using the Metilo application which automatically conducts a quantitative image analysis. 3. Research results The examined coatings have demonstrated a correct macrostructure homogeneous with no surface defects, had the prescribed thickness. The coating sprayed using the OSU system with an open nozzle had visibly higher roughness. Examples of the structure of the obtained coatings are presented in Figures 1-3. Figure 1. The structure of coatings sprayed with the AT400 device. Figure 2. The structure of coatings sprayed with the Smart ARC device. The coating obtained using the OSU system with an open nozzle exhibited an unsatisfactory structure - discontinuity in the coating and was excluded from further tests. The structures of coatings obtained with the use of Smart ARC and AT 400 after an adjustment of spraying parameters had comparable hardness indices and void ratios (Table 4). The coating obtained using the Smart ARC system was examined on a scanning microscope (Fig. 4) and its chemical composition was determined with the application of EDX. 3

5 Figure 3. The structure of coatings sprayed with the OSU device. a) b) c) Figure 4. The structure of the coating sprayed using Smart ARC: a) SEM, b) BSE, c) EDX. The structure (Figs 2, 4) and the results of the EDX analysis of the chemical composition indicate high homogeneity and compactness of the coating which adheres to the substrate very well. The obtained microsections were used for tests of porosity. For this purpose, the automatic image analysis methods (the Metilo software) were used. Hardness of the coatings was also determined. The results are presented in Table 5. 4

6 Table 4. Properties of the obtained coatings. Device Coating parameters Porosity (%) Hardness ( HV) other Smart ARC AT OSU - - fractures Table 5. Chemical composition of the obtained coatings atomic %. Area Al Si Cr Mn Fe pt pt Conclusions Coatings obtained by means of arc thermal spraying are distinguished by a correct structure (low porosity, good adhesion to the substrate) and very high hardness which is comparable to carbide coatings applied through high velocity oxygen fuel spraying. It is possible to produce relatively thick coatings (up to ca. 500 m) which are characterised by high resistance to abrasive and corrosion wear [9, 14]. Therefore, it is possible to use them for the production of coatings which protect different components against erosion and corrosion wear, while also operating at elevated temperatures. Such coatings are frequently used to protect the walls of combustion chambers in power boilers. Examples of such applications are shown in Figure 5. Figure 5. Arc spraying of coatings made of Fe-Cr-B material on the components of power boilers. Unfortunately, coatings of this type tend to be susceptive to the formation of single grain fractures vertical to the surface or fractures between grains. This phenomenon is characteristic for the examined coating material, however, a proper selection of spraying parameters significantly reduces this defect. A correction of the chemical composition of the coating material provides better possibilities of avoiding fractures in the material layers. For example, an addition of Al improves, among others, plastic properties of the coating [15-17]. The application of additional sealing coatings [18] is advantageous for components working at high temperatures and in an aggressive environment. 5

7 Systems which are designed to spray "hard" wires should be used to spray coatings of the Fe-Cr-B type. Coatings produced using the OSU system with an open nozzle do not meet the strict requirements set upon the coatings to be applied in the power engineering industry. 5. References [1] Formanek B, Szymañski K, Hernas A et. all 2002 Erosion damage and protective coatings for water walls of CFB boilers, AMME. [2] Hoop P J and Allen C 1999 Wear [3] Stringer J 1995 Wear [4] Wang B Q 1996, Erosion corrosion of thermal sprayed coatings in FBC boilers, Wear [5] Formanek B, Szymañski K, Hernas A, Letsko A and Ilyuszenko A 2011, Thermally sprayed composite coatings for corrosion protection of power energy boilers, 2 International Symposium «Surface Engineering. New Powder Composition Materials. Welding» March 23-25, Minsk, Belarus. [6] Higuera Hidalgo V, Belzunce Varela J, Ceries Mendez A and Poreda Martinez S 2001 Wear [7] Higuera Hidalgo V, Belzunce Varela J, Ceries Mendez A and Poreda Martinez 2001 Tribology Int [8] Buta Singh Sidhu and Puri SPrakash D 2004 Mater. Sci. Eng. A [9] Formanek B, Szymañski K and HernAS A 2011 Material Engineering No [10] Wang B Q and Luer K 1994 Wear [11] Verstak A, Wang B Q, Baranovski V and Bielaiev A 1998 Corrosion 98, paper no 193, NACE, Houston. [12] Formanek B 1997 Biuletyn Instytutu Spawalnictwa w Gliwicach (The Bulletin of Welding Institute in Gliwice) No 5. [13] Szymañski K, Formanek B and Kuczowitz B 2009 Binding [14] Formanek B and Szymañski K 2006 Problems of corrosion and corrosion protection of structural materials Special Issue [15] Pokhmurskii V, Student M, Pokhmurska V H, Wielage B and Wank A 2005 Influence of solid lubricant addition on tribological properies of coatings arc sprayed with cored wires based on Fe- Cr-B-Al and Fe-Cr-C-Al systems, Internationale Tagung Thermisches Spritzen, Wroclaw, , ISBN , 122. [16] Tillmann W, Vogli E and Ferreira 2008, Proc. International Thermal Spraying Conference. Mastritcht, [17] Pokhmurska H, Wielage B, Grund T, Schuberth S, Pokhmurskii V, Student M and Karpenko G V 2009 Gas-Abrasive Resistance of Arc Sprayed Dispersion-Hardened Coatings Obtained from Iron- Based Cored Wires, International Thermal Spray Conference 2009; Las Vegas USA. [18] Formanek B, Szymañski K and Szczucka-Lasota B 2005, New generation of protective coatings intended for the power industry, Journal of Materials Processing Technology