european commission technical steel research. interaction of free-cutting steel by c. vasey, a....

7/27/2019 European Commission Technical Steel Research. Interaction of Free-Cutting Steel by C. Vasey, A. Turner, C. Better

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* +ISSN 1018-5593

European Commission

t e c h n i c a l s t e e l r e s e a r c hProperties and in-service performance

Interaction of free-cutting steelmicrostructure with machining technology

STEEL RESEARCH


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European Commission

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Properties and in-service performance

Interaction of free-cutting steelmicrostructure with machining technology

C. Vasey, A. Turner, C. Betteridge, C. ElliotBritish SteelSwinden Technology CentreMoorgateRotherham S60 3ARUnited Kingdom

Contract No 7210-MA/8171 October 1990 to 30 September 1993

Final report

Directorate-GeneralScience, Research and Development1998 EUR 17840 EN


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LEGAL NOTICENeither the European Commission nor any person acting on behalf of the Commissionis responsible for the use which might be made of the following information.

A great deal of additional information on the European Union is available on the Internet.It can be accessed through the Europa server (http://europa.eu.int).Cataloguing data can be found at the end of this publ ication.Luxembourg: Office for Official Publications of the European Communities, 1998ISBN 92-828-1687-7 European Communit ies, 1998Reproduction is authorised provided the source is acknowledg ed.Printed in LuxembourgPRINTED ON WHITE CHLORINE-FREE PAPER


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I N T E R A C T I O N O F F R E E C U T T I N G S T E E L M I C R O S T R U C T U R E W IT H M A C H I N I N GT E C H N O L O G YB r i t i s h S tee l p i cE C S C A g r e e m e n t N o . 7 2 1 0 . M A / 8 1 7S U M M A R YT h e a d v a n c e s i n c u t t i n g t o o l s c a p a b l e of o p e r a t i n g a t h i g h e r c u t t i n g s p e e d s a n d t e m p e r a t u r e s m a y p l a c ed i f f e r e n t d e m a n d s o n s t e e l w h i c h m u s t r e s p o n d t o t h e i n c r e a s e d d e f o r m a t i o n r a t e s a n d i n c r e a s e dt e m p e r a t u r e s . I n t h i s pr o je c t t h e e m p h a s i s h a s b e e n a f i n i s h i n g o p e r a t io n w h e r e h i g h c u t t i n g s p e e d s a n ds ma l l dep th s of cu t a r e o f im por tanc e , r a th e r tha n heavy me ta l r em ova l u s ing s lower s pee ds , h igh f eedra te s and s ubs tan t i a l dep th s o f cu t . Th e ma jo r objec t ives of the work have been to de te rm ine the r e s pon s eo f d i f f e r en t too l /workp iece com bina t ions for op t im um pe r fo r ma nce , and to exa mi ne the po ten t i a l f o r ap r e d i c t i v e m o d e l i n a s s e s s i n g t h e m a c h i n a b i l i t y o f t h e s e c o m b i n a t i o n s , i n o r d e r t o r e d u c e e x t e n s i v ema ch in ab i l i ty t e s t in g in the fu tu r e in th e s ea r ch fo r idea l comb ina t ions o f s t ee l , too l and cu t t in g cond i t ion .Th e work encom pas s e d th r e e bas i c typ es of s t ee l s - l ow ca rbon , med ium ca rbon low a l loy and aus ten i t i cs t a i n l e s s s t e e l s , i n c l u d i n g v a r i o u s f re e c u t t i n g a d d i t i v e s a n d d e o x i d a t i o n t r e a t m e n t s , w i t h t h e e m p h a s i s o nt h e l o w c a r b o n s t e e l s a n d s t a i n l e s s g r a d e s r e p r e s e n t i n g c o n t r a s t i n g g r a d e s k n o w n t o m a c h i n e e a s i l y a n dwi th s ome d if f i cu l ty . Al l of the s t ee l s we re t e s t ed u s ing coa ted ca rb ide , ce r am ic , and ce rm e t cu t t in g too l s .T h e to o l w e a r p r o d u c e d b y a u s t e n i t i c s t a i n l e s s s t e e l s w a s d e t e r m i n e d l a r g e l y b y t h e a d h e r e n c e o fworkp iece ma t e r i a l . Th i s wa s s eve re w i th coa ted ca rb id e too l ing , r e s u l t ing in decohes ion of the coa t ingand a ma rke d de t e r io r a t io n in s u r f ace f in is h. I n the r e s u lphu r i s ed aus te n i t i c s t a in le s s s t ee l ce r am ictoo l ing p roduced the bes t pe r fo rmance , bu t , ove ra l l , c e rme t too l ing p roved advan tageous wi th a l l o f theaus ten i t i c s t a in le s s g r ades a s the r e was l i t t l e t endency fo r the adhe rence o f workp iece ma te r i a l .S m a l l vo lum es o f m an ga ne s e s u lph ide were found to be of va lue in AIS I 304 qua l i t i e s , s ug ges t ing th a teven in non - r e s u lphu r i s ed s t ee l s the s u lphu r con ten t s hou ld be t igh t ly con t ro l l ed fo r p roduc t cons i s t encya n d q u a l i t y w i t h r e s p e c t t o m a c h i n i n g .In low ca rbo n s t ee l s a t h igh cu t t in g s peeds the bene f i c i a l e ff ec t o f m ang ane s e s u lph ide inc lu s ions wasap pa ren t wi th a l l cu t t ing tool types in bo th p ro long ing too l l if e and im prov ing s u r f ace f in is h. How ever , t hem e t a l l i c a d d i t i v e s , l e a d , a n d a c o m b i n a t i o n of l e a d , b i s m u t h , a n d t e l l u r i u m d i d n o t o ff er f u r t h e rim pro ve me n t s in r edu c ing too l we a r , a l th oug h ch ip fo rm and s u r f ace f in is h we re impro ved . S u lp h ideinc lu s io ns were a l s o bene f i c ia l i n p rom ot in g ch ip b r e ak i ng in low ca rbon s t ee l s .F o r the un res u lphu r i s ed p la in C Mn s t ee l t he ce rme t cu t t ing too l gave the bes t pe r fo rmance , and th i scu t t in g too l type a l s o p roved to be s a t i s f ac to ry w i th the r e s u l phu r i s e d ba lance d fr ee cu t t ing s t ee l g r ade s .Wi th s i l i con -k i l l ed f r ee cu t t ing s t ee l s ce rme t cu t t ing too l s p roduced a poo re r qua l i ty s u r f ace f in i s h thanth a t gene ra te d wi th coa ted ca rb ide and ce r amic too l ing . I n a s i l icon k il l ed ca lc ium - t r ea ted qua l i ty th i swa s s hown to be due to the adh e re nce o f ma nga nes e s u lph id e depos i t s to coa ted ca rb ide and ce r am ictoo l ing , s ug ges t ing thes e too l s s hou ld be p r e f e r r ed for k i l led fr ee cu t t ing s t ee l s . S uch depos i t s were no tobs e rved on ba lanced f r ee cu t t ing s t ee l s .I nc r e as ed f eed r a t e s p ro longed too l l if e fo r a g iven r a t e o f me ta l r emov ed .A w o r k p i e c e / t o o l t h e r m o c o u p l e t e c h n i q u e p r o v e d t o b e i n a d e q u a t e i n c h a r a c t e r i s i n g t h e t e m p e r a t u r e so b t a i n e d d u r i n g m a c h i n i n g . T h e t e m p e r a t u r e s m e a s u r e d di d n o t c o r r e s p o n d w i t h t h o se o b t a i n e d f ro mcom pres s io n t e s t in g fo r s im i l a r s t r e s s es and s t r a in s a s de t e rm ined in a mod i f ied Me rch an t mode l . Them o d e l p r o v e d t o b e i n s u f f ic i e n t l y p r e c i s e t o d e fi n e m a t e r i a l p e r f o r m a n c e a c c u r a t e l y . A l i m i t e dinve s t iga t ion o f too l v ib r a t ion in the 0 -40 kHz r ange s ugg es ted no ef fect on too l we a r . Al tho ugh ch a t t e rresu l ted in a deter iora t ion in sur face f in ish i t was not found to be detr imenta l to the tool l i fe of the coatedc a r b i d e i n s e r t e m p l o y e d .I t i s ap pa ren t f rom th i s r e s ea rch tha t the r e i s s cope for the deve lop men t of new f r ee cu t t in g s t ee l s o f f e ringthe capab i l i ty o f enhanced mach inab i l i ty wi th a wide r ange o f cu t t ing too l s and cu t t ing cond i t ions .


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C O N T E N T S P A G E1 . I N T R O D U C T I O N 1 12. B A C K G R O U N D 1 12 .1 T r e n d s i n M a c h i n i n g T e c h n o l o g y 1 12 .2 Def in i t ion of M ach inab i l i ty 122 .3 Me t a l l u r g y o f F r e e M a c h i n i n g S t e e l s 122 .4 M od ern Tool M ater ia l s 143. M A T E R I A L S F O R E X A M I N A T I O N 153.1 Steel Qu ali t i es 153 .2 Tool M ater ia l s 154. R E S U L T S O F E X P E R I M E N T S 164 .1 Tool W ear Te s t s 164 .2 M a c h i n a b i l i t y Mo d e l f o r N o n - O r t h o g o n a l C u t t i n g C o n d i t i o n s 194 .3 Sc a n n i n g E l e c t r o n Mi c r o s c o p y 2 14 .4 Tool Li fe Te s t s - Un coa ted C arb id e To ol ing 214 .5 T o o l T e m p e r a t u r e M e a s u r e m e n t s 224 .6 Ch ip Fo rm 224 .7 M e t a l l o g r a p h y a n d A u t o m a t i c I m a g e A n a l y s i s 2 34 .8 Me c h a n i c a l P r o p e r t i e s 2 44 .9 Tool Vib ra t io n 255. D I SC U SSI O N 2 75.1 Au s ten i t ic S ta in les s S tee l s 275 .2 M edium C arb on Low Al loy Stee l s 285 .3 Low Ca rbo n Stee l s 285 .4 M ech anic a l Mo del l ing 305 .5 Vi bra t ion 316 . C O N C L U SI O N S 3 2

R E F E R E N C E S 3 3T A B L E S 3 5F I G U R E S 4 6A P P E N D I X 1 M A C H I N A B I L IT Y M O D E L F O R 121N O N - O R T H O G O N A L C U T T I N G C O N D I T I O N SA P P E N D I X 2 T O O L T E M P E R A T U R E D E T E R M I N A T I O N 133A P P E N D I X 3 V E R I F I C A T I O N O F A C C E L E R O M E T E R 137O U T P U T


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LIST OF TABLE S1. Chem ical Com positions.2. Material Condit ion.3. Tool Mate rials .4. Chip Form and Colour.5. Inclusion Size and M aterial H ardn ess.6. Mechanical Prop ert ies: Tensi le Da ta7. Mechanical Prope rt ies: Compression Test Data.8. Cutt ing Tip Fai lure Modes Coated Carbide Tips.9. Surface Rou ghne ss M easu rem ents Made by Taly Surf for Ba rs Cut with Coated Carbide Tips.L I ST O F A P P E N D I C E S1. M achina bility Model for Non -Orthog onal Cu tting Conditions.2. Tool Temperature Determinat ion3. Verification of Accelerometer Output


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LIST OF FIGU RES1. W ear Test Data : AIS I304 ; Coated Carbid e Tooling.2. Wear Test Data: AISI 304: Cermet Tooling.3. W ear Test Data : AISI 304; Cera mic (K060) Tooling.4. W ear Test Da ta: AISI 304 Ca; Coated Ca rbide Tooling.5. Wear Test Data: AISI 304 Ca; Cermet Tooling.6. W ear Test Da ta: AISI 304 Ca; Ce ram ic (K090) Tooling.7. Wear Test Data: AISI 303; Coated Carbide Tooling.8. W ear Test Data: AISI 303; Cermet Tooling.9. W ear Test Da ta: AISI 303; Ce ram ic (K060) Tooling.10 . Wear Test Date: A ISI 303; Coated C arbide Tooling; 0.31 and 0.5 mm/rev Feed Rate s.11. Wear Test Data: AISI 303; Ce rm et Tooling; 0.31 and 0.5 mm/rev Feed Rates.12. Wear Test Data: AISI 303; Cer am ic (K060) Tooling; 0.31 and 0.5 mm /rev Feed R ates.13 . Wear Test Data: 709M40; Cutting Speed: 260 m/min.14 . W ear Test Data: 709M 40Ca; Cu tt ing Speed: 260 m/min.15 . Wear Test Data: CMn; Coated Carbide (GC415MF) Tooling.16. W ear Test Data: CMn; C ermet Tooling.17. W ear Test Data: CMn; C eramic Tooling.18 . W ear Test Data: 230M07; C oated Carbide (GC415P) Tooling.19 . W ear Test Data: 230M07; C oated Carbide (GC415MF) Tooling.20 . Wear Test Data: 230M07; C ermet Tooling.21. Wear Test Data: 230M07; Ceramic Tooling.22 . W ear Tes t Data: 230M07; Coated Ca rbide (GC415P) Tooling, 0.31 and 0.5 mm /rev FeedRates.23 . W ear Test Da ta: 230M07; C erm et Tooling, 0.31 and 0.5 mm /rev Feed Rate.24 . W ear Test Da ta: 230M07; Cera mic Tooling, 0.31 and 0.5 mm/rev Feed Rate.25 . W ear Test Data: 230M07Pb ; Coated Carbide (GC415P) Tooling.26 . W ear Test Data: 230M07Pb; C erme t Tooling.27 . W ear Test Data: 230M07Pb; Ce ramic Tooling.


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28. W ear Test Data: 230M07Pb; Coated Carbide (GC415P) Tooling, 0.31 and 0.5 mm /rev F eedRates .29. W ear Test Data: 230M 07Pb; Cerm et Tooling, 0.31 and 0.5 mm /rev Feed Rates .30 . W ear Test Data: 230M07Pb; Ceramic Tooling, 0.31 and 0.5 mm/rev Feed R ates.31. Wear Test Data: 230M07PbBiTe; Coated Carbide (GC415P) Tooling.32 . W ear Test Data: 230M07PbBiTe; C ermet Tooling.33. W ear Test Data: 230M07PbBiTe; C eramic Tooling.34. Wear Test Data: 230M07S; Coated Carbide (GC415MF) Tooling.35. Wear Test Data: 230M07S; Cermet Tooling.36. Wear Test Data: 230M07Si; Ceramic Tooling.37 . Wear Test Data: 230M07SiCa; Coated Carbide (GC415P) Tooling.38. Wear Test Data: 230M07SiCa; Cermet Tooling.39 . Wear Test Data: 230M07SiCa; Ceramic Tooling.40 . M erch ant Model Test Data : AISI 304.41. M erchant Model Test Data: AISI304C a.42 . M erch ant Model Test Da ta: AISI 303.43. Merchant Model Test Data: 709M40.44 . Me rchant Model Test Data: 709M40Ca.45. Me rchant Model Test Data: CMn.46 . Merchant Model Test Data: 230M07.47. Merchant Model Test Data: 230M07Pb.48. Merchant Model Test Data: 230M07PbBiTe.49. M ercha nt Model Test Da ta: 230M07 + S.50 . M ercha nt Model Test Data: 230M07 + Si + Ca.51. Tool W ear and Deposits.52 . Tool W ear and Deposits.53. Chip Form s, AISI 303, Feed Rate = 0.189 mm /rev, Depth of Cut/1.0 mm .54 . (a) Flan k Wear Data: CMn; P30 Carbide.

(b) Cra ter Depth W ear Data: CMn; P30 Carbide.


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55. (a) Fla nk W ear Data: 230M07; P30 Carbide.(b) Cr ate r Depth W ear Data: 230M07; P30 Carbide.56 . (a) Fla nk W ear Data: 230M07Pb; P30 Carbide.(b) Cr ate r Dep th W ear Data: 230M07Pb; P30 Carbide.57 . (a) Fla nk W ear Data: 230M07P bBiTe; P30 Carbide.(b) Cr ate r Depth W ear Data: 230M07PbBiTe; P30 Carbide.58. (a) Fla nk W ear Data: 230M07 + Si; P30 Carbide.(b) Cr ate r Depth W ear Data: 230M07 + Si ; P30 Carbide.59. (a) Flan k W ear Data: 230M 07+S i + Ca; P30 Carbide.

(b) Cra ter Depth W ear Data: 230M07 + Si +C a; P30 Carbide.60 . Unco ated C arbide Tool Life (Flank W ear) of Low Carbon S teels.61. Unco ated C arbide Tool Life (Crater W ear) of Low Carbon Steels.62 . Cu tt ing Temp erature s for Austeni t ic Stainless Steel.63 . Cu tting Tem pe ratu res for Low Carbo n Steel Qualities:- Carbide Cu tting Tool.64 . Cu tt ing Tem peratu res for Low Carbon Steel Qualit ies:- Cermet C utt ing Tool .65. Oxide Inclusion Para me ters - CMn Quali ty.66 . Inclusion Morphology.67 . Stress-Strain -Tem perature Surface: AISI 304 Quali ty.68 . Stress-Strain -Tem perature Surface: 230M07 Quali ty.69 . Tool Holder Vibrat ion Spectrum Cerm et Tip; 95 mm Tool Holder Ove rhang.70 . Mac hined B ar Surface Profiles AISI 303.71. Ma chined Ba r Surface Profiles: Low Carbon Steels.72. Coated Carbide Tools: Pa ram ete rs after 3000 m Cut at 260 m/min, Au stenitic Stain lessSteels .73. Coated Carb ide Tools: Pa ram ete rs after 3000 m Cut at 260 m/m in, Low Carbon Steels.74. Cermet Tools: Parameters after 3000 m Cut at 260 m/min, Austenitic Stainless and MediumCarbon Steels .75. Cer me t Tools: P ar am ete rs after 3000 m Cut at 260 m/m in, Low Carbon Steels.76 . Ceramic Tools: Pa ram eter s af ter 3000 m Cut at 260 m/min, Austeni t ic Stainless and MediumCarbon Steels .77. Cera mic Tools: P ar am ete rs after 3000 m Cut at 260 m/m in, Low Carbon Steels.78. Cerm et Tools: Pa ram ete rs af ter 3000 m Cut at 260 m/min, Calcium Treated Quali t ies


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I N T E R A C T I O N O F F R E E C U T T IN G S T E E L M I C R O S T R U C T U R E W I T H M A C H I N I N GTECHNOLOGYBritish Steel picECSC Agreement N o. 7210.MA/817FINAL TECHNICAL REPOR T

1. INTRODUCTIONRecent advances in the machining of steels have included developments in workpiece metallurgy, tooltechno logy and the widespread automat ion v ia computer numer ica l cont ro l , o f many machiningoperat ion s. In part icula r , new tool varian ts al low much higher m etal rem oval rate s to be a chieve d.However, performance is crucial ly dependent on the mechanical and chemical interact ions between thetool an d the w orkpiece under the selected cutting conditions.The objective of the research was to identify a new method for developing improved steels suitable foradvanced cutting processes and to identify the optimum cutting conditions and tool design for these newsteels . To achieve this objective the work was aimed at achieving a better understanding of the cuttingmec hanism and i ts interact ion with the steel microstructure. One of the aim s of the work was to estab lisha numerical description of the microstructural distribution and cutting forces for each phase in the steel,and the phase interfaces. Based on this nume rical description of the m icro stru ctu re a further aim of thework was to obtain a mathematical model of the machining process capable of making predictions of highspeed machining characteristics under different cutting/tooling conditions.There a re indicat ions tha t under high speed mac hining conditions that the co nventional addit ives such aslead may be less effective and that under these conditions other phases, such as complex silicates mayprove to be more useful in improving m achina bility.This programme has examined the responses obtained from a variety of tool/workpiece combinations usingsingle-point turn ing . Cu tting speeds and feed rate s were selected to allow high er ra tes of me tal remo val tobe assessed at depths of cut typical of finishing operations, where surface finish tolerances and part sizeare a t a prem ium . This has enabled recom mend at ions to be ma de fo r opt imised too l /workpiececombinations.2. BACKGROUND2.1 Trend s in Ma chining Technology

The increases in productivi ty available from NC/CNC technology have led to demands for increasedperformance from b oth workpiece materials and tooling to maximise the potential of these machines. Onemethod of increasing productivi ty is to machine at higher rates of metal removal , thus reducing partprodu ction tim es. The economies forthcoming will be offset, at some critica l rat e of me tal rem oval, byincreasing tooling costs owing to rapid wear*1). However, whilst the capital and labour costs involved inma chinin g operat ions have r isen, tooling costs have decreased over recent y ears in term s of performance price, such tha t tool life ma y not be the dom inant conside ration in m an y situat ion s. Indeed, Stjernbergand Thelin reasoned that , with new production technology, rel iabil i ty and reproducibil i ty may be moreimp ortan t tha n ut i l is ing the maximum capacity of each cutt in g edge*2).Therefore, i t is possible tha t higher rates of metal removal ma y assum e some prominence in future, andthis concept is often described as "high speed mach ining'. Th is is a relat ive t er m in view of the v ast ran geof speeds at which different materials can be effectively machines, and it is difficult to arrive at acomprehensive definition.

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In the case of s ingle-point turn ing , Ho shi has described cu rre nt an d ant ic ipa ted futu re m ach inin gconditions as follows*3).

Work Mater ia lLow C Stee lsHigh C SteelsSteels (General)

Operat ionRoughingRoughingFin ish ing

V (m/min) X f (mm/rev)Presen t

350 X 0.6200 X 0.4450 X 0.15

Fu t u r e5 0 0 X 0 . 6300 X 0.4700 X 0.15

These figures are necessari ly speculat ive, but serve to underl ine the increa ses in cut t ing speeds which areant icipated.2.2 Defini t ion of Mac hinabi l i tyT h e c o n c e p t o f ' m a c h i n a b i l i t y ' o f a g i v e n m a t e r i a l t a k e s o n d i f f e r e n t m e a n i n g s u n d e r v a r i o u scircumstances and a precise definit ion cannot be mad e. In the past , machinab il i ty rat ings have beendeveloped to describe the rela tive e ase of difficulty of transfo rmin g a raw m ate rial into a finished product,based on tool life predictions* 4) . However any me asure of mach inabil i ty should relate to the minimu mtotal cost requ ired to produce a satisfacto ry part*5).Consequently, it is impossible to devise a single all-embracing test to evaluate 'machinability' , since theconcept is relia nt upon costing inform ation which will vary according to circum stance . An a ltern ativ eapproach is to generate scientific data on the response of a tool/workpiece combination to a wide range ofmachining condit ions for a given operat ion and then determine 'machinabil i ty ' by imposing selected'acceptance cr i ter ia ' on the information thus generated to determine the maximum acceptable rate ofmetal removal. The criteria may include constraints on rate of tool wear, surface finish, part size etc. tosui t .2.3 Metal lurgy of Free Machining Stee lsThe objective underlying free machining steel design is to increase the ease with which metal can beremoved by cut t in g operat ions. Increasing ly, interact ion s between the tool and the workpiece must beconsidered in order to achieve opt imu m performance. However, enhancem ents in machining performanceare often at the expense of mechanical properties.2.3.1 Steel M a t r i xThe cutting behaviour of carbon and alloy steels is influenced by chemical composition, microstructure,quan ti ty and type of inclusions and work hard enin g rate . In normalised ferr i tic /pearl i tic s teels , increasingcarbon contents reduce tool lives rapidly, with chip formation changing from a flow mechanism to a shearmechanism*6). How ever, for steels con tain ing less tha n 0.15% C, larg e built-up edges of m ate ria l m aydevelop on the tool ra ke face, lead ing to unp redic table ma chin ing characteristics* 7).Reductions in s train hardening rates can induce lower tool wear , shorter chip lengths and a superiorsurface finish, and this can be promoted by cold work or increases in phosp horus or nitrogen leve ls. Ifqua ntitie s of ha rd second pha ses (e.g. bain ite) a re pre sen t instea d of pe arl ite, the region of she ar in front ofthe tool tip is restricted, leading to higher temperatures and lower feed forces* 8). Quenched and temperedmartensi t ic /baini t ic s t ructu res exhibi t infer ior m achinin g character is t ics because of high hardn ess levels ,although most medium carbon steels are machined in this condition* 9).

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Austenitic stainless steels are generally regarded as difficult to machine in comparison to low carbonferr i te /pearl i te s t ructures because of their high duct i l i ty , toughness, high work hardening rate and poorthermal conductivity* 10 ). Con sequen tly, tools tend to run ho tter with a tendency to develop larg e b uilt-upedges and the high wo rk hard enin g rat es can cause problem s if feed rates are low or cutting is in terru pted .2.3.2 Effect of Inc lusio nsFor a given ma tr ix s truc ture, one of the most popular methods of enhancing machining performance is toman ipulate the n atu re an d distr ibut ion of inclusions present within the s teel . Inclusions can be broadlyclassified in te rm s of th eir influence on cu tting b ehav iour a s follows*11). De tr ime ntal (abrasive, hard): AI2O3, S1O2, Cr 20 3, TiN e tc. Favo urable: MnS, MnSe, MnTe, Pb, Bi . Li ttle or no effect: FeO , Mn O, soft basi c silic ate s.2.3 .2 .1 Su lphid e Ph ase sOne of the most common techniques for enhancing machining performance is to elevate the sulphur level,thereby increasing the volume of deformable MnS inclusions. Typically, the highest commercial S levelsare ~-0.35%*12) and s ulph ur is the cheapest m achinabil i ty additive*13 ). Additions are beneficial in severalways*14-15 ).(i) Th e sh ea rin g work involved in chip forma tion is reduced.(ii) A stro ng lubr icatin g effect occurs at the tool/chip interface.( ii i) Chip em bri t t lem ent is promoted.(iv) Pro tectiv e sulphide depo sits form on carbid e tools over a wide ran ge of cuttin g speeds.(v) Bu ilt-up edge stability is increa sed and size is reduced, resu lting in improved surface finish.Machinability is optimised by a uniform distribution of large globular sulphides, whose distribution isstrongly influenced by castin g and deoxidation practices*16 ).Addit ions of selenium and/or te l lur ium , which are vir tual ly insoluble in s teel , are also used. Bothelem ents can sub stitu te for sulph ur to form m ixed inclusions and their h igh surface ac tivities lead to lowersurface energies at matrix/sulphide interfaces, thereby encouraging microvoid formation and cracking.Tel lurium may also reside as a thin f i lm at interphased boundaries , thereby reducing resis tance to shearstill further*12 ).2.3 .2 .2 Lead /Bismu th Ad dit ionsAlthough the solubility of lead in molten steel at 1550C is ~ 0 .3 % , i t is virtu ally insoluble in the solid sta teand forms a random dispersion of inclusions in low sulphur steels* 12 ) . In resulphurised grades, leadaddit ions also form ' ta i ls ' on MnS inclusions. Lead acts as an internal lu bric ant , redu cing fr ic tion,promoting chip em bri t t lem ent , impro ving chip form and surface f inish. How ever, a t high cu t t ingtem peratu res leaded steels begin to behave similar ly to non-leaded va riants . Und er these conditions it hasbeen suggested that lead becomes inefficient in the liquid state* 17 ) , but as rake face temperatures aregene rally in excess of the me lting point of lead, this con tention m ay be in some doubt.However lead can act ively promote the beneficial globularisat ion of sulphides leading to enhancedmachining performance*18 ). Add itions beyond 0.35% are not normally feasible owing to segregationpheno men a; how ever, th e co-introduction of up to 0.10% Bi, which is closely related to lead, allows the

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tota l insoluble phase conten t to be elevated further*19 ) . If no lead is presen t , bismuth is normally prese ntas ta ils to MnS inclusions and as a fine dispersion*12 ).2.3.2.3 Oxid e Mod ificationInclusion s of AI2O3 and S1O2 are believed to restrict s he ar an d impede pla stic flow in sh ea r zon es, ther ebyelevat ing cut t ing temperature and reducing tool l i fe* 19). Alum ina is also likely to be abrasi ve to mostcu ttin g tools. One stra teg y involves the modification of thes e oxides to form complex silicate inclusions viasmal l ca lc ium addi t ions , which can sof ten and behave in a s imi la r manner to su lph ides a t thetem pera tures genera ted by carbide cut t ing tools a t high speeds*16) . Favourable phases include anorthi te( C a O . A l 20 3 .2Si0 2) with a melting point of 1350C, and gehlenite (2CaO.Al2 0 3 .S i0 2) with a slightly lowerm eltin g point. Good deoxidation practice is a prere quisite . In the case of tools con tainin g a surface laye rsof TiC/N/(C, N), the p lastic oxides form chffusion-inhibiting deposits on the tool, reducin g th erm al wear*18 ).However, in the case of alumina coatings, or for alumina-based ceramic tooling, calcium and silicon candiffuse into the tool t ip , causing softening and degradat ion* 2 6) . T h e r e f or e , u l t i m a t e m a c h i n i n gperformance is s t rongly dependent on interact ions between the tool and workpiece at the cut t ingtempera ture genera ted .2.4 Mod ern Tool MaterialsIn the field of single-point tur nin g, the rang e of tools available is vast, with m ost ma ter ial s bein g a vaila blein a wide rang e of index able ins ert geom etries which are selected to suit the desired op eratio n. The m ostcommon materials include carbides, which consist principally of tungsten carbide embedded in a cobaltbased matrix, coated carbides, in which single and multiple layers of TiN, TiC and AI2O3 are added toenhance wear and crate r resis tance allowing higher rate s of metal removal , and cerm ets . This la t tercategory consists of composite materials with high TiC contents in a nickel-based binder, resulting in veryhigh therm al deformation resis tances and hardnesses. Inserts made in these categories are usual ly offeredin ei the r 'plain ' or 'chipbreaking ' geometr ies; chipbreaking grooves are m oulded into the r ak e facesallowin g chip control and leading to reduction s in cuttin g forces. Specialised chipb reak ing geo me trieshav e been developed to suit par ticu lar w orkpiece m ate rials , operations or tool grades*20 ).2.4.1 Nov el Tool Materials2 .4.1.1 Alum ina-Ba sed CeramicsAlumina tool ing has been under development s ince the 1940's with the main object ive of opt imisingthermo mech anical prope rt ies . Thre e main categories have evolved;(i) AI2O3-T1C.(ii) Al 2 0 3 - Z r 0 2(iii) AI2O3 reinforced wit h SiC whiske rs*21 ).The aim of the modif icat ions to the alumina matr ix is to increase fracture toughness and thermal shockresis tan ce, a l though this m ay be at the expense of ul t im ate hardn ess. Tools reinforced with SiC aregenerally restricted to the machining of superalloys owing to the high reactivity of SiC, par t icu la r ly wi thferrous workpieces.Alum ina-based tools excel a t high cut t ing speeds (>3 00 m/min) and cut t ing f luids are p ermissible in mosti n s t a n c e s . H o w e v e r a s n o t ed p r e v i o u s ly , c h e m i c a l i n t e r a c t i o n s l i m i t s t h e i r a p p l i c a b i l i t y w i t hcalcium-treated s teels . Although some simple chipbreaking geometr ies have been dem onstrate d, theseare not yet avai lable on commercial inserts .

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2.4.1.2 Sil ico n Nitride-Based MaterialsTooling based on silicon nitride has attracted great attention owing to a combination of excellent hightempera ture mechanica l p roper t ies and res i s tance to ox ida t ion and thermal shock* 21) . However ,production problem s have, until recently, limited availability. The the rm al co ndu ctivity of S13N4 isapproximately double that of AI2O3-T1C whilst the coefficient of thermal expansion is lower by a factor of2, resul t in g in a much improved therm al shock resis tance. A vast range of a l loyed varia nts h ave beenresearched with common additives including AI2O3, A1N a nd S1O2.2.4.1.3 Po lycry stall in e Cubic Boron NitrideFollowing diamond, polycrystalline cubic boron nitride is the second hardest material known to man, andin contrast to diamond, i t is thermally s table in air and in contact with ferrous workpieces* 2 2 ' .Consequently, these tools have found favour in application where many identical operations are to beperformed a t a minim al rate of tool wear in order to ensure reproducibility, and also for the machining oful tra har d ferrous ma terials where other tool ma terials are inadequate . However, owing to the na ture ofthe production route, the cost of PCBN inserts is higher by at least an order of magnitude than that ofcarbide tooling, and it is unlikely that such tools are comp etitive in the cutti ng of free m ach ining steels.3. MATERIALS FOR EXAMINATION3.1 Stee l Qu alit iesThe steels were acquired for testing as forged 63 or 130 mm diameter bars, which were heat treated asappr opria te to the quality . The three austen itic stainless steels at the 130 mm section - AISI 304, AISI 304calcium treat ed, and AISI 303 resulph urised - were examined in the solution trea ted cond ition. Th emedium carbon engineering grades, 709M40 and 709M40 calcium treated, were examined in a 63 mmsection in the oil quench ed and tempered condition. The six low carbon va ri a n ts - base , S, Pb , SPbB iTe,silicon killed and silicon killed calcium treated, were norm alised prior to the m achin ing tes ts. The ba rswere p repared for ma chin ing by skim min g to remove all surface scale, with a final pass at a 1 mm dep th ofcut to avoid excessive surface ha rden ing of the workpiece.The chemical compositions of the m ater ials and ma terial conditions are give n in Tab les 1 and 2 . W ith th eexception of the Si and SiCa low carbon free cutting steels, which were produced un der vacu um as 1 tonnem elts to en sur e cleann ess of a comm ercial quality, all of the ma teria ls were of production o rigin. Thecalcium treatment of the silicon killed low carbon free cutting steel was attempted by taping calciumm etal to the side s of the ingo t mould.3.2 Too l Ma terialsOf the ra ng e of tool m ate rial s described above the following were selected as being of most relev ance ;

Coated Carbides - Plain and Chipbreaking Geom etries;Cerm ets - Chipbreaking only;Alu min a Based Ceramics - Plain Only.

The performance of S13N4 based ceram ics was not regarded as sufficiently know n to justify inclusion, andPCB N ins erts w ere not considered to be competitive for free ma chinin g app lication s.The tool m ate ria ls acquire d are detailed in Table 3, together with their app lication w ithin th e project. Allof the tools were 12 m m square inserts , 4 mm thick, with a 0.075 mm nose radius. The inserts were ei therplain or of the M F chipb reak er geom etry to limit the range of tool geom etry u sed.

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4. RESULTS O F EXPER IM ENTSMachinabil i ty data were obtained by long turning under s tandard condit ions for each qual i ty to amaximum distance cut of 5100 m. Tool forces were monitored using a 3-axis Kistler dynamometer withthe output displayed on a portable microcom puter. Th e tool forces were logged imm edia tely prior tointerr uptio n of the cut at 300 or 600 m inte rva ls when surface finish an d tool we ar we re d eterm ined .4.1 Tool Wear Tes ts4.1.1 Au stenitic Stain less SteelWear tests were conducted on the three qualities of austenitic stainless steel using the grades of toolingund er investigation. A stan dar d feed rat e of 0.2 mm /rev wa s used at four cut ting s peeds (170, 260, 420,650 m/min) with a 1 mm de pth of cut and no coolant. Tool wear w as monitored b y the d eterm inatio n offlank wear at 300 or 600 m intervals, with determination of surface finish and tool force at the sameintervals . The resul ts are presented graphical ly in Figs. 1-12 which give the variat ion of f lank wear,surface finish, feed force (x) and c utting force (z) for the var ious tool m ate rial com binatio ns.The standard grade austenitic stainless steel caused rapid tool failure of all three tool grades at cuttingspeeds of 420 m/min, Figs. 1-3. At a cu ttin g speed of 260 m/m in tool failure wa s rapid on the ceram ictooling, and neither cermet nor coated carbide tool achieved the aim cutting distance of 5100 m, with toolfailure at ~ 300 m for both tools. How ever, cutting forces we re lower for the cerm et tooling an d surface finishbette r and more consistent; the surface finish w ith the coated carbid e tooling det erio rate d rap idly withincreasing wear.At a cutting speed of 170 m/min, coated carbide tooling completed the aim cutting distance of 5100 m butf lank wear measurements became large and could not be rel iably determined, and surface f inishdeteriorated progressively. The cermet tool almost completed the full test distance at 170 m/min, with abette r surface finish tha n the coated carbid e and with sim ilar tool forces. The cera m ic tooling, even at170 m/min, failed quickly.At a cutting speed of 260 m/min, all of the tools cut for 5100 m on the resulphurised austenitic stainlesssteel. Cerm et and ceramic tooling performed for the full du ratio n of the test a t 420 m/m in, but only th eceram ic tooling was adequ ate at 650 m/m in, Figs. 8-10. The surface finish obtain ed by cerm et and c eram ictooling was quite uniform over the tests to 5100 m, but the finish obtained with the coated carbide toolingdeter iorated after 3000 m, rising from 1.5 pm to 3.5 urn. Th e lowest feed and cut tin g forces were ob tainedwith the cerm et tooling, with coated carbid e and ceram ic tooling high and sim ilar.The calcium-treated austeni t ic s ta inless s teel could not be machined sat isfactori ly with the ceramictooling at any of the three cutting speeds, with rapid flank wear and deteriorating surface finish on alltest s followed by tool failure, Figs . 4-6. T he coated carbid e tooling also failed r apid ly a t 420 m/m in, and at260 m/min flank wear increased substantially throughout the test with the surface finish being poor andvaria ble. Tool forces were sim ilar to those obtained with the stan da rd aust enit ic grad e. At 170 m/m insurface finish deteriorated very markedly with distance cut, with tool forces similar to those at 260 m/min.The cerm et tool performed satisfactorily a t 170 and 260 m/m in, w ith low flank we ar, good surface finishand lower cutting forces than for the coated carbide tooling.To widen the cut t ing condit ions used, further tests were conducted on the austeni t ic s ta inless qual i ty atcut ting speeds of 260 and 420 m/min wi th feed ra tes of 0.31 and 0.50 mm /rev. Th e hig her feed rate of0.31 mm/rev at the lower cutting speed of 260 m/min is equivalent in metal removal rate to 420 m/min at0.2 mm/rev and that of 0.5 mm/rev equivalent to 650 m/min at 0 .2 mm/rev and to 420 m/min at0.31 mm /rev. As shown by com paring Fig s. 7 and 10, longer tool lives can be obtain ed by incre asing thefeed rat e while m ainta inin g metal remo val rat e, but at the expen se of increased tool forces. The surfacefinish using the coated carbide tooling at the higher feed rates remained similar to some of the valuesobtained at 0.2 mm /rev due to nose we ar removin g the nose radi us. W ith cerme t tooling the re sul tssuggest a s imilar pat tern, but with a deter iorat ion in surface f inish as the nose radius maintained

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inte grity de spite the high feed rates. Using ceramic tooling all of the tes ts completed the 5100 m te stleng th, with forces increasing with feed rate , but with erra tic surface finish valu es o btained.4.1.2 Medium Carbon Low Alloy Steels (709M40)The size of the bars restr ic ted testing to a maximum c utt ing speed of 260 m/m in. In the s ta nd ard ,calcium-free quality at this cutting speed only the ceramic (K060) cutting tool was capable of completingthe full te st, with both coated carbide and cermet tools failing a t about one -third of the t est distan ce. W iththe calcium-treated quality there was little difference in performance, with flank wear rates higher on theceram ic tool. W ith ceramic tools, cutting forces increased with tool we ar but s urfac e finish did n otdeteriorate. The surface finish obtained on the calcium-treated steel was inferior to that of the standardgra de, Figs. 13 and 14.4.1.3 Low Carbon Stee ls4.1.3.1 Carbon-ManganeseThe plain carbon-manganese steel was tested at 260, 420 and 650 m/min using a feed rate of 0.2 mm/rev.Under these conditions, all of the tools completed the test distance of 5100 m at 260 and 420 m/min, butonly the cermet tooling endured a t 650 m/min, Figs. 15-17. On all of the tools, flank wear inc reased withcu ttin g speed and distance cut. Surface finish was ~2-3 pm w ith all of the tools and con ditions, with cu ttin gforces also very similar for all conditions.4.1.3.2 Low Carbon Bala nced Free Cutting SteelOn the unleaded LCFCS, using plain geometry coated carbide tools, flank wear increased steadily withdistanc e cut, but with little variatio n with cuttin g speed for eith er 260 or 420 m/min , Fig. 8. A t the h igh estcutt ing speed used, 650 m/min, flank wear increased rapidly betwe en 2000 and 3500 m of cut. The surfacefinish (between 2 and 3 pm) varied little with cutting speed, but also tended to increase with the distan cecut. The m easured tool forces did not show a consistent variation with cut ting sp eed, but also tended toincre ase with cuttin g distanc e. Similar tests conducted usin g the coated carbid e tool with a b uil t-inchip break er, gave similar resu lts, but the tool forces were ~50 N/m m lower, Fig. 19. At the lowest cu ttingspeed of 260 m/min flank wear was higher than for the CMn steel, but substantially lower at the twohigher c ut t ing speeds.The flank wear on the cermet cutting tool increased with both cutting speed and distance cut, Fig. 20, asdid surface rough ness after the distance cut exceeded 4000 m and at the hig hes t cutt ing speed. The cuttin gforces increased with distance cut, and were comp arable to those obtained w ith the coated carbide. Thetool we ar tended to be slightly higher than th at on the CMn steel, bu t with be tter surface finish. Theceram ic cutting tool gave results sim ilar to those obtained w ith the coated carbid e tool, Fig. 21, but with abe tter surface finish at the high cutting speeds. The wear on the ceram ic tooling tended to be lower t hanwith the plain CM n steel.The use of higher feed rates, Figs. 22-24, showed reduced tool wear for similar metal removal rates, butsurface finish deteriorated and tool forces increased.4.1.3.3 Lead ed Low Carbon Balan ced Free Cutting SteelThe leaded free cutting steel generated flank wear on the plain coated carbide cutting tools similar to thatof the unleaded grade at cutting speeds of 260 and 420 m/min, Fig. 25, but severe spike and nose wearoccurred at 650 m/m in, prev entin g satisfactory m easu rem ents of flank wea r. The une ven tool wear led to asevere deterio ration in surface finish after a cutting distance of 2000 m. At 260 m/m in surface finish wassim ilar to the unlead ed grade, but at 420 m/min surface finish imp roved w ith cu tti ng dis tan ce . At260 m/min and 400 m/min the feed and cutting forces tended to be lower for the leaded free cutting steelthan the unleaded grade, but at a 650 m/min, the heavy wear with the leaded steel resulted in higher toolforces.

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With the cermet tooling, flank wear increased with both cutting speed and distance, Fig. 26, but at a lowerrate than that on the unleaded steel. However surface finish on the leaded steel was generally inferior tothat of the unleaded grade, especially at 420 m/min with surface finish ~3.5 pm compared with 1.5 pm.Both feed and cuttin g forces were slightly lower than for the unlea ded steel when u sing the ce rm et tooling,with the lowest forces at 420 m/min.The flank wear on the ceramic tool was similar to that of the cermet tool, Fig. 27, increasing with bothcutt ing speed and distance cut but a t a lower rate t han on the un leaded q ual i ty. The lowest cut t ing speed(260 m/min) exhibited the poorer overall surface finish 2.3 pm, slightly inferior to that of the unleadedsteel. A t the higher cutt ing speeds the leaded steel gave surface finish valu es of2 pm w hich were slig htlyinferior to the unleaded grade . W ith the ceram ic tools the 420 m/m in cutt ing speed gave the highes t forcesand the 260 m/m in speed the lowest, though th ese w ere errati c. Ov erall , the leaded steel tende d to produceslightly lower cutt ing forces tha n the unlead ed g rade.The use of increased feed rates of 0.31 and 0.5 mm/rev did not affect flank wear on coated carbide tools,Fig. 28. The surface finish deteriorated as would be anticipated, but at the highest feed rate the surfacefinish 8.5 pm w as below the theoretical finish of 10 pm. The increased feed rates resul ted in more errat iccutting forces but only the 0.5 mm /rev feed r ate produced a significant incre ase in cut ting force.Using cermet cutting tools, flank wear was increased by both increased cutting speed and increased feedrate, w ith feed ra te inc reasin g both cuttin g forces and feed forces, Fig . 29.The ceramic tooling showed increased flank wear with increased feed rate, together with increased feedand cutting forces, Fig. 30. At a cutting speed of 260 m/min and a feed rate of 0.31 mm/rev the surfacefinish was consistently better th an th e value expected as a function of the nose rad ius an d feed ra te .4.1 .3 .4 Lead-Bismuth-Tel lurium Treated Low Carbon Ba lance d Free Cutt ing Stee lUsing plain coated carbide inserts , f lank wear increased most rapidly at the highest cut t ing speed,650 m/m in, with little difference in wear rat e at 260 and 420 m/m in, Fig. 31 . The increa sed wea r at th ehighest cutting speed led to a deterioratio n in surface finish. Cu ttin g forces were relatively unaffected byvariatio ns in speed or tool wear. W ith cerm et tools, wear incre ased with cuttin g speed, with significantdifferences between the n speeds used, Fig. 32, bu t surface finish w as not notably affected by the increasedtool wear. Cu tting forces were lower with the cerm et tool, but varied little with speed. The ceram iccutting tool, K060, gave similar flank wear behaviour to the cermet tool, with cutting forces similar tothose of the carbide tool, Fig. 33. The surface generated using all three tools varied little with cuttingspeed, but the ceram ic cutting tool gave the best overall perform ance.4.1.3.5 Sil icon Kil led Low Carbo n Fre e Cutting Stee lWea r test s were conducted using similar conditions to other low carbo n steel va rian ts. All of the toolingcompleted the full test distance under all conditions, with a performance similar to the balanced 230M07grade on the coated carbide and ceram ic tools. Th e cermet tool gener ated lower wear and forces but poorersurface finish, Figs. 34-36.4.1.3.6 Sil icon Kil led and Calcium Tr eated Low Carb on Fre e Cutting SteelThe silicon and calcium treated grade gave a broadly similar performance to the silicon killed equivalent,though flank wear ten ded to be lower on all of the cutt ing tools.4.1.3.7 Tool Sel ect ionThe results of the wear tests and surface finish measurements suggest the following material/workpiececombinations for use in high speed finish ma chi nin g op erations :

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QualityAustenitic Stainless

AISI304AISI 304CaAISI303Medium Ca rbon Low Alloy Steel709M40709M40CaLow C arbon

CMn230M07230M07Pb230M07PbBiTe230M07 + S230M07 + Si+ Ca

Tool Type

C e r m e tCermetCeramic

Cermet /CeramicCermet /Ceramic

CermetCoated Carbide/Cermet/CeramicCermet /CeramicCermet/CeramicCoated Carbide/CeramicCoated Carbide/Ceramic

Suitable Cutting Speedm/min

12015025 0

25 0250

30045 045 045 060 060 0

4.2 Machinabil ity Model for Non-Orthogonal Cutting ConditionsThe modification of the Merchant model to cater for non-orthogonal cutting conditions is given inAppendix 1.Machining tes ts were performed at a feed ra te of 0.2 mm /rev, with a 3.0 mm de pth of cut and cuttin g speedsrang ing from 110-650 m/min for a helical distanc e cut of 150 m. Imm ediate ly prior to the end of the cut arecord of tool forces was tak en an d a chip sam ple obtain ed. A new tool tip was used for each test, w ith tes tsconducted using coated carbide, cerm et and ceramic tooling. The chip sample s were mounted in coldsetting resin. After polishing, selected cross sectional are as were determine d using an image a nalyse r(IBAS 2) to obtain values for use in the m achin ability model.4.2.1 Austeni t ic Stainless Stee lsShear AngleThe shear angle determ ined for the stan da rd aus tenitic sta inless gr ade (AISI 304) was 20-30 at the slowercutting speeds increasing to 25-35 at t he high er cu tting speeds, Fig. 40. Repe at testin g to determine thesignificance of the anomaly apparent for the K060 cutting tool shows that the scatter in the data obtainedwas quite large . Calcium trea tm en t did appear to affect she ar ang le, Fig. 41 , but an increas e in sulphurcontent raised t he sh ear an gle to 30/40, Fig. 42.Shear StressShear s tresses were lowest for the resulphurised s teel -500 /700 N/m m 2 compared with -500/800 N/mm 2for the AISI 304 g rades.Shear StrainShear stra in calculated for all of the steels were fairly sim ilar - 2 . 5 , with the resulp huris ed steel tending tobe the lowest.

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Friction CoefficientThe calculated coefficient of friction were in the range 0.3 - 0.4, with the cermet tool giving the lowestvalue for all of the austenitic stainless grades.4.2 .2 Med ium Carbon Low Al loy Stee lsShear AngleThe values on both the calcium and sta nda rd 709M40 qual i t ies were very s imilar a t 35, Figs. 43 and 44.Shear StrainShear s t ra ins were 2.4 for both grade s.Shear Stres sThe she ar str ess for the steels was 700-800 N/m m 2 .Frict ion Coeff ic ientThe friction coefficients obtained, un like the other param eter s, were significantly different, ran gin g from0.25for th e cerm et tool to 0.35 for the coated carb ide, and 0.45 for the ceram ic. T hese re su lts w ere s im ilarfor both steel typ es.4.2.3 Low Carb on Ste elsShear AngleOn the CMn steel, the she ar an gle ten ded to increa se w ith cutting speed incre asing from 10-15 to 20 at650 m/m in, Fig. 45. The resulph urised 230M07 steel had a higher ran ge of shear ang le increa sing from 13-20 at 110 m/min to 19-25 at 650 m/min, Fig. 46, with generally similar results for the leaded andlead-bismuth-tel lur ium variants , Figs. 47 and 48. The silicon killed LCFCS showed a similar shear angle,Fig. 49, with a wider scat ter on the calcium treated varian t , but shear angles of the sa me ma gnitude ,Fig. 50. A repeat test on the resulphurised steel suggested results on the low carbon steel could be morereproducible than those on the austen i t ic grad es.Shear StrainThe shear s trains determined on the CMn steel were qui te variable ranging from 4.5 to 6.3 at the lowerspeeds general ly decreasing to 3.2 at 650 m/min. On the resulphurised qual i ty shea r s tr ain s were lower,decreasing from 3.3-4.6 to 2.8-3.4 at the higher speeds, with shear strains of similar magnitude on theother low carbon steel .Shear Stres sShe ar stre sse s were sim ilar on all of the low carbon v aria nts a t 500 N/ mm 2 , showing l i t t le variat ion withcutt ing speed.Friction CoefficientThe friction coefficients determined for the CMn steel decreased with increased cutting speed from-0.4 8/0 .58 to 0.28/0.48 at 650 m /min.The friction coefficients tended to be lower for the free cutting steels with 0.35-0.45 at a cutting speed of110 m/m in and 0.15-0.37 at 650 m/ min . Th e cerm et cu ttin g tools gave the lowest friction coefficients,especially with two silicon killed free cutt ing s teels.

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4.3 Sca nnin g Electron Microscop ySelected tools used to perform both short and long term machining tests on some of the qualities wereexamined for wear and deposits.4.3.1 Au sten itic Stain less SteelsFor th e AISI 304 stee l, only sho rt term test s at 260 m/min were exam ined a s the majority of the tests endedin tool failure. Wo rkpiece m ate rial was observed adh ering to all thr ee tool typ es espe ciall y to thechipbreaker, and to the nose of the ceramic tool. Severe damage to the coating was occurring even aftersho rt time s on the coated carbide, with the coating being peeled from the cu tting edge to reve al th e ca rbidesub strate . Similar damag e occurred with the AISI 304Ca steel af ter a much longer cut t in g t im e,Fig. 51(a). No depo sits of m ang ane se sulph ide were found on the coated carbide tools with eith er of theAISI 304 qual i t ies .On the cermet cutting tool no massive deposits of workpiece material were observed with the AISI 304Casteel , with only thin layers of adherent workpiece material present , together with t races of manganesesulphide. W ith the ceramic tool ing al l the tests terminated qui te rapid ly, with workpiece m ate ria lad he rin g to the tools, and the catastroph ic nat ur e of the failures made evalu ation difficult. How ever, on atest with a K090 ceramic tool, failure was limited to the nose area and the adherent workpiece depositsand nature of the tool wear could be examined, Fig. 51(b), revealing a deeply grooved flank face, withgrooves contain ing significant am ou nts of calcium as well as silicon and alum iniu m.With the AISI 303 steel, workpiece material was observed adhering to areas of the tool where the coatinghas been badly worn, such as the flank wear band, Fig. 51(c). Across the rake face a layer of manganesesulphid e thick enough to prev ent detection of the underlyin g tool m ate rial w as observed. The su lphidelayer displayed a pattern of cracking suggestive of contraction of a solidifying layer during cooling aftercuttin g. On the cerm et tool a similar lay er of sulphide was observed on the rak e face, Fig. 52(a), with nosignificant w orkpiece mate rial deposits present. The ceramic tool, Fig. 52(b), also had no significantworkpiece material deposits, and although a thin layer of MnS was prese nt across the rake face there wasno craz ing of the laye r. The flank wea r was very uniform with a patte rn of sma ll vertical grooves.4.3.2 Low Car bon Stee lsAt 260 m/min there was very little wear or deposits present on any of the tools, and only the standard230M07 and th e calcium-silicon-killed 230M07 steels were exam ined in any de tail after 5100 m of cut.On the 230M07 stee l, both cerm et and coated carbide cutting tools were free of deposits of eith er workpiecem ate ria l or ma nga nes e sulphide. The ceramic tool had developed nume rous shallow grooves along theflank wear band and there was a thin smear of manganese sulphide over the bevel of the ceramic cuttingtool edge.The silicon killed calcium treated 230M07 steel showed little tool wear on the coated carbide tool, andthere was a crazed layer of manganese sulphide, similar to that illustrated for the AISI 303 steel, coveringthe rak e face of tool. 'Fin gers ' of sulphide exten ding across the tool from the ge neral are a of chip contact,Fig. 52(c), were thick enough to prev ent detection of the un derlying tool coating. On the flank face, a layerof iron was observed ad her ing to the tool wh ere the outer TiN layer had worn away. On the cerme t cu ttingtool there was no general covering of manganese sulphide, and where sulphide was detected i t wasinsufficient to pre ven t detection of the und erlyin g tool m ate rial. In con trast, the ceram ic (K060) cuttin gtool had a coherent layer of manganese sulphide suff icient to prevent detect ion of the underlyingalum inium , and behind the c ut t ing area deposi ts r ich in calcium, s i licon, and alu min ium were observed.4.4 Tool Life Tes ts - Un coate d Carbide ToolingTo provide a link with more conventional data, cutting tool wear tests were conducted on the 130 mmdiameter low carbon steel bars using SPGN120308 ES30 (P30) uncoated carbide tooling with lubrication

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using a 0.25 mm/rev feed rate and 2.5 mm depth of cut as per IS03865:1977 condition B. The flank wearcurve s and crate r depth we ar curves, Figs. 54-59 were used to det erm ine th e tim es to a flank w ear c riterionof 300 pm, and crater depth w ear criterion of 100 pm . Thes e valu es ar e shown a s Taylor plo ts in Figs. 60and 61 to obtain 15 min tool lives for each criterion.Fl an k wear resu lts on the CMn steel were quite scatte red, w ith poor tool lives from th e inn er re gion s of thebar , Fig. 60. O verall the V i 5 value was -1 8 6 m/min. The 230M07,230M 07Pb and 230M07PbBiTe qua l i t iesall gave similar resu lts within a rang e of 250-266 for the V15 valu e. The silicon-k illed ste els gave asubstan t ial ly higher values at 511/513 m/min. The crater depth we ar cr i te r io n, F ig. 61 , shows th e230M07, 230M07Pb and 230M07PbBiTe steels to form a similar group as for the flank wear criterion, butthe slopes of the CMn, and silicon killed steel curves were steeper, with the calcium treated steel havingthe least crater w ear . Although few of the tests on the CMn steel a t ta in ed th e 100 pm cri ter ion, th e s teepslope of the curve h as been inferred from the dat a show n in Fig. 54(b).4.5 Tool Tem perature Mea surem entsThe tool /workpiece temperatures were determined using the experimental arrangement described inAppendix 2, using uncoated carbide cuttin g tools and cer me t cutti ng tools.4.5.1 Au stenitic Sta inless Stee lsTool tempe ratures were determine d on the s tainless grad es us ing a feed ra te of 0.2 mm /rev and a depth ofcut of 1 mm with carbide cutting tools. A new tool tip was used for each test an d the em f recorded a s soonas a s teady state was achieved. Th ere was l it t le difference betw een the thre e grad es with tem pera tures~500C for a cutting speed of 110 m/min, r is ing to ~900C at 420 m /min, Fig. 62.4.5.2 Low Carb on SteelsCu tt ing tempera tures were determined on al l of the low carbon grades, Fig. 63. The lowe st cut t ingtempe ra ture of 330C at 110 m/min wa s the 230M07PbBiTe steel , and this gave con sistent ly lower cu t t ingtemp era tures than al l of the o ther resu lphur i sed s tee l s , r i s ing to 700C a t 420 m/m in . The o therresulphurised s teels a l l performed similar ly, with cut t ing temperatures increasing from 500C at110 m/m in to 800C at 420 m/min. The plain CM n steel gave the lowest tem pe rat ur es a t cut ting speeds inthe ra ng e 260 - 420 m/min.Fo ur of the steels were tested with a cerme t tool, Fig . 64. Th e tool tem pe ra tur es ob taine d on the plainCMn, 230M07PbBiTe, and silicon killed free cutting steels were similar in rank order to those obtainedwith the carbide tool ing, and also very s imilar in term s of absolute c ut t ing te m per atu re obtained, Fig. 64.4.6 Ch ip FormThe chip form and colour observed in the wear tests is summarised in Table 4. The temper colours cannotbe t reated as an absolute measure of temperature as the oxidat ion layers in chips are generated in veryrapid heat ing and are also in influenced by al loy content . How ever, the relat ive tem pe ratu res denoted canbe used on a comparat ive basis , with tempe rature s increasin g from si lver-gold-p urple-blue- grey.4.6.1 Au stenitic Sta inles s Ste elsThe AISI 304 steel produced a well broken chip form in most of the tests using chipbreaker tools, withsigns of hea ting of the chip towards th e end of some tes ts show n by the o ccurrence of gold/purple oxidationcolours. At 170 m/min using the coated carbide tool chip breakup was poor with some snarled ribbon andhelical chips. With the ceramic tool the absence of a chipbreaker led to the occurrence of snarled ribbonchips.Snarle d ribbon chips were not observed in any of the te sts on the A ISI 304 steel, an d oxid ation of chips w asless than e vident in the base AISI 304 grade .

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The resulphurised AISI 303 steel had loose chips in all of the tests, with chip localisation severe at thehighest cutting spaces, resulting in fine fragm entation of some chips, Fig. 53. This phen om enon was mostpronounced for the ceram ic tools.4.6.2 Medium Carbon Low Alloy SteelsChip breakup was good when controlled by the chipbreaker geometries, but snarled ribbon chips wereproduced w ith the ceram ic tools.4.6.3 Low Carb on SteelsThe plain CMn steel was machined using a chipbreaker geometry for the coated carbide as snarled ribbonchips from plain tooling gave difficulty in cutting. Chip colours indicated t ha t the c erm et tools producedhotter chips tha t the coated carbide tooling. The ceramic tooling gave some problems w ith snar led ribbonchips at 420 and 650 m/min, but at 260 m/min a w ell broken he lical form was produced.The standard 230M07 resulphurised grade gave no swarf breakup problems with any tool , and thecomparison of plain and chip breaker tools showed the la tter to be associated w ith a sma ll reduc tion in chiptempera ture .The leaded resulp hurise d steel showed less oxidation of the chips for all tool grad es, with heav ily oxidisedblue chips only evident at the highe st cutting speed on the ceram ic cutting tool. The re su lp hu ris edPb-Bi-Te free cutting steel showed a similar chip form and colour to the leaded grade, with someindications in chip colour that the sw arf may h ave been slightly cooler.The silicon killed low carbon free cutting steel gave swarf breakup problems with the plain carbidegeometry so tests were conducted using the MF chipbrea ker g eome try. The con sistent silver colour of thechips showed chip temperatures to be comparable or lower than for the balanced steels, with only theceramic tool producing heavily oxidised swarf.The calcium treated silicon killed steels also produced chips with little oxidation, and comparison of thechips produced by the ceramic tooling suggested lower chip temp era ture s th an experienced with th e siliconkilled grad e.The tests us ing P30 uncoated carbide tooling showed a range of chip form and colour. The CMn and230M07 steels both gave blue or blue/purple chips showing tha t the sw arf tem pe rat ure of these grades washotter than that of the leaded and resulphurised silicon treated variants, which had chips which weresmaller and m ore broken, with gold the predom inant colour suggestive of a reduced am oun t of oxidation.4.7 Metal lography and Automatic Image An alys isLongitudinal sect ions were taken from one bar of each qual i ty for the assessment of hardness andinclusion size by autom atic image a nalys is.Each sam ple was assess ed for inclusion s categorised into four types :-

Free oxideFree sulphide h Isolated/non-touching inclusionsAssociated oxide - an oxide inclusion in contact with or enca psu lated by a sulph ide inclusion.Associated sulphide - sulphide inclusion in contact w ith an oxide inclusion.

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4.7 .1 Au steni t ic Sta in less Stee lThe polished metallo grap hic sp ecime ns were sca nned in a single block of 100 fields usin g a mag nificationof 500 X over an area of 1.46 mm 2 . The oxide are a w as the highest in the AISI 304 steel, Tab le 5, withinclusions more num erous but sma ller in the resulphu rised quali ty. Sulphide area s observed related tothe sulphur content of the steels, with the AISI 304Ca having a sulphide area 5X that of the AISI 304grade and the AISI 303 being 7 X higher aga in .4.7 .2 M edium Carbon Low Al loy Stee lsThe calcium-treated quality had an oxide content 5 X that of the calcium-free variant, with the oxide inthe calcium treate d steel associated w ith the sulph ide. Despite the high oxide volume most of the sulphidewithin the steel was not associated with oxide but present as elongated inclusions.4.7.3 Low Carb on Ste elsThe CMn steel was characterised by a high globular oxide inclusion content, which increased in sizetowards the centre of the bar, Fig. 65. The silicon killed free cutting steel variants were both quite clean,with relat ively few oxide inclusions compared with the balanced free cutt ing steels which containedsubs tant ia l vo lumes o f duct i le manganese s i l ica tes , wi th h ighes t f requencies in the leaded andlead-bism uth-tel lurium var iants . The bulk of these si l icates was presen t in associat ion with sulphides,often occurring as inclusion tails.The sulphide morphology of selected grades is illustrated in Fig. 66. In the austenitic stainless steels thesulphides were quite globular, with the pattern of sulphides in the AISI 304Ca variant showing evidence ofthe as cast s truc ture, Fig. 66(b). T he m edium carbon engine ering steels had very f ine elongated sulphides.In the low carbon steels , the ki l led variants both had elongated sulphides, but the sulphides in thebalanced steels were much more globular , with the tel lu rium treated va riant showing the least deformedinclusions of the resulp hurised grades .4 .8 Mech an ica l Proper t i e s4.8.1 AmbientThe room temperature mechanical propert ies of al l of the steels were determined on the remains of barsused for the ma chining tests .In austenit ic stainless grades, the resulphurised AISI 303 quali ty had the lowest ducti l i ty and highesttensi le strength, Table 6. The yield streng ths of the thre e steels were in the rang e 194-250 N/m m 2 , withthe calcium-treated grad e having the highest yield stre ngth .The medium carbon low al loy steels were similar in ducti l i ty, but the calcium-treated steel had asignificantly lower stre ng th. Of the low carbo n resulp hu rised g rade s, the Pb-Bi-Te quality had the lowestyield strength, and both sil icon treated steels had higher stren gths and g reater ducti l i ty. The CMn steelwas similar in stren gth to the resulph urised grades but had a much greate r ducti l i ty.4.8 .2 Elevated Tem peratu reStress-strain-temperature curves were determined by compression test ing on 15 X 10 mm diametercylindrical compression specim ens machin ed from ma teri al of the AISI 304 and 230M07 steels. Sam pleswere heated to various ini t ial temperatures between ambient and 700C and subject to compressiontesting as described previously* 24 ) . The comp ress ion fo rces , tem per a tu res and d isplacem ents weremonitored througho ut the test and recorded on a microcomputer . The da ta was used to define th estress-s train-tem peratu re surface assu min g a const i tut ive relat ionship of the form:-

= exp ( A ^ + A 2 2 + A 3 T3)

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The curves are shown in Figs. 67 and 68. Superimposed on these curves are the instability loci when therate of work hard enin g corresponds to the rate of the rm al softening. The adiaba tic strai nin g locus wasalso superimposed on the curves to define the adiabat ic , thermally aided instabi l i ty s train, a t theintersection with the instability locus.The shear stresses and strains for metal cutting were defined for various speeds using shear stressesdetermined from the cutting forces obtained by the modified Merchant model and shear strains calculatedfrom the chip thickne ss. The data were transfo rme d to axial stress es and stra ins and are recorded inTable 7, together with temp erature s determ ined on the plain carbide ins erts .4.9 Tool Vibra tionA subsidiary aim of the project was to assess the significance of vibration in single point machinabilitytests and to dete rm ine wh ether it is responsible for the poor reprod ucibility of such tests. Tria ls weretherefore arranged in which the major machining parameters (cutting speed, depth of cut and feed rate)were fixed bu t tool geometry was varied in order to provide different v ibratio n regim es. A description ofthe accelerometer and its associated ins trum ent atio n is given in Appendix 3.All of the tes ts were performed on Type 304 stainle ss s teel.The tool geometry was varied by changing the distance the tool holder projected from the clampingarrange me nt . This dis tance was referred to as the 'overhanging length ' and was mea sured from thecutting tip of the tool to the point where the tool holder en tered t he clam p. Two ove rhan g leng ths w ereused in the tria l, i.e. 43 mm and 95 mm. The lat ter value was excessive in relationsh ip to the cross sectionof the tool holder (25 mm square) and preve nted th e norm al tool post arra ng em en t from be ing used. A newclamp was therefore b uilt specifically for this t rial.The following parameters were constant throughout the trial for all three tool types:1. Cu tting speed:- 260 m/m in2. Feed rate:- 0.2 mm/rev3. Depth of cut:- 1m mTest were performed using the thr ee different types of tool tip adopted in the res t of this project.Surface finish and flank wear mea sur em ents w ere made after a cutti ng distance of 300 m. The tip 'scutting corner and the tool holder overhang position were then returned to their former positions and theprocess repea ted. This continued u ntil a catastro phic tool failure occu rred (i.e. the tip broke) or some othe revent occurred th at w ould in practice re sult in a tool chan ge.An accelerom eter was screwed into the underside of the tool holder to m eas ure vibration .The purpose of the accelerom eter w as twofold:1. To verify that changing the overhanging length of the tool holder al tered i ts vibrat ioncharacter is t ics .2. To monitor any chang es in the vib ration of the tool holder as th e test progressedEach tip type was tested twice; a total of six tes ts were therefore performed. The first three tests w ereperformed on the coated carbide tip, the cermet tip and ceram ic tip respectively. Th is sequence w asrepeated for the final thr ee tes ts.

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4.9.1 Sur face Finis hThe coated carbide results, suggested that the CLA measurements were similarly erratic for both toolholder positions, although in the second test, there was a slight indication that the surface finish wasworse w ith the tool holder in the 43 mm ove rha ng position.Th e CLA resu lts for cerm et tips, were mu ch mo re consiste nt. However, it was aga in difficult to discernan y significant difference in the CLA v alue s caused by cha ngin g the tool ove rhan g position.The first test on the ceram ic tool tip was mo re successful. It indicated that the CLA resu lts were of sim ilarorder for both overhang posi t ions, a l thoug h the ini t ia l performance w as somewhat bet ter when the 43 m mtool overhang position was used.4.9.2 Flank WearIn the first test, on coated carbide tools, the flank wear measurements for both tool holder positions werealmost identical. However, in the repeat test, there was an indication that the flank wear was reduced forthe sho rter tool overha ng distance .The flank wear measurements for the cermet tips, were virtually identical, irrespective of the tool holderposition.The results from the first tests on the ceramic tip again suggested that the tool overhang length had littleeffect on the flank wear. Un fortu nate ly the re are too few data from the repea t test to corrobo rate t hisfinding.4.9.3 VibrationA typical Fourier t ransform of the accelerom eter m easure me nts is shown in Fig. 69. C hatter only occurredwith the coated carbide tips and then only when the gre ate st tool overha ng position was used. Fou riertransform clearly demonstrated resonant behaviour in these cases with an initial peak at 1.4 kHz of theorder 40 m/s2 when the tip was new but this increased to over 4000 m/s 2 at the end of the test when the tipwas worn. The magn itude of the tool holder 's vibrat ion also increased when the shorter ove rhang distancewas used but i ts magnitud e only increased from 150 m/s 2 to 550 m/s 2 , and ch atte r did not occur.4.9 .4 Ult imate Fai lure Mo desThe individual tests were ended by a variety of different failure modes. These are catalogued in Table 7.None of the tools failed catastrop hical ly du ring the tests on the coated carbide t ips . The tes ts wereeventual ly hal ted because there seemed l i t t le point in continuing them after so much f lank wear hadoccurred. Consequently these part icu lar tests did not determine whether catastrophic tool fai lure waspossible.Catastrophic tool failure occurred in three out of the four tests on the cermet tips, the exception being forone of the tes ts with th e tool holder in its 43 mm ov erha ng position. The corner of the tips chipped off aftercu ttin g distance of 2700 m and 2400 m w hen the tool holder w as in its 95 mm ove rhan g position and after2100 m when it was in the 43 mm ov erha ng position. Th us if the performances of the tool holder p ositionsare ranked in terms of the tips' resistance to fracture it can be seen that the 43 mm position occupies thefirst last position. Th is sugg ests th at the tool holder position has little effect on this mode of failure,al thou gh the evidence is rathe r l im ited.All the ceramic tips failed catastrophically but unlike the cermet tips, the entire edge chipped off ratherth an jus t the corne r. This beha viou r resulte d in both of the tip's corners being removed when th e firstceramic tip failed (Test 3). Separate tips were therefore used in the repeat tests to avoid a recurrence ofthi s behaviour. They failed after cu ttin g distances of 1500 m and 900 m for the 43 mm and 95 m move rhan g position respectively. Th is limited evidence sugg ests that the vibration regime associated w iththe 95 mm tool ove rha ng dec reased tool life.

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5. DISCUSSIONThe advances made in cutting tool technology have enabled machinists to contemplate the use of highercut t ing speeds as the tool ma terials are capable of withstandin g the higher tem peratu res gen erated. Thediffering tool types examined in this work may show a variety of responses to the differing iron matricesbeing cut , whe ther austeni t ic , marten si t ic , or ferr it ic . I t is conceivable th at s teels m ay need to bedeveloped which perform advantageously under these conditions.In selecting the appropriate tool for machining, the machinist must have regard for its performance in anum ber of aspects, of which tool life and surface finish are im porta nt features.5.1 Au sten itic Stain less SteelThe cermet cutting tools produced the most satisfactory performance with the two AISI 304 grades,achievin g the lowest tool wea r and tool force, togethe r with the be tte r over all surface finish. Theworkpiece was less adh ere nt to those tools, and the SEM examin ation conducted suggests tha t this featureis the contro lling factor of the machin ing of stainless steel unde r the cutt in g condition s use d. Theadherence of the workpiece material was particularly marked on the coated carbide tools, and it wouldseem that this tooling is vulnerable to damage by peeling of the coated layers, with workpiece materialthen ad he ring ve ry strongly to the exposed carbide tool. Althoug h one of the steels was calcium tr eat ed tomodify the inclusions it is more likely that differences in wear between the two 304 qualities was due tothe difference in sulphide content, with the AISI 304Ca quality having 5 X the volume of manganesesulphide inclu sions as th e base AISI 304 qua lity. No layers of calcium bea ring inclusions were observed onany of the c uttin g tools. The differences in chip form, with snarled ribbon chips evident in lower s ulph ursteels can also be attr ibu ted to differences in sulphide volume, resultin g in inhere ntly poor chipbrea king .The examination of the ceramic tools used to cut the AISI 304Ca steel strongly indicated evidence ofchem ical attac k, w ith the calcium probably reac ting with the alumina of the tool, so tha t these tools had aneven shorter tool life than with the base 304 steel, offsetting the potential benefits of the increasedsulphide volume.W ith the resu lph urise d a usten itic stainless steel the ceramic cutting tool gave the bette r tool lives, with nosignificant material deposits adhering to the cutting edge, and surface finish was much better than withcoated carb ide, reflecting the poor performances of this tool due to adheren t deposits, Fig. 70. The cerm etcuttin g tool also showed little tendency for workpiece mate rial to adhere strong ly, and th is was reflected inthe lowest forces with this tool type.The modified Merchant model used did show the advantage of sulphurisation in reducing shear stress,shear strain and friction, but the scatter on the data was high, principally due to difficulties in assessingthe area of chips which suffered from co nsiderable distortion and fragmen tation. The trends for s hearlocalisation are w ell illus trate d by the chip form of the resulp hurised grade. The degree of segm entation isincreased by cutting speed and was most prevalent with alumina tools and least with the cermet tooling.Such segmentation is caused by localisation of the shear in the chip, and the effect of tool type would beassociated with th e efficiency with which the tool conducts hea t from th e cutting zone. As the the rm alconductivity of alumina tools is substantially lower than cermet tools, which have a metallic matrix, agre ater propo rtion of he at will rem ain in the chip, thus incre asing the rate of the rm al softening. In thes ecircumstances the Merchant model is invalid, and while the results obtained using the modified model doshow some gene ral tren ds , the difficulties encoun tered suggest th at the model is not an appro priate one foruse with a uste nitic stain less steels. Any model devised will need to take in to account the localisation ofshear due to the developmen t of critical strain instability.The experiment aimed at using higher feed rates showed that this technique can be used to reduce wear,but surface finish, which depends principally on feed rate and nose rad ius, deter iorate s un less a tool with areduced nose radiu s is used. However, this would increase th e risk of severe nose wear a nd on the ba sis ofthese res ult s would not be practical w ith coated carbides. In these tests the cermet tool did m ain tai n its

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integrity but this tool type can be liable to brittle fractures* 20 ) . However, under finishing conditions thelower tough ness of cerm et tools compared with coated carbides should not be critic al.The reasons for the reduced adherence of the austeni t ic s ta inless grade to cermet materials are notimm ediately apparent , and further rese arch work needs to be conducted on this topic . For interm ediateand rou ghing cuts cermets could be unsui table , and the benefi ts of the tou gher coated carbide is req uired.However, some considerat ion should be given to the development of a carbide cut t ing tool with acerm et-type co ating or sim ilar so tha t the benefits of these tool types m ay be combined.5.2 Medium Carbon Low Allo y SteelThe wear tests which were conducted on the medium carbon engineering grades did not produce surfacefinish value s close to the theoretical v alue anticipa ted by the com bination of nose radiu s and feed ra te. Inrespect of tool wear and tool force there was little difference between the standard and calcium-treatedgra des , despite the lower strengt h of the latte r. The surface finish of the calciu m- treated steel was poo rertha n th at of the s tand ard grade, possibly due to i ts lower s treng th an d highe r duct i l i ty . The ceram ic toolsshowed the lowest wear, but the cermet tools exhibited the lowest cutting forces, though part of thisdifference would be a feature of the differing tool tip geom etries. The re sul ts obtaine d u sing th e modifiedMe rchant m odel a lso showed no differences between the grades other th an the fr ic t ion me asu rem entswhich indicated the cerm et tool to have the lowest friction and the ceram ic the high est. Alth ough thecalcium-treated steel did have a higher oxide content, which is usually the case for such steels, the lowproduct calcium observed could be suggestive of partial rather than full inclusion modification practice.On this basis it would be unwise to draw too many generalised conclusions on the relative merits ofcalcium-treated and non-calcium-treated steels from this data.5.3 Low Carbon SteelsIn terms of tool wear, the best composition was the calcium-treated, silicon killed free cutting steel, withall of the other low carbon compositions, including the CMn steel, having similar wear responses, withsome minor variations e.g. at high cutting speeds the coated carbide tool suffered severe nose wear withthe leaded free cutting steel. In respect of surface finish, the stand ard b alanced free cu ttin g steel wasbet ter than the CMn steel but no further advantages were noted with the leaded and tel lur ium treatedgrad es. The surface finish data sugge st th at when using advance d cu ttin g tools leaded and p rem iumgrades of free cutting steels do not offer any advantages, but resulphurisation does retain some benefits,talys urf profiles for the K060 tooling at 260 m/min does sugges t tha t lead does con tribute to a mo re r eg ula rsurface finish even at high cu tting speeds, Fig. 71 .The calcium treated silicon killed low carbon free cutting steel was the only variant showing a positiverespon se to tool wear when compared w ith the plain CMn steel. This respon se was also confirmed by t hecooler chip colour of these steels and appears to be associated with the tendency of this steel to formsulphid e deposits on both coated carbide and ceram ic cutting tools. The oxide inclusion s pres ent also had atenden cy to adh ere to the ceram ic tool. Despite a wide variation in steel cleann ess over the ra ng e of lowcarbon steels tested the tool wear did not appear to be affected, suggesting that the inclusion speciespre sen t did not have sufficient har dn ess difference compared with th e tool to produce significant abra sion .Although the CMn steel did produce a good performance compared with free cutting grades, it should benoted that on the coated carbide tooling a chipbreaker was used rather than a plain insert, as the use of thelat te r resu l ted in poor chipbreaking so tha t swarf was a considerable p roblem.The resul ts on the uncoated carbide tools using IS03685 standard test ing techniques did show upsignificant differences between the low carbon steels. A difference in performance betw een th e inter iorand surface of the bar of the CMn steel sug gests th at abra sion effects, w hich were of no significance withthe coated carbid e, were detrim enta l to the uncoated tool. The balance d free cu tting stee ls all gave sim ilarresu l t s , sugges t ing a s imi la r wear behaviour be tween unleaded , l eaded and lead-b ismuth te l lu r iumva rian ts, and th at und er the conditions used in this project the presen ce of lead has a lim ited benefit to toollife. These r esu lts confirm the findings of Mills and Ahk tar*17). The difference in wear behaviour betweensilicon killed and balanced free cutting steels suggests a different wear mechanism dependent on the level

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of deoxidation, including the low sulph ur stee l, which would in itself be worthy of furth er study. It ispossible tha t a cur rent ECSC*27) project being conducted by Unimetal will elucidate this aspect of materialperformance.The m easured tool tem peratu res with cermet and carbide cutt ing tools were very simila r for correspondingtest on similar qualities, indicative of tool temperature being generated almost wholly by deformation ofthe workpiece m aterial rather than by intera ct ion betw een workp iece and tool . The m ost no tabledifferences between the quali t ies was for the CMn steels , which gave the lowest overal l cut t ingtem peratur es, and the lead-bismuth-tel lurium steel which produced the second lowest tem pera tures . Theincrease in cutt ing tem pera ture can be calculated from a simple shear strain model:

whereYc

======

11p cCut t ing tempera tureShe ar flow stre ssShear strainDensityHeat capacity

increase

Calcula t ions based on th is re la t ionship us ing the modif ied Merchant model da ta sugges t cut t ingtem per atu res ~620C for the CMn steel, - 500C for the silicon killed LC FCS and ~460C for the SPbB iTe freecutt ing steel at a cutt ing speed of 420 m/min. The calculated tem pera tures for the resulp hurised steels areat variance with the measured te mp eratures which were 300C higher. The reduced tem pera ture of thePbBiTe steel compared to the other resulphurised steels can be at tr ibuted to a lower shea r stre ss, but thehigher shear strain of the CMn quali ty should have produced the highest measured temperature, not thelowest as observed.The significance of the tool temperature measured by the tool-work thermocouple technique is alwayssubject to some doubt due to the steep temperature gradient that exist across the secondary shear zone* 6).The chip colour temperatures do suggest that the CMn did machine hotter, as would be expected from theMerchant model data, so the temperatures determined on the CMn steel , and also on the resulphurised230M07 quality ap pear to be in error. It can be postulated th at this difference will be due to differences inflow behav iour at the secondary she ar zone and the a rea of contac t of chip and tool, but this a spect requ iresfurther exam ination. I t is imperat ive for any model of the mach ining behaviour th at the tem peratu redistr ibution within the materia l is known so tha t f low stress-strain behaviour can be ut i l ised. These datashow that the temperatures measured by the workpiece/tool thermocouple method are not sufficientlyreliable to use in such circumsta nces. It is possible th at they could be used in conjunction with oth ermethods of chip tem peratu re determina tion to give est im ates of the tem per ature s involved. The lowcutting temperature observed on the CMn steel does suggest that such a tool would have a reduced level ofcra ter w ear by diffusion, and the tool life criterion for crat er we ar does suggest a difference in behav iour atcutting speeds 200-250 m/min when compared with resulphurised steels. However, investigation of thewear ch aracter istics of the P30 tooling was not within the r em it of this project.With such a wide range of microstructure employed it is difficult to obtain direct comparisons undersimilar cutting conditions, whilst at

european commission technical steel research. interaction of free-cutting steel by c. vasey, a....

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