Design of steels for high speed machining

S. V. Subramanian, H. O. Gekonde, X. Zhang, and J. Gao

lead are used as in AISI 1215 and AISI 12L14. Theincompatibility of deformation between the soft inclusionsHistorical analysis of metal cutting shows that metaland the steel matrix promotes damage events leading toremoval rates have been increasing in the course ofductile fracture, i.e. void nucleation, void growth, and voidthe century, predicated by the advancement in toolcoalescence.1,2 At high cutting speeds, the chip fracturematerials but the steel design has lagged behind.process is facilitated by strain rate hardening of the matrix,This paper examines the mechanisms of chipwhich obviates the need for a large volume fraction of softformation and tool wear as a function of cuttinginclusions.3 However, at high cutting speeds, chips exhibitspeed in metal cutting. Chemical wear is identifiedshear localisation, which causes chemical crater wear of theas the dominant mechanism of tool wear at hightool. The crater wear is caused by dissolution of the toolcutting speeds caused by temperature rise due tointo the chips, which occurs by diffusion mechanism.4 Theshear localisation in the primary and secondaryorigin of shear localisation, its consequences on chipshear zones of chip. Shear localisation in theformation, tool wear, and surface finish, and the control ofprimary shear zone is shown to be influenced byshear localisation in metal cutting through steel design areboth microstructural parameters, i.e. matrixdiscussed in this paper.hardening and second phase particles, and metal

cutting variables, i.e. cutting speed (strain rate) andfeed (pressure). Shear localisation in the secondary ORIGIN OF SHEAR LOCALISATION IN METALshear zone is caused by the tribological conditions CUTTINGof seizure at the tool/chip interface. Chemical crater

Theoretical and experimental investigations on shear local-wear is caused by the dissolution of tool into theisation in metal cutting were carried out at McMasterworkpiece (chip) by diffusion mechanism and canUniversity in a range of iron alloys with varying matrixbe prevented by suppressing the tribologicalhardening and volume fraction of inclusions.5 The micro-condition of seizure. The design of steel for highstructural changes in the chips were characterised usingspeed machining is based on engineering glassyoptical metallography, SEM, TEM, and X-ray diffractionoxide inclusions in steel, which are designed totechniques. Contact length and crater wear were charac-form a viscous layer in situ at the tool/chip interfaceterised using optical microscopy and SEM. The forceat high cutting speeds. The viscous layer lubricatesmeasurements were carried out. Salient observationsthe tool/chip interface and prevents the occurrencefrom the recent research investigations are presented andof seizure, thereby suppressing chemical craterdiscussed.wear. In comparison with the large volume fraction

of inclusions required for promoting ductile fractureat low cutting speeds, the amount of inclusions Analysis of shear localisation in metal cuttingrequired for lubricating the tool/chip interface is Matrix and second phase particles are important micro-very small and is in the range that is typical of clean structural parameters that influence shear localisation.steel. Thermodynamic modelling is shown to be a Thermal softening potential of the matrix, i.e. the changepowerful tool to engineer glassy oxide inclusions in in flow stress with temperature, causes thermoplastic shearsteel. I&S/1480 localisation in accordance with the mechanism originally

proposed by Zener and Hollomon about 50 years ago.6The authors are in the Department of Materials Science and When an increment in stress due to strain hardening of theEngineering, McMaster University, Hamilton, ON, Canada L8S 4L8. matrix is overcome by a decrease in stress due to thermalThis paper was presented at the 4th International Conference on

softening, plastic deformation becomes unstable and the‘Behaviour of materials in machining’, organised by The Instituteof Materials and held in Stratford-upon-Avon, UK on 12–13 homogeneous deformation gives way to a localised bandlikeNovember 1998. deformation to form adiabatic shear bands. The incompati-

bility of deformation between the second phase particles© 1999 IoM Communications Ltd.and the matrix causes void nucleation and growth andheterogeneous deformation leading to ductile fracture. Theflow localisation caused by the instability strain in thepresence of voids is referred to as geometrical softening.

BACKGROUND The general prediction in the literature for the onset ofinstability is defined by a maximum in shear stress or inAt low cutting speeds, the tribological condition at the

tool/chip interface is sliding. The chip morphology tends terms of critical strain. Bai and Dodd7 have concluded thatinstability is a necessary but not an adequate condition forto be flow or continuous type. Physical wear processes

dominate. Abrasion of the tool by inclusions or second localisation. Meyers8 has postulated that shear localisationis caused by a sharp decrease in flow stress accompanyingphase particles in the workpiece which are harder than the

tool contributes significantly to tool flank wear. Elimination a major microstructural event such as dynamic recrystallis-ation. Dodd and Atkins9 have analysed flow localisation inof the hard abrasive deoxidation inclusions such as alumina

or their modification into soft inclusions using calcium shear deformation under conditions of thermal softening,geometrical softening, and a combination of both. Thistreatment are essential features of the modern deoxidation

practice of steel to minimise tool flank wear. At low cutting approach has been extended in the present work to analyseshear localisation occurring in metal cutting.5 The changespeeds, chip disposal tends to be a problem even with chip

breakers. A large volume fraction of soft inclusions is in flow stress accompanying phase transformation is shownin the present work to cause shear localisation in metalengineered into the steel to promote ductile fracture.

Traditional free machining additives such as sulphur and cutting.5

ISSN 0301–9233 Ironmaking and Steelmaking 1999 Vol. 26 No. 5 333

334 Subramanian et al. Design of steels for high speed machining

a

b

a

b

1 a Flow chip morphology of hardened Fe–28·9Ni–0·10C 2 a Partially segmented chip morphology of hardenedalloy at cutting speed of 25 m min−1, exhibiting shear Fe–28·9Ni–0·10C alloy, at cutting speed of 150 mlocalisation, and b tool crater wear after 10 s cutting min−1, exhibiting shear localisation in primary andat 25 m min−1 (note crater well removed from cutting secondary shear zone, and b tool crater wear afteredge) 10 s cutting at 150 m min−1

Studies on matrix effect on shear localisation andits consequencesStudies on the influence of thermal softening potential ofthe matrix on shear localisation were carried out in aFe–28·9Ni–0·1C alloy that was heat treated to obtain a fullymartensitic structure. Shear localisation is revealed metallo-graphically by the white transformation band formedby the reverse martensite transformation to austenite.Figure 1a shows continuous chip morphology obtained ata cutting speed of 25 m min−1, with the shear localisationoccurring in the secondary shear zone at the tool/chipinterface as delineated by the white band, and Fig. 1bshows the corresponding tool crater wear which can bebarely detected. On increasing the cutting speed to75 m min−1, the chips exhibit the onset of shear localisationin the primary shear band in addition to transformationshear band in the secondary shear zone. Significant craterwear occurs, but the crater is located at some distance fromthe cutting edge. On increasing the cutting speed to150 m min−1, the chip exhibits shear localisation in boththe primary and secondary shear zones (Fig. 2a) andthe crater draws closer to the cutting edge (Fig. 2b). Onannealing the alloy to the fully austenitic condition beforemachining, the chip reverts to fully continuous chipmorphology, which demonstrates that in the absence ofthermal softening potential of matrix shear localisation

a

bdoes not occur and hence the chip morphology tends to be

3 a Flow chip morphology of annealed Fe–28·9Ni–0·10Cflow type. Figure 3a shows the flow chip morphology ofalloy at cutting speed of 350 m min−1 and b tool craterFe–28·9Ni–0·1C alloy in the annealed condition at a cuttingwear after 10 s cutting at 350 m min−1

speed of 350 m min−1, and Fig. 3b shows the correspondingtool crater wear located well away from the cutting edge.Theoretical analysis shows that the temperature in the ation shear band envelops the segmented chip in

Fe–28·9Ni–0·1C alloy. The white band is the region that hasprimary shear zone will increase linearly with cuttingspeed once the shear is localised.7 Figure 4a is an optical undergone phase transformation from martensite to austen-

ite. Figure 4b shows severe damage to the cutting edge ofmicrograph of an almost fully segmented chip obtained ata cutting speed of 456 m min−1, showing that transform- the tool, caused by chemical wear due to interaction of the

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Subramanian et al. Design of steels for high speed machining 335

a

b

a

b

4 a Nearly fully segmented chip of hardened Fe–28·9Ni– 5 a Fully segmented chip of ferritic ductile iron at0·10C alloy at cutting speed of 456 m min−1 and cutting speed of 350 m min−1 (shear localisation inb interaction of primary shear zone at high primary shear zone caused by geometrical softening)temperature with cutting edge, resulting in loss of and b tool crater wear, crater localised at cutting edgecutting edge of tool at 456 m min−1 of tool caused by shear localisation in primary shear

zone

primary shear zone at high temperature with the cuttingedge of the tool. At 456 m min−1 and above, the temperature wear at the cutting edge of the tool. At a lower cutting

speed of 200 m min−1, the chip is partially segmented andrise in the primary shear zone is adequately high to localisethe crater at the cutting edge of the tool, which extends the crater is localised at some distance from the cutting

edge, and the crater is caused by shear localisation in therapidly to the flank face of the tool. The loss of cuttingedge of the tool causes ploughing of the tool onto the secondary shear zone.workpiece, resulting in microstructural damage to themachined surface and loss of surface finish. It should be Effect of metal cutting variables on shearemphasised that, even though the onset of shear localisation localisationin the primary shear zone occurs at 75 m min−1, the chips

The effects of cutting speed, feed, depth of cut, and coolantstend to be partially segmented over a wide range of cutting

on Fe–28·9Ni–0·1C alloy were studied. The microstructuralspeeds up to 400 m min−1 and the crater wear is localised

changes accompanying phase transformation were used toat some distance away from the cutting edge. The chemical

map shear localisation. Cutting speed and feed are foundwear is controlled by shear localisation in the secondary

to be important metal cutting variables that influenceshear zone over a wide range of cutting speeds correspond-

shear localisation. On reducing the feed from 0·397 toing to partially segmented chip morphology. Thus, it should

0·055 mm rev−1, the shear localisation in the primary shearbe possible to minimise chemical crater wear by suppressing

zone is suppressed, and the chip morphology changesshear localisation in the secondary shear zone through

from fully segmented to flow type at a cutting speed ofin situ lubrication and coating of the tool.

350 m min−1 in hardened Fe–28·9Ni–0·1C alloy (see Fig. 6aand b). The effect of lowering the feed is to increase

Studies on geometrical softening effect on shear the compressive stress in the primary shear zone, which,localisation in turn, suppresses the geometrical softening effect. The

microstructural changes at low feed suggest more uniformThe effect of second phase particles on shear localisationwas investigated by comparing the behaviour of ferritic heating of the chip. Further investigations are required to

clarify the effect of feed on thermal and mechanical loadingductile iron containing a large volume percentage (~10%)of graphite nodules in a ferritic matrix with AISI 1020 steel. of the chips. In any case, the decrease in feed is shown to

be effective in suppressing shear localisation in the primaryFigure 5a is the segmented chip morphology exhibitedby ductile iron at a cutting speed of 350 m min−1. Chip shear zone, even in hardened matrix.

The strain rate in the primary shear zone increases withsegmentation is caused by geometrical softening due to alarge volume fraction of graphite nodules in ferritic matrix. cutting speed. The critical shear strain for strain localisation

decreases with increasing strain rate or cutting speed.Figure 5b shows the localisation of the crater at the cuttingedge of the tool caused by shear localisation in the primary The critical shear strain is influenced by contributions

from thermal and geometrical softening. The actual strainshear zone. The crater wear on the tool is localised at thecutting edge. Thus, irrespective of whether shear localisation occurring in the primary shear zone during metal cutting

decreases with strain rate (cutting speed). The actual strainis caused by thermal or geometrical softening, the localtemperature rise in the primary shear zone causes chemical can be calculated from the shear angle, which is a free

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336 Subramanian et al. Design of steels for high speed machining

7 Regimes of chip morphology as function of strain rate(cutting speed) during machining of typical hardenedsteel

in the secondary shear zone is sheared under the action ofcompressive and shear forces acting on the rake face of thetool. This results in thermoplastic shear localisation at thetool/chip interface, which raises the local temperature.Thermally activated processes set in, causing interdiffusionof solutes at the tool/chip interface, as demonstrated byIngle.4 The physical model for seizure based on atomic

a

b

6 a Full segmentation of chip caused by large feed contact at the tool/chip interface allows the prediction of(0·397 mm rev−1) at cutting speed of 350 m min−1 and the critical speed for the onset of seizure at the tool/chipb flow chip morphology caused by low feed interface. The onset of seizure is predicted when the normal(0·055 mm rev−1) at same speed stress at the tool/chip interface exceeds the yield strength

of the asperities subjected to the temperature rise in theprimary shear zone. The model is validated by Gekonde5boundary problem involving mechanics of metal cuttingby carrying out force measurement while machiningand material behaviour. Figure 7 shows schematically theAISI 1045 steel with cemented carbide tool. The chip bodyshear instability regimes as a function of cutting speed andtemperature and the normal stress increase with the cuttingits relation to chip morphology. If the actual strain exceedsspeed. Based on the experimental data, the critical speedthe critical strain, the onset of shear localisation is predicted.for the onset of seizure is derived. The onset of seizure atAn important consequence of shear localisation is chipthe critical cutting speed was confirmed by measuring thesegmentation. Based on the phenomenological observationsconcentration depth profile of tungsten penetration intoof chip morphology and the actual strain as a function ofthe chip, a consequence of the tribological condition ofcutting speed, the critical strain for shear localisation canseizure at the tool/chip interface.be deduced, which is shown schematically. Theoretical

analysis shows that the critical strain for shear localisationshould decrease with strain rate, as experimentally con- PREVENTION OF TOOL CRATER WEARfirmed by Vinh et al.10

Tool crater wear is the dominant mode of tool failure inmachining clean steel at high cutting speeds. Crater wear

Shear localisation in secondary shear zone is caused by the tribological phenomenon of seizure atthe tool/chip interface. Shear localisation raises the localWith the increase in cutting speed, there is a transition in

the tribological condition at the tool/chip interface from temperature at the tool/chip interface. Dissolution of thetool material into the workpiece (chips) occurs by diffusionsliding (asperity contact) to seizure (atomic contact). The

tribological phenomenon of seizure sets in when the normal mechanism. Quantitative analysis of diffusional wear hasconfirmed that high diffusivity paths are involved. Thestress (compressive) at the tool/chip interface exceeds the

yield strength of the asperities. It should be pointed out maximum depth of crater coincides with phase transform-ation temperature. Hence it is proposed that crater wear isthat the chip material is subjected to a temperature rise in

the primary shear zone before it flows over the rake face phase transformation coupled and that dislocations gener-ated during deformation concomitant with phase transform-of the tool, where the chip is subjected to secondary shear.

In consequence the asperities are deformed or squeezed to ation are effective in providing high diffusivity paths thatenhance diffusional wear.15 Chemical crater wear can bemake atomic contact at the tool/chip interface. The recent

work on nanotribology has emphasised that the nature of eliminated by preventing the tribological phenomenon ofseizure at the tool/chip interface by in situ lubrication.chemical bonding at the interface is important.11,12 The

essential feature of seizure in metal cutting is that atomic External lubricants are not effective once atomic contact(seizure) is established at the tool/chip interface.16 The toolcontact is established at the tool/chip interface and the

nature of the bonding is considered to be essentially dissolution crater wear can be prevented by in situlubrication at the tool/chip interface through inclusionsmetallic. Relative motion occurs within the softer (chip)

material. The layer of the chip in contact with the tool is engineered into the workpiece or by coating of the tool.Since dissolution crater wear is diffusion controlled, thestationary and relative motion takes place in the adjacent

layers with the shear velocity gradually increasing until the coating strategy is based on controlling factors thatsuppress diffusion at the tool/chip interface. In thebulk chip speed is reached. These views were expressed

by Trent, but quantitative evidence was not provided.13,14 diffusional wear analysis, the local equilibrium concen-tration of solute and the square root of the diffusionOnce seizure occurs, the chip material of the seized region

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Subramanian et al. Design of steels for high speed machining 337

coefficient of the solute in the chip matrix are the keymaterial parameters that determine the diffusional wear.By choosing a tool or coating which has the least solubilityin the workpiece, it should be possible to minimisediffusional wear.17 The diffusion coefficient can be loweredby decreasing the temperature through cryogenic machining.

Inclusion engineering of steels (workpiece) tominimise crater wearTheoretical analysis has shown that glassy inclusions canform a viscous layer at the tool/chip interface, therebysuppressing dissolution wear. The viscous layer formed atthe tool/chip interface must be adequately thick for theshear to be accommodated within the viscous layer ratherthan at the solid/viscous layer interface in order to minimisefriction.13 The lubricating layer needs to be about 10 atomlayers thick. It should be emphasised that the volumefraction of inclusions required to lubricate the tool/chipinterface is very small (~10−4), which is in the regime ofclean steel. The efficacy of the viscous layer in suppressingthe tribological phenomenon of seizure was investigated infree cutting steels. Glassy oxide inclusions engineered intoAISI 1215 was demonstrated to be more effective thanhafnium nitride (HfN) coating in suppressing dissolutioncrater wear.18

Thermodynamic modelling of inclusion engineeringof steelsThe pioneering research carried out over the years atIRSID on the thermodynamic modelling of slag–metalequilibria and the characterisation of the rheology ofdeoxidation inclusions have paved the way for inclusion

(a)

(b)

AISI 1045: [%C]=0·45, [%Si]=0·30, [%Mn]=1·0, [%S]=0·01,engineering in a number of applications including improved[%P]=0·01machinability.14–16 Recent advances in ladle metallurgy

8 a Composition region of target glassy inclusion inhave made the control of calcium treatment feasible.Si–Mn deoxidised AISI 1045 steel at 1495?C, where

Thermodynamic modelling capabilities have advanced in isoactivity of Al corresponding to inclusion definesrecent years to predict the composition of exogenous window of soluble Al required by slag–melt treatment,inclusions that form directly in liquid steel prior to and and b isoactivity of Al in melt after equilibration withsubsequent to slag–metal treatment, just prior to solidifi- CaO–SiO

2–Al

2O

3slag of eutectic E of anorthite

cation and the indigenous inclusions formed during solidi- composition at 1550?C in same steelfication. The use of these models in the control of therheology and composition of target inclusions that form aneffective lubricating layer at the tool/chip interface during Inclusion engineering of AISI 1045 steelhigh speed machining of AISI 1045 steel is discussed. In a 1045 grade steel, the silicon content is sufficiently high

The application of thermodynamic and kinetic models (0·30 wt-%) and the residual aluminium window for targetfor inclusion engineering of steels for improved machin- spessartitic inclusion is 0·25–2·5 ppm at 1495°C (see Fig. 8a).ability at high cutting speeds comprises the following steps. Since the aluminium in the melt is invariably high, a slag–

1. Identify the target inclusions that form in situ a metal treatment at 1550°C is required to control theviscous layer of adequate thickness at the tool/chip interface residual aluminium content in the melt within the targetduring moderate and high speed machining to prevent the window. By equilibrating with a slag of eutectic compositiontribological phenomenon of seizure. The rheology of the of anorthite in CaO–Al2O3–SiO2 ternary system at 1550°C,target inclusions is glassy. The composition of the target the residual aluminium is controlled to 2·5 ppm and theinclusions is defined by their viscosity at the temperature residual oxygen to 35 ppm (see Fig. 8b). On cooling toand pressure at the tool/chip interface during machining. 1495°C after slag–metal treatment, the residual aluminium

2. Design the base chemistry of the melt with respect to has decreased to be well within the target window forsilicon and manganese so that the inclusions in equilibrium spessartitic formation. The slag–metal equilibration offerswith the melt are controlled to be within the target region a niche over other methods for the control of residualof glassy spessartitic phase. concentration of reactive elements, such as aluminium, to

3. Select the composition of CaO–SiO2–Al2O3 slag for ultralow levels required for the precipitation of indigenousslag–metal treatment to control the residual aluminium glassy inclusions during solidification.and oxygen to the desired levels to form glassy inclusions.The inclusions that form during cooling of the melt to the

CONCLUSIONSliquidus temperature after slag–metal treatment are referredto as primary deoxidation inclusions or exogenous 1. At low cutting speeds, the tribological phenomenon

of sliding operates at the tool/chip interface. The chipinclusions. The composition of the inclusions is slagcontrolled. The target is to produce glassy anorthitic morphology is flow type and the mechanism of tool wear

tends to be physical. The design of steel for low cuttinginclusions.4. Control the residual aluminium and oxygen in the speeds is based on engineering a large volume fraction of

soft indigenous inclusions that promote ductile fracture ofmelt to the required levels to form glassy spessartiticinclusions during solidification of the melt. These indigenous chip formation process, and the elimination of hard abrasive

oxide inclusions that cause physical wear by abrasion.inclusions are melt controlled.

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338 Subramanian et al. Design of steels for high speed machining

2. At moderately high cutting speeds, the tribological of the authors (HOG) acknowledges research supportreceived from the International Development Researchphenomenon at the tool/chip interface changes over to

seizure involving atomic contact. Thermoplastic shear Centre (IDRC), Ottawa. Helpful discussions withDr Z. Basinski, Dr R. Sowerby, Dr J. D. Embury,localisation at the tool/chip interface raises local temper-

ature which causes dissolution of the tool into the chip Dr M. Elbestawi, and Dr G. R. Purdy are gratefullyacknowledged.by diffusion mechanisms. Dislocations generated during

deformation concomitant with phase transformation con-tribute to enhanced diffusional wear. Diffusional wear can REFERENCESbe lowered by the design of a coating of the tool with a

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