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Tool life and wear mechanisms of TiN coated tools in an intermittent cutting operation
Journal of Materials Processing Technology 116 (2001) 10±15
Tool life and wear mechanisms of TiN coated tools in an intermittent cutting operation E.O. Ezugwu*, C.I. Okeke School of Engineering Systems and Design, Machining Research Centre, South Bank University, 103 Borough Road, London SE1 0AA, UK
Abstract A P20-30 PVD TiN coated cemented carbide inserts with sharp edges were used to perform an external right hand thread cutting operation on two grades of steel up to a cutting speed and feed rate of 225 m min 1 and 0.44 mm rev 1, respectively. The test results show that cutting speed and feed rate had the most signi®cant in¯uence on tool life. A V-shape type of wear along the nose region of the tool face with adhering ¯ake-like oxide debris and micro/macro-chipping are the dominant failure modes while abrasion and plastic deformation of the sharp cutting edge were the main wear mechanisms affecting tool performance, particularly at higher cutting conditions. The ¯ake-like oxide debris were observed to increase when machining workpiece material with higher nickel and chromium content in their composition. Tool life was also found to have functional correlation between the cutting force, hardness and length of wear (AFM) along the nose region. # 2001 Elsevier Science B.V. All rights reserved. Keywords: PVD TiN coating; Flake-like oxide debris; Micro/macro adhesion and abrasion; Plastic deformation
1. Introduction PVD TiN coating deposited on sharp tool edges enhance machining productivity and also generate acceptable surface ®nish with moderate power consumption during machining operation. When properly applied, PVD coating can extend tool life by up to three times relative to uncoated tools [1]. The credibility of the hard temperature-stable coating materials is attributed to good impact strengths and suf®cient hardness and toughness which improve abrasion resistance, thus enhancing performance during machining. Assessing the applicability and performance of coating materials for various metal cutting operations require that the cemented carbide substrate should possess the correct balance of toughness and hardness to minimise premature tool failure and also be hard enough to adequately support the coating materials. Moderate Co content with ®ner WC grain size is more effective in terms of higher hardness and greater abrasion resistance with slight reduction in transverse rupture and impact strengths of carbide tools. Processing of carbide tools along this direction will ensure wider application in various machining operations. A good knowledge of the machining characteristics of a workpiece material is essential in harnessing the bene®ts of PVD coated carbide tools. For example, machining work* Corresponding author. Fax: 44-171-815-7699. E-mail address: [email protected] (E.O. Ezugwu).
piece materials prone to work hardening can generate high temperatures, welding on the tool surface in addition to high resistance to metal removal because of their high shear strength during machining [2,3]. These usually lead to a shorter tool life, pronounced chipping/premature failure of the cutting edge, severe surface abuse and the production of long continuous chips [4±7]. Maintaining acceptable ¯ank wear, below the rejection criterion, is very essential to avoid excessive surface and sub-surface damages on machined components. This paper presents results of the threading of inclusion modi®ed (IQ) EN 19T (708M40T) and EN 24T (817M40T) steels with PVD TiN coated inserts. It also highlights predominant failure modes and mechanisms responsible for tool failure at various cutting conditions. 2. Experimental procedures The machining trials were carried out on a 470 mm long and 150 mm diameter hardened steel bars. The nominal chemical composition and hardness of the workpiece materials are given in Tables 1 and 2, respectively. PVD TiN coated carbide tools with sharp edges were used for the machining operation. The nominal composition of the substrate and properties of the TiN coating are given in Tables 3 and 4, respectively. The thread quality, effective tool life and rejection criteria for both tool grades were based on physical examination of the thread surface to check for smooth
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 0 8 5 2 - 4
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
11
Tables 1 Chemical composition of EN 19 steel (wt.%) C
Si
Mn
S
P
Cr
Mo
Ni
Ca
Fe
Hardness
0.36 0.44
0.15 0.30
0.70 1.00
0.04 Max
0.04 Max
0.90 1.10
0.20 0.50
± ±
0.001 0.004
Bal. Bal.
273 HV Measured average
Table 2 Chemical composition of EN 24 steel (wt.%) C
Si
Mn
S
P
Cr
Mo
Ni
Ca
Fe
Hardness
0.36 0.44
0.10 0.40
0.45 0.70
0.4 Max
0.40 Max
1.10 1.40
0.20 0.35
1.30 1.70
0.001 0.004
Bal. Bal.
285 HV Measured average
Table 3 Chemical composition of the substrate of the threading tools (vol.%) Tool
WC
TiC TaC
Co
ISO code
PVD TiN
68.5
21.0
10.5
P20-30
silvery surface ®nish using a simple gauge to measure thread pro®le. A 5% growth in thread root (GTR) was stipulated as the criterion for tool rejection. The effective tool life (ETL) is the time corresponding to average ¯ank wear of about 0.587 mm measured along the tool nose. Wear above this value generally has an adverse effect on thread quality and can also cause premature failure of the threading insert during machining. These data (5% GTR and 0.587 mm AFW) were obtained from the formula relating the GTR, ¯ank wear and a clearance angle of 68 [8]. A tool holder with 68 clearance angle was used in the threading test since higher clearance angle usually has a greater in¯uence on tool performance when machining steels. An electronically controlled centre lathe with a steplessly variable 11 kW motor drive, which provides a torque of 1411 N, was used for the machining trials. The cutting conditions employed during machining are listed below: cutting speed (m min 1): 150, 175, 200 and 225; feed rate (mm rev 1): 0.40 and 0.44; depth of cut (mm): 1.5. Each insert was clamped on a standard 25 mm shank tool holder designated AL 25-3C RH to provide a 68 clearance angle during threading. All the machining trials were carried out without coolant to prevent shattering of the inserts due to
thermal shock. A 3-phase piezo-electric dynamometer was used for recording component forces during machining. Tool wear was monitored with a travelling microscope connected to a digital readout device. Tool rejection criteria employed, based on British standard (BS 5623: 1979), are given below: 1. Average flank wear: 0.587 mm. 2. Maximum flank wear: 1.18 mm. 3. Notching/groove at the depth of cut line or tool nose: 1 mm. 4. Surface finish: non-smooth and non-silvery colour. 5. Excessive chipping (flaking) or catastrophic fracture of the cutting edge.
3. Results and discussions 3.1. Effect of cutting speed and feed rate Table 5 shows recorded tool life at the cutting conditions investigated. There was a gradual reduction in tool life with increasing cutting speed up to 200 m min 1. Further increase in cutting speed up to 225 m min 1 resulted in a rapid drop in tool life. Threading the EN 24T steel generally gave about 20% less tool life than the EN 19T steel. This is due to the difference in hardness as well as the presence of hard abrasive carbide particles such as Mo2C, FeMo2C and FeMo2C6 [8,9] in the matrix of the steel grades. Table 5 also shows the effect of feed rate when threading EN 19T and EN 24T steels. A 10% increase in feed rate from 0.40 to 0.44 mm rev 1 reduced the tool life by about half when machining at speeds up to 200 m min 1. Increase in cutting
Table 4 Properties of the PVD TiN coating material Tool
Coating thickness (mm)
Grain size (mm)
TRS (N/mm2)
Density (g/cm3)
Hardness (HV)
PVD TiN
3±4
2.0
1700
12.4
91.1
12
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
Table 5 Tool life obtained when threading EN 19T and EN 24T steels at various cutting conditions
wear in excess of the stipulated rejection criterion (AFM: 0.587 mm) at lower tool life [8].
Speed (m min 1)
Feed rate (mm rev 1)
Tool life (min)
3.3. Wear pattern
EN 19T steel
EN 24T steel
150 175 200 225 150 175 200 225
0.40 0.40 0.40 0.40 0.44 0.44 0.44 0.44
41 39 37 5.2 22 21 17 2.6
35 32 31 2.5 17 15 13 2.3
parameters indicates that the interaction of process energies, including temperature, increases generally and can therefore promote various wear mechanisms, which accelerate tool wear with the consequent reduction in tool life. 3.2. Cutting speed and flank wear rate Figs. 1 and 2 show the comparison between the cutting speed and ¯ank wear rate when machining these two grades of steel with PVD TiN coated carbide inserts. Flank wear rate is calculated as the average ¯ank wear (mm) divided by the effective tool life (min). In both ®gures, an increase in cutting speed gave fairly uniform ¯ank wear rate up to a cutting speed of 200 m min 1 and increased rapidly with further increase in cutting speed to 225 m min 1. The `V' shape type of wear (Fig. 3), which concentrates along the tool nose, encourages erosion of the coating layers, alteration in edge geometry, loss of coercive strength and thermal cracking (Fig. 4) of the substrate at higher speed conditions. The higher ¯ank wear rate also increases the average ¯ank
A `V'-shape type of ¯ank wear (parabolic shape) along the tool nose was the dominant failure mode observed during threading (Fig. 3). The wear curvature on the tool faces also varied according to the length of wear along the tool face and width of the wear along the edge, where the wear along the width is about one-third of that along the nose. This is due to the alternating cycle of the cutting edge during threading, thus making the cutting edge to wear evenly in a `V' pattern as the angle of the thread pro®le is 608. Flake-like oxide debris adhered on to the worn edges of the inserts and generally increased with increasing cutting conditions as illustrated in Fig. 3. Formation and adhesion of the ¯ake-like oxide debris is a result of the metallurgical reaction arising from the interaction between the tool and the workpiece materials during machining. This is more likely to be in¯uenced by the composition of the work material and temperature generated. The EN 24T with higher nickel and chromium content (Table 2) had more ¯ake-like oxide debris on the tool face after threading. Chipping and ¯aking of the tool particles occurred at the initial tool entry when threading with a sharp edged insert. This phenomenon is more severe at higher speed conditions, thereby encouraging early erosion of the coating layer and subsequent exposure of the substrate to thermal shrinkage stress due to transient temperature during the idling period when the cutting edge is temporarily redundant. Fig. 4 shows thermal cracks on the exposed substrate after machining EN 24T steel at a high cutting speed and a feed rate of 200 m min 1 and 0.44 mm rev 1, respectively. Formation of thermal cracks at high cutting speed weakens the cohesive bond strength of the substrate.
Fig. 1. Cutting speed versus flank wear rate when threading EN 19T with PVD TiN coated inserts at a feed rate of 0.44 mm rev 1.
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
13
Fig. 2. Cutting speed versus flank wear rate when threading EN 24T with PVD TiN coated inserts at a feed rate of 0.44 mm rev 1.
Fig. 5 shows the plot of cutting speed against cutting force, hardness and tool life when threading EN 19T steel with PVD TiN coated inserts. The ®gure clearly shows an increase in cutting forces with increasing cutting speed, contrary to expectation. Trent [1] reported that the cutting force decreases on increasing the cutting speed partly due to the softening of the workpiece material under the high temperature generated and partly due to decreased tool± chip contact area as a result of thinner chips produced in single point turning. Analysis of the cutting forces in threading have shown that this is not the case. This can
be attributed to the perceived rubbing effect between the two contacting bodies, especially at the pullout stage as well as the effect of forces exerted on the tool nose and clearance surface [10]. As mentioned earlier, most of the wear concentrates along the tool nose, meaning that there will be a high concentration of temperature on the tool nose region. This may increase the likelihood of spalling and consequently shorter tool life at higher speed conditions (Figs. 1 and 2). Additional load (tool nose force) exerted on the cutting edge can also increase the cutting force at higher cutting speeds. In addition, increase in cutting speed during threading causes increased ¯ank wear rate, localised plastic deformation of the cutting edge, alteration in tool nose radius and an increase in tool±workpiece contact length.
Fig. 3. V-shape type of wear and adhering flake-like debris at the cutting edge after threading EN 24T steel.
Fig. 4. Thermal cracks on the exposed tool substrate after machining EN 24T steel for 13 min at a speed of 200 m min 1, feed rate of 0.44 mm rev 1 and depth of cut of 1.5 mm.
3.4. Relationship between cutting forces, hardness and tool life during threading
14
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
Fig. 5. Functional relationship between the cutting force, hardness and tool life when threading EN 19T steel at a feed rate of 0.40 mm rev 1.
These combine to increase the cutting forces at higher speed conditions. Fig. 5 also shows a 3±5% reduction of the initial hardness (273 HV) of the material after threading at various cutting speeds investigated. This indicates softening of the work material due to elevated temperature at the tool nose, thus a reduction in tool life and an increase in the cutting force at higher cutting speeds. This suggests that PVD TiN coated threading inserts with sharp edges encounter higher stresses and higher temperature per unit area at the nose region, which increases the tool±workpiece contact length/area hence, higher ¯ank wear rate and lower tool life when threading under high speed conditions.
to additional forces exerted on the tool nose (tool nose force) and clearance surface as a result of the rubbing effect between the two contacting bodies, especially at the pullout stage.
Acknowledgements The authors acknowledge the support from Damsthall Tools and Macreadys Carbon and alloy steel which enabled this work to be carried out. References
4. Conclusions 1. The test results show that acceptable tool life can be achieved when machining EN 19T and EN 24T steels with PVD TiN coated inserts with sharp edges using cutting speed and feed rate up to 200 m min 1 and 0.44 mm rev 1, respectively. 2. Increase in cutting parameters promote the interaction of process energies, including temperature, which in turn accelerates various wear mechanisms, resulting in more severe tool wear and reduced tool life. 3. A V-shape type of wear along the nose region with adhering flake-like oxide debris was the dominant tool failure mode while micro/macro-adhesion and abrasion wear as well as plastic deformation of the sharp cutting edge are the wear mechanisms affecting tool performance, particularly at higher cutting conditions. 4. When threading EN 19T and EN 24T steels with PVD TiN coated carbide inserts under high speed conditions, cutting force increases with increasing cutting speed due
[1] E.M. Trent, Metal Cutting, 3rd Edition, Butterworth/Heinmann, London, 1991. [2] E.O. Ezugwu, Z.M. Wang, Performance of PVD and CVD coated carbide tools when machining Ti±6Al±4V, Tribol. Trans. 40 (1997) 81±85. [3] A. Jawaid, E.O. Ezugwu, Cutting tools in manufacturing: a review, Proceedings of the Fifth International Conference in China, April 1991, pp. 286±296. [4] V. Solaja, Wear of carbide tools and surface finish generated in finish turning of steel, Wear 919 (1958/59) 40±58. [5] E.O. Ezugwu, C.I. Okeke, Machining of nickel-based (C-263) alloy with TiAlN and TiZrN coated carbide tools, in: C. Dingchang, N. Norihiho, Y. Yasuo, C. Wuyi (Eds), Progress of Cutting and Grinding, International Academic Publishers, 1998, pp. 13±19. [6] E.O. Ezugwu, C.I. Okeke, Analysis of component forces when threading Ni±Cr±Mo steel with coated carbide inserts under high speed conditions, in: R.W. Bainess, A. Taleb-Bendiab, Z. Zhao (Eds.), Proceedings of the 14th National Conference on Advances in Manufacturing Technology, University of Derby, 1998, pp. 43± 50. [7] A.A. Minevich, Wear of cemented carbide cutting inserts with multi-layer Ti-based PVD coating, Surf. Coat. Technol. 53 (1992) 161±170.
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15 [8] E.O. Ezugwu, C.I. Okeke, A.R. Machado, High speed threading of inclusion-modified steels using coated carbide inserts, J. Mater. Process. Technol. 86 (1999) 216±225. [9] E.O. Ezugwu, C.I. Okeke, A.R. Machado, Surface defects/abuse when threading inclusion-modified steels with coated carbide inserts
15
under high speed condition, in: D.J. Hargreaves, W. Scott (Eds.), Proceedings of Australian Tribology Conference, December 6±9, 1998, pp. 583±588. [10] M.E.A. Moem, Effect of the clearance angle on wear of high speed tools, Wear 72 (1981) 1±11.
Tool life and wear mechanisms of TiN coated tools in an intermittent cutting operation E.O. Ezugwu*, C.I. Okeke School of Engineering Systems and Design, Machining Research Centre, South Bank University, 103 Borough Road, London SE1 0AA, UK
Abstract A P20-30 PVD TiN coated cemented carbide inserts with sharp edges were used to perform an external right hand thread cutting operation on two grades of steel up to a cutting speed and feed rate of 225 m min 1 and 0.44 mm rev 1, respectively. The test results show that cutting speed and feed rate had the most signi®cant in¯uence on tool life. A V-shape type of wear along the nose region of the tool face with adhering ¯ake-like oxide debris and micro/macro-chipping are the dominant failure modes while abrasion and plastic deformation of the sharp cutting edge were the main wear mechanisms affecting tool performance, particularly at higher cutting conditions. The ¯ake-like oxide debris were observed to increase when machining workpiece material with higher nickel and chromium content in their composition. Tool life was also found to have functional correlation between the cutting force, hardness and length of wear (AFM) along the nose region. # 2001 Elsevier Science B.V. All rights reserved. Keywords: PVD TiN coating; Flake-like oxide debris; Micro/macro adhesion and abrasion; Plastic deformation
1. Introduction PVD TiN coating deposited on sharp tool edges enhance machining productivity and also generate acceptable surface ®nish with moderate power consumption during machining operation. When properly applied, PVD coating can extend tool life by up to three times relative to uncoated tools [1]. The credibility of the hard temperature-stable coating materials is attributed to good impact strengths and suf®cient hardness and toughness which improve abrasion resistance, thus enhancing performance during machining. Assessing the applicability and performance of coating materials for various metal cutting operations require that the cemented carbide substrate should possess the correct balance of toughness and hardness to minimise premature tool failure and also be hard enough to adequately support the coating materials. Moderate Co content with ®ner WC grain size is more effective in terms of higher hardness and greater abrasion resistance with slight reduction in transverse rupture and impact strengths of carbide tools. Processing of carbide tools along this direction will ensure wider application in various machining operations. A good knowledge of the machining characteristics of a workpiece material is essential in harnessing the bene®ts of PVD coated carbide tools. For example, machining work* Corresponding author. Fax: 44-171-815-7699. E-mail address: [email protected] (E.O. Ezugwu).
piece materials prone to work hardening can generate high temperatures, welding on the tool surface in addition to high resistance to metal removal because of their high shear strength during machining [2,3]. These usually lead to a shorter tool life, pronounced chipping/premature failure of the cutting edge, severe surface abuse and the production of long continuous chips [4±7]. Maintaining acceptable ¯ank wear, below the rejection criterion, is very essential to avoid excessive surface and sub-surface damages on machined components. This paper presents results of the threading of inclusion modi®ed (IQ) EN 19T (708M40T) and EN 24T (817M40T) steels with PVD TiN coated inserts. It also highlights predominant failure modes and mechanisms responsible for tool failure at various cutting conditions. 2. Experimental procedures The machining trials were carried out on a 470 mm long and 150 mm diameter hardened steel bars. The nominal chemical composition and hardness of the workpiece materials are given in Tables 1 and 2, respectively. PVD TiN coated carbide tools with sharp edges were used for the machining operation. The nominal composition of the substrate and properties of the TiN coating are given in Tables 3 and 4, respectively. The thread quality, effective tool life and rejection criteria for both tool grades were based on physical examination of the thread surface to check for smooth
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 0 8 5 2 - 4
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
11
Tables 1 Chemical composition of EN 19 steel (wt.%) C
Si
Mn
S
P
Cr
Mo
Ni
Ca
Fe
Hardness
0.36 0.44
0.15 0.30
0.70 1.00
0.04 Max
0.04 Max
0.90 1.10
0.20 0.50
± ±
0.001 0.004
Bal. Bal.
273 HV Measured average
Table 2 Chemical composition of EN 24 steel (wt.%) C
Si
Mn
S
P
Cr
Mo
Ni
Ca
Fe
Hardness
0.36 0.44
0.10 0.40
0.45 0.70
0.4 Max
0.40 Max
1.10 1.40
0.20 0.35
1.30 1.70
0.001 0.004
Bal. Bal.
285 HV Measured average
Table 3 Chemical composition of the substrate of the threading tools (vol.%) Tool
WC
TiC TaC
Co
ISO code
PVD TiN
68.5
21.0
10.5
P20-30
silvery surface ®nish using a simple gauge to measure thread pro®le. A 5% growth in thread root (GTR) was stipulated as the criterion for tool rejection. The effective tool life (ETL) is the time corresponding to average ¯ank wear of about 0.587 mm measured along the tool nose. Wear above this value generally has an adverse effect on thread quality and can also cause premature failure of the threading insert during machining. These data (5% GTR and 0.587 mm AFW) were obtained from the formula relating the GTR, ¯ank wear and a clearance angle of 68 [8]. A tool holder with 68 clearance angle was used in the threading test since higher clearance angle usually has a greater in¯uence on tool performance when machining steels. An electronically controlled centre lathe with a steplessly variable 11 kW motor drive, which provides a torque of 1411 N, was used for the machining trials. The cutting conditions employed during machining are listed below: cutting speed (m min 1): 150, 175, 200 and 225; feed rate (mm rev 1): 0.40 and 0.44; depth of cut (mm): 1.5. Each insert was clamped on a standard 25 mm shank tool holder designated AL 25-3C RH to provide a 68 clearance angle during threading. All the machining trials were carried out without coolant to prevent shattering of the inserts due to
thermal shock. A 3-phase piezo-electric dynamometer was used for recording component forces during machining. Tool wear was monitored with a travelling microscope connected to a digital readout device. Tool rejection criteria employed, based on British standard (BS 5623: 1979), are given below: 1. Average flank wear: 0.587 mm. 2. Maximum flank wear: 1.18 mm. 3. Notching/groove at the depth of cut line or tool nose: 1 mm. 4. Surface finish: non-smooth and non-silvery colour. 5. Excessive chipping (flaking) or catastrophic fracture of the cutting edge.
3. Results and discussions 3.1. Effect of cutting speed and feed rate Table 5 shows recorded tool life at the cutting conditions investigated. There was a gradual reduction in tool life with increasing cutting speed up to 200 m min 1. Further increase in cutting speed up to 225 m min 1 resulted in a rapid drop in tool life. Threading the EN 24T steel generally gave about 20% less tool life than the EN 19T steel. This is due to the difference in hardness as well as the presence of hard abrasive carbide particles such as Mo2C, FeMo2C and FeMo2C6 [8,9] in the matrix of the steel grades. Table 5 also shows the effect of feed rate when threading EN 19T and EN 24T steels. A 10% increase in feed rate from 0.40 to 0.44 mm rev 1 reduced the tool life by about half when machining at speeds up to 200 m min 1. Increase in cutting
Table 4 Properties of the PVD TiN coating material Tool
Coating thickness (mm)
Grain size (mm)
TRS (N/mm2)
Density (g/cm3)
Hardness (HV)
PVD TiN
3±4
2.0
1700
12.4
91.1
12
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
Table 5 Tool life obtained when threading EN 19T and EN 24T steels at various cutting conditions
wear in excess of the stipulated rejection criterion (AFM: 0.587 mm) at lower tool life [8].
Speed (m min 1)
Feed rate (mm rev 1)
Tool life (min)
3.3. Wear pattern
EN 19T steel
EN 24T steel
150 175 200 225 150 175 200 225
0.40 0.40 0.40 0.40 0.44 0.44 0.44 0.44
41 39 37 5.2 22 21 17 2.6
35 32 31 2.5 17 15 13 2.3
parameters indicates that the interaction of process energies, including temperature, increases generally and can therefore promote various wear mechanisms, which accelerate tool wear with the consequent reduction in tool life. 3.2. Cutting speed and flank wear rate Figs. 1 and 2 show the comparison between the cutting speed and ¯ank wear rate when machining these two grades of steel with PVD TiN coated carbide inserts. Flank wear rate is calculated as the average ¯ank wear (mm) divided by the effective tool life (min). In both ®gures, an increase in cutting speed gave fairly uniform ¯ank wear rate up to a cutting speed of 200 m min 1 and increased rapidly with further increase in cutting speed to 225 m min 1. The `V' shape type of wear (Fig. 3), which concentrates along the tool nose, encourages erosion of the coating layers, alteration in edge geometry, loss of coercive strength and thermal cracking (Fig. 4) of the substrate at higher speed conditions. The higher ¯ank wear rate also increases the average ¯ank
A `V'-shape type of ¯ank wear (parabolic shape) along the tool nose was the dominant failure mode observed during threading (Fig. 3). The wear curvature on the tool faces also varied according to the length of wear along the tool face and width of the wear along the edge, where the wear along the width is about one-third of that along the nose. This is due to the alternating cycle of the cutting edge during threading, thus making the cutting edge to wear evenly in a `V' pattern as the angle of the thread pro®le is 608. Flake-like oxide debris adhered on to the worn edges of the inserts and generally increased with increasing cutting conditions as illustrated in Fig. 3. Formation and adhesion of the ¯ake-like oxide debris is a result of the metallurgical reaction arising from the interaction between the tool and the workpiece materials during machining. This is more likely to be in¯uenced by the composition of the work material and temperature generated. The EN 24T with higher nickel and chromium content (Table 2) had more ¯ake-like oxide debris on the tool face after threading. Chipping and ¯aking of the tool particles occurred at the initial tool entry when threading with a sharp edged insert. This phenomenon is more severe at higher speed conditions, thereby encouraging early erosion of the coating layer and subsequent exposure of the substrate to thermal shrinkage stress due to transient temperature during the idling period when the cutting edge is temporarily redundant. Fig. 4 shows thermal cracks on the exposed substrate after machining EN 24T steel at a high cutting speed and a feed rate of 200 m min 1 and 0.44 mm rev 1, respectively. Formation of thermal cracks at high cutting speed weakens the cohesive bond strength of the substrate.
Fig. 1. Cutting speed versus flank wear rate when threading EN 19T with PVD TiN coated inserts at a feed rate of 0.44 mm rev 1.
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
13
Fig. 2. Cutting speed versus flank wear rate when threading EN 24T with PVD TiN coated inserts at a feed rate of 0.44 mm rev 1.
Fig. 5 shows the plot of cutting speed against cutting force, hardness and tool life when threading EN 19T steel with PVD TiN coated inserts. The ®gure clearly shows an increase in cutting forces with increasing cutting speed, contrary to expectation. Trent [1] reported that the cutting force decreases on increasing the cutting speed partly due to the softening of the workpiece material under the high temperature generated and partly due to decreased tool± chip contact area as a result of thinner chips produced in single point turning. Analysis of the cutting forces in threading have shown that this is not the case. This can
be attributed to the perceived rubbing effect between the two contacting bodies, especially at the pullout stage as well as the effect of forces exerted on the tool nose and clearance surface [10]. As mentioned earlier, most of the wear concentrates along the tool nose, meaning that there will be a high concentration of temperature on the tool nose region. This may increase the likelihood of spalling and consequently shorter tool life at higher speed conditions (Figs. 1 and 2). Additional load (tool nose force) exerted on the cutting edge can also increase the cutting force at higher cutting speeds. In addition, increase in cutting speed during threading causes increased ¯ank wear rate, localised plastic deformation of the cutting edge, alteration in tool nose radius and an increase in tool±workpiece contact length.
Fig. 3. V-shape type of wear and adhering flake-like debris at the cutting edge after threading EN 24T steel.
Fig. 4. Thermal cracks on the exposed tool substrate after machining EN 24T steel for 13 min at a speed of 200 m min 1, feed rate of 0.44 mm rev 1 and depth of cut of 1.5 mm.
3.4. Relationship between cutting forces, hardness and tool life during threading
14
E.O. Ezugwu, C.I. Okeke / Journal of Materials Processing Technology 116 (2001) 10±15
Fig. 5. Functional relationship between the cutting force, hardness and tool life when threading EN 19T steel at a feed rate of 0.40 mm rev 1.
These combine to increase the cutting forces at higher speed conditions. Fig. 5 also shows a 3±5% reduction of the initial hardness (273 HV) of the material after threading at various cutting speeds investigated. This indicates softening of the work material due to elevated temperature at the tool nose, thus a reduction in tool life and an increase in the cutting force at higher cutting speeds. This suggests that PVD TiN coated threading inserts with sharp edges encounter higher stresses and higher temperature per unit area at the nose region, which increases the tool±workpiece contact length/area hence, higher ¯ank wear rate and lower tool life when threading under high speed conditions.
to additional forces exerted on the tool nose (tool nose force) and clearance surface as a result of the rubbing effect between the two contacting bodies, especially at the pullout stage.
Acknowledgements The authors acknowledge the support from Damsthall Tools and Macreadys Carbon and alloy steel which enabled this work to be carried out. References
4. Conclusions 1. The test results show that acceptable tool life can be achieved when machining EN 19T and EN 24T steels with PVD TiN coated inserts with sharp edges using cutting speed and feed rate up to 200 m min 1 and 0.44 mm rev 1, respectively. 2. Increase in cutting parameters promote the interaction of process energies, including temperature, which in turn accelerates various wear mechanisms, resulting in more severe tool wear and reduced tool life. 3. A V-shape type of wear along the nose region with adhering flake-like oxide debris was the dominant tool failure mode while micro/macro-adhesion and abrasion wear as well as plastic deformation of the sharp cutting edge are the wear mechanisms affecting tool performance, particularly at higher cutting conditions. 4. When threading EN 19T and EN 24T steels with PVD TiN coated carbide inserts under high speed conditions, cutting force increases with increasing cutting speed due
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