Wear 258 (2005) 203–207

Comparing the tribological behaviour of an austenitic steelsubjected to diverse thermal treatments

A. Garcıa∗, A. Varela, L. Garcıa, M.C. Rıo, S. Naya, M. Suarez

Department of Industrial Engineering II, Escola Politecnica Superior, University of Coru˜na, C/Mendiz´abal s/n, 15403 Ferrol, Spain

Received 22 December 2003Available online 13 October 2004

Abstract

It is known that manganese steels respond well in situations where a high wear resistance is required.The manganese content provides an austenitic structure with both wear resistance and high tenacity values under wear. In its working life,

this austenitic structure becomes martensitic as a result of impact and the phenomenon of friction, thus explaining an increase in hardness andits response under wear.

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Wear resistance of an austenitic steel with a 5% manganese content is determined following the regulation ASTM G105 “Staethod for conducting wet sand/rubber wheel abrasion test”, consistent with subjecting samples to an abrasive wear test unde

onditions. These wear tests were carried out on samples that had been previously subjected to various heat treatments.Once the wear tests were completed, the hardness of the tested samples was measured. The results were then compared with

or samples without wear. Moreover, the microstructure and impact test of each sample were examined.2004 Elsevier B.V. All rights reserved.

eywords:Mn–steel; Abrasive wear; Hardness; Microstructure; Heat treatment

. Introduction

Wear of materials is a great problem in general indus-ry, but causes particular concern in the case of coal millst power plants. The need to replace parts because of the

riction-induced damage leads to an increase in operationalosts: for the spare itself and the additional expense of stop-ing activity during its replacement. Thus, it is very important

o select the appropriate material for those applications thatequire good behaviour in wear conditions[1,2].

Traditionally, the hardening of carbon or alloyed steel byeat treatment (quenching and tempering) was employed inoal mill linings. The wear behaviour of these materials is dueo the hardness achieved with such treatments. Nevertheless,s hardness is increased, toughness is reduced, and this meanshigher risk of breaking[3].

∗ Corresponding author. Tel.: +34 981 337400x3221;ax: +34 981 337410.

E-mail address:agd@cdf.udc.es (A. Garcıa).

Therefore, on most occasions, mechanical propertieneeded in addition to good wear response. Austeniticganese steel satisfies the requirements of both wear resiand appropriate toughness. The wear resistance of thiterial is a result of the austenitic phase’s transformationmartensite because of the friction and impact on its su[4–6].

Austenitic manganese steel works well under wearditions because of the relative speed with which the masite is created and worn out[7]. One way to improve bottribological and mechanical behaviour may be heat-treathe austenitic manganese steel. In this study, the influenheat treatment on an austenitic manganese steel’s behis analysed.

2. Materials

The material under study was an austenitic mangasteel subjected to different heat treatments. The che

043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2004.09.003

204 A. Garcıa et al. / Wear 258 (2005) 203–207

composition of this steel was determined by means of sev-eral analytical techniques, such as atomic absorption (AA),X-ray fluorescence (XRF), induced couple plasma (ICP) andcarbon/sulphur analyser, and the results are given inTable 1.

Different heat treatments were carried out. The influenceon mechanical and tribological properties of the quenchingspeed was established through the following treatment: heat-ing to 1100◦C, maintaining this temperature for 1 h and cool-ing in different media: water, oil and air.

Also, after quenching in water, tempering treatments atseveral temperatures were carried out. The treatment temper-atures were, in each case, 250, 300, 350, 400 and 450◦C.Once samples were kept at these temperatures for 2 h, theywere air-cooled.

3. Experimental

The experimental tasks of this study included metallo-graphic analysis, abrasive wear resistance determination,hardness measurement and impact tests for the untreated steeland the different treated samples.

3.1. Metallographic analysis

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surface. Before and after the other three steps, the sampleis weighed. At each stage, the sample surface is markedabove the mark of the previous step.

• Sample mass loss measurement is normalized accordingto the following expression:

Normalized mass loss (g)

= 177.8 × 12.7 × actual mass loss (g)

actual diameter (mm)× actual width (mm)

where 177.8 and 12.7 are the theoretical diameter andwidth, respectively, as indicated in the ASTM G105.

• The logarithm of normalized mass loss is plotted againstthe actual rubber wheel hardness, and the abrasive wear re-sistance is determined as the lost mass value correspondingto a rubber hardness value of 60± Shore A.

An examination of the surface of the samples was doneby scanning electron microscopy after the wear test.

3.3. Hardness measurements

Hardness measurements were carried out in the RockwellC scale. The applied load for these tests was 150 kg and theindenter was a conical diamond.

The hardness value was determined for each sample twice:t oncem

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After heat treatment, the samples were ground andshed in order to obtain a suitable surface for their microscbservation. Prior to this surface observation, the sample

aces were etched with Nital 5%.

.2. Abrasive wear resistance

The abrasive wear resistance for each sample wasined by following the guidelines in the ASTM G105 “Staard test method for conducting wet sand/rubber wheelion test”[8]. According to this procedure, a controlled wrocess is to be carried out in the following way:

The abrasive material is sand and water mixed in theportions specified.The surface under study is placed against that of a ruwheel, and the abrasive passes between those two suThe wear process has four stages. In each stage, therotates 1000 turns, and the rubber hardness is 50± ShoreA, 50± Shore A, 60± Shore A and 70± Shore A, respectively. The first step’s purpose is to prepare the sam

able 1hemical composition of the austenitic steel under study

lement Percentage (%

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he first time after the heat treatment was finished, andore after the abrasive wear test was carried out.

.4. Impact test

Impact energy values were obtained by means oharpy impact test. Sample sizes were 10 mm× 10 mm×5 mm, and the usual V-notch geometry. The applied imnergy was 300 J.

. Results

.1. Microstructure

Fig. 1 shows the microstructure of the untreated sabove on the left), quenched in water (above on the riempered at 250◦C (bottom on the left) and 450◦C (bottomn the right). The former presents an austenitic structurconsequence of the chemical composition (5% of Mn)recipitated carbides along the grain boundary. As a res

riction undergone by the samples in the grinding andshing stages, needles of martensite can be observed a

ost of the carbides along the boundary grains disappeang the quenching treatment in the manganese steel, aseen in the figure above on the right. The sample tempe50◦C presented a different structure; in this case, the fricpplied in grinding and polishing did not cause an aust

o martensite transformation. Its micrograph shows someide along the grain boundary; nevertheless, the numb

hese carbides is noticeably lower than the carbides obs

A. Garcıa et al. / Wear 258 (2005) 203–207 205

Fig. 1. Above, microstructure of steel without treatment (on the left) and microstructure of the steel quenched in water (on the right). Below, microstructure ofthe steel tempered at 250◦C (on the left) and microstructure of the steel tempered at 450◦C (on the right).

in the untreated steel microstructure. The higher the temper-ature, the lower the amount of carbides observed along thegrain boundary. In the case of the sample tempered at 450◦C,carbides could hardly be observed.

4.2. Abrasive wear resistance

The abrasive wear resistance of the heat-treated samplesare shown inTable 2. The results are expressed as an in-crement of the wear resistance with regard to that of the un-treated steel. Thus, it is possible to observe how the heat treat-ment modifies the tribological behaviour of the manganesesteel.

A comparison of the wear surface of the steel withouttreatment and the one quenched in oil can be observed inFig. 2. The first picture presents a surface with more irreg-ular and deeper marks and it is possible to observe materialloss along the surface. This fact is in agreement with the re-sults obtained in the abrasive wear test, which indicated thatthe abrasive wear resistance of the sample quenched in oilis bigger than that for the untreated steel. When the wearresistance increases, material removal is more difficult, thepresence of pores and irregularities decrease and the markson the surface appear more clearly.

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4.3. Hardness

Fig. 3shows the hardness values for the different samples,and shows a comparison between these values and those ob-tained after the wear test was carried out on each sample. Itis possible to observe how the friction modifies this mechan-ical property. Values presented in this graph are average val-ues. In all the cases, the standard deviation falls in the range0.00–0.92, with an average value of 0.56. Further, Pearsoncoefficient (a measurement of the variability) was calculated,being the average coefficient for this kind of test 0.01. Asconclusion of these statistical data, it is possible to say thatthe hardness measurement variability is not high.

4.4. Impact energy

Table 3presents the impact energy obtained for the differ-ent samples by means of the Charpy test. As in the previouscase, average values are presented in the table. In the case ofthe impact energy measurements obtained by Charpy test, thestandard deviation values were in the range from 0 to 1, withan average value of 0.44 and a Pearson coefficient average of0.04. Again, the variability of the test was small.

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able 2ncrement of abrasive wear resistance for treated samples with regardalue obtained for the steel without treatment

uenching media Tempered temperatureir Oil Water 250◦C 300◦C 350◦C 400◦C 450◦C0% 41% 54% 39% 22% 12% 33% 46%

. Discussion

The high manganese content (5%) leads to an austicrostructure. In the untreated steel micrograph showig. 1 (above on the left), it is possible to observe marite needles on the austenitic background, which are a cuence of the austenite transformation because of the fr

206 A. Garcıa et al. / Wear 258 (2005) 203–207

Fig. 2. Wear surface of manganese steel without treatment (on the left) and quenched in oil (on the right).

force applied in the grinding and polishing stages. Themartensite phase is responsible for the steel’s wear resistance.Fig. 3compares the sample’s hardness values before and af-ter the abrasive wear test. When the steel is left untreated orquenched in air and oil, its hardness increases after the wearprocess once again due to phase transformation. Neverthe-less, this growth is not seen in the samples quenched in wateror tempered. In the former case, the steel’s hardness beforethe wear test is 50 HRC, which is the same value that wasobtained with the other quenched and untreated samples afterthe test. The cases of tempered samples are different, and thehardness values remain constant without undergoing changesas a consequence of the wear. This fact can be attributed tothe austenite stabilization through heat treatment, so thesesamples do not present martensite in their microstructure.

The other consequence of heat treatments in the steel mi-crostructure evolution is the modification of the carbides pres-ence and distribution. In the untreated steel, carbides were ob-served along the grain boundary, but in the quenched samples,these carbides disappeared during the heating to 1100◦C.Nevertheless, when the steel, after quenching in water, wassubjected to a tempering at 250◦C, carbides appeared againin the grain boundary, but in small numbers than in the steelwithout treatment. As tempering temperatures increased, theamount of carbides in the grain boundary decreased.

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F ifferents

ing causes the abrasive wear resistance to rise. In heat treat-ment, the sample is first heated. Then, as cooling is accel-erated, the steel’s abrasive wear resistance grows. Thus, inair-cooling, the wear resistance is 10% greater than the valueobtained for the untreated steel, while the biggest incrementis for water-cooling, at 54%.

When the steel is subjected to tempering, there is an im-provement in wear behaviour, but there is no clear relation-ship between treatment temperatures and a percentage rise inwear resistance values.

Assuming that wear resistance is a function of hardness,one might suppose that the tempered samples’ wear responseshould be similar to that for the steel quenched in air (itshardness is about 40 HRC). Nevertheless, the improvement inwear resistance is bigger for tempered samples. Particularly,samples tempered at 250 and 450◦C present a wear resistancesimilar to the steel quenched in oil.

A further analysis of the sample wear surfaces by means ofSEM shows how wear marks are bigger when the hardnessis lower. This fact is due to the facility to remove materialfrom the softer surfaces. When a portion of material is takenoff, a pore is created in the surface, and this induces morematerial removal. Thus, for the most wear-resistant samples,the marks on the wear surface are smaller.

Fig. 4shows how hardness and wear resistance are mod-i hesep ard tot e un-t s ares tancei ance isa resis-t ay, ah ed ino ound4

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Heat treatment modifies the tribological behaviour ofanganese steel in a way that can be seen inTable 2. Quench

ig. 3. Hardness measurements before and after wear test for the damples.

fied by means of heat treatment. The improvement in troperties is expressed as a percentage rise with reg

he values obtained in hardness and wear tests for threated sample. As can be seen in this figure, both trendimilar. An increment in hardness means that wear resisncreases, and when hardness decreases, wear resistlso reduced. However, the changes observed in wear

ance are superior to the increase in hardness. In this wardness increment of about 25% for the sample quenchil corresponds with a wear resistance increment of ar0%.

able 3mpact energy values obtained in the Charpy test

ntreated Quenching media Tempering temperature

Air Oil Water 250◦C 300◦C 350◦C 400◦C 450◦C

0 J 12 J 7 J 6 J 10 J 13 J 9 J 7 J 5 J

A. Garcıa et al. / Wear 258 (2005) 203–207 207

Fig. 4. Hardness and wear resistance percentage rise with regard to the steelwithout treatment values.

The impact energy value for the untreated steel is 10 J, andthis value increased to 12 J when the sample was quenched inair. Quenching in oil and water means a reduction in the mea-sured values, which are higher when cooling rate increases.For tempered samples, increased impact energy values wereobtained for 250 and 300◦C. For temperatures above 300◦C,impact energy values decrease again. This fact can be ex-plained by an embrittlement phenomenon in this temperaturerange.

6. Conclusions

An austenitic steel with 5% manganese subjected to dif-ferent heat treatments of quenching and tempering has beenstudied. The influence of heat treatment on microstructure,hardness and tribological behaviour has been analysed.

The hardness of quenched samples increased with coolingrate, while tempered samples present hardness values that arvery similar, between 40 and 42 HRC.

For untreated steel or the one quenched in air and oil, thereis an increase in hardness after the wear test. This increase isdue to the austenitic transformation into martensite because

of the friction forces applied during the test. This incrementis not appreciated for samples quenched in water and thosethat are tempered. In the latter case, heat treatment causes theaustenitic phase to stabilize.

For quenched samples, tribological behaviour improveswith cooling speed, but there is no clear relationship betweentempering temperature and abrasive wear resistance.

Wear results for steel tempering to 250 and 450◦C arebetter than expected. Abrasive wear resistance in these casesare similar to that for the sample quenched in oil.

Hardness and abrasive wear resistance present the sametrend in function of heat treatment; wear response showsgreater improvement than hardness does.

Impact energy values follow a decreasing trend with cool-ing speed in quenching treatment, which increases for sam-ples tempered to 250 and 300◦C. Tempering treatment above300◦C cause a greater brittleness in the steel.

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8] ASTM G105-89, Standards test method for conductingsand/rubber wheel abrasion tests, ASTM Standards.