system enables metals and alloys with high melting pointup to about 2000'C to be deposited on the target surface.These features are suitable for a deposition of corrosionresistant coatings (T.Sidhu et a\2A05A).In the HVOFprocess the powder/wire material is melted and propelledat a high velocity toward the surface with the use ofoxygen and fuel gas mixtures as shown in fig.l. The mostimportant parameter regarding the coating quality ispowder 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 to1000m/s for HVOF spraying. HVOF process of thermalspray have particle high speed that produce low porositycoatings , oxides ,better bond strength and hardness ascompared 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:l0 fiffi rm8 r6Dm

0ffi0flFtffiffiliar 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 ofHVOF process with its other counterparts.The highresistance of high-chromiuffi, nickel-chromium alloys tohigh+emperature oxidation and corrosion allows them tobe widely used as welded and thermally sprayed coatingsin fossil fuel-fired boilers, waste incineration boilers, andelectric furnaces. The HVOF process is often used todeposit high-chromium, nickel-chromium coatings ontothe outer surfaces of various parts of boilers to prevent thepenetration of hot gases, molten ash, and liquids to lessnoble carbon steel boiler tubes (T.Sidhu et a1,2005B) .TheHVOF coatings have higher bond strengths than for theplasma-spray coatings by 25%. The better adhesionstrength of the HVOF coatings is attributed to the bettermechanical interlocking of the sprayed droplets with thesubstrate due to the high kinetic energy experienced by theimpingirg particles. Hence, the HVOF coatings canperform very well in corrosive environments as comparedto other processes (T.Sidhu et a1,2005C).

ry. HVOF CoernqcsC4C2-NiCr coating was deposited on SAE-3 47H boiler

steel by HVOF spray process and investi gated at 700"Cfor 50 cycles in Na2Soa-Fez(So+) molten salt, as well asair environments. The HVOF spray C4C2-NiCr coatingwas found to be successful in maintaining its adherence inboth the environments. The surface oxide scales were alsofound to be intact. The formation of ctromium rich oxidescale might have contributed for the better hotcorrosion/oxidation resistance in the coated steel (M. Kauret al, 2a0D. The Fe-based superalloy Superfer 800H wasused as a substrate material and coating alloys CryC2-NiCrC alloy powders, namely as- atomtzed powder with aconventional coarse-grained structure and cryomilledpowder with a nanocrystalline structure, were employedon medium steel with HVAF process. Heat treatment wasconducted at 650 "C in air. Samples were removed after10, 30, 50, 100, 150 and z}a hr. Both the as-sprayedNicrc coatings possessed a compact microstructure whichexhibited a more homogeneous morphology withuniformly distributed fine carbide dispersions and a muchhigher microhardness. The nanostructure coating exhibitedexcellent thermal stability, whose average grain sizestabilized at about 100 nm after 50 h of exposure at650 "C (K.Tao et aI,20A5).

NiCr and Stellite-6 coatings have been formulated onboiler tube steels namely ASTM-SA-2l0 Grade Al,ASTM-SA213-T-11 and ASTM-SA2 t3-T-22 by HVOFtechnique using LPG as fuel gas. The results of Stellite-6coating were better than those of the NiCr coatings for lowvalue of porosity and surface roughness. Microhardnessmeasurement across the cross-section of coating showedthat the Stellite-6 coating has higher hardness as comparedto the NiCr coating, although both coatings have highhardness values compared to the substrate steels(H.S.sidhu et al, 2010). HVOF process was used to depositNi-based hardfacing NiCrFeSiB alloy powder on boiler

lnterrational Conference on Advances in Materials and Manufacturing Technology-Z}Ll83