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Machining of high chromium hardfacing materials
Journal of Materials Processing Technology 115 (2001) 423±429
Machining of high chromium hardfacing materials X.J. Rena,b,*, R.D. Jamesa, E.J. Brookesa, L. Wanga a
School of Engineering, University of Hull, Hull HU6 7RX, UK College of Material Science and Engineering, Yanshan University, Qinhuangdao, Hebei, PR China
b
Received in revised form 23 April 2001; accepted 2 August 2001
Abstract Hardfacing materials with large chromium carbide are widely used in industry to increase the wear resistance of the component. Its manufacturing process is an important aspect of the overall economy of this technology. Inclusion of large chromium carbides in its microstructure makes these materials very dif®cult to machine. Development of PCBN tools has opened up the possibility to machine this type of engineering material by conventional turning process to increase the productivity. In this work, the deformation behaviour of the hardfacing material and the wear of PCBN tools are reported. The chip formation was studied by a quick-stop device, and the chip root and the chip were examined to investigate the deformation of the large chromium carbide in the cutting process. It was found that the machining process has involved fracture of large carbides ahead of the cutting edge and bending and cracking of the carbide underneath the transient surface. The main modes of tool wear were identi®ed as edge chipping and ¯ank wear, and mechanical loading and the abrasiveness of the carbide particle were the main cause. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Chip formation; Cutting; Quick-stop; Hardfacing; PCBN tools; Chromium carbides
1. Introduction There is a widespread need for abrasion-resistant materials in industries as diverse as mining and food processing [1,2]. Iron-based hardfacings are a popular choice and their wear-resistance is attributable to microstructures in which hard carbides are dispersed in a relatively soft matrix [3]. The alloys forming chromium carbides are particularly effective because these carbides are usually present as relatively large micro-constituents, which offer large surface areas to the passage of abrasive material. Hardfacing materials are often applied to a substrate by welding to provide wear-resistant layers several millimetres thick. Subsequent machining is usually necessary to achieve required standards of dimensional accuracy and surface ®nish. However, the existence of high content of large chromium carbide makes machining of these hardfacings very dif®cult other than by grinding. The development of cutting tools based on polycrystalline cubic boron nitride (PCBN) have opened up the possibility of turning and milling as alternatives, and this has offered great improvement of the manufacturing process of hardfaced component in terms of high productivity, ¯exibility, etc. [4]. *
Corresponding author. Present address: School of Engineering, University of Exeter, Exeter EX4 4QF, UK. E-mail addresses: [email protected], [email protected] (X.J. Ren).
Cubic boron nitride (CBN) is the second hardest material in the world and PCBN tools have found application in machining of a large range of hard ferrous materials including hardened steel, cast iron and Co/Ni based alloys, etc. [5,6]. Extensive research work has been done to investigate the machining process of these materials [7±10]. Machining of iron hardfacing materials with large carbide constituents is relatively new application ®eld for hard turning technique with PCBN tools and the machining process were less understood. Therefore, an investigation into the machining process, e.g. chip formation, is essential to further improve the manufacturing process of these materials. Chip formation is one of the most important aspect of the cutting process, factor such as tool wear, was related how the behaviour of the workpiece material around the cutting edge. Quick-stop device is one of the well-established method for fundamental studies of a cutting processes, by which, the cutting was stopped suddenly and details of the chip formation process were kept undisturbed with the chip attached to the workpiece [11±13]. It was a successful tool in understanding the cutting mechanism of a material in the very early stage in metal cutting research and has been successfully used in studying the machining mechanism of hard materials (e.g. hardened steel [10]) and materials with second particles (e.g. SiC reinforced Al/SiC metal matrix composite [14,15]). In these tests, not only the general chip formation modes were successfully captured but also the
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 0 2 9 - 9
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X.J. Ren et al. / Journal of Materials Processing Technology 115 (2001) 423±429
deformation behaviour of the second phase (particle or whisker) was revealed. These informations were critical in understanding the machinability of the workpiece and fundamental mechanism of the tool wear process. In this work, the quick-stop method was used to study the chip formation of typical hardfacing materials; the deformation behaviour of large chromium carbides and their effect on the chip formation process of the workpiece were investigated by examining the chip root obtained and the subsurface of the transient plane. Chips under different cutting speeds were studied; characteristics such as the shear plane, cutting ratio (the ratio between the undeformed chip thickness and the chip thickness) were examined. Tool wear modes were revealed using scanning electron microscope (SEM) and the effect of cutting parameters was also studied by performing series of cutting tests. 2. Experimental procedure 2.1. Workpiece and tool materials A chromium carbide based hardfacing layer, nominally 6 mm thick, had been deposited on a mild steel bar (é100, L300 mm) using a ¯ux cored arc welding (FCAW) machine. The mild steel bar was preheated to about 4008C before being coated with the hardfacing and the component was cooled down slowly after the deposition to avoid cracking. Samples were pre-turned to remove the rough welded skin before the quick-stop and cutting tests. The hardfacing materials used in these tests belong to the Fe±Cr±C composition system. As shown in Fig. 1, it is characterised by a very coarse structure. A large portion of primary carbides (about 50 vol.%) has formed with the remaining part transformed into mixture of austenite and
Fig. 1. Microstructures of the hardfacing workpiece materials: 1, primary carbide; 2, fine carbides in g matrix.
more ®ne carbides. In the solidi®cation process, the primary carbides have exhibited columnar growth with a hexagonal cross-section [16], so the carbide shows different sections depending on the sampling plane. The measured bulk hardness was ranged for HK 7.12 GPa (HRC55-58), with the hardness for the carbide were HK 14.0 GPa. The PCBN material was dense PCBN aggregate with a Knoop hardness of HK 36.5 GPa at room temperature. 2.2. Test facility and procedure The quick-stop facility was mounted in place of the front tool post of the Churchill `Compturn' 290 CNC lathe used for the cutting tests. It utilises a humane killer gun to rapidly disengage and remove the tool from the cutting position so that the chip remains attached to the workpiece, i.e. the cutting action is effectively frozen. Details of the facility has been introduced in a previous report [8]. Rapid acceleration of the tool away from the workpiece is achieved when an explosive charge is ®red. The mean acceleration of the tool from the chip bottom could reach over 32.5E7 mm/s [17]. Hence, for the cutting speed used in this test, the removal of the tool no longer affects the chip ¯ow after a very short distance from the tool root. RNGN070300 inserts were held on the quick-stop tool holder with a negative rake of 68. A tailstock was used in all the tests in order to make the system more rigid. The tool holder is pivoted and supported by a shear pin and a captive bolt gun is used to break the shear pin and to accelerate the tool holder away from the workpiece. The cutting condition was set as speed, 32±85 m/min; feed rate, 0.15±0.25 mm/ rev; a depth of cut of 0.65 mm and without coolant. In the quick-stop, once the steady-state cutting was established, the cutting action was suddenly stopped by ®ring the gun. The sample, the chip and its connecting part of the workpiece (as schematically shown in Fig. 2), was separated from the parent bar using a spark electron
Fig. 2. Schematic diagram to show the chip attached to the parent workpiece after the quick-stop.
X.J. Ren et al. / Journal of Materials Processing Technology 115 (2001) 423±429
425
Fig. 3. Metallographic section through quick-stop specimen. (a) General view showing the deformation of microstructural constituents during the cutting process and the microstructure of the workpiece in relation to the cutting edge: primary carbide about perpendicular to the cutting edge (A), primary carbide about parallel to the cutting edge (B). (b) Enlarged view of edge region showing cracking of the primary carbide around the cutting edge.
discharging machine (EDM) and etched in Vilella's reagent (5 ml HCl, 1 g picric acid, 100 ml ethylalcohol) to show carbides and the matrix after grinding and polishing. In order to understand the behaviour of the constituents (carbides and matrix) of the hardfacing during the cutting process, sections normal and parallel to the cutting direction (Vt) on the transient plane of the workpiece material were also prepared metallurgically and observed on optical microscope and SEM. 3. Results and discussions 3.1. Deformation of the carbides in the cutting process Fig. 3a shows a microstructure of a longitudinal section of a quick-stop sample. On this plane, the large carbides showed different section shape in relation to their orientation to the cutting edge. The columnar carbides (A) are carbides about perpendicular to the cutting edge, while the hexagonal carbides (B) are those about parallel to the cutting edge. As shown in the sample, saw-tooth type chips are formed and the deformation is concentrated along a shear plane orientated to the cutting direction of 458. The free edge of each chip segment were parallel to the cutting direction. This suggested that the chip segment has undergone limited rotation movement as in the case for hardened steel [10]. The mechanism of this type of chip formation was due to the stress condition ahead of the cutting edge. As a result of lower level of compressive stress and at the same time higher shear stresses, a crack would form in the surface of the workpiece at the point where the critical stress is ®rst exceeded [9,18]. Due to the microstructural heterogeneity of the hardfacing material, the crack initiation would be in¯uenced by the local microstructure, thus resulted in variable size chip segments.
Apart from the general chip formation mode, the deformation of the large carbides was also successfully captured. As shown in Fig. 3a and highlighted in Fig. 3b, some large columnar primary carbide around the cutting edge have cracked before they actually touch the cutting edge. The fracture mode of the carbide is cleavage as shown in Fig. 3b, but no apparent crack of the eutectic matrix was observed. After fracture, the fragment of the carbide moved into the chip in the following chip formation process and no apparent movement, e.g. rotation, of these large fractured segments of the carbide has been observed. A cross-section of the transient surface was also studied to further reveal the deformation of carbides and the matrix underneath the transient surface, which was contacting the ¯ank face of the insert in cuttings. As shown in Fig. 4, cracking and bending of columnar primary carbides has happened within the near surface region, and possibly,
Fig. 4. Deformation of primary carbides immediately underneath the transient surface.
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plastic deformation of the carbides has involved during the cutting process. Most of the cracks are parallel to the cutting direction and has propagated about half-way the carbide. De-cohesion of the carbide and matrix has also occurred due to bending of the carbide. The fracture of these carbides around the cutting edge are related to the stress pattern generated in the cutting process and the mechanical property of the carbide itself, especially its lower fracture toughness [16]. In cutting, there is a steep stress gradient in front of the tool and a strong stress concentration in the form of the relative sharp cutting edge [19]. When the matrix is not enough to absorb the strain energy, the bending moment from the cutting force could exert a very high loading on the large carbide. This is believed to be the main energy source which has caused
the carbides to crack. Another possible energy source may be delivered by the cutting chamfer. When the insert was forced into the workpiece, it will generate a high indentation force around the edge region; this will result in the fracture of the carbide due to its lower fracture toughness compared with the eutectic matrix. 3.2. Metallurgical analysis of chips The basic form of the chips is semi-circular with the side near the minor cutting edge severely segregated as shown in Fig. 5a. The general form of the chips did not vary signi®cantly with changes in cutting speed or feed rate. However, the colour of the chips changed from bright to dark with increased cutting speed. This re¯ected that the cutting
Fig. 5. Typical chips of the hardfacing material: (a) general chip form; (b) longitudinal section of the chip.
X.J. Ren et al. / Journal of Materials Processing Technology 115 (2001) 423±429
temperature increased with cutting speeds and the consequent higher oxidation tendency of the chip, which agrees with the cutting temperature test results [20]. Fig. 5b shows a typical view of the longitudinal crosssections of the chips. The angle of the primary deformation plane with respect to the cutting direction for the materials were at about 458 and this angle was kept roughly constant irrespective of the variations in the size of chip segments. As shown in the ®gure, much less deformation has occurred in the bulk of the chip segment than in primary and secondary deformation zones. This type of saw-tooth chip or cyclic chips has been reported in some workpiece including titanium alloys, hardened 4340 steel, Case carburized steel, Inconel 718 [9,12]. The formation mechanism of this type of chip depends on the thermal properties and metallurgical state of the workpiece material, as well as on the dynamics of the machine structure and cutting process [12]. The overall effect of these thermomechanical±metallurgical characteristics of the work material is to concentrate deformation in a narrow region and resist deformation in the bulk of the segment [21]. For the hardfacing materials in this work, the microstructure of the hardfacing comprises of large quantity of carbides, which could hold their strength at a higher temperature and inhibit deformation within the chip segment, so that the shearing or cracking process to form the chip segment was limited to a narrow region extending from the tool edge to the free surface of the workpiece. Apart from the overall effect of the carbides to the chip formation process, individual large carbides and their orientation could strongly control the chip formation process. As shown in Fig. 5b, a carbide, which is large enough, cover the one or more chip segments, has kept its original form, thus restraining the ability of the material to deform. 3.3. Chip reduction ratio and the chip velocity The average chip thickness was determined by measuring the dimension of the mid-cross-section and the cutting ratio was calculated (r t=tc , where t is the undeformed chip thickness and tc the chip thickness). As shown in Fig. 6, the cutting ratio was not affected by cutting speed signi®cantly, and the ratio is about 1. This value was much higher than that of other engineering materials (0.05±0.5) depending on cutting conditions [19,22]. In machining of ductile materials, a higher cutting ratio normally indicates good machinability. This is not the case for the hardfacing material used in this work. Similar phenomenon has been reported in the machining of a nickel aluminide intermetallic alloy, in which the chip reduction coef®cient was found not to be an appropriate indicator of the magnitude of the deformation undergone by the workpiece material [22]. As shown in the chip section (Fig. 5b), the soft matrix was not continuous enough to avoid direct contact between the base of the chip and the tool rake face. In cutting, these abrasive hard carbide particles slide over the tool face. The velocity of these particles would be an important factor need
427
Fig. 6. Cutting ratio and the chip velocity.
to be considered [23], and this could be represented by the chip velocity (Vc), which is the velocity of the chip relative to the tool and directed along the tool face. Thus, the travelling speed of the abrasive particles could be estimated using the cutting ratio by Eq. (1) [19]: Vc rV
(1)
where V is the surface cutting speed, r the cutting ratio and Vc the velocity of the chip. As shown in Fig. 6, the velocity of the chip increased linearly with cutting speed, the proportionality was correlated with the constant value of the cutting ratio at the cutting speed range. This is probably due to that the chip formation was initiated by fracture rather than shear instability [9,18]. This result suggested that, even though the cutting was performed in lower cutting regime (S 45±85 m/min), the cutting edge and tool face would still suffer from the high load from the fast moving chips. 3.4. Tool wears characteristics and effect of cutting parameters Fig. 7a shows the tool failure mode of the PCBN tool. Flank wear and edges chipping were the main wear mode. Severe damage has occurred on the cutting edge, which has lost its original geometry and lowered by the chipping process. As shown in Fig. 7b, particle loss has occurred in the ¯ank wear region in addition to the grooving. Fig. 8 shows the wear of the tools at different cutting speeds (cutting for 1 min). The scale of chipping has increased with cutting speed while ¯ank wear was lowest at a surface speed of 45 m/min. It has been established that mechanical, thermal loading and chemical effects may all contribute to the wear of CBN tools in machining hard ferrous material [24±26]. In this case, the deformation mode of the carbide in the cutting process, as revealed by the quick-stop test and chip morphology study, suggested that high mechanical loading have played more active role. Although the constituents of the
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Fig. 7. Typical tool wear when machining hardfacing: (a) general view; (b) ridged surface on the flank of the tool showing abrasive wear process.
tool materials (CBN and AlN) are harder than the carbides and the matrix of the workpiece material, it is still possible that the tool will suffer `soft abrasive' wear [27]. As shown in Fig. 8, ¯ank wear was at minimum level at a surface speed of 45 m/min, at speeds higher or lower than this, the wear rate increased. This is due to different in¯uences of cutting speed on the ¯ank wear. At lower speeds, the temperature of the workpiece is relatively low and these results in increased mechanical loading and rapid ¯ank wear. At higher speeds, thermal-induced softening of the workpiece is not suf®cient to offset the effect of the speed-induced wear. 4. Conclusions Fig. 8. Tool wear vs. cutting speed (VBmax: maximum width of flank wear, CHmax: maximum width of edge chipping).
The machinability of chromium carbide based hardfacing appears to be strongly related to their microstructure
X.J. Ren et al. / Journal of Materials Processing Technology 115 (2001) 423±429
properties and, in particular, to the presence and deformation characteristic of the large carbides. The machining process has involved fracture of large carbides ahead of the cutting edge and bending and cracking of the carbide underneath the transient surface. The machining yielded thinner chip with high cutting ratio (1). In cutting, carbide inhibits the bulk deformation of the chip segments and resulted in saw-tooth chip formation. Cracking was the main mechanism in the formation of the chip segment. The main tool wear modes were found to be edge chipping and ¯ank wear, and mechanical loading and the abrasiveness of the carbide particle was the main cause. Flank wear showed a maximum at the speed range tested chipping increased with cutting speed. References [1] Wear Resistant Surface in Engineering: A Guide to their Production, Properties and Selection, International Research and Development Co. Ltd., Department of Trade and Industry, London, 1985. [2] R. Menon, New developments in hardfacing alloys, Weld. J. (February) (1996) 43±48. [3] S. Atamert, H.K.D.H. Bhadeshia, Microstructure and stability of Fe±Cr±C hardfacing alloys, Mater. Sci. Eng. A 130 (1990) 101±111. [4] X.J. Ren, R.D. James, E.J. Brookes, Application of PCBN tools in machining welded hardfacing materials, in: Proceedings of the Third Grinding and Machining, Ohio, USA, 1999, pp. 511±525. [5] P.J. Heath, Structure, properties and applications of polycrystalline cubic boron nitride, in: Proceedings of the 14th North American Manufacturing Research Conference, Minneapolis, MN, 1986, pp. 66±80. [6] A.M. Abrao, D.K. Aspinwall, M.L.H. Wise, A review of polycrystalline cubic boron nitride cutting tool developments and application, in: Proceedings of the 13th International MATADOR Conference, Manchester, UK, 1993, pp. 169±180. [7] R.M. Hooper, C.A. Brookes, Microstructure and wear of cubic boron nitride aggregate tools, in: Proceedings of the Second International Conference on Science Hard Material, Rhodes, 1984, pp. 907±917. [8] C.A. Brookes, R.D. James, F. Nabhani, A.R. Parry, Structure, composition, and integrity of workpiece: tool interfaces in metal cutting and grinding, in: Proceedings of the IMechE Seminar on Tribology in Metal Cutting and Grinding, 1992, pp. 41±48.
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[9] M.C. Shaw, A. Vyas, The mechanism of chip formation when hard turning steel, Ann. CIRP 47 (1) (1998) 77±82. [10] G. Poulachon, A. Moisan, A contribution to the study of the cutting mechanisms during high speed machining of hardened steel, Ann. CIRP 47 (1) (1998) 73±77. [11] E.M. Trent, Metal Cutting, 3rd Edition, Butterworths, London, 1991. [12] D.A. Stephenson, J.S. Agapiou, Metal Cutting Theory and Practice, Marcel Dekker, New York, 1996. [13] S.H. Yeo, W.W. Lui, V. Phung, A quick-stop device for orthogonal machining, J. Mater. Process. Technol. 29 (1992) 41±46. [14] O. Quigley, J. Monaghan, P. O'Reilly, Factors affecting the machinability of an Al/SiC metal±matrix composite, J. Mater. Process. Technol. 43 (1994) 21±36. [15] N.P. Hung, S.H. Yeo, K.K. Lee, K.J. Ng, Chip formation in machining particle-reinforced metal matrix composite, Mater. Manuf. Proc. 13 (1) (1998) 85±100. [16] S. Lee, S. Choo, E. Baek, S. Ahn, N. Kim, Correlation of microstructure and fracture toughness in high-chromium white iron hardfacing alloys, Metall. Mater. Trans. A 27 (1996) 3881±3891. [17] F. Nabhani, The performance of ultra-hard cutting tool materials in machining aerospace alloy TA48, Ph.D. Thesis, University of Hull, Hull, UK, 1991. [18] W. Konig, A. Berktold, J. Liermann, N. Winands, Top quality components not only by grinding, Ind. Diamond Rev. 3 (1994) 127±132. [19] M.C. Shaw, Metal Cutting Principles, Clarendon Press, Oxford, 1984. [20] X.J. Ren, Ph.D. Thesis, Univesity of Hull, Hull, UK, 2000. [21] R. Komanduri, T.A. Schroeder, On shear instability in machining a nickel±iron based super alloy, Trans. ASME J. Eng. Ind. 108 (1986) 93±100. [22] S. Chatterjee, Machinability of a nickel aluminide intermetallic alloy, J. Mater. Eng. Perform. 2 (1) (1993) 101±105. [23] I.M. Hutchings, Tribology, Friction and Wear of Engineering Materials, Edwards Arnold, 1992. [24] S.A. Klimenko, Y.A. Mukavoz, V.A. Lyashko, A.N. Vashchenko, V.V. Ogorodnik, On the wear mechanism of cubic boron nitride base cutting tools, Wear 157 (1992) 1±6. [25] W.Y. Chen, The machining of hardened steel using superhard CBN tooling and CBN tipped rotary cutting tools, Ph.D. Thesis, University of Birmingham, 1993. [26] W. Konig, A. Neises, Wear mechanisms of ultrahard, non-metallic cutting materials, Wear 162±164 (1993) 12±21. [27] Y.K. Chou, C.J. Evans, Tool wear mechanism in continuous cutting of hardened tool steels, Wear 212 (1997) 59±65.
Machining of high chromium hardfacing materials X.J. Rena,b,*, R.D. Jamesa, E.J. Brookesa, L. Wanga a
School of Engineering, University of Hull, Hull HU6 7RX, UK College of Material Science and Engineering, Yanshan University, Qinhuangdao, Hebei, PR China
b
Received in revised form 23 April 2001; accepted 2 August 2001
Abstract Hardfacing materials with large chromium carbide are widely used in industry to increase the wear resistance of the component. Its manufacturing process is an important aspect of the overall economy of this technology. Inclusion of large chromium carbides in its microstructure makes these materials very dif®cult to machine. Development of PCBN tools has opened up the possibility to machine this type of engineering material by conventional turning process to increase the productivity. In this work, the deformation behaviour of the hardfacing material and the wear of PCBN tools are reported. The chip formation was studied by a quick-stop device, and the chip root and the chip were examined to investigate the deformation of the large chromium carbide in the cutting process. It was found that the machining process has involved fracture of large carbides ahead of the cutting edge and bending and cracking of the carbide underneath the transient surface. The main modes of tool wear were identi®ed as edge chipping and ¯ank wear, and mechanical loading and the abrasiveness of the carbide particle were the main cause. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Chip formation; Cutting; Quick-stop; Hardfacing; PCBN tools; Chromium carbides
1. Introduction There is a widespread need for abrasion-resistant materials in industries as diverse as mining and food processing [1,2]. Iron-based hardfacings are a popular choice and their wear-resistance is attributable to microstructures in which hard carbides are dispersed in a relatively soft matrix [3]. The alloys forming chromium carbides are particularly effective because these carbides are usually present as relatively large micro-constituents, which offer large surface areas to the passage of abrasive material. Hardfacing materials are often applied to a substrate by welding to provide wear-resistant layers several millimetres thick. Subsequent machining is usually necessary to achieve required standards of dimensional accuracy and surface ®nish. However, the existence of high content of large chromium carbide makes machining of these hardfacings very dif®cult other than by grinding. The development of cutting tools based on polycrystalline cubic boron nitride (PCBN) have opened up the possibility of turning and milling as alternatives, and this has offered great improvement of the manufacturing process of hardfaced component in terms of high productivity, ¯exibility, etc. [4]. *
Corresponding author. Present address: School of Engineering, University of Exeter, Exeter EX4 4QF, UK. E-mail addresses: [email protected], [email protected] (X.J. Ren).
Cubic boron nitride (CBN) is the second hardest material in the world and PCBN tools have found application in machining of a large range of hard ferrous materials including hardened steel, cast iron and Co/Ni based alloys, etc. [5,6]. Extensive research work has been done to investigate the machining process of these materials [7±10]. Machining of iron hardfacing materials with large carbide constituents is relatively new application ®eld for hard turning technique with PCBN tools and the machining process were less understood. Therefore, an investigation into the machining process, e.g. chip formation, is essential to further improve the manufacturing process of these materials. Chip formation is one of the most important aspect of the cutting process, factor such as tool wear, was related how the behaviour of the workpiece material around the cutting edge. Quick-stop device is one of the well-established method for fundamental studies of a cutting processes, by which, the cutting was stopped suddenly and details of the chip formation process were kept undisturbed with the chip attached to the workpiece [11±13]. It was a successful tool in understanding the cutting mechanism of a material in the very early stage in metal cutting research and has been successfully used in studying the machining mechanism of hard materials (e.g. hardened steel [10]) and materials with second particles (e.g. SiC reinforced Al/SiC metal matrix composite [14,15]). In these tests, not only the general chip formation modes were successfully captured but also the
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 0 2 9 - 9
424
X.J. Ren et al. / Journal of Materials Processing Technology 115 (2001) 423±429
deformation behaviour of the second phase (particle or whisker) was revealed. These informations were critical in understanding the machinability of the workpiece and fundamental mechanism of the tool wear process. In this work, the quick-stop method was used to study the chip formation of typical hardfacing materials; the deformation behaviour of large chromium carbides and their effect on the chip formation process of the workpiece were investigated by examining the chip root obtained and the subsurface of the transient plane. Chips under different cutting speeds were studied; characteristics such as the shear plane, cutting ratio (the ratio between the undeformed chip thickness and the chip thickness) were examined. Tool wear modes were revealed using scanning electron microscope (SEM) and the effect of cutting parameters was also studied by performing series of cutting tests. 2. Experimental procedure 2.1. Workpiece and tool materials A chromium carbide based hardfacing layer, nominally 6 mm thick, had been deposited on a mild steel bar (é100, L300 mm) using a ¯ux cored arc welding (FCAW) machine. The mild steel bar was preheated to about 4008C before being coated with the hardfacing and the component was cooled down slowly after the deposition to avoid cracking. Samples were pre-turned to remove the rough welded skin before the quick-stop and cutting tests. The hardfacing materials used in these tests belong to the Fe±Cr±C composition system. As shown in Fig. 1, it is characterised by a very coarse structure. A large portion of primary carbides (about 50 vol.%) has formed with the remaining part transformed into mixture of austenite and
Fig. 1. Microstructures of the hardfacing workpiece materials: 1, primary carbide; 2, fine carbides in g matrix.
more ®ne carbides. In the solidi®cation process, the primary carbides have exhibited columnar growth with a hexagonal cross-section [16], so the carbide shows different sections depending on the sampling plane. The measured bulk hardness was ranged for HK 7.12 GPa (HRC55-58), with the hardness for the carbide were HK 14.0 GPa. The PCBN material was dense PCBN aggregate with a Knoop hardness of HK 36.5 GPa at room temperature. 2.2. Test facility and procedure The quick-stop facility was mounted in place of the front tool post of the Churchill `Compturn' 290 CNC lathe used for the cutting tests. It utilises a humane killer gun to rapidly disengage and remove the tool from the cutting position so that the chip remains attached to the workpiece, i.e. the cutting action is effectively frozen. Details of the facility has been introduced in a previous report [8]. Rapid acceleration of the tool away from the workpiece is achieved when an explosive charge is ®red. The mean acceleration of the tool from the chip bottom could reach over 32.5E7 mm/s [17]. Hence, for the cutting speed used in this test, the removal of the tool no longer affects the chip ¯ow after a very short distance from the tool root. RNGN070300 inserts were held on the quick-stop tool holder with a negative rake of 68. A tailstock was used in all the tests in order to make the system more rigid. The tool holder is pivoted and supported by a shear pin and a captive bolt gun is used to break the shear pin and to accelerate the tool holder away from the workpiece. The cutting condition was set as speed, 32±85 m/min; feed rate, 0.15±0.25 mm/ rev; a depth of cut of 0.65 mm and without coolant. In the quick-stop, once the steady-state cutting was established, the cutting action was suddenly stopped by ®ring the gun. The sample, the chip and its connecting part of the workpiece (as schematically shown in Fig. 2), was separated from the parent bar using a spark electron
Fig. 2. Schematic diagram to show the chip attached to the parent workpiece after the quick-stop.
X.J. Ren et al. / Journal of Materials Processing Technology 115 (2001) 423±429
425
Fig. 3. Metallographic section through quick-stop specimen. (a) General view showing the deformation of microstructural constituents during the cutting process and the microstructure of the workpiece in relation to the cutting edge: primary carbide about perpendicular to the cutting edge (A), primary carbide about parallel to the cutting edge (B). (b) Enlarged view of edge region showing cracking of the primary carbide around the cutting edge.
discharging machine (EDM) and etched in Vilella's reagent (5 ml HCl, 1 g picric acid, 100 ml ethylalcohol) to show carbides and the matrix after grinding and polishing. In order to understand the behaviour of the constituents (carbides and matrix) of the hardfacing during the cutting process, sections normal and parallel to the cutting direction (Vt) on the transient plane of the workpiece material were also prepared metallurgically and observed on optical microscope and SEM. 3. Results and discussions 3.1. Deformation of the carbides in the cutting process Fig. 3a shows a microstructure of a longitudinal section of a quick-stop sample. On this plane, the large carbides showed different section shape in relation to their orientation to the cutting edge. The columnar carbides (A) are carbides about perpendicular to the cutting edge, while the hexagonal carbides (B) are those about parallel to the cutting edge. As shown in the sample, saw-tooth type chips are formed and the deformation is concentrated along a shear plane orientated to the cutting direction of 458. The free edge of each chip segment were parallel to the cutting direction. This suggested that the chip segment has undergone limited rotation movement as in the case for hardened steel [10]. The mechanism of this type of chip formation was due to the stress condition ahead of the cutting edge. As a result of lower level of compressive stress and at the same time higher shear stresses, a crack would form in the surface of the workpiece at the point where the critical stress is ®rst exceeded [9,18]. Due to the microstructural heterogeneity of the hardfacing material, the crack initiation would be in¯uenced by the local microstructure, thus resulted in variable size chip segments.
Apart from the general chip formation mode, the deformation of the large carbides was also successfully captured. As shown in Fig. 3a and highlighted in Fig. 3b, some large columnar primary carbide around the cutting edge have cracked before they actually touch the cutting edge. The fracture mode of the carbide is cleavage as shown in Fig. 3b, but no apparent crack of the eutectic matrix was observed. After fracture, the fragment of the carbide moved into the chip in the following chip formation process and no apparent movement, e.g. rotation, of these large fractured segments of the carbide has been observed. A cross-section of the transient surface was also studied to further reveal the deformation of carbides and the matrix underneath the transient surface, which was contacting the ¯ank face of the insert in cuttings. As shown in Fig. 4, cracking and bending of columnar primary carbides has happened within the near surface region, and possibly,
Fig. 4. Deformation of primary carbides immediately underneath the transient surface.
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plastic deformation of the carbides has involved during the cutting process. Most of the cracks are parallel to the cutting direction and has propagated about half-way the carbide. De-cohesion of the carbide and matrix has also occurred due to bending of the carbide. The fracture of these carbides around the cutting edge are related to the stress pattern generated in the cutting process and the mechanical property of the carbide itself, especially its lower fracture toughness [16]. In cutting, there is a steep stress gradient in front of the tool and a strong stress concentration in the form of the relative sharp cutting edge [19]. When the matrix is not enough to absorb the strain energy, the bending moment from the cutting force could exert a very high loading on the large carbide. This is believed to be the main energy source which has caused
the carbides to crack. Another possible energy source may be delivered by the cutting chamfer. When the insert was forced into the workpiece, it will generate a high indentation force around the edge region; this will result in the fracture of the carbide due to its lower fracture toughness compared with the eutectic matrix. 3.2. Metallurgical analysis of chips The basic form of the chips is semi-circular with the side near the minor cutting edge severely segregated as shown in Fig. 5a. The general form of the chips did not vary signi®cantly with changes in cutting speed or feed rate. However, the colour of the chips changed from bright to dark with increased cutting speed. This re¯ected that the cutting
Fig. 5. Typical chips of the hardfacing material: (a) general chip form; (b) longitudinal section of the chip.
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temperature increased with cutting speeds and the consequent higher oxidation tendency of the chip, which agrees with the cutting temperature test results [20]. Fig. 5b shows a typical view of the longitudinal crosssections of the chips. The angle of the primary deformation plane with respect to the cutting direction for the materials were at about 458 and this angle was kept roughly constant irrespective of the variations in the size of chip segments. As shown in the ®gure, much less deformation has occurred in the bulk of the chip segment than in primary and secondary deformation zones. This type of saw-tooth chip or cyclic chips has been reported in some workpiece including titanium alloys, hardened 4340 steel, Case carburized steel, Inconel 718 [9,12]. The formation mechanism of this type of chip depends on the thermal properties and metallurgical state of the workpiece material, as well as on the dynamics of the machine structure and cutting process [12]. The overall effect of these thermomechanical±metallurgical characteristics of the work material is to concentrate deformation in a narrow region and resist deformation in the bulk of the segment [21]. For the hardfacing materials in this work, the microstructure of the hardfacing comprises of large quantity of carbides, which could hold their strength at a higher temperature and inhibit deformation within the chip segment, so that the shearing or cracking process to form the chip segment was limited to a narrow region extending from the tool edge to the free surface of the workpiece. Apart from the overall effect of the carbides to the chip formation process, individual large carbides and their orientation could strongly control the chip formation process. As shown in Fig. 5b, a carbide, which is large enough, cover the one or more chip segments, has kept its original form, thus restraining the ability of the material to deform. 3.3. Chip reduction ratio and the chip velocity The average chip thickness was determined by measuring the dimension of the mid-cross-section and the cutting ratio was calculated (r t=tc , where t is the undeformed chip thickness and tc the chip thickness). As shown in Fig. 6, the cutting ratio was not affected by cutting speed signi®cantly, and the ratio is about 1. This value was much higher than that of other engineering materials (0.05±0.5) depending on cutting conditions [19,22]. In machining of ductile materials, a higher cutting ratio normally indicates good machinability. This is not the case for the hardfacing material used in this work. Similar phenomenon has been reported in the machining of a nickel aluminide intermetallic alloy, in which the chip reduction coef®cient was found not to be an appropriate indicator of the magnitude of the deformation undergone by the workpiece material [22]. As shown in the chip section (Fig. 5b), the soft matrix was not continuous enough to avoid direct contact between the base of the chip and the tool rake face. In cutting, these abrasive hard carbide particles slide over the tool face. The velocity of these particles would be an important factor need
427
Fig. 6. Cutting ratio and the chip velocity.
to be considered [23], and this could be represented by the chip velocity (Vc), which is the velocity of the chip relative to the tool and directed along the tool face. Thus, the travelling speed of the abrasive particles could be estimated using the cutting ratio by Eq. (1) [19]: Vc rV
(1)
where V is the surface cutting speed, r the cutting ratio and Vc the velocity of the chip. As shown in Fig. 6, the velocity of the chip increased linearly with cutting speed, the proportionality was correlated with the constant value of the cutting ratio at the cutting speed range. This is probably due to that the chip formation was initiated by fracture rather than shear instability [9,18]. This result suggested that, even though the cutting was performed in lower cutting regime (S 45±85 m/min), the cutting edge and tool face would still suffer from the high load from the fast moving chips. 3.4. Tool wears characteristics and effect of cutting parameters Fig. 7a shows the tool failure mode of the PCBN tool. Flank wear and edges chipping were the main wear mode. Severe damage has occurred on the cutting edge, which has lost its original geometry and lowered by the chipping process. As shown in Fig. 7b, particle loss has occurred in the ¯ank wear region in addition to the grooving. Fig. 8 shows the wear of the tools at different cutting speeds (cutting for 1 min). The scale of chipping has increased with cutting speed while ¯ank wear was lowest at a surface speed of 45 m/min. It has been established that mechanical, thermal loading and chemical effects may all contribute to the wear of CBN tools in machining hard ferrous material [24±26]. In this case, the deformation mode of the carbide in the cutting process, as revealed by the quick-stop test and chip morphology study, suggested that high mechanical loading have played more active role. Although the constituents of the
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Fig. 7. Typical tool wear when machining hardfacing: (a) general view; (b) ridged surface on the flank of the tool showing abrasive wear process.
tool materials (CBN and AlN) are harder than the carbides and the matrix of the workpiece material, it is still possible that the tool will suffer `soft abrasive' wear [27]. As shown in Fig. 8, ¯ank wear was at minimum level at a surface speed of 45 m/min, at speeds higher or lower than this, the wear rate increased. This is due to different in¯uences of cutting speed on the ¯ank wear. At lower speeds, the temperature of the workpiece is relatively low and these results in increased mechanical loading and rapid ¯ank wear. At higher speeds, thermal-induced softening of the workpiece is not suf®cient to offset the effect of the speed-induced wear. 4. Conclusions Fig. 8. Tool wear vs. cutting speed (VBmax: maximum width of flank wear, CHmax: maximum width of edge chipping).
The machinability of chromium carbide based hardfacing appears to be strongly related to their microstructure
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properties and, in particular, to the presence and deformation characteristic of the large carbides. The machining process has involved fracture of large carbides ahead of the cutting edge and bending and cracking of the carbide underneath the transient surface. The machining yielded thinner chip with high cutting ratio (1). In cutting, carbide inhibits the bulk deformation of the chip segments and resulted in saw-tooth chip formation. Cracking was the main mechanism in the formation of the chip segment. The main tool wear modes were found to be edge chipping and ¯ank wear, and mechanical loading and the abrasiveness of the carbide particle was the main cause. Flank wear showed a maximum at the speed range tested chipping increased with cutting speed. References [1] Wear Resistant Surface in Engineering: A Guide to their Production, Properties and Selection, International Research and Development Co. Ltd., Department of Trade and Industry, London, 1985. [2] R. Menon, New developments in hardfacing alloys, Weld. J. (February) (1996) 43±48. [3] S. Atamert, H.K.D.H. Bhadeshia, Microstructure and stability of Fe±Cr±C hardfacing alloys, Mater. Sci. Eng. A 130 (1990) 101±111. [4] X.J. Ren, R.D. James, E.J. Brookes, Application of PCBN tools in machining welded hardfacing materials, in: Proceedings of the Third Grinding and Machining, Ohio, USA, 1999, pp. 511±525. [5] P.J. Heath, Structure, properties and applications of polycrystalline cubic boron nitride, in: Proceedings of the 14th North American Manufacturing Research Conference, Minneapolis, MN, 1986, pp. 66±80. [6] A.M. Abrao, D.K. Aspinwall, M.L.H. Wise, A review of polycrystalline cubic boron nitride cutting tool developments and application, in: Proceedings of the 13th International MATADOR Conference, Manchester, UK, 1993, pp. 169±180. [7] R.M. Hooper, C.A. Brookes, Microstructure and wear of cubic boron nitride aggregate tools, in: Proceedings of the Second International Conference on Science Hard Material, Rhodes, 1984, pp. 907±917. [8] C.A. Brookes, R.D. James, F. Nabhani, A.R. Parry, Structure, composition, and integrity of workpiece: tool interfaces in metal cutting and grinding, in: Proceedings of the IMechE Seminar on Tribology in Metal Cutting and Grinding, 1992, pp. 41±48.
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