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system enables metals and alloys with high melting point up to about 2000'C to be deposited on the target surface. These features are suitable for a deposition of corrosion resistant coatings (T.Sidhu et a\2A05A).In the HVOF process the powder/wire material is melted and propelled at a high velocity toward the surface with the use of oxygen and fuel gas mixtures as shown in fig.l. The most important parameter regarding the coating quality is powder particle velocity, which Frr*l:*i*fi ***ti*g Sieffi :*$d: :*lto*k"*.a+*,s .d' Cro,.', F..*,ot;t#.*f , :g;tt* -,t S_+fir*f gge, Fig-l Schematic diagram, of Hi?t ffiiry oxy tuel process ( v. hotea et ranges from 100m/s for powder combustion spray to 1000m/s for HVOF spraying. HVOF process of thermal spray have particle high speed that produce low porosity coatings , oxides ,better bond strength and hardness as compared to its other counterparts as shown in frg.2 ( GR.heath et al , 2008 ). Fig-Z Characteristics of the HVOF and standard Ffrqx. FlarfiaFfa*ma Trmper*iu,re fCl lffi:l 0 fiffi rm8 r6Dm 0ffi0flFtffiffi liar F*rtklrVefseiry gTrtsl I.. Fig-3 Comparison of various processes (Dorfman M., 2002)plasma- process coatings: (I) hardness, (II) porosity (III) oxide content, (fD bond srrength (V) maximum thickness. (T.Sidhu et a1,2005B) Fig-3 shows the velocity and temperature comparison of HVOF process with its other counterparts.The high resistance of high-chromiuffi, nickel-chromium alloys to high+emperature oxidation and corrosion allows them to be widely used as welded and thermally sprayed coatings in fossil fuel-fired boilers, waste incineration boilers, and electric furnaces. The HVOF process is often used to deposit high-chromium, nickel-chromium coatings onto the outer surfaces of various parts of boilers to prevent the penetration of hot gases, molten ash, and liquids to less noble carbon steel boiler tubes (T.Sidhu et a1,2005B) .The HVOF coatings have higher bond strengths than for the plasma-spray coatings by 25%. The better adhesion strength of the HVOF coatings is attributed to the better mechanical interlocking of the sprayed droplets with the substrate due to the high kinetic energy experienced by the impingirg particles. Hence, the HVOF coatings can perform very well in corrosive environments as compared to other processes (T.Sidhu et a1,2005C). ry. HVOF Coernqcs C4C2-NiCr coating was deposited on SAE-3 47H boiler steel by HVOF spray process and investi gated at 700"C for 50 cycles in Na2Soa-Fez(So+) molten salt, as well as air environments. The HVOF spray C4C2-NiCr coating was found to be successful in maintaining its adherence in both the environments. The surface oxide scales were also found to be intact. The formation of ctromium rich oxide scale might have contributed for the better hot corrosion/oxidation resistance in the coated steel (M. Kaur et al, 2a0D. The Fe-based superalloy Superfer 800H was used as a substrate material and coating alloys CryC2- NiCrC alloy powders, namely as- atomtzed powder with a conventional coarse-grained structure and cryomilled powder with a nanocrystalline structure, were employed on medium steel with HVAF process. Heat treatment was conducted at 650 "C in air. Samples were removed after 10, 30, 50, 100, 150 and z}a hr. Both the as-sprayed Nicrc coatings possessed a compact microstructure which exhibited a more homogeneous morphology with uniformly distributed fine carbide dispersions and a much higher microhardness. The nanostructure coating exhibited excellent thermal stability, whose average grain size stabilized at about 100 nm after 50 h of exposure at 650 "C (K.Tao et aI,20A5). NiCr and Stellite-6 coatings have been formulated on boiler tube steels namely ASTM-SA-2l0 Grade Al, ASTM-SA213-T-11 and ASTM-SA2 t3-T-22 by HVOF technique using LPG as fuel gas. The results of Stellite-6 coating were better than those of the NiCr coatings for low value of porosity and surface roughness. Microhardness measurement across the cross-section of coating showed that the Stellite-6 coating has higher hardness as compared to the NiCr coating, although both coatings have high hardness values compared to the substrate steels (H.S.sidhu et al, 2010). HVOF process was used to deposit Ni-based hardfacing NiCrFeSiB alloy powder on boiler lnterrational Conference on Advances in Materials and Manufacturing Technology-Z}Ll 83
