Investigations on machinability aspects of hardened AISI 4340 steel at different levels of hardness using coated carbide tools

Investigations on machinability aspects of hardened AISI 4340 steel at different levels of hardness using coated carbide tools

Int. Journal of Refractory Metals and Hard Materials 38 (2013) 124–133 Contents lists available at SciVerse ScienceDirect Int. Journal of Refractory...

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Int. Journal of Refractory Metals and Hard Materials 38 (2013) 124–133

Contents lists available at SciVerse ScienceDirect

Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Investigations on machinability aspects of hardened AISI 4340 steel at different levels of hardness using coated carbide tools Satish Chinchanikar ⁎, S.K. Choudhury Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India

a r t i c l e

i n f o

Article history: Received 26 August 2012 Accepted 21 January 2013 Keywords: Turning Machinability Hardened steel Coated carbide tools Chip morphology Tool life

a b s t r a c t This study investigates the effect of workpiece hardness, cutting parameters and type of coating (coated tool) on different machinability aspects like, the tool life, surface roughness, and cutting force and chip morphology during turning of hardened AISI 4340 steel at different levels of hardness. Cutting forces observed to be higher for harder workpiece and for CVD applied multi-layer MT-TiCN/Al2O3/TiN coated carbide tool. Better surface finish observed for harder workpiece and for PVD applied single-layer TiAlN coated carbide tool. However, better tool life obtained by CVD coated tool can be attributed to its thick coating and the protective Al2O3 oxide layer formed during cutting, which has protected the tool from severe abrasion at elevated temperatures. Modified Taylor tool life equation indicated that the workpiece hardness followed by the cutting speed and depth of cut as the most influencing factors on tool life. The better performance of CVD coated tool under study is obtained by limiting the cutting speed to 300 and 180 m/min for workpiece hardness of 35 and 45 HRC, respectively. However, the upper limit is of 200 m/min when using PVD coated tool. It has been observed that the tool wear form and the wear mechanism(s) by which the tool wear occurred are influenced by the workpiece hardness, cutting conditions and the type of tool. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Development in the cemented carbide grades, coating materials and coating deposition technologies, have extended the application of coated carbide tools to hardened steel machining and stands out as an economical alternative to costly ultra-hard tool materials like CBN and ceramic tools. However, higher workpiece hardness imposes certain restrictions on cutting conditions to be employed during machining. Further, performance of these tools largely depends upon the physical properties and chemical compositions of the coated tools; especially the coating materials and their characteristics. Lima et al. [1] evaluated the machinability of hardened steels at 50 and 42 HRC, using PCBN and coated carbide inserts, respectively. They observed lower cutting forces when turning harder material and better finish when using PCBN tool. In another study [2], they observed distinct behavior of thrust and feed forces at different levels of workpiece hardness. Chakraborty et al. [3] observed better performance of ceramic tools as against WC tools during hardened steel machining. El-Tamimi and El-Hossainy [4] evaluated the influence of cutting variables on tool life in view of improving the machined surface quality. Suresh et al. [5] analyzed the effect of cutting parameters and machining time on various machinability aspects during turning of hardened AISI 4340 steel using coated carbide tools. They observed better surface finish at lower values of feed, machining time and at higher cutting ⁎ Corresponding author. Tel.: +91 512 2597270; fax: +91 512 2597408. E-mail address: [email protected] (S. Chinchanikar). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.01.013

speed. However, lower values of cutting parameters and machining time resulted in better tool life. Garcia et al. [6] investigated the wear behavior of functionally graded outer-surface of cemented carbide inserts by sintering process. They observed better cutting results for the nitrided substrate coated with CVD multi-layer coating. Avila et al. [7] carried out hard machining with coated cemented carbide inserts. They observed higher wear rate with TiAlN coated tools followed by TiN and TiCN coated tools. However, Khrais et al. [8] observed better performance of the TiAlN coated tools by limiting the cutting speed to 260 m/min. Kurniawan et al. [9] also observed better performance of these tools during hard machining of stainless steel (48 HRC). Dogra et al. [10] obtained comparable tool life by multi-layer TiCN/Al2O3/TiN coated carbide tool against CBN tool; especially at low cutting speed. Noordin et al. [11] observed the better functioning of multi-layer TiCN/Al2O3/TiN carbide inserts against coated cermet inserts during dry turning of tempered tool steel. Sahoo et al. [12] also reported on better functioning of these multi-layer carbide inserts during hard turning. However, Kyung-Hee et al. [13] noticed no significant benefit in resisting flank wear by multi-layer coating. Gaitonde et al. [14] explored the consequences of machining parameters on various machining responses using ceramic inserts. Cutting force and power was observed highly susceptible to feed rate. Suresh et al. [15] determined the eventuality of process variables on different aspects of machinability using taguchi technique during hard turning. El-Hossainy et al. [16] observed the dependency of cutting force and surface roughness on machining time. These factors were sensitive to time and claimed that the time factor must be considered not only for

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flank wear, but also for the power consumed and product quality. Their study of analysis concluded that the surface finish and cutting force obtained can be used as a tool changing criterion. Aouici et al. [17] examined the inference of cutting parameters and workpiece hardness on cutting force and surface roughness using CBN tools. Jindal et al. [18] evaluated the metal cutting performance of PVD TiN, TiCN, and TiAlN coated cemented carbide tools while turning Inconel 718, SAE 1045 steel and ductile iron. For all the work combinations and cutting conditions used, they found that the TiAlN coated tools performed better followed by the TiCN and TiN coated tools. It is reported that TiAlN and TiCN coatings characterized by high hot hardness [19] and TiAlN coating is the first choice for thermally influenced processes and TiCN for mechanically influenced processes, and Ti-N can work as a good compromise between the two coatings [20]. Similarly, ceramic coatings Al2O3 exhibit good resistance to abrasive wear and have high thermal stability [21]. Sufficient literature is available on hardened steel machining using CBN or ceramic tools. Although some investigations are available using coated carbide tools but, performance of these tools considering the effect of work material hardness, cutting parameters, and type of coating material (coated tool) is rarely reported. This study investigates different machinability aspects like, the tool life, surface roughness, three components of cutting force and chip morphology during turning of hardened AISI 4340 steel at three levels of hardness: 35, 45 and 55 HRC, respectively, using differently coated carbide tools; PVD applied single-layer TiAlN and CVD applied multi-layer MT-TiCN/ Al2O3/TiN. This study also establishes a correlation among the cutting parameters, workpiece hardness and tool life. Different tool wear forms and wear mechanisms considering the effect of work material hardness, cutting parameters and type of tool are discussed with the images taken by digital and scanning electron microscope. 2. Experimental details 2.1. Workpiece materials, cutting inserts and experimental procedure Turning tests were performed on hardened AISI 4340 steel at three levels of hardness: 35, 45 and 55 HRC, respectively. Hardness was maintained uniform throughout the cross section with a maximum variation of ± 1 HRC by a hardening and tempering process. The workpiece used has a length and diameter of 400 mm and 90 mm, respectively. Experiments were performed using cemented carbide inserts (ISO class P10) coated with single-layer TiAlN (Kennametal KC5010) and CVD coated multi-layer MT-TiCN/Al2O3/TiN (Kennametal KC9110), designating hereafter as tool number T1 and T2, respectively. T1 is PVD coated with Titanium aluminum nitride (TiAlN) with 6% cobalt binder in the substrate with an average thickness of 2 microns. T2 is CVD coated with three main layers of coating having overall thickness of 18 microns. The order of coating layers and their individual average thickness can be seen from Fig. 1(a). A tool holder and insert geometry, having ISO designation as PCBNR 2020 K12 and CNMG 120408, respectively, were employed with tool geometry as follows: including angle = 80°, inclination and side rake angles= −6°, clearance angle = 5° and approach angle=75°. The insert and cutting edge geometry for multilayer and single-layer coated tools are shown in Fig. 1(a). Cutting edge radius for both the inserts is less than 20 μm (rb 20 μm). However, edge radius varies with the coating layer thickness for multi-layer coated tool can be seen from Fig. 1(a). Details of the chip-breaker geometry for both the coated inserts are shown in Fig. 1(b). It is designated as –MP and –MN for PVD and CVD coated inserts, respectively. Both these geometries are recommended for medium machining. It can be seen that chip-breaker geometry for PVD coated tool has a positive cutting edge style and has a negative, stable cutting edge style for CVD coated tool. During experiments, tool height, its overhang and tool geometry were kept constant. Flank wear and its growth was monitored at regular intervals of length of

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cut. Digital microscope and FEI QUANTA 200 ESEM were used to evaluate the wear on flank surfaces. Surface roughness and cutting forces were assessed by using Qualitest TR100 and a three-component dynamometer (KISTLER Type 9257BA), respectively. 2.2. Cutting conditions Dry cutting experiments were carried out on a HMT centre lathe. Cutting conditions were selected on the basis of machine capability, literature review and tool manufacturer's recommendation. Initially, cutting force experiments using the cutting conditions shown as run numbers A, B and C in Table 1 and tool life experiments using the cutting conditions shown as run numbers a and b in Table 2, respectively, were performed on 35 HRC workpiece using both the coated tools. Tool life experiments showed better performance for T2 as against T1 even when cutting speed exceeded above 300 m/min when turning softer workpiece. Therefore, experimental investigations at higher levels of hardness were carried out using T2. However, cutting speed was limited to 200 m/min as premature tool failure occurred above this cutting speed. Even with lowest cutting condition, T2 was unable to cut 55 HRC workpiece and failed catastrophically in a very short time of cutting. Experiments were not performed at this level of hardness. Cutting force and surface roughness tests on 35 and 45 HRC workpiece were performed using common cutting conditions shown as run numbers 1 to 7 in Table 1. Similarly, Tool life tests on 35 and 45 HRC workpiece were performed in the wide range cutting conditions as shown in Table 2. 3. Results and discussion In this section, actual findings are summarized. Plots showing the force components, tool life and surface roughness at two levels of workpiece hardness are presented. Referring to ISO 3685–1977 (E), the tool life was considered to be over when the maximum flank wear or maximum end clearance wear or nose wear reached 0.2 mm. Different tool wear forms, wear mechanisms, and chip morphology is explained with the SEM and a digital microscope images. 3.1. Force components and surface roughness Effect of type of coating (type of tool) and the cutting parameters on cutting forces was investigated according to experimental runs A, B and C (Table 1) while turning workpiece hardened to 35 HRC. Fig. 2(a) depicts the variation of tangential (P1), axial (P2) and radial (P3) cutting force components with cutting speed using constant feed and depth of cut values (Run A, Table 1). It can be seen that the cutting forces are higher for T2. The tangential component of cutting force (P1) is largest in magnitude followed by the axial (P2) and radial force (P3). Decrease in tangential component with the increase in cutting speed can be seen when using T1. However, for T2, the plot remains practically unaltered in the higher cutting speed range. Similar behavior can be seen for axial and radial components and may be because of change in frictional conditions at the tool flank due to increased tool wear rate at higher cutting speeds. The lower magnitude of force components for T1 can be attributed to minimum friction provided by PVD applied coating to the flowing chip [22]. Experimental observations of force components with varying feed and depth of cut are shown in Fig. 2(b)–(c), respectively (Run B and C, Table 1). It can be seen that cuttting forces vary almost linearly with depth of cut and feed. However, at low value of depth of cut or feed, the radial component has become almost equal or dominates the axial component. At these cutting conditions, machining mostly takes place by the tool nose section than the straight section of the side cutting edge, resulting in an increase in radial force component than axial component.

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Fig. 1. (a) Cutting edge geometry for coated inserts (b) chip-breaker geometry for coated inserts.

Further, variation in cutting force components considering the effect of workpiece hardness and cutting parameters was assessed according to experimental runs 1 to 7 (Table 1), using CVD coated tool (T2). From Fig. 3(a) and (b), it can be seen that the cutting forces are higher for the harder workpiece. However, reduction in cutting forces approaching near to the level of softer workpiece can be seen at higher feed (Table 1, Run 5) or at higher depth of cut (Table 1, Run 7). Reduction in cutting forces occured because of the softening of the workpiece due to higher elevated temperatures; especially when turning harder workpiece. Axial and radial components initially decrease with cutting speed but tend to increase when cutting speed exceeds 150 m/min for harder workpiece (Fig. 3(a) and (b)) (Table 1, Run 1 to 3). This indicates increase in tool wear rate when turning harder workpiece beyond this cutting speed. However, for softer workpiece, these components initially decrease and remains practically unaltered in the higher cutting speed range. From the cutting force experiments, it can be seen that, lower cutting forces are incurred by PVD coated tools. Cutting forces affected mostly by depth of cut followed by feed and get little affected by cutting speed. Cutting forces recorded higher for harder workpiece. However,

Table 1 Cutting conditions for cutting force and surface roughness test. Run number

Cutting speed (m/min)

Feed (mm/rev)

Depth of cut (mm)

Hardness (HRC)

A

0.2

1.5

35

B

100–142–200–265– 300 200

C

200

1, 2 and 100–150–200 3 4 and 5 150 6 and 7 150

0.1– 0.15–0.2–0.25– 1.5 0.3 0.2 0.5–1–1.5– 2–2.5 0.2 1.5

35

0.1–0.3 0.2

35–45 35–45

1.5 0.5–2.5

35 35–45

reduction in cutting forces occurred at higher depth of cut and feed due to thermal softening of the workpiece. Similarly, variation in surface roughness considering the effect of cutting parameters, workpiece hardness and type of tool was investigated according to experimental runs 1 to 7 (Table 1). Experimental observations of surface roughness produced using T1 and T2 when turning 35 HRC workpiece and when turning 45 HRC workpiece using T2 as shown in Fig. 3(b). Plots indicate lower values of surface roughness for T1, which can be attributed to lower cutting forces and minimum friction offered by these tools between the tool face and the chips. It can be seen that surface roughness decreases with the increase in cutting speed (Table 1, Run 1 to 3). However, it is almost unaltered beyond the cutting speed of 150 m/min when turning 45 HRC workpiece and may be due to increase in axial and radial components of force beyond this cutting speed. Surface roughness increases sharply at higher feed value (Table 1, Run 5) or at higher depth of cut (Table 1, Run 7), which is in line with the classical theory of metal machining. At higher feed and depth of cut, the force components increase sharply resulting into more vibrations and hence, chatter marks on the machined surface. However, this effect can be seen as more prominent when using T1. Because of growing crater wear on these tools, reduce the metal available to support the tool, which promotes more chatter. Also, this tool was Table 2 Cutting conditions for tool life test. Run number

Cutting speed (m/min)

Feed (mm/rev)

Depth of cut (mm)

Hardness (HRC)

Tool type

a b c d e f g

142–265–345–487 142–200–265 100–200–300 100–150–200 142 150 200

0.125 0.2 0.2 0.2 0.2 0.1–0.3 0.1–0.3

0.8 1.5 0.8 0.8 0.5–1.5 0.8 0.8

35 35 35 45 35–45 45 35

T1–T2 T1–T2 T2 T2 T2 T2 T2

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Fig. 3. Effect of workpiece hardness on (a) tangential and axial force (b) radial force and surface roughness.

Fig. 2. Variation of cutting force components with (a) cutting sped (b) feed (c) depth of cut.

observed to be more sensitive to changes in cutting conditions, discussed in Section 3.2. In summary, when surface finish is a criterion for tool selection, PVD coated tool is the better option. However, higher feed and depth of cut is to be avoided. Surface roughness gets affected mostly by feed and depth of cut and cutting speed showed little influence; especially when turning harder workpiece. 3.2. Tool life Applicability of differently coated carbide tools to hardened steel machining (35 HRC) was assessed in the wide range of cutting speeds and at two levels of feed and depth of cut values (Run a and b, Table 2), respectively. Experimental observations showed an increase in flank wear land with machining time generally confined to three distinct regions, namely, break-in period, steady-state wear region and the failure region. The comparative of tool life obtained with both the coated tools is shown in the histogram, in Fig. 4(a). It can be seen that T2 performed better than T1 at all the tested conditions. A better tool life of T2 can be attributed to its thicker coating, and the protective Al2O3 oxide layer formed during cutting, which has protected the tool from severe abrasion at elevated temperatures. T2 (CVD coated tool)

contains three main layers of coating, MT-TiCN (medium temperature titanium carbonitride), Al2O3 (aluminum oxide) and TiN (titanium nitride), respectively. The inner layer MT-TiCN provides better adhesion to the tool substrate and renders protection against flank wear. Intermediate Al2O3 layer is chemically inert with low thermal conductivity provides protection against the elevated temperatures and crater wear. The outer layer TiN/TiCN of 2 μm thickness provides additional wear resistance [23]. It can be seen that higher tool life is obtained using lower cutting speed, feed and depth of cut values (Fig. 4(a)). However, this effect is more prominent when using T1 and can be explained with respect to the tool life exponent and constant of Taylor tool life equation calculated based on experimental results by fitting linear trend line. The tool life exponent values obtained at two different cutting conditions (Runs a and b, Table 2) showed a significant difference for T1 and a marginal difference for T2, indicate the sensitivity of T1 with respect to the cutting conditions. Similarly, smaller values obtained of the tool life equation constant and exponent for T1 indicates lower tool life and sensitivity of these tools to the cutting speed as compared to T2. As seen in Fig. 5, lower flank wear values were recorded for T1 in the initial stage of machining. However, after some time of machining when the thin layer of TiAlN coating gets removed from the substrate; most probably by adhesion and abrasion wear mechanisms, a rapid increase in the flank wear rate of T1 subsequently dominating the flank wear rate of T2 can be seen due to oxidation-dominated and diffusion wear mechanisms. Lower values of flank wear of T1 can be attributed to better heat isolation of the TiAlN coating from the tool cutting edge and better thermal properties of the TiAlN coating than the outer TiN layer of multi-layer MT-TiCN/Al2O3/TiN coating scheme [24]. Further, lower coefficient of friction which is a characteristic of the PVD applied coating process [22], higher hardness and thermal conductivity of the TiAlN film [19,20] restrains the higher strains, cutting temperature and compressive stresses at the secondary deformation zone [24]. It can be seen

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Fig. 4. Histogram of tool life for (a) tools, T1 and T2 when turning 35 HRC workpiece (b) when turning 35 and 45 HRC using tool T2.

that a sizable tool life can be obtained by limiting the cutting speed to 200 m/min when using T1. The multi-layer CVD coated tools (T2) showed better performance in terms of tool life at all the tested conditions, which can be attributed to its thick triple layer of coating. It is reported that higher hardness of the coating layer(s) clubbed with the higher thickness of the coating provides better stability and mechanical support to the cutting edge; especially when machining at higher cutting speeds [24]. However, with continual machining chipping off of the coating layers from the tool surface, severe damage at the nose and crater wear and pitting on the rake face was observed; especially when working at higher cutting speeds. As multi-layer coated tool (T2) performed better as compared to single-layer coated tool (T1) during turning the softer workpiece (35 HRC), machining performance of this tool (T2) was further assessed at the higher level of workpiece hardness of 45 HRC. However, cutting speed was limited to 200 m/min as premature tool failure occurred above this cutting speed. Effect of workpiece hardness and cutting parameters on tool life (of T2) was investigated using the cutting conditions as given in Table 2. The comparative performance of tool life when turning hardened steel at two different levels of hardness (35 and 45 HRC, respectively), as shown in the histogram, in Fig. 4(b). Almost same tool life obtained can be seen when machining the softer and the harder workpiece at a cutting speed of 300 and 150 m/min, respectively. Similarly, decrease in tool life of about 65% and 46% can be seen when cutting speed is elevated from 100 to 200 m/min (in case of harder workpiece) (Table 2, Run d) and 100 to 300 m/min (in case of softer workpiece) (Table 2, Run c), respectively. Increase in depth of cut (0.5 to 1.5 mm) (Table 2,

Fig. 5. Flank wear progression of T1 and T2 when turning 35 HRC workpiece.

Run e) or feed value (0.1 to 0.3 mm/rev) causes decrease in the tool life of about 54% and 35% can be seen when turning 45 and 35 HRC workpiece, respectively (Table 2, Run g and f).

Fig. 6. Tool life variation (predicted by model) with (a) cutting speed (b) feed and c) depth of cut.

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Fig. 7. Tool images (a) T1 (V = 142 m/min, 35 HRC) (b) T2 (V = 345 m/min, 35 HRC) (c) T2 (V= 200 m/min 45 HRC) and (d) T2 (V = 300 m/min 35 HRC).

Since the tool life gets affected by level of workpiece hardness and cutting conditions, relative importance of each factor can be obtained by formulating a model. A modified Taylor tool life equation was developed to know the most affecting cutting parameter(s) on tool life within

the range of parameters considered under the present study. Equations were developed based on the experimental observations of tool life for 35 and 45 HRC workpiece, respectively. However, tool life results at 345 and 487 m/min were not considered in developing the equations as

Fig. 8. SEM images of T2 when working on 45 HRC workpiece (a) T2 (V = 150 m/min, f = 0.3 mm/rev and d = 0.8 mm). (b) Magnified view of marked rectangle in Fig. 7(a). (c) Magnified view of rectangle A in Fig. 7(b). (d) Magnified view of marked rectangle B in Fig. 7(b).

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The tool life results were analyzed using the least error square method. The unknown coefficients were determined by using DataFit software. Model R-squared values are shown in the bracket. It can be seen that cutting speed followed by depth of cut become the most influencing parameters on tool life (from exponents and constants of Eqs. (1)–(2)). However, this effect is more prominent for harder workpiece, which can be seen from Fig. 6(a) to (c). Also, can be confirmed from Eq. (3), which shows that workpiece hardness followed by cutting speed and depth of cut become the most influencing parameters, on tool life. It can be seen that a sizable tool life (>10 min) can be obtained by limiting the cutting speed to 300 m/min and 180 m/min (using Eqs. (1)–(2)) for 35 and 45 HRC workpiece, respectively, even using highest feed and depth of cut.

3.3. Wear mechanisms and failure mode

Fig. 9. SEM image of flank face of T2 when working on 45 HRC workpiece at V = 142 m/min, f = 0.2 mm/rev and d = 1.5 mm.

most of the experiments were carried in the cutting speed range of 100 to 200 m/min for 45 HRC and up-to 300 m/min for 35 workpiece, respectively. Developed equations are expressed as below: a) For workpiece hardness: 35 HRC

V T

1:778

f

0:5967

d

0:8634

¼ 5:2968  10

  2 R ¼ 0:90

4

ð1Þ

½100≤V ≤300

b) For workpiece hardness: 45 HRC

V T

0:6619

f

0:3208

d

0:4606

  2 R ¼ 0:86

¼ 697:7056

ð2Þ

½100≤V ≤200

c) For workpiece hardness: 35–45 HRC

6

T ¼ 2:9454  10  V H

−2:4168

−0:605

−0:3758

−0:4452

d  f R ¼ 0:88 ½100≤V ≤200 2

ð3Þ

In this section, tool wear form and wear mechanisms are discussed with the images taken by digital and scanning electron microscope. Flank wear, plastic deformation of the cutting edge and crater wear was observed as a dominant wear form when using PVD coated single-layer TiAlN coated tool (T1). However, flank wear, nose wear, chipping at the nose and clearance face; especially when using lower depth of cut and plastic deformation of the cutting edge was observed as a dominant wear form when using CVD coated multi-layer MT-TiCN/Al2O3/TiN carbide tool (T2). Different wear forms observed on tool flank and rake faces at the end of cutting are shown in Fig. 7(a)–(d). Crater wear as a dominant wear form when using T1 (at V = 142 m/min, f = 0.2 mm/rev and d = 1.5 mm), and chipping of the protective Al2O3 oxide layer along with the coating layers and nose wear when using T2 (at V = 345 m/min, f =0.125 mm/rev and d = 0.8 mm) can be seen from Fig. 7(a) and (b), respectively. However, Fig. 7(c) depicts the image of T2 when turning 45 HRC workpiece (at V = 200 m/min, f = 0.2 mm/rev and d = 0.8 mm). Chipping of the coating layers from the nose of the tool can be seen. Fig. 7(d) depicts the image of T2 when turning 35 HRC workpiece (at V = 300 m/min, f = 0.275 mm/rev and d = 0.8 mm). A severe damage/chipping at the clearance face can be seen. The tool wear mechanism(s) by which the tool wear occurred can be explained with SEM images considering the effect of workpiece hardness, cutting parameters and type of coating. Fig. 8(a) shows the rake and flank faces of T2 at the end of cutting when turning 45 HRC workpiece (at V = 150 m/min, f = 0.3 mm/rev and d = 0.8 mm). Fig. 8(b) depicts the magnified view of the marked rectangle in Fig. 8(a). The enlarged view of marked rectangle ‘A’ and ‘B’ in Fig. 8(b) is shown in Fig. 8(c) and (d), respectively. A severe damage at the nose of the tool, pitting on the rake and flank faces resulted in exposing the tool

Fig. 10. SEM image of T2 when working on 35 HRC workpiece (a) T2 at V= 200 m/min, f = 0.3 mm/rev and d = 0.8 mm (b) magnified view of rectangle in Fig. 9(b). (c) Elemental analysis at position A shown in Fig. 9(b).

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Fig. 11. SEM image of T2 when working on 35 HRC workpiece. (a) Flank face of T2 at V = 300 m/min, f = 0.2 mm/rev and d = 0.8 mm (b) after the removal of coating layers. (c) Magnified view of rectangle in Fig. 10(b).

substrate, coating delamination, crater wear and metal adhesion can be seen. Abrasion marks parallel to the metal flow direction indicates wearing of the coating layers due to abrasion of the hard particles of the workpiece, which eventually led into removal of the coating can be seen from Fig. 8(d). Metal adhesion can be seen from Fig. 8(c). After a brief cutting, this adhered metal gets dislodged and comes out as a loose fragment along with the coating material resulted in pitting on the flank and rake faces subsequently exposes the tool substrate. At comparatively higher depth of cut flank wear was observed as a dominant wear form. A typical wear observed at flank face of the T2 when turning 45 HRC workpiece as shown in Fig. 9 (at V=142 m/min, f=0.2 mm/rev and d=1.5 mm). Abrading of the different coating

layers by the hard particles of the workpiece and subsequently getting fractured due to plucking of the adhered material can be seen. However, when turning softer workpiece adhesion was observed as a dominant wear mechanism when using PVD and CVD coated tools. Fig. 10(a) and (b) depicts the condition of CVD coated tool (T2) when turning 35 HRC workpiece (at V=200 m/min, f=0.3 mm/rev and d=0.8 mm). Severe nose damage and pitting on the rake face can be seen. Fine abrasion marks completely filled by thin layers of adhered material can be seen. Removal of this adhered metal from the tool surface due to fast flowing chips is responsible for fragmentation of tool material. However, deep abrasion marks covered with metal adhesion when turning at higher cutting speeds can be seen from Fig. 11(a). This adhered metal

Fig. 12. SEM image of T1 when working on 35 HRC workpiece. (a) Flank and rake face when working at V= 265 m/min, f = 0.2 mm/rev and d = 1.5 mm (b). (c) Rake face when working at V = 265 m/min, f = 0.125 mm/rev and d = 0.8 mm.

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Fig. 13. Chip morphology at V = 150 m/min and d = 1.5 mm.

eventually plucked from the tool surfaces under the high sliding speed resulted in forming shallow pockets as seen in Fig. 11(b). Removal of the hard particles from the tool flank surface which are responsible for cutting are shown by marked circles in Fig. 11(c) indicates the signs of three body abrasion. These removed particles acted as a medium of three-body abrasion between the workpiece and the tool flank surface. Rapid deterioration of the cutting edge eventually leads to catastrophic failure are normally observed just after the removal of the coating layer(s); especially when using T1. Comparatively smooth flank surface of T1 (PVD TiAlN) (Fig. 12(a)) as against the rough flank surface of T2 (CVD multi-layer) after coating removal/delamination (Fig. 11(c)) shows the better adhesion strength of the MT-CVD TiCN coating layer of T2 to the tool substrate. Fig. 12(a) depicts the condition of PVD coated tool (T1) after cutting for 4.5 minute (at V = 265 m/min, f = 0.2 mm/rev and d = 1.5 mm). The complete spalling of coating from the flank and rake faces and plastic deformation of cutting edge can be seen. High compressive stresses and cutting speed in this case have resulted in plastic deformation of the cutting edge. Further, fracturing of the adhered metal from the flank and rake surfaces might have resulted in the removal of coating. A rough surface in the crater wear region observed for these tools indicates a severe wear mode. A similar observation was reported while machining with coated and uncoated carbide tools [7]. This may be probably have occurred by diffusion and adhesion wear mechanisms

which are active on the rake face. Inter-diffusion of tool and workpiece elements (Fe and Co) occurs when fast flowing chip sticks to the tool surface resulting in the weakening of the binder. This causes fracturing of small elements of the tool being carried away along with the chips resulting in the formation of rough surface in the crater as can be seen from Fig. 12(b). In summary, abrasion and adhesion when using CVD coated tools and adhesion, abrasion and diffusion when using PVD coated tools are the dominant wear mechanisms when turning hardened AISI 4340 steel. However, abrasion wear mechanism is more prominent when turning harder workpiece. Plastic deformation of the cutting edge takes place due to increase in stresses and temperature in the cutting region when cutting continued with the dull cutting tool (cutting edge with severe nose damage or broken cutting edge(s)) as observed when using CVD coated tools or when cutting continued with a tool having severe crater wear as observed when using PVD coated tools. 3.4. Chip morphology Chips produced significantly influence the surface finish of the workpiece, tool life and overall cutting operation. In the present work, the resulting chip-form was strongly influenced by the work material hardness, type of coated tool and cutting conditions; especially feed rate. The physical appearance of the chips obtained by T1 and T2 when turning

Fig. 14. SEM chip images (1) and (2) free surface (using T1) (3) and (4) back surface (using T1). (5) and (6) free surface (using T2) (7) and (8) back surface (using T2).

S. Chinchanikar, S.K. Choudhury / Int. Journal of Refractory Metals and Hard Materials 38 (2013) 124–133

workpiece at hardness levels of 35 and 35–45 HRC, respectively, at two feed values of 0.1 and 0.3 mm/rev and at cutting speed and depth of cut of 150 m/min and 1.5 mm, respectively, is shown in Fig. 13. Taking ISO-based chip form categorization as a guide [11], snarled long ribbon-chips (Fig. 13(a) and (b)) and snarled tubular-chips (Fig. 13(c)) are obtained at low feed rate when turning steel hardened to 35 and 45 HRC, respectively. At higher feed rate, short spiral (Fig. 13(d)) and loose arc chips (Fig. 13(e)) are produced by T1 and T2, respectively, when turning 35 HRC workpiece and short helical chips (Fig. 13(f)) are produced when turning 45 HRC workpiece. However, chips obtained are more closely curled in case of harder workpiece. It becomes hard to break resulting in higher cutting forces. The similar observation was reported while turning AISI 1045 steel under dry and wet cutting condition [25]. Better surface finish produced by using T1 can be explained in terms of chip morphology. Chip has two surfaces; one is the back surface of a machined chip which closely contacts with the tool rake face. Second is the free surface which has a jagged, rough appearance caused due to shearing mechanism. When the chip slides up the rake face, its back surface experiences high contact pressure. In addition, rubbing of the flowing chips with the rake face generates high temperature and high frictional forces resulting in polished and bright back surface [26,27]. The typical free and back surfaces of the chip produced using coated tools T1 and T2, respectively, are shown in Fig. 14 (1–8). More or less polished back and smooth free surfaces can be seen on the chip produced when using T1 (Fig. 14 (1–4)). However, rough back surface showing some parallel stripes and damaged free surface can be seen when using T2 (Fig. 14 (5–8)). Rough back surface is resulted from the hard particles of protective Al2O3 layer formed during cutting. Similar observation was reported in [28]. Polished and bright chip back surface obtained when using T1 shows that minimum friction was offered by these tools during cutting resulted in lower cutting forces and better surface finish as against higher values when using T2. 4. Conclusions • Cutting forces presented higher values for harder workpiece and for CVD coated multi-layer MT-TiCN/Al2O3/TiN carbide tool. However, in case of harder workpiece reduction in cutting forces occurred; especially at higher feed and depth of cut due to thermal softeneing of the workpiece. • Cutting forces varied almost linearly, with the feed and depth of cut but showed different behavior with cutting speed. Initially, the cutting forces decreased with the increase in cutting speed but almost unaltered in higher cutting speed range. • Surface roughness presented lower values for PVD coated tool; indicated by polished and bright back surface of the chip as against the rough surface showing some parallel stripes when using CVD coated tool. CVD coated tools produced better finish when turning harder workpiece. • Developed modified Taylor tool life equation showed that the tool life gets affected mostly by workpiece hardness followed by cutting speed and depth of cut. Higher tool life was obtained using CVD coated tools. Tool life was observed to be more sensitive to changes in cutting conditions; especially when using PVD coated tools. • Flank wear, chipping of the coating layers from the nose and the clearance face, when using CVD coated tool and crater wear when using PVD coated tool, were dominant wear forms. Abrasion and adhesion when using CVD coated tool and abrasion, adhesion and diffusion when using PVD coated tool, were dominant wear mechanisms. Plastic deformation was observed for both the coated tools towards the end of tool life. • Better performance of the PVD and CVD coated tools under study was at any cutting speed less than 200 and 300 m/min, respectively, when turning steel hardened to 35 HRC. However, it was less than 180 m/min when turning steel hardened to 45 HRC.

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