ORIGINAL ARTICLE

Performance of mixed ceramics and CBN tools during hardturning of martensitic stainless steel

Kehinde Sobiyi & Iakovos Sigalas & Guven Akdogan & Yunus Turan

Received: 9 June 2014 /Accepted: 14 October 2014# Springer-Verlag London 2014

Abstract This paper describes the wear behavior of a ceramicand a PcBN cutting tool during the turning of AISI 440Bstainless steels at different machining conditions. Experimentalresults showed that the wear mechanism for ceramic cutting toolis predominantly abrasive wear and for cBN tools was adhesivewear and abrasive wear. The abrasive wear is as a result of hardcarbide particles in the workpiece material resulting in groovesformed on the flank face. There was formation of transferredlayer followed by plastic deformation on the rake face of PcBNtool when cutting at low speed and feed rate. Better surfacefinish (Ra) was recorded for ceramics but with deterioratingsurface topography. The results also show that good dimensionalaccuracy can be achieved with cBN tools using CNC machinewith high static and dimensional stiffness coupled with high-precision hard turning.

Keywords PcBN .Mixed ceramics . Hard turning .

Cutting tool

AbbreviationsSEM Scanning electron microscopePcBN Polycrystalline cubic boron nitrideTiC Titanium carbide

1 Introduction

In the last few decades, hard turning has rapidly becomean attractive alternative route for finish production of partsmade from hardened steels as opposed to grinding. Hardturning is a finishing or semi-finishing operation thatdepends on geometrical accuracies, surface topography,and integrity of sub-surface layer [1, 2]. Generally, formany hardened parts applicable in the automotive, tool,and die industry, a relatively high accuracy can beachieved during hard turning but sometimes importantproblems arise with surface integrity, which reduce thefatigue life of the machined surfaces [3]. The quality ofthe surfaces of the machined part is highly dependent ontool wear and wear resistance of the cutting tool employedduring the machining operation [4, 5].

In hard turning operations, many cutting tools are usuallyemployed such as cermets, cemented carbide, ceramics, andpolycrystalline cubic boron nitride (PcBN). The most pre-ferred tools in machining hard materials such as cast irons,alloy steels, bearing steels, and case-hardened steels are ce-ramics and PcBN due to their high hot hardness and wearresistance [5–9].

Their hardness and chemical stability enable them towithstand the high thermal and mechanical loads gener-ated during hard turning operations. However, PcBNcutting tool materials have higher hardness, better abra-sion resistance, and excellent retention of strength com-pared to ceramic tool materials at both low and hightemperatures [10].

Many studies have been carried out on hard turning usingceramics and PcBN tools, with focus on the effects of toolcompositions, cBN content, tool geometry, cutting tempera-ture, cutting forces, cutting conditions, and frequency of in-terruption on tool life, on wear mechanisms, surface integrity,and dimensional accuracy [1–3, 7, 8, 11–23].

The total tool wear generally observed in any cutting toolcan be divided into two main broad categories, the chemical

K. Sobiyi (*) : I. SigalasSchool of Chemical andMetallurgical Engineering, University of theWitwatersrand, Private Bag 3, Wits, Johannesburg 2050, SouthAfricae-mail: kk_sob@yahoo.com

I. Sigalase-mail: iakovos.sigalas@wits.ac.za

G. AkdoganDepartment of Process Engineering, University of Stellenbosch,Private Bag X1, Matieland, Stellenbosch 7602, South Africae-mail: gakdogan@sun.ac.za

Y. TuranCastco Precision Castings, P.O. Box 19133, Tygerberg 7505, SouthAfrica

Int J Adv Manuf TechnolDOI 10.1007/s00170-014-6506-z

wear (dissolution or diffusion wear) and mechanical wear(adhesion, abrasion, and fracture wear), which are normallyassociated with the worn area on the cutting tool [24, 25].

The main wear mechanisms in PcBN during turning ofhardened steels are generally considered to be a combinationof adhesion, abrasion, and diffusion, and the wear mechanismis dependent on the cutting conditions, tool geometry, andmaterial properties of the tool and the workpiece [17, 19, 23,26]. The ceramic wear mechanism involve abrasion, plasticflow, adhesive fracture, and tribological effects which alldepend on the mechanical and thermal conditions generatedduring the machining test [3].

The aim of this study is to investigate the feasibility ofusing PcBN and Al2O3-based cutting tools for hard turning ofmartensitic AISI 440B stainless steel by observing the effectof variation of cutting parameters on the tool wear, surfaceintegrity, dimensional accuracy, and chip formation.

2 Experimental procedures and method

The workpiece material was martensitic stainless steel AISI440B (supplied by Bohler steel) widely used in engineeringapplications due to its good mechanical properties resultingfrom its high chromium and carbon content [27]. Its chemicalcomposition (in wt%) is given in Table 1.

The workpiece was through-hardened followed by a tem-pering process by yielding a microstructure of a temperedmartensite to attain hardness between 40 and 44 HRC(Rockwell hardness). Hardness was measured with a digitaldurometer (DM2-D 390).

Dry finish turning tests were performed on the martensiticstainless steel using two different cutting tools. Those werecommercially available cutting tools made frommixed ceram-ic (Al2O3-based) and polycrystalline cubic boron nitride(PcBN). The composition of the cutting tool materials is givenin Table 2 below as given by the supplier. The average grainsize was measured using the scanning electron microscope.

A CNC Turn Master LA200 LLiouya-Hsing lathe with a14.72-kW power output and maximum speed of 3,500 rpm,using a Fanuc Oi-TB CNC controller, was used to conduct theturning tests.

The cutting tool inserts were secured onto a tool holderwhich matches the insert types used (SNGN 090308S-01020).The geometry of the inserts results in a rake angle of −6°, aback rake angle of −6°, clearance angle of 6°, and an approachangle of 45°. The mixed ceramics and PcBN inserts were bothsquare shaped with 0.8-mm nose radius (honed, chamfer0.1 mm×20°).

The turning conditions are given in Table 3. The cuttingconditions used for the experiments were appropriate forfinish cutting.

The diameter of the workpiece tested was 50 mm and thecutting length was 200mm. Before actual testing, the oxidizedlayer present on the workpiece surface was removed by re-moving the outmost 1 mm. After each pass, the average toolwear was measured. The flank wear failure criterion for cut-ting tool wear scars was used according to JIS B4011-1971standard. According to this standard, cutting tool failure is setat 200 μm wear on the flank face, that being the point beyondwhich the tool can no longer achieve a good finish on theworkpiece surface.

The tool wear was evaluated using an optical micro-scope (Olympus BX 41 M) and the worn tool was exam-ined with an ultra-high resolution field emission scanningelectron microscope (SEM), Joel JSM 7600 F, in order tounderstand the wear modes and mechanisms that affectthe tool performance. Qualitative observations were usedto confirm plastic deformation and adhesion based on thebuildup of material on the flank and rake faces coupledwith EDX analysis.

Instantaneous roughness criteria measurements (arithmeticmean roughness, Ra), for each cutting condition, are obtainedby means of a Bondetec roughness meter. The length exam-ined was 3 mm with a basic span of 0.8 mm. This roughnesswas directly measured on the workpiece held between thechuck and tailstock, without dismounting from the lathe, inorder to reduce uncertainties caused by resumption operations.The measurements were repeated three times and taken atreference lines equally positioned at 120°. An average of thesevalues was taken as the result.

The chip morphology was observed using an optical mi-croscope (Olympus BX41M), followed by an analysis of thechip forms using the scanning electron microscope.

Table 1 Chemical composition of the AISI 440B stainless steel (wt%)

C Cr Mo V Si Mn Fe

0.9 17.5 1.10 0.10 0.45 0.40 Balance

Table 2 Composition of cutting tool materials

Cutting tool Supplier Composition

PcBN (CBN 100) SECO tools cBN (50 %)+TiC (40 %)+WC (6 %)+AlN, AlB2 (4 %) average grain size of cBN is 2 μm

Mixed ceramic (CC 650) Sandvik Coromant Al2O3 (70 %)+TiN (22.5 %)+TiC (7 %) average grain size of TiC is 4 μm

Int J Adv Manuf Technol

3 Results and discussion

3.1 Flank wear of tool materials

The flank wear with the cutting time progression with thecutting speed as the parameter, at depth of cut of 0.1 mm andfeed rate of 0.1 mm/rev, is shown in Fig. 1. The cutting speedsselected were moderate as recommended by the suppliers ofthe cutting tools. The results show that the flank wear in-creases with increasing cutting speed for both cutting tools.Generally, the cutting speed shows greater effect on the toollife of any cutting tool, thus this is the most significant factoraffecting the tool life during machining of hardened steel [15,28]. The PcBN cutting tool, however, showed a better wearresistance for all cutting speeds; this is due to its fracturetoughness and ability to retain its hardness at elevated tem-perature (higher hardness at both low and high temperatures),and thus, it can withstand the high temperature generated atthe tool-chip interface [28, 29]. For the ceramic cutting tool, atthe tool chip interface, the cutting temperature observed washigh resulting from the cutting speed, and the layer on the toolface becomes soft with more accelerated wear, since the flankface can be easily abraded by the hard particles in the workmaterial [8].

At cutting speed of 200 m/min, PcBN cutting tool removesmore material from the workpiece when compared to themixed ceramic at speed of 100 m/min. This makes it moresuitable to use in terms of wear resistance.

Figure 2 shows the average flank wear as a function ofcutting length, at the much higher cutting speed of 600 m/min,with same depth of cut and feed rate as used in the previousmeasurements. At this cutting speed, as the tool enters andleaves the workpiece, the tool is exposed to rapid mechanicaland thermal shocks. The wear rate for PcBN cutting tool waslower at initial cuts, but toward the end of the tool life, thecutting tool failed by excessive chattering of the flank faceresulting from its poor impact resistance and mechanicalstrength which is caused by ineffective crystal to crystalbonding of the PcBN (low cBN content) tool [30].

Figures 3 and 4 show the flank wear as a function of cuttinglength for mixed ceramic and PcBN, respectively, with feedrate as the parameter. It is seen that the flank wear decreaseswith increasing feed rate for mixed ceramic and also for cBNcutting tool. There is a significant effect of feed rate increaseon the wear rate most especially with feed of 0.15 mm/rev.More material can easily be cut within the shortest timeinterval, while the wear rate improves. For cBN, the flankwear decreases initially with an increase in the feed to 0.1 mm/rev, and later increases with feed of 0.15 mm/rev.

Table 3 Turning conditions

Tool Mixed ceramic, PcBN

Cutting speed, Vc (m/min) 100, 150, 200

Feed, f (mm/rev) 0.05, 0.1, 0.15

Depth of cut, d (mm) 0.1

0

50

100

150

200

0 5000 10000 15000 20000 25000 30000

Ave

rage

Fla

nk w

ear

Vb

(µm

)

Cutting length (mm)

Mixed ceramic-100 Mixed ceramic-150 Mixed ceramic- 200

cBN-100 cBN-150 cBN-200

Fig. 1 Effect of cutting speed on flank wear, at feed=0.1 mm/rev andDOC=0.1 mm

0

50

100

150

200

250

0 500 1000 1500 2000 2500 3000

Ave

rage

Fla

nk w

ear

Vb

(µm

)

Cutting length (mm)

Mixed ceramic cBN-100

Fig. 2 Flank wear at 600 m/min, at feed=0.1 mm/rev and DOC=0.1 mm

0

50

100

150

200

0 2000 4000 6000 8000

Ave

rage

Fla

nk w

ear

Vb

(µm

)

Cutting length (mm)

feed-0.05 mm/rev feed-0.1 mm/rev feed-0.15 mm/rev

Fig. 3 Flank wear at 100 m/min, for mixed ceramic

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Generally, PcBN cutting tool showed a better perfor-mance in all the cutting conditions investigated due to itssuperior wear resistance, and ability to retain its strengthat higher cutting temperatures, but at low feeds, the toolwear for PcBN is very high as a result of it lower tough-ness. Thus, it cannot withstand high machine vibrationwith small feeds. However, for both cutting tools, the feedof 0.1 mm/rev is mostly recommended due to its moderateflank wear and better surface finish. There is a highimpact loading on the tribological system, which iscaused by microgrooves and macrogrooves on the work-piece surfaces, accompanied by high temperature at thechip-tool interface. The high cutting temperature generat-ed at the chip tool interface resulting from high speedsand feeds leads to the accelerated tool wear of the mixedceramic due to the decrease of its hot hardness, poorshock resistance, and fracture toughness [31].

3.2 Morphology of cutting tool

Figures 5 and 6 show the optical micrographs and Figs. 7 and8, the SEMmicrographs of the worn cutting tool surfaces. Thefigures show the cutting tools’ flank face and rake face. Themost dominant wear modes in both cutting tools are flank andcrater wear, which can be seen easily in the SEMmicrographs.Abrasive wear mechanism is more predominantly observedfor both cutting tools on the flank face but is moderate at thechip-tool interface on the rake face. Other wear mechanismssuch as chipping, adhesion, and plastic deformation were alsoobserved on the cutting tools.

Figure 5a–f clearly shows a smooth abrasive wear (marksparallel to the cutting direction) on the flank face and evidenceof built up layer on the rake face of the PcBN cutting tool,observed at all cutting speeds and feeds. The grooves formedon the flank face of the PcBN tool result from the detachmentof cBN grains from the bond caused by the hard particles inthe work material [8]. The smooth wear is normally observedwith this particular cutting tool when cutting hardened steel asa result of the small feeds and strength of the cutting tool [32].The metal to metal contact and high temperature at the chip-tool interface creates the environment where diffusion cantake place in the cutting tool; however due to the temperaturesgenerated, there is a possibility of chemical interaction be-tween the cutting tool and the workpiece material. This wearphenomenon is normally reported when machining hardenedsteels using PcBN [7, 23, 33]. There is evidence of grooves onthe top surface of the flank face at cutting speed of 100 m/minat initial cuts, and this is later covered by adhered, plasticallydeformed material. The clearance face of the cutting tool atthis speed shows brittle chipping. At feeds of 0.05 mm/rev,evidence of chipping on the clearance face was clearly ob-served. When the feeds and speeds were further increased, thechipping was reduced. On the rake face, at 100 m/min, there is

0

20

40

60

80

100

120

0 2000 4000 6000 8000 10000 12000

Ave

rage

Fla

nk w

ear

(µm

)

Cutting length (mm)

feed-0.1 mm/rev feed-0.05 mm/rev feed-0.15 mm/rev

Fig. 4 Flank wear at 100 m/min for PcBN cutting tool

Chipping

Transferred layer

Moderate abrasiona) b) c)

e)d) f)

Fig. 5 Optical micrograph of theworn cBN cutting tool at varyingcutting speed, a 100 m/min, b150 m/min, and c 200 m/min; andvarying feed rate, d 0.05 mm/rev,e 0.1 mm/rev, and f 0.15 mm/rev

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evidence of built up edge, moderate abrasion (cratering), andadhesion with transferred layer as shown in Fig. 7b, c. Thecomposition of the adhered metal on the cutting tool edge was

confirmed by EDAX analysis. The adhered material consist ofthe constituents of the workpiece material with elements suchas, Fe, Si, Cr, Mn, and oxygen formed during oxidation. From

Abrasion marks

Grooves

a) b) c)

e)d) f)

Fig. 6 Optical micrograph of theworn mixed ceramic cutting toolat varying cutting speed, a 100 m/min, b 150 m/min, and c 200 m/min; and varying feed rate, d0.05 mm/rev, e 0.1 mm/rev, and f0.15 mm/rev

Smooth abrasion

Chipping

Flank wear

Adhesion followed by

plastic deformation

Crater

b)a)

100 µm

Transferred material followed by plasticdeformation

10 µm

c)

Fig. 7 SEM micrograph and EDAX of failed cBN cutting tool, speed 100 m/min, feed 0.1 mm/rev, and DOC 0.1 mm

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Fig. 8c, there is evidence of grain pull out from the cuttingtool, which is carried away from the flank face.

Figures 6 a–f and 8a–c show the micrographs of the wornmixed ceramic cutting tool. The main wear mechanism ob-served is abrasive wear, with deeper grooves when comparedwith the parallel marks on the flank face of the cBN cutting

tool. This could be a result of fragments of the carbide grainsthat are pulled out from the tool surface, dragged across theflank face, thus removing the tool material. However, thedeeper marks, which are a result of the carbide grains in themixed ceramic, are found to be bigger than the grains in thecBN. On the rake face, there exist adhesion and crater wear,

Abrasion

Cratering with

transferred material

Grain pull out

a)

c)

b)

1 µm

100 µm 10 µm

Fig. 8 SEM micrograph and EDAX pattern of failed mixed ceramic cutting tool, speed 100 m/min, feed 0.1 mm/rev, and DOC 0.1 mm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1000 2000 3000 4000

Su

rfa

ce r

ou

gh

ness R

a (

µm

)

Cutting length (mm)

Mixed ceramic cBN

Fig. 9 Surface roughness (Ra) produced at 100 m/min cutting speed,feed 0.1 mm/rev, and DOC 0.1 mm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 500 1000 1500 2000 2500 3000 3500

Su

rfa

ce r

ou

gh

ness R

a (µ

m)

Cutting length (mm)

Mixed ceramic cBN

Fig. 10 Surface roughness (Ra) produced at 150 m/min cutting speed,feed 0.1 mm/rev, and DOC 0.1 mm

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caused by rapid shearing of the successive layers of the cuttingtool, thus forming deep craters.

The predominant abrasive wear occurring on the flank faceof both cutting tools is as a result of the hard carbide particleswithin the martensitic matrix of the workpiece material suchasM23C (predominant carbide in this workpiece), SiC, Cr7C3,VC, Mo2C, and Fe3C as described by many authors as themajor cause of wear during machining of hardened steels [4,5, 7, 20, 23].

3.3 Effect of machining parameters on the surface roughness

Diameter error, geometric deviation, and surface integrity arethe key quality characteristics in any turning operation.Surface roughness in metal cutting is sometimes used forevaluating the cutting tool failure and also a good indicationof the quality of the machined surface. The surface finishproduced during hard turning while using the PcBN andmixed ceramic was found to be generally satisfactory. Theroughness value for PcBN cutting tool was similar at cuttingspeed of 100 and 150 m/min, but it improved at 200 m/min.For the mixed ceramic cutting tool, the Ra value was better at100 m/min compared to higher cutting speeds. The minimumand maximum values of Ra 0.51 and 1.13 μm, respectively,were recorded for the mixed ceramic tool and 0.63 and 1.44for cBN tool.

The surface finish produced while using mixed ceramiccutting tool is found to be better compared to that obtainedwith PcBN cutting tool at all the cutting speeds (100, 150, and200 m/min) investigated. Figures 9, 10, and 11 show thesurface roughness variation with depth of cut. In Fig. 10, atcutting speed of 200 m/min, mixed ceramics produce an Ra inthe range of 0.5 μm in the initial cuts, and later, the surfacestarts deteriorating as the flank wear increases. At averageflank wear of 150 μm in case of mixed ceramic inserts, the Raobtained was about 1 μm while at about the similar Ra value,the flank wear of cBN was about 76 μm at cutting speed of100 m/min.

The effect of increase in the cutting speed results in adecrease in the surface finish produced by the cBN cuttingtool, whereas for mixed ceramic, the surface finish is moderateat the initial cuts but increases with cutting speed as the tool isworn-out, resulting in chatter marks and debris on the ma-chined surfaces as shown in Figs. 12a and 13a. The effect oftool vibration and tool wear has a significant effect on thesurface finish of the machined component [9].

Figures 12 and 13 show the typical surface produced on theworkpiece after machining using PcBN and mixed ceramic.Observed in Figs. 12b and 13b are deeper feed marks on thesurface generated with PcBN machining. This shows thereason for a higher surface roughness obtained with cBNcompared to mixed ceramic cutting tool. However, the surfaceintegrity obtained with the mixed ceramic has more defectscompared to the one obtained using PcBN cutting tool. Thedefects are more on the surface produced at increased cuttingspeed. Layers of debris that have undergone severe deforma-tion are observed on the machined surface. This defect may bea result of unstable built up edge adhering unto the machinedworkpiece surface caused by the high cutting temperatures atthe cutting tool interface [4].

The difference between the measured diameter and thedesigned diameter, where a positive error indicates undercut-ting of a cylindrical workpiece, is known as the diameter error.For turned component parts especially where cylindrical fitsare involved, the dimensional error is an important qualitycharacteristic [34]. The average diameter of the workpiecewas checked for dimensional deviation after subjected toturning using the cutting tools tested in this study. The result

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000 2500 3000

Surf

ace

roug

hnes

s Ra

(µm

)

Cutting length (mm)

cBN 100Mixed ceramic

Fig. 11 Surface roughness (Ra) produced at 200 m/min cutting speed,feed 0.1 mm/rev, and DOC 0.1 mm

a) b)Fig. 12 Machined surfaces. aMixed ceramic and b cBN atcutting speed of 100 m/min, feed0.1 mm/rev, and DOC 0.1 mm

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could be used for determining the most appropriate time forthe tool to be discarded when considering the tolerance.Figure 14 shows the increase in the geometrical deviation(concentricity) of the workpiece as cutting progresses withthe flank wear (measured in terms of length of cut). At thefinal ninth pass, the dimensions of the final piece compared tothe expected ones had increases by about 15 μm with PcBNcutting tool and 32 μm for the mixed ceramic cutting tool.This is as a result of the edge of the cutting tool that is nolonger sharp due to the wear of the edges. At the end of thenine passes, the average tool wear for the cBN cutting tool was76 and 165 μm for cBN and mixed ceramic, respectively, atthe same cutting time. The results obtained show clearly thatthe cutting tool wear affects the geometrical deviation; as thetool becomes worn, the tool tip geometry changes [35, 36].There is a loss of effective depth of cut resulting from the wearof the tool tip on the clearance side, leading to change ofalignment between the cutting tool and the workpiece asdimensional and form errors are induced [36]. Generally,PcBN produces better dimensional accuracy due to its betterwear resistance [3].

3.4 Chip morphology

Figure 15 shows the optical micrograph of the typicalcutting chip produced by the cutting tools. The cutting

chip produced by PcBN at cutting speed of 100 m/min isbrown in color while that produced by the mixed ceramicis darker and purple in color. The color change is due tohigh temperature experienced at the chip tool interface forboth inserts. PcBN has a higher thermal conductivity,reducing the cutting temperature at the tool edge, whichgives it the ability to conduct more heat away from thecutting zone than the mixed ceramic tool [37].

At low cutting speeds, continuous cutting chips were pro-duced by both cutting tools (see Fig. 16). Only the colordiffers at speeds above 150 m/min. The cutting chips pro-duced by mixed ceramic become serrated at 150 m/min whilethis only happens with PcBN at speeds above 200 m/min. Atextreme high speeds, segmented cutting chips were formed byboth cutting tools.

Figure 16c, d shows the upper surface of the chips, which isobserved as rough, with minute corrugations. Greater slantingcorrugations with more periodical cracks and breaking up ofthe outer edges into series of segments are produced by themixed alumina tool.

The underside of the cutting chips produced by mixedceramic and PcBN at cutting speed of 100 m/min is shownin Figs. 17 and 18, respectively. The underside of mixedceramic produced chips shows evidence of feed marks, debrisfrom the transferred layer, and small pores. Some of the chipremoved is attached unto the surface of the cutting tool by

a) b)Feed direction

Fig. 13 Machined surfaces. aMixed ceramic and b cBN atcutting speed of 200 m/min, feed0.15 mm/rev, and DOC 0.1 mm

-35

-30

-25

-20

-15

-10

-5

0

0 500 1000 1500 2000 2500 3000

Δd

(µ

m)

Cutting Length (mm)

Mixed ceramic cBNFig. 14 Effect of tool wear ongeometric dimensional deviation,speed 200 m/min, feed 0.1 mm/rev, and DOC 0.1 mm

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adhesion during the shearing process, through plasticdeformation of the chip. Similar trends was also ob-served with PcBN cutting tool as shown in Fig. 18,except that the feed marks were not pronounced andformation of new compounds was observed in the mi-crostructure of the cutting chip.

4 Conclusions

The wear mechanism of mixed ceramic and cBN cuttingtools during hard turning of martensitic AISI 440B stain-less steel was studied. The following conclusions can bedrawn:

& The cutting speed has the more significant effect on theflank wear rate; it increases with increase in cutting speedfor both cutting tools. At increasing feed rate, the flankwear increases with increase in feeds with mixed ceramictool, but with cBN cutting tool, the opposite occurs mostespecially at feed of 0.05 mm/rev due to excessive tool

vibration. However, feed rate of 0.1 mm/rev is recom-mended for finish turning due to its moderate tool wear.

& cBN tool has a higher metal removal rate as a result of itsbetter wear resistance due to its high hot hardness and abilityto retain its strength at higher cutting speeds and feed rates.

& Tool wear is terminated by the wear on the flank face. Thefailure of mixed ceramic is predominantly by abrasivewear and cratering, while cBN fails by abrasive, adhesive,and diffusion wear. The abrasive wear is caused by hardcarbide particles within the martensitic matrix of the workmaterial and pulled out grains from the tool material.

& Better surface roughness Ra is produced by the mixedceramic tool, but this material yields poorer surface qualitywith inclusions and debris on the machined surface, whencompared to the surface produced by the PcBN cutting tool.

& The cutting chips produced are long, continuous, andserrated at low cutting speeds for both the tools but ser-rated and segmented as the cutting speed increases beyond150 m/min. However, the chips formed with the mixedceramic tool show a darker surface, resulting from oxida-tion caused by the higher temperatures generated duringthe machining process.

Fig. 15 Typical chips formedduring hard turning of martensiticstainless steel. a cBN and bmixedceramic

a) b)a)

c) d)

10 µm 10 µm

Fig. 16 Optical micrograph ofcutting chip by a mixed ceramicand b cBN, and SEM imaging, cmixed ceramic and d cBN at100 m/min, feed 0.1 mm/rev, andDOC 0.1 mm

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Feed marks Micro pores

100µm 10µm

Fig. 17 Underside SEMmicrograph and EDAX pattern ofcutting chips produced by mixedceramic at cutting speed of 100m/min

Small debris

100µm 10µm

Fig. 18 Underside SEMmicrograph and EDAX pattern ofcutting chips produced by cBN atcutting speed 100 m/min

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Acknowledgments The authors wish to thank DST-NRF Centre ofExcellence in Strong Materials (CoE-SM) and Element Six Pty Ltd. forfunding this work and Tshwane University of Technology, Pretoria, forproviding characterization facilities.

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