Progress in the application of cold gas dynamic spraying to repairing continuous casting molds

Baosteel Technical ResearchVolume 5, Number 1, March 2011, Page 1repairing continuous casting mold as dynamic spraying toProgress in the application of coldZHANG Junbao", LIANG Yongli"and ZHANG Huabin1)Advanced Technology Division, Research Institute, Baoshan Iron Steel Co, Ltd, Shanghai 201900, China2)Silicon Steel Department, Baoshan Iron Steel Co, Ltd, Shanghai 200941, ChinaAbstract: A new continuous casting mold repairing method-cold gas dynamic spraying( CGDS is introduced. Thetudy investigates the advantages of the CGDs process regarding repairing operation, such as convenient, in-siturepairation, little heat delivery, microstructural and dimensional stability and other special applications. Microstructure andmechanical properties of the copper alloy coating, nickel coating, ceramic composite coating, and their interface to thesubstrates, which are usually used in repairing operation have been researched by means of optical microscopy( OM)g electron microscopy(SEM), transmission electron microscopy(TEM) and micro-hardness tests. Experimentalresults have demonstrated the relative density of the copper alloy coating is as high as 98. 7%, and that no obviousdifference can be observed between the CrZrCu substrate and the Cu alloy coating in terms of microstructures; thus theinterface is quite difficult to be identified. The bonding strength and micro-hardness of the Cu alloy coating reach up to37 MPa and 310 HVo2, respectively. The interface between the copper alloy coating and the nickel coating is either zigzagor wave shaped, and the cohesion is relatively good. As-sprayed nickel coating is dominated by severely deformedparticles, and the relative density is up to 98. 5%. Complete recrystallization occurred after annealing at 900C for onehour, while its micro hardness remains as high as 124. 1 HVo?. All these results have indicated that CgDs is a promisingtechnology for repairing the continuous casting mold and that its future development is prosperous as wellKey words: cold gas dynamic spraying CGDS ) continuous casting mold; repairing; coating; microstructuremechanical propertydoi:10.3969/jiss.1674-3458.2011.01.004Therefore a new method which needs to be1 Introductionconvenient, in-situ and harmless to the CrzrCu body ofcontinuous casting molds is much demanded. Cold gascasting.A mold is made of copper or copper-llbrs dynamic spraying( CGDS )is a potential process forMolds are one type of key equipments for continuerepairing continuous casting molds. The CGDs processmaterials with a nickel or nickel- base alloy layer plated of repairing operation looks after three aspects: thehereon. Sometimes, a chromium layer may additionally copper alloy coating and its interface to the CrZrCube plated on. In the continuous casting of aluminum substrate, the interface between the copper alloand steel, various Cu molds are used, which have coating and the nickel layer, and nickel layer itselfshown damages and wear. When the surface plated This study presents research on all the three aspects bylayer is worm and scratched, it is usually subjected to a means of the new technical method CGDsrepairing operation. The conventional way of repairingis to completely abrade and remove the nickel ornickel-base alloy layer together with some CrZrCu 2 CGDS technology and its advantages ofbody from the surface and thereafter, apply a new repairing continuous casting moldssurface on the surface. However. there has been aninsufficient adhesion problem between the two metalsThe CGDs process was originally developed by Aof the same kind plated on each other and a Papyrin and his colleagues in the mid-1980s at thedegradation problem in use of the nickel or nickel-base Institute of Theoretical and Applied Mechanics of thealloy plated layer. The copper body of the mold will Russian Academic of Science in Novosibrisk. Theybecome thinner and thinner as the abrading operation successfully deposited a wide range of pure metalsgoes on, and thus, it will soon have to be completely metal alloys, and composites onto a variety of substratereplaced. Furthermore, the mold has to be disassembled materials, and demonstrated the feasibility of CGDS forin every repairing process. Another repairing method is various applications. In the CGDS process[2,31, a typethermal spray. Unfortunately, the amount of heat of gas is accelerated to the supersonic velocity in a dedelivered to the coated part is a lot, leading to Laval nozzle, i中国煤化工 nozzle. Themicrostructural changes and deformation of the CrZrCu coating materialtream in thesubstrate materialform of powderCNMHG. acceleratedCorrespondingauthor:ZHANGJunbao;E-mail;zhangjunbao@baosteel.comBaosteel Technical Research, Vol 5. No I. Mar. 20/1by the gas in the nozzle, and propelled towards the paintingsubstrate to be coated. Above a certain particleAs a consequence, the deleterious effects of highvelocity, which is characteristic for each respective temperature oxidation, evaporation, melting, crystalpowder material and its properties, the particles form a lization, residual stresses, debonding, gas release, anddense and solid adhesive coating on the substrate other common problems incurred traditional thermalsurfacespray methods are minimized or eliminatedUpon the impact, the particles must undergoAs a method of repairing the mold for continuoussufficient deformation to be able to adhere to the casting CGDs enables the partial repair of a moldsurface. The gas is heated to a temperature up to 923 K which has never been done and can, as a result, attain a(650C)before entering the nozzle. This increases the reduction in the manufacturing costs of the mold andparticles temperature and velocity, and thus, assists the extend the service life of the mold. a damaged part ofdeformation upon the impact. However, the gas's the surface of the mold, whose base material is hightemperature at the inlet is clearly below the melting strength Cu alloy like Cr-Zr-Cu, can be repaired by thetemperature of the coating material, which means the CGDS process. Only damaged parts need to beparticles can not be melted in the gas jet. In repaired, leaving the functioning and smooth partscomparison to other thermal spray processes, drawbacks unchanged. The thickness of the coating is able toundesirable phase transformations, can be avoided in The coating formed on the surface of repaired part isthe CGDs process. Nitrogen is favored as the made of Cr, Ni and Ni alloys, or the combination of Niprocessing gas, which can be used to spray most or Ni alloys, and the rest is made of high-hardnessmaterials suitable for this application without causing ceramics, and some oxide of Cr, Ni or Ni alloysoxidation. The use of nitrogen-helium( He mixtures Moreover, all the repairing operations are in-situ andor He alone results in higher gas velocities for the same disassembling the mold body is not necessary. Theinlet conditions and nozzle geometry, which also mold is restored to its designed shape and its serviceproduces higher particle velocities. Therefore, it is life is prolongedpossible to spray materials that require higher kineticenergy.The most important advantages over the thermal3 CGDs repairing experimentsspray material deposition technologies are(1)A relatively small amount of heat is delivered to won area of the mold is illustrated in color( Fig. 1)the coated part, so that microstructural changes to the The won CrZrCu substrate, destroyed Ni layer andsubstrate material can be reduced to the minimal or special layer should all be repaired using the CGDsavoidedmethod by Cu alloy coating, pure Ni coating and Ni2)Due to the absence of in-flight oxidation andceramic composite coating, respectively.other chemical reactions,, thermally-sensitive andoxygen-sensitive depositing copper can be CGDSed/ Special layerwithout any significant material degradation;Pure Ni coatingNi layer(3 )Nano phase, and intermetallic and amorphousmaterials, which are not amenable to conventionalCu alloy coatithermal spray processes( due to a major degradation ofhe depositing materials ), can be CgDsed;CrzrCu substrate(4)Formation of the embrittling phases can beavoided:(5)Macro-segregation and micro-segregation of thealloying elements during the solidification, whichFig 1 Cross section illustration of CGDS repairing part forworn mold areaaccompanies conventional thermal spray techniques andcan considerably compromise materials properties, do 3. 1 Copper alloy coating on the CrZrCu substratenot occur during CGDS. Consequently, attractiveA gas-atomized Cu alloy( its chemical compositionproperties are retained in the CGDSed bulk materials;includes 2. 5%Ni, 0. 15% Si, 1. 5% Cr, 0. 3%Zr,(6)The"peening"effect of the impinging solid 0. 15% Sn and 0. 15% Mg) powder was used as feedparticles can give rise to potentially beneficialstock. The characterization of the powders is shown incompressive residual stresses in CGDs depositedmaterials in contrast to the highly detrimental tensile Fig. 2. The morphology of the Cu alloy powder isresidual stresses induced by solidification shrinkageparticulates are nearly perfectly round as shown in theaccompanying conventional thermal-spray proce- left bottom corner in Fig. 2(a). The particle sizedistribution of中国煤化工inFg2(b)(7)CGDS of the materials like copper, solder andThe averagepolymeric coatings offers exciting new possibilities fortal particlesCNMHn 80% of thecost-effective and environmentally friendly altermativesP………SlzeThe CGDs experiments were performed with theto technologies such as electroplating, soldering and cold gas technology( CGT) Kinetic spray 3 000M.AZHANG Junbao, et al. Progress in the application of cold gas dynamic spraying to repairing continuous casting molds 19de-Laval nozzle with a round-shaped exit was used. 150 mms with a standoff distance of 25 mm. aThe diameter of the nozzle exit was 8 mm, the ratio of mixture of 92%N2 +8% He with a stagnationthe exit diameter to the throat diameter was about temperature of 500C and a stagnation pressure of3. 08 mm, and the length from the throat to the exit 3.0 MPa was used as the processing gas Nitrogen waswas 68 mm. All the coating specimens were produced used as the carrier gas which was set as 6% of theby traversing the nozzle across the surface at a speed of processing gasSingle volume fraction162100(a)SEM image of morphology(b) Particle size distributionFig 2 Characterization of the Cu alloy powdersThe microstructure of the feedstock powder and theThe SEM image of the copper alloy coating showscoatings were characterized using an Zeiss Axioplan2 the microstructure of the CrZrCu substrate, the copperoptical microscopy OM)or a Hitachi $4200 fieldemission scanning electron microscopy(SEM) at 15alloy coating and their interface in Fig. 4( a). It isworth noting that there is no obvious difference20 kV. The transmission electron microscopy(TEM) between the substrate and thecoating:thus,theJEM200CX TEM, operating at a voltage of 160 kv.interface is quite difficult to be identified. Themicrostructure of the copper alloy CGDs coating afterHardness measurements for the powder and the coating ctching is shown in Fig 4(b). It is quite clear that themounted particles using a CSM micro-combi testercoating is uniform and composed of severely deformedIt can be seen that the picture of the copper allparticles. Its microstructure is quite homogeneous andcoating on a 10 mm thick CrzrCu substrate in Fig. 3s relative density is higher than that of the thermalThe CrZrCu substrate is a specimen delivered tospray due to the fact that no big pores can be observedBaosteel together with the CrzrCu plate to fabricatein theThe main mechanical properties of the copper alloycontinuous casting molds. It is quite clear that the coating cared for the continuous casting mold are listedthickness of the copper alloy coating fabricated byCGDS is up to 6mm, covering most worn depth of thein Table 1. The density of the coating was tested and itcontinuous casting mold in use. It can be seen thatwas 98. 7% after etching. The bonding strength is asthere was no heat influence on the Crzrcu substratehigh as 37 MPa, and the data are obtained by thewhich was proved by the fact that the color andlor and breaking of the bonding glue which is not in thedimensional shape of the substrate remainedthe bonding strength in practice of the coating may beunchanged. That is to say, both the partial scratched even higher. The micro-hardness is as high asgroove and the worn surface may be repaired andthe damaged parts were amended, leaving the310 HVo.2, which is higher than that of the CrzrCuareas unchainedbody. In the continuous casting process, the temperatureof the CrZrCu body usually is lower than 500Cbecause one side of the CrzrCu body is cooled downhighifcoating may decrease in use, it will be high enough toperform steel castingAbove all, from the microstructure and importarmechanical properties, it can be indicated that CGDs isa proper method to repair damage in the continuouscasting molds中国煤化工3. 2 InterfaceTHtnickel coatingCNMHGIng and theSteel continuous casting molds are made of theFig 3 Copper alloy coating on the CrZrCu substrateCrzrCu alloy body with a nickel or nickel -base alloBaosteel Technical Research Vol 5. No I Mar. 20/1Cr/rusCualloy coating200u(a) SEM image of the interface(b)SEM image of the coatingFig 4 Morphology of the copper alloy coatingTable 1 Basic mechanical properties of copper alloy coating has to be repairedfor the continuous casting molda gas-atomized, 99. 99% pure annealed nickelMechanicalpowder with a mean particle size of 20. 64 um waspropertydensity/% strength/ MPa hardness(Hv) used as feedstock. The characterization of the as-saidpowderCopper alloy98.7310nickel powder is spherical particulates of an even sizeThe particle size distribution of the nickel powder isillustrated in Fig. 5(b). The average size is 20. 64 umlayer plated thereon, so after repairing the CrZrCu alloy and more than 90% of the total particles are less thanbody, a nickel or nickel-base alloy layer plated thereon 40 um.Single volume fractionCumulative volume fraction86020Size(a)SEM morphology(b) Particle size distributioFig 5 Characterization of the Ni powdersThe CGDS experiments of the nickel coatingrecombined on a copper alloy coating. From the fineerformed with the same equipment and sameSEM image of the interface, similar conclusion ismeter of the copper coating. The only difference was obtained due to no considerable differences can behat stagnation temperature changed from 500c to identified, as shownin Fig. 6(b)550C because the micro-hardness of the nickel powderIn order to test the bonding strength, thick specimensis as high as 140up to 15 mm for both the copper alloy coating and theThe morphologies of interface between the copper nickel coating were fabricated together and the diametercoating and the nickel coating are shown in Fig. 6. The of the tested specimen was 10 mm. The bonding strengthcopper coating is red and the nickel coating is grey in of the as-sprayed state is up to 75 MPa as measuredFig. 6(a). It is quite clear that the interface between which is high enough for the continuous casting moldthe two coatings can be easily identified by color and platethe interface is not a smooth line but a zigzag path ofwaves. Adhesion between the two coatings, further-3. 3 Microstructure and mechanical properties ofmore, is quite good and no gap can be seen in the the nickel coatinginterfaced area. Some nickel particles can be seenNickel coatIy contant linmid steel in usinserted in the copper coating due to those nickel resulting in中国煤化工nnealing stateparticles mixed in the CgDs process, adhesion round thus, microstrCN MH Gertie variationthe nickel particles is also quite good, as shown in at different temperatures are Important properties forFig 6(a),on the bottom side. It is noted that using continuous casting mold plates. Annealing treatmentthe same CGDs a well-bonded nickel coating can bewas conducted in an electrical furnace for one hour atZHANG Junbao, ef al. Progress in the application of cold gas dynamic spraying to repairing continuous casting molds 21(a) Color optical picture(b)SEM imageFig 6 Morphology of the interface between the copper alloy coating and the nickel coatingtemperatures of 300C, 500C, 600C and 900C in an particles are observed! i. The density of the coating isargon atmosphere to analyze the microstructures and up to 98. 5%. In Fig. 7(b), a typical TEM micro-mechanical properties of the nickel coating in use. The structure of the as-sprayed nickel coating is shown. Themicrostructures of the nickel coating are shown in microstructure also exhibits heavily deformed featuresFig. 7. An etched cross-section of the as-sprayed nickel The grains with an average size of about 200 nmcoating is shown in Fig. 7(a)and(b) o. Clearly, the contain many dislocation walls, dislocation tangles oras-sprayed coating is dominated by the appearance of dislocation cells, and most of boundaries are curvedseverely deformed microstructures. The majority of the irregular or ill-defined. All of these characteristics aicats,and some interfaces between the deposited plastic deformation in the CGDS process d to severedeposited particles are highly deformed into elongated typical features of those materials subject(a)SEM image of as-sprayed state(b)TEM image of as-sprayed state(c) SEM image of the nickel coating after (d)TEM image of the nickel coating afterannealing at 900C for one hourannealing at 900C for one hourFig 7 Microstructures of the nickel coatingFig. 7(c)shows the SEM microstrf the recrystallization has already been completednickel coating after annealing at 900C for one hourFig. 8 shows the coatings micro-hardness beforeThe newly formed grains are evolved instead of and after annealing at various temperatures In the aselongated grains in the as-sprayed state, and thereforesprayed state, the coating exhibits a comparably highthe former particle interfaces are no longer observed hardness of 235.2 HVo. 2. With the increase of thefrom the cross-section. The mean grain size after annealing temperature, the hardness decreases smoothlyannealing at 900C reaches about 5. 5 um. The grainfirstly at low annealing temperature. and then dropssize difference is also confirmed by the TEM rapidly in a ter中国煤化工00~500℃observation, as shown in Fig. 7( d ).In addition, the When annealingrains in the coating, annealed at 900, are much decline in harCNMHGn60℃,theantituoig annealinglarger and the dislocation density seems to be much temperature is less pronounced, and reaches a value oflower. Therefore, it can be concluded that the 124. 1 HVo? at 900C, which is higher than thermalBaosteel Technical Research. Vol 5. No. 1. Mar. 201/spray nickel coating and it is high enough for thecontinuous casting mold. Usually, the origin ofpronounced decline in hardness during annealing canbe attributed to recrystallization. It suggests that therecrystallization start temperature may be in the rangebetween 300-500C, which is lower than the typicalrecrystallization temperature of pure nickel(600C).Itis probably due to the high deformation energy storedin the as-sprayed coatingMoreover the variance of hardness statistical resultsdecreases with an increasing annealing temperature, asnown in Fig 8. It incISan effective way of reducing the amount and sizes ofAnnealing temperature /Cthe cold-forged" interfaces, because the presence of Fig. 8 Micro-hardness vs annealing temperature for thethese interfaces that are not well-bonded in coatings is CGDS nickel coatingsmainly responsible for the scattering of the coatingsa relatively lower thermal conductivity to the CrzrCuhardness-13.The temperature of the nickel coating is body. this case, high-hardness ceramics can be addedabout 600c in use the micro-hardness of the CGDsnickel coating after annealing at 600t is still as high shows a composite coating with% ceramics andas 149.8 HVo.2, which is high enough, and this 85% Ni coaling141As foror even mechanical propertyindicates that CGDS is a proper method to fabricate the the whole surface of the mold including the repairednickel coating for the continuous casting moldarcas and un-repaired areas can be coated with one3. 4 Composite and multilayer coating on the kind of material, for example nickel. All the coatingCrzrCu substratelayers are banded very well, as shown in Fig 9(b)Some areas of the mold for continuous casting prefermm(a) Ceramics composite coating(b)Multilayer coatingFig 9 SEM photos of multilayer coating4 CrzrCu body repairing operationstayed unchanged. That is to say, both the partialscratched groove and the wom surface can be repairedFig 9 shows repairing of a continuous casting moldbody using CGDs method. It can be seen fromFig 10(a)that on the top of the picture shows the steelcontinuous casting mold body with size of 300 mm x900 mm was worn on the entire surface about 1 mmthick. In the middle of Fig. 10(a), one can see the moldbody was cleaned by milling to the Crzrou base, and onthe bottom of Fig 10(a), a 3 mm to 8 mm thick copperlayer is coated to the body according to the different0worm depths. All the coatings are adhered even and(a) Worn mold body and the (b)Milling different depthscooing suface will be milled to he ori nad dg easionsCGDS copper alloy coatingig. 10 Copper alloy repod to be milled中国煤化工Fig. 10(b). It is quite clear that the coating fabricated byAs a summawn regardingCGDS is as thick as up to 8mm and without having any the continuoYHCNMHGring coatingsheat influence on the substrate, which was proved by the fabricated by the CGDs method. Copper alloy andfact that the color and dimensional shape of the substrate nickel coatings can be prepared on the continuousZHANG Junbao, ef al. Progress in the application of cold gas dynamic spraying to repairing continuous casting molds 23casting mold using thecomethod to repair theand mechanical properties of nanocrystalline Ni coatingsdestroyed areas of thebody and the nickelproduced by cold gas dynamic spraying[ J]. Surface andcoating.The relative density of the copper coating is asCoating Technology, 2006, 201(3-4): 1166-1172.high as 98.7%, there is no clear difference between the [4] Borchers C, Gartner F and Stoltenhoff TMicrostructuralthe interface is quite difficult to be identified The [5Coatings[J]. J Appl Phys. 2003, 93: 10064-10070 coppercopper body and the coating in the microstructure, andand macroscopic properties of cold sprayed copperKang Hyun Ki and Kang Suk Bong. Tungsten/copperbonding strength and micro-hardness are as high as upcomposite deposits produced by a cold spray[J]. Scriptato 37 MPa high and 310 HVo.2 respectively. TheMater..2003,49:l169-1174.interface between the copper coating and the nickel 6] Choi H, Yoon S, Kim G, et al. Phase evolutions of bulkoating is a zigzag path of wave and its adhesion isamorphous NiTiZrSiSn feedstock during thermal andquite strong. As-sprayed nickel coating is dominated bykinetic spraying processes [J]. Scripta Materialia, 200553(1):125-130heavily deformed particles, and the relative density is [7] Shukla V, Elliott G S and Kear B H. Hyperkineticup to 98.5%. Completely recrystallization occurs afterdeposition of nanopowders by supersonic rectangularannealing at 900C for one hour, while its microimpingement[J]. Scripta Maters, 2001, 44: 2179-2182.hardness is still as high as 124. 1 HVo?. All these [8] Stoltenhoff T, Borchers C, Gartner F, et al.Micro-results indicate that the CgDs is a potential technologystructures and key properties of cold-sprayed andfor continuous casting mold repairingthermally sprayed copper coatingsSurface andCoating Technology, 2006, 200(16-17): 4947-4960[9] Kim H J, Lee C H and Hwang S Y Fabrication of wc-5 CconcluSionCo coatings by cold spray deposition [J]. Surface andCoating Technology, 2005, 191(2-3): 335-340CGDS offers new applications for spray technology[10] Zhang H B, Zhang J B, Liang Y L, et al. Effect ofwhich makes it possible to produce metal coatings withannealing treatment on microstructures and mechanicalproperties of cold-sprayed nickel coating[ J].Baosteelvery little porosity and oxygen content. That isTechnical Research, 2009, 3 (2): 46-51beneficial to the physical properties such as the [11] Assadi H, Gartner F, Stoltenhoff T, et al. Bondingelectrical conductivity and corrosion resistance. Themechanism in cold gas spraying [J]. Acta Materialiaprocess does not require any melting of the spray2003,51(15):43794394material, the severe plastic deformation of the particles12 Gartner F, Stoltenhoff T, Voyer J, et aL.Mechanicalupon the impact causes a significant amount of heatingproperties of cold-sprayed and thermally sprayed coatingsresulting in adhesion, which is a common feature of the[J]. Surface and Coating Technology, 2006, 200(24677046782spray material and its plastic deformation charac- [13] Zhang H L, Huang P Z, Sun J, ef al. Morphologicalterristicshealing evolution of penny-shaped fatigue microcracks inAs for a method to repair the mold for continuouspure iron at elevated temperatures [J]. Applied Physicscasting, CGDs is convenient, in-situ repairation, littleLetters,2004,85(7):1143-1145amount of heat delivered. microstructural and [14] Lima R S, Karthikeyan J, Kay CM, et al. Microstructuraldimensional stability and harmless to the CrzrCu bodycharacteristics of cold-sprayed nanostructured wC-Cotogether with special applications. As a result, CGDscoatings[ J]. Thin Solid Films, 2002(416): 129-135will soon become one of the best methods to repair themold for continuous castingReferences[1] Minami Kenji, Kawata Yoshimasa, Ushio Tetsuji, et alMethod for repairing a mold for continuous casting ofel:US,4502924[P].198503052] Stoltenhoff T, Kreye H and Richter H J. An analysis ofthe cold spray process and its coatings[ J]. Journal ofThermal Spray Technology, 2002, 11: 542-550ZHANG Junbao LIANG YongliZHANG Huabin[3] Ajdelsztajn L, Jodoin B and Schoenung J M. Synthesis中国煤化工CNMHG