D velopCen ratiHardwa

The galvanizing of steel sheet by continu­ous hot dipping in a molten bath of zinc

containing various amounts of aluminum isthe most efficient and economical method ofproviding corrosion protection to most steelcompositions. However, the performance ofgalvanizing molten· metal bath hardware canstrongly influence both the downtime experi­enced by a line and the coating quality.Typical galvanizing lines operate for an aver­age of two weeks prior to required downtimefor hardware maintenance. In 1996 and 2001,the International Lead Zinc Research Organi­zation (ILZRO) in Research Triangle Park,N.C., conducted two surveys among worldwidecontinuous hot dip production lines. The sur­veys' results show that, for most companies,the most frequent cause of line stoppage is pothardware problems that are related to one ormore of the following three issues:

• Performance of bearings supportingrotating components, such as the sinkroll, stabilizer roll and deflector roll.

• Corrosion of pot hardware in moltenzinc, including corrosion of materialssubjected to sliding contact.

• Nucleation and growth of dross (inter­metallic particles) on pot hardware,especially roll surfaces causing cosmeticdefects in strip coating.

The ILZRO surveys, which included hotdip production lines around the world, alsosummarized the state of the art in materialstechnology for molten metal bath hardware.The rolls, including sink rolls and stabilizerrolls, are made. primarily of stainless steel316L, which is a low-carbon version of 17Cr­12Ni austenitic grade. Stainless steel 316L isalso used for snout tips. Roll bearings are gen­erally made of a cobalt-based superalloy, com-

of the Nextath

t i Is

mercia~ly trademarked as Stellite. In somecases, WC-Co coatings are applied to the rollsurface for improved resistance to wear andcorrosion.

Many research efforts on various aspects ofcontinuous hot operation have been reported

A fr.equent cause of line stoppage on galvanizing lines

is pot hardware problems. A cooperative program to

improve pot· hardware materials and designs has led

to significant cost savings for companies that operate

galvanizing lines, along with many of their suppliers.

from Europe, Australia and China in pastyears. Some examples are studies on the cir­culation patterns of molten metal in the zincmolten metal bath of a continuous strip galva­nizing line. Surface segregation of minor ele­ments, including C, Si, Mn, P, S, Cr, Ni, AI, Cuand Ca, affecting the quality of hot dip coat­ing has also been reported. Steel companiesin Japan conducted research on the develop­ment of molten metal bath hardware. KobeSteel has attempted the development ofHVOF (high-speed oxygen flame) .sprayedWC-Co coating for sink rolls in a galvanizingbath. Hitachi Ltd. has investigated the feasi­bility of using ceramics or ceramic-coatedsteels for the molten metal bath hardware inhot dip conditions.

Since 2001, a more fundamental approachto the improvement of pot hardware materialsand designs, aimed at creating entirely newclasses of pot hardware materials, has beenunder way in a cooperative program co-fund­ed by the U.S. Department of Energy with a

Authors

Ever J. Barbero, professor and chairman, Dept. of Mechanical and Aerospace Engineering, Carl Irwin, director ­Industries of the Future-West Virginia Program, and Xingbo Uu, research assistant professor, West Virginia University,Morgantown, W.Va.; Vinod Sikka, materials processing group leader, Oak Ridge National Laboratories, Oak Ridge,Tenn.; and Frank Goodwin (pictured), vice president of materials science, International Lead Zinc ResearchOrganization, Research Triangle Park, N.C. (ever.barbero@mail.wvu.edu)

Iron and Steel Technology 10 (2004) 31-37. October 2004 .. 31

Co-sponsors of the "Improved Materials for PotHardware" Program

Company

AK Steel Corp.

ASB Industries

Bethlehem Steel Corp. (now ISG)

California Steel Industries

Deloro Stellite

Duraloy Electroalloys

Ellison Surface Technologies

Fontaine Engineering

ILZRO

Metaullics Systems

National Steel Corp. (now U. S. Steel)

Praxair

Steel Dynamics

Stoody Thermadyne

Teck COMINCO

Vesuvius

Location

Middletown, Ohio

Barberton, Ohio

Bethlehem, Pa.

Fontana, Calif.

St. Louis, Mo.

Scottdale, Pa.

Hebron, Ky.

Bridgeport, W.Va.

Research Triangle Park, N.C.

Solon, Ohio

Mishawaka, Ind.

New Castle, Pa.

Butler, Ind.

Bowling Green, Ky.

Toronto, Ont., Canada

Beaver Falls, Pa.

total budget of $4.6 million. The goal of thisprogram is to increase the lifetime of pothardware components. Along with improvingquality, such hardware improvements wouldhave a significant impact on costs; a typicalannual cost for line downtime for a NorthAmerica galvanizing line is about $800,000per year. Thus, for the 57 hot dip galvanizinglines operating in the U.S., this figure wouldbe calculated as $45.6 million per year.Cooperating in this program are 19 sponsors,listed in Table 1. These include U.S. steelcompanies that operate galvanizing lines,along with many of their pot hardware andcoating suppliers.

The research team includes West VirginiaUniversity (WVU), Oak Ridge NationalLaboratory (ORNL) and the InternationalLead Zinc Research Organization (ILZRO). Asteering committee will manage all aspects ofthe project. The national laboratory, universi­ty research organization and industry partici­pants will carry out the technical aspects of theproject. Figure 1 illustrates the project man­agement and coordination plan.

Organizational plan for project coordination and management.

I Steering Committee IH )~ ~ ~

, t W ~

Oak Ridge West International LeadNational Virginia Steel Zinc Research

Laboratory Futures Organization

Steel Companies SuppliersBethlehem Steel Duraloy

West Virginia • • California Steel StoodyUniversity Steel Dynamics

GalvProNational SteelWeirton SteelWheeling-Nisshin

Project Priorities and Work PlanThis program focuses on three topics, eachrelated to a specific galvanizing bath composi­tion. Bearing wear is being considered mainlyin galvanizing baths, with minor work alsobejng done in galvanneal baths. Bearing mate­rials include both monolithic materials, suchas metal alloys and ceramics, and coated mate­rials using plasma spray and other processes.The second topic is dross buildup, largelyfocusing on galvanneal baths with a minoremphasis on galvanizing baths. Here themechanisms of dross buildup are being stud­ied along with optimization of coatings for rollsurfaces that can minimize dross buildup. Thethird topic, being given less emphasis than theother two, is dross formation in the Galvalumebath. Here, phase relations are of importance,as well as the interaction of roll surfaces withGalvalume dross to minimize Galvalume linestoppages.

To address these priorities, a total of sixtasks are under way, as listed in Table 2. Tasks1, 4 and 6 are led by West Virginia University,while tasks 2 and 3 are led by Oak RidgeNational Laboratory. West Virginia Universityis also cooperating with six steel mills on Task5, involving in-plant testing and trials of newmaterials. Figure 2 shows the work breakdownstructure of the program.

As seen in the breakdown, there are threerounds of testing being conducted, with eachround lasting about six months. The firstround of testing focuses on the current com­mercial available materials and sets up thebaseline of the materials development, while

Follansbee, W.Va.

Weirton, W.Va.

Weirton, W.Va.

t

West Virginia Steel Futures

Wheeling-Nisshin

Weirton Steel (now ISG Weirton)

32 ... Iron & Steel Technology

" Taliler~"_ ,u~

...- ~ v ;.

Tasks of the Research Program, "Improved Materials for Pot Hardware"Task No. Title Investigator

Failure analysis and materials characterization West Virginia University

2 Materials and process modeling Oak Ridge National Laboratory

3 Materials development Oak Ridge National Laboratory

4 Materials testing and analysis West Virginia University

5 In-plant testing and trials Joint with sponsors

6 Meetings and reports West Virginia University

the second and third rounds of testing focuson the newly developed materials.

nism of dross buildup on Galvalume rolls canalso be found.

Note: * (decision points: go/no go)

Materials Testing and AnalysisThere are four groups in the research teamconducting testing and analysis of materials forpot hardware application. The lab scale liquidmetal corrosion test is carried out by OakRidge National Laboratory, while the in-plant

ID Task Year 1. Year 2 Year 3

1 Failure Analyses andMaterials Characterization

1.1 Industrial survey and review

1.2 Failure analyses andmaterials characterization

2 MaterialslProcess Modeling

2.1 Thennodynamics andkinetics n10deling

3 Materials 'Development

3.1 Bulk alloys

3.2 Surface treatlnents/coatings

4 Corrosion Testing/Analyses

4.1 Coupon testing

4.2* Lab scale dynamic testing

4.3 Friction and wear testing

5 In-Plant Testing and Trials

5.1* Pilot scale Inaterials testing

5.2 Component testing

6 Meetings and 'Reports

6.1 Hold at least two technicalmeetings per year

6.2 Complete final report

Failure Analysis and MaterialsCharacterizationSeveral program sponsors generously donat­ed to this program a number of used pothardware materials for study. Early .in theexamination of pot roll materials, it becameclear that dross buildup occurred inareas where the pot roll surfacedoes not touch the strip. Even smallareas of noncontact between stripand roll surface become locationswhere dross buildup occurs. Whenbuildup first occurs, the dross parti­cles are isolated from each other bythe surrounding liquid zinc. Withtime , additional dross particlesbuild up and the roll surfacechanges to an area with increasinglyless liquid zinc fraction. As drossparticles build up on each other,the liquid fraction moves progres­sively outward to the new contactingsurface. Thus, cross-sections ofdross buildup on rolls that havebeen in service for a time show acompletely solid layer of built-updross at the roll/dross interface.

Moving outward toward the built­up surface, the liquid fractionbecomes increasingly higher, out tothe roll surface where a dilute mix­ture of interconnected dross parti­cles are mixed with the galvanizingalloy. This is the surface conditionthat steel strip sees when it passesover the pot rolls in areas ofbuildup. The dross particles in thesemisolid pot roll surface layer canoccasionally break off and adhere tothe strip surface, or compact and bemore massive, depending on localequilibrium conditions. This is illus­trated in Figure 3, where bottomdross on both the sink and stabilizerrolls ~ apparenL A ~milar mech~ _~_o_r~k_b~r_e~a_k_d_o_w_n~~_r_u_ct_u_r_e_.~~~~~~~~~~~~~~~~~~~~_

October 2004 • 33

100

screen and evaluate the pot hard­ware materials. The results of thefirst round of tests are reported andsummarized as follows.

In-plant Liquid Metal Corrosion Test - Thefirst round of candidate materials were chosenfor testing. These included four substratealloys and three hardfacing alloys. Initial trialsat the six cooperative galvanizing lines of thisstudy were conducted with these test materi­als. The initial substrate alloys were chosen as:

• CF-3M, the cast variant of stainless steel316L, which is a baseline roll materialfor testing in the program.

• Stellite 6, a cobalt-based superalloy thatserves as the baseline material for bear­ing materials.

Lab-scale Liquid Metal CorrosionTest - Commercially availablematerials, along with a wide range ofinitial candidate materials, wereinvestigated using static corrosiontesting in liquid zinc at Oak RidgeNational Laboratory. Rankings of allmaterials tested with a preoxidationtreatment are shown in Figure 4.Their superior performance com-

pared to the as-machined specimens can beseen by comparing Figure 4 to 5. Of note isalloy Tribocor 532N, an alloy consisting of50% Nb-30% Ti and 20% W, as well as Alloy 4,containing 20% Cr-6.5% Al-0.5% Ti, with thebalance being Fe, along with small additionsof Si, Mn, C and Y. Both of these alloys weredeveloped at Oak Ridge National Laboratory.The other compositions shown in Figure 5 arecommercially available. To meet the goal ofthe program, which was significant improve­ment in pot hardware performance, it wasdeemed necessary that the testing materialshave significant improvement over the per­formance of stainless steel 316L in the staticcorrosion tests, at least for screening purpos­es. This means that the material would needto have a corrosion rate of around 10 mg/m2

in 500 hours. Seven materials from the statictests have been found to achieve this:Tribocor with a nitrided surface, the ACDceramic, Tribaloy T800, a tungsten-20%molybdenum weld overlay, the Metaullics2012 and 2020 alloys, and a tungsten weldoverlay. If a preoxidation treatment is given,then nine additional materials also meet thistarget, and all are variants of Oak RidgeNational Laboratory's Alloy 4 with oxidationtreatments at 1,100°C between one and twohours. Such treatments are compatible withpreheats given by line operators to pot hard­ware rigs before they are brought into service.

101<0

(b)(a)

Alloy Ranking

500h Static test inZn-O.16AI at 465°C

20

40

corrosion test is conducted by WVU. In addi­tion, two sets of bearing and pot roll surfacetesting apparatus have been developed byWVU. One of the bearing testers, originallyconstructed by Duraloy Technologies to con­duct immersion testing on real-scale bearingsamples in a 500-pound zinc pot, was donatedto WVU' at the beginning of the project. It wasmodified to allow roll surfaces to oppose eachother in the zinc bath, thus modeling drossbuildup conditions. Another small lab-scalebearing tester was constructed for screeningexperiments at WVU. This consists of a ball oftest material that is pressed against a socket ofa second material, all immersed in liquid zinc.Wear between the ball and socket and the fric­tion coefficient will be measured in order to

N'-"" 80

Eo'""'-0)

E·'-'" 60.cool{)

c:eneno

...J

Alloy ranking after ORNL lab-scale static corrosion test, preoxidationtreatment condition.

Dross buildup in GI bath: (a} Zn bath side and (b) roll face side.

~4 .... 'rnn l\J <::tAAI T~rhnnlnD\l

1IIIIaII _

oo<:t

100

10 10 f"- CC) 0')

0 ci0

"0

~ ~~

1:CD 0 en 0>

~J:s 8 > ~ ~ az 0 N @ a> as -J t-O) "0 @ > C5 0 (i; U ~

D- C') l co v ~ ~ CD

~ 0 >-~

@ 0 CD Z .1Il co co ...t ;; >- ~0 0 >- >- "0 os 0 CJ)

~>-

~>. >.

Q) .Q ....I >- (,)CD < :a .2 .Q Q;

~ 9 C\I ..Q .Q .QQ. « <0 ..Q .85 'C

0 ;;( « ~~

< i « i « « ~ ~ « 'C0 t- :E CJ) CD t-jg @ 3: en a.~c ~ ~

O'C\IC\I.

500h Static test inZn-O.16A1 at 465°C

200 '"' ·..·?· • ~ ·..,..·i "..i · •..·~· · ~ ..• •.. ~, ..·..· i ·~· · ~ ''' ·r ~..·..· ·~· .." 1 · l'·,,·..•..•..·~· •..! i

300

400 I- ,..~ + ~ ~ ~ ~ · 1·..· ·r· '· !· ~ · ·t· t· t ·..·..~ · ··..·r·..· j, •..•..·"i · ·1 · !

~ 4.E-03 ...--------------­Q)

~ III GI/GA bath Ii~ 2.E-03s::o'(i)o......8 O.E+OO ...---------.

2.E-02-----------------..II GL bath

.J::.ooL()

.5eneno--1+J..c0>

'CDS

Corrosion rates of the alloys in Gl/GA bath.

Alloy Ranking

Alloy ranking after ORNL lab-scale static corrosion test, as-machinedcondition.

• Corrosion in the GL Bath. The corrosionrates of the alloys in the Galvalume bathare shown in Figure 7. Although Stellite 6and MSA 2012 behave very well in GI/GAbath, the corrosion rates of MSA 2012 isthe fastest among the four alloys listed inthe figure, which is around 1.5 x 10-2 inch­es/week. After 12 weeks of immersion inthe GL bath, the lower 3 inches of MSA2012 have disappeared. The corrosionrate of Stellite 6 is about 75 percent ofCF3M, and the rate of ORNL-4 is about 1.4times that of CF-3M.

• Oak Ridge .Alloy 4.• Metaullics Alloy MSA 2012.

The three hardfacing alloys are:

• Laser-clad tungsten carbide.• Spray-coated tungsten carbide.• SiAlON ceramic.

These materials are first being tested by stat­ic immersion at the cooperating galvanizinglines. California Steel Industries and WeirtonSteel are operating galvanizing lines, NationalSteel and AK Steel are operating galvanneal­ing lines .and Wheeling-Nisshin andBethlehem Steel are operating Galvalumelines. The samples are long, thin strips ofeither the substrate material, CF3M spraycoated or laser coated with the tungsten car­bide coating, or Oak Ridge Alloy 4 spray coat­ed with the tungsten carbide coating. Resultsfrom these tests are as follows:

• Corrosion in the· GI/GA Bath. The corro­sion rates of the alloys were calculatedbased on the data at 1 inch from the bot­tom of the specimens. Figure 6 illustratesthe corrosion rates of the base alloys andcoating in the GI/GA bath. There is nomeasurable thickness change for Stellite 6and MSA 2012 specimens immersed up tofour weeks in different baths. In the mean­time, the corrosion rate of CF-3M alloys isaround 3.5 x 10-3 inches/week for all fourGI/GA baths in this investigation. The cor­rosion rate of Spray-WC coating is around1.9 x 10-4 inches/week, which is about1/18 of the corrosion rate of CF-3M.

• Uniform Dissolution Versus SelectiveCorrosion. In general, a weight lossmethod is often used to evaluate the pothardware materials' corrosion resistance,since it is simple and easy to conduct. Thismethod assumes that the corrosion is auniform dissolution process and that theless material that is lost, the better corro­sion resistance it offers. However, theresults from this investigation show thatthere are two kinds of corrosion. The

1.E-02

O.E+OOStellite 6 MSA2012 CF-3M ORNL-4

October 2004 ... 35

Elemental map of Stellite 6 in a GA bath.

SEM/BSI picture of Stellite 6 afterimmerging in a GL bath for four weeks isshown in Figure 8. The bottom right cor­ner is Stellite 6 alloy, and the freezing bathis on the left. There is only one inter­metallic layer between the bath and thealloy, which indicates that the corrosion ofStellite 6 in a GL bath is a uniform disso­lution process. However, contrary to thealloy in a GL bath, the corrosion of Stellite6 in a GI/GA bath is a selective corrosionreaction. Fe and AI in the bath will segre­gate toward the alloy and react with thematrix to form an intermetallic com­pound. Cr and Co in the alloy will diffuseout of the alloy and react with the molten

36 .. Iron & Steel Technology

metal (Figure 9). Therefore, the weightloss method is not a valid corrosion meas­urement method under these conditions,because the procedure is not a uniformdissolution process.

Lab-scale Wear Testing - Screening of candi­date materials was conducted using the WVUsmall-scale zinc pot bearing materials tester.The materials were tested for wear and coeffi­cient of friction. The testing was done whilethe materials were submerged in a 860°F gal­vanize bath. Figure 10 illustrates a comparisonof the wear rates of various material pairs test­ed. All tests were performed at a contact pres­sure of 200 psi and a contact velocity of 10.4inches/second. The test duration for mosttrails was 24 hours. Figure 11 shows the coeffi­cient of friction between the two materialsfound using the lab-scale tester. It can be seenfrom a comparison of friction coefficient andwear rate that like materials produce high fric­tion and low wear, whereas unlike materialshave high wear and low friction.

SummaryReliable performance of galvanizing pot hard­ware is essential to the productivity of a hotdip galvanizing line and the quality of coatingsproduced. The most frequent cause of galva­nizing line stoppage is pot hardware prob­lems. In order to increase the lifetime of pothardware components by an order of magni­tude, since 2001 a cooperative program wasco-funded by the U.S. Department of Energyand the galvanizing-related industries in theU.S., with a total budget of $4.6 million. Inthis paper, the program was briefly intro­duced, along with a summary of the firstround of test results.

Three topics are being focused on in thisprogram: liquid metal corrosion, bearing wearand dross buildup. In the first round of tests,static corrosion tests of CF3M (casting versionof 316L), Stellite 6 and MSA 2012 in industri­al galvanizing (GI), galvanneal (GA) andGalvalume (GL) baths were carried out, andthe samples were investigated by opticalmicroscope and SEM/EDAX. It was foundthat there are two types of corrosion behav­iors: dissolution and diffusion-controlledintermetallic compound formation.

The wear of bearing materials was studiedby two unique test facilities specificallydesigned for this project. One is a small-scalemultifunctional wearing tester, which can testwear rate and friction coefficient of the mate­rials in a short period of time. Another is a500-pound zinc bath to conduct a long-termbearing wear test. In the first round, the wearof several commercially available bearingmaterials was tested, and the microstructure

1IIImmI _

316L StainlessSteel Ball on a316L Stainless

Steel Seat

ILaser-Clad Tungsten Carbide 8al1 on a MSA 2012 Seat

II

I

I

MSA 2020 Ball on a MSA 2012 Seat

iMSA 2020 Ball on a Laser-Clad Tungsten Carbide Seat

IMSA 2012 Ball on a Laser-Clad Tungsten Carbide Seat

IMSA 2012 Ball on a Stellite #6 Seat

I I ILaser-Clad Tungsten Carbide Ball on a Stellite #6 Seat

I I II Stellite #6 Ball on a Stellite #6 Seat

Triballoy T-800 Ball on a Triballoy T-800 Seat! I~

I

o 0.001 0.002 0.003 0.004 0.005 0.006

Wear Rate (inches/hour)

Material wear rates using the WVU small-scale tester.

Material coefficient of friction using the WVU small-scale tester.

Friction Coefficient

0.50.450.40.350.3

Ii Triballoy +-800 Ball

I on a Triballoy T-800

\- II I

MSA 2020 Ball on a MSA 2012 Seat

I iMSA 2012 Ball on a MSA 2012 Seat

II!iII

ILaser-Clad Tungsten

Carbide Ball on a

Stellite #6 Seat

0.25

I

I I I I316L Stainless Steel Ball on a 316L Stainless Steel Seat

0.20.150.10.05

and .wearing surface of the materials

after the test were studied by optical

microscope and SEM. A unique

dross buildup setup that consists of

two sleeves counter-rotating against

each other was used to simulate the

dross buildup in the productionline.

Through collaboration with the

U.S. Department of Energy, the U.S.

steel industry and its suppliers have

opened the possibility of creating an

entirely new class of pot hardware

materials that have the potential to

provide significant performance

improvements over existing materi­

als. The multipronged attack being

taken by Oak Ridge National

Laboratory and West Virginia

University, with additional technical

support from Teck Cominco, has

the potential of meeting these goals,

as already shown by a number of

materials in initial screening tests.

Readers are invited to view the cur­

rent status of the project at:

http://iofwv.nrcce.wvu.edu/index-

steel.cfm. ..

This paper was presented at the 2003 Iron and Steel Exposition

and AISf Annual Convention, Pittsburgh, Pa.

October 2004 .. 37