Machinability evaluation in hard turning
of AISI 4340 steel with different cutting
tools using statistical techniques

U

¸Cayda¸s

Department of Mechanical Education, Technical Education Faculty, Firat University, Elazig 23119, Turkey. email:
[e-mail]

The manuscript was received on 15 September 2009 and was accepted after revision for publication on 25 November 2009.

DOI: 10.1243/09544054JEM1822

Abstract: This paper is concerned with the continuous turning of AISI 4340 steels with
hardness values in the range between 360 and 460 HV using cubic boron nitride, ceramic, and
P10 grade carbide tools. The objective of this paper is to study the performance of these cutting
tools and the effect of workpiece hardness on machinability responses. The samples were oil
quenched and tempered at 250, 350, and 500

C for 1 h in order to obtain different hardness

values. Surface roughness, tool flank wear, maximum tool–chip interface temperature, and
microhardness variations after machining are assessed. A Taguchi orthogonal array was used to
guide the experiments. Cutting speed, feed rate, depth of cut, workpiece hardness, and tool
types were varied in the experiments. A detailed statistical investigation was made by using
analysis of variance, Tukey–Kramer comparison, and correlation tests. The best combination of
cutting parameters for determining optimal conditions and the influence of each factor on the
response were revealed and are discussed in detail.

Keywords: hard turning, statistical analysis, cubic boron nitride tools, AISI 4340 steel

1 INTRODUCTION

AISI 4340 steels with hardness values between 28 and
38 HRC are extensively used in industry. High
strength low alloy (HSLA) steel is very cheap, com-
pared to expensive high alloy steels, and it has an
appropriate hardness value combined with a very
high toughness and tensile strength. For use in cer-
tain applications, HSLA steels must be austenized,
quenched, and tempered using a a time–temperature
cycle so that they achieve a hardness greater than 52
HRC and an acceptable toughness [1]. Normally,
parts with a hardness greater than 52 HRC are
machined using a grinding process. However, grind-
ing has the drawbacks that it requires the use of a
coolant and is a time-consuming process. Hence,
more environmentally friendly processes are desired.
Hard turning has emerged as a possible replacement
for grinding for certain applications since it offers
many benefits including lower equipment costs, a
shorter set-up time, a reduced number of process
steps, and better surface integrity [2]. Since high-
hardness workpieces are involved in finish hard

turning, a high toughness and wear resistance are
required for the tools. Cemented carbide tools are not
suitable for hard turning since their toughness and
wear resistance levels deteriorate rapidly at high
temperatures [3]. Therefore, hard turning is per-
formed using ceramics and polycrystalline cubic
boron nitride (PCBN, more commonly CBN) cutting
tools. Ceramics tend to possess a high melting point,
excellent hardness levels, and good wear resistance
and CBN has an excellent fracture toughness beha-
viour [4]. The performance of ceramics and CBN
cutting tools and the quality of the machined sur-
faces are dependent on the cutting conditions, i.e.
cutting speed, feed rate, depth of cut, workpiece
hardness, machining time, and tool nose radius,
which significantly influence the surface roughness,
white layer formation, cutting temperature, and tool
wear [5–7].

1.1 Studies on the turning of hard materials

Various studies have been conducted to investigate
the performance of CBN and ceramic tools in the

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machining of various hard materials. Luo et al. [8]
studied wear behaviour in the turning of AISI 4340
hardened alloy steels using CBN and ceramic tools. It
was reported that the main wear mechanism for CBN
tools was abrasion whereas the adhesion and abra-
sion were dominant for ceramic tools. Additionally,
tool life was increased with the cutting speed for both
cutting tools due to the formation of a protective layer
on the tool–chip interface. An increase in workpiece
hardness lead to lower wear rates. Yallase et al. [9]
experimentally investigated the behaviour of CBN
tools during the hard turning of AISI 52100 tempered
steel. The surface quality obtained with the CBN tool
was significantly better than that obtained using
grinding. A relationship between flank wear and sur-
face roughness was deduced from a parametric ana-
lysis based on extensive experimental data.

Davim and Figueira [10] carried out an experimental

investigation using ceramic tools, composed approxi-
mately of 70 pet cent Al

2

O

3

and 30 per cent TiC, in

surface finish operations on AISI D2 cold work tool
steel, heat treated to a hardness of 60 HRC. A com-
bined technique using an orthogonal Taguchi array
and analysis of variance (ANOVA) was used in the
study. The test results showed that it was possible to
achieve surface roughness levels as low as R

a

< 0.8 mm

with an appropriate choice of cutting parameters that
eliminated cylindrical grinding. Lima et al. [11] eval-
uated the machinability of hardened AISI 4340 and D2
steels at different levels of hardness by using a range of
cutting tool materials. The 4340 steels were hardened
to 42 and 48 HRC and turned by using coated carbide
and CBN inserts. The results indicated that when AISI
4340 steel was turned using low feed rates and depth of
cut, higher cutting forces were recorded and lower
surface roughness values were observed for softer
workpiece materials as cutting speed was elevated and
they deteriorated with feed rate.

Quiza et al. [12] presented an investigation into

predicting ceramic cutting tool wear in hard
machining of AISI D2 steel using neural networks.
Two models were adjusted to predict tool wear for
different values of cutting speed, feed rate, and
machining time, one of them was based on statistical
regression and the other was based on a multilayer
perception neural network. The neural network
model had a better performance than the regression
model in its ability to make accurate predictions of
tool wear. Tamizharasan et al. [13] analysed the tool
life, tool wear, material removal rate, and economy of
hard turning of a hardened petrol engine crank pin
material by using three grades of CBN tools.

Oliviera et al. [14] investigated the hard turning of

AISI 4340 steel with a 56 HRC hardness value in
continuous and interrupted cuts with PCBN and
whisker-reinforced cutting tools. The results indi-
cated that, the longest tool life in continious turning,

was achieved by the PCBN tool; however, similar tool
life values were obtained in interrupted turning for
both the PCBN and ceramic tools. In terms of
roughness, PCBN showed better results.

Jiang et al. [15] reported a study that addressed

the surface morphology, surface roughness, coating
cross-section, chemical composition, crystal structure,
microhardness, adhesion, and wear life of CBN-based
coating deposition on carbide inserts (SNMG 120408)
for finish hard turning of hardened AISI 4340 steel
bars. The surface quality of the machined workpieces
in terms of their surface roughness and white layer
formation were also analysed and results were pre-
sented. Aslan [16] explored the performance and wear
behaviour of TiN-coated tungsten carbide, TiCN

þ

TiAlN-coated tungsten carbide, TiAlN-coated cermet,
a mixed ceramic of Al

2

O

3

þ TiCN and CBN tools in

high-speed cutting of AISI D3 cold work tool steel
hardened to 62 HRC. The CBN tool exhibited the best
cutting performance in terms of both flank wear and
surface finish. Additionally, the highest volume of
material removal was obtained with the CBN tool.

Grzesik [17] reported an extensive characterization

of the surface roughness generated during hard
turning operations of AISI 5140 (60 HRC) steels with
conventional and wiper ceramic tools at variable
feed rate conditions. Gaitonde et al. [18] attempted to
analyse the effects of depth of cut and machining
time on machinability aspects such as machining
force, power, specific cutting force, surface rough-
ness, and tool wear by using second-order mathe-
matical models during the turning of high chromium
content AISI D2 cold work tool steel with CC650,
CC650WG, and GC6050WH ceramic inserts. All the
ceramic tools had the same chemical composition of
70 per cent Al

2

O

3

and 30 per cent TiC. In the para-

metric analysis, the CC650WG wiper insert per-
formed better with respect to surface roughness and
tool wear, whereas the CC650 conventional insert
was useful in reducing the machining force, power,
and specific cutting force.

Arsecularatne et al. [19] performed an experimental

investigation on the machining of AISI D2 steel of
hardness 62 HRC with PCBN tools. It was reported that
the most feasible feeds and speeds fall in the ranges
from 0.08 to 0.20 mm/rev and from 70 to 120 m/min
respectively, and that the tested PCBN tools failed
mainly due to flank wear. Kumar et al. [20] conducted
machining studies on hardened martensitic stainless
steel (60 HRC) to analyse the effect of tool wear on tool
life of alumina ceramic cutting tools. Tool wear such as
flank wear, crater wear, and notch wear were noted.
Multiple regression models were developed to predict
the tool wear mechanisms.

More et al. [21] experimentally investigated the

effects of cutting speed and feed rate on tool wear,
surface roughness, and cutting forces in the turning

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of AISI 4340 hardened steel using CBN–TiN-coated
carbide inserts and analysed the cutting conditions
using the ANOVA technique for maximum tool life. In
addition, a cost analysis based on the total machining
cost per part was performed for the economic com-
parison of CBN–TiN-coated and PCBN inserts.

Shi and Liu [22] developed a finite element model

to predict chip formation and phase transformations
in orthogonal hard machining of hardened 52100
steel (62 HRC) using PCBN tools. It was found that
high temperatures around the secondary shear zone
caused fast re-austenization and martensitic trans-
formation, while other parts of the chips retained the
original tempered martensitic structure. Singh and
Rao [23] conducted an investigation to determine the
effects of cutting conditions and tool geometry on the
surface roughness in finish hard turning of a bearing
steel (52100). Mixed ceramic and TiCN tools with
different nose radii were used. Mathematical models
for the surface roughness were developed using a
response surface methodology.

It is clear from this literature survey that although

numerous studies have been conducted on the per-
formance of CBN and different kinds of ceramic tools
on hard turning of different hardened steels, no sys-
tematic study related to the statistical effects of
turning parameters and other conditions i.e. work-
piece hardness and tool types on machining aspects
of AISI 4340 steel such as surface roughness, tool–
chip interface temperature, and tool flank wear is
available. Therefore, the aim of this paper is to fill this
gap. For this purpose, an orthogonal array, ANOVA,
and Tukey–Kramer statistical tests, box plots and
factor comparisons are made using the Minitab 15
and JMP 5.0 softwares.

2 EXPERIMENTAL DETAILS

2.1 Workpiece material

AISI 4340 alloy steel was chosen as the workpiece
material for this research. AISI 4340 is a medium
carbon, heat-treatable, oil-hardening, low alloy steel.
It has gained wide acceptance in numerous indus-
tries for applications such as shafts, gears, and air-
craft landing gear, and it is chosen for any application
where high strength, fatigue, and creep resistance are
needed, even at elevated temperatures [24]. Solid
bars with a diameter of 25 mm and a length of
130 mm were prepared. The specimens were heat
treated at 870

C (austenization temperature) for

30 min, quenched in oil, then tempered at 250, 350
and 500

C for 1 h. It is well known that after

quenching engineering steels are often unsuitable for
industrial applications because of their higher hard-
ness and brightness. Thus, in this study tempering is

used to gain the required toughness levels, and dif-
ferent tempering temperatures are used to create
different hardness levels to enable the exploration of
the effect of hardness (as a machining condition) on
the machinability of AISI 4340 steel. The nominal
chemical composition of the as-received material is
shown in Table 1, and the heat treatment conditions
and hardness values are listed in Table 2.

2.2 Cutting inserts

Standard grade white ceramics (Al

2

O

3

þ ZrO

2

) with the

designation of DNGN 150708 were commercially
obtained for the dry turning tests. This geometry type
was selected because CBN tools are available that have
a similar geometry allowing the machining results to
be compared under similar cutting conditions. The
CBN tools (TiC

þ Al

2

O

3

) were DNGA 150608. For

comparison purposes, P10 grade carbide tools (inserts:
TNMG 160408, tool geometry: –6, –6, 6, 6, 30, 0, 0.8)
were also used. The CBN and ceramic tool inserts were
mounted on the left-hand tool holders PDNNL 2525-
M15 and SVVCN 2525-M16 respectively.

2.3 Machining experiments and analytical

techniques

The hard turning experiments were performed using
a John Ford T 35 industrial-type CNC lathe which has
a 10 kW spindle power and a maximum spindle speed
of 3500 r/min. The independent variables and their
factor levels are given in Table 3. Normally, a large
number of experiments are needed for five factors,
each varied at three levels. In order to save both costs
and time, a L

27

Taguchi orthogonal array was chosen

(Table 4) to guide the experimental choices. The
cutting time was fixed at 5 min for all test conditions.
The tool–chip interface temperature was recorded

Table 1

Chemical composition of AISI 4340 steel (wt %)

C

Si

Mn

P

S

Cr

Ni

Mo

Fe

0.425 0.343 0.692 0.014 0.007 0.850 1.461 0.220 Balance

Table 2

Heat treatment conditions and hardness of AISI
4340 steels

Heat treatment conditions

Work hardness
(HV)

As-received sample

286

870

C / 30 min / oil quenched

435

870

C / 30 min / oil quenched / tempered at

250

C for 1 h

460

870

C / 30 min / oil quenched / tempered at

350

C for 1 h

435

870

C / 30 min / oil quenched / tempered at

500

C for 1 h

360

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Machinability evaluation in hard turning of AISI 4340 steel

1045

using an IMPAC pyrometer (PortaWin 1.11). The
accuracy of the temperature measurement was

– 1

C

between 250 and 1800

C, and the response time was

0.01 ms. In order to avoid errors during measure-
ments, which could be caused by incorrect emissivity
values, the infrared pyrometer was calibrated by
heating the cutting tools (CBN, ceramic, and P10
carbides) containing a physically embedded ther-
mocouple in a furnace. The average emissivity value
was determined to be 0.85. Figure 1 shows the
pyrometer mounted on the turret of the CNC lathe.
The beam was focused on the tool–chip interface as
shown in the figure. The spot size and the distance
between the pyrometer and focus point were adjus-
ted to 7.5 and 400 mm, respectively. The extent of tool
flank wear was monitored using a Scheer-ST-Tumico
microscope (0.002 mm resolution). The surface
roughness of the machined bars was measured at a
sampling length of 0.8 mm using a Mitutoyo SJ – 201

portable device. A Nicon MA 100 optical microscope
was used to study the microstructures of the samples
after heat treatments. The microhardness of the
machined samples was measured in 100

mm steps

below the surface by using a Future – Tech FM – 700
hardness tester. The applied load and the indentation
time were 100 g and 10 s, respectively.

ANOVA was applied to the experimental data by

using the JMP 5.0 and Minitab R15 statistical soft-
wares to determine the effects of cutting speed, feed
rate, depth of cut, workpiece hardness, and cutting
tool types on the machined surface roughness, tool
flank wear, and tool–chip interface temperature.
Tukey–Kramer test, pair means comparison, and box
plot studies were performed to determine the degree
of difference among the data.

3 RESULTS AND DISCUSSION

3.1 Microstructure

The influence of tempering temperature on the
microstructure of AISI 4340 steel is depicted in Fig. 2.
As shown in Fig. 2(a), the morphology of hardened
(870

C

þ oil quenching) sample consists mostly of

dislocated martensite laths. When the material is
tempered at 250

C for 1 h, the microstructure

becomes generally finer and corresponds to a low-
tempered martensite (Fig. 2(b)). At this temperature,
«-carbide precipitates are found at the martensite
lath. The structure of medium-tempered martensite
at 350

C corresponds to interlath cementite pre-

cipitates (Fig. 2(c)). In this microstructure, a partial
transformation of the

«-carbide to cementite can be

anticipated.

When

the

tempering

temperature

increases up to its highest value (500

C), the micro-

structure becomes coarser and consists of equiaxed
grains of ferrite and martensite (Fig. 2(d)). In brief, at

Table 4

Taguchi experimental design using a L

27

ortho-

gonal array

Experiment

Control factors and levels

Designation

A

B

C

D

E

1

60

0.08

0.1

360

CBN

2

60

0.08

0.1

360

Ceramic

0

3

60

0.08

0.1

360

P10

þ

4

60

0.16

0.2

435

CBN

000

5

60

0.16

0.2

435

Ceramic

0000

6

60

0.16

0.2

435

P10

000 þ

7

60

0.24

0.3

460

CBN

þ þ þ

8

60

0.24

0.3

460

Ceramic

þþþ0

9

60

0.24

0.3

460

P10

þ þ þ þ

10

90

0.08

0.2

460

CBN

0

0 þ

11

90

0.08

0.2

460

Ceramic

0

0 þ 0

12

90

0.08

0.2

460

P10

0

0 þ þ

13

90

0.16

0.3

360

CBN

00

þ

14

90

0.16

0.3

360

Ceramic

00

þ 0

15

90

0.16

0.3

360

P10

00

þ þ

16

90

0.24

0.1

435

CBN

0

þ 0

17

90

0.24

0.1

435

Ceramic

0

þ 00

18

90

0.24

0.1

435

P10

0

þ 0 þ

19

120

0.08

0.3

435

CBN

þ þ 0

20

120

0.08

0.3

435

Ceramic

þþ00

21

120

0.08

0.3

435

P10

þ þ 0 þ

22

120

0.16

0.1

460

CBN

þ 0 þ

23

120

0.16

0.1

460

Ceramic

þ0þ0

24

120

0.16

0.1

460

P10

þ 0 þ þ

25

120

0.24

0.2

360

CBN

þ þ 0

26

120

0.24

0.2

360

Ceramic

þþ00

27

120

0.24

0.2

360

P10

þ þ 0 þ

Table 3

Factors and their respective levels

Symbol Control factor

Level 1 Level 2

Level 3

A

Cutting speed (m/min) 60

90

120

B

Feed rate (mm/rev)

0.08

0.16

0.24

C

Depth of cut (mm)

0.1

0.2

0.3

D

Workpiece hardness

(HV)

360

435

460

E

Tool type

CBN

Ceramic P10 carbide

Fig. 1 Temperature measurement setup

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low tempering temperatures, the microstructures of
samples are oversaturated and strengthened by car-
bon. An increase in tempering temperature leads to
coarse carbides, and therefore, the hardness of sam-
ples become much lower [25], as shown in Table 2.

3.2 Statistical analyses of experimental results

The experimentally measured results are tabulated in
Table 5. Additionally, Fig. 3 shows the performances
of the tools in terms of surface roughness, tool flank
wear, and tool–chip temperature. Box plots of main
surface roughness values that depend on the hard
turning parameters are given in Fig. 4. Additionally
Tukey–Kramer tests on the surface roughness, tool
flank wear, and maximum tool–chip interface tem-
perature are given in Tables 6, 7, and 8, respectively.
In addition, statistical differences between each fac-
tors and their levels for machining outputs are given
in these tables. One can easily find the optimum
machining settings from these tables. These tests are
exact

a-level tests if the sample sizes are the same

and are conservative if the sample sizes are different.
By examining Fig. 4 and Table 6, it is possible to
compare each pair of groups to find how the com-
parison circles intersect. The lowest average surface
roughness values are obtained with CBN tools at
the following settings (2.8581

mm): 0.08 mm/rev feed

rate, 0.1 mm depth of cut, material within hardness of
360 HV, and cutting speed of 120 m/min. The per-

formance of ceramic tools was followed by CBN
tools, and the P10 grade carbide tools gave the

a)

b)

c)

d)

Fig. 2 Influence of tempering temperature on the microstructure of AISI 4340 steel (a) 870

C

þ oil

queching; (b) after quenching, tempered at 250

C for 1 h; (c) after quenching, tempered at 350

C

for 1 h; and (d) after quenching, tempered at 500

C for 1 h

Table 5

Experimentally measured results

Experiment

Surface
roughness
(

mm)

Tool flank
wear (mm)

Maximum
temperature
(

C)

1

0.480

0.062

304.2

2

1.538

0.082

347.8

3

2.046

0.932

381.6

4

2.872

0.100

432.4

5

3.676

0.160

462.2

6

9.304

0.752

503.4

7

1.190

0.068

493.7

8

1.456

0.120

501.8

9

15.958

0.476

527.0

10

1.520

0.130

471.1

11

1.642

0.168

500.8

12

4.118

1.243

514.2

13

1.152

0.146

466.0

14

2.426

0.224

494.1

15

3.620

0.551

529.6

16

3.856

0.300

420.4

17

7.572

0.424

493.6

18

9.543

0.852

558.0

19

1.604

0.642

462.7

20

3.910

0.943

564.8

21

1.757

1.363

634.6

22

1.685

0.238

423.4

23

2.000

0.488

510.3

24

2.732

0.996

548.5

25

2.371

0.310

427.6

26

4.600

0.490

537.4

27

5.064

0.756

690.4

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Machinability evaluation in hard turning of AISI 4340 steel

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worst results. The circle for P10 grade carbide tool
(Fig. 4(e)) with a thick grey pattern indicates that the
average surface roughness values obtained by this
tool is significantly different from the whole average
values. An increase in feed rate from 0.08 to 0.24 mm/
rev leads to a significant increase in surface rough-
ness (27 per cent). This significant effect of the feed
rate can be attributed to helicoid furrows, resulting
tool shapes, and the helicoid movement of the

tool–workpiece interface which are generated by this
increase. These furrows become deeper and broader
as the feed rate increases. For this reason, a low feed
rate must be used during turning [26]. On the other
hand, higher cutting speeds improve the surface
roughness as expected. When the cutting speed is
low, a low temperature is recorded at the tool–chip
interface and the material becomes hard to cut.
On the other hand, if the cutting speed is high, the

0

4

8

12

16

0

5

10

15

20

25

30

Expe r im e ntal r un

S

u

rf

a

c

e

r

o

ughne

s

s

(µ

m

)

CBN

Ceramic

P10

0,0

0,4

0,8

1,2

1,6

0

5

10

15

20

25

30

Expe r im e ntal r un

T

o

o

l F

lan

k We

ar

(

m

m

)

CBN

Ceramic

P10

200

400

600

800

0

5

10

15

20

25

30

Expe r im e ntal r un

M

a

x

imu

m T

o

o

l -

C

h

ip

T

e

mp

e

ra

tu

re

(°

C

)

CBN

Ceramic

P10

a)

b)

c)

Fig. 3 Cutting tool performance in terms of (a) surface roughness; (b) tool flank wear; and (c) maximum

tool–chip wear

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Fig. 4 Analysis of average surface roughness values and Tukey–Kramer tests related to cutting conditions

(a) effect of cutting speed; (b) effect of feed rate; (c) effect of depth of cut; (d) effect of workpiece
hardness; and (e) effect of cutting tool type

Table 6

Comparison of means for all pairs using the
Tukey–Kramer test on the average surface
roughness

Cutting parameter

Level 1

Level 2

Level 3

Cutting speed

4.280 0000

3.938 7778

2.858 1111*

Feed rate

2.068 3333*

3.274 1111

5.734 4444

Depth of cut

3.494 6667*

3.907 4444

3.674 7778

Workpiece hardness

2.588 5556*

4.899 3333

3.589 0000

Cutting tool type

1.858 8889*

3.202 2222

6.015 7778

q

¼ 2.497 29 a ¼ 0.05. Levels not connected by same letter are

significantly different.
* Optimum level.

Table 7

Comparison of means for all pairs using the
Tukey–Kramer test on the average tool flank
wear

Cutting parameter

Level 1

Level 2

Level 3

Cutting speed

0.305 777 78*

0.448 666 67

0.691 777 78

Feed rate

0.618 333 33

0.406 111 11*

0.421 777 78

Depth of cut

0.486 000 00

0.456 555 56*

0.503 666 67

Workpiece hardness

0.394 777 78*

0.615 144 44

0.436 333 33

Cutting tool type

0.221 777 78*

0.344 333 33

0.880 111 11

q

¼ 2.497 29 a ¼ 0.05. Levels not connected by same letter are

significantly different.
* Optimum level.

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generated heat makes the material soft enough to
escape the cutting edge [27]. The workpiece is easily

deformed at high temperatures and high cutting
speeds. The effects of depth of cut and workpiece
hardness on surface roughness are subtle. By chan-
ging the depth of cut between 0.1 and 0.3 mm, the
surface roughness fluctuates insignificantly. The
depth of cut is usually a major factor that influences
the surface roughness. However, in this case, the
influence is not significant, which could be due to
the very small values of cutting depth. Thus, it is
concluded that the effect of depth of cut is almost
negligible. By increasing the workpiece hardness, the
surface roughness first increases and then slowly
falls off.

Figure 5 depicts the effect of cutting conditions on

tool flank wear. Figure 5 and Table 7 show that the

Table 8

Comparison of means for all pairs using the
Tukey–Kramer test on the average tool–chip
temperature

Cutting parameter

Level 1

Level 2

Level 3

Cutting speed

439.344 44*

494.200 00

533.300 00

Feed rate

464.644 44*

485.544 44

516.655 56

Depth of cut

443.088 89*

504.388 89

519.366 67

Workpiece hardness

464.300 00*

503.566 67

498.977 78

Cutting tool type

433.500 00*

490.311 11

543.033 33

q

¼ 2.497 29 a ¼ 0.05. Levels not connected by same letter are

significantly different.
* Optimum level.

Fig. 5 Analysis of average tool flank wear values and Tukey–Kramer tests related to cutting conditions (a)

effect of cutting speed; (b) effect of feed rate; (c) effect of depth of cut; (d) effect of workpiece
hardness; and (e) effect of cutting tool type

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highest maximum average tool flank wear obtained
by, considering cutting speed, feed rate, depth of cut,
and workpiece hardness is the 0.880 11 mm for the
P10 tool with the ceramic tools taking second place.
The minimum tool flank wear value was obtained for
the CBN tool at settings of a 60 m/min cutting speed,
a 0.16 mm/rev feed rate, a 0.2 mm depth of cut, and a
material hardness of 360 HV. Higher cutting speeds,
and middle levels of feed rate and depth of cut and
softer materials gave the best results.

On the other hand, the measurement of tempera-

ture during machining is directly related to the
quality of parts, which is as important as other per-
formance characteristics such as surface roughness
and tool wear because diffusion, chemical reactions,

and thermal softening are exponentially dependent
on temperature. Thus, productivity and efficiency
of material removal operations are adversely affected
by an increase in temperature.

The effects of turning parameters on maximum

tool–chip interface temperature are given in Fig. 6
and Table 8. As can be seen from the results, the feed
rate, depth of cut, and cutting tool materials are
proportional to the interface temperature. Apart
from the turning parameters, the interface tempera-
tures recorded by the infrared pyrometer gradually
increase with machining time, as shown in Fig. 7. The
same trends were recorded in all the tests. Here, the
third experiment is given as a representative result.
Machining with P10 tools (Fig. 6(e)) at 120 m/min

Fig. 6 Analysis of maximum interface temperature values and Tukey–Kramer tests related to cutting

conditions (a) effect of cutting speed; (b) effect of feed rate; (c) effect of depth of cut; (d) effect of
workpiece hardness; and (e) effect of cutting tool type

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Machinability evaluation in hard turning of AISI 4340 steel

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cutting speeds (Fig. 6(a)) leads to significantly differ-
ent average temperatures. In the open literature the
maximum working temperatures for CBN, ceramic,
and P10 carbides are reported as 1150, 1700, and
875

C, respectively [28–30]. The cutting tools do not

lose their cutting properties, phase structures, or
toughness until these temperatures are reached.
Equation (1) can be used to determine the tempera-
ture difference (in per cent) between the maximum
working and actual machining conditions for a tested
tool material

100

T

MWT

T

EMT

T

MWT

· 100

ð1Þ

where T

MWT

is the maximum working temperature

(in centigrade) of the cutting tool and (in centigrade)
T

MWT

is the maximum experimentally measured

temperature. Using equation (1), the temperature
differences for CBN, ceramic, and P10 carbides
were obtained as 42.935, 33.22, and 78.85 per cent,
respectively. The lower the percentage difference the
more durabile is the tool against temperature.

The correlation between surface roughness, tool

flank wear, and maximum tool–chip temperature in
hard turning of hardened/tempered AISI 4340 steels
was examined, and a positive correlation was found
between the responses (Fig. 8). The correlation coef-
ficients between the surface roughness and tool flank
wear was found to be 0.2551, that between the sur-
face roughness and the interface temperature was

found to be 0.3320, and that between the tool flank
wear and interface temperature was found to be
0.5725. The pairwise correlation coefficients show
that there is a stronger correlation between tool flank
wear and interface temperature (P

¼ 0.0018). With a

worn tool, more surface contacts occur between the
tool and workpiece, and an increase in sliding friction
and rubbing causes higher temperatures.

3.3 ANOVA tests

ANOVA tests were made to analyse the statistical
effects of hard turning conditions on the total var-
iance of the results. Tables 9 to 11 show the results of
the ANOVA tests for surface roughness, tool flank
wear, and tool–chip interface temperature, respec-
tively. These analyses were carried out at a 5 per cent
significance level, i.e. at a 95 per cent confidence
level. The last column of each of these tables shows
the percentage values of each factor contribution P
(in per cent) to the total variation, thus indicating the
degree of influence on result [10].

An analysis of Table 9 leads to the conclusion that

the cutting tool type (27.14 per cent) and feed rate
(21.05 per cent) are statistically significant for surface
roughness. The workpiece hardness influences the
surface roughness only to a minor extent (8.10 per
cent). Other factors are not statistically significant for
surface roughness. The effect of depth of cut is neg-
ligible. Similarly, it can be seen from Table 10 that the

Fig. 7 The relationship between cutting time and interface temperature (test 3)

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cutting tool type (58.59 per cent) and cutting speed
(18.204 per cent) are statistically significant for tool
flank wear; however, the other factors are not statis-
tically significant. Finally it can be seen from Table 11
that the cutting tool type is again the most important
factor for the tool–chip interface temperature (32.38
per cent). The cutting speed is the second-most
important factor (24.04 per cent), and the depth of
cut (17.62 per cent) is the third-most important fac-
tor. The effects of feed rate and workpiece hardness

seem to be statistically unimportant for the tool–chip
interface temperature.

3.4 Changes in microhardness

Figure 9 shows the microhardness profiles of the
machined surfaces (tests 13, 14, and 15). The micro-
hardness measurements were performed at 100

mm

steps down to a depth of 1 mm below the machined
surface (as shown in Fig. 10) and the average result of
three measurements for each depth was recorded and

Fig. 8 Correlations between surface roughness and tool flank wear, surface roughness and maximum

interface temperature, and tool flank wear and maximum interface temperature

Table 9

ANOVA for average surface roughness

Source

Degrees
of
freedom

Sum of
squares

Mean
square

F ratio

P (%)

Cutting

speed

2

9.918 12

4.9591

0.4125

3.32

Feed rate

2

62.842 53

31.4213

3.2008

21.05

Depth of

cut

2

0.770 88

0.3854

0.0311

0.25

Workpiece

hardness

2

24.172 67

12.0863

1.0576

8.10

Cutting

tool type

2

81.001 09

40.5005

4.4703

27.14

Error

24

278.8992

11.6208

–

40.14

Total

34

457.604

100

Table 10

ANOVA for tool flank wear

Source

Degrees
of
freedom

Sum of
squares

Mean
square

F ratio

P (%)

Cutting

speed

2

0.685 5487

0.342 774

2.6707

18.204

Feed rate

2

0.251 7534

0.125 877

0.8597

6.68

Depth of

cut

2

0.010 1956

0.005 098

0.0326

0.3

Workpiece

hardness

2

0.246 7054

0.123 353

0.8412

6.55

Cutting

tool type

2

2.206 4414

1.103 22

16.9787

58.59

Error

24

3.085 7589

0.128 50

–

9.676

Total

34

6.4864

100

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Machinability evaluation in hard turning of AISI 4340 steel

1053

plotted. Under the same machining conditions, the
hardness profiles of the machined surfaces tend to
follow a similar trend for all the tested cutting tools.
The top surface hardness is lower than that of the
bulk material and in general it could be considered to
be tempered martensite. It can also be seen from
Table 5 that the interface temperature for tests 13,

14, and 15 are in the range from 466 to 529.6

C. When

the workpiece material is subjected to a high cutting
temperature and high cutting pressure during dry
machining, a competing process between work
hardening and thermal softening takes place which
affects the fundamental behaviour of the workpiece
material [27]. It can be concluded that the instability
or alteration of microstructure in the form of plastic
deformation caused by the high temperatures created
during dry machining leads to the softening of the
martensite (metallurgical alteration). At 250

mm

below the machined surface, the hardness reaches its
maximum values. After the peak point is reached, the
average microhardness value gradually decreases
until almost all the bulk material hardness is attained
at about 500

mm below the machined surface. Then, it

remains essentially stable up to a depth of 1mm
below the machined surface. Thermal softening is
most pronounced for the P10 tool because of the
higher temperatures they create as shown in Fig. 9.

4 CONCLUSION

The effects of the cutting speed, feed rate, depth of
cut, workpiece hardness, and cutting tool type on
surface roughness, tool flank wear, and maximum
tool–chip interface temperature during an orthogonal
hard turning of hardened/tempered AISI 4340 steels
was investigated. The experiments were guided using
a Taguchi design approach. The following con-
clusions can be drawn from the presented analyses.

1. On the basis of the microstructural results it can

be concluded that an increase in tempering tem-
perature leads to a decrease in hardness of the
samples.

2. The best surface roughness was obtained with

CBN tools followed by ceramic and then the P10
grade carbide tools.

3. Based on the Tukey–Kramer tests and box plots,

the optimal hard turning conditions for surface
roughness were obtained with CBN tool at
A3B1C1D1, (see Table 3 for explanation of factors
and levels) while the minimum tool flank wear
and tool–chip interface temperature values were
recorded at A1B2C2D1E1 and A1B1C1D1E1 set-
tings, respectively.

4. Based on the ANOVA results, the contributions of

the hard turning parameters on surface roughness
follow the order cutting tool type (factor E), feed
rate (factor B), workpiece hardness (factor D),
cutting speed (factor A), and depth of cut (factor
C). Similarly, the order of factors’ effects on tool
flank wear and tool–chip interface temperature
were obtained as E

> A > B > D > C and E > A >

C

> B > D, respectively.

Table 11

ANOVA for maximum tool–chip interface tem-
perature

Source

Degrees
of
freedom

Sum of
squares

Mean
square

F ratio P (%)

Cutting

speed

2

40 096.77

20 048.4

3.7982

24.04

Feed rate

2

12 329.60

6164.80

0.9580

7.40

Depth of

cut

2

29 400.97

14 700.5

2.5682

17.62

Workpiece

hardness

2

8296.43

4148.22

0.6282

4.98

Cutting

tool type

2

54 014.06

27 007.0

5.7481

32.38

Error

24

137 949.344

5747.8893 –

13.58

Total

34

282 087.174

100

200

300

400

500

600

0

100

200

300

400

500

600

700

800

900

1000

Depth (µm)

Microhardness (HV)

P10

Ceramic

CBN

Fig. 9 Microhardness

variations

of

machined

cross-

sections with different cutting tools (tests 13, 14,
and 15)

Fig. 10 Microhardness measurements beneath the mach-

ined surface

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5. There were positive correlations between resp-

onses and their interactions. The most correlated
pair was obtained between tool flank wear and
interface temperature with correlation coefficient
of 0.5725 (P

¼ 0.0018).

6. The microhardness results show that a thermal

softening occurs on the top surface of machined
samples because of high temperatures.

Authors 2010

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