The pin-on-disk wear tests were performed in elevated temperature air at 350 ºC for cast steelwith different composition. Morphology, structureand oxidation films were measured by usingXRD, SEM and TEM.The mechanism of wear was thus clarified. Under elevated temperatureair at 350 ºC, typical oxidation wear was presented in the cast steels. Duringsliding oxidation ofworn surface takes place. The oxide film formed is one of the main factor in determining wearrate which correlates with Quinn’s oxidation wear theory.
INTRODUCTIONSteels are thermo dynamically unstable likeother metals under atmospheric condition.Oxide film on the sliding surface are formed atelevated temperature due to dry sliding wearof steels.
Oxide film breaks off in the sliding areawhen the critical thickness of sliding isreached. This process is continually andperiodically repeated. This formof wear iscalled as oxidation wear. The role of oxidationin the wear of steels was first found by Fink in1930 (Fink, 1930). In 1956, Archard and Hirstproposed a classificationof wear into mild andsevere wear (Archard and Hirst, 1956). Quinnet al. carried out intensive research onoxidation wear of ferroalloy (Quinn, 1978 and
ISSN 2278 – 0149 www.ijmerr.comVol. 3, No. 3, July 2014
1 Department of Mechanical Engineering, Manipal Institute of Technology, Manipal University, Manipal, Karnataka 576104, India.
1998; and Quinn et al., 1979 and 1980). Theystudied the influences ofsliding velocity andload on mild wear and found an expressionfor the wear rate under mild wear conditionsat room or slightly elevated temperatures.Wilson et al. studied the influence of wearoxide debris particles in reducing wear,particularly at higher temperature (Wilson etal., 1980). Oxide film may decrease metal-metal contact during the relative sliding motionof metallic parts, protecting them against wear.Oxide films have been found significant indetermining wear rate. Due to changes inoxide composition different transition in wearrate are formed. The formation of oxide asprotective layer to reduce wear was found inmost of the research works on oxidation wear.
Oxidation wear behavior is closely related tooxide film. The wear mechanism is dependentin the condition of testing. Microstructuralchanges of steel have no effect on the wearmechanism transition. Thus no differences inwear volume were found due tomicrostructure. In this research, wearperformances in air environment at elevatedtemperature were studied for the cast alloysteels. The wear rate of cast alloy steel withdifferent composition and microstructurewere measured under elevated temperatureof 300 ºC. The microstructure, morphologyand oxidation films were measured by usingXRD, SEM and TEM. The wearcharacterstics were also analysed.
EXPERIMENTAL PROCEDURECast alloy steels are widely used in got forgingdie steels. Cast steel were studied using hotforging die. To investigate the effects of alloyelements on elevated-temperature wear, thenominal compositions of the caststeel weredesigned as follows:
0.1-0.2C wt.%, 2-3 Cr wt.%, 3 Mo wt%, 0.1-1.1 V wt%. Other composition include: 0.2-0.4% Si, 0.3-0.4%Mn, 0.1% S and 0.05% P.
The cast steels were melted in 30 kgmedium frequency induction furnace with non-oxidation method. At 1450 ºC, the melt wasdeoxidized with Al, and then poured. Finallythe hedge samples were cast. The steelswere austenitized at 920 ºC for 45 minutesand quenched in oil, then tempered at 500ºC. In this case, the microstructures of the caststeels are tempered with grain size 15-20 µm.The variations of alloy elements mainly leadto the precipitation of different secondarycarbides.
RESULTS AND DISCUSSIONThe wear tests were performed on apin-ondiskhigh-temperature wear tester. The dryweartest rig was used in this work. The weartests for all the cast steels were carried outunder elevated temperature at 350 ºC with testparameters: 50 N normal load; 0.5 m/s slidingspeed and 1600 m sliding for sliding speed.
Pins are made of cast steels with dimensionof 4 mm * 10 mm and disks of 52 HRC withdimension of 50 mm * 6 mm. From the pinspecimens all of the data of wear weremeasured. Pins and disks were polished anddegreased before tests. The pin specimenswere cleaned with acetone and dried beforeand after test. The wear rate is calculated byusing the formula:
Ws = V/Pd
where V is the wear volume loss (m3); d is thesliding distance (m); P is load (N). Themorphology, composition and structure of wornsurfacewere analyzed with SEM and X-raydiffractometer (XRD). Different wear rates ofcast steel persists with different composition.The wear rates of most of the cast steels arerelatively low, ranging from 3.9 * 10-15 m3/Nmto 4.28 * 10-14 m3/Nm. In most of the cast steelsoxidation wear should be classified as mildwear. Fe3O4 and Fe2O3 on worn surfaces ofpin specimens areidentif ied as thepredominant oxides for all the cast steels byX-ray diffraction analysis. Thus we canconclude that the wear of cast steel underelevated-temperature air at 300 ºC is a typicaloxidation wear. Comparison of the wear-ratedata indicates that mild wear predominatedthroughout the test, with some intrusions intothe severe wear. The wear rate of oxidation
wear is not always independent of matrix.Some variations of matrix in compositiondefinitely results in transition of mild wear toseverewear.
High ambient temperature and friction heatcause oxidation of sliding surface underatmospheric condition. Plastic deformation ofworn surface accelerates oxidation process.Hence oxidation speed should be higher thanthat of staticoxidation. Plastic deformation ofworn surface accelerates oxidation process.Hence oxidation speed should be higher thanthat of staticoxidation. Formation of oxide filmon worn surface requires a period of time inthe process of elevated-temperature wear. Anoxidational wear mechanism was establishedfor all the steels tested after a short running-inperiod. Oxygen content on worn surface istoolow to form enough thick oxide film inrunning-in period. The oxide film cannot provideprotective film for reducing wear. Thus severewear occurs. After running-in period, oxygencontent increases enough toform thick oxidefilm and spread over the whole area. But whenoxide film reaches critical thickness, oxide filmbecomes unstable and breaks up to form weardebris because of brittleness and internalstress of oxide. A steady oxidational wearcontinues with alternating process of oxide-filmformation and delamination. The fluctuation offriction coefficients corresponds to variationof sliding surface state resulted fromdelamination of oxide film. When certain-thickness oxide film is formed, sliding surfacestate is relatively smooth with lower coefficientof friction. When oxide film breaks off slidingsurface state is relatively rough with highercoefficient of friction. The wear rate mainlydepends on oxide-film delamination, which
decides amount of generated oxide debris. Itis well known that oxide film has an importantrole in oxidation wear. The oxide films on wornsurfaces of all the cast steels were examinedusing SEM.
Figure 1: SEM Images of Wear Debrisof the Cast Steel
In low wear-rate specimens, a thick oxidefilmwas formed on the worn surface withclosely bonding interface. In high wear-ratespecimens, exceptfor a thick oxide film formedon the worn surface, crack andoxide in crackwas found to exist in matrix under oxide film.At high ambient temperature, oxide film has athick oxide layer which directly grows on thesteel surface.
In the most of cast steels, delamination ofoxide film occurs inside oxide film or atinterface between oxide film and matrix. A newdelamination pattern of oxide film is found inthe specimens with high wear rate. It can beseen that crack initiates and propagates inmatrix under oxide film. This is different fromoxide-film delamination pattern of mildoxidation wear. Consequently, the wear rateis obviously elevated. This should beclassified as severe wear.
Figure 2 (a) Variation of Sn/S atomic ratiowith substrate temperature (the source
temperature was kept constant at 300 °C anddeposition time constant at 1.5 min).
CONCLUSIONAbove-experimental results show that lowerwear rates were found for the most cast steels.This is oxidation-dominated mild wear.However, under the same testing condition,considerable higher wear rates were observedfor the special steels. This transition of mild
wear to severe wear should be attributed tomicrostructure resulting from the variation ofcomposition. The transition of mild wear tosevere wear is realized through changes ofdelamination pattern of oxide film.Delamination from inside oxide film or oxide/matrix interface was consider to be normalpattern in oxidation wear. This pattern showthat wear behavior mainly depends on oxidefilm. Oxidation of worn surface and fatiguedelamination of oxide film proceedalternatively during wear. As there are notcoarse second phases in steel, delaminationof oxide film takes place inside of oxide filmor at interface of matrix and oxide film, whichis classified as mild oxidation wear with lowerwear rate. The experimental results conformto Quinn’s oxidation wear theory.
ACKNOWLEDGMENTI would also like to dedicate this research workto my father late R S Mishra and mother K LMishra.
REFERENCES1. Archard J F and Hirst W (1956), “The
Wear of Metals Under UnlubricatedConditions”, Proc. R. Soc. A, Vol. 236,pp. 394-410.
2. Childs T H C (1980), “The Sliding WearMechanisms of Metals, Mainly Steels”,Tribol. Int., Vol. 13, No. 12, pp. 285-293.
3. Fink M (1930), “Wear Oxidation—A NewComponent of Wear”, Trans. Am. Soc.Steel Treat., Vol. 18, pp. 1026-1034.
4. Quinn T F J (1978), “The Division of Heatand Surface Temperatures at SlidingSteel Interfaces and their Relation toOxidational Wear”, Trans. ASLE, Vol. 21,pp. 78-86.
Figure 2: Delamination Patterns of OxideFilm in the Cast Steels with Low Wear
Rate
Figure 3: Delamination Patterns of OxideFilm in the Cast Steels with High Wear
5. Quinn T F J (1998), “Oxidational WearModelling Part 3: The Effects of Speedand Elevated Temperatures”, Wear,Vol. 216, pp. 262-275.
6. Quinn T F J, Sullivan J L and Rawson D M(1979), “New Development in theOxidation Theory of the Mild Wear ofSteels”, in Proc. Int. Conf. Wear Mater.,Dearborn.
7. Quinn T F J, Sullivan J L and Rowson D M(1980), “Developments in the Oxidational
Theory of Mild Wear”, Tribol. Int., Vol. 13,No. 4, pp. 153-158.
8. Sullivan J L and Athwal S S (1983), “MildWear of a Low Alloy Steel at Temperaturesup to 500 ºC”, Tribol. Int., Vol. 16, No. 3,pp. 123-131.
9. Wilson J E, Stott F H and Wood G C(1980), “The Development of WearProtective Oxides and their Influence onSliding Friction”, Proc. R. Soc. A, Vol. 369,pp. 557-574.
![Analysis of Wear Particles Formed in Boundary-Lubricated ... · tionally, it is believed that the wear debris also reflects the wear mechanism [14–16], and therefore, significant](https://static.documents.pub/doc/80x56/5ed0021901eb6c0e46324747/analysis-of-wear-particles-formed-in-boundary-lubricated-tionally-it-is-believed.jpg)
