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Available online at www.sciencedirect.com

Wear 264 (2008) 788–799

Friction and wear behaviour of high strength boron steelat elevated temperatures of up to 800 ◦C

J. Hardell ∗, E. Kassfeldt, B. PrakashLulea University of Technology, Department of Applied Physics and Mechanical Engineering, SE- 971 87 Lulea, Sweden

Accepted 2 December 2006Available online 14 June 2007

bstract

There has been a growing usage of high strength steels, particularly in automobile applications mainly as structural parts in view of their lighteight and high strength properties. These materials are also being considered for dynamic applications. However, the understanding of their

ribological behaviour vis-a-vis their hot forming and also as tribological materials is highly inadequate. The present work thus aims at creatingew knowledge about the tribological characteristics of high strength steels and bridging this existing gap. High temperature tribological studiesn different tool steels (with and without surface treatment) sliding against high strength boron steel (with and without coating) and studies onelf-mated hardened high strength boron steel under dry reciprocating sliding conditions have been conducted. High temperature tribologicaltudies keeping in view the hot metal forming aspects were conducted by using an SRV machine whereas a two-disc machine was employed fornvestigating their fundamental friction and wear behaviour. The results from the high temperature studies indicate that the friction is dependentn temperature since a reduced friction level was observed with increasing temperature. The wear of the tool steels increased with increasing

emperature and nitriding of the tool steels provided better protection against severe wear. The results from the study on self-mated hardenedigh strength boron steel showed that sliding speed has a marginal effect on friction whereas the effect of contact pressure is more pronounceddecreasing friction with increased contact pressure). The specific wear rate decreased with increased sliding speed.

2007 Elsevier B.V. All rights reserved.

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eywords: Friction; Wear; High strength boron steel; Tool steel; High tempera

. Introduction

The increased usage of high strength steels (HSS) began in the970s during the oil crisis when the automobile industry realisedhe need to develop lighter and fuel-efficient vehicles. Further,s the demands on reduced emissions and enhanced crashwor-hiness grew, the automobile industry considered employingight weight and high strength materials in order to meet theseemands. High strength steels, also known as advanced hightrength steels offered the possibilities of significant weighteduction coupled with enhanced strength of automobile struc-ural parts. These days, HSS are commonly employed for various

tructural reinforcements and crash protection systems in dif-erent automobile applications. High strength steels are alsomployed in several other applications such as agricultural

∗ Corresponding author. Tel.: +46 920 491774; fax: +46 920 491047.E-mail address: jens.hardell@ltu.se (J. Hardell).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2006.12.077

ribology

mplements and mining equipments that are prone to high wear.ne of the major problems associated with manufacturing HSS

omponents is their high strength which creates problems dur-ng metal forming. Since most components, especially in theutomotive industry, have complex geometries, problems suchs increased spring back, tendency to work-harden and higheroads on the tooling are commonly encountered during the metalorming operation [1,2]. In order to overcome these problemsnd also to improve the mechanical properties through hard-ning, the HSS components are usually produced through hotetal forming processes. The hot metal forming of HSS compo-

ents in turn introduces new problems such as oxidation, scaleormation on the workpiece surface and increased wear of theorming tools. To alleviate some of these problems, Al–Si coatedSS sheets are sometimes used. The Al–Si coating, besides pre-

enting scale formation, also provides other advantages such asrevention of decarburisation of the work piece, reduced toolear and improved corrosion resistance [3]. Significant tribolog-

cal research pertaining to cold metal forming has been carried

ear 264 (2008) 788–799 789

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J. Hardell et al. / W

ut but the main focus has mainly been on lubricated formingperations. Schedin [4] looked at the initiation of galling in sheetetal forming and concluded that various tool surface defects

ct as initiation sites for material transfer and that it seems to bendependent of the material combination as well as the presencef lubricants. Alinger and Van Tyne [5,6] studied the evolution ofhe tribological characteristics of several die forming materialsnd found that tungsten carbide performed best of all the inves-igated materials both regarding the evolution of wear as well asriction. An investigation on uncoated electron beam-texturedteel sheets was performed by Wihlborg and Gunnarsson [7]nd the results showed that the surface roughness was an impor-ant factor in determining the frictional behaviour in the mixedubrication regime. Adhesive wear of tool steels is a commonroblem in metal forming and Fontalvo et al. [8] conducted atudy on microstructural aspects of adhesive wear. They foundhat the carbide content and the spacing between the carbidesre the main controlling parameters influencing the adhesiveear of tool steels. An investigation using acoustic emission

echnique was done by Skare and Krantz [9] and the resultsndicated that hot-dip galvanised HSS sheets induced less wearn the tools as compared to uncoated HSS. The results also high-ighted the importance of the tool surface roughness in reducingear. As mentioned earlier, the reported work on tribology ofot metal forming is rather limited in comparison to the stud-es conducted on cold forming operations. There are many newroblems encountered at elevated temperatures such as thermalatigue, oxidation, etc. Pellizzari et al. [10] investigated the tri-ological behaviour of hot rolling rolls and found that the wearf the rolls at elevated temperatures occurred mainly due to oxi-ation and abrasion and that the hot hardness of the tool steelas an important parameter. Beynon [11] states in his review

hat many aspects of tribology in hot metal forming have noteen adequately studied yet and research studies, both throughaboratory experimentation as well as computer simulations areecessary to enhance the knowledge within this field.

The present applications of HSS are mostly as stationarytructural parts. These high strength boron steels in view ofheir light weight and high strength properties may also haveotential in pertinent dynamic applications. However, a carefulurvey of the published work indicates that there are hardly anyesults pertaining to the tribological properties of these materi-ls available in the open literature. It is therefore necessary tonvestigate the friction and wear characteristics of these hightrength steels with a view to exploit the useful properties ofhis interesting category of materials for utilisation in variousribological applications.

The present work thus aims at bridging some of the exist-ng gaps and creating new knowledge regarding the tribologicalehaviour of high strength boron steels. This work focuses onribological investigations both on high strength boron steel –ool steel tribological pairs at elevated temperatures – as wells self-mated hardened high strength boron steel tribological

airs. The aim of this study is to gain new knowledge that wille useful from the view points of tribology of hot metal form-ng and enhanced usage of high strength boron steel in variousribological applications.

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ig. 1. Schematic of the SRV test configuration employed in the high tempera-ure tribological study.

. Experimental

.1. Test equipment

Two different experimental equipments have been used in thistudy. For tribological studies at high temperatures, the OptimolRV reciprocating friction and wear test machine was utilised.n this machine, tests can be conducted up to a temperature of00 ◦C. The SRV machine utilises an electromagnetic drive toscillate an upper specimen against a stationary lower specimens shown in Fig. 1 The upper specimen is loaded against theower specimen by means of a servo motor. Friction is mea-ured by using a pair of piezoelectric force transducers mountedt the base of the lower specimen block and the coefficient ofriction is obtained from the output signal of these transducersnd the applied normal load. A computerised data acquisitionnd control system is utilised to control and measure differentarameters during the tests.

The other test equipment was a Wazau UTM2000 two-discachine which was used in the studies pertaining to fundamen-

al tribological properties of self-mated hardened HSS. In thisachine, the disc test specimens are mounted on the shafts of two

eparate servo motors. Fig. 2 shows the schematic of the contactonfiguration and the direction of oscillatory rotation. One motors mounted on a rigid base and the other one is mounted on alide so as to enable loading of the disc specimens against eachther by means of the lever loading arrangement. The speedsf the two motors can be controlled independently in order toonduct tests under pure sliding, rolling/sliding or pure rollingonditions. Load, frictional torque, wear and contact resistancean be measured continuously throughout the test duration. Theoefficient of friction is calculated from the measured frictionalorque as follows:

oefficient of friction = FN · r

T(1)

here FN is the applied normal load (N), T the measured torqueN m) and r is the radius of the disc specimen (m).

790 J. Hardell et al. / Wear 26

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ig. 2. Orientation of the discs relative to each other. The arrows show theirection of rotation (rotational angle α = 150◦).

.2. Test materials and specimens

In the high temperature tests, the upper test specimens wereade from tool steels of three different alloying compositionsith and without a nitriding surface treatment. In the casef surface-treated tool steels, the test specimens were treatedhrough a multi-step process involving both plasma nitridingnd gas nitro-carburising. The depth of the nitrided layer wasetween 0.25 and 0.3 mm. Nitriding of steels to improve theirorrosion and wear resistance through formation of hard nitrideurface layers is a well-known method [12,13]. The upper spec-mens were in the form of cylindrical pins of Ø 10 mm and0 mm height with one spherical end. During tests, the sphericalnd of the pin was in contact with the lower flat disc specimensØ 24 mm and 7.85 mm thick) which were made of uncoateds well as Al–Si coated high strength boron steel. The thick-

ess of the coating was approximately 25 �m. This geometryesulted in a point contact configuration (initially at the start ofhe tests) and enabled application of high contact pressures typ-cally encountered in metal forming. The composition, the type

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able 1lloying composition in (wt.%), hardness values, initial surface roughness and surfac

aterial C Mn Cr Si B P

oron steel 0.2–0.25 1.0–1.3 0.14–0.26 0.2–0.35 0.005 >0.oated boron steel 0.25 1.4 0.3 0.35 0.005 –ool steel 1 0.37 1.4 2.0 0.3 – –ool steel 1 0.37 1.4 2.0 0.3 – –ool steel 2 0.31 0.9 1.35 0.6 – 100ool steel 3 0.39 0.4 5.2 1.0 – –

a Hardness value is for the coating.b The coating thickness was approximately 25 �m.

able 2ardness, initial surface roughness and alloying elements in (wt.%) of the test specim

aterial C Mn Cr Si

ardened boron steel 0.25–0.3 1.0–1.3 0.15–0.25 0.2IC 860-O 900Aa 0.67 1.0 0.028 0.3

a Close equivalent to EN 13674–1260.

4 (2008) 788–799

f surface treatment, hardness values and initial surface rough-ess of different test specimens used in this work are given inable 1.

The disc test specimens pertaining to tribological propertiesf self-mated hardened high strength boron steel were maderom hardened high strength boron steel with an outer diameterf 45 mm and 10 mm thickness. The hardening of the specimensas done by tempering and quenching in water. A common rail

teel grade, 900A, was chosen as a reference material since it iswell known wear resistant rail material and the reference testsere also performed with self-mated 900A tribological pairs.he chemical composition, hardness values and initial surface

oughness of these materials are given in Table 2. The hardnessalues were measured on the test surface, i.e. the circumference,nd these are higher than the bulk hardness of the materials. Thisigh hardness of the discs circumferential surfaces is a result ofork hardening during the machining of the test specimens.The wear of the test specimens has been presented in terms

f specific wear rates which is defined as:

pecific wear rate = V

FN · S(2)

here V is the volume worn away (mm3), FN the normal loadN) and s is the sliding distance (mm).

.3. Test procedure

.3.1. Elevated temperature tests on tool steels and hightrength boron steel by using SRV machine

In the high temperature tests, all specimens and specimenolders were cleaned in industrial petrol in an ultrasonic cleaner,insed with ethanol and dried before the tests. After mountinghe specimens in the test rig, the heating of the lower specimen

as started. The heating was done while the test specimens were

till separated in order to avoid heating of the upper specimenrior to testing. This was basically done with a view to simulatehe conditions prevalent during the forming process where the

e treatment of the test specimens

S Ni Mo V HV300g Ra (�m) Treatment

03 >0.01 – – – 234 0.323 Untreated– – – – 85a 1.06 Al–Si coatingb

– 1.0 0.2 – 559 1.29 Plasma nitrided– 1.0 0.2 – 389 1.27 Untreated

ppm 40 ppm 0.7 0.8 0.145 794 2.31 Plasma nitrided– – 1.4 0.9 634 1.83 Plasma nitrided

ens

P S B HV300g Ra (�m)

–0.35 0.03 0.025 0.003 670 0.5351 0.012 0.015 – 418 0.428

J. Hardell et al. / Wear 264 (2008) 788–799 791

Table 3Test parameters used in the high temperature tribological tests

Test parameters Point contact tests

Load (N) 20Temperature (◦C) 500, 600, 800Stroke length (mm) 2FD

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Table 4Parameters used in the self-mated hardened high strength steel tribological tests

Test parameters Sliding velocity tests Contact pressure tests

Contact pressure (MPa) 250 110, 250, 500, 750Sliding velocity (mm/s) 2, 4, 6, 8 4Rotational angle (◦) 150 150TD

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requency (Hz) 50uration (min) 15

he test equipment was an SRV reciprocating friction and wear tester.

ork piece is heated and the tool gets heated only when it isrought in contact with the work piece. When the desired tem-erature of the lower test specimen was reached, the upper testpecimen was brought in contact and loaded against the lowerpecimen and the test was started. The test parameters used inhis study are listed in Table 3. Tests were performed to studyoth friction and wear characteristics of different tribologicalairs. The friction tests were performed with a view to obtainhe initial friction as well as steady state friction results fromifferent pairs of various tool steels and work piece materials atifferent temperatures. The initial friction is important since theool is in contact with the hot work piece for a very short timeefore it is removed. All experiments have been repeated andood reproducibility of the results was observed. As an exam-le at 600 ◦C the experimental uncertainty for the wear resultsere −6.036 × 10−8 ± 8.814 × 10−9 and in the friction results.025 ± 0.035.

.3.2. Tests on self-mated hardened high strength boronteel by using two-disc machine

The same cleaning procedure as explained above wasmployed in the studies on self-mated high strength boron steelribological pairs. The tribological studies were carried out underry oscillatory sliding conditions as it provides severe conditionsesulting in high wear and therefore an effective way to evalu-te the wear properties of the test materials. The test parametershosen for these studies are listed in Table 4. All experiments

ave been repeated and good reproducibility of the results wasbserved. The experimental uncertainty for the wear results ishown in Figs. 17 and 19 and as an example from the frictionesults at 750 MPa the uncertainty was 0.432 ± 0.008.

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ig. 3. Coefficient of friction as a function of time during the initial 60 s of tests on (and boron steel pair (load: 20 N; stroke: 2 mm; frequency: 50 Hz).

emperature R.T. R.T.uration (h) 1 1

he test equipment was a Wazau UTM2000 two-disc machine.

. Results and discussion

.1. Frictional behaviour of tool steel and high strengthoron steel at elevated temperatures

The short duration friction results of different tool steels bothith and without surface treatment and coated and uncoatedigh strength boron steel specimens at elevated temperaturesre given in Figs. 3–5.

The friction coefficients of plasma nitrided tool steel 1 andoron steel pairs are lower at higher temperature (see Fig. 3(a)).t lower temperatures (500 and 600 ◦C), the initial friction isery high and it is then followed by a drop to a lower valuehereas at the higher temperature (800 ◦C) the friction is almost

onstant during the initial stage. It can also be noted that nocuffing tendencies are observed in these short duration tests atny of the test temperatures. In case of untreated tool steel 1nd boron steel pairs, the overall friction is higher and frictionncreased as the tests progressed. This pair involving untreatedool steel tends to seize towards the end of the short duration testt 500 ◦C as shown in Fig. 3(b).

The frictional behaviour of plasma nitrided tool steel 2 andoron steel pairs is similar to that of plasma nitrided tool steeland boron steel pairs except that in the pair involving plasma

itrided steel 2, there are signs of scuffing at 800 ◦C as seen inig. 4(a). Plasma nitrided tool steel 3 and boron steel pairs haveesulted in very high friction coefficients and have also shown

cuffing tendencies both at 500 and 600 ◦C, respectively.

The tests conducted on Al–Si coated boron steel slidinggainst plasma nitrided tool steels 1–3, respectively, at 500 ◦Cave resulted in very high and unstable friction as can be seen

) plasma nitrided tool steel 1 and boron steel pair and (b) untreated tool steel 1

792 J. Hardell et al. / Wear 264 (2008) 788–799

Fig. 4. Coefficient of friction as a function of time during the initial 60 s of tests on (a) plasma nitrided tool steel 2 and boron steel pair and (b) plasma nitrided toolsteel 3 and boron steel pair (load: 20 N; stroke: 2 mm; frequency: 50 Hz).

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ig. 5. Coefficient of friction as a function of time during the initial 60 s for t00 ◦C and (b) at 800 ◦C (load: 20 N; stroke: 2 mm; frequency: 50 Hz).

rom Fig. 5(a). It is however interesting to note that friction forhe Al–Si coated boron steel at 800 ◦C decreased to almost halff its value at 500 ◦C. The friction was also a lot more stable andery similar for all three tool steels at 800 ◦C.

As mentioned earlier, the steady state frictional behaviourf different tribological pairs was also investigated. In plasma

itrided tool steel 1 and boron steel pairs, the friction is lowert higher temperatures but at the highest temperature of 800 ◦C,riction rapidly increased after about a minute and resulted ineizure as shown in Fig. 6(a). The tribological pair involving

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ig. 6. Coefficient of friction as a function of time for tests on (a) plasma nitrided tooload: 20 N; stroke: 2 mm; frequency: 50 Hz).

n plasma nitrided tool steels 1–3 against Al–Si coated boron steel pairs at (a)

ntreated tool steel 1 has shown higher initial friction but anpposite behaviour as compared to that seen in tribologicalair involving plasma nitrided steel 1, i.e. seizure occurred at00 ◦C and relatively lower and stable steady friction at 800 ◦Cs seen in Fig. 6(b). The frictional behaviour of tribologicalairs involving plasma nitrided tool steels 2 and 3 is somewhat

nomalous as can be seen in Fig. 7. Plasma nitrided tool steelhas a lower initial friction at 600 ◦C but the final friction is

he same as that for 500 ◦C. At 800 ◦C, there are early signs ofcuffing and seizure occurs after about 200 s. Plasma nitrided

l steel 1 and boron steel pair and (b) untreated tool steel 1 and boron steel pair

J. Hardell et al. / Wear 264 (2008) 788–799 793

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ig. 7. Coefficient of friction as a function of time for tests on (a) plasma nitrideair (load: 20 N; stroke: 2 mm; frequency: 50 Hz).

ool steel 3 has a very high initial friction at both 500 and00 ◦C that decreases and ends up at a similar level. At 800 ◦C,ts behaviour is similar to that of plasma nitrided tool steel

and seizure occurs after approximately 200 s. The observedehaviour of decreasing friction with increasing temperaturean possibly be attributed to formation of compacted layers ofxide and wear debris due to oxidation at elevated tempera-ures. Several authors have reported the formation of such layers14–16].

Friction coefficients obtained from the tests conducted onl–Si coated boron steel during sliding against the three plasmaitrided tool steels at 500 ◦C are very high and only tests involv-ng plasma nitrided tool steel 3 lasted the entire test duration. Thether two seized after a short time as can be seen from Fig. 8(a).nterestingly, test conducted on Al–Si coated boron steel andlasma nitrided tool steel 1 pair at 800 ◦C resulted in consid-rably lower friction which increased significantly towards thend of the test, see Fig. 8(b). This behaviour can possibly bettributed to the formation of a hard Fe–Al layer at 800 ◦C.he formation of high melting point Fe–Al phase has also been

eported by Suehiro et al. [3]. Zhu et al. have also reported the

ormation of Fe–Al and Fe3Al intermetallics at elevated temper-tures [17]. They attributed the decrease in friction to formationf lubricious oxide layers on the Al–Si coated high strengthoron steel at elevated temperatures.

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ig. 8. Coefficient of friction as a function of time for tests on plasma nitrided toolload: 20 N; stroke: 2 mm; frequency: 50 Hz).

ig. 9. Coefficient of friction as function of sliding distance for self-mated hard-ned high strength boron steel at different sliding velocities (contact pressure:50 MPa; test duration: 1 h; temperature: R.T.).

.2. Frictional behaviour of self-mated hardened hightrength boron steel

The frictional characteristics of self-mated hardened hightrength boron steel pairs for different oscillating speeds arehown in Fig. 9. It can be seen that initially during the running-n stage, the speed does not have any significant effect on

steels 1–3 against Al–Si coated boron steel pairs at (a) 500 ◦C and (b) 800 ◦C

794 J. Hardell et al. / Wear 26

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ig. 10. Coefficient of friction as a function of sliding distance for self-matedardened high strength boron steel at different contact pressures (sliding veloc-ty: 4 mm/s; test duration: 1 h; temperature: R.T.).

riction but after a sliding distance of about 5 m, the frictionends to decrease slightly in tests conducted at a speed ofmm/s. It is also interesting to note that the hardened high

trength boron steel pairs run-in rather quickly and frictionecreases to considerably lower values as compared to that of00A steel. The overall friction, both during running-in andhereafter is significantly lower for the hardened high strengthoron steel self-mated pairs as compared to the 900A steelairs.

The influence of contact pressure, particularly on the fric-ional characteristics of hardened high strength steel pairs is

uch more pronounced as compared to that of speed. At theowest contact pressure of 110 MPa, full running-in of the hard-ned high strength boron steel pair is not accomplished evenfter 1 h of sliding and the coefficient of friction value is highers compared to those from other tests conducted at higher contactressures. When the contact pressure is increased the surfaces

un-in rather quickly and the friction coefficients decrease toower and more stable values as can be seen in Fig. 10. The fric-ional behaviour of the reference material 900A pairs as functionf load is similar but the overall friction coefficients are consid-

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ig. 11. Specific wear rates of (a) plasma nitrided tool steel 1 and boron steel and (b) u0 N; stroke: 2 mm; frequency: 50 Hz; test duration: 15 min).

4 (2008) 788–799

rably higher as compared to those of the hardened high strengthoron steel pairs.

.3. Wear behaviour of tool steel and high strength boronteel at elevated temperature

In order to compare the wear resistance of the tool steelsnd high strength boron steel after being subjected to recipro-ating sliding at different elevated temperatures, specific wearates were calculated for all tribological pairs. In tribologicalests at elevated temperatures, the oxidation of test specimenslays an important role in determining their friction and wearehaviour. In order to account for the oxidation in wear results athe highest test temperature, a stationary oxidation test was con-ucted by using the same equipment as in the tests and heatinghe lower specimen to 800 ◦C and maintaining it at that temper-ture for the same duration in contact with the upper specimens in the tribological tests. The oxidation was then quantifiedy weighing the specimens after the test. It was found thathe oxidation was negligible on the tool steel specimens pos-ibly owing to a small surface area. The work piece specimensn the other hand experienced a weight gain. This value washen subtracted from the wear values obtained in the tests at00 ◦C. The boron steel experienced a negative wear duringests against the plasma nitrided tool steel 1 at the two loweremperatures and a positive wear at the highest temperatures can be seen in Fig. 11(a). This negative wear means trans-er of material from the mating tool steel surface. The transferf material from the mating surface has also been confirmedy SEM/EDS analysis. The oxidation at elevated temperatureslso contributes to the weight gain of the high strength boronteel discs. The wear of the plasma nitrided tool steel 1 is alsoigher at higher temperatures. In the case of untreated toolteel 1, the wear is relatively lower at higher temperature buthe corresponding wear of the boron steel increased at higheremperature, i.e. from negative wear at low temperature to a

ositive wear at high temperature. For plasma nitrided tool steel, the temperature does not seem to have any significant effectn its wear, see Fig. 12(a). However, there is a significantlyigher wear of the boron steel at 800 ◦C as compared to that

ntreated tool steel 1 and boron steel at 500, 600 and 800 ◦C, respectively (load:

J. Hardell et al. / Wear 264 (2008) 788–799 795

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ig. 12. Specific wear rates of (a) plasma nitrided tool steel 2 and boron steel anload: 20 N; stroke: 2 mm; frequency: 50 Hz; test duration: 15 min).

t other temperatures. The wear behaviour in case of tribo-ogical pairs involving plasma nitrided tool steel 3 is similaro that of plasma nitrided tool steel 2 except that the wear inlasma nitrided tool steel 3 at 800 ◦C is negligible as shown in

ig. 12(b). This behaviour may be possibly caused due to mate-ial transfer and surface oxidation at elevated temperatures. Astated by Stott [18], there are several ways that oxides or oxi-ised wear debris can influence the wear behaviour. The oxide

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ig. 13. Specific wear rates of Al–Si coated boron steel and plasma nitrided tool steelest duration: 15 min).

Fig. 14. SEM images of (a) boron steel disc wear scar and (b) plasma n

lasma nitrided tool steel 3 and boron steel at 500, 600 and 800 ◦C, respectively

ear debris may be removed or get retained in the contact toorm surface protective layers. These can also act as third bod-es or get embedded into the surface and abrade the counter body.his is one possible explanation of the observed behaviour of

ncreasing wear with increasing temperature since the contactas covered by wear debris after testing. Increasing wear with

emperature has also been reported by Wang et al. [19] as aesult of increased abrasive wear due to softening of the spec-

s 1–3 at (a) 500 ◦C and (b) 800 ◦C (load: 20 N; stroke: 2 mm; frequency: 50 Hz;

itrided tool steel 1 pin wear scar from a test performed at 500 ◦C.

796 J. Hardell et al. / Wear 264 (2008) 788–799

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Fig. 15. SEM images of (a) boron steel disc wear scar and (b) pla

men thus providing insufficient support for the oxide layersormed.

The tests with coated boron steel resulted in negative wear onhe Al–Si coated boron steel disc specimens for almost all com-inations and temperatures (Fig. 13). For all the tool steels, theear increased manifolds when the temperature was increased

rom 500 to 800 ◦C. This may again be attributed to the forma-ion of a hard Fe–Al layer on the work piece specimen whichould increase the wear on the counter body. The wear on the

ool steels at 800 ◦C is very similar suggesting that the compo-ition of the tool steel does not have significant effect on theear results. The lowest wear at 500 ◦C was found for plasmaitrided tool steel 2 but that test also had the highest negativeear of the coated boron steel. It is also important to note that thexidation of the specimens also affects the wear results. Sincehe test duration in tests varies owing to early seizure in someases, the influence of oxidation on wear results will also differ.

The SEM images of the worn surfaces of the boron steeliscs and the mating plasma nitrided tool steel 1 pin specimens

t 500 and 800 ◦C are given in Figs. 14 and 15, respectively. TheEM images of the worn surfaces of the boron steel disc and

he mating untreated tool steel 1 pin from the test conducted at00 ◦C are given in Fig. 16.

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Fig. 16. SEM images of (a) boron steel disc wear scar and (b) untrea

itrided tool steel 1 pin wear scar from a test performed at 800 ◦C.

During reciprocating sliding tests on boron steel againstlasma nitrided tool steel 1 at 500 ◦C, the worn surface of theoron steel is characterised by some scoring marks and typi-al adhesive wear features as seen in Fig. 14(a). The scoring orbrasive wear marks may be caused due to the harder asperitiesf the upper plasma nitrided tool steel pin specimen. The weart 800 ◦C is predominantly adhesive in nature. The possibilityf some softening of the tool steel material at higher tempera-ure may reduce the abrading action of the asperities and result inncreased adhesive wear, see Fig. 15(a). The appearance of adhe-ive wear marks also suggests the possibility of some materialransfer from the pin specimen to the disc. The SEM/EDS analy-is of the disc worn surface has also confirmed this phenomenon.ince the boron steel does not contain any Ni, the presence of ai peak in the obtained EDS spectrum clearly shows the transferf material from the tool steel pin specimen as it contains Ni.he SEM image of the worn surface of the boron steel from the

est conducted against untreated tool steel 1 at 500 ◦C indicateshe occurrence of severe adhesive wear, see Fig. 16(a). It may be

entioned that this particular test stopped after about 180 s due

o seizure. It is thus evident that the plasma nitriding of tool steeloes provide protection against severe adhesive wear, seizure oralling.

ted tool steel 1 pin wear scar from a test performed at 500 ◦C.

J. Hardell et al. / Wear 264 (2008) 788–799 797

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ig. 17. Total specific wear rates for self-mated hardened high strength boronteel during tests with varying sliding velocity (contact pressure: 250 MPa; testuration: 1 h; temperature: R.T.).

.4. Wear behaviour of self-mated hardened high strengthoron steel

The wear results for the self-mated hardened high strengthoron steel pairs at different speeds and also that of the reference00A steel at 4 mm/s are shown in terms of specific wear ratesn Fig. 17. The specific wear rate of the hardened high strengthoron steel pairs seems to be decreasing with increasing slidingpeed. The wear resistance of the boron steel as compared to thatf the reference 900A steel is a lot more superior. The decrease inpecific wear rate with increasing sliding velocity may result dueo formation of oxides or some other protective layers owing toncreased frictional heating and consequently higher interfacialemperatures.

Typical worn surfaces were also analysed by usingcanning electron microscopy/energy dispersive spectroscopySEM/EDS) techniques with a view to get an insight into the

ear process. The SEM micrographs in Fig. 18 indicate that

he wear modes of the specimens from the tests conducted atifferent speeds are quite similar and that the material removals predominantly caused by sliding (or adhesive) wear mecha-

shas

ig. 18. SEM images of worn self-mated hardened high strength boron steel discs. Sluration: 1 h; temperature: R.T.).

ig. 19. Total specific wear rates for self-mated hardened high strength boronteel during tests with varying contact pressure (sliding velocity: 4 mm/s; testuration: 1 h; temperature: R.T.).

ism. In Fig. 18(a), a part of the original surface is also visiblen the left part of the image since the wear scar does not coverhe entire width of the disc.

The results pertaining to the wear behaviour as function ofontact pressure are shown in Fig. 19. These results show thathere is no clear trend of the specific wear of the hardened hightrength boron steel pairs as function of contact pressure. Onhe other hand, the specific wear rate increased significantly forhe reference 900A steel pair when the contact pressure wasncreased from 250 to 500 MPa.

The worn surfaces from the tests performed on the hardenedigh strength boron steel pairs at 110 and 750 MPa, respec-ively, were analysed by using SEM/EDS and the corresponding

icrographs are shown in Fig. 20. As can be seen from theseicrographs, the wear is rather mild in nature at low contact

ressure, characterised by fine adhesive wear marks as well asome traces of the original surface. At the higher contact pres-

ure of 750 MPa, the worn surface is plastically deformed andave large grooves formed by the removal of relatively largedhered fragments. There is no visible trace of the originalurface.

iding velocity in (a) 2 mm/s and in (b) 8 mm/s (contact pressure: 250 MPa; test

798 J. Hardell et al. / Wear 264 (2008) 788–799

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ig. 20. SEM images of worn self-mated hardened high strength boron steel dest duration: 1 h; temperature: R.T.).

. Conclusions

Experimental studies pertaining to the friction and wearehaviour of different boron steels (with and without coatings)uring rubbing against different tool steels (with and withouturface treatments) have been carried out at elevated tempera-ures (500–800 ◦C) and some new results have been obtained.hese results indicate that the operating temperature consid-rably influences the friction and wear behaviour during theribological interaction of high strength boron steel and toolteel. It has also been seen that the plasma nitriding of theool steel specimens is effective in reducing friction and provid-ng protection against severe adhesive wear (seizure or galling)uring rubbing against high strength boron steel. The Al–Sioating on boron steel has been effective in reducing frictionuring rubbing against plasma nitrided tool steel 1 at 800 ◦C butesulted in significantly higher friction at 500 ◦C. Further, thel–Si coated boron steel has generally resulted in lower wearf plasma nitrided tool steel. It may be pertinent to mentionere that these are the initial results and further research is inrogress. The future work will focus more on investigating theribochemical changes occurring on mating surfaces at elevatedemperatures such as the type of oxides, borides or any other tri-ochemical layers in order to explain the observed tribologicalehaviour.

The tribological properties of hardened high strength boronteel have been studied under dry oscillatory sliding condi-ions. The influence of contact pressure, speed and interactionf wear particles on friction and wear has been investigated.ome tests on a 900A steel have also been conducted for com-arison of results. The results have shown that the friction andear characteristics of the hardened high strength boron steel

re considerably superior to that of the 900A steel. The influ-nce of sliding speed on the frictional behaviour of self-matedigh strength boron steel pairs was marginal but friction tends

o decrease at higher contact pressures. The specific wear ratesf the high strength boron steel pairs decreases with increasingliding speed whereas the influence of contact pressure on thepecific wear rate do not reveal any clear trend. The mechanisms

[

[

ontact pressure in (a) 110 MPa and in (b) 750 MPa (sliding velocity: 4 mm/s;

f friction and wear of high strength boron steels are still unclearnd further research is required to investigate these mechanisms.

cknowledgements

This work has been carried out with financial support fromorrbottens Forskningsrad and the authors gratefully acknowl-

dge their support. The authors would like to thank Accra teknikB and Scania AB for providing test specimens for these studies

nd also thank them for their active interest in this work.

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