MACHINABILITY OF MARTENSITIC STEELS
IN MILLING AND THE ROLE OF HARDNESS

U. Persson and H. Chandrasekaran

Swedish Institute for Metals Research

Drottning Kristinasv. 48

114 28 Stockholm

SWEDEN

Abstract

Results from machinability studies of some newly developed martensitic
steels in milling is presented in this paper. Two new steels (TOOLOX 33
and TOOLOX 44) in the hardness range of 300 and 400 HV30 were com-
pared with two commercial steels (CS1 and CS2) in the same hardness range.
Face milling with button type coated cemented carbide inserts was used in
the milling tests. Cutting speed was varied at constant tooth feed to obtain
different tool life. The tool wear was monitored at regular intervals. Based
on standard V-T curves the Taylor machinability index n and C was com-
puted using a tool life criterion of VB = 0,3 mm. At both hardness levels the
milling machinability of the new grades was superior to the existing commer-
cial grades. Severe scatter in the tool wear was observed for all the materials
after a specific flank wear. The improved machinability or reduction in tool
wear associated with the new steels may be attributed to the influence of alloy
content on tool wear mechanism during intermittent cutting.

Keywords:

machinability, martensitic steels, milling.

INTRODUCTION

Good machinability is a critical requirement to extend the market share

and identify new applications for through hardened low alloyed marten-
sitic steels. The market for these steels is mainly in the tool and mould
industry and hence good machinability is economically very attractive. It is
also well known that martensitic steels of required hardness could be pro-

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duced through different processing routes from the molten state. Traditional
through hardened martensitic steels intended for wear resistant applications
are not normally the candidate materials in the die and mould making indus-
try. However, recently newer martensitic steels are appearing in the market
where good machinability in operations like milling is also incorporated
through alloy design and steel processing. It is proposed present the results
from the machinability study of such steels at two hardness levels and com-
pare with the machinability of commercially available steels aimed at the
same market.

Face milling using button type coated cemented carbide inserts under

conditions comparable to ISO-8688-1/1994 was carried out. Our earlier ex-
perience [1] has shown that within a class (composition based) of martensitic
steels the hardness and machinability in milling to have good correlation.
These were also based on face milling using button inserts, which displayed
a combined micro-chipping along with flank wear limiting the tool life. The
machining tests were not aimed at identifying the optimum tool or cutting
conditions, but to compare two groups of martensitic steels. Accordingly
tool wear was monitored at suitable intervals and a tool wear criterion of 0,3
mm of flank wear was used. Low power optical microscopy was used for
wear measurements. The classical Taylor constants were evaluated for all
the materials. These results will be presented now.

Within the overall objective of mapping the machinability of some marten-

sitic steels, the specific aim of this project is to evaluate the machinability of
two groups of martensitic steels with hardness of ∼ 300 and 400 HB respec-
tively. Two commercial steels (CS1 and CS2) and a new grade, TOOLOX
33, all with a hardness of about 310 HV30 formed the first group. One new
grade, TOOLOX 44 with a hardness of 460 HV30 formed the second group.
It is proposed to use the same face milling test procedure as before. Our face
milling test is a modified version of the standard ISO-8688-1/1994.

MATERIALS USED AND THE MACHINABILITY TEST
METHOD

THE WORK MATERIAL AND TOOLS

Four martensitic steels namely two commercial grades CS1 and CS2 and

two new grades TOOLOX 33 and TOOLOX 44 (SSAB Oxelösund) were
evaluated in our study. The nominal composition and hardness of the steels

Machinability of Martensitic Steels in Milling and the Role of Hardness

1227

Table 1.

Nominal chemical composition (wt %)and hardness (HV30) of the steels

Material

C

Si

Mn

P

S

Cr

Ni

Mo

V

CS1

0,39

0,33

1,46

0,006

0,062

1,84

—

0,15

—

CS2

0,38

0,26

1,39

0,010

0,009

1,86

0,94

0,17

—

TOOLOX 33

0,28

0,57

0,91

0,008

0,0019

1,18

0,71

0,39

0,117

TOOLOX 44

0,30

0,61

0,89

0,010

0,0009

1,23

0,66

0,79

0,145

Material

Ti

Cu

Al

Nb

B

N

HV30

CS1

—

—

—

—

—

—

339

CS2

—

—

—

—

—

—

307

TOOLOX 33

0,025

0,02

0,044

0,002

0,0019

0,0046

326

TOOLOX 44

0,026

0,04

0,045

—

0,0016

0,0036

458

are shown in Table 1. Both the TOOLOX grades contain less S, but more
Mo and V than the grades CS1 and CS2. The last two materials have the
same composition, but the S content of CS1 is greater than CS2, which
contains some Ni also. The test plates TOOLOX 33 and TOOLOX 44
were austenitized at 925℃ followed by quenching in water and subsequent
tempering at 660℃ for 120 min for TOOLOX33 and at 600℃ for 120 min
in the case of TOOLOX 44.

All the work materials were supplied in the form of rectangular blocks

of 600 × 180 × 60 mm after heat treatment. The length of the specimen
coincides with the rolling direction and face milling was carried out along
the length and across the width of 60 mm in all cases.

Face milling was carried out using 80 mm diameter cutter Coromill 200,

R200-068Q27-12L carrying 4 round/button type tool inserts with the ge-
ometry RCHT1204 MO-PL and based on the recommendations of the tool
supplier the Coromant GC1025 grade was used in the studies. The grade
GC1025 is PVD (TiCN+TiN) coated grade with fine grained carbide sub-
strate.

TEST PROCEDURE

The milling tests were carried out at the machining laboratory of SIMR

using a CNC milling machine (Abene 550 VHF-CNC ) with a spindle power
of 6 kW. As mentioned earlier face milling was carried out along the length of

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6TH INTERNATIONAL TOOLING CONFERENCE

Table 2.

Information about the milling machine, cutter and insert grade

Milling Machine

CNC milling machine Abene

Cutter

Coromill 200 ∅ = 80 mm

Insert dimensions and geometry

RCHT1204MO-PL

Insert grade and number of teeth

GC1025,

z = 4

Tool geometry

γa; Edgeradius

14°; 15 µm

Axial depth of cut

a

p

= 2 mm

Width of cut

W = 60 mm

Coolant

none

the 60×600 mm face in all cases. As the work material was already prepared
by milling no special surface preparation to eliminate heat affected layer (as
in gas cutting) was required. In our machining tests to avoid transient effects
associated with the variation in the width of cut at the two ends of the work
piece, this region (

D/2 = 40 mm) was milled by a separate tool. Thus, the

effective milling length in our studies was 520 mm per pass. Full information
about the milling test is given in Table 2, while a schematic view of the face
milling configuration used in our study is shown in Fig. 1.

After selecting a suitable reference speed, the cutting speed was varied at

a constant reference tooth feed to obtain the V-T curves. Subsequently tests
were also carried out at one additional feed for one of the cutting speeds.

Figure 1.

Schematic view of the vertical face milling operation. The milled length for

tool life studies was 520 mm. Schematic view of flank wear scar on the round inserts and
the flank wear observed in this study. A flank wear criterion of VB = 0,3 mm, was used to
stop the test.

Machinability of Martensitic Steels in Milling and the Role of Hardness

1229

TOOL WEAR MEASUREMENT

The flank wear of the insert was measured at regular intervals for each

of the four teeth used. Tool wear was measured using a low power optical
microscope and image capture to a PC using software. Since the wear scar
was non-uniform, the maximum flank wear was used as the parameter, Fig. 2.
Based on the wear of the 4 teeth the average wear and its variation were
calculated. A flank wear criterion of VB = 0,3 mm, was used to define tool
life. The average tool wear was plotted as a function of milled length. Tool
life calculation was carried out from the wear results through interpolation.

Along with the measurement of the flank wear the appearance of the

worn region was also recorded in some instances. This was used later on to
compare the nature of tool wear in a qualitative manner.

The progress of tool wear was similar for all the four work materials. This

indicates that the operating wear mechanisms remain unchanged. However,
more detailed analysis of the tools using SEM would be required to ascertain
the initiation and progress of the controlling wear mechanisms.

Figure 2.

TOOLOX 33 machined at V

c

= 400

m/min and f = 0, 15 mm/tooth. In a) after

8,6 min of milling and in b) the same insert after 11,5 min.

RESULTS FROM TOOL WEAR TESTS

FLANK WEAR

Typical curves showing the progress of tool wear when milling steels C1

and TOOLOX 33 as a function of milled length are shown in Fig. 3 and 4.
Based on the criterion of a minimum life of a few minutes the upper speed

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6TH INTERNATIONAL TOOLING CONFERENCE

limit was chosen and this was followed by two additional speeds at lower
levels. An attempt was made to choose at least one speed common to each
group.

Figure 3.

Flank wear as a function of milling length for three speeds. Material CS1. At

400 m/min the insert failed after 2950 mm and for 280 m/min it held 18150 mm. The feed
used in the tests was 0,15 mm/tooth.

Figure 4.

Flank wear as a function of milling length for three speeds. Material TOOLOX

33. At 500 m/min the insert reached VB=0,3 mm after 4950 mm and for 350 it held 18700
mm. The feed used in the tests was 0,15 mm/tooth.

In all the cases the milling tests displayed severe scatter between the results

of the 4 inserts. This was especially severe when VB > 0,1 mm. This was

Machinability of Martensitic Steels in Milling and the Role of Hardness

1231

Table 3.

Tool life obtained from tool wear results in face milling for the three cutting speeds

and feed 0,15 mm/tooth. The influence of a greater feed (0,2 mm/tooth) at one speed is also
shown

CS 1

CS 2

TOOLOX 33

TOOLOX 44

V

c

Tool life

V

c

Tool life

V

c

Tool life

V

c

Tool life

[m/min]

[min]

[m/min]

[min]

[m/min]

[min]

[m/min]

[min]

280

27,2

320

25,9

350

22,4

200

34,6

350

6,6

350

13,2

400

11,5

250

8,3

400

3,1

400

6,9

500

4,1

300

3,8

280

4,3

320

10,2

350

16,3

200

11,2

(0,2)

(0,2)

(0,2)

(0,2)

also the observation made during the previous study [1]. One reason for the
scatter is the cutter run-out associated with insert mounting and adjustment
of all the 4 inserts. In order to limit this the radial run out of the inserts was
adjusted to be within 0,02 mm before each machining test For each material
the tool life at VB = 0,3 mm was obtained from the interpolation of the flank
wear curve such as shown in Fig. 3 and 4. The consolidated tool life results
computed from the wear curves are shown in Table 3. The resulting tool life
varied widely from as much as 34,6 min for TOOLOX 44 at

V

c

=200 m/min

to shorter life of. 3,1 min for the steel CS1 at

V

c

= 400 m/min.

The resulting machinability curves, ( log

V log T plot) for these materials

shown in Fig. 5 display comparable gradient, as may be expected within a
given class of steels. The line fitting is not statistical.

MACHINABILITY INDEX

The most widely used tool life equation is the Taylor tool life equation,

which relates the tool life T in minutes to the cutting speed V [m/min] through
an empirical tool life constant,

C as shown in equation (1)

V T

n

=

C

(1)

Rewriting equation 1 and taking logarithms we get

log

V = log C − n log T

(2)

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Figure 5.

V –T curves from dry face milling of the five martensitic steels using round

coated carbide (GC1025) inserts; tool life criterion used was VB =0,3 mm; feed f

t

= 0

, 15

mm/tooth.

In equation 2,

n is the slope of the tool life curve. In our results neglecting

tool life values below 3 minutes the machinability constants were obtained
from Fig. 5, and tabulated in Table 4.

Table 4.

Machinability constants,n and C in face milling for the four materials

Material

Machinability constants

Machinability [m/min]

Index

n

Constant C

V

10,0,3

V

30,0,3

V

45,0,3

CS1

0,16

476

329

276

258

CS2

0,17

558

377

313

292

TOOLOX 33

0,21

669

412

327

301

TOOLOX 44

0,18

379

246

205

191

As may be expected the index

n mainly controlled by the tool material

varies much less than the constant

C, which is primarily controlled by the

workmaterial. Once the index

n and C are known, the cutting speeds for

required tool life are readily computed. Cutting speed computed accordingly
for the four steels corresponding to tool life of 10, 30 and 45 minutes, is
shown in Table 4. The cutting speed at 10, 30 and 45 minutes lifetime was
then plotted as a function of milling time. In Fig. 6 it can be seen that if the
milling time for TOOLOX 33 increases from 10 min to 45 min as the cutting
speed is reduced from 412 m/min to 301 m/min, a reduction of 26%.

Machinability of Martensitic Steels in Milling and the Role of Hardness

1233

Figure 6.

Calculated cutting speed versus milling times, 10, 30 and 45 min.

Additional tests to identify the sensitivity of the machinability results to

the feed level used were also carried out. A feed rate of

f

z

= 0

, 2 mm/tooth

and an intermediate cutting speed was used here. The results are shown in
Table 5 and Fig. 7. CS1 appears to be most sensitive to increase in feed
(85% reduction tool life), while TOOLOX 33 displayed the least sensitivity
to feed increase (27% reduction in tool life).

Table 5.

The effect of tooth feed f

z

= 0

, 2, mm/tooth, on tool life at intermediate cutting

speed

Material

V [m/min]

Tool life T [min]

f

z

= 0

, 2

f

z

= 0

, 15

CS1

280

4,3

27,2

CS2

320

10,2

25,9

TOOLOX33

350

16,3

22,4

TOOLOX 44

200

11,2

34,6

DISCUSSION OF RESULTS

For a given tool and cutting operation (turning, milling, drilling etc),

machinability in terms of tool life can be related to their hardness [2]. Cor-
relation is often better within the same class of steel alloys (similar mi-

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 7.

Flank wear VB as a function of milling length for feed f

z

= 0

, 2 mm/tooth and

a cutting speed of 280 m/min (CS1), 320 m/min (CS2), 350 m/min (TOOLOX 33) and 200
m/min (TOOLOX 44).

crostructure). With strong microstructural difference even for same hard-
ness the machinability may differ (often different tool wear mechanism).
Since our investigations with a similar class of steels reported earlier [1]
also resulted in similar tool wear mechanism (predominant micro chipping)
it was considered reasonable to correlate the hardness of all the 4 steel alloys
tested. We have used the machinability criterion of V

10,0,3

to correlate with

hardness and the results are shown in Fig. 8. The correlation appears to
be good indicating that machinability improves with a reduction in hard-
ness. However, at the same level of hardness (∼300 HV30) the 3 materials
TOOLOX 33, CS2 and CS2 display clear difference. We shall now try to
interpret the machinability variation terms of known factors related to their
composition, microstructure, and hardness.

The first feature is the clearly better machinability of alloy CS2 than the

harder alloy CS1 (339 HV30) in spite of the greater content of S and Si in
CS1, both of which may be considered positive from machinability point of
view. However, it should be mentioned that the role of S in machinability
improvement is most effective when the operating wear mechanism is abra-
sion under continuous cutting operation. But in our case the chip loading
is intermittent and the dominant operating wear mechanism appears to be

Machinability of Martensitic Steels in Milling and the Role of Hardness

1235

Figure 8.

Relation between hardness (HV30) and milling machinability (V

10,0.3

) for

alloys tested. The materials divided into different groups. TOOLOX and commercial steels.

micro-chipping, which is partly adhesion induced. Thus the effectiveness
of S in machinability improvement here is clearly impaired.

The next feature of interest is the highest machinability observed in the

case of alloy TOOLOX 33, while at somewhat lower hardness the alloy
CS2 containing more S (traditionally favourable from machinability point of
view) displayed lesser machinability (refer to Table 4 and Fig. 8. Based on the
experience of milling other martensitic tool steels [3] this may be attributed
to the positive effects of increased Si content, although the mechanism for
this is not clear as yet. Additional studies with greater insight into the
microstructure and wear mechanism are required to clarify this point.

CONCLUSIONS

Based on the present study the following conclusions can be drawn.

The face milling test using round cemented carbide inserts in a test
mode comparable to that in the standard is able to grade the machin-
ability of low alloyed martensitic steels.

For the present class of martensitic steel alloys good correlation be-
tween bulk hardness and milling machinability (as for example V

10,0,3

)

could be observed.

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Within the group of steels in the hardness range of 310 340 HV30, the
new grade TOOLOX 33 displayed the best machinability, exceeding
the commercial grades CS1 by almost 300% and CS2 by 80% at a
cutting speed of 350 m/min.

In general all wear results displayed scatter when the flank wear VB>
0,1 mm, probably due to the milling mode and associated wear mech-
anisms.

ACKNOWLEDGMENTS

The present work was financed by SSAB Oxelösund AB. Thanks are

due to Anders Skirfors and Jonny Myrbakk (SSAB) for close interaction.
Thanks are also due to the keen interest shown by Per Hansson and Christer
Offerman (both from SSAB Oxelösund) in the test results and discussions.
Rickard Sundström (Sandvik Coromant AB) is thanked for the supply of
tools. Timely help from Christer Eggertson (SIMR) is also greatly acknowl-
edged.

REFERENCES

[1] U. PERSSON and H. CHANDRASEKARAN (2000) Evaluation of machinability index

in milling for some HARDOX steels, Report from Swedish Institute for Metals Research,
IM-2000-807.

[2] H. CHANDRASEKARAN (1998) Machinability of ferrous alloys and the role of mi-

crostructural parameters – a literature survey, Report from Swedish Institute for Metals
Research, IM-3664.

[3] U. PERSSON and H. CHANDRASEKARAN (2001) Finish milling of some alloy steels

in the hardened state, Report from Swedish Institute for Metals Research, IM-2001-520.