Tribological behavior of coatings for continuous casting of steel

Surface and Coatings Technology 146–147(2001) 55–64

0257-8972/01/$ - see front matter� 2001 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž01.01475-X

Tribological behavior of coatings for continuous casting of steel

Alejandro Sanz*

Danieli & C. SpA, Centro Research and Development (CRD), Via Nazionale 41, 33042 Buttrio (Ud), Italy

Abstract

There are a large number of steel making processes in which great demands are made on the surface behavior of severalcomponents that come into direct contact with steel under various conditions. Continuous casting is mainly a heat-extractionprocess. The mold must rapidly transfer heat from the steel to the cooling water. In continuous casting, steel solidification startswhen it comes in contact with the mold liner’s interior surface. The key job of every mold consists in cooling the molten steelin a controlled way. The mold is a major element in the overall economics of a continuous casting plant which explains thenumber of innovative approaches to increase the working life(length of time during which the mold shows acceptable dimensionalstability to meet the quality standards) or to satisfy the new demands to be met by the mold liners. Coating the mold inner is afirmly established practice, in particular with electrolytic surface modification treatments, to cope with the various operating needsincluding low wettability, high hardness, good wear resistance and low cost. Several pin-on-disk tests were carried out to determinethe friction and wear behavior of different coatings. The friction partner for all coatings was a K30(WC–Co 9%) chip, thesliding speed was 10 cmys at a temperature of 2508C and a load of 5 N. The sliding time was 200 h(720 000 revolutions for aradius of friction 16 mm). Additional tests for shorter times allowed verification of the morphological evolution of the wear track.All coatings were also evaluated using a scratch test. This test introduces stresses at the interface between the coating and thesubstrate as the sample is displaced at constant speed. The critical load(L ) recorded from the scratch test translates the complexc

intrinsic properties of a specific coating into a very reproducible figure of great practical significance. This paper presents atribological characterization of conventional electrolytic coatings, bare copper alloys and some new surface solutions for continuoussteel casting molds.� 2001 Elsevier Science B.V. All rights reserved.

Keywords: Continuous casting; Mold inner coatings; Pin-on-disk tests; Scratch tests; Electrolytic deposits; Thermal spray coatings; Chemicallymodified layers

1. Introduction

Continuous casting is mainly a heat-extraction proc-ess. The mold must rapidly transfer heat from the steelto the cooling water. The function of the mold liner isto shape the strand and to produce a sufficiently strong,sound, low stressed shell that contains the liquid coreas it continues to solidify that can withstand the pullingstresses. The conditions for creating a homogeneousshell are an efficient and uniform heat transfer. Therequirement for high thermal conductivity leads to thenatural choice of copper and its alloys as the basematerials for mold substrates. Heat transfer along the

* Tel.: q39-0432-97-1821; fax:q39-0432-97-1821.E-mail address: [email protected](A. Sanz).

mold wall is not uniform with the maximum flux located20–50 mm below the meniscus. More than 60% of theheat exchange by the mold occurs in the mold top-half.Achieving improved heat conduction is at least asimportant as reducing friction for achieving high castingspeedsw1–4x.Friction phenomena occur when two moving surfaces

are rubbed directly against each other. The traditionalsolution (only applicable if casting with a submergednozzle) is the use of the casting fluxes(if not, castingoils are used as lubricants). Friction between the solid-ifying steel and the mold is basically sliding(with asmall fraction of sticky friction). The frictional forcebetween the strand and the mold can be analyzed bycalculating the thermal shrinkage and the shearingstrength of the meniscus. The frictional force between

56 A. Sanz / Surface and Coatings Technology 146 –147 (2001) 55–64

Table 1Materials and coatings under study with main relevant properties

Materials Deposition Thickness Hardness Thermal(abbreviation method (mm) (HV) conductivityused) (WymK)

Cu–Cr–Zr(Cu) 120 340Hard chromium Electrolytic 80 1000 67Nickel Electrolytic 120 220 55Ytria stabilized Super D-gun 200 960 3.3zirconia(YSZ)

YSZqchemicalhardening(YSZqCH) Super D-gun 230 1268 4.5Al O qCr C (FGM)2 3 3 2 D-gun 250 580 9CoqWCqchemical Plasmaslurry hardening Chemical slurry(WC–CoqCS) qheating

(several cycles) 100 1500 15CoqCr C3 2 Electrolytic 80 500 17.5(CoqCrC)

Amorphous A HVOF 120 1200 4.5Amorphous B HVOF 120 850 4.5

steel and the copper mold increases with increasingtemperature.The mold inner coating is a firmly established prac-

tice, in particular with electrolytic surface modificationtreatments(mainly hard-Cr, Ni and their combination).Under the new Environmental Protection Agency(EPA)rules, hexavalent chromium compounds are increasinglysubjected to maximum exposure limits leading toincreasing costs. This trend will consistently continueupward, and, therefore, there is a focus on hard chromereplacementw5–7x. Several interesting options exist, butin most cases their line-of-sight characteristic does notmake them feasible for tubular molds. The requirementsfor the highest possible standards in terms of workinglife, operational safety and low maintenance leave thepossibility of some localized antiwear applications. Thesurface solutions for increasing the wear resistance arenormally not compatible with the high thermal conduc-tivity required for the mold’s primary function. However,in some areas of the mold, the heat extraction capacityis in excess of that required or the wear-prone areas aremeaningless in the global heat extraction process whichallow protection of some local critical areasw7–9x.

2. Experimental

Nine different types of coatings were tribologicallyevaluated under conditions close to those found in ahigh-speed bloom caster at 6 mymin. The coating rangecovers those already in service in several melt shops aswell as several innovative coatings aimed at futureapplications. Different deposition and hardening tech-niques have been explored in the present work. Tables1 and 2 give a summary of the tribological experimentalset-up covering the materials tested as well as thegeometry and features of the equipment used.

Copper based alloy was used as a reference for baremolds. This same alloy was used as a substrate for allcoatings with the exception of the electrolytic CoqCrCcoating.Three electrolytic coatings were evaluated. The elec-

troplated cobalt was co-deposited with a fine chromiumcarbide powder to enhance wear properties. This CoqCrC coating was deposited on an AISI 316 stainlesssteel to avoid bath poisoning with copper ions. Hardchromium is the most commonly used coating for billetsand blooms(long products). Electrolytic nickel is usedfor slab and thin slab mould(flat products).The thermal sprayed materials included thermal bar-

riers, amorphous coatings and functionally graded mate-rials. Ytria stabilized zirconia is a thermal barrier thatcould be used to limit the heat flux peak at the meniscuslevel in a casting mold. The amorphous coatings producesmooth, hard surfaces with high abrasive and erosivewear resistance. The difference between the amorphousA coating and the amorphous B coating resides in ahigher chromium content in the latter which leads to avery low porosity and an increased corrosion resistance.The functionally graded material is a 150-mm-thickseries of layers grading an Al O™Cr C transition.2 3 2 3

The FGM was deposited on a 50-mm-thick, Co-basebond coat.Two types of chemically hardened coatings were

explored. Chemically hardened YSZ was obtained byheating to 2608C and quenching into phosphoric acid(left immersed for several hours), then slowly heated to4808C followed by a slow cooling. The WC–CoqCS isa thermochemically formed composite coating. Afterapplying a WC–17% Co coating, a water-based chemicalslurry with chromium oxide, silicon dioxide and alumi-num oxide was applied on the surface. The component

57A. Sanz / Surface and Coatings Technology 146 –147 (2001) 55–64

Table 2Tribological testing set-up: nature of the tests and sensibility of the measuring devices

Wear test Adhesion tests

Test configuration Pin-on-disk Test configuration Scratch testsPin chip material WC–Co 9% Indenter RockwellLoad 5 N Indenter radius 200mmSliding speed 10 cmys Indenter material DiamondMean estimated 5Ny8.4 mm s2

pressure(assuming 0.6 Nymm (6 bars)2 Speed 10 mmyminperfect flat contact)

Chamber temperature 2508C Loading rate 100 NyminRadius of friction 16 mm Scratch length 10 mmSliding distance 200 h or Load range 0–100 N

720 000 rev.(100%)160 h or L andLc1 c2 Optical microscopy576 000 rev.(80%) determination method Average of 3 scratches100 h or Acoustic sensor Vallen type360 000 rev.(50%)60 h or Penetration LVDT sensor216 000 rev.(30%) monitoring

Friction sensor LVDT up to 10 N Fn, Dx, AE sampling rate 1000 HzSensor sensitivity 0.1 mVyVymm Fn, Dx, AE channel filtering 10 HzSensor resolution 0.02mm Fn: frictional force,

Dx: displacement,AE: acousticemission

was dried and heated to 5008C. By repeating thisprocedure several times, a totally dense, pore-free coat-ing was produced that was chemically bonded to thesubstrate. The coated surfaces were very smooth(R sa0.5mm).Tribological tests were performed on a high temper-

ature pin-on-disk tribometer with a chip-on-disk config-uration. A pin (chip slider) is pressed with a certainload on a disk, which is in rotation under well-definedconditions. During the rotation of the pin, friction forcesapply a force on the arm where the slider is fixed.Friction force is determined by direct measurement(withan inductive sensor) of the tangential force applied onthe flexible arm. The friction coefficient is given by theratio between the friction force and the normal force(applied load). The pin-on-disk tests were run for amaximum of 200 h which is considered the reference(100%) and for three shorter times(160, 100 and 60 hequivalent to 80, 50 and 30%) to clearly understand theevolution of the interfacial phenomena at the contact.Disk wear was calculated at the end of the test with

a profilometer. Chip wear was measured by opticalmicroscopy. The wear rate,t (see equation below), wasdetermined for the disk and was calculated according tothe standard DIN 50324. This calculation includes vol-ume loss, applied load, and the total sliding distance ofthe test.

V 3w xts m yNm (1)F Øln

whereV (m ) is the worn volume,F (N) is the normal3n

force applied andl (m) is the total sliding distance.Three wear measurements were performed with a

Taylor–Hobson profilometer to get an average sectionof the track. The worn volume was determined byintegration of the section. The worn volume of the diskwas calculated with the following equation:

3Ž .V s2prS mm (2)disk

where S (mm ) is the worn area calculated with the2

program ‘sillon’ of the Talysurf andr (mm) is theradius of the wear trackw10–13x.Scratch testing consists of introducing stresses at the

interface between the coating and the substrate. This isachieved by pressing a diamond stylus on the samplesurface with a normal loadF . As the sample isN

displaced at constant speed, the resulting stresses at theinterface cause flaking or chipping of the coating. Thesmallest load at which a specific failure event is recordedis called the critical load(L ). L translates the complexc c

intrinsic properties of a specific coating system into avery reproducible figure of great practical significance.The scratch tester provides cross-referenced data onLc

by simultaneously recording three different effects:(a)tangential force variations;(b) acoustic emission fluc-tuations; and(c) microscopic deformationsw14–17x.A series of six scratches were performed on the

samples before and after annealing at 2508C for 200 h.Three scratches were made perpendicular(H) and theother three scratches parallel(≤) to the grinding direction


Table 3Pin-on-disk wear test results

Material Test Disk Chip wear mmin mmean mmax

duration wear rate wworn surface(mm )2

(h) (m yN)2 yworn thickness(mm)x

Cu 200 h 6.14 E-14 8.41(max)y0.60 0.61 0.80 0.99Hard Cr 200 h 1.20 E-13 10.8y0.65 0.60 0.82 0.99

160 h 9.50 E-14 6.8y0.35 0.60 0.69 0.79100 h 2.10 E-13 9.6y0.58 0.53 0.68 0.8360 h 1.70 E-13 4.72y0.22 0.53 0.65 0.77

Ni Few ) 4

YSZ 200 h 2.30 E-14 3y0.34 0.47 0.70 1.04160 h 2.13 E-14 2.8y0.38 0.36 0.73 0.98100 h 6.51 E-14 5.8y0.33 0.44 0.73 0.9560 h 6.40 E-14 2.5y0.13 0.72 0.88 1.04

YSZqCH 200 h 1.14 E-14 5.2y0.2 0.20 0.77 1.1160 h 3.10 E-14 6y0.27 0.46 0.80 1.02100 h 2.20 E-14 4.9y0.17 0.50 0.80 1.0060 h 1.50 E-14 3y0.1 0.46 0.60 0.80

FGM 200 h 5.63 E-16 4y0.11 0.34 0.80 1.04160 h 2.29 E-15 3.9y0.135 0.56 0.75 0.94100 h 1.36 E-15 2.8y0.1 0.48 0.65 0.8560 h 3.79 E-15 4.1y0.16 0.46 0.75 0.82

(scratches)WC–Coq CS 200 h Not measurable Not measurable 0.35 0.45 0.49

160 h by profilometry by optical 0.36 0.39 0.43277.7 microscopy 0.42 0.47 0.54

CoqCrC 200 h 3.89 E-14 7.4y0.51 0.30 0.60 0.84160 h 2.53 E-14 4.9y0.49 0.20 0.50 0.81100 h 3.00 E-14 4.5y0.34 0.30 0.70 0.9060 h 3.98 E-14 4.0y0.27 0.34 0.70 0.89

Amorphous A 200 h 5.33 E-14 Not measurable 0.42 0.52 0.60160 h 5.21 E-14 by optical 0.40 0.51 0.65100 h 5.00 E-14 microscopy 0.28 0.50 0.6660 h 5.93 E-14 0.42 0.52 0.63

Amorphous B 200 h 2.89 E-13 1.6y0.065 0.40 0.65 0.81160 h 3.11 E-13 1.3y0.055 0.44 0.65 0.79100 h 1.60 E-13 Not measurable 0.36 0.40 0.5660 h 1.62 E-13 Not measurable 0.36 0.41 0.58

of the substrates. The critical loadsL (H) and L (≤)c1 c2

are determined by optical observations which are morereliable than acoustic emission.

3. Results and discussion

Tables 3 and 4 present a general summary of theresults of the tribological evaluation of the differentcoatings. Fig. 1 shows the microstructure cross-sectionof an Al O™Cr C functionally graded material, which2 3 3 2

is considered as a potential candidate for future moldsurfaces.From the data of the copper alloy disk wear rate as a

function of test duration, it can be noted that chip wearis very important generating significant quantities ofdebris particles. A thick black oxide layer, probablyCuO, was formed on the disk. It seems that the highsolubility between Co(from WC) and Cu plays animportant role of wear and increases the wear of thetwo partners.

The hard-chromium wear rate was similar for 200and 160 h, and higher for 100 and 60 h. This behaviorwas certainly due to a high wear rate at the beginningof the test because of the high pressure of the chip onthe coating(small contact area). Optical and microscop-ic observations showed the cracked nature of the Cr-layer(mostly after the 200-h at 2508C annealing). Therewas a preferential wear of the chip in the slidingdirection (Fig. 2). The chips exhibited wear thatincreased with the test duration except for the test at100 h that exhibited very high wear. There was asignificant difference in the friction behavior betweenthe test at 200 h and the three other tests. The meanestimated pressures(at the end of the test) were 4.6 barfor the 200-h test, 7.3 bars for the 100-h, 5.2 bar forthe test at 50% and 10 bar for the shortest test. Thescratch test samples showed some little cracks in thecoating outside the track without any important delam-ination and several transverse cracks through the track.The scratch tests confirmed the brittle nature of the


Table 4Scratch test results

Material Sample L : 1st smallc1 L :1st bigc2

cohesive failure(H) cohesive failure(≤)

CuHard Cr Reference 60 N 50 N

Annealed 22 N 25 NNi Reference 11 N 13 N

Annealed 6 N 16 NYSZ Reference 48 N 64 N

Annealed 48 N 72 NYSZqCH Reference 66 N )100 N

Annealed 47 N 57 NFGM Reference 43.5 N 48.5 N

Annealed 42 N 53 NWC–CoqCS Reference 68 N Not measurable

Annealed 64 N Not measurableCoqCrC Reference Not measurable Not measurable

Annealed Not measurable Not measurableAmorphous A Reference 35 N 23 N

Annealed 35 N 35 NAmorphous B Reference 63 N 60 N

Annealed 48 N 33 N

Fig. 1. Metallographic cross-section of the FGM coating.

coating. There were no significant differences betweenthe two tests(before and after 2508C) and between thetwo grinding directions. The scratch behavior of thecoating before and after 200 h at 2508C does not seemto be very different even though the critical load washigher before the heating procedure. The cracks werevery small, comparable between the two tests, anddifficult to quantify with accuracy.The other often-used coating in steel casting molds is

electrolytic nickel. Three tests were performed andstopped after a few revolutions because of very badwear and friction behavior. Deep scratches were

observed on the coating during the initial test cycle. Thefriction force increased very significantly and saturatedthe electrical signal(friction coefficient )4). Underthese conditions, it was impossible to continue the testsfor the expected duration. Therefore the test series hasbeen stopped. The scratch tests showed important defor-mation of the coating due to the applied load. Thecoating seems to be very soft with no cracks observed.It was difficult to determine exactly the critical load ofthe coating because no important failures were observed,just a large plastic deformation of the coating. Never-theless, there were no significant differences in failures,


Fig. 2. Pin-on-disk wear tracks on a hard-chromium coating after 720 000 revolutions(200 h). Temperature 2508C, speed: 10 cmys, load: 5 N.(a) Wear of the chip;(b) wear track of the coating; and(c) detail of the wear track.

on one hand between the two grinding directions, andon the other hand before heating and after annealing(200 h) of the coating at 2508C. The influence of theannealing was relatively weak.The wear and friction behavior of the electrolytic Ni

coating and hard Cr coating were very different. Thefirst is very soft and is plastically deformed with thepin-on-disk tests, whereas the second is brittle andexhibit numerous cracks at high temperature and underhigh stresses. The hardness of the electroplated Cr fallssteadily at the mold working temperatures leading tohardness values of approximately 750 HV after 1 h at3008C. The average thickness of a hard chromiumcoating at the mold level ranges from 35 to 120mm. Insome cases, hard chromium plating is preceded by anickel plating. The hard chromium provides a wear andchemical resistant layer, and, nickel provides a layerwith a high strain-to-fracture feature and with a thermalexpansion coefficient similar to that of copper. The latteraspect allows the deposition of relatively thick Ni-layers(1–5 mm) with different thickness profiles. In somecases Ni is used alone(without any external Cr layer)in thick layers, but it normally shows some chemicalreactivity with the mold fluxes. It is more common tosee Ni as an intermediate layer or applied in the final2y3 of the mold length so as to prevent direct contactwith molten slag.Thermal barriers YSZ and YSZqCH are similar to

those chemically hardened at a later stage. For the YSZthere is a remarkable difference between the two long-term tests and the two short-term ones. This is probably

due to the fact that the pressure is more important atthe beginning of the test(i.e. the contact area is smaller),and the asperities of the coating are sharper, so theabrasion effect and scratches are more important. Therewas no significant difference in friction, except the testat 30% of the total time where the friction is slightlyhigher than for the other tests(ms0.88). Mean esti-mated pressure: 5 Ny3.5 mms1.43 Nymm (14.32 2

bar). The scratch behavior of the YSZ coating beforeand after the test at 2508C does not seem to be verydifferent. The difference in wear rate of the YSZqCHcoating between the beginning and the end of the testwas less important than for YSZ coating. Moreover, thewear rate was a factor 2–3 lower for the YSZqCHcoating. This better behavior is probably due to thechemical hardening and the associated densification ofthe coating. The friction coefficient of the YSZqCHsystem became less and less stable when increasing thetest duration. The YSZqCH scratch resistance decreasesafter the thermal treatment. Optical micrographs showedthat delamination appears in the track at a lower loadfor the test after 2508C.The wear rate of the functionally graded material

(FGM) was more important during the first part of thetest (60 h) and decreased during the test duration until200 h. Concerning optical microscopy, the wear mech-anism seemed to be the same all along the test. Thetrack was worn progressively without large scratchesand without thick oxide layers. There was just a differ-ence for 200 h where a small oxide layer in the middleof the track was observed. Important friction at the


Fig. 3. Pin-on-disk wear tracks on a WC–CoqCS coating after 1 000 000 revolutions(277.7 h). Temperature 2508C, speed: 10 cmys, load: 5 N.(a) Wear of the chip; and(b) wear track of the coating.

beginning of the test was responsible for the wearappearance at first in the sliding direction on the edgeof the chip. The contact area remained relatively constantif compared to different tests(except for the 200-h test).The wear behavior seems to be the same for all chipsexcept for the 200-h test(scratches are observed). Therewas no significant difference in friction behavior for thedifferent tests, except in the first part of the curve ofthe 200-h test where the friction coefficient was a littlehigher than for the other tests. Pressure was relativelyconstant during the tests. Contact area was the same forall tests(from 30 to 100% of the total duration), andthe average contact area was 3.7 mm . The estimated2

pressure was 5 Ny3.7 mms1.35 Nymm (13.5 bar).2 2

The wear behavior of the WC–CoqCS coating wasvery good because no wear tracks were visible withoptical microscopy. There was only a little polishingeffect of the surface, which was perceptible to the eyes.Wear of the chip could not be measured. Nevertheless,some scratches were visible in the sliding direction ofthe test(Fig. 3). The wear behavior was so good, thatthe general arrangement of the test for this material wasshifted to 1 000 000 revolutions under the pin-on-disktest (equivalent to 277.7 continuous test hours). Therewas no significant difference in the friction behavior ifcompared with the other three tests. The friction wasvery stable and relatively low. Mean estimated pressurewas 5 Ny9 mms0.55 Nymm (5.6 bar). The scratch2 2

behavior of the coating before and after the test at 2508Cdid not seem to be very different. Optical microscopyanalysis of the scratch tests showed that the failure is

not measurable outside the track. Some small cohesivecracks were found. Cracks in the track were measurable.The critical loadsL and L were determined byC1 C2

optical observations.The CoqCrC coating contain 15–25% chromium

carbide with an average size of 2–5mm. At the workingtemperature, the carbides resist wear by supporting loads(and in case of glaze formation, they help to anchor it).Some carbide changes at high temperature(Cr C™3 2

M C ) allow doubling the carbide fraction volume and7 3

therefore hardness is increased up to 650 HV. The testfor pin-on-disk test for 200 h showed a wear behaviorwhich was worse than for the other tests. This behaviorcould have resulted from a change in the wear mecha-nism, which induced a higher wear. Indeed, the frictioncurve showed a behavior totally different after 300 000revolutions. Friction was more stable, but higher(m;0.6) than at the beginning of the test. There wasno significant difference in the friction behavior amongthe different tests. The first part of the curve shows lowfriction, followed by an unstable part where the frictionincreases and decreases quickly, then followed by amore stable but higher friction. The mean estimatedpressure was 5 Ny5.2 mms0.96 Nymm (9.6 bar). A2 2

series of three scratches were performed on the substratebefore and after heating at 2508C for 200 h. Only plasticdeformation of the coating without any delaminationsor important cracks was observed(Fig. 4).The general form of the microstructure of the two

analyzed amorphous deposits is a two-phase solid withdendritic chromium borides in a matrix of iron–chro-


Fig. 4. Scratch test on a CoqCrC coating.

mium or nickel–chromium(depending on the type ofcoating) which was predominantly amorphous(40–50%). Fully amorphous structures are moderately stable(crystallizing under service conditions and not providingsignificant wear improvements). There was no signifi-cant difference in the friction behavior of the differenttests for the amorphous A coating. The curve was verystable and the mean friction coefficient was approxi-mately 0.5. The wear of the chip was not measurable( just a few scratches). Some small cracks in the coatingoutside the track without any important delaminationsand a high density of transverse cracks through the trackwere observed. The coating seems to be very brittle.There was no significant difference between the twotests of the metallic amorphous A coating, before andafter 2508C, and between the two grinding directions.The scratch behavior of the coating before and after thetest at 2508C seemed quite similar. The wear behaviorof amorphous coating B is different from what wasobtained for the other coating. The friction curves forthe two longest tests showed several periods of highand stable friction followed by short periods of lowfriction. This high friction would explain the high wearrate, whereas friction was more stable and lower for thetwo other tests. There was a preferential wear of thechip in the sliding direction for the two longest testsand not measurable wear for the tests at 60 and 100 h.This was due to the fact that the wear mechanism wasdifferent along the tests. The mean estimated pressurefor the longest test(200 and 160 h) was: 5 Ny1.5mm s3.3 Nymm (33 bars). The mean estimated2 2

pressure for the shorter tests(100 and 60 h) is: 5 Ny8.41 mms0.6 Nymm (6 bars). The scratch test track2 2

of metallic glass B(Fig. 5) showed some small cracksin the coating outside the track without any importantdelaminations and a lot of transverse cracks through the

track. The wear behavior of amorphous coating A wasmuch better than amorphous B coating, and the frictionbehavior was more stable. The information obtainedwith scratch tests showed that type A was more brittlethan type B (radial cracks through the coating werenumerous).

4. Conclusions

Chromium plating is widely used as a confirmedsolution to cope with the various operating needs includ-ing low wettability, high hardness, good wear resistanceand low cost. The as-deposited Cr-hard plating has ahardness of approximately 1000 HV. The deposit is verybrittle and the high degree of internal tensile residualstresses causes it to be microcracked spontaneously.The requirements for the highest possible standards

in terms of working life, operational safety and lowmaintenance leave the possibility for some, localized,antiwear applications. The surface solutions for increas-ing wear resistance are normally not compatible withthe high thermal conductivity required for the mold’sprimary function. In some areas of the mold, the heatextraction capacity is in excess to what is desired orwear-prone areas are meaningless in the global heatextraction. Among these areas that can be improved,could be cited the lateral plates of slab and thin slabcasters(also called end-plates), the bottom end of themold (where the air-gap is already formed and heattransfer is limited to radiation) and guiding devices(grip plates or foot-rollers) at the exit of the mold. Thereduction in porosity plus the thermal treatment associ-ated with the chemical slurry treatment leads to fullydense coating chemically bonded to the substrate. Chem-ically hardened metal based coatings additionally showfriction coefficients lower than those of electroplated


Fig. 5. Scratch test on an amorphous coating(amorphous B). (a) Scratch perpendicular to the roughness direction; and(b) scratch parallel to theroughness direction.

hard chromium. The WC–CoqWS materials have nowbeen introduced in the above mentioned areas of themold with an excess of heat extraction capacity andunder severe wear conditions. Ceramic based chemicallyhardened coatings show better wear behavior than con-ventional hard-chromium layers with similar frictioncoefficients. Materials like the YSZqCH have a simplerprocessing schedule(single chemical slurryqannealingat the end) than the WC–CoqWS materials but cannotmatch the wear resistance of the latter. The high thermalresistance of both the YSZ and YSZqCH does notallow their use on the whole mold surface but just someselected areas.Some new electrodeposited Co alloys are harder than

Ni deposits and are becoming increasingly interestingfor casting purposes. Co rich plating leads to theformation of wear resistant surfaces able to promote theformation of adherent cobalt oxide in sufficient quantityto produce a glaze and to prevent metal-to-metal contact.Electrochemical solutions for depositing CoqCrC coat-ings are very sensitive to bath contamination by metals(Cu in particular). This family of coatings could beused on top of a nickel bond coat with proper maskingof the areas not to be coated(a similar procedure to theone followed with the NiqCr electrolytic coatings incontinuous casting molds).The amorphous coatings family is limited to applica-

tion where the working temperature of the surface iskept below 6008C (in order to avoid crystallization).The electroplated amorphous coating candidate them-selves as hard-Cr plating alternatives, but not for high-

speed caster where maximum peak temperature couldbe harmful for the coating.The working life of a mold depends on several

different variables(steel grade, quality standards, moldfluxes, casting parameters, etc.), but coatings can givea significant impulse toward high wear resistance, chem-ical inertness and higher productivity through increasedmachine availability and through strand retention ofnominal dimensions.

VitaeAlejandro Sanz: Materials Science Engineering

Degree at the Simon Bolivar University. Master Science(DEA) at the Ecole Nationale Superieure de Chimie´(Institut National Politechnique). Ph.D. at Ecole Nation-al Superieure de l’Aeronautique et de l’Espace. Current´ ´position executive manager of materials developmentfor the Danieli & C group.

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