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Wear 274 275 (2012) 84 93

Contents lists available at ScienceDirect

Wear

j o ur nal ho me p age: www.elsev ier .com/ locate /wear

omparative research on wear characteristics of spheroidal graphite cast ironnd carbon steel

.X. Weib, S.Q. Wanga,b,, X.H. Cuib,

School of Environment and Materials Engineering, Yantai University, 264005 Yantai, ChinaSchool of Materials Science and Engineering, Jiangsu University, 212013 Zhenjiang, China

r t i c l e i n f o

rticle history:eceived 27 December 2010eceived in revised form 2 August 2011ccepted 8 August 2011vailable online 25 August 2011

a b s t r a c t

Wear characteristics of a spheroidal graphite cast iron and a carbon steel were studied under atmosphericconditions at 25400 C. The spheroidal graphite cast iron presented obviously different wear behaviorsfrom the carbon steel, which may be attributed to the presence of graphite. With an increase of ambienttemperature, tribo-oxides of carbon steel substantially increased and its substrate softened, thus severewear, oxidative mild wear, oxidative wear and extrusive wear took turns to prevail. However, comparedeywords:liding wearteelast ironribochemistry

with carbon steel in the same case, tribo-oxides were markedly reduced in the spheroidal graphite castiron, thus oxidative mild wear and oxidative wear did not appear due to the lack of oxides. It is suggestedthat less tribo-oxides in the spheroidal graphite cast iron may be attributed to the reduction of graphiteto tribo-oxides during sliding.

2011 Elsevier B.V. All rights reserved.

lectron microscopy

. Introduction

FeC alloys are oxidized due to their thermodynamically insta-ility in air. During sliding, especially at a higher sliding velocity,ormal load and/or ambient temperature, oxidation is intensifiednd tribo-oxides form on worn surfaces [1,2]. The developmentf a tribo-oxide layer would markedly affect wear behaviors andechanisms of FeC alloys [14]. Hence, oxidative wear plays an

mportant role in the wear process, and the research on oxidativeear requires more attention.Steels and graphite cast iron as important FeC alloys are

opularly adopted in various engineering applications. Extensiveesearch on oxidative wear of steels has been carried out and theear behavior and mechanism of steels are clearly understood.owever, there has been less study of graphite cast iron and hencehe wear behavior and mechanism of graphite cast iron are lesslear.

Quinn et al. studied the oxidative wear of ferroalloys under mild

ear conditions at room- and slightly elevated temperatures androposed a mild oxidative wear model [1]. Wilson et al. studiedhe influence of wear oxide debris particles on reducing wear and

Corresponding author at: School of Environment and Materials Engineering,antai University, 264005 Yantai, China. Tel.: +86 5356706039;ax: +86 5356706039. Corresponding author. Tel.: +86 51188797618; fax: +86 511 8791919.

E-mail addresses: shuqi wang@ujs.edu.cn (S.Q. Wang),ueixianghong@sohu.com (X.H. Cui).

043-1648/$ see front matter 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2011.08.015proposed another important oxidative wear model [2]. The wearoxide debris particles have been shown to form nano-structuredglaze layers at a higher temperature to enhance wear resistance[3]. They shared the same viewpoint that the tribo-oxides play animportant role in oxidative wear; tribo-oxides can form on wornsurfaces and prevent the metalmetal contact and adhesion, thusreducing wear rate.

Most subsequent studies on oxidative wear originated fromQuinn et al.s work, and tribo-oxides were found to reduce wearby forming a protective layer [46]. In most cases, oxidative wearmeans a mild wear. However, when the matrix does not haveenough strength to support tribo-oxide layer, tribo-oxides may notreduce wear and tribo-oxides were reported not to prevent wearin some studies [7,8]. Inman et al. pointed out that when no glazeformed due to worse sinterablity of oxides, severe wear prevailed[9]. Wang et al. further studied oxidative wear of steels in severetesting conditions and pointed out that tribo-oxides did not alwaysprevent wear, especially when the mild-severe wear transitionoccurred in oxidative wear [10,11]. Additionally, we suggested thatthe two types of oxidative wear should be distinguished and termedoxidative mild wear and oxidative wear [12]. Inman et al. also sug-gested a division of oxidative wear/mild wear into abrasive mildwear and protective mild wear as well as severe wear into standardsevere wear and abrasion-assisted-severe-wear [9,13]. Abrasion-

assisted-severe-wear meant that in some cases tribo-oxides did notprevent wear, on the contrary accelerated wear.

Riahi and Alpas [14] produced the first wear map for grey castiron, where wear mechanisms were classified into ultra-mild, mild

dx.doi.org/10.1016/j.wear.2011.08.015http://www.sciencedirect.com/science/journal/00431648http://www.elsevier.com/locate/wearmailto:shuqi_wang@ujs.edu.cnmailto:cueixianghong@sohu.comdx.doi.org/10.1016/j.wear.2011.08.015

M.X. Wei et al. / Wear 274 275 (2012) 84 93 85

Table 1Chemical compositions of the steels and spheroidal graphite cast iron used in this study.

Materials Element (wt.%)

C Cr Mo V Si Mn S P

0.25 0.15

aewdtiastscrrgwissdeeoigWbmca

towwaowoib

800-2 3.403.80 1045 0.420.50

86 M.X. Wei et al. / Wear 274 275 (2012) 84 93

Fig. 2. Wear rate vs. load curves of the carbon steel (a) and spheroidal graphite cast iron (b) sliding at various ambient temperatures.

mH

3

3

apartwi1(

tw4s

spgs

icroscope (SEM). Hardness of pin specimen was measured by anR-150A type Rockwell apparatus.

. Results and analysis

.1. Wear and friction behaviors

Fig. 2 shows wear rate vs. load curves of the carbon steelnd spheroidal graphite cast iron sliding at various ambient tem-eratures. The wear rate of the carbon steel at 25 C increasedpproximately linearly with increasing load. At 200 C, the wearate abruptly decreased as the load was raised from 50 to 100 N,hen increased again slightly with further increasing load. Theear rate at 400 C increased gradually at first with increas-

ng load, then started to rise rapidly once the load surpassed25 N, exceeding the detection limit of the testing machineFig. 2a).

At 25200 C, the spheroidal graphite cast iron presented almosthe same wear rate vs. load curves, only the wear rate at 200 Cas slightly higher than that at 25 C. However, the wear rate at00 C was markedly higher than those at 25200 C, and increasedharply once the load surpassed 125 N (Fig. 2b).

Comparisons of wear rates between the carbon steel and

pheroidal graphite cast iron sliding at various ambient tem-eratures are shown in Fig. 3. The wear rate of the spheroidalraphite cast iron at 25 C was lower than that of the carbonteel at loads below 150 N, but higher once over 150 N (Fig. 3a).

Fig. 3. Comparisons of wear rate between the carbon steel and spherAt 200 C, the same phenomenon happened again, only the tran-sition load was 100 N (Fig. 3b). At 400 C, the wear rate ofthe spheroidal graphite cast iron was marginally higher thanthat of the carbon steel at loads below 100 N, and their dif-ference gradually became greater with further increasing load(Fig. 3c).

The variation of friction coefficient with sliding distance for thecarbon steel and spheroidal graphite cast iron sliding under dif-ferent conditions is shown in Fig. 4. In most cases, the spheroidalgraphite cast iron presented a lower friction coefficient than thecarbon steel. An increase in load resulted in a decrease in frictioncoefficient of the materials at every ambient temperature. The fluc-tuation of friction coefficient of the carbon steel was slight at 25 Cand under 150 N at 400 C, enlarged under 100200 N at 200 C andunder 50100 N at 400 C. The spheroidal graphite cast iron wasof small fluctuation of friction coefficient in most cases and pre-sented a large fluctuation of friction coefficient merely under 50 Nat 400 C.

3.2. XRD analysis for worn surfaces

X-ray diffraction patterns of worn surfaces of the carbon steeland spheroidal graphite cast iron sliding under various condi-

tions are shown in Fig. 5. At 25 C, only trace tribo-oxides wereidentified on worn surfaces of the carbon steel. The amount oftribo-oxides gradually increased with increasing ambient tempera-ture from 25 C to 200 C; a small amount of tribo-oxides appeared

oidal graphite cast iron sliding at 25 (a), 200 (b) and 400 C (c).

M.X. Wei et al. / Wear 274 275 (2012) 84 93 87

(c) an

ats1Itost4

3

ssa(wra2atwadl(

a4ocaw

Fig. 4. Friction coefficient vs. sliding distance curves of the carbon steel (a)

t 200 C (Fig. 5a and b). However, at 400 C, a large amount ofribo-oxides were generated; the peaks corresponding to oxidesignificantly surpassed those of iron. But once the load reached50 N, the amount of tribo-oxides markedly decreased (Fig. 5c).t was observed in the spheroidal graphite cast iron that merelyrace tribo-oxides formed at 25200 C and a small amount of tribo-xides appeared at 400 C (Fig. 5df). Compared with the carbonteel, the amount of tribo-oxides generated on worn surfaces ofhe spheroidal graphite cast iron was obviously reduced, even at00 C.

.3. SEM morphology of worn surfaces

SEM morphologies of worn surfaces of the carbon steel andpheroidal graphite cast iron sliding under various conditions arehown in Figs. 6 and 7, respectively. At 25 C, obvious adhesivend a trace of abrasive mark were identified in the carbon steelFig. 6a and b). In addition, tribo-oxides sparsely scattered on theorn surface under high load, which can be confirmed by XRDesults and section morphology of worn surfaces. The amountnd area of tribo-oxides increased as the load surpassed 100 N at00 C; tribo-oxide layer formed with some delaminated cratersnd a trace of abrasive mark (Fig. 6c and d). At 400 C, a smoothribo-oxide layer with delaminated craters was found across thehole worn surface (Fig. 6e and f). However, at a load of 150 N,ny trace of smooth tribo-oxide layers and delaminated cratersisappeared, instead only a small amount of fine grooves paral-el to the sliding direction and many tiny oxide particles appearedFig. 6g).

For the spheroidal graphite cast iron at 25200 C, adhesive traceppeared without tribo-oxides on worn surfaces (Fig. 7ad). At00 C, although a small, sparse amount of tribo-oxides appeared

n the partial worn surface, adhesive was still predominant wornharacteristic (Fig. 7e). As the load reached 150 N, the worn surfacelso became smooth just as happened in the carbon steel, but thereere no oxide particles (Fig. 7f).d the spheroidal graphite cast iron (d)(f) sliding under various conditions.

3.4. Section morphology of worn surfaces and subsurfaces

In order to ascertain the formation of tribo-oxides on worn sur-faces, section morphologies of worn surfaces and subsurfaces ofthe carbon steel and spheroidal graphite cast iron sliding undervarious conditions were investigated, as shown in Figs. 8 and 9,respectively. For the carbon steel at 25 C, tribo-oxides did not formunder a load of 50 N (Fig. 8a), but appeared under a load of 100 N andhigher. Such oxides were distributed sparsely on worn surfaces andreached a thickness of less than 3 m (Fig. 8b). At 200 C, no tribo-oxide layer appeared was observed under a load of 50 N (Fig. 8c),however, once the load reached 100 N or above, a tribo-oxide layerwith a thickness of about 810 m existed (Fig. 8d). At 400 C, alarge amount of tribo-oxides formed and the thickness increasedto 2025 m (Fig. 8e). However, as the load reached 150 N, thethickness of tribo-oxide layer substantially decreased to less than0.5 m (Fig. 8f). This may explain why the peaks corresponding toiron surpassed those of oxides (Fig. 4c).

For the spheroidal graphite cast iron, no tribo-oxide layerappeared on worn surfaces at 25 C (Fig. 9a and b). At 200 C, therewas reduced tribo-oxides formation (compared to carbon steel)with a reduced thickness of less than 3 m (Fig. 9c and d). Dueto the presence of graphite in the spheroidal graphite cast iron,rupture seemed to appear readily on the worn surface, as shownin the inserted image of Fig. 9b. At 400 C, the amount of tribo-oxides correspondingly increased, and tribo-oxide layer was stillthin (less than 5 m) and discontinuous (Fig. 9e and f). At 150 N, theelongated graphite appeared companied by massive plastic defor-mation in the sub-surface region.

4. Discussion

At 25 C, almost no or trace tribo-oxides appeared on worn sur-

faces of the carbon steel and spheroidal graphite cast iron, thusmetalmetal contact was not avoided. Shear stress resulted fromfriction force would cause deformation of contacting asperities,leading eventually rupture inside the softer pin resulting in mass

88 M.X. Wei et al. / Wear 274 275 (2012) 84 93

(c) a

lamihsiwcgd

Fig. 5. X-ray diffraction patterns for worn surfaces of the carbon steel (a)

oss [18,19], thus a severe wear prevailed. Archard [19] proposedn equation to evaluate wear resistance in this regime, whereaterial wear loss is directly proportional to normal load and slid-

ng distance, but inversely proportional to the softer materials realardness (the pin specimen in this study). Although the carbonteel and spheroidal graphite cast iron possessed the same orig-nal hardness (40 HRC), they presented different wear behaviors,

hich might be due to the graphite in the spheroidal graphiteast iron. At loads below 150 N, the wear rate of the spheroidalraphite cast iron was slightly lower than that of the carbon steelue to graphite lubrication. However, on application of a 200 Nnd spheroidal graphite cast iron (d)(f) sliding under various conditions.

load, the lubrication effect lessened and even disappeared. In thiscase, the presence of graphite led to easy fracture of worn surfacesand the wear rate of the spheroidal graphite cast iron thus rapidlyincreased.

For carbon steel, as ambient temperature was elevated to200 C, severe wear still operated under 50 N load. With increas-ing load, more tribo-oxides started to form on worn surfaces due

to increased frictional heating. These tribo-oxides did not cover thewhole worn surface, and preferentially formed at contacting asper-ities [10]. When the tribo-oxide layer reaches a thickness of about10 m, they could prevent metalmetal contact and adhesion,

M.X. Wei et al. / Wear 274 275 (2012) 84 93 89

F itions(

a[Qcai

s

ig. 6. Morphologies of worn surfaces of the carbon steel sliding under various condf) 100 N, 400 C; (g) 150 N, 400 C.

nd protect the underlying material against mechanical damage10,11]. In this case, the oxidative mild wear regime proposed byuinn et al. [1] and Wilson et al. [2] prevailed. The wear rate of thearbon steel therefore dropped abruptly on a raising load to 100 N

nd above this, the wear rate increased slowly and steadily withncreasing load.

However, tribo-oxides generated on worn surfaces of thepheroidal graphite cast iron were too little and to thin to avoid: (a) 50 N, 25 C; (b) 200 N, 25 C; (c) 50 N, 200 C; (d) 200 N, 200 C; (e) 50 N, 400 C;

metalmetal contact and adhesion, thus the severe wear stilldominated. During sliding, the increasing friction heat and highambient temperature would result in the decrease in the hardnessof worn surface and subsurface, thus the wear rate of the spheroidal

graphite cast iron at 200 C further increased compared with thatat 25 C. Therefore, the original material hardness in Archardsequation [19] should be replaced by the real surface hardness. Insevere wear of the spheroidal graphite cast iron at 200 C, graphite

90 M.X. Wei et al. / Wear 274 275 (2012) 84 93

F der va(

lwo

asamipNpmo[miotl

ig. 7. Morphologies of worn surfaces of the spheroidal graphite cast iron sliding une) 50 N, 400 C; (f) 150 N, 400 C.

ubrication still functioned and affected wear behavior. Thus, theear rate of the spheroidal graphite cast iron was lower than thatf the carbon steel at loads below 100 N.At 400 C, a dense glaze tribo-oxide layer with a thickness of

bout 2025 m formed over the whole worn surface of the carbonteel, thus the oxidative mild wear prevailed at low load. In such

case, tribo-oxides are considered to play a decisive role in deter-ining wear behavior, irrespective of matrix microstructures. With

ncreasing load of 100125 N, the matrix was thermally softenedrobably due to dynamic recovery and recrystallization [10,11].ot enough mechanical support for the tribo-oxide layer could berovided due to a large scale plastic deformation of the softenedatrix. Both tribo-oxide and matrix affect wear behavior, and thexidative wear, rather than oxidative mild wear began to prevail1012]. At 150 N, high compressive and shear stresses caused aassive plastically deformed region due to severe thermal soften-ng of 1045 steel, the uppermost material was liable to extrusionut from the contact surface, resulting in a high wear rate. Althoughribo-oxides were inevitably produced at 400 C, the formed tribo-ayer of less than 0.5 m was too thin to offer wear protection.rious conditions: (a) 50 N, 25 C; (b) 200 N, 25 C; (c) 50 N, 200 C; (d) 200 N, 200 C;

This wear could be classified as extrusive wear as reported bySo et al. [20].

The level of tribo-oxides observed on the worn surface of thespheroidal graphite cast iron also increased on raising the tem-perature to 400 C, however, there were still not present to levelsseen under equivalent conditions with carbon steel. Only a smallamount of tribo-oxides partially covered the worn surface and theiramount and thickness (less than 5 m) was insufficient to eliminatemetalmetal contact completely. It is possible that the tribo-oxidesmight have reduced wear to some extent, but severe wear wasstill the dominant wear mechanism in this case. It is worth not-ing that the lubrication effect of graphite seemed to disappear andso the wear rate of the spheroidal graphite cast iron was slightlyhigher than that of the carbon steel at loads below 100 N. On appli-cation of a 150 N load, the extrusive wear also prevailed for thespheroidal graphite cast iron. Unlike the extrusive wear regime

observed with carbon steel, fracture of worn surface at the inter-face between graphite and matrix was noted. As a result, therewas a greater mass loss at high load compared with that of car-bon steel, due to lower strength and plasticity of the spheroidal

M.X. Wei et al. / Wear 274 275 (2012) 84 93 91

F iding u2

gfl

mgso

gcww

2

4

3

r

ig. 8. Section morphologies of worn surfaces and subsurfaces of the carbon steel sl00 C; (e) 50 N, 400 C; (f) 150 N, 400 C.

raphite cast iron. It appears that the graphite cast iron did notacilitate any advantage during elevated-temperature wear due tooss of graphite lubrication.

The spheroidal graphite cast iron possessed the sameicrostructure and hardness as the carbon steel, except forraphite. These different wear behaviors and wear regimes of thepheroidal graphite cast iron should be attributed to the presencef said graphite.During sliding, it is suggested that the graphite in the spheroidal

raphite cast iron was gradually exposed, then ground down toarbon powder to cover worn surface. Graphite as a strong reducerould cause the following reactions during elevated-temperatureear:

C + O2 2CO (1)CO + Fe3O4 3Fe + 4CO2 (2)

CO + Fe2O3 2Fe + 3CO2 (3)These reactions could occur during sliding, i.e. graphite powder

eacts first with oxygen to produce carbon monoxide, which thennder various conditions: (a) 50 N, 25 C; (b) 200 N, 25 C; (c) 50 N, 200 C; (d) 200 N,

reduces the tribo-oxides generated on the worn surface. The lev-els of tribo-oxide on the worn surface of the spheroidal graphitecast iron may therefore be substantially reduced by these reac-tions, resulting in the differently observed wear behaviors and wearregimes.

Except for its indirect function (on tribo-oxides), graphite woulddirectly affect wear behavior of the spheroidal graphite cast iron.The role of graphite in the spheroidal graphite cast iron can besummarized as follows. Graphite reduced wear as a solid lubricantunder the low-load conditions (at 25200 C), however, acceler-ated wear under a high load due to this high load causing fractureof the worn surface at the interface between graphite and matrix.At 400 C, it is suggested that tribo-oxides were chemically reducedby the described chemical reactions, hence explaining the markedlyreduced levels of tribo-oxide and graphite on worn surfaces. Hence,the ability of both graphite and tribo-oxides to reduce wear were

substantially impaired.

It must be noted that graphite chemically reduced tribo-oxideduring wear is a suggested mechanism. This is because graphitewould physically affect oxide sintering and layer formation

92 M.X. Wei et al. / Wear 274 275 (2012) 84 93

F phite2

dwf

5

(

(

(

ig. 9. Section morphologies of worn surfaces and subsurfaces of the spheroidal gra00 C; (d) 200 N, 200 C; (e) 50 N, 400 C; (f) 150 N, 400 C.

uring sliding. If graphite worsen oxide sintering, tribo-oxidesould be reduced. Thus, the role of graphite during wear needs

urther investigation.

. Conclusions

1) With increasing ambient temperature, the wear rate of the car-bon steel first decreased (with a minimum at 200 C), thenincreased, while the wear rate of the spheroidal graphite castiron continuously increased. At 400 C, once a higher load of150 N was applied, the wear rates of both materials rapidlyincreased.

2) For the carbon steel, severe wear prevailed at 25 C and 50 N,at 200 C. Oxidative mild wear operated under a load rangingfrom 100 to 200 N at 200 C and also at 50 N only at 400 C.Oxidative wear occurred within a load range of 100125 N at

400 C. Extrusive wear appeared with further increasing load.

3) For the spheroidal graphite cast iron, tribo-oxides weresubstantially reduced due to the suggested reduction of tribo-oxides by graphite. Thus severe wear was always dominant cast iron sliding under various conditions: (a) 50 N, 25 C; (b) 200 N, 25 C; (c) 50 N,

regime at 25400 C, except for a load of 150 N at 400 C, whereextrusive wear with fracture prevailed.

(4) Graphite reduced wear under the conditions of lower load(at 25200 C), however, accelerated wear under a high loadbecause of fracture at the interface between graphite andmatrix. At 400 C, it is suggested that tribo-oxides were chem-ically reduced by the described chemical reactions. Hence, theability of both graphite and tribo-oxides to reduce wear wassubstantially impaired.

Acknowledgements

Financial supports of our work by National Natural ScienceFoundation of China (No. 51071078) and Natural Science Fund ofJiangsu Province (No. BK2009221) are gratefully acknowledged.References

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Comparative research on wear characteristics of spheroidal graphite cast iron and carbon steel1 Introduction2 Experimental procedure2.1 Materials and heat-treatment2.2 Wear test2.3 Evaluation techniques

3 Results and analysis3.1 Wear and friction behaviors3.2 XRD analysis for worn surfaces3.3 SEM morphology of worn surfaces3.4 Section morphology of worn surfaces and subsurfaces

4 Discussion5 ConclusionsAcknowledgementsReferences