Machinability investigations in hard turning of AISI D2 cold work tool steel with conventional and wiper ceramic inserts

Machinability investigations in hard turning of AISI D2 cold work tool steel with conventional and wiper ceramic inserts

Int. Journal of Refractory Metals & Hard Materials 27 (2009) 754–763 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & H...

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Int. Journal of Refractory Metals & Hard Materials 27 (2009) 754–763

Contents lists available at ScienceDirect

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

Machinability investigations in hard turning of AISI D2 cold work tool steel with conventional and wiper ceramic inserts V.N. Gaitonde a,*, S.R. Karnik b,1, Luis Figueira c, J. Paulo Davim c,2 a

Department of Industrial and Production Engineering, B.V.B. College of Engineering and Technology, Vidyanagar, Hubli-580 031, Karnataka, India Department of Electrical and Electronics Engineering, B.V.B. College of Engineering and Technology, Vidyanagar, Hubli-580 031, Karnataka, India c Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal b

a r t i c l e

i n f o

Article history: Received 2 July 2008 Accepted 11 December 2008

Keywords: Hard turning AISI D2 cold work tool steel Ceramic inserts Machinability Response surface methodology

a b s t r a c t Hard turning with ceramic cutting tool has several benefits over grinding process such as elimination of coolant, reduced processing costs, improved material properties, reduced power consumption and increased productivity. Despite its significant advantages, hard turning can not replace all grinding due to lack of data concerning surface quality and tool wear and hence there is a need to study the machinability characteristics in high precision and high-hardened components. An attempt has been made in this paper to analyze the effects of depth of cut and machining time on machinability aspects such as machining force, power, specific cutting force, surface roughness and tool wear using second order mathematical models during turning of high chromium AISI D2 cold work tool steel with CC650, CC650WG and GC6050WH ceramic inserts. The experiments were planned as per full factorial design (FFD). From the parametric analysis, it is revealed that, the CC650WG wiper insert performs better with reference to surface roughness and tool wear, while the CC650 conventional insert is useful in reducing the machining force, power and specific cutting force. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Precision hard turning, an alternative to conventional grinding, is a cost-effective, high productivity and flexible machining process for ferrous metal components, which are often hardened above 45 HRC [1,2]. The material removal rate (MRR) in hard turning is much higher than grinding even though smaller depth of cut and feed rates are required [3]. It has also been reported that the resulting machining time reduction is as high as 60% in hard turning [4]. The turning of hardened components are used in many applications such as gears, shafts, bearings, cams, forgings, dies and molds, which significantly reduce the manufacturing costs, lead times and improve overall product quality [5–8]. The hard turning is generally performed without a coolant using ceramics and cubic boron nitride (CBN) cutting tools due to the required tool material hardness. The cutting tools required for hard turning are relatively expensive as compared to grinding operations and hence there is a need to investigate the tool life to assure the economic justification for hard turning. As reported by Byrne et al. [9] and Klocke et al. [10], the hard turning can provide a

* Corresponding author. Tel.: +91 836 2378272; fax: +91 836 2374985. E-mail addresses: [email protected] (V.N. Gaitonde), [email protected] (S.R. Karnik), [email protected] (J. Paulo Davim). 1 Tel.: +91 836 2378312; fax: +91 836 2374985. 2 Tel.: +351 234 401566; fax: +351 234 370953. 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.12.007

relatively high accuracy for hardened components but the problems occur with surface finish and tool wear. The hard turning may induce deep compressive residual stress in the subsurface, while it may also produce white layer on the component surface [11]. Huang and Liang [12] reported that the properties and composition of tool materials are crucial to the behavior of machining forces, which in turn may affect the surface finish and tool life. Chou and Evans [13], Chou et al. [14], Thiele and Melkote [15], Thiele et al. [16] and Ozel et al. [17] identified various factors affecting cutting forces, surface roughness, tool wear and surface integrity by conducting several experiments in hard turning of various grades of steels using CBN tools. The effect of microstructure of hardened steels on tool wear mechanisms was studied by Poulachon et al. [18]. EI-Wardany et al. [19,20] investigated the effects of cutting conditions and tool wear on chip morphology during high speed turning of AISI D2 work tool steel and further studied the quality and integrity of the machined surfaces. Kishawy and Elbestawi [21] described the tool wear characteristics and surface integrity during hard-speed turning of AISI D2 cold work tool steel. Chou and song [22] stated that a large tool nose radius gives better surface finish but generates deeper white layers in finish turning of AISI 52100 bearing steel using alumina titanium-carbide tools. Benga and Abrao [23] and Kumar et al. [24] observed a better surface quality in turning of hardened steel components using aluminaTiC ceramic tools. Lima et al. [25] analyzed the of effects of cutting speed, feed rate and depth of cut on cutting forces, tool wear and sur-

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face roughness in hardened AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel materials. The detailed 2D and 3D analysis of surface finish in hard turning was presented by Grzesik and Wanat [26,27] and Klocke and Kratz [28]. Grzesik [29] reported the surface roughness characterization during hard turning operations with conventional as well as wiper ceramic cutting tools at variable feed rates. Schwach and Guo [30] investigated the surface topography, surface roughness, micro-hardness, subsurface microstructure and residual stresses of turned AISI 52100 components. Pavel et al. [31] observed the effect of tool wear on surface finish in interrupted and continuous hard turning. The best tool material and tool cutting edge micro geometry to turn continuous, semi interrupted and interrupted surfaces of AISI 4340 hardened steel in terms of tool wear and tool life was found by Diniz and Oliveira [32]. Ozel and Karpat [33] used regression and artificial neural network models for predicting the surface roughness and tool wear in hard turning of AISI H13 steel using CBN inserts. Quiza et al. [34] also compared the statistical models with neural network models for predicting the tool wear in hard turning of AISI D2 steel using conventional ceramic inserts. Davim and Figueira [35] investigated the influence of cutting speed and feed rate on flank wear, specific cutting pressure and surface roughness in hard turning of AISI D2 cold work tool steel with conventional ceramic inserts using statistical techniques. The multi-radii tool nose shaping, known as wiper geometry has been introduced for substantial improvement of surface finish in machining of hardened components using ceramic inserts [36,37]. As can be seen from the literatures, no systematic study has been reported on the machinability aspects in hard turning process. Further, the effects of depth of cut and machining time on machinability characteristics during turning of hardened components are also not studied. Hence, an attempt has been in this paper to investigate the effects of depth of cut and machining time during turning of hardened AISI D2 cold work tool steel using both conventional and wiper ceramic inserts on various aspects of machinability such as machining force, power, specific cutting force, surface roughness and tool wear by developing second order mathematical models based on response surface methodology (RSM). The process modeling by RSM using design of experiments is proved to be an efficient modeling tool [38]. The RSM not only reduces the cost and time but also gives the required information about the direct and interaction effects of process parameters. 2. Experimental procedure 2.1. Planning of experiments To develop the mathematical model based on RSM, a careful planning of experimentation is very much essential. In the present study, depth of cut and machining time are identified as the factors, which affect the responses such as machining force, power, specific cutting force, surface roughness and tool wear. Three levels are defined for each of the factors and the ranges of the factors were selected based on preliminary experiments. Thus nine trials based on full factorial design of experiments (FFD) were planned [38]. The factors and their levels in the present investigation are presented in Table 1 and the experimental layout plan as per FFD for the present investigation is given in Table 2.

Table 2 Experimental layout plan as per FFD. Trial no.

Levels of input parameters

Actual setting values of input parameters

d

t

d (mm)

t (min)

1 2 3 4 5 6 7 8 9

1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3

0.2 0.2 0.2 0.4 0.4 0.4 0.6 0.6 0.6

5 10 15 5 10 15 5 10 15

2.2. Experimentation and machinability evaluation The high chromium AISI D2 cold work tool steel was used as a work material, the chemical composition is illustrated in Table 3. The average hardness of 59/61 HRC was maintained through a heat treatment process (quenching in a vacuum atmosphere at 1000–1040° C). In order to assure the required stiffness of chuck/ workpiece/cutting tool system, the ratio of cylindrical turning length to the initial diameter of workpiece was approximately kept as 4. The ceramic inserts with TiN coating of ISO code-CNGA 120408 T01020 were used in the current study. Fig. 1 shows 3 different types of ceramic inserts, namely, conventional CC650 and wiper (multiple point radii) of CC650WG and GC6050WH, which were used in the present investigation. The mixed alumina insert has a chemical composition of Al2O3 (70%) and TiC (30%) with tool geometry as follows: rake angle: 6°; clearance angle: 6°; major cutting edge angle: 95° and cutting edge inclination angle: 6°. The ‘DCLNL2020K12’ (ISO) type tool holder was employed throughout the work. CNC lathe of type ‘Kingsbury MHP 50’ was employed to conduct the experiments. The CNC lathe is equipped with 18 kW spindle power and a maximum spindle speed of 4500 rpm. The cutting speed of 80 m/min and feed rate of 0.10 mm/rev was kept constant through out the investigation. The experiments were performed as per FFD and the trials were randomized. Three different components of forces, namely, cutting force (Fc), feed force (Ff) and depth force (Fd) were measured through a ‘KistlerÒ’ piezoelectric dynamometer (model 9121). The values were monitored continuously and recorded through a three-channel charge amplifier (model 5019) with data acquisition. The ‘HommelwerkeÒ T1000’ type profilometer was used for the measurement of surface roughness on turning surfaces, with a cut off of 0.8 mm in accordance to ISO/DIS 4287/1E. Each surface roughness measurement was repeated thrice and the average was taken as an arithmetic surface roughness (Ra). The flank tool wear was evaluated by a ‘MitutoyoÒ TM-500’ tool makers’ microscope with 30 magnification and 1 lm resolution. The admissible wear was established according to ISO 3685 standard (1993) and measured at corner radius (VBC). Due to lower depths of cut (0.2–0.8 mm) compared to tool nose radius (0.8 mm), VBC is used instead of VBB. The machining force (Fm), power (P) and specific cutting force (Ks) are determined from the following equations:

Table 1 Factors and levels. Factor

Depth of cut (d), mm Machining time (t), min

Level 1

2

3

0.2 5

0.4 10

0.6 15

Table 3 Chemical composition of AISI D2 work material (wt%). C

Si

Mn

Cr

Mo

V

1.55

0.30

0.40

11.80

0.8

0.8

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Fig. 1. Ceramic inserts used in the experimentation.

Table 4 Experimental results for AISI D2 cold work tool steel machining with CC650, CC650WG and GC6050WH ceramic inserts. Trial no.

1 2 3 4 5 6 7 8 9

Fm ¼

Machining force, Fm (N)

Power, P (kW)

CC650

CC650

135.22 223.67 244.44 222.13 322.47 352.56 142.06 264.44 394.31

CC650

GC60

WG

50WH

181.48 249.79 289 349.39 414.22 485.68 393.04 551.12 881.99

272.33 262.7 277.99 279.23 396.9 362.26 399.21 540.34 810.52

0.11 0.16 0.18 0.18 0.24 0.25 0.08 0.18 0.27

CC650

GC60

WG

50WH

0.13 0.17 0.19 0.26 0.27 0.29 0.35 0.37 0.51

0.18 0.18 0.19 0.19 0.3 0.29 0.3 0.42 0.63

Specific cutting force, Ks (MPa)

Surface roughness, Ra (microns)

Tool wear, VC (mm)

CC650

CC650

CC650

CC650 WH

50WH

0.086 0.134 0.19 0.06 0.097 0.13 0.07 0.163 0.249

0.053 0.083 0.113 0.078 0.108 0.147 0.095 0.152 0.178

0.077 0.111 0.164 0.045 0.082 0.107 0.041 0.077 0.178

3951.91 6149.04 6667.59 3446.71 4575.2 4710.34 1035.78 2219.41 3349.97

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi F 2c þ F 2f þ F 2d

P ¼ Fcv Fc Ks ¼ f d

CC650

GC60

WG

50WH

4884.21 6299.24 7308.82 4939.46 5028.03 5485.92 4345.32 4657.04 6406.29

6803.49 6876.04 7218.18 3559.46 5542.4 5449.47 3748.04 5281.68 7922.52

0.39 0.52 0.57 0.63 0.95 1.34 0.86 1.33 1.94

CC650

GC60

WG

50WH

0.17 0.23 0.26 0.4 0.56 0.52 0.5 0.47 0.82

0.53 0.57 0.55 0.3 0.4 0.39 0.43 0.58 0.88

2

GC60

ð1Þ

Y ¼ b0 þ b1 d þ b2 t þ b12 dt þ b11 d þ b22 t2

ð2Þ

where Y is the desired response and b0, b1,. . . b22: regression coefficients of polynomial equation to be determined for each response. The regression coefficients of linear, quadratic and interaction terms of mathematical models are determined by [38]:

ð3Þ

where, d is the depth of cut. The computed values of machining force (Fm), power (P), specific cutting force (Ks) and measured values of surface roughness (Ra) and tool wear (VC) are given in Table 4.

b ¼ ðX T XÞ1 X T Y

ð4Þ

ð5Þ

where, b is the matrix of process parameter estimates; X: is the calculation matrix, which includes linear, quadratic and interaction terms; XT is the transpose of X and Y is the matrix of desired response. The mathematical models determined by multiple regression analysis for predicting the machining force, power, specific cutting force, surface roughness and tool wear during hard turning of AISI D2 cold work tool steel using different ceramic inserts are given by:

3. Development of RSM based machinability models In the present investigation, the second order RSM based mathematical models for machining force (Fm), power (P), specific cutting force (Ks), surface roughness (Ra) and tool wear (VC) were developed with depth of cut (d) and machining time (t) as the process parameters. The response surface equation considering two factor interactions is given by [38]:

CC650 2

F m ¼ 120:297 þ 1107:592 d þ 19:48567t þ 35:7575 dt  1625:75d  0:8696t 2 2

P ¼ 0:08333 þ 0:966667 d þ 0:011t þ 0:03 dt  1:5d  0:0006t 2

ð6Þ ð7Þ

2

K s ¼ 2224:798  496:425 d þ 613:2657t  100:372 dt  8711:67d  18:1667t 2

Ra ¼ 0:252222 þ 0:725 d  0:04167t þ 0:225 dt  0:95833d þ 0:000867t

2

2

ð8Þ ð9Þ

2

VC ¼ 0:239333  1:1875 d þ 0:004667t þ 0:01875 dt þ 1:3255d  0:00002t 2

ð10Þ

CC650WG 2

F m ¼ 300:3844  191:475d  33:7603t þ 95:3575dt þ 199:3333d þ 1:002133t 2 2

P ¼ 0:157778 þ 0:1d  0:01633t þ 0:025dt þ 0:333333d þ 0:000733t 2

ð12Þ

2

K s ¼ 6912:646  11640:9d þ 17:24533t  90:91dt þ 12475:42d þ 9:342667t 2

Ra ¼ 0:08667 þ 2:066667d  0:02533t þ 0:0575dt  2:125d þ 0:001t 2

ð11Þ

2

VC ¼ 0:00222 þ 0:0625d þ 0:0077t þ 0:00575dt þ 0:033333d  0:00015t 2

2

ð13Þ ð14Þ ð15Þ

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V.N. Gaitonde et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 754–763

GC6050WH 2

F m ¼ 597:8922  1854:28d  24:1197t þ 101:4125dt þ 2026:292d þ 0:011067t2

ð16Þ

2

P ¼ 0:382222  1:26667d  0:01467t þ 0:08dt þ 1:416667d  0:00013t2

ð17Þ

2

K s ¼ 13208:81  41845d  66:7957t þ 939:9475dt þ 36447:04d  4:66053t 2

Ra ¼ 1:422222  5:40833d  0:02167t þ 0:1075dt þ 5:666667d  0:00013t

2

ð18Þ

2

ð19Þ

2

VC ¼ 0:211333  0:77167d  0:00507t þ 0:0125dt þ 0:75d þ 0:00048t 2

ð20Þ

Table 5 Results of ANOVA of machinability models for CC650 ceramic insert. Machinability characteristic

Machining force Power Specific cutting force Surface roughness Tool wear *

Sum of squares

Degrees of freedom

Mean square

F-ratio

Regression

Residual

Regression

Residual

Regression

Residual

61345 0.0304667 24512764 2.02361 0.0286569

1494 0.0013333 743919 0.00634 0.0017251

5 5 5 5 5

3 3 3 3 3

12269 0.0060933 4902553 0.40472 0.0057314

498 0.0004444 247973 0.00211 0.0005750

24.64* 13.71* 19.77* 191.37* 9.97*

Significant at 95% confidence interval.

Table 6 Results of ANOVA of machinability models for CC650WG ceramic insert. Machinability characteristic

Machining force Power Specific cutting force Surface roughness Tool wear * **

Sum of squares

Degrees of freedom

Mean square

F-ratio

Regression

Residual

Regression

Residual

Regression

Residual

331073 0.105211 6445224 0.28856 0.0128160

12611 0.003944 1244618 0.03204 0.0001489

5 5 5 5 5

3 3 3 3 3

66215 0.021042 1289045 0.05771 0.0025632

4204 0.001315 414873 0.01068 0.0000496

15.75* 16.00* 3.11*** 5.41** 51.66*

Significant at 95% confidence interval. Significant at 90% confidence interval. Significant at 75% confidence interval.

***

Table 7 Results of ANOVA of machinability models for GC6050WH ceramic insert. Machinability characteristic

Machining force Power Specific cutting force Surface roughness Tool wear *

Sum of squares

Degrees of freedom

Mean square

F-ratio

Regression

Residual

Regression

Residual

Regression

Residual

242287 0.170978 17403078 0.210869 19.9171

11992 0.005378 962044 0.013353 0.1212

5 5 5 5 5

3 3 3 3 3

48457 0.034196 3480616 0.042174 3.9834

3997 0.001793 320681 0.004451 0.0404

12.12* 19.08* 10.85* 9.48* 98.64*

Significant at 95% confidence interval.

Table 8 R2 values of machinability models.

Table 9 % Prediction error of machinability models for the experimental data.

Machinability characteristic

CC650

CC650WG

GC6050WH

Machinability characteristic

CC650

CC650WG

GC6050WH

Machining force Power Specific cutting force Surface roughness Tool wear

0.9762 0.9581 0.9706 0.9969 0.9432

0.9633 0.9639 0.83815 0.9000 0.9885

0.9528 0.9695 0.9476 0.9404 0.9267

Machining force Power Specific cutting force Surface roughness Tool wear

4.33 5.81 7.20 3.20 13.60

8.83 7.19 6.02 10.77 3.24

7.59 6.49 5.04 6.88 12.30

V.N. Gaitonde et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 754–763

Experimental Predicted

3

4

5 6 Trial no.

7

8

6000 4000 Experimental

2000

Predicted

0

Tool:CC650WG Experimental

800

2

3

4

Predicted

600 400

0 3

4

5 6 Trial no.

7

8

9

Tool:GC6050WH 1000 Experimental

800

Predicted

600

8

9

7

8

9

7

8

9

6000 4000 Experimental 2000

Predicted

0 1

2

7

Tool:CC650WG

8000

2

3

4

200

1

5 6 Trial no.

9

Specific cutting force (MPa)

2

1000

Machining force (N)

Tool:CC650

8000

1 1

Machining force (N)

Specific cutting force (MPa)

Tool:CC650

450 400 350 300 250 200 150 100 50 0

Specific cutting force (MPa)

Machining force (N)

758

5 6 Trial no.

Tool:GC6050WH

10000 8000 6000 4000

Experimental

2000

Predicted

0 1

400

2

3

4

5 6 Trial no.

200 Fig. 4. Experimental and RSM predicted values of specific cutting force.

0 1

2

3

4

5 6 Trial no.

7

8

9

Fig. 2. Experimental and RSM predicted values of machining force.

Tool:CC650

Experimental

0.25 Power (kW)

Surface roughness (microns)

Tool:CC650 0.3

Predicted

0.2 0.15 0.1 0.05 0 1

2

3

4

5

6

7

8

2.5 Experimental

2

Predicted

1.5 1 0.5 0 1

9

2

3

4

Trial no.

Predicted

Surface roughness (microns)

Power (kW)

Experimental

0.4 0.3 0.2 0.1 0 1

2

3

4

5

6

7

8

0.8

Experimental

0.6

Predicted

1

Surface roughness (microns)

Power (kW)

0.4 0.2 0 3

4

5

6

8

9

7

8

9

0 2

3

4

7

5 6 Trial no.

Tool:GC6050WH

Predicted

2

7

0.2

9

Experimental

1

9

0.4

Tool:GC6050WH

0.6

8

1

Trial no.

0.8

7

Tool:CC650WG

Tool:CC650WG 0.6 0.5

5 6 Trial no.

8

9

Trial no.

Fig. 3. Experimental and RSM predicted values of power.

1

Experimental

0.8

Predicted

0.6 0.4 0.2 0 1

2

3

4

5 6 Trial no.

Fig. 5. Experimental and RSM predicted values of surface roughness.

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where, d in mm; t in min; Fm in N, P in kW; Ks in MPa; Ra in microns; VC in mm. The analysis of variance (ANOVA) has been applied to check the adequacy of the developed machinability models [38]. The ANOVA table consists of sum of squares and degrees of freedom. The sum of squares is performed into contributions from the polynomial model and the experimental error. The mean square is the ratio of sum of squares to degrees of freedom and F-ratio is the ratio of mean square of regression model to the mean square of the experimental error. As per ANOVA, the calculated value of F-ratio of developed model should be more than F-table for the model to be adequate for a specified confidence interval. The results of ANOVA for machinability models using CC650, CC650WG and GC6050WH ceramic inserts are summarized in Tables 5–7 respectively. The goodness of fit of the developed machinability models was also tested by computing the coefficient of determination (R2). The R2is the proportion of variation in the dependent variable explained by the polynomial model. Table 8 gives the R2 value of developed machinability models, which indicate a very good correlation between the experimental and the predicted values of machinability aspects. The Eqs. (6)–(20) are used to test the accuracy of the developed machinability models using the experimental data of FFD. The% prediction accuracy of the model is given by:

The developed machinability models (Eqs. (6)–(20) are used to predict the machining force (Fm), power (P), specific cutting force (Ks), surface roughness (Ra) and tool wear (VC) by substituting the values of depth of cut (d) and machining time (t) within the ranges of the process parameters selected. The variations of machining force, power, specific cutting force, surface roughness and tool wear are exhibited in Figs. 7–11, respectively. Fig. 7 illustrates the variation of machining force with depth of cut at different values of machining time. It is observed that, in case of hard turning of AISI D2 cold work tool steel with CC650WG and GC6050WH wiper ceramic inserts, the machining force almost

Tool : CC650

where, yi,expt is the measured value of response corresponding to i trial; yi,pred is the predicted value of response corresponding to ith

Tool:CC650 0.3 0.25 0.2 0.15 0.1 0.05 0

Machining force (N)

ð21Þ th

5 min 10 min

400

15 min

300 200 100 0

Experimental

0.2

Predicted

0.3 0.4 0.5 Depth of cut (mm)

0.6

Tool : CC650WG 1000 1

2

3

4

5 6 Trial no.

7

8

9

Tool:CC650WG 0.2 0.15

Machining force (N)

Tool wear (mm)

4. Results and discussion

500

   n  100 X yi;exp t  yi;pred  D¼    n i¼1  yi;pred

Tool wear (mm)

trial and n is the number of trials in FFD. The prediction error of the developed machinability models is given in Table 9 and is found to be within 14%. The comparison of the experimental and the predicted values of Fm, P, Ks, Ra and VC for the experimental data of FFD during hard turning of AISI D2 cold work tool steel with different ceramic inserts are depicted in Figs. 2–6, respectively.

0.1

5 min

800

10 min

600

15 min

400 200 0

Experimental

0.05

0.2

Predicted

0.3 0.4 0.5 Depth of cut (mm)

0.6

0 2

3

4

5 6 Trial no.

7

8

9

Tool wear (mm)

Tool:GC6050WH 0.2

Experimental

0.15

Tool : GC6050WH 1000

Machining force (N)

1

Predicted

0.1 0.05

5 min

800

10 min

600

15 min

400 200 0

0 1

2

3

4

5 6 Trial no.

7

8

Fig. 6. Experimental and RSM predicted values of tool wear.

9

0.2

0.3 0.4 0.5 Depth of cut (mm)

0.6

Fig. 7. Effect of depth of cut and machining time on machining force.

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V.N. Gaitonde et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 754–763

Tool : CC650 0.3

Power (kW)

0.25 0.2 0.15

5 min

0.1

10 min

0.05

and GC6050WH wiper ceramic inserts. Further, it is observed that the specific cutting force increases with decrease in depth of cut in case of hard turning of AISI D2 cold work tool steel with CC650 and CC650WG ceramic inserts. The reason might be at lower values depth of cut the shear model does not fit adequately to the chip formation process, as the material is subjected to lower strain rates and hence specific cutting force increases. It is also seen from figure that, hard turning with GC6050WH wiper ceramic insert machining, the specific cutting force increases beyond 0.45 mm depth of cut. The interaction effect due to depth of cut and machining time on surface roughness is given in Fig. 10. The surface roughness exhibits a linear relationship with depth of cut in case of hard turning with CC650 conventional ceramic insert, whereas it follows almost a non-linear trend in case of CC650WG and GC6050WH wiper ceramic inserts. It is seen that, the minimum surface roughness occurs at lower values of depth of cut and machining time and vice versa in case of CC650 and CC650WG inserts. This is because, an increase in depth of cut increases the thrust force, generating more heat and thus resulting in higher surface roughness. It is also ob-

Specific cutting force (MPa)

linearly increases with depth of cut. The increase in machining force is probably due to increase in contact area between the workpiece and cutting tool with increase in depth of cut. But in case of hard turning with CC650 conventional ceramic insert, the machining force initially increases with increase in depth of cut up to 0.45 mm and then decreases with increase in depth of cut beyond 0.45 mm. Further, it is also revealed that the machining force is highly sensitive to machining time at higher values of depth of cut for all the ceramics tested. Fig. 8 depicts the estimated power in relation to process parameters. It is seen from the comparison plots that the interaction effect of depth of cut and machining time on power is almost the same as that of the machining force for all the ceramic inserts. At slower depth of cut, there is a small resistance to cutting tool, while at the larger depth of cut; the work material offers more resistance to cutting tool increasing the friction. Hence, the cutting force increases due to increase in friction, which in turn increases the power. The variation of depth of cut with machining time on specific cutting force is presented in Fig. 9. The comparison reveals that, in case of CC650 conventional ceramic insert, the specific cutting force is highly sensitive to depth of cut as compared to CC650WG

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Depth of cut (mm) Fig. 8. Effect of depth of cut and machining time on power.

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Fig. 9. Effect of depth of cut and machining time on specific cutting force.

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served from figure that in case of hard turning with GC6050WH insert, the surface roughness is minimal at a center level of 0.4 mm depth of cut. Further, it can also be seen from this figure that the influence of machining time on surface roughness is less at lower values of depth of cut. Fig. 11 shows the behavior of tool wear in relation to the process parameters. It is observed that there exists a synergetic interaction due to depth of cut and machining time on tool wear for AISI D2 machining with all the ceramic inserts tested. The tool flank wear linearly increases with depth of cut for all values of machining time selected in the range 5–15 min in case of CC650WG inserts. The increase in tool flank wear is probably due to abrasion at the rake face of the cutting tool as the machining time progresses. Further, it is observed that in case of CC650 and GC6050WH ceramic inserts the tool wear initially decreases with increase in depth of cut up to 0.4 mm and then increases. Fig. 12 shows the tool wear of GC6050WH ceramic insert observed with scanning electron microscopy (SEM) after the machining test for a depth of cut of 0.2 mm and a machining time of 15 min. The flank tool wear presents the grooves generated

Tool wear (mm)

V.N. Gaitonde et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 754–763

0.15 0.1 0.05 0 0.2

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Fig. 11. Effect of depth of cut and machining time on tool wear.

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Fig. 10. Effect of depth of cut and machining time on surface roughness.

Fig. 12. SEM image of tool wear of GC6050WH ceramic insert after machining (d = 0.2 mm and t = 15 min).

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Fig. 13. Example of EDS analysis in the flank face of GC6050WH ceramic insert.

through abrasive mechanism and adhesion of material, confirmed through energy dispersive X-ray spectroscopy (EDS) analysis as illustrated in Fig. 13. The presence of Fe and Cr is clearly evident in the material deposited in the flank wear area of ceramic tool face. From the above discussions on the interaction effects, it is clear that for a specified machining time, the effect of depth of cut variation is different for various aspects of machinability and further depends on type of inserts employed. For all the inserts tested, the influence of variations in machining time on all machinability characteristics is comparatively less at lower values of depth of cut. It is worth mentioning here that low machining time is essential to improve all the machinability characteristics, which are considered in the present investigation. However, different value of depth of cut is required to minimize each of these aspects. It is also important to note that, the value of depth of cut required to minimize one machinability characteristic may not be suitable to minimize the other aspect. A comparison study reveals that, the performance of CC650 conventional ceramic insert is better as compared to both CC650WG and GC6050WH wiper ceramic inserts with reference to machining force, power and specific cutting force. However, the CC650WG wiper ceramic insert is very useful in reducing tool wear and surface roughness as compared to other two inserts. Hence, a trade–off is inevitable for the selection of both ceramic insert as well as depth of cut, which depends on the priority of the machinability characteristic. 5. Conclusions In the present paper, an investigative study on machinability aspects of machining force (Fm), power (P), specific cutting force (Ks), surface roughness (Ra) and tool wear (VC) has been carried out during hard turning of high chromium AISI D2 cold work tool steel with CC650, CC650WG and GC6050WH ceramic inserts. The response surface methodology (RSM) based mathematical models were developed to analyze the effects of depth of cut (d) and machining time (t) on machinability. The experiments were planned as per full factorial design (FFD) and the adequacy of the models was tested through analysis of variance (ANOVA). Based on the experimental results and parametric analysis, the following conclusions are drawn.

 The machining force and power are highly sensitive to machining time at higher values of depth of cut. Both machining force and power linearly increase with depth of cut for machining with CC650WG and GC6050WH wiper ceramic inserts. On the other hand, AISI D2 cold work tool steel machining with CC650 conventional ceramic insert, the machining force and power both increase with increase in depth of cut up to 0.45 mm and then suddenly decrease beyond 0.45 mm.  The specific cutting force is highly sensitive to depth of cut in case of hard turning with CC650 conventional insert as compared to CC650WG and GC6050WH wiper ceramic inserts. The specific cutting force decreases with increase in depth of cut in case of CC650 and CC650WG inserts machining, while the specific cutting force increases beyond 0.45 mm depth of cut in case of GC6050WH machining.  The surface roughness is minimal at lower values of depth of cut and machining time in case of CC650 and CC650WG inserts machining, while the minimum surface roughness occurs at 0.4 mm depth of cut for hard turning with GC6050WH insert.  The tool wear decreases with increase in depth of cut up to 0.4 mm and then suddenly increases in case of CC650 and GC6050WH inserts. On the other hand, the tool wear increases linearly with depth of cut in case of CC650WG insert.  The CC650WG wiper ceramic insert performs better with reference to surface roughness and tool wear, while CC650 conventional ceramic insert is useful in minimizing the machining force, power and specific cutting force. References [1] Konig W, Berktold A, Koch KF. Turning versus grinding–a comparison of surface integrity aspects and attainable accuracies. Ann CIRP 1993;42(1):39–43. [2] Tonshoff HK, Arendt C, Ben Amor R. Cutting of hardened steel. Ann CIRP 2000;49(2):547–66. [3] Tonshoff HK, Wobker HG, Brandt D. Tool wear and surface integrity in hard turning. Prod Eng 1996;3(1):19–24. [4] Tonshoff HK, Wobker HG, Brandt D. Hard turning–Influence on the workpiece properties. Trans NAMRI/SME 1995;23:215–20. [5] Zou JM, Anderson M, Stahl JE. Identification of cutting errors in precision hard turning process. J Mater Process Tech 2004;153–154:746–50. [6] Rech J, Moisan A. Surface Integrity in finish hard turning of case hardened steels. Int J Mach Tool Manuf 2003;43:543–50. [7] Destefani J. Technology key to mold making success. Manuf Eng 2004;133(4):59–64.

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