105 

AdvancesinProductionEngineering&Management ISSN1854‐6250

Volume12|Number2|June2017|pp105–114 Journalhome:apem‐journal.org

https://doi.org/10.14743/apem2017.2.243 Originalscientificpaper

  

Analysis of corrosion resistance when turning martensitic stainless steel X20Cr13 under chilled air‐cooling 

Kostadin, T.a,*, Cukor, G.b, Jakovljevic, S.c aKarlovac University of Applied Sciences, Karlovac, Croatia bUniversity of Rijeka, Faculty of Engineering, Rijeka, Croatia cUniversity of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia   

A B S T R A C T   A R T I C L E   I N F O

Amongstdifferentmethodsofcoolinginmachining,applicationofairiscon‐sideredtobethecleanestandmostenvironmentallyfriendlyone.Theaimofthispaperwastoexplorethepossibilityofchilledair‐coolingconsideringthecorrosionresistancewhenturningmartensiticstainlesssteelX20Cr13.Thus,acomparisonbetweenanalternativecoolingtechniquewherecompressedairisrefrigeratedandjettedtothecuttingzonebymeansofRanque‐Hilschcoun‐ter‐flow vortex tube and the conventional flood cooling with oil‐in‐wateremulsion was done. The corrosion resistance was determined by electro‐chemical testingandthesurfaceconditionwasanalysedwithbothScanningElectronMicroscopy(SEM)andEnergyDispersiveX‐raySpectroscopy(EDS).From the results presented herein, it can be concluded that the specimencooledwithchilledairhasbetterresistancetocorrosionactivityafteracer‐tainlongertimeinrelationtothespecimencooledwithemulsion.Inaddition,thespecimencooledwithchilledairhaslowersurfaceroughness,whichhasapositiveeffectonthecorrosionresistance.Hence,inthescopeofenvironmen‐tallyfriendlymachiningthevortextubebasedchilledair‐coolingcanbesuc‐cessfullyappliedwhenturningdifficult‐to‐cutmartensiticstainlesssteel.

©2017PEI,UniversityofMaribor.Allrightsreserved.

  Keywords:TurningStainlesssteelX20Cr13CorrosionresistanceChilledair‐coolingVortextube

*Correspondingauthor:tihana.kostadin@vuka.hr(Kostadin,T.)

Articlehistory:Received3February2017Revised20April2017Accepted24April2017 

  

1. Introduction 

Stainlesssteelsarenotablefortheircorrosionresistance.Themachinabilityofstainlesssteelsissubstantiallydifferent from thatof carbonor alloy steels andothermetals and theyare com‐monlyknownasdifficult‐to‐cutmaterials.Theuseofcutting fluids isgenerallyadvised inma‐chiningofstainlesssteels inorder toovercometheirwork‐hardening tendencyaswellas lowthermalconductivitythatrestrictstheheatflowawayfromthemachinedsurfaces.Overheatingof stainless steel surfaceduringmachining, characterisedby the formationofheat tinting col‐ours,mustbeavoidedsincethecorrosionresistancecanbedeteriorated.Atissue,picklingthesurfacecanbeusedtorestorecorrosionresistanceonthefinishedworkpiece.Anotherreasonthatcuttingfluidsareusuallynecessaryforcuttingstainlesssteelsistheirtendencyofformingthelongchips,whichadheretothetoolrakefaceandfrequentlyformbuilt‐upedgeleadingtoapoor surface finish.Floodsuppliedoil‐in‐wateremulsionsarenormallyused formachiningathighercuttingspeedswithcarbidetooling.Toallowself‐passivationofthestainlesssteelsurfac‐es,alltracesofthecuttingfluidshouldberemovedfromtheworkpieceaftermachining[1].

Thegrowingchallengeinindustryistodomachininginagreenenvironmentwithoutsacri‐ficingthemachiningperformance.Conventionalliquidcuttingfluidsinso‐calledwetmachining

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create several health and environmental problems. Especially, their degradation and ultimatedisposalarethemaindifficulties.Therefore,usingthegasasacoolantmayworkasanalterna‐tive to conventional liquid coolants inwetmachining.Nowadays, themost exploited gases ingas‐cooledmachining are air, carbon dioxide and nitrogen. The air is a free natural resourcereadilyavailableeverywhereandwithoutanychemicalexposurerisks tohumansor theenvi‐ronment.Inaddition,therearenocoolantdisposalproblemsandassociatedcostssincetheairismergedintotheatmosphere.Amongstavailablemethodsofcoolinginmachining,applicationofairisrecognizedtobethecleanestandmostenvironmentallyfriendlyone[2].

Numerous firms have implemented chilled air‐cooling as an alternative cooling method,achievingdifferentbenefitsinsubstitutingconventionalliquidcuttingfluidssuppliedbyflood‐ing.Anenvironmentallychilledairsupplycanbeperformedbyvortextubeforcoolingandjet‐ting thecompressedair to thecuttingzone,by thermoelectricaircoolerorbycryogeniccom‐pressedaircooler[3].

Ranque‐Hilschvortextube,orsimplyvortextube,isamechanicaldevicethatisusedforsim‐ultaneousgenerationofcoldandhotairstreamsoutofasinglesourceofcompressedair.Fur‐thermore,thecostofavortextubewithaccessoriesisbetween500‐1000€,soitisreasonablypricedforsmallmanufacturingfirms[4].Generally,itcanbeclaimedthatsuchasystemisabletodeliver thermalcoolingperformanceverymuchcomparable toconventional liquidcoolantsand produces bettermachining performance compared to drymachiningwithout any cuttingfluid[5].InturninghypereutecticAl‐Sialloyschilledairincomparisontodrymachiningreducestheflankweardependinguponcuttingconditions(upto20%),thetemperatureinthetool‐chipinterface(upto7%)andcuttingforcesmostlyowingtolesseningofadhesionandbuilt‐upedge[6].InhardturningofTi6Al4Vtitaniumalloy,chilledair‐coolingincomparisontodrymachiningsimplygeneratesthewrinkledandbreakingchips,providessignificantlylowercuttingtempera‐ture(upto57.1%),diminishesthetoolwearanddecreasestheaveragevalueofsurfacerough‐ness(upto27.6%)[7].Insoftmaterialsendmilling,thevaluesofsurfaceroughnessforchilledair‐cooling are lesser than those indrymachining and are greater than those in conventionalwetmachining(floodcooling)whilethevaluesofflankwearforchilledair‐coolingareneartothoseinconventionalwetmachining[8].Similarresultsoftheconductedinvestigationsinmill‐ingandhardmillingof42CrMo4steel also confirmedhighefficiencyof the chilledair‐coolingnotonlyincomparisonwithdrymachining,butalsoincomparisonwithconventionalfloodcool‐ingusingliquidcuttingfluids[9,10].Inmillingofcarbonfibrereinforcedplastic(CFRP),thetoollifeprolongationwhenutilizingchilledairasareplacementfordrymachiningisfrom12.1%atlowestcuttingspeedandfeedto45.6%athighestcuttingspeedandfeedwiththeimprovementof delamination factor too [11]. In cylindrical grinding of spindle shaft materials SCM4 andSCM21,chilledair‐coolingcanreducedboth theresultingcompressiveresidualstressesat theworkpiecesurfaceandthesurfaceroughness[12].Hence,apart fromitsenvironment friendli‐nessoption,vortextubebasedchilledair‐coolingsystemsfindapplicationinmachiningdifficult‐to‐cutmaterials.

Heatremoval,i.e.coolingisoneofthemostsignificantrolesofacuttingfluid.Moreefficientremovalofheatleadstobettertoollifeanddimensionalaccuracyofworkpiece.Incomparisontooil,waterpossessesgreatercapacityforremovingtheheat.Yet,watercausescorrosiononthefreshlymachinedmetal,which isanuisanceforbothuserandmanufacturerofwatermisciblecutting fluids. Corrosion inhibitors are an integral part of their formulationswith purpose topreventcorrosiononworkpieces,machinetoolpartsandtoolsmadeofmetals.However,corro‐sioncanalsoappearindrymachiningsinceitoccursnotonlyduetouseofwatermisciblecut‐tingfluids[13].Consequently,itisworthtostudytheinfluenceofchilledair‐coolingduringthecuttingprocesson thecorrosionresistanceofamachinedsurface. It shouldbenoted that,ac‐cordingtothedataavailabletotheauthorsofthisstudy,therearenoprofessionalorscientificpapersonthistopicsofar.

Therefore,theobjectiveofthispaperwastocomparealternativechilledair‐coolingvs.con‐ventionalfloodcoolingconsideringthecorrosionresistancewhenturningmartensiticstainlesssteel X20Cr13. Flood cooling was carried out by the commercial oil‐in‐water emulsion andchilledairwasgeneratedbyRanque‐Hilschcounter‐flowvortextube.Selectionofthetestmate‐

Analysis of corrosion resistance when turning martensitic stainless steel X20Cr13 under chilled air‐cooling 

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rialwasdonebearinginmindthatthemartensiticstainlesssteelshavethehigheststrengthbutalso the lowest corrosion resistance in comparison with the austenitic and ferritic grades ofstainless steels [14]. The presented results show that environmentally friendly turning of themartensiticstainlesssteelcanbeperformedandecologicalburdenofconventionalfloodcoolingcanbeeliminated.

2. Electrochemical testing of corrosion resistance 

In standard EN ISO 8044:1999 [15], corrosion is defined as "physicochemical interaction be‐tweenametalanditsenvironment,whichresultsinchangesinthepropertiesofthemetal,andwhichmayleadtosignificantimpairmentofthefunctionofthemetal,theenvironment,orthetechnicalsystemofwhichtheseformapart".Corrosionresistanceisapropertyofthematerialresistancetotheactionofthesurroundingmedium,whichmeansthatthecorrosionmorestablematerialistheoneinwhich,inthesameexternalconditions,thereislessintensivedestructiononthesurface,orlessundesirablechangesinmicrostructure[16].

Corrosionresistanceofmaterialcanbedeterminedbyusingdifferentmethods.Responsive‐ness to low corrosion rates, short experimentation andwell‐founded theory are themain ad‐vantagesofelectrochemicaltechniques.Measurementsofanelectrochemicalpotentialisoneofthemorebasicmeasurementsinelectrochemistry.Intestingpractice,abasicpolarizationcellisusedconsistingofanelectrolytesolution,aworkingelectrode(metalspecimenwhosecorrosionrate should be defined), a reference electrode (saturated calomel electrode) and an auxiliaryelectrode(madeofplatinumorgraphite).Theelectrodesareconnectedtoapotentiostat,whichenablesboththecontrolledchangesof themetalspecimenpotential (voltage)andthecurrentflowsmeasurementsasafunctionofpotential.

As a rule, corrosion potentials pertain to open circuit (no current flow)measurements. Incorrosionliterature,theopencircuitpotentialandthecorrosionpotentialareusedinterchange‐ably.Ittakesacertainamountoftimeforaspecimeninagivenelectrolytetoreachastablecor‐rosion potential at which the cathodic reduction rate is equal to the anodic dissolution rate(metalcorrosion).Corrosionmeasurementsshouldbemadeonlyafterthestablecorrosionpo‐tentialhasbeenreached[17].

Corrosionpotentialisusedasacriterionforthecorrosionbehaviour.Duringtheopencircuitmeasurements,potentialvs.timeofexposureplotcanbeobtained.Positivevaluesofcorrosionpotentialindicatethespecimenstabilityinthetestelectrolyte,negativevaluesindicatedissolu‐tion(corrosion),andifthevalueschangefromnegativetowardspositiveones,thenaspontane‐ous passivation of specimen occurs. In addition, the specimenwith lower corrosion potentialwilldissolvefasterinthetestelectrolyte.

Tafel extrapolationmethod [18] is probably themost commonly used polarization‐testingmethodformeasuringcorrosionrateandawidevarietyoffunctions.Itreliesgreatlyonthede‐terminationofso‐calledTafellinesfromcathodicandanodicpolarizationbranchesrepresentedinE‐logjplotwhereEistheappliedpotentialandjistheappliedcurrentdensity.Theintersec‐tion of both extrapolated Tafel lines determines the value of corrosion potentialEcorr and thecorrosion currentdensity jcorr, Fig. 1.Once jcorr is known, it is possible to define the corrosionrate.

ThebaseofTafelextrapolationmethodisButler‐Volmerequation,whichdesignatesthede‐pendenceofthecurrentdensityatanelectrodeonitspotential,assumingthatbothreactions(acathodicandananodic)takeplaceonthesameelectrode:

(1)

where ja and jc are the individual anode and cathode current densities respectively, α is thetransfercoefficient,zisthenumberofelectronsincludedintheelectrodereaction,FistheFara‐dayconstant,Ristheuniversalgasconstant,Tistheabsolutetemperatureandη=E–Ecorristhe

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ba

bc

Ecorr

jcorr

Currentdensitylogj,A/cm2

PotentialE,V

Anodic

Cathodic

AnodicTafelslope

CathodicTafelslope

Fig.1AschematicillustrationoftheTafelextrapolationmethod

activationoverpotential.ForE>>EcorrtheButler‐VolmerequationsimplifiestotheTafelequa‐tionfortheanodicreaction:

log (2)

Analogously,forE<<EcorrtheTafelequationforthecathodicreactionis:

log (3)

Constants(aa,ac)andslopes(ba,bc)ofanodicandcathodicTafellinesaregivenasfollows:

2,3031

log ;2,3031

;2,303

log ;2,303

(4)

3. Vortex tube cooling 

GeorgesJ.Ranque,aFrenchphysicsstudent, inventedthevortextubebyaccident in1928.Hemadeanumberofeffortstocommercialisethetechnologyincludingapatentin1934.However,thetechnologyonlybecamewidelyknownafteraGermanphysicist,RudolphHilschpublishedascientificpaper in1945entitledWirbelröhre (literally, vortex tube).Hence thenameRanque‐Hilschvortextube.

Vortextubeisamechanicaldevicethatsplitsacompressedairintocoldandhotstreams.Inthevortextube,suppliedcompressedairflowsthroughouttangentiallyarrangednozzlestoaninternalcounterbore.Thenozzlescauseavortexmotionoftheair.Thisspinningstreamofairturns90°andflowsdownthetubelikeaspinningshell,similartoatornado.Anamountoftheheatedairescapesthroughavalveathotendofthetube.Theremainedairmovesbackintheformofasecondvortexlocatedwithinthelow‐pressureareaofthelargervortex.Theinternalvortexcoolsdownandreleasesatcoldendofthetube.Thehotstreamofaircanattaintempera‐tureof100°Cwhilethecoldstreamofaircanreacheven−50°C.Fig.2showsaschematicviewofsuchcounter‐flowRanque‐Hilschvortex tube.Aparallelvortex tube(duetoairflow) isalsoavailable,buttheefficiencyislower.

Since it is very simple, compact, light and quiet, the vortex tube can be used as coolingequipmentofCNCmachinetools.Furthermore,therearenomovingpartstobebrokenorwornoutandhencethevortextubeneedsslightservicing[19].Thevortextubefitswellthecoolingrequirementsofcuttingtools.Afastcoldstreamofairensurescoolingaswellasremovalofthe"chip"thatisproducedbythetool.Besides,compressedaircanbefoundineverymachineshop.

Analysis of corrosion resistance when turning martensitic stainless steel X20Cr13 under chilled air‐cooling 

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Fig.2Ranque‐Hilschcounter‐flowvortextube–schemeofworkingprinciple

Themajordisadvantageisthelackoflubrication,whichcanbereplacedbychoosingappropri‐atetoolscoatedwithself‐lubricatingcoatingsthatenablemachiningintheseconditions[9,20].Inaddition,itrequiresahighairflowrateandacompressoroflargecapacitytobackupthecool‐ingactionofthevortextube[3].

4. Experimental work 

Experimentalresearchincludedboththecorrosiontestingandthesurfaceanalysisofthemate‐rialpriorandaftermachining.

For the electrochemical corrosion testing, three specimens from the martensitic stainlesssteelX20Cr13withhardnessof272HBwereprepared.Thechemicalcompositionoftestmate‐rialisspecifiedinTable1.Firstspecimenwasinthebasic(un‐machined)conditionandmarkedOS.Thesecondonewasmachinedusingthefloodcoolingwithoil‐in‐wateremulsionINABU7and marked EM. The third one was machined using the chilled air‐cooling by means of theRanque‐Hilschcounter‐flowvortextubeNexFlowTMFrigid‐XCoolerSystemc/w57025ADwithaninletairpressureof0.69MPaandmarkedVO.TheexperimentswereperformedontheCNClathePrvomajskaTU360intheLaboratoryforMachiningProcessesattheUniversityofRijeka,Faculty of Engineering. The selected cutting parameterswere: cutting speed vc = 220m/min,feedratef=0.3mm/revanddepthofcutap=0.4mm.TheworkpiecediameterforturningwithcuttinginsertsSECODNMG150608‐MF‐4TP2501wasØ80mm×463mm.

Table1ChemicalcompositionofmartensiticstainlesssteelX20Cr13

Chemicalelement

Fe C Si Mn P S Cr Mo Ni V Nb Cu

% 85.85 0.236 0.352 0.683 0.044 0.023 11.97 0.125 0.299 0.053 0.07 0.195

ElectrochemicaltestingofcorrosionratewascarriedoutintheLaboratoryforChemicalTest‐ingattheUniversityofRijeka,FacultyofEngineering.Twomethodswereapplied:opencircuitpotentialvs.timeofexposuremeasurementsandTafelextrapolation.ThePARSTAT2263poten‐tiostatandthePowerSuitesoftwarewereused.Electrolytewas3.5%aqueoussolutionofNaCl.

Thecorrosionpotential(opencircuitpotential)wasdeterminedforthefirstspecimen(OS)andthenthecorrosioncurrentdensityrequiredfordefiningthecorrosionratewasobtainedbythe Tafel extrapolation method. The same procedure was repeated for the second specimen(EM)andthethirdspecimen(VO).

Togaininsightintothesurfaceconditionofthespecimensaftercorrosiontesting,twoanalyt‐ical techniques were applied: Scanning ElectronMicroscopy (SEM) using VEGA TS 5136MMmicroscope and Energy Dispersive X‐ray Spectroscopy (EDS) using Oxford Instruments Si(Li)INCAX‐sightdevice.SurfaceanalysisofspecimenswasperformedintheLaboratoryforMateri‐alographyattheUniversityofZagreb,FacultyofMechanicalEngineeringandNavalArchitecture.Inaddition,theachievedsurfaceroughnesswasmeasuredusingHommelTesterT1000measur‐ingdevice.

Pressurized gas inlet 

(up to 1 MPa, 20 °C) 

Control valve 

Cold gas outlet (–50 °C) 

Hot gas outlet (100 °C) 

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Basicspecimen(OS)

Emulsioncooling(EM)

Vortextubecooling(VO)

Basic specimen (OS) 

Vortex tube cooling (VO) 

Emulsion cooling (EM) 

5. Results and discussion 

5.1 Corrosion testing 

Open circuit potential vs. time of exposure summary plot as well as polarization behavioursummaryplotforallthreespecimens(OS,EMandVO)inelectrolytearegiveninFig.3andFig.4respectively.Table2demonstratestheresultsoftheTafelextrapolationmethod.

Fig.3Opencircuitpotentialvs.timeofexposureforspecimensin3.5%aqueoussolutionofNaCl

Fig.4Polarizationbehaviourforspecimensin3.5%aqueoussolutionofNaCl

Table2ResultsofcorrosiontestingobtainedbytheTafelextrapolationmethod

Specimen Ecorr[mV] jcorr[mA/cm2] ba[mV] bc[mV]Corrosionrate[mm/year]

OS ‐380.736 1.94 106.705 241.747 0.0218EM ‐332.724 0.555 131.619 153.754 0.00625VO ‐359.706 3.56 208.859 458.957 0.04

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Testresultsshowthatthecorrosionpotentialofthebasicspecimen(OS)isstableandtherearenomajordeviations.Inaddition,thevalueofthecorrosionpotentialofthisspecimenisthemostnegativeofallthreeobservedspecimens,whichindicatesthelowestcorrosionresistance.The emulsion‐cooled specimen (EM)during the turning operation demonstrates in the begin‐ning the value of the corrosion potential nearest to zero and consequently better corrosionpropertiesincomparisonwithotherspecimens.Thiscanbeexplainedbythepresenceoftheoilfilm,whichafter awhile "breakdown"and then the corrosion resistanceof the specimende‐creases(curveE‐ttendstofall).Thevortextubecooledspecimen(VO)duringtheturningopera‐tionindicatesslightdeclineofE‐tcurveatthebeginningofmeasurementandthen,afteratime,thecreatingofprotectivelayerincreasesthecorrosionresistance(curveE‐tonanuptrend).Toendwith, theTafel extrapolationmethodobtainedslightlyhigher corrosion rateof thevortextubecooledspecimenincomparisontotheemulsion‐cooledspecimen.

5.2 Surface analysis 

TheimagesofsurfacesforthespecimenswithoutandafterexposuretotheelectrolytetakenbythescanningelectronmicroscopearepresentedinFig.5andFig.6respectively.TheresultsofEDS analysis for identification of specific chemical elements and their proportionswithin ob‐servedsurfacesonspecimensarepresentedinFig.7,Fig.8andFig.9.

Withrespect to thebasic specimen(i.e.un‐machined), theanalysis results showthediffer‐ence in surface condition considering the presence of chemical elements. The surface of thespecimenthatwasnotexposedtotheelectrolyte(Fig.7a)containschemicalelementsthatarein

Fig.5Imagesofsurfacesforthespecimenswithoutexposuretotheelectrolyte:a)basicspecimen,

b)emulsion‐cooledspecimen,c)vortextubecooledspecimen

Fig.6Imagesofsurfacesforthespecimensafterexposuretotheelectrolyte:a)basicspecimen,

b)emulsion‐cooledspecimen,c)vortextubecooledspecimen

b)  c) a) 

b)  c) a) 

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the chemical compositionof the testmaterial X20Cr13.However, the surfaceof the specimenthatwasintheelectrolyte(Fig.7b)showsthepresenceofotherchemicalelements,suchasoxy‐gen(O)andsodium(Na),becauseoftheelectrolytecomposition.

Fig.7EDSanalysisofbasicspecimen:a)withoutandb)afterexposuretotheelectrolyte

Fig.8EDSanalysisofemulsion‐cooledspecimen:a)withoutandb)afterexposuretotheelectrolyte

C  21.73 % Si  0.35 % Cr  10.57 % Fe  67.36 % 

a) 

C  16.50 % O  2.83% Na  0.23 % Si  0.36 % Cr  10.46 % Mn  0.91 % Fe  67.36 % b) 

C  24.63 %O  3.73 % Al  0.37 % Si  0.42 % Cl  0.24 % Ca  0.38 % Cr  9.74 % Mn  0.78 % Fe  59.70 %

a) 

C  24.14 % O  3.35 % Si  0.28 % Cr  9.83 % Mn  0.75 % Fe  61.26 % Ni  0.40 % b) 

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Fig.9EDSanalysisofvortextubecooledspecimen:a)withoutandb)afterexposuretotheelectrolyte

Thesurfaceoftheemulsion‐cooledspecimenduringturningoperationthatwasnotexposedtotheelectrolyte(Fig.8a)containsotherchemicalelementsinadditiontotheonesthatarepartof the testmaterial.Here itshouldbehighlightedthepresenceofcalcium(Ca),probably fromtheemulsioncomposition.Surfaceanalysisofthespecimenthatwasintheelectrolyte(Fig.8b)alsoshowsthepresenceofoxygen(O)andnickel(Ni).

The surface of the vortex tube cooled specimenduring turning operation thatwas not ex‐posedtotheelectrolyte(Fig.9a)comprisesbasicchemicalelementswiththeadditionofoxygen(O),duetotypeofcooling.Specimenthatwasintheelectrolyte(Fig.9b)containsaslightlyhigh‐erpercentageofoxygen(O).

Finally,thevortextubecooledspecimensurfacehaslowermeanroughnessincomparisontothesurfaceofemulsion‐cooledspecimen(Ra(VO)=1.576μmvs.Ra(EM)=1.645μm),whichhasapositiveeffectonthecorrosionresistance.Thecorrosionresistanceisbetterforlesservalueofsurfaceroughness.

6. Conclusion 

The conducted study provides valuable insights on the possibility of applying Ranque‐Hilschcounter‐flowvortextubebasedchilledair‐coolinginsteadoftheconventionalfloodcoolingwithoil‐in‐water emulsion considering the corrosion resistancewhen turningmartensitic stainlesssteelX20Cr13.Themostinterestingfindingstandsout:thespecimencooledwithchilledairhasbetter resistance to corrosion activity after a certain longer time in relation to the specimencooledwithemulsion,probablyduetocreationofaprotectivelayer.Thisphenomenonrequiresmoreresearcheffort.Itshouldbepointedoutthattheuseofchilledair‐coolingcanberecom‐mendedwhenturninggivenmaterial,especiallyinviewoflowerachievablesurfaceroughnessvalues incomparisontotheconventional floodcooling.Thisrecommendationshouldbetakenintoaccountregardingtheenvironmentalaspectsofmachining.Sincethecoolingmediumisair,therearenochemicalexposureriskstohumansortheenvironment.

C  18.57 % O  3.03 % Si  0.44 % Cr  10.80 % Mn  0.64 % Fe  66.53 %

b) 

C  15.99 % O  2.22 % Si  0.36 % Cl  0.22 % Cr  11.27 % Mn  0.74 % Fe  69.19 % a) 

Kostadin, Cukor, Jakovljevic

In further research, it is suggested to establish the mathematical model between the corro-sion resistance and the surface roughness of the tested material considering different cooling techniques. In addition, their impact on the tool life needs to be examined as well as the econom-ic viability of introducing the chilled air-cooling. Finally, the optimization of cutting parameters, namely cutting speed, feed rate and depth of cut, would also be welcomed.

Acknowledgement This work was financially supported by the University of Rijeka, Croatia, No. 13.09.1.2.10.

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