MACHINABILITY AND TOOL WEAR DURING

THE HIGH SPEED MILLING OF SOME HARDENED
TOOL STEELS

H.Chandrasekaran and U. Persson

Swedish Institute for Metals Research

Drottning Kristinas väg 48

114 28 Stockholm

Sweden

Abstract

High speed milling of tool steels in the hardened state is an expanding

field. Results from machinability and tool wear studies during the finish
milling of some hot and cold working tool steels using both cemented carbide
and CBN tool inserts are presented in this paper. Recommended cutting
speeds (75 to 600 m/min) and feeds (0,025 to 0,15 mm/tooth) were used
to compare tool performance as well as the surface integrity of the milled
surface. Microstructural study of the work material and SEM investigation
of the worn tools also formed part of this investigation.

While cemented carbide tools appear to be capable of milling the different

steels in our study, use of proper CBN tools and cutting conditions could im-
prove the milling productivity appreciably in the case of cold working steels.
However, in the case of hot working steels (50 HRC) the optimal cutting
conditions associated with CBN tools seem to be very sensitive to nominal
compositional changes in the work material. Obvious microstructural dam-
age or tensile residual stress was not seen during the present studies. The
limited SEM study of the tools indicated that adhesion induced micro chip-
ping could be the probable limiting factor during the high speed milling of
hotworking steels. The critical role of alloy content and structure was also
seen to play an important role on their machinability during hard milling.

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INTRODUCTION

The demand on the quality and productivity of dies, moulds and press tools

for metal and plastic forming industry is increasing continuously. Such cold
and hot working tools are traditionally manufactured through rough ma-
chining, finish machining followed by heat treatment. Grinding and EDM
(electro discharge machining) are often the final operation carried out. How-
ever, cutting tool and machine tool developments give us the possibility to
machine materials in the hardened state. This technology of hard machining
is capable of enhancing the overall production economy, arising from reduc-
tion in lead time and elimination of the grinding operation [1] . Improvement
in working environment due to the elimination of a cutting fluid is an addi-
tional outcome affecting the overall production economy very favourably.

Successful implementation of this technology however, requires the se-

lection of proper tools and cutting conditions [2]. The effective range of
cutting conditions for the milling of hardened steels (50 ∼ 60 HRC) with
conventional or super hard tool materials is rather narrow. Despite this,
industrial experience shows that similar tool steels with marginal compo-
sitional difference do display substantial difference in machinability in the
hardened state. Since it is possible to achieve the required hardness both
through composition and processing, the resulting effect on machinability is
difficult to predict, mainly because available information about the specific
effect of microstructural features such as matrix, carbide/other particles on
hard machinability is very little. This is one of the prime motives for the
present research effort.

Reliable information about the role of material microstructure on the

milling machinability of tool steels in the hardened state could be very useful
in practice and the present work is a step in this direction. The general objec-
tive of this study is to expand our knowledge base in the field of high speed
milling of typical tool steels in the hardened state. As both technical and
practical demands indicated, for our study we have chosen the fine milling
of these steels in the hardened state. The work material being the main pa-
rameter of interest, standard tools and cutting conditions were used during
the controlled milling tests. Systematic mapping of the evolution of tool
wear was followed by an examination of the worn tools and the machined
surface. Both SEM and optical microscopy was used in these investigations.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...

1239

Limited surface integrity investigations were also carried out. These results
will be presented now.

WORK MATERIALS USED

It was proposed to use standard hot and cold working tool steels from the

market in the study and included the following materials, namely

five hot working tool steels, namely ORVAR Supreme, THG2000 and
3 variants of the grade DIEVAR

five cold working tool steels namely VANADIS 4 and CALDUR, P/M
HSS ASP 2023 and ASP2023S and bearing steel 100Cr6.

All the five hot working grades were heat treated to a hardness of 50

HRC. The hardness of the cold working grades varied from 61 HRC (CAL-
DUR), 62HRC (VANADIS 4, ASP 2030 and ASP2030S) and 63 HRC (steel
100Cr6).

Chemical analysis of the steels indicated that THG 2000 and ORVAR to

differ mainly in Si, and S content, while DIEVAR contains more Mo, but
lesser amounts of Si and V.

All the other grades contain much more C. The P/M grades VANADIS 4

and ASP2023 have comparatively higher alloy content, especially V. Cold
working grades CALDUR and the ball bearing steel 100Cr 6 are low alloyed
carbon steels.

The microstructural features of the work materials used in our study were

obtained from metallographic sections of the work material after nital (4%)
etching. All the tool steels investigated have a fine martensitic microstructure
in the hardened state. The P/M grade VANADIS 4 contains in addition a fine
distribution of primary carbides of CrC and VC, while ASP2023 contains
mainly primary carbides of WC and VC.

MILLING CUTTERS AND TOOL MATERIALS USED

Since both hot and cold working steels were involved, the required tool

material and recommended geometry also varied. Since the number of ma-
terial variants were many, it was proposed to compare their machinability
under one cutting condition in terms of speed, feed and depth of cut, suitable
for the chosen tool. The tool holders and the tools (material type and geome-
try) as well as the cutting conditions for the milling tests were selected on the

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recommendations of the tool supplier. Bulk of the milling was carried out
using different grades of super hard tool material, namely cubic boron nitride
(CBN) from more than one supplier. For one material (ASP2023) other ce-
mented carbide and cermet grades were also used. Finish face milling using
end mills/face mills was the main test mode.

MILLING TESTS

Finish milling in the hardened state is characterised by small feeds and

depths of cut using small diameter cutters rotating at high speeds. In view
of the large number of material and tool combinations involved in our study
high speed milling tests were carried out of different laboratories, namely
Machining laboratory of Uddeholm Tooling AB at Hagfors, Secotools AB,
Fagersta and Sandvik Coromant AB, Stockholm. All tests were carried
out in the dry condition. A flank/notch wear of 0,2 mm (VB = 0,2 mm)
was the evaluation criterion used in these studies. In cases of uneven wear
the tests were stopped when the maximum wear level reached 0,2 mm.
Similarly, when the tool wear was too little the test was stopped to conserve
the resources. Tool wear was monitored at regular intervals using low power
toolmakers microscope.

The consolidated test conditions used are shown in Table 1. As a step

in mapping the observed tool wear and machinability behaviour of the tool
the worn tools were examined in SEM. Further an effort was also made to
map the surface integrity parameters of the milled surfaces. This included
measurement of surface quality (Profilometer), state of microstructure (met-
allographic section), subsurface deformation (microhardness) and residual
stresses (X-ray diffraction). These investigations were carried out only for
chosen test conditions only. We will now present the tool wear results and
the results obtained from SEM only.

RESULTS FROM TOOL WEAR STUDIES

Evolution of flank and notch wear during the milling of three DIEVAR

variants using CBN inserts is shown in Fig. 1. If both notch and flank wear
were present these were monitored. It can be readily seen that in the initial
stages both wear modes follow similar path. However, the resulting tool
wear for the three DIEVAR variants clearly shows that with marginal mate-
rial variation even at comparable hardness the tool wear could be affected.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...

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Table 1.

The cutting conditions used for tool wear studies in milling

Tool steels tested

Tool grade

z

V

f

z

a

p

a

e

used

mm/tooth

m/min

mm

mm

ORWAR supreme

BN300

1

600

0,08

0,5

7

THG 2000
DIEVAR (1,2,3)
ASP 2023

CBN 20

2

400

0,15

0,3

4

ASP 2023S

CBN 20

2

400

0,15

4

0,3

VANADIS 4

CBN 20

2

400

0,15

4

0,3

ASP 2023S

F30M

2

75

0,15

4

0,6

ASP 2023S

390 CB50

2

400

0,15

1,5

0,3

ASP 2023S

390 CT530

2

75

0,15

1,5

0,3

ASP 2023S

390 1025 E

2

75

0,15

1,5

0,3

ASP 2023S

390 1025 M

2

75

0,15

1,5

0,3

CALDUR

CBN20

2

600

0,025

3

0,3

100Cr6

CBN20

2

600

0,025

3

0,3

CALDUR

CBN100

1

400

0,05

0,15

47

100Cr6

CBN100

1

400

0,05

0,15

47

CALDUR

CBN300

1

400

0,05

0,15

47

100Cr6

CBN300

1

400

0,05

0,15

47

Similar wear development was also seen in the case of other hotworking
grades. From these curves the tool life was computed using a flank wear
criterion of VB = 0,2 mm.

The consolidated tool life results for the five hotworking grades is shown

in Fig. 2. The machinability difference between the nominally similar (hard-
ness) grades in hard milling at identical cutting conditions could still differ
by almost two orders of magnitude as in the case of ORVAR and one of the
DIEVAR variants.

In the case of the cold working grades the four P/M processed variants, as

the workmaterial was in the form of circular bars, circular interpolation mode
was used to face mill using a milling cutter with two cutting edges. Both
CBN and a cemented carbide grade were used. The consolidated results are
shown in Fig. 3. The critical role of tool grade selection was also evident in
the milling of the cold working grades. An almost 100% increase in tool life
was possible through the use of correct grade of CBN as shown in Fig. 4.

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Figure 1.

Evolution of tool wear (flank and notch) for three variants of DIEVAR (E10615,

E10685 and E10283) using CBN (BN300) inserts; Vc=600 m/min, fz=0,08 mm/tooth, a

p

=7 mm and a

e

=0,5 mm.

This was also evident in the case of cemented carbides shown in the case of

milling ASP2023S using CBN and a set of three cemented carbides (Fig. 5).
Evidently the cermet grade (CT530) is unsuitable for this workmaterial.

DISCUSSION OF RESULTS

In order to understand and interpret the observed machinability behaviour

in our studies additional investigation of the tools in SEM was carried out.
In the absence of systematic observation of the tools as a function of time
the observed difference in the appearance of the tools at the end of tool life
was used to draw qualitative information.

This indicated that in most cases associated with long tool life a stable

evolution of wear along both rake and flank surface was evident. This was
typical in the cases of ORVAR-CBN (BN300), CALDUR/100Cr6 – CBN
(CBN300).

Machinability and Tool Wear During the High Speed Milling of Some Hardened...

1243

In the absence of this the progress of wear was often through one or a

combination of following modes, namely abrasive wear along with progres-
sive micro-chipping and final destruction of edge, Fig. 6, severe notching
(many of the DIEVAR variants) and localised chipping. In many instance of
poor machinability tendency for notch formation and adhesion of the work
material to the tool was evident. In other words due to the interaction of
the microgeometry with the workmaterial at the prevalent cutting conditions
(cutting speed, feed and depth of cut), thermally induced tool-work interac-
tion enhancing the tendency for workmaterial adhesion takes place. If this
is postponed for a long time as in the case of 100Cr6 –CBN300 shown in
Fig. 7, the tool life is also prolonged. Indirect evidence for the above hy-
pothesis also came from the appearance of the chips. With progressive tool
wear the chip morphology and colour changed (thinner and wider) indicating
increased temperature. Under these conditions the notching at the depth of
cut line is also facilitated due to increased interaction with air (oxygen).

The improvement in the machinability of S alloyed ASP2023S grade is

another strong evidence of the traditional role of machinability improvement
additives. Stable presence of MnS layer could protect the tool from the large
primary carbides in the workmaterial [3]. The influence of other elements
likes Si and the intermittent nature of the contact phenomenon are additional
features affecting tool wear. This is also the possible reason for the observed
variations in the machinability of the hot working grades in our case.

CONCLUSIONS

Based on our experimental results and discussion, the following conclu-

sions are in order.

The finish milling of hardened tool-steel poses no technical problems
as such. But to achieve practical tool life optimal tools (material and
micro- geometry) and cutting conditions are critical.

Successful milling of most of the hardened steels with cemented car-
bide tools appears feasible. However, correct selection of cutting
conditions and type of CBN could result in high productivity for some
of the steels.

For a given hot working grade (50 HRC) apparently nominal com-
positional difference could affect machinability significantly. Milling

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the grade ORVAR SUPREME with the same CBN grade at the same
cutting conditions resulted in the tool life variations of 2 orders of
magnitude.

Even at comparable bulk hardness level, increase in carbide content
contributes to the deterioration in the machinability of the cold work-
ing tool steels, as evidenced by 100Cr6 and VANADIS 4.

There is some evidence indicating that the tool-work interaction and
the formation of protective inclusion layers or reduction in the adhe-
sion of work material is closely related to the alloy content.

ACKNOWLEDGMENTS

We are thankful to VINNOVA, Uddeholm AB, Secotools AB, Sandvik

Coromant AB, Ovako AB and Erasteel Kloster AB for jointly financing this
project. Thanks are also due to Uddeholm AB (Staffan Gunnarsson), Sec-
otools AB (Bengt Högrelius) Sandvik Coromant AB (Rikard Sundström),
Erasteel Kloster (Jan Tiberg) and Ovako AB (Tomas Andersson) for the
supply of materials and milling tests.

REFERENCES

[1] K. KÖNIG, A. BERTHOLD and K-F. KOCH, (1993), Turning versus grinding – A

comparison of surface integrity aspects and attainable accuracies, Ann. of CIRP 42/1,
pp. 39–43.

[2] S. GUNNARSSON, (1996) Machining of hardened steels, "Progress in Tool Steels –

Proc. of 4th Int. Conf. On Tooling, Ruhr University, Bochum, Sept. 11–13, 1996, pp.
457–467.

[3] 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.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...

1245

Figure 2.

Tool life in milling of five hot working steels using CBN(BN300) inserts using

a criterion of VB =0,2 mm; Vc=600 m/min, fz=0,08 mm/tooth, a

p

=7 mm and a

e

=0,5 mm.

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Figure 3.

Tool life in milling of four P/M cold working steels with CBN (BN20) and ce-

mented carbide inserts using a criterion of VB =0,2 mm; Vc=400 m/min, fz =0,15 mm/tooth,
a

p

=1,5 mm and a

e

=0,3 mm. Notice the changed cutting conditions for the carbide insert

F30M; Vc = 75 m/min and a

e

=0,6 mm.

Machinability and Tool Wear During the High Speed Milling of Some Hardened...

1247

Figure 4.

Tool life results for two cold working grades using different type of CBN inserts;

cutting conditions as in Table 1.

Figure 5.

Tool life as a function of tool material type in hard milling (circular interpolation)

of ASP2023S; cutting conditions as in Table 1.

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(a)

(b)

Figure 6.

SEM view of CBN (BN300) tool edge after 496 min of milling in ORVAR;

observe the even flank wear (a), notch wear and chipping on the rake (b).

(a)

(b)

Figure 7.

SEM view of the cutting edge after milling 100Cr6 for 420 min.

using

CBN(CBN300) showing some material adhesion (a) and the same region at a higher magni-
fication (b).