Surfaceand CoatingsTechnology,30 (1987)73 - 81 73

UNDERWATER PLASMA SPRAYING OF HARDSURFACING ALLOYS*

E. LUGSCHEIDER, B. HAUSER and B. BUGSEL

Material ScienceInstitute, Technical University Aachen, Templergraben55, D-5100Aachen(F.R.G.)

Summary

Basic investigationsshow that plasmasprayingis a suitable processtoproduce wear- and corrosion-resistantcoatings under water. Underwaterplasma spraying (UPS), first carried out at low water depth, offers someadvantagesover air plasma spraying concerningnoise,radiation and dustreduction, as well as in increasing the coating quality, as demonstratedusing self-fluxing nickel-based hardsurfacing alloys. The coating qualitywas investigatedby meansof microstructureanalysesand by measurementof microhardness,porosity and adhesionstrength. Owing to the variableenvironmental conditions the spraying parametershave to be optimizedcarefully, particularly the sprayingdistance.In further developmentsunder-water plasmaspraying at greaterwater depthswill be dealt with for appli-cationsmainly in the offshoreindustry.

1. Introduction

Investigations using the plasma-sprayingprocess to apply coatingsdirectly under water are described in this paper. The underwaterplasma-spraying (UPS) process,first carried out at low water depth,offers attrac-tive possibilitiesfor the depositionof wear- and corrosion-resistantcoatingson surfacesimmersed in water. Underwater processingdrastically reducesnoise,radiationand dust and at the sametime offers asubstantialimprove-mentin quality.

Self-fluxing nickel-basedhardsurfacingalloys were used to determinethe basic processdata for this technology.Other coating materials,such asoxides, intermetallics, corrosion-resistantalloys and high alloy steels arecurrently underinvestigation.

The developmentof the underwaterplasma-sprayingprocessat greaterdepths with regard to applications in marine and offshore structuresatstationary or mobile plants is to be the subject of further investigations.

*paper presentedat 13th International Conferenceon Metallurgical Coatings, April

7 -11, 1986, San Diego, CA, U.S.A.

0257-8972/87/$3.50 © ElsevierSequoia/Printedin The Netherlands

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Using the UPS technologysurfacescan be protectedboth by carrying outrepairson thecoatingandby partially recoating.

Automatic underwaterplasma spraying under hyperbaric conditionscould offer many advantagescompared with underwatercoating pro-cessesusingchambersystemsor divers, who havealreadycarried out flamespraying[1].

2. Sprayingequipment

Industrial plasma-sprayingequipment is required for spraying underwater. The equipmentwas composedof a 40 kW power unit, a dual roto-feedhopper,a control unit and a plasmatorch. Spraying testswerecarriedout in a water tank about 1 m3 in volume. To effect visual control the tankwas made from transparentsafety glass. Preparedsampleswere fitted to atable which was mounted at the bottom of the tank. Relative movementbetween the torch and the sample was achievedby moving the torch viaa manipulationdevice which was locatedoutsidethe tank andwasoperatedsemiautomatically(Fig. 1).

Fig. 1. Manipulation deviceand watertank.

3. Spraypowderandsubstratematerial

Three different types of self-fluxing wear-resistanthard nickel alloys(Colmonoy 5, Colmonoy 52 and Colmonoy 42) were employed for theseinvestigations. With the metalloids boron and silicon, oxide films on thebasemetal can be reduced,forming boron and silicon oxides [2]. Theseox-ides migrate to the outerzoneof the coating soanundisturbedmetallurgical

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bonding betweenthe coating and the base metal can be expectedevenunderwet conditions.

The influence of the powder particle shapeon the sprayingresults,i.e. on the layer quality, was investigated for spherical and for brokenpowders(Figs. 2 - 4). Thecompositionof the powdersis shownin Table 1.

±it0~m]Fig. 2. Scanningelectronmicrographof Colmonoy42 powder.

Fig. 3. Scanningelectronmicrographof Colmonoy 52 powder.

:~~t~f

iw~ 20 .IniFig. 4. Scanningelectronmicrographof Colmonoy 5 powder.

TABLE 1

Chemicalcomposition of thealloys

Alloy Elementalcomposition(wt.%) Powderform

[C] [B] [Si] [Fe] [Cr] [Ni]

Colmonoy 42 0.45 2.0 2.25 2.5 10.0 82.8 SphericalColmonoy 52 0.65 2.5 3.75 4.25 11.5 77~35 SphericalColmonoy 5 0.65 2.5 3.75 4.25 11.5 77.35 Broken

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Various sizes and forms of base metals were chosen.Coatings weresprayed on sheetmetal 100 mm X 35 mm X 6 mm in size for microscopicanalysesand on rods with diametersof 40 mm and 20 mm for mechanicalinvestigations.Austenitic andcarbonsteelwereusedassubstratematerials.

4. Plasmasprayingunderwater

4.1. Influenceof theenvironmenton the sprayingparametersThephysical limiting conditionswith UPSaredifferent from thosewith

air plasmaspraying.Because of the changedphysical conditions under water nearly all

spraying parametershad to be elaboratedin order to get excellent layerqualities. Besidesother important sprayingparameters,e.g. current density,plasma gas pressure,plasma gas composition, traversespeedof the torch,powder feed setting etc., the most crucial step in developing UPS tech-nology is the correct setting of the spraying distance.Comparedwith airplasmasprayingthe sprayingdistanceis greatlyreduced.

Harris and Waldie [31 carried out UPSusingaluminium powdersandusedalmostthe samesprayingdistanceas that in this presentwork.

Furthermore,it is found that the processwas sensitiveto evensmallvariations in the stand-off distancebetweenthe spray torch and the sub-strate. Even small deviations in the stand-off distanceproducedsignificantchangesin the layer quality. A specialgeometricsamplewasusedto deter-mine the distance(Fig. 5). This sampleeffecteda sprayingdistancewhichincreasedsteadily from 0 to 30 mm. With all otherparametersheld constantan optimized rangeof stand-off distancecould be determined.Theseresultswere usedas the basis for further testson flat samplesandthe realoptimumdistancewasdetermined.

l”i14. ~.. Sl)v~iaI saniplv tisod 1~dt~I~rIni nt~I h~’‘ptimum sprayingrange.

4.2. SprayingprocessAlthough the sprayingprocessis carried out in water it is absolutely

necessaryto providea dry basemetal surfaceto produceexcellentcoatings.

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Dry conditions underwater are effected by the plasmaprocessitself. Theplasmagasesstreamingout of the plasmatorch at high velocity and highpressurepush away the surrounding water. Any remaining moisture onthe samplesurface is vaporized in very short time as a result of the veryhigh temperatureof the plasmabeam.So a dry spot on the samplesurfaceat the moment of coating is guaranteed,provided the plasma sprayingdistanceis properlyadjusted.

The coating of all specimenswas carried out with a vertical sprayingdirection on ahorizontally fixed sample.The substrateand the torch wereimmersed in the water to a depth of about 300 - 500 mm. The coatingdirection, i.e. the trajectory of the powder particles, was downwards.Incontrast with the set-up used by Karpinos et al. [4], who had to start upthe plasmatorch in air before it could be immersedin the water to performUPS, in the presentwork the plasmawas ignited underwater, as would beneededin practicalmaintenanceoperations.

In order to achievegood coating qualitieswith high adherencea pre-treatmentof the substratewas required. This involved sandblastingwithalumina,which effectedcleaned,roughenedand activatedsurfaces.Directlyafterwardsthe samplewas fixed in the water tank andcoated.Sandblastingsystemscanalso be usedsuccessfullyunderwater [5].

As mentioned above, relative movement betweenthe torch and thesubstratewas controlled outsidethe tank. The surfacewas scannedin linesvia the semiautomaticmanipulation device. Hence, coating of the wholesurfaceareaof thesamplewasguaranteed.

As with air plasmaspraying, the thickness of the coating mainly de-pendson the traversespeedof the torch, the numberof passes,the powderfeed rate and the powder gas pressure.The coating thicknessvaried from100to 300 ~m for a singletraverse,dependenton the chosenparameters.

4.3. CoatingpropertiesThe most remarkable characteristic of underwater plasma-sprayed

coatings is the low porosity within the coating. Provided all parametersare set correctly, this property can be achievedin coatingsproduced frommetal powder.Good adhesivestrengthbetweenthelayer andthe basemetalis required as well as low porosity to gain wear- and/orcorrosion-resistantcoatings.Typical microstructuresof layersproducedusing the UPS processare shown in Figs. 6 - 8. The micrographsshow coatingsproduced fromvarious powders,namely Colmonoy 5, Colmonoy 42 andColmonoy 52.Thecorrespondingsprayingconditionsare indicated in Table 2.

All the coatings presenta homogeneousuniform microstructurewithgood bonding properties. The coating—basemetal transition zonesof thecomposites are characterizedby superior metallurgical interactions. Nomicrocracks inside the layer or peeling from the substratewere detected.Theporevolume obtainedin somecaseswasextremelylow.

The microhardnesswas subject to large variations. In the transitionzone between the substrateand the Colmonoy 42 coating the hardness

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50 1111 )O p11-

Fig. 6. Unetched Colmonoy 42 coating plasma sprayedunder wateron austenitic sub-stratematerial.

Fig. 7. Unetched Colmonoy 52 coating plasmasprayedunder water on austenitic sub-stratematerial.

Fig. 8. Unetched Colmonoy 5 coatingplasmasprayedunder wateron austeniticsubstratematerial.

TABLE 2

Spray parameters

Powder Colmonoy42 Colmonoy 52 Colmonoy5

Plasmagasflow rate, argon (I min~) 46 71 35Plasmagasflow rate, helium (1 min

1) — — 8Power(kW) 27 24 19Sprayingdistance(mm) 21 20 20Feedhoppersetting(1 min’) 0.40 0.30 0.30Powdergas (1 min~1) 0.4 0.4 0.4Traversespeed(mm min1) 600 500 600Grain sizerange (SUm) 45-63 45-63 <45

valuewas about 300 HVO.05. The value for the coatingitself was higher at450 HVO.05 andat the surfaceits valuewas 525 HVO.05.The averagemicro-hardnessof the coatingwas425 HVO.05.

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The variation in the microhardnessthrough the coatings producedfrom Colmonoy 42 powder was also analysed.Microhardnessmeasuredinthe transition zonehad values of about 310 HVO.05. The valueincreasedsteadily from the transition zone to the outer surfacezone. The coatingitself had a valueof 480 HVO.05 and at the outer surfacezoneit was 520HVO.05. The mean value of the microhardnessfor this coating was 455HVO.05.

The microhardnessmeasurementresults for coatings produced fromColmonoy 5 powder were almost the same valuesas those obtained forColmonoy 52 powder.

The cooling rate of the externalzone is influencedby the high thermalconductivity (Table 3) of water so it differs from the cooling rate of thetransition zone. This might influence residual stressesin the layer andresult in peelingfrom substrate,but in spiteof the highquenchratesachiev-able in UPS, coatingswith excellent adherencepropertiescan be producedunderwater.

TABLE 3

Somephysicalpropertiesof the environmentwhensprayingin air or under water

Environment Densityp ThermalconductivityA Specificheat cp(kg m

3) (W m~1K1) (kcal kg1 K1)

Air 1.25 0.026 0.240Water 1000.0 0.60 1.0

5. Results

The metalloids boron and silicon were found to have no significantinfluence on the feasibility of plasmasprayingunderwater. In all casesthequality of coating achievedunder wet conditionswas excellentand in someaspectssuperiorto air plasmaspraying.

Powderswith variousparticleshapesbut with identical chemicalcom-positions (Table 1) wereapplied. Providedthat the sprayingconditionsareset up properly for eachpowdersimilar results were obtainedwhethertheparticleswerebrokenor spherical.

The ratio of surfacearea to volume of the broken powder is higherthan that of the spherical powder, hence theseparticles melted faster. Inspite of this, similar coating qualitiescan be guaranteedwith brokenpow-dersif theplasmaenergyis decreased.

To obtain dataabout the adherenceproperties of the Colmonoy 42and 52 alloys, powderswith identical grain size distributions were testedon both austeniticandcarbonsteel basemetals.As with all other sprayingprocesses,adhesion is one of the most important criteria for suitable

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Base Metal Austenibs Steel Base Metal Carbon Steel

70 70

INcLmm22~ 50 ~ 50

1:

11 1 120 20

~‘o~ ~ ~o\ O\

Fig. 9. Adhesion strength determinedaccording to DIN 50160 for Colmonoy 42 andColmonoy 52 plasmasprayedon austeniticsteelunderwater.

Fig. 10. Adhesion strength determined according to DIN 50160 for Colmonoy 42 andColmonoy 52 plasmasprayedon carbonsteelunderwater.

coatings. Adhesion strongly determinesthe performanceof coated partsin actual operation conditions.The adhesionstrength was ascertained,inaccordancewith DIN 50160 [6].

As requiredin DIN 50160,for each systemfive sampleswere stresseduntil they failed.

As can be seen from Figs. 9 and 10, the tensile testresults obtainedwith UPS are quite encouraging.Colmonoy 42 and Colmonoy 52 powdersapplied to carbon steel andaustenitic steelgave different adhesionstrengthvalues.

As well as the dispersion zone (lowest and highest values) of eachsystemindicated by the unshadedarea, the averagevalue is marked by achain line.

The Colmonoy 42 powder with 35 - 40 HRC has a higher adhesionstrengthvaluethan has Colmonoy 52 with 45 - 50 HRC, irrespectiveof thesubstrate material. Owing to higher hardnessvalues of the Colmonoy 52powder (dependenton higherchromium and boron contents)the ductilityof the coating which was stressedin the tensile test was less favourable.

Fig. 11. Fracturesurfaceof a test pieceproducedaccordingto DIN 50160.

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The test results obtained for coatings on carbon steel were even higherthan those for coatings on austenitic substrate material. However, theresults showthat excellentadhesioncanbe achievedunderwater.

As indicated in Fig. 11 the fracturesurfaceis characterizedby cohesivefailure. The failure modeis abreakingwithin the layer.

6. Summarizingcomments

The extension of plasma-sprayingprocessesinto underwaterregionsmakesit possibleto apply coatingsdirectly underwater.In order to producesuch coatingsthe sprayingconditions andparametershaveto be optimizedcarefully, with due considerationgiven to the changedphysical limitations.In particular, the sprayingdistancemust be set up properly. Underwaterplasma-sprayedself-fluxing nickel-based hardsurfacing alloys are free ofcracksand characterizedby homogeneousmicrostructureswith low poros-ity. With regard to the metallurgicalreactions in the transition zone theadhesionstrengthresultsarequite encouraging.

References

1 H. G. Schafstall and P. Szelagowski,DVS Rep. 80, 1983 (Deutscher Verband fürSchweisstechnik,Düsseldorf).

2 0. Knotek, E. Lugscheiderand H. Eschnauer,StahleisenBücher, Verlag Stahleisen,Düsseldorf,1975.

3 W. K. Harris and B. Waldie, Proc. 7th ISPCEindhoven,Eindhoven, 1985, PaperNo.B-7-3.

4 D. M. Karpinos, V. G. Zillberbergand S. Yu. Sharivker,PoroshkovayaMetallurgiya, 4(1973).

5 B. Donkerand U. Richter, Hansa-Schiffahrt-Schiffbau-Hafen,20 (1982) 119.6 GermanDIN Stand.50160,1979.