Wear 250 (2001) 576–586 Tool-wear mechanisms in hard turning with polycrystalline cubic boron nitride tools G Poulachon a,∗ , A Moisan b , I.S Jawahir c a LaBoMaP, ENSAM, 71250 Cluny, France MécaSurf, ENSAM, 13617 Aix-en-Provence, France Center for Robotics and Manufacturing Systems, University of Kentucky, Lexington, KY 40506, USA b c Abstract Hard turning is a turning operation performed on high strength alloy steels (45 < HRC < 65) in order to reach surface roughness close to those obtained in grinding (R a ∼ 0.1 m) Extensive research being conducted on hard turning has so far addressed several fundamental questions concerning chip formation mechanisms, tool-wear, surface integrity and geometric accuracy of the machined components The major consideration for the user of this relatively newer technology is the quality of the parts produced A notable observation from this research is that flank wear of the cutting tool has a large impact on the quality of the machined parts (surface finish, geometric accuracy and surface integrity) For components with surface, dimensional and geometric requirements (e.g bearing surfaces), hard turning technology is often not economical compared with grinding because tool-life is limited by the tolerances required (i.e high flank wear rate) The aim of this paper is to present the various modes of wear and damage of the polycrystalline cubic boron nitrides (PCBN) cutting tool under different loading conditions, in order to establish a reliable wear modeling Flank wear has a large impact on the quality of the parts produced and the wear mechanisms have to be understood to improve the performance of the tool material, namely by reducing the flank wear rate The wear mechanisms depend not only on the chemical composition of the PCBN, and the nature of the binder phase, but also on the hardness value and above all on the microstructure (percentage of martensite, type, size, composition of the hard phases, etc.) of the machining work material The proposed modeling is in a generalized form of the extended Taylor’s law allowing to prediction of the tool-life as a function of the cutting parameters and of the workpiece hardness The effects of these factors on tool-wear, tool-life and cutting forces are discussed in the paper © 2001 Elsevier Science B.V All rights reserved Keywords: Hard machining; CBN tool; Chip formation; Tool-wear Introduction The requirement of manufactured parts in terms of complexity, shape, material, size, etc inevitably enforces the manufacturing firms to adapt new optimization strategies for the manufacturing processes, allowing the use of latest manufacturing techniques The part sequences in the manufacturing process, are generally diverse, and are directly linked to the part shaping process A process can include sequences of moulding, forming, shearing, thermal treatment and finish machining Optimization strategies can avoid or delete certain sequences, according to the initial shapes and the necessary requirement of the final part Other strategies have the objectives to reduce the time required for certain or all sequences by the ∗ Corresponding author Present address: LaBoMaP, ENSAM, 71250 Cluny, France Tel.: +33-385-595-330; fax: +33-385-595-370 E-mail address: gerard.poulachon@cluny.ensam.fr (G Poulachon) • utilization of the new technologies; • elimination of sequences; and • substitution or combination of sequences leading to manufacturing cost reduction The actual optimization strategies aim at increasing the productivity, quality, or the cost reduction by searching • optimal material removal; • improving machining accuracy; • reduction of the number of operations and the machining allowances (i.e near netshape); and by • introducing of efficient and flexible sequences Hard machining makes a major contribution to search flexibility during the machining of hard alloys or high mechanical strength materials In fact, intermediate operations such, as grinding operations can be eliminated This, in most cases, leads to substantial cost reduction in manufacturing, and therefore hard turning operations are developing wide applications in industry 0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V All rights reserved PII: S 0 - ( ) 0 - G Poulachon et al / Wear 250 (2001) 576–586 The development and improvement of new cutting tool materials offer remarkable performance improvements These cutting tool materials include especially coated micrograin carbides, ceramics and polycrystalline cubic boron nitrides (PCBN) The cutting geometry is generally characterized by a large negative rake angle, a reinforced edge chamfer to avoid the chipping of the cutting edge due to the large compressive stresses, and an edge radius generally obtained by honing Under the conditions of finish machining, the chip formation will occur on the chamfer face where the main cutting force will be the thrust force In hard turning, the property of cutting tool materials must be improved to reduce the flank wear rate to make it suitable for such precision applications The lack of knowledge concerning the process mechanics in hard turning leads us to be interested in the study of large stresses that the tool can withstand In particular, the specific cutting force applied on the rake face, the slip velocity at the tool-chip interface, the high dynamic stresses endured by the cutting tool due to the chip formation mechanisms are of considerable interest to researchers today These different statements lead us to study precisely the evolution of the wear and the damage caused to the cutting tool in order to establish a reliable wear model This model can allow us to predict tool-life as a function of the cutting parameters and the work material properties PCBN applications: state-of-art In many applications, the cutting of ferrous materials in their hardened conditions can replace grinding to give significant savings in costs and increase in productivity PCBN and ceramics tools are widely used in the manufacturing industry for the cutting of various hard materials: high speed tool steels, die steels, bearing steel, alloys steels, case-hardened steels, white cast irons, and alloy cast irons The hardness values considered are in the range of HRC 50–70; which can be achieved by the following two mechanisms: • martensitic transformation hardening; and • carbide precipitation hardening Hardening by martensitic transformation is typical within the surface layer of case-hardened steels, generally used to combine the wear resistance of the surface with the toughness of the core (gears, shafts, hubs, etc.) Hardening by carbide precipitation mechanisms is the result of the formation of carbides in the microstructure In this case, the hardness value depends on the type, amount, and distribution of the carbides The abrasion resistance of the carbides involves difficulties in the machining of dispersion-hardened work materials White cast irons, die steels, high speed tool steels, high alloy steels are examples of this category Various studies have been conducted to investigate the performance of PCBN and ceramic tools in hard 577 machining and especially to predict the effects of hardness on the tool-wear rate Narutaki and Yamane [1] showed the case of machining soft steels, where the wear resistance of the tool is controlled by the bonding strength of the tool grains For the case of high speed steels, the work material contains many ultra-hard carbide grains which have strong abrasive action on the tool In such case, they consider that the tool-wear is mainly controlled by abrasion, and the resistance of the tool increases with the increase of PCBN grain content in the tool In conclusion they show that PCBN with low content of CBN grain (∼60%) produces an excellent performance in machining hard materials not containing many of ultra-hard grains (low alloy steels, case-hardening steels, tool steels, etc.) On the other hand, PCBN with high content of CBN grains (∼90%) offers a better performance for abrasive wear and is suitable for machining hardened high speed steel products Also, they point out that the cutting temperature increases with the increase of the hardness value of the machined workpiece until HRC 50 When the workpiece hardness exceeds this value, the cutting temperature decreases with the increase in hardness They explain these phenomena by the change in the cutting mechanisms They also show that the possibility of diffusion wear of CBN tool seems to be relatively low since the cutting temperature is not high enough and CBN grains are chemically stable for iron Nakayama et al [2] indicate that the cutting forces in the machining of hard material are not necessarily high compared with those of soft materials A high shear angle and the formation of saw-toothed chips due to poor ductility reduce the forces despite the high strength of hard material They mention that for cutting tests of 0.25% carbon steel at varying hardness up to HV 500, when 0◦ rake tool is used, the cutting forces are almost independent of the hardness On the other hand, when −20◦ rake tool is used, the increase in workpiece hardness decreases both cutting and thrust forces In the machining of hardened steels (HV 760), the larger negative rake angle significantly increases the thrust force, whereas the increase in cutting force is little Ohtani and Yokogawa [3] state that the main wear mechanism of CBN and ceramic tools in the machining of cold work tool SKD11 (hardness range HRC 18–60) is abrasion by hard alloy carbide particles contained in the workpiece structure The lifespan of carbide tools decreases as workpiece hardness increases, while the ceramic and CBN tool-life shows the opposite results Matsumoto et al [4] performed cutting tests on AISI 4340 steels with hardness range HRC 29–57 using ceramic (Al2 O3 –TiC) tool inserts They observed when steels in the hardness range HRC 30–50 were cut, continuous chips were observed, and an increase in hardness caused a decrease in cutting force This phenomenon was also observed by Chao and Trigger [5] When the hardness exceeded HRC 50, chip segmentation appeared and there was a sudden increase in cutting forces Similar results were obtained by Eda et al [6] while cutting maraging steels of various hardness 578 G Poulachon et al / Wear 250 (2001) 576–586 Chryssolouris [7] carried out experimental cutting tests with four different work materials with the same hardness (HRC 55), but with different structures, in order to determine the influence of the work material structure on the wear behavior of CBN cutting edges He showed that the cutting times for the same wear criterion for these various materials were different A large difference in wear behavior was highlighted: ratio 1/2 for flank wear criterion (300 m) and 1/4 for crater depth criterion (50 m) Davies et al [8] performed similar experiments to compare the tool-wear rates for three steels of the same chemistry but different microstructures They showed that the tool-wear rate decreases with decreasing CBN grain size and noted however, that bulk hardness and transverse rupture strength also increase with decreasing grain size Luo et al [9] studied the wear behavior in turning AISI 4340 hardened alloy steel with a hardness range HRC 35–60 using CBN tools (TiC and Al2 O3 bonds) and ceramics tools (Al2 O3 + TiC) They also found a limiting hardness value at HRC 50 for the cutting force, cutting temperature and the tool-life The main wear mechanism for CBN tools was shown as the abrasion of the binder material by hard carbide particles of the workpiece, while for ceramic tools, the adhesion wear and abrasion wear mechanisms are shown as dominant König et al [10] investigated tool-life in drilling of several different steels (16MnCr5E, 31CrMo12, X100CrMoV5-1, X210CrW12, S6-5-2) in order to assess the machinability of these materials with a hardness in excess of HRC 60 with respect to the influence of different hardening processes and structural compositions They found that for each workpiece material, there is an optimum cutting speed They indicated that the feed force increases with coarse carbides (S6-5-2), while the cutting force rises with fine-grain, uniform structures (16MnCr5E) They highlighted that PCBN cannot be used economically for machining materials with a high ferrite content and hardness below HRC 50 Table The composition and heat treatment conditions of 100Cr6 Composition (wt.%) Composition (wt.%) C Si Mn S P 1.03 0.32 0.4 [...]... principally controlled by the bonding strength of the tool grains Above this limit, the material is considered hard , and in this case the tool- wear is mainly controlled by abrasion, and the wear resistance of the tool increases with the increase of CBN grain content Acknowledgements The major part of this research was supported by Sandvik Company who provided the PCBN cutting tool and Burgundy Council (France)... Poulachon et al / Wear 250 (2001) 576–586 • The choice of the CBN content and binder phase play an important role in tool- life depending on the work material • According to the bibliography and experimental results of chip formation, a limiting value of hardness at HRC 50 is clearly defined Below this limit, the cutting forces decrease In this case of soft material, the wear resistance is principally controlled... Viens, B.L Laube, Wear 209 (1997) 241–246 [13] W König, A Neises, Wear 162–164 (1993) 12–21 [14] F.W Taylor, Trans ASME 28 (1907) 31–279 [15] G Poulachon, A Moisan, Ann CIRP 47/1 (1998) 73–76 [16] G Poulachon, A Moisan, Trans ASME J Eng Ind 122 (3) (2000) 406–412 [17] G Poulachon, A Moisan, S Bordes, L Colombet, in: Proceedings of the C.E.M Symposium on Machinability and Cutting Tool Damage Mechanisms, Personnal... (1951) 777–792 [6] H Eda, K Kishi, H Hashimoto, in: Proceedings of the American Society for Metals, Metals Park, Ohio, 1980, pp 265–286 [7] G Chryssolouris, J Appl Metalworking 2 (2) 1982 [8] M.A Davies, Y Chou, C.J Evans, Ann CIRP 45/1 (1996) 77–82 [9] S.Y Luo, Y.S Liao, Y.Y Tsai, J Mater Process Technol 88 (1999) 114–121 [10] W König, M Klinger, R Link, Ann CIRP 39/1 (1990) 61–64 [11] G Poulachon,... research was supported by Sandvik Company who provided the PCBN cutting tool and Burgundy Council (France) The authors wish to thank S Bordes and L Colombet for their help in carrying out a part of experimental work during their final year project References [1] N Narutaki, Y Yamane, Ann CIRP 28/1 (1979) 23–28 [2] K Nakayama, M Arai, T Kanda, Ann CIRP 37/1 (1998) 89–92 [3] T Ohtani, H Yokogawa, Bull... Ann CIRP 47/1 (1998) 77–82 [19] M.A Elbestawi, A.K Srivastava, T.I El-Wardany, Ann CIRP 45/1 (1996) 71–76 [20] R Komanduri, R.H Brown, Trans ASME J Eng Ind 103 (1981) 33–51 [21] R Komanduri, T Schroeder, B.F Von Turkovich, O.G Flom, Trans ASME, J Eng Ind 104 (1982) 121–131 ... cutting temperatures induce a rapid tool- wear The worn flank surface increases the cutting 4.2 Tool- wear At the early stage of cutting, initial breakdown in cutting edge with the edge rounding... steel in the hardness range HRC 10–50 were cut, continuous chip was produced [4–6] with a decrease in cutting forces when hardness increases in this range In fact, an increase in steel hardness in. .. grain content in the tool In conclusion they show that PCBN with low content of CBN grain (∼60%) produces an excellent performance in machining hard materials not containing many of ultra-hard