Applied Surface Science 258 (2012) 4977 4982

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Applied Surface Science

j our nal ho me p age: www.elsev ier .com

A supe deof bear

Xiaojing nga State Key Lab emy ofb Graduate Uni

a r t i c l

Article history:Received 2 NoReceived in re18 December 2Accepted 24 JaAvailable onlin

Keywords:Bearing steelHigh temperatDecarburizatioCoatings

eous erciecarbmatixidat

1. Introduction

In recenwidely usedformance, cHowever, Agenerally caScales are foing in a lowesuch as lowsurface impucts due to datoms fromface with lothe surface.changes of hcase of stee1.5% carbonstrength duDecarburizarates are hnomena: ox

CorresponE-mail add

Therefore, it has a lot of difculties to prevent steel from oxidationand decarburization simultaneously.

0169-4332/$ doi:10.1016/j.t years high carbon chromium bearing steel has been because it has the advantages of comprehensive per-onvenient operation and low production cost [1,2].nnealing and reheating treatments on bearing steels arerried out in furnaces having an oxidizing atmosphere.rmed on the steel materials during their heating, result-red product yield, accompanied with various problems,ering in the commercial value of the products due toerfection and lowering in the strength of the steel prod-ecarburization [3,4]. Decarburization is a loss of carbon

the surface of the work pieces, thereby producing a sur-wer carbon content than at some other distance beneath

It implies changes of mechanical properties as, e.g., theardness and of fatigue resistance [5]. Particularly in the

l materials, such as bearing billets, which contain about, the inuence on the steel quality by the lowering ine to the surface decarburization is very signicant [6].tion and oxidation occur simultaneously. The reactionigh, and there is competition between the two phe-idation rapidly consumes the decarburized metal [3].

ding author. Tel.: +86 10 62588029; fax: +86 10 82544919.ress: sfye@home.ipe.ac.cn (S. Ye).

The aim of this work is to fabricate a high temperature corrosionresistant coating. The microstructure and sintering behaviors of thecoatings were investigated. The corrosion resistant mechanism alsois discussed.

2. Experimental

2.1. Preparation of the coating

The chemical composition (in weight percent) of the bearingsteel used in the present study is C 1.00%, Cr 1.50%, Mn 0.30%, Si0.25%, S 0.01%, P 0.01%, and Fe balance. Specimens with dimensionsof 10 mm 10 mm 10 mm were prepared by rolling and interme-diate annealing (at 500 C) operations. Subsequently, the specimenswere mechanically polished by abrading on SiC papers in succes-sion up to 800 grit followed by cleaning and rinsing in supersonictreatment and dried for coating protective layers.

The coating materials, consisted of dolomite, bauxite and sili-con carbide, were mixed in various proportions. The compositionsof the dolomite and bauxite are listed in Tables 1 and 2. The mix-ture was diluted with water to secure the proper viscosity, whichenabled deposition by painting or spraying. The addition of thebinding agent (citric acid) was necessary for good adhesion to thesurface of the steel after drying.

see front matter 2012 Elsevier B.V. All rights reserved.apsusc.2012.01.135rcial coating to improve oxidation anding steel at high temperature

Wanga,b, Lianqi Weia, Xun Zhoua,b, Xiaomeng Zhaoratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Acadversity of Chinese Academy of Sciences, No. 19(A) Yuquan Road, Beijing 100049, China

e i n f o

vember 2011vised form011nuary 2012e 2 February 2012

ure oxidationn

a b s t r a c t

The coating material consisted of aqua coating material when applied suptance but also helps in inhibiting the dthermal analysis revealed that the forreducing atmosphere formed by the oization./ loc ate /apsusc

carburization resistance

a, Shufeng Yea,, Yunfa Chena

Sciences, P. O. BOX 353, Beijing 100190, China

slurry of dolomite, bauxite and silicon carbide mixture. Suchally on the steel surface not only enhances oxidation resis-urization even up to 1250 C. Metalloscope, XRD and TG-DTA

on of a newly densied coating comprised of spinels and theion of SiC improved the resistance of oxidation and decarbur-

2012 Elsevier B.V. All rights reserved.

4978 X. Wang et al. / Applied Surface Science 258 (2012) 4977 4982

Table 1Compositions of the dolomite.

Component MgO CaO SiO2 Fe2O3 Al2O3

Content [wt.%] 39.82 58.72 0.64 0.46 0.36

Table 2Compositions

Component

Content [wt

2.2. Experim

Two typweighted, hand then w

m = m1

Steel loss

Anti-oxidat

where m1 aoxidation wof the bare

Every spthem was petched by 4Finally, thefor metallolayer.

Anti-decarb

where dbarethe coated s

2.3. Charac

Oxidatiocarried outwhich was sensitivity oemployed wEnergy chaTG-DTA thement Factoobserved bLP/P, Leica,Pro, Philipsphases in th

3. Results

3.1. The effe

The coateffects of a60 min wer

Dolomitnesium oxiframeworkdecarburizaadmixed in

Table 3Compositions and the corrosion resistance of the coatings.

Parts by weight A B C D E

Dolomite 64 64 64 64 64 carbide 20 20 0 20 40e 20 0 20 40 20acid 7 7 7 7 7d water 29 25 25 37 37xidation effect, 68.00 64.00 56.00 48.00 68.00

ecarburizationct, %

100.00 14.03 3.36 100.00 100.00

use the ne SiC powder was oxidized and changed gradu-protective cristobalite-SiO2 layer which acted as an excellent

to oxygen diffusion from atmosphere. The protective SiO2t formed from the SiO2 which was added initially but wasformed through the oxidation process of the SiC. Graphites also formed due to the oxidation of SiC, keeping a reduc-osphere even at quite high temperatures [7]. The presenceiderable graphite in this way indicated that the diffusion ofO is .parablyr ante and2O3, eveermbecaby trature (2Ftic wxide

did ned t

expltion. tical carbual de

e eff

ting g temm bo

of thof the bauxite.

Al2O3 SiO2 TiO2 Fe2O3 CaO K2O

.%] 63.96 29.15 3.36 2.41 0.76 0.36

ental method

es of bearing steel specimens, bare and coated, wereeated for 0120 min at temperatures of 9501250 C,eighed again without scale.

m2 (1)

= m1 m2m1

100% (2)

ion effect oxi =mbare mcoated

mbare 100% (3)

nd m2 are the weights of the specimens before and afterithout scale, g. mbare and mcoated are the weight lossspecimen and the coated one.ecimen was cut into two parts by cutter bar and one ofolished by abrading on SiC papers. The specimens were

vol% admixture of nitric acid and absolute ethyl alcohol.y were dehydrated by absolute ethyl alcohol and usedgraphic observation to determine the decarburization

urization effect dec =dbare dcoated

dbare 100% (4)

and dcoated are the decarburized depths of the bare andpecimens, m.

terization

n-kinetics studies of bare and coated specimens were by a thermobalance (RZ, Luoyang Precondar, China)equipped with a continuous weighing capacity of 500 g,f 1 mg and data recorded every 60 s. The heating rateas 10 C/min up to a nal temperature of 1250 C.

nge with the temperature rising was detected by armal analysis system (TG-DTA, Beijing Scientic Instru-ry, China). Microstructures of the decarburization werey an optical microscope (polarization microscope, DM

Germany). Data of X-ray diffractometer (XRD, XPert, Netherlands) were collected to analyze the change ofe coating.

and discussion

cts of coating components

SiliconBauxitCitric distilleAnti-o

%Anti-d

effe

is becaally to barrierwas nonewly (C) waing atmof consC and vented

Comremarka bettebauxitFeOAl

Howthat ovdown duced tempefayalita eutecof the otance consumcan beburizatheoreical depractic

3.2. Th

Coaheatin

Froweighting materials were mixed according to Table 3 and thenti-oxidation and anti-decarburization at 1250 C fore evaluated.e was chosen as the main raw material because mag-de has high melting point and can be regarded as the

of the coating. As shown in Table 3, the oxidation andtion prevention were obviously improved when SiC isto the coating by comparing the specimens A and C. This Fig. 1impeded and the decarburization was effectively pre-

ed B (without bauxite in the coating), the specimen A increased the decarburization resistance and also hadi-oxidation effect. This is based on the decomposition of

its reaction with Fe and dolomite. The products SiO2,-Al2O3 and spinel were nonpermeable to oxygen [8,9].r, by comparing specimens A and D, it has been founduch bauxite would make the oxidation resistance dropuse the anti-oxidation effect of the silicon oxide pro-he decomposition of bauxite actually deteriorated ates in excess of 1200 C. This is due to the formation ofeOSiO2), which had a low melting point. Fayalite formedith wustite above 1177 C that promoted the rapid grow

by liquid oxide attack [7,10]. The decarburization resis-ot decrease due to the increase of oxide scale whichhe decarburized layer produced below 1177 C [3]. Thisained by the competition between oxidation and decar-As shown in Fig. 1, the practical oxidation equals to theoxidation. The depth difference between the theoret-rization and the theoretical oxidation determines thecarburization.

ects of heating temperature

A in Table 3 was applied in the test and the effects ofperature are shown in Table 4.th the curves in Fig. 2, which shows the difference in thee cleaned specimens before and after heating, it can be. The competition between oxidation and decarburization.

X. Wang et al. / Applied Surface Science 258 (2012) 4977 4982 4979

Table 4The oxidation and decarburization resistance of coating A heated for 0120 min at temperatures of 9501250 C.

Temperature, time oxi , % dec ,% Temperature, time oxi , % dec ,%

950 C, 60 min 44.20 6.45 1250 C, 0 min 67.19 65.141050 C, 60 min 60.43 6.85 1250 C, 30 min 67.17 100.001150 C, 60 min 61.93 7.05 1250 C, 90 min 60.43 100.001250 C, 60 min 64.44 100.00 1250 C, 120 min 63.60 100.00

observed that the steel loss increased with increasing temperature,for both types (bare and coated) of specimens. The steel loss of thecoated specimen was smaller than that of the bare one during theheating process. The amount of SiO2 in bauxite was much higherthan that in mullite; the excess SiO2 together with the impurities inbauxite formed an amorphous SiO2 and cristobalite to accompanythe formation of mullite at temperature about 1000 C [11].

Bauxite is composed of diaspore (Al2O3H2O) and kaolinite(Al2O32SiO22H2O). The reactions (5) and (6) occurred becausealumina and metakaolinite (Al2O32SiO2) were formed when dias-pore and kaolinite were heated up to 500 C [1214]. Heating at980 C leaded to a direct formation of -alumina and amorphousSiO2 or spinel (SiAl2O4) according to reactions (7) and (8). Mullitephase rst appeared at a temperature around 1100 C accordingto reactions (9) and (10), its amount increased with the increaseof temperature. The amorphous SiO2 changed to cristobalite above1200 C according to reaction (11).

Al2O3H2O (Al2O32SiO2

Al2O3

Al2O32SiO2 Al2O3

Al2O32SiO2 SiAl2O

SiAl2O4 (spi

1/3(3A

Al2O3 (-alu

1/3(2A

Fig. 2. Steel loscaling.

3Al2O32SiO2 (mullite) + 4SiO2 (amorphous) 3Al2O32SiO2 (mullite) + 4SiO2 (cristobalite) (11)The amorphous SiO2, cristobalite and mullite had certain effect

in impeding the diffusion of oxygen. However, the coating was notcompact enough that oxidation still existed [15]. At higher tem-peratures, the protection was much effective (Table 4), becauseon one hand, graphite (C) was formed due to the oxidation of SiC,keeping a reducing condition within the coating even at quite hightemperatures [7]. On the other hand, new sintered phases werenonpermeable to oxygen at high temperature. Fig. 3 shows the newphases formed at 1150 C, including several spinels (MgCr2O4, (Mg,Fe) (Cr, Al)2O4, MgAl2O4 and Fe (Cr, Al)2O4) [8,9].

From Fig. 4 it is evident that the decarburization of the barespecimen increased below 1150 C. With increasing temperatureabove 1150 C, the growth rate of the decarburized layer decreased

e thef theraturting

re spre sp, con

coat a ce

the a(4). AurizatingC, asyer, als toary d0 C hdiaspore) Al2O3 (-alumina) + H2O (5)2H2O (kaolinite)

2SiO2 (metakaolinite) + 2H2O (6)

(metakaolinite)

(-alumina) + 2SiO2 (amorphous) (7)

(metakaolinite)

4 (spinel) + SiO2 (amorphous) (8)

nel) + SiO2 (amorphous)l2O32SiO2) (mullite) + 4/3SiO2 (amorphous) (9)

mina) + 2SiO2 (amorphous)l2O32SiO2) (mullite) + 4/3SiO2 (amorphous) (10)

becausscale otempethe coathe baThe ba(Fig. 2)for thetion inphase,to Eq. decarbthe coa1050

ized lamaterisecondat 125ss of GCr15 bearing steel specimens during heating for 60 min after Fig. 3. XRD pa1150 C. diffusions of C and O were retarded by the thick oxide bare specimen in a certain degree when heated at highe [16]. Below 1150 C, the anti-decarburization effect of

was less than 7.05%, because the decarburized depth ofecimen dbare was not large after the scale was cleaned.ecimen generated more serious oxidation on the surfacesuming certain depth of decarburized layer. Althoughed specimen, the coating protected it from decarburiza-rtain degree due to the formation of mullite and glassynti-decarburization effect dec was still small accordingt higher temperature, above 1150 C, the protection oftion was much effective, because the main reactions in, which gave the protective properties, started at around

evident from the DTA curve. The scale and decarbur-formed at the previous stage, reacted with the coating

produce the newly densied lm, which hindered theecarburization. Therefore the coated specimen heatedad no decarburization after scaling.tterns of bare specimen (a) and coated specimen (b) after heating at

4980 X. Wang et al. / Applied Surface Science 258 (2012) 4977 4982

Fig. 4. Decarburization growth of bare and coated specimens during heating for60 min.

Fig. 5 shows the bare specimen experienced endothermic pro-cess at 780 C because of austenization. And at about 950 C, anevident exothermal process happened because of the oxidation. Bycontrast, the coated specimen experienced an endothermic pro-cess below 100 C because of the dehydration of coating. At 175 C

Fig. 5. DTA cupartial enlarge

Fig. 6. Isothermal oxidation kinetics of bare and coated specimens at 1250 C in air.

a stronger endothermic reaction started, assumed to be the meltingprocess of citric acid. The exothermic reactions are observed from450 C to 540 C due to the decarburization of the excessive citricacid and the combustion of the residual carbon [17,18]. However,metakaolinan endothe[1214]. Thdecomposit790 C, wasthe bare sption proces(12). The ereaction (1was overlabe seen at mullite phaaccording t

CaMg(CO3)

CaCO3 C

The reactartelayer srve of specimens with and without coating: (a) complete and (b)d. Fig. 7.ite were formed at around 500 C, therefore, there wasrmic peak separating the exothermal peak of citric acide reactions (12) and (13) are the two steps of thermalion of dolomite. The reaction (12), happened at around

not been clearly expressed in Fig. 5 because similar withecimen, the coated specimen experienced austeniza-s at 780 C and its peak covered the peak of reactionndothermal peak can be seen at around 935 C due to3) and the peak of the decomposition of metakaolinitepped with the reaction (13). The exothermal peak canaround 1100 C and 1200 C because the formation ofses according to reactions (9) and (10) and cristobaliteo reaction (11).

2 CaCO3 + MgO + CO2 (12)

aO + CO2 (13)

tion of coating materials with scale and decarburizedd above 1000 C. Steel loss of GCr15 bearing steel specimens at 1250 C in air.

X. Wang et al. / Applied Surface Science 258 (2012) 4977 4982 4981

Fig. 8. Decarburization growth of bare and coated specimens at 1250 C in air.

3.3. The effects of holding time

Coating A in Table 3 was applied in the test and the effects ofholding time are shown in Table 4.

Fig. 9. Microsfurnace: (a) ba

The isothermal oxidation kinetic curves of bare and coated spec-imens at 1250 C in air for 6 h were plotted as a graph of weight gainversus time and are illustrated in Fig. 6. It can be seen the coatingreduce oxidation rate of GCr15 bearing steel.

Fig. 7 shoquite a lot omarked redcess when tbased on thto oxygen a

It can bbare specimthe contrardecreased wjust increaslayer formematerials, wlow temperformed by certain effestill existed

When thmaterials, din the formously generso compactoxygen at hdepth of theat 1250 C. Icompletelysteel surfac

4. Conclus

1. A novel bauxite, and deca

2. The perfglassy pmullite aoxidationenough t

igheuse iC, kee and

the3. At hbecaof Sscalwithtructure of steel GCr15, after heating for 2 h at 1250 C in a mufere and (b) coated.

above 11could hin

Acknowled

The authKey TopicsAcademy oNational Scyear Plan PScience Fou

References

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[2] S. Gangulanti-frictogy, 2002ws the bare specimens severely suffered oxidation, andf steel was lost in the form of oxide scale. There was auction in loss of steel volume in the whole heating pro-he specimens were coated. The effect of the coating wase formation of spinels, which exhibit low permeabilityt temperature up to 1250 C.e observed in Fig. 8 that the decarburization of theen increased with the extension of holding time. On

y, the decarburization of the coated specimen inverselyith the increase of holding time. When the temperature

ed to 1250 C, the coated specimen had decarburizationd below 1250 C, because the reactions of the coatinghich produced new densied coating, did not start at

ature. Despite the glassy phase, cristobalite and mullite,the decomposition of bauxite at low temperature, hadct in impeding the diffusion of oxygen, decarburization

when holding time was less than 30 min.e holding time was prolonged at 1250 C, the coatingecarburization layer and the scale quickly took partation of effective lm and the decarburization, previ-ated, disappeared quickly as a result. Then the lm was

that it blocked the secondary diffusion of carbon andigh temperature. Fig. 9 shows that the decarburization

bare specimen reached 839.51 m after heating for 2 hn contrast, the coated specimen had no decarburization. The coating could diminish the decarburization on thee for up to 100% (Table 4).

ions

coating was successfully fabricated by using dolomite,silicon carbide and binding agent to prevent oxidationrburization during reheating process of bearing steels.ormed tests conrmed that SiO2 in bauxite formed ahase and cristobalite to accompany the formation oft 1000 C. The products had certain effect in preventing

and decarburization, but the coating was not compacthat oxidation and decarburization still existed.r temperature, the protection was much effective,on one hand, graphite was formed due to the oxidationeping a reducing atmosphere. On the other hand, the

decarburized layer, formed at previous stage, reacted coating materials to produce the newly densied lm50 C. The lm was mainly composed of spinels and itder the secondary oxidation and decarburization.

gments

ors gratefully acknowledge the nancial supports from in Innovation Engineering Funded by the Chinesef Sciences (No. KGCX2-YW-224), Key Projects in theience & Technology Pillar Program in the Eleventh Five-eriod (No. 2006BAC02A14), and the National Naturalndation of China (No. 50774073).

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A superficial coating to improve oxidation and decarburization resistance of bearing steel at high temperature1 Introduction2 Experimental2.1 Preparation of the coating2.2 Experimental method2.3 Characterization

3 Results and discussion3.1 The effects of coating components3.2 The effects of heating temperature3.3 The effects of holding time

4 ConclusionsAcknowledgmentsReferences