THE INFLUENCE OF STEEL GRADE AND STEEL
HARDNESS ON TOOL LIFE WHEN MILLING IN
HARDENED TOOL STEEL

S. Gunnarsson, B. H¨ogman and L. G. Nordh

Uddeholm Tooling AB

Research and Development

683 85 Hagfors

Sweden

Abstract

The rapid development of cutting tools and machining processes have made it
possible to machine hardened tool steel. Milling operations that would have
been impossible to carry out only a few years ago are today used in die and
mould production. However, the chemical composition of the tool steel and
the steel hardness, has a very big influence on which cutting tool life that
can be achieved when machining in hardened tool steel. In this investigation
coated solid cemented carbide milling cutters have been tested in different tool
steels at hardness levels up to 62 HRC. The cutting speed has been varied in
an attempt to find the optimal tool life for best productivity. The investigation
shows significant difference in production cost between two steels with the
same hardness, but different chemical analysis.

Keywords:

Hardened Die and Mould Steel, High Speed Machining

INTRODUCTION

Machining of moulds and dies direct in hardened state is possible today

due to the rapid development of cutting tools and machine tools [1]. A
common way to machine a die in the past, was first to rough machine the
die in soft condition, followed by hardening and then to spark erode the
die to the finished dimension. This procedure will create a hard re-melted
layer on the mould surface, which, if it is a die casting mould, must be

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polished away to give an acceptable mould surface. This polishing is a very
time consuming operation, and can sometimes be difficult to carry out on the
whole surface if the mould has deep grooves and pockets. A typical example
is a die casting mould for escalator steps, which contains a lot of ribs and
grooves. At one customer who is using our material DIEVAR hardened to
51 HRC, a comparison was done between to spark erode (EDM) the mould
and to machine it, using the high speed milling technique (HSM) [2]. It
took 300 hours to EDM the mould, which was followed by 400 hours of
polishing to get rid of the re-melted layer. To machine the mould direct in
hardened state using the HSM technique took 80 hours. The example shows
that huge time saving can be achieved when machining with cutting tool
direct in hardened tool steel. However to succeed with this new technique,
the following factors are of very big importance:

Mould and die design

Cutting tool

Machine tool

Tool passes

Work material

The hardness of the work material

This paper shall present what influence the chemical composition of the

steel and the hardness has on the tool life when milling with solid cemented
carbide milling cutters

TEST PERFORMANCE

EXPERIMENTAL

The tests have been performed in a vertical Modig MD 7200 machining

centre at the R & D at Uddeholm Tooling AB, see Fig. 1. The machine has a
maximum spindle speed of 18 000 rpm and the power 20 kW. It is equipped
with a HSK 63 taper and the carbide tools were mounted in a collet chuck.

The machining trials were made in hardened steel samples with the dimen-

sion 200×200×50 mm. A cavity was machined with the size 120×120×40 mm.
The walls of the cavity were angled 45

°.

The Influence of Steel Grade and Steel Hardness on Tool Life when Milling in...

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Figure 1.

Machining centre Modig MD7200.

The cutting speed used in the trials is the true cutting speed calculated

on the working diameter of the milling cutter. When milling with small
depth of cut with a ball nose end milling cutter, the working diameter on
the cutter is small. In the finishing milling trial the axial depth of cut was
0.15 mm. This means that the working diameter for a ∅ 10 mm cutter with
radius 5 mm was only 2.4 mm. This explains, why the maximum cutting
speed in the finishing milling trial only was 135 m/ min, as this cutting speed
represents 18 000 rpm in spindle speed, which was the maximum limit in
the machining centre.

Two different kinds of milling operations were investigated, rough milling

and finishing milling. The tool paths used in the investigation were of con-
stant Z-level pocketing strategy beginning from the centre of the work piece
and out towards the cavity sides. After finishing one level a helical interpo-
lation was done in the centre of the work piece to the next cutting level, see
Fig. 2. Compressed air was used to evacuate the chips.

The cutting tools used were two fluted coated ∅ 10 mm solid carbide

ball nose end mills VC2-SSB manufactured by Kobelco. The flank wear
was measured frequently during the tests, and the trial was stopped when a

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flank- or notch wear of 0,2 mm on the milling cutter was reached For rough
milling the measuring unit was the machined volume and for the finishing
step the machined surface area. The cutting speed was varied, the remaining
parameters were kept constant as follows:

Rough milling
Axial depth of cut, ap:

1.0 mm

Radial depth of cut, ae:

2.5 mm

Tooth feed, fz:

0.06 mm/tooth

Finishing milling
Axial depth of cut, ap:

0.15 mm

Radial depth of cut, ae:

0.15 mm

Tooth feed, fz:

0.15 mm/tooth

WORK MATERIAL

For the trials, tool steels with different properties and application areas

were chosen [1]. The analyses of the materials are given in Table 1.

DIEVAR and HOTVAR are steels used for hot work applications such as

die casting, extrusion and warm forging of metals. They are conventionally
made by ingot casting but also electro slag remelted (ESR), thus they have

Figure 2.

Milling strategy used in the investigation.

The Influence of Steel Grade and Steel Hardness on Tool Life when Milling in...

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Table 1.

Analysis and hardness of the investigated tool steels

Steel grade

C

Si

Mn

Cr

Mo

V

W

HRC

DIEVAR

0,4

0,3

0,5

5,0

2,3

0,6

51

HOTVAR

0,5

1,0

0,8

2,6

2,2

0,8

56

STAVAX ESR

0,4

0,9

0,5

13,6

0,3

50

CALMAX

0,6

0,3

0,8

4,5

0,5

0,2

54

CALDUR

0,7

1,0

0,8

2,5

2,1

0,5

60

ELMAX

1,7

0,8

0,3

18,0

1,0

3,0

56

VANADIS 6

2,1

1,0

0,5

6,8

1,5

5,4

60

VANADIS 10

2,9

1,0

0,5

8,0

1,5

9,8

62

VANADIS 23

1,3

0,5

0,3

4,2

5,0

3,1

6,4

61

a very low amount of inclusions in the steel structure. Working hardness is
in the range 44–52 HRC for DIEVAR and 54–58 HRC for HOTVAR.

STAVAX ESR is a conventionally made martensitic stainless steel, also

ESR-treated, and the steel is used for plastic injection moulds. The working
hardness is in the range 45–52 HRC.

CALMAX and CALDUR are conventionally made cold work tool steels,

which can be used for punching and pressing of plate. The working hardness
is in the range 56–62 HRC. The chosen materials are of the type with no
primary carbides in the microstructure.

ELMAX, VANADIS 6, VANADIS 10 and VANADIS 23 are powder met-

allurgical made tool steels, and they all contain a lot of hard primary carbides
in the steel structure. ELMAX is due to its high chromium content, corrosion
resistant and is used for plastic moulds where there is a need for good wear
resistance. VANADIS 6 and VANADIS 10 are cold work tool steels mainly
used for punching of plates and VANADIS 23 is a high speed steel, which
can be used for cutting tools. The working hardness for the PM steels is in
the range 56-64 HRC depending on type of steel grade and application.

All the tested steels were hardened and tempered to the hardness given in

Table 1.

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RESULTS

FINISHING MILLING

When finishing milling, the machined surface area in cm

2

as a function

of cutting speed was chosen as the unit of comparison. The results can
be seen in Fig. 3. In the figure results of two different cutting speeds are
given. The values without brackets are the true cutting speed calculated on
the working diameter, the values in brackets are the cutting speed calculated
on the maximum diameter on the cutter (10 mm).

For the PM steels different cutting speeds have a small influence on the

tool life when looking at machined surface area. The machined surface area
was more or less the same with different cutting speeds. The machined
surface area was also quite small, in the range 120–600 cm

3

.

Figure 3.

Machined surface as function of cutting speed when finishing milling (wear

criteria 0,2 mm flank wear).

The reason for the short tool life when milling in PM tool steels is firstly,

that the steels contain a lot of hard and abrasive carbides that produce a high
wear on the cutting tool, secondly, the steels are hardened to a high hardness.

The cold work materials containing no primary carbides have a quite high

hardness but due to the absence of primary carbides the machined surface
area is higher than for the PM steels. The biggest area machined is obtained
with the cutting speed 100 m/min. The tool life is longer for CALMAX than
for CALDUR probably due to the hardness difference.

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The plastic mould steel STAVAX ESR gave, in spite of the low hardness,

relatively low tool life.

The hot work tool steel DIEVAR gave the highest tool life of all the

investigated materials. At the cutting speed 100 m/ min, the tool lasted for
20 hours before it was worn out. One of the reasons for the long tool life is
probably that this steel had a low hardness. The second hot work tool steel
tested, HOTVAR, gave a considerable shorter tool life, which is due to the
higher hardness in this material.

ROUGH MILLING

Some of the materials, which were tested at the finishing milling, have also

been tested in rough milling. The results showed big differences between
different tool steels even tough the hardness was the same.

The results from the rough milling as the removed volume of work material

as function of cutting speed can be seen in Fig. 4. The cold work material

Figure 4.

Machined volume as function of cutting speed when rough milling (wear criteria

0,2 mm flank wear).

CALDUR and the PM steel VANADIS 6 have a rather low machinability
and the tool life is decreasing when the cutting speed is increased already
from 40–60 m/ min. Even tough the hardness is about the same for the PM
steel and CALDUR, the slope is at a lower level for the PM steel. This is
mainly due to the high primary carbide content in this material.

For the two steels with the lower hardness, DIEVAR and CALMAX, the

tool life is longer in CALMAX, which is opposite to the result obtained in

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the finishing milling trials. The reason for this can be that rough milling
gives more heat into the milling cutter. Earlier investigation [3] has shown,
that the tool life obtained when milling in DIEVAR is very dependent on the
cutting edge temperature. A high cutting edge temperature will give a low
tool performance.

Nevertheless, for all the materials tested in the rough milling stage, the

cutting speed has a rather strong influence on the tool life. If a high cutting
speed is chosen it will shorten the tool life considerable.

DISCUSSIONS AND ECONOMICAL ASPECTS

The investigation has shown that it is a big difference in machinability

between different tool steels when machining them in hardened condition.
Two materials with the same hardness can give quite different cutting tool
life. This means also that the cost to produce a die from PM tool steel will be
higher compared to a die produced from low alloyed tool steel. A customer
that today is using hot work tool steel for a die, for instance a die for a case
for a mobile phone, can have a tool life up to twenty hours on the milling
cutter when finishing milling. If he requires a more wear resistant material
in the die, for instance the PM steel ELMAX, the tool life will decrease to
around 40 minutes.

The cost to finish mill a specific surface area in relationship to the cutting

speed for the different materials is shown in Fig. 5. In the example it has been
calculated with machine costs of 110 Euro/hour and a cost for the milling
cutters of 110 Euro/cutter. The time for tool change is assumed to be 10
minutes.

For three materials (VANADIS 10, VANADIS 6 and VANADIS 23)

100 m/ min seems to be the cutting speed that gives the lowest production
cost for respectively material. For the other materials the lowest cost is at a
cutting speed of 135 m/ min or higher.

The example shows also the huge difference in cost to produce a specific

surface area for different materials. For the steels with the best machinability
the cost to mill one square centimetre is about 0,3 Euro, while to machine
the same surface in the material VANADIS 10 costs about 1,4 Euro

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Figure 5.

Total cost / cm

2

as function of cutting speed when finishing milling in the

different tool steels.

CONCLUSION

The new technique to machine a mould cavity direct in hardened steel has

much to offer regarding shortening of the lead times from design until the
mould run in production. This investigation has shown that the production
costs to machine a mould or die direct from hardened steel, is very dependent
on the work material. Not only the steel hardness has an influence, but also
the steel analysis has an important influence on the total cost to produce the
mould.

In most cases the higher production cost for a mould made from a high

alloyed tool steel be can neglected, compared to the better performance and
longer life of the mould in production of parts.

REFERENCES

[1] SANDVIK Coromant; Application Guide, Die & Mould Making.

[2] ASM INTERNATIONAL; Metals Handbook, Ninth edition, Volume 16, Machining.

[3] Internal report; Forskningsmeddelande, FM00-160-3