HIGH TEMPERATURE EROSION OF THERMAL SPRAYINGPROCESSES ON AISI-310 BOILER STEEL SUBSTRATE

ARSHPREET KAUR¹, JASMANINDER SINGH GREWAL² AND DAANVIR KARAN3

¹GNDEC, Ludhiana, Punjab, India, Email: arshpandher89@gmail.com²GNDEC, Ludhiana, Punjab, India, Email: jsgrewal23023@gmail.com3Email: karandhir198@gmail.com

Abstract: In the present investigation two different types of coatings Cr3C2NiCr and WC-12Co were depositedon selected AISI-310 boiler tube steels. The erosion studies were conducted on uncoated as well as thedetonation gun and high velocity oxy-fuel coated samples. The erosion experiments were carried out usingan air-jet erosion tester at impingement angles of 30Ú and 90Ú. Alumina particles of size 50 microns wereused as erodent. The coating was characterized by X-ray Diffracto-meter (XRD), Scanning Electron Microscope(SEM), equipped with an energy dispersive X-ray analyzer (EDAX). The eroded surface was also analyzedby SEM/EDAX. The WC-12Co coating has undergone minimum erosion in both the spraying process ascompared to Cr3C2NiCr and bare AISI-310. Detonation Gun sprayed coatings gave better results as comparedto High Velocity Oxy Fuel sprayed coatings with respect to erosion.

Keywords: Solid Particle Erosion, Thermal Spraying Processes, Physical vapour deposition.

1. INTRODUCTIONThe behavior of the material is therefore greatlydependent on the surface of the material, surfacecontact area and the environment under whichthe material must operate. The surfacecharacteristics of engineering materials have asignificant effect on the serviceability and life ofa component thus cannot be neglected in design.Erosion is the process of wear in which materialare removed from the solid surface by the actionof solid particles impinging on it. This type of wearis common in many industrial devices includingboilers. Coatings are provided on the surface ofmaterials so as to reduce the impact onenvironmental factors on the precious resourcesi.e., materials. A coating can be defined as a layerof material, formed naturally or depositedartificially on the surface of an object made ofanother material, with an aim of obtainingrequired technical or decorative properties.Different coatings are used to prevent/reduce theloss of materials by erosion, erosion-corrosion andwear. Such as Chemical vapor deposition, physicalvapor deposition and thermal spraying. Thermalspraying is one of the most effective and low costmethod to protect the material from wear, hightemperature corrosion, stresses and erosion, thus

increasing the life of material in use. It may becategorized as either combustion or electricprocesses. Combustion processes include flamespraying, HVOF spraying, and detonation flamespraying. The Detonation gun basically consistsof a long water cooled barrel with inlet valves forgases and powder. Oxygen and fuel (acetylenemost common) is fed into the barrel along with acharge of powder. A spark is used to ignite thegas mixture and the resulting detonation heatsand accelerates the powder to supersonic velocitydown the barrel. A pulse of nitrogen is used topurge the barrel after each detonation. Thisprocess is repeated many times a second. TheHVOF (High Velocity Oxygen Fuel) ThermalSpray Process is basically the same as thecombustion powder spray process (LVOF) exceptthat this process has been developed to produceextremely high spray velocity. There are a numberof HVOF guns which use different methods toachieve high velocity spraying. One method isbasically a high pressure water cooled HVOFcombustion chamber and long nozzle. Fuel(kerosene, acetylene, propylene and hydrogen)and oxygen are fed into the chamber, combustionproduces a hot high pressure flame which is forceddown a nozzle increasing its velocity. Powder may

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8(1) 2017 : January-June • pp. 77-84

78 Arshpreet Kaur, Jasmaninder Singh Grewal and Daanvir Karan

be fed axially into the HVOF combustion chamberunder high pressure or fed through the side oflaval type nozzle where the pressure is lower.Another method uses a simpler system of a highpressure combustion nozzle and air cap. Fuel gas(propane, propylene or hydrogen) and oxygen aresupplied at high pressure, combustion occursoutside the nozzle but within an air cap suppliedwith compressed air. The compressed air pinchesand accelerates the flame and acts as a coolantfor the HVOF gun. Powder is fed at high pressureaxially from the centre of the nozzle.

1.1. Literature ReviewA comparison has been brought out between twohigh velocity coating processes namely highvelocity oxy-fuel (HVOF) and detonation gunspraying process (DS). A WC-10Co-4Cr powderhas been sprayed on a medium carbon steel usingthe above mentioned high velocity sprayprocesses. Some of the coated specimens werefurther ground by a diamond wheel withcontrolled parameters. The coating in both ‘as-coated’ and ‘as-ground’ conditions has been testedfor solid particle erosion behavior. The erosionexperiments were carried out using an air-jeterosion test rig with silica erodent’s at a velocityof 80m/s. It has been found that surface grindingimproved the erosion resistance. It represents thedetailed characterization of the WC-Co-Cr coatingin both ‘as-coated’ and ‘as-ground’ form. A detailedanalysis indicates that the increase in residualstress in the ground specimen is a possible causefor the improvement in erosion resistance[5].During the hot erosion behavior of HVOFchromium carbide metal cermets coatings sprayedwith different powders, all of the coatings testedexhibited similar brittle erosion behavior as aresult of a cracking and chippingmechanism[10].A large varieties of detonationsprayed coatings have been categorized and eachcoating over a range of process parameters. Thesecoatings are characterized in terms of phasecontent and distribution, porosity, micro hardnessand evaluated for erosion, abrasion and slidingwear resistance. From the results andexperimental data they conclude that the coatingsare substantially poorer than bulk material ofidentical composition and that the hardness andtribological properties of coatings are morestrongly influenced by the process parametersthemselves rather than micro structural

parameters like phase content and distribution,porosity, etc [9].The evaluated characteristics andphase composition of HVOF sprayed WC-Co andCr3C2-NiCr coatings applied on boiler steelsubstrates and concluded that high microhardness and low porosity values of these coatingsis due to that these coatings preserve a highvolume of carbide that is well-dispersed in thematrix [8].

2. MATERIALS AND METHOD

2.1. Selection of Substrate MaterialSelection of the substrate material for the presentstudy has been made to extend the working lifeof boiler tubes. AISI-310 stainless steel is usedas it is the base material for boiler tubes. Table 1Chemical composition (wt%) of the AISI-310Stainless steel.

Table 1Chemical Composition of AISI-310

Element Composition (wt%)

Ni 19-22

Cr 24-26

C 0.25

Mn 2

Si 1.50

S 0.03

P 0.045

Specimens with dimensions of 20 15 5 mmwere prepared from the substrate material. Thespecimens were polished using emery papers of220, 400, and 600 grit sizes and subsequentlypolished on 1/0, 2/0, 3/0 and 4/0 grades. Thesespecimens are then grit blasted with Al2O3 (meshsize 30) prior to deposition of coatings byDetonation Gun Spray and high velocity oxy fuelprocess.

2.2. Development of CoatingsTwo types of coating powders namely (A) WC-12CO and (B) Cr3C2NiCr are selected fordetonation Spray Coating Process and HighVelocity Oxy Fuel Spray Coating Process aftercomprehensive literature survey. Research papersshows that the mentioned coatings have excellentbond strength and wear resistance.

• Tungsten Carbide Cobalt (WC-12CO):Tungsten Carbide Cobalt powder,

High Temperature Erosion of Thermal Spraying Processes... 79

when sprayed by using Detonation Sprayprocess, it produces coatings very denseand homogeneous having excellentabrasive wear resistance. The powder issupplied by H C Starck, Germany underthe product code Amperit 584. Thepowder is agglomerated and sinteredhaving particle size as -45+20 microns.

• Chromium Carbide-Nickel ChromiumPowder (Cr3C2NiCr):Chromium Carbideand Nickel Chromium powder, whensprayed using the Detonation Sprayprocess, it produces coatings, which arevery hard, dense and excellent bonded.Chromium Carbide and NickelChromium coating is recommended forresistance to wear by fretting, abrasiveparticles oxidation. The powder isagglomerated and sintered havingparticle size as 10-45 microns.

2.3. Deposition of Coatings by DetonationSpray Process

The WC-12CO and Cr3C2NiCr powders weresuccessfully deposited on AISI-310 substratematerial by the Detonation Spray Process. Priorto the application of the coatings, the specimenswere grit blasted using Al2O3 (mesh size 30) tothinly remove the metal surface layer and tocreate a rough contour on the surface necessaryfor the adhesion of the coatings. The blastedsurfaces, with the surface roughness of about 8-10ìm Ra, were then thermally sprayed with twoabove said powders as shown in Table 3.

2.4. Deposition of Coatings by HVOF Process

The WC-12CO and Cr3C2NiCr powders weresuccessfully deposited on AISI-310 substratematerial by the High Velocity Oxy Fuel Spray

Process. The process was selected as it gives highdensity, high bond strength, improved toughness,higher coating thickness, beneficial residualstress, excellent wear resistance, superb corrosionresistance, fine surface finishes with low coatingporosity as shown in Table 2.

2.5. Erosion Studies in Simulated CoalFired Boiler Environment

Erosion testing was carried out using an air jeterosion tester at the Yadawindra College of

Table 2Summary of High Velocity Oxy Spray Flow Process

Pressure of Fuel gas (Oxygen) 9 bar

Pressure of Fuel gas (L.P.G.) 7.5 bar

Pressure of Air 6 bar

Flow rate of Fuel gas (Oxygen) 250 LPM

Flow rate of Fuel gas (L.P.G) 60LPM

Flow rate of Air 600 LPM

Spray Distance 180mm

Coating thickness 150-200 microns

Table 3Summary of Detonantion Gun Spray Process

Fuel Gas Oxygen and AcetyleneCarrier gas Nitrogen

Pressure of Fuel gas (oxygen) 2-3 bar

Pressure of Fuel gas (Acetylene) 1-1.5 bar

Pressure carrier gas (Nitrogen) 2.3 bar

Flow rate of fuel gas (oxygen) 2800-5120LPM

Flow rate of fuel gas (Acetylene) 2240-2420LPM

Flow rate of carrier gas (Nitrogen) 720-960LPM

No of shots per second 3

Stand-off distance 150-180mm

Maximum substrate temperature 110°C

(a)

(b)

80 Arshpreet Kaur, Jasmaninder Singh Grewal and Daanvir Karan

Engineering, Talwandi Sabo. The rig consisted ofan air compressor, a particle feeder, an air particlemixing an accelerating chamber. Dry compressedair was mixed with the particles, which were fed ata constant rate from a conveyor belt type feeder inthe mixing chamber and then accelerated by passingthe mixture through a tungsten carbide convergingnozzle of 4mm diameter. These accelerated particlesimpacted the specimen, which could be held atvarious angles with respect to the impactingparticles using an adjustable sample holder. Theimpact velocities of the particles could be varied byvarying the pressure of the compressed air

3. RESULTS AND DISCUSSION

3.1. Characterization of CoatingsIt is observed from the optical macrographs of D-gun sprayed as coated specimens that theCr3C2NiCr coating has deep blackish greyappearance whereas WC-12Co appears grayish.Furthermore cracks were observed on the coatingsurface during visual examination. The eroded

samples WC-12Co by Detonation Gun and highvelocity oxy fuel spray processes are shown in Fig.5 (a) and (b) and Fig. 6. (a) and (b).

3.2. Porosity and Coating ThicknessDue to intermittent process during D-gunspraying, very few pores/voids appear in the D-gun sprayed coatings as compared to HVOFcoating process. The average porosity of assprayed coatings is less than 1.0%. The averagethickness of Cr3C2NiCr and WC-12CO coatings

(c)

(d)

Figure 1: Cross sectional image of as coated on on AISI-310 boiler Steel: (a) Cr3C2NiCr deposited by D-gun;(b) WC-Co deposited b y D-Gun;(c)Cr3C2NiCrdeposited by HVOF;(d) WC-Co deposited by HVOF

Figure 2: Experimental set-up for erosion to simulatedcoal-fired boiler environment

Figure 3: Main body of the experimental set-up forerosion to simulated coal-fired boiler environment

High Temperature Erosion of Thermal Spraying Processes... 81

Figure 4: Assembly holders and sample holders of theexperimental set-up for erosion to simulated coal-firedboiler environment

(a) (b)

Figure 5: Eroded samples of:(a) Cr3C2NiCr depositedby D-gun (b) WC-Co deposited by Detonation GunProcess

(a) (b)

Figure 6: Eroded samples of:(a) Cr3C2NiCr depositedby High Velocity Oxy-fuel Process; (b) WC-Codeposited by High Velocity Oxy-fuel Process

on AISI-310 measured by coating thicknesstesting gauge (Minitest-600B) lies between 100-150ìm by both the thermal spraying processes.

3.3. SEM/EDAX analysis of the D-gun assprayed coatings

The SEM micrographs as well as EnergyDispersive Spectrum (EDS) with elemental

(a)

(b)

(c)

Figure 7: SEM/EDAX by detonation gun spray process(a) Cr3C2NiCr eroded at 30°;(b) Cr3C2NiCr eroded at90°; (c) Element distribution of Cr3C2NiCr eroded at30° and90°.

(a)

composition of eroded samples for Detonation gunand HVOF sprayed Cr3C2NiCr and WC-12Cocoatings onAISI-310 are shown in Fig. 7 – 10 (forangles 90° & 30°).

82 Arshpreet Kaur, Jasmaninder Singh Grewal and Daanvir Karan

3.4. X-Ray Diffraction (XRD) analysisThe X-ray diffractograms for detonation sprayedCr3C2NiCr and WC-12CO coatings on AISI-310are shown in Fig. 11 and Fig. 12 respectively.

(b)

Figure 8: SEM/EDAX by detonation gun sprayprocess (a) WC-12Co eroded at 30°; (b)WC-12Coeroded at 90 °

(b)

(a)

(c)

Figure 9: SEM/EDAX by HVOF process: (a)Cr3C2NiCr eroded at30°; (b)Cr3C2NiCr eroded at90°; (c) element distribution of Cr3C2NiCr erodedat 30° and 90°.

(b)

(a)

(c)

Figure 10: SEM/EDAX by HVOF process (a) WC-12Coeroded at 30° (b) WC-12Co eroded at 90° (c) elementdistribution of WC-12Co eroded at 30° and 90°

Figure 11: X-ray diffraction pattern of coated AISI-310substrate with Cr3C2NiCr coating

High Temperature Erosion of Thermal Spraying Processes... 83

Table 4Experimental Data Obtained on Erosion Test Rig

Coating Time Angle Initial Final Cumu.(hr) Weight Weight Weight

(gm) (gm) (gm)

Uncoated 3 90° 13.7917 13.7865 0.0052AISI-310 30° 15.5558 15.5443 0.0115

WC-12Co 3 90° 13.6550 13.6468 0.0082by HVOF 30° 14.2320 14.2179 0.0141

Cr3C 2NiCr 3 90° 11.5785 11.5629 0.0156by HVOF 30° 13.3569 13.3343 0.0226

WC-12Coby 3 90° 15.5508 15.5470 0.0038D-gun 30° 18.0700 18.0597 0.0103

Cr3C2 NiCr by 3 90° 17. 7589 17.7491 0.0098D-gun 30° 17.5819 17.5621 0.0198

In the erosion testing, the material is erodedby continuous impact of erodent particles theerosion starts at centre first, and then proceedstowards the edges of the samples, the shape ofdeveloped scar depends on the angle of impact.When erodent strikes the surface at an impactangle of 90°, material is eroded creating a circulardepression; while at a 30° impact angle, materialis eroded forming an elliptical shape depression.The weight loss for un-coated and coatedAISI-310boiler at an impact velocity of 35msÉ1 andimpingement angle of 30° and 90° whentemperature was 400° C and surrounding air at700° C is shown in table 4. The weight loss forcoated Cr3C2-NiCr by HVOF at 30° impact ismore and the weight loss of WC-12CO by D-gunat 90° impact is less. Hence Cr3C2NiCr showsbetter erosion resistance as compared to WC-12Coin both the deposition processes i.e. D-Gun andHVOF. Further, the SEM/EDAX analysis (Fig. 5.a, b D-gun and Fig 9. a,b HVOF) indicates that incase of AISI-310 coated with Cr3C2NiCr

respectively by HVOF and D-gun the coating isstill there after three hours continuous erodentin pacts at an impact velocity of 35 msÉ1 andimpingement angle of 30° and 90° when substratetemperature was 400°C and surrounding air at700°C. This coating has also successfullyprotected the substrate. SEM/EDAX analysis(Fig.8. a,b /Fig 10.a,b) indicate that in case ofAISI-310 coated with WC-12CO respectively byD-gun and HVOF the coating could not withstandafter three hours continuous erodent in pacts atan impact velocity of 35 msÉ1 and impingementangle of 30° and 90° when substrate temperaturewas 400°Cand surrounding air at 700°C and thecoating failed. The solid particle erosion rate ofthe uncoated and coated AISI-310 boiler steel asshown in table 4. Authors suggested that thematerial subjected to erosion initially undergoesplastic deformation and is later removed bysubsequent impact of the erodent on the surface.The ploug hing occurs by the impact of erodentparticles and lips or ridges are formed at the bankof the craters. These lips are fractured or removedwith further erosion. At no erosion temperatures,at high impact velocities and feed rates there isno oxide scale. Even if there is any oxide scale, itwill be very thin and it will be able to deform inthe same manner as that of the substrate target.Under evaluated temperature erosion of metalswith minimum or negligible oxidation, erosiontakes place from the metallic surface and thismechanism of erosion is called metal erosion [9].Most of the metallic material irrespective oftemperature of erosion, exhibit a ductile behavior,i.e. a maximum erosion rate at oblique angles. Theerosion behavior in this regime is similar to theambient temperature erosion behavior of metallicmaterials. The erosion response in the metallicerosion regime is ductile as described by Bellmanand Levy (1981). Authors suggested that thecreation of platelet-like edges by impact extrusionprotrude outward over adjacent material and theloss of these platelets appears to be responsiblefor the mass loss. Authors have further explainedthe repeated deformation of craters and formingof new craters is common and some platelets areextrude several times before getting knocked off.The solid particle erosion rate of the coatedsamples in Fig. 11. and Fig. 12. Indicate thatmaximum erosion took place at 30° impact angle,which indicate ductile behavior and erosion tookplace at 90° impact angle, which indicate brittlebehavior. In case of brittle material; loss in solid

Figure 12: X-ray diffraction pattern of coated AISI-310substrate with WC-12CO coating

84 Arshpreet Kaur, Jasmaninder Singh Grewal and Daanvir Karan

particle erosion occurs predominantly through theformation and interaction of a sub-surface micro-crack network. At low impact, the kinetic energyof the erodent particles contribute mainlythroughtheploughing mechanism and very littleto normal repeated impact. The ploughingmechanism is associated with the plasticsmearing and cutting of the materials while therepeated impact mechanism is responsible forinitiating and propagating the grain boundarymicro cracks. Brittle materials are not easilyplastically deformed. Hence the material removalrate is low in case of brittle and higher in ductilematerials. Brittle nature of the material allowsthe cracks readily to propagate to form cracknetwork. The subsequent impacts will easilyremove the surface material via the ejection ofthe upper layer grains. Hence the materialremoval rate is high in brittle material.

4. CONCLUSIONS

Based upon experimental results obtained in thepresent work, the following conclusions have beendrawn:

• The Cr3C2NiCr and WC-12Co coatingshave been successfully deposited on AISI-310 Stainless steel by using Detonationgun spray and High Velocity Oxy Fuelspray methods.

• The Detonation Gun Spray Coating isbetter than High Velocity Oxy FuelCoating with respect to erosion rate.

• WC-12Co shows better erosion resistanceas compared to Cr3C2NiCr in both thedeposition processes i.e. D-Gun and HVOF.

• The maximum erosion takes place at anangle of 30° as compared to 90°in all thecoated samples which shows the ductilebehavior at 30°.

• The erosive wear resistance at an angleof 30° and 90° for coating-substrateprocess combinations in their increasingorder is given by WC-12Co (DG)

Deposited on - AISI-310> Uncoated AISI-310>WC-12Co (HVOF)Deposited on AISI-310>Cr3C2NiCr (DG) Deposited on AISI-310> Cr3C2NiCr (HVOF) Deposited onAISI-310.

REFERENCES[1] M. Ahmad, M. Schatz &M, V. Casey (2013),

“Experimental investigation of droplet size influenceon low pressure steam turbine blade erosion”, Wear,2013.

[2] A. Maksim, R. Veinthal , H.S. E.,I.Hussainavo,A.Vallikin, M.Lelis& J.Priss, “Effect ofoxidation on erosive wear behaviour of boiler steels”,Tribology International, 2013.

[3] J.S.Grewal, J. P.Singla& V.Chawla, “ A survey of thedetonation gun sprayed wear resistant coatings”National Conference of Mechanical and Materialengineering,2010.

[4] Hidalgo, V.H. Varela, F.J.B., Menender, A.C.Martinaz& S.P., “A Comparative study of HighTemperature Erosion Wear of Plasma SprayedNiCrBSiFe and WCNiCrBSiFe Coatings underSimulated Coal Fired Boiler Conditions”. TribologyInternational, Vol. 34, 2001, pp. 161-69.

[5] J.K.N.Murthy, S.Bysakh, K.Gopinath&B.Venkataraman, “Microstructure dependent erosionin Cr3C2-20(NiCr) coating deposited by detonationgun”, Surface and coatings technology, Volume 202,2007, issue 1. Pp. 1-12.

[6] S.Schuldt,G.Arnold, Roschy, Y.Schneider& H.Rohm,“Defined abrasion procedures for cutting blades andcomparative mechanical and geometrical wearcharacterization”, Wear, Vol 300, 2013, pp 38-43.

[7] V. N.Shukla, V. K.Tewari& R. Jayaganthan.,“Comparison of tribological behavior of Cr 3C2/NiCrcoatings deposited by different thermal spraytechniques – A review”, Material science andengineering, Vol 2, 2011, issue 1-2.

[8] H.Singh,B.S. Singh &Prakash, “Mechanical andmicrostructural properties of HVOF sprayed WC-Coand CrçC2-NiCr on the boiler tube steels using LPGas the fuel gas”, Journal of Materials ProcessingTechnology, Vol 171, 2006, Issue 1, pp. 77-82.

[9] G.Sundaranjan, D. Sen & G. Sivakumar ,” Thetribologicalbehaviour of detonation sprayed coatings:the importance of coating process parameters “, Wear,Vol 258, 2005, pp. 377-391.

[10] W., B. Qian. & S. Z.Rong,” Hot erosion behavior ofcarbide-metal composite coatings,” Journal of MaterialsProcessing Technology, Vol. 143-144, 2003, pp. 87-92.