oe wear performance of Inconel 617, Stellite 6 alloys and X32CrMoV33 hot

tigated. The wear resistance of the latter is degraded at 750 1C due to itsce.

ue

e sp

50 1

manufactured by forging owing to their superior mechanical

rging pto seich coowerctivefor Alechnofor forganding

under thermal cycling. The conventional hot work tool steels were

superior thermal fatigue performance than conventional hot

conelish ofl pot,unitwith

immersed into the sand. The wear test samples were tted to a

Contents lists available at ScienceDirect

els

Tribology Int

Tribology International 43 (2010) 22222230wear test.E-mail address: yucel.birol@mam.gov.trset up an appropriate process for the production of steel parts onan industrial scale [8].

mild steel rod attached to an electric motor assembly capable ofrotating the specimens in hot sand at 220 rpm for 6 h. Measureswere taken to maintain a trouble-free sand ow aroundthe samples. The present wear test is thus a three-body abrasive

0301-679X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.triboint.2010.07.011

n Tel.:+90 262 6773084; fax: +90 262 6412309.shown to rapidly deteriorate under such severe conditions [810].Suitable die materials with extended service life are required to

an average particle size of 163 mm (Fig. 1c). The zirconia sand washeated to the desired test temperature before the test sample wasmodest owing to a mushy feedstock [7]. The wear caused by theimpact of the a-Fe globular particles in the slurry and hightemperature oxidation not only impair the quality of the diesurface, but also introduce potential sites for crack nucleation

samples (25 mm25 mm) machined from X32CrMoV33, In617 and Stellite 6 alloys (Table 1) and ground to a surface n500 grit sand paper. The wear test rig comprised of a stee150 mm120 mm|, placed inside a resistance heating(Fig. 1a and b). The steel pot was lled with zirconia sandprocess temperatures often exceeding 1300 1C [2,3]. The surface-to-interior temperature gradients and the thermal stresses thusgenerated across the die are much larger than with Al and Mg[46]. The principle die failure mechanism is thus claimed to bethermal fatigue as the mechanical loading on the tooling is

The wear test was designed to mimic the conditions encoun-tered in steel thixoforming where a-Fe particles of the semi-solidfeedstock move over the die cavity surface once forced into thedie. The die materials were represented by 6 mm thick squareical and fatigue properties. The rathenergy costs of the conventional foprompted the part manufacturerstechnologies. Semi-solid forming, whof casting and forging at much lcompetitive cost, is indeed an attramatured into an industrial practiceinnovative near-net shape forming tmarket and provide lightweighting

Thixoforming of steel is very demrocess, however, haveek alternative formingmbines the advantagesforming forces and atoption. Having alreadyand Mg alloys [1], thislogy could upgrade theed steel parts.on tool materials with

temperature oxidation, erosion and wear [16]. The present workwas undertaken to investigate the high temperature abrasivewear resistance of these materials and rate their performancewith that of the conventional hot work tool steel employed in hotforging of steel components [17].

2. Experimentalproperties since the castings fail to provide the required mechan-er high tooling, material and

work tool steels [7,1115] these alloys are claimed to be attractivecandidates owing to their outstanding resistance also to highHigh-temperature abrasive wear testingthixoforming of steels

Yucel Birol n

Materials Institute, Marmara Research Center, TUBITAK, Kocaeli, Turkey

a r t i c l e i n f o

Article history:

Received 19 April 2010

Accepted 15 July 2010Available online 23 July 2010

Keywords:

Three-body abrasion

Sliding

High temperature

Wear

a b s t r a c t

High temperature abrasiv

work tool steel was inves

inferior oxidation resistan

substantial material loss d

abrasion without extensiv

other hand, improves at 7

1. Introduction

Steel parts for drive units and chassis components are often

journal homepage: www.Extensive oxidation co-occuring with abrasive wear at 750 1C leads toto the lack of a protective oxide scale, sufciently ductile to sustain the

alling. The wear resistance of the Inconel 617 and Stellite 6 alloys, on the

C owing to protective oxides that sustain the abrasion without spalling.

& 2010 Elsevier Ltd. All rights reserved.

Several high temperature alloys were tested recently for theirpotential to withstand the steel thixoforming environment for aneconomically acceptable life [7,1114]. Shown to possess af potential tool materials for

evier.com/locate/triboint

ernational

d Stellite 6 alloys used in the present work.

Al Co Cu Nb Ti V W Fe

X32CrMoV33 0.281 0.190 0.200 3.005 2.788 0.221 0.025 o0.010 0.1651 0.0015 o0.001 0.413 0.020 92.63Inconel 617 0.080 0.945 0.513 21.88 8.177 53.861 0.167 10.872 0.304 0.010 0.211 2.850

0.094 58.241 0.033 0.009 4.512 2.660

K-typethermocouple

heatingelements

sample

zirconia sand

motor

stainlesssteel pot

ork. (c) Scanning electron micrograph of the zirconia sand used in abrasive wear tests.

Y. Birol / Tribology International 43 (2010) 22222230 2223An assessment of the wear resistance has to take into account

Stellite 6 1.089 1.099 1.154 28.272 0.004 2.802

Fig. 1. (a) Photo and (b) sketch of the abrasive wear test set-up used in the present wTable 1Chemical composition of the X32CrMoV33 hot work tool steel and Inconel 617 an

Alloy C Si Mn Cr Mo Nithe hardness of the material [18]. Hot work tool steels are knownto resist tempering and retain their hardness up to approximately650 1C [19]. Hence, the wear tests were performed below andabove this temperature, at 625 and 750 1C, respectively, in orderto fairly describe the impact of temper resistance of the hot worktool steel on its wear resistance. The latter is the maximumtemperature attained at the surface of the die during thixoform-ing of steels [20]. The temperature of the sand was maintained at625710 and 750710 1C throughout the wear tests. Hardness ofthe samples was measured with a Vickers hardness tester with1000 g load and a dwell time of 15 s.

Weight loss was used as the measure of wear. Electronicbalance of 0.1 mg accuracy was used to measure the weight ofspecimens before and after each test. The worn surfaces werecharacterized with a JEOL 6335F model eld emission gunscanning electron microscope (FEG-SEM) tted with an OxfordInstruments INCA model energy dispersive X-ray analyzer (EDS).Thermo-gravimetric analysis (TGA) were performed with aSETARAM TG/DTA unit to determine the oxidation behaviour ofthe materials tested in the present work. Powder samplespulverized via mechanical means, with an average grain size of45 mm, placed in deep platinum pans were heated in owingoxygen at 10 1C min1 until 850 1C. A Shimadzu XRD-6000 modelX-ray diffractometer with Cu Ka radiation was employed for theidentication of oxides on the surfaces of the samples submittedto high temperature wear tests.

3. Results and discussion

The hardness and weight loss of the samples submitted to hightemperature wear tests are illustrated in Fig. 2. The hardness dropof the hot work tool steel upon a 6-h long thermal exposure at

hard

ness

(HV

1)

before testing after testing at 625C after testing at 750C

0

100

200

300

400

500

600

0

10

20

30

40

50

60

70

80

90

100

wei

ght l

oss

(mg)

after testing at 625C after testing at 750C

Stellite 6 Inconel 617X32CrMoV33

Stellite 6 Inconel 617X32CrMoV33

Fig. 2. (a) Hardness and (b) weight loss values of the three alloys tested at 625 and750 1C.

625 1C is negligible suggesting that 625 1C is near, yet below thetemper softening temperature of this material. Likewise, theweight loss of the X32CrMoV33 tool steel due to abrasive wear isthe smallest at this temperature. The hardness drop of Inconel617 alloy, on the other hand, is the largest. This substantialsoftening may be responsible for the weight loss it has suffered,measured to be the highest at this temperature. With anintermediate hardness at 625 1C, the abrasive wear performance

of the Stellite 6 alloy falls in between. The weight loss andhardness values are inversely proportional at 625 1C suggestingthat the wear resistance of the three alloys tested in the presentwork is closely linked with their hardness.

The response to abrasive wear testing of the candidatematerials is markedly different at 750 1C. X32CrMoV33 tool steelwhich has largely retained its hardness and resisted softening at625 1C, responded to thermal exposure at 750 1C with a sharp

Fig. 3. Photos of (a) the X32CrMoV33 hot work tool steel and (b) Stellite 6 samples after abrasive wear tests at 750 1C.

10m

10m

s of

Y. Birol / Tribology International 43 (2010) 222222302224Element C K O K Al K Si K V K Cr K Fe KMo L

Wt% 17.49 28.05 0.22 0.21 0.11 0.64

52.860.41

Fig. 4. Scanning electron micrograph, EDS spectrum and EDS analysis of the corner

abrasive wear test at 625 1C.Element C K O K Al K Si K V K

Cr K Fe K Mo L

Wt% 6.47 16.81 0.29 0.42 0.37 2.50 70.42 2.33

(a) the surface and (b) the corner of X32CrMoV33 hot work tool steel sample after

hardness drop (Fig. 2a). This is not surprising since the temperresistance of hot work tool steels is claimed to be around 650 1C[19]. Inconel 617 alloy, on the other hand, has experienced onlyslight softening with a further increase in the test temperature to750 1C, suggesting that it has already almost fully softened at625 1C. The Stellite 6 alloy appears to soften gradually over thistemperature range (Fig. 2a).

The weight loss of the X32CrMoV33 sample after the wear testat 750 1C was measured to be 91 mg evidencing a markedreduction in its wear resistance. The oxide scales, which spalledfrom the surface soon after the sample cooled to roomtemperature, is responsible for much of this weight loss(Fig. 3a). The wear resistance of the Inconel 617 and Stellite 6alloys, on the other hand, appears to have improved at 750 1C. Theweight loss of these two alloys due to abrasive wear at 750 1C isconsiderably less than at 625 1C (Fig. 2b). It is also interesting tonote that the differences between the Inconel 617 and Stellite 6alloys in terms of hardness and weight loss are almost completelyerased after the wear tests at 750 1C. The two alloys seem to haveattained similar hardness and have suffered nearly equal weightloss after testing at 750 1C.

The abrasive wear was most prominent along the leadingedges, particularly around the bottom corners, of the sampleswhere the pressure head and thus the abrasion capacity of thesand was the highest (Fig. 3b). These locations were thus roundedoff evidencing that the wear at the corners and edges isresponsible for much of the material loss incurred on the samples.The surfaces of the samples on the other hand, hardly revealedany wear damage as the sand particles tended to slide over theat surfaces under the present circumstances. The specimen

Element C K

10m

Wt% 2.51

s of

Element C KO KAl KSi KV KCr KFe KNi KMo L

1mm1mm

Wt% 32.8431.030.340.540.362.2728.530.963.12

Fig. 5. Scanning electron micrograph, EDS spectrum and EDS analysis of a heavilyworn location at the corner of X32CrMoV33 hot work tool steel sample after

abrasive wear test at 750 1C.

Y. Birol / Tribology International 43 (2010) 22222230 2225O K Al K Si K V K Cr K Mn K Fe K Mo L

19.010.20 0.35 0.62 4.54 0.1767.054.62

Fig. 6. Scanning electron micrograph, EDS spectrum and EDS analysis of the corner

abrasive wear test at 750 1C.geometry and the features of the wear test employed in thepresent work facilitated the assessment of the relative impact ofoxidation and abrasive wear in generating material loss duringelevated temperature wear testing.

The X32CrMoV33 hot work tool steel submitted to wear test at625 1C shows markedly different features at the surface and at thecorner. The surface is in an oxidised state while the features of thecorner are typical of three-body abrasion with multiple indenta-tions (Fig. 4). The wear features such as craters, extrusion lips aremuch ner with respect to the erodent size and instead scale withthe microstructure. The EDS analysis of the respective regions hasshown stronger oxygen peaks at the surface than at the corner,implying that oxide at the corner has been thinned by abrasion.The marked difference between the C levels at the surface and atthe worn corner seems to underline the impact of carbides in theoxidation process. The EDS analysis always revealed high C levelsfor those sites where O levels were also high. This is true alsofor the sample corner locations where the wear damage was

Element C K O K Al K Si K V K Cr K Mn K Fe K Mo L

10m

Wt% 7.57 37.230.370.600.595.590.3843.094.59

(a) the surface and (b) the corner of X32CrMoV33 hot work tool steel sample after

substantial (Fig. 5). Carbides appear to be involved in theoxidation process more than the matrix phase and are thusclaimed to be more prone to oxidation at high temperatures. It isthus inferred that the carbides oxidise more readily and are thusremoved from the surface more rapidly under abrasion.

Both the surface and the corners of the X32CrMoV33 hot worktool steel sample submitted to wear test at 750 1C were heavilyoxidised as evidenced by the SEM micrographs (Fig. 6). The wearfeatures, readily identied around the corners of the X32CrMoV33sample tested at 625 1C, were entirely concealed by oxidationwhen tested at 750 1C. The oxide scale was shown by XRD analysisto be Fe3O4 (Fig. 7) and, by much higher oxygen levels in the EDSanalysis, to be relatively thicker. This contrasts the case of wear at625 1C where the oxide scales at the corners were thinner. This

ElementC K O KAl K Si K Cr K Mn K Fe K Co K Ni K Mo L

10 m

10.20 Wt%

24.51 0.29 1.18 19.70 0.84 1.847.00 28.44 6.01

Fig. 9. Scanning electron micrograph, EDS spectrum and EDS analysis of the corners oat 750 1C.

C K O K Al K Si K Cr K Mn K Fe K Co K Ni K Mo L

Element Element C K O K Al K Si K Cr K Mn K Fe K Co K Ni K Mo L

10m 10m

Wt% Wt% 13.59 8.350.340.5616.520.522.2710.3041.496.07

10.25 14.53 0.26 1.28 17.56 0.85 1.90 8.33 35.32 9.74

Fig. 8. Scanning electron micrograph, EDS spectrum and EDS analysis of the corners of (a) the surface and (b) the corner of Inconel 617 sample after abrasive wear testat 625 1C.

2 (degrees)

inte

nsity

(a.u

)

100 908070605010 20 30 40

Fig. 7. XRD pattern of X32CrMoV33 hot work tool steel sample submitted toabrasive wear test at 750 1C: ~, a-Fe; , Fe3O4.

Y. Birol / Tribology International 43 (2010) 222222302226C K O KAl K Si K Cr K Mn K Fe K Co K Ni K Mo L

10 m

Element Wt% 21.73 11.43 0.220.46 15.47 0.54 1.90 8.00 36.03 4.22

f (a) the surface and (b) the corner of Inconel 617 sample after abrasive wear test

O K Al K Si K Cr K Mn K Mn K Fe K Co K Ni K Mo L

O K Al K Si K Cr K Mn K Fe K Co K Ni K Mo L

C K O K Si K Cr K

Fe K Co K Mo L W M

C K O K Si K Cr K Mn K Fe K Co K Mo L W M

Element

1

2

1

2

5 m 5 m

1

22

1 13.480.300.7217.470.592.5410.8045.328.79

16.730.470.9117.340.233.069.9643.397.91

5.7010.750.5721.24

2.0754.050.134.60

6.029.890.5220.760.872.5955.020.104.24

Wt% Element Wt%

Element Wt%Element Wt%

0.89

Fig. 11. Scanning electron micrographs, EDS spectrums and EDS analysis of indicated locations corners at the corner of (a) Inconel 617 and (b) Stellite 6 sample afterabrasive wear test at 750 1C.

ElementC K O K Si K Cr K Mn K Fe K Co K W M

ElementC K O K Si K Cr K Mn K Fe K Co K W M

10 m 10 m

Wt%4.8320.390.8628.232.222.5337.573.38

Wt%8.7112.520.4919.671.002.1151.404.09

Fig. 10. Scanning electron micrograph, EDS spectrum and EDS analysis of the corners of (a) the surface and (b) the corner of Stellite 6 sample after abrasive wear testat 750 1C.

Y. Birol / Tribology International 43 (2010) 22222230 2227

observation conrms the synergy between wear and oxidation atelevated temperatures. Wear, dominant at the corners, generatesfresh surfaces and promotes oxidation which in turn leads toincreased material loss under the present wear test conditions.Once again high C levels are linked with high oxygen levelsconrming the higher tendency of carbides for oxidation.

The Inconel 617 sample which has suffered the highestmaterial loss at 625 1C shows at the corners features quite similarto those observed in the hot work tool steel. Multiple indentationsover the corners imply a three-body type abrasive wear (Fig. 8).However, there is hardly any evidence of wear damage on the atsurfaces of the sample as evidenced by the survival of the gritpaper marks introduced before the wear test. The slightcolouration of the surfaces implies oxidation as conrmed bythe EDS analysis. The oxides are too thin, however, to be identiedon cross-sections at optical microscope resolutions and togenerate Bragg reections in the XRD analysis. The EDS analysisshows the oxides to be relatively thicker at the surface suggestingthat oxides have been abraded at the corners. Surface and cornerfeatures of the Inconel 617 sample after the abrasive wear test at

20

10

0

O, w

t%

15

10

5

0

Si,

wt%

40

0

10

20

30

Cr,

wt%

10

%

1 m

Fig. 13. (a) Transverse section micrograph of the surface region of the Inconel 617 sampthe surface oxide.

10 50 70 902 (degrees)

inte

nsity

(a.u

)

20 30 40 60 80 100

Fig. 12. XRD patterns of (a) Inconel 617 and (b) Stellite 6 samples submitted toabrasive wear tests at 750 1C: &, g; D, Cr2O3.

Y. Birol / Tribology International 43 (2010) 2222223022285

0

Fe, w

t

15

10

5

0

Co,

wt%

40

30

20

10

00 1 3

Ni,

wt%

2m

le submitted to abrasive wear test at 750 1C and (b) line scan EDS spectrums across

750 1C are quite similar. Oxidised surfaces and indented cornersare the dominant features (Fig. 9). It is worth noting, however,that oxidation has progressed at this higher test temperature asinferred from the higher oxygen levels in the EDS analysis. Theoxide at the surface is, once again, thicker than at the corner,suggesting that the oxide at the corner has been partially removedby abrasion.

The features of the Stellite 6 samples submitted to abrasivewear tests are very similar. The SEM micrographs and the EDSanalysis of the surface and the corners of the sample tested at750 1C are shown in Fig. 10. The chemistry is quite uniform acrossthe abraded corners and is thus claimed to be independent of themorphological features for both alloys (Fig. 11). Reduced materialloss at a higher wear test temperature, in spite of furtheroxidation, is taken to imply that the oxides of the Inconel 617and Stellite 6 alloys are protective and work against material loss,possibly by the reducing friction coefcients [21]. This contraststhe response to high temperature wear of the hot work tool steelthat deteriorates with increase in temperatures.

The X32CrMoV33 steel responds to abrasive wear at 625 1C viaabrasion of the oxidised surfaces. Oxidation becomes thepredominant failure mechanism in the abrasive wear testperformed at 750 1C [2224]. Synergy is generated between theoxidation process and wear at elevated temperatures withabrasion leading to enhanced oxidation and oxidation, in turn,

leading to further material loss. The two mechanisms co-occuringat 750 1C has led to material loss possibly more than would beproduced if either process acted independently as suggested byother researchers [25,26]. An adhesive and protective oxide scale,sufciently ductile at 750 1C to sustain the abrasion withoutextensive cracking or spalling, is clearly missing in X32CrMoV33[27,28]. The morphology of the X32CrMoV33 wear sample istypical of eroded oxides at 625 1C but reects the severity of theoxidation process at 750 1C. The loss of mechanical strength withincrease in test temperature is also believed to be instrumental inthe substantial material loss suffered by the hot work tool steel.

The adhesive and protective oxides growing slowly on Inconel617 and Stellite 6 alloys, on the other hand, has sustained theabrasion without spalling and is claimed to be responsible for theimproved wear resistance of these alloys with increase in testtemperatures [27,28]. High-temperature alloys rely on a mini-mum of 20 wt% Cr to form protective Cr2O3 scales [29,30]. TheXRD analysis has shown Cr2O3 to be the predominant oxide on thesurface of both Inconel 617 and Stellite 6 samples (Fig. 12a and b).The plasticity of the oxides, essential to sustain abrasive wear, hasbeen reported to be adequate for Cr2O3 at high temperatures[23,27,28,31]. The surface oxides found by the EDS were relativelyricher with Si and with Si and Al in the case of Inconel 617 andStellite 6 samples, respectively, implying that Si and Al are alsoinvolved in the development of surface oxides (Figs. 13 and 14).

10

10

5O, w

t%

20

10

5

4

6

8

2

4

6

8

1

10

20

30

234

5

0

Si,

wt%

Cr,

wt%

Mn,

wt%

ple s

Y. Birol / Tribology International 43 (2010) 22222230 2229Al,

wt%

Co,

wt%

Mo,

wt%

1 m

Fig. 14. (a) Transverse section micrograph of the surface region of the Stellite 6 sam

surface oxide.2

2

4

6

8

2

4

6

0

10

5

Ni,

wt%

Fe, w

t%W

, wt%

10 2 3 4 5 10 2 3 4 5m m

ubmitted to abrasive wear test at 750 1C and (b) line scan EDS spectrums across the

resistance. While the response of the X32CrMoV33 steel to wear

References

Y. Birol / Tribology International 43 (2010) 222222302230at 625 1C is abrasion dominated at 625 1C, oxidation becomesthe predominant mechanism at 750 1C. Extensive oxidationco-occuring with abrasive wear at 750 1C leads to substantialmaterial loss. The wear resistance of the Inconel 617 and Stellite 6alloys, on the other hand, improves at 750 1C.

The tribological behaviour is strongly affected by the nature,the thickness, the adherence and the morphology of the oxidescales [32,33]. The poor adherence and limited ductility of Fe3O4promote the failure of the thick oxide scale on the tool steelsample impairing its resistance to wear at elevated temperaturesOxides of aluminium and silicon are also well established to behighly protective for alloys intended for high temperatureapplications [30]. With a Cr content only as much as 3 wt% andwith hardly any Si and Al, the hot work tool steel, on the otherhand, lacks a continuous protective oxide and cannot enjoy suchprotection. The resistance to oxidation of the X32CrMoV33 toolsteel is clearly inferior with respect to the Inconel 617 and Stellite6 alloys that behave very similarly over this temperature range(Fig. 15).

4. Conclusions

The abrasive wear performance of the X32CrMoV33 hot worktool steel, better than those of Inconel 617 and Stellite 6 alloys at625 1C, is degraded at 750 1C due to its inferior oxidation

0100

101

temperature (C)100 200 300 400 500 600 700 800

Fig. 15. Weight gain as a function of temperature of the three alloys used in thepresent work.102

103

104

105Stellite 6

wei

ght g

ain

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Acknowledgements

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High-temperature abrasive wear testing of potential tool materials for thixoforming of steelsIntroductionExperimentalResults and discussionConclusionsAcknowledgementsReferences