NEW B ¨

OHLER POWDER METALLURGY HIGH

SPEED STEEL WITH EXCELLENT HOT HARDNESS

I. Maili, R. Rabitsch, W. Liebfahrt, H. Makovec and E. Putzgruber

BÖHLER EDELSTAHL GMBH & Co KG

Mariazellerstr. 25

A – 8605 Kapfenberg

AUSTRIA

Abstract

High Performance Cutting – that means reducing production costs by using
higher cutting speeds and by reducing the amount of coolant and lubricant
(for example using dry cutting).

For the cutting tools this means a much higher requirement for strength

and hot hardness than under normal cutting conditions.

Böhler has therefore developed a new super-hard powder metallurgy high

speed steel which is more highly alloyed than state-of-the-art high speed
steels. This new Böhler grade has a well-balanced chemical analysis and
is produced via the new Böhler-Uddeholm-Powder-Technology route, which
includes the most modern production facility available for powder metallurgy
steels. This brings the advantages of high hardness up to 70 HRc, high wear
resistance, superior hot hardness in combination with very good machinability
and a better ductility than comparable Powder Metallurgy High Speed Steels.

With all these advantages and especially with the advantage in its hot

hardness, the new Böhler steel is excellent for cutting tools such as end mills,
milling cutters, hobs, etc. Due to its high wear resistance and very high
compression yield strength this steel grade can also be used for special cold
work applications.

Keywords:

Powder metallurgy, High speed steel, Hot hardness

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6TH INTERNATIONAL TOOLING CONFERENCE

INTRODUCTION

The development of the first high-speed steels started over 100 years ago.

High speed steels were developed to machine metals at high temperatures
and with minimum tool wear.

New high speed steels with different chemical compositions were con-

tinuously developed to meet the ever increasing demands placed on cutting
tools. Today, there are definite trends in the machining industry towards
cost reduction, which means increasing the cutting speed and decreasing the
amount of lubricant as far as dry cutting. These extreme requirements for
cutting tools have led to the fact that state-of-the-art high-speed steels are
often not sufficient, so that cemented carbide tools are used although there
are numerous disadvantages. A cemented carbide tool has a very high price;
it is much more difficult to machine and because of its modest toughness
a tool made of cemented carbides is susceptible to impact which leads to
untimely breakage.

This is the reason that higher alloyed high speed steels are still in demand

[1, 2]. These highly alloyed steels can only be produced by powder metal-
lurgy methods, which is why Böhler Edelstahl reacted by installing the most
modern PM-plant in the world [3].

Accordingly for Böhler Edelstahl this resulted in the development of a new

highly alloyed steel grade, which is distinguished in an optimum combination
of relevant properties.

NEW STEEL GRADE: BÖHLER S290PM

MICROCLEAN

BACKGROUND TO THE ALLOY DESIGN AND
CHEMICAL COMPOSITION

At the moment, Böhler’s top benchmark in the field of PM-HSS is Böhler

S390PM Microclean. With its advantages of a hardness up to 68 HRc, a very
good wear resistance and hot hardness in combination with good toughness
and best machinability, Böhler S390PM Microclean has fixed its position in
the market. New trends in the cutting industry, such as high-speed-cutting
or dry cutting, increase the demands placed on cutting-tools regarding wear
resistance and hot hardness. For this reason, cemented carbides are used
even though the machinability and the toughness of the tools are inadequate.

New B¨ohler Powder Metallurgy High Speed Steel with Excellent Hot Hardness

379

Böhler saw the necessity to develop a new high-speed-steel which can fill

the gap between cemented carbides and Böhler S390PM, see Fig. 1. This

Figure 1.

Background to the development of the new PM-HSS grade S290PM Microclean.

is the reason that the target was set to produce a new PM-HSS with the
following properties:

a hardness capability of about 70 HRc

a much higher thermal stability than that of S390PM

adequate toughness properties

similar machinability to S390PM and better machinability than other
highly alloyed PM-HSS with more than 9 wt% cobalt

The new alloy was designed with an increased volume fraction of primary

carbides compared to Böhler S390PM. But the high hardness capability was
not only reached because of a very high carbide content.

By a specific intervention in the chemical composition of the matrix,

the microstructural coarsening and the growth of the secondary carbides
were positively influenced regarding hot hardness. A good toughness was
achieved by balancing the chemical composition, which influences the ma-
trix and leads to a limited number of primary carbides.

The chemical composition of the new PM-HSS grade S290PM Micro-

clean in comparison to Böhler S390PM is shown in Table 1.

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6TH INTERNATIONAL TOOLING CONFERENCE

Table 1.

Chemical composition of the new PM-HSS grade S290PM Microclean in com-

parison to Böhler S390PM Microclean

Steel grade

% C

% Cr

% W

% Mo

% V

% Co

S390PM

1.6

4.8

10.5

2.0

5.0

8.0

S290PM

2.0

3.7

14.5

2.5

5.0

11.0

PROPERTIES OF THE NEW STEEL GRADE BÖHLER

S290PM MICROCLEAN

Hardening and Tempering Behaviour.

Figure 2 shows the harden-

ing and tempering behaviour of Böhler S290PM in comparison to Böhler
S390PM for austenitising temperatures ranging from 1150

◦

C up to 1210

◦

C .

Figure 2.

Hardening and tempering behaviour of the new PM-HSS grade S290PM Mi-

croclean in comparison to Böhler S390PM Microclean.

All of the samples were austenitised in a vacuum furnace, quenched by N

2

gas down to room temperature, and tempered three times for 2 hours each.
As can be seen in Fig. 2, within the temperature range relevant for highly
alloyed high-speed-steels, hardness values up to 70 HRc are achievable.

New B¨ohler Powder Metallurgy High Speed Steel with Excellent Hot Hardness

381

All of the samples austenitised at 1190

◦

C and 1210

◦

C achieved about

70 HRc after tempering 3 times at 560

◦

C , which is the minimum tempering

temperature usually recommended for the elimination of retained austenite
and optimum toughness. At this stage the hardness of the S290PM is clearly
higher than that of the S390PM. Hardening from 1150

◦

C and tempering 3

times at 540

◦

C or 560

◦

C resulted in a hardness of 69+ HRc which is an excel-

lent value when considering that this heat treatment is a typical commerical
heat treatment for non-cutting tool applications.

Toughness.

The toughness of the new PM-HSS grade S290PM Micro-

clean is positively affected by:

a very well balanced chemical analysis of the matrix

a very homogeneous microstructure

very fine and homogeneously distributed carbides

very low non-metallic inclusion content (K0 = 0 – 1 ) [4]

Figure 3 compares the toughness, in the form of impact energy, of the

new steel-grade to Böhler S390PM and its competitors.

Figure 3.

Toughness of the new PM-HSS grade S290PM Microclean in comparison to

Böhler S390PM Microclean and its competitors.

All of the samples were hardened from 1150

◦

C in vacuum, cooled by

N

2

-gas quenching to room temperature and were tempered three times to a

hardness value of 67 HRc.

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6TH INTERNATIONAL TOOLING CONFERENCE

Böhler S390PM has a well-known reputation for a very good toughness

even at a high hardness value. As is shown in Fig. 3, it reaches an impact
energy value of more than 30 J/cm

2

at the chosen heat treatment parameters.

Böhler S290PM Microclean is of course not as tough as the Böhler

S390PM Microclean because of its higher alloying element content – but
it reaches an impact energy value of more than 20 J/cm

2

which is an amaz-

ing result especially when compared to its highly alloyed competitors.

Hot Hardness.

Figure 4 was constructed by testing the materials using

the laboratory facility at the Materials Center Leoben [5].

Figure 4.

Hot hardness of the new PM-HSS grade S290PM Microclean at a temperature

of 650

◦

Cin comparison to its competitors.

The effect of isothermal tempering on the hardness changes of the new

PM-HSS grade S290PM Microclean in comparison to its competitors is
shown in Fig. 4.

All the samples were hardened at 1190

◦

C in a vacuum furnace and tem-

pered to a hardness value of 68 HRc. The hot hardness shown in Fig 4
was measured at a temperature of 650

◦

C over time. The temperature of

650

◦

C was chosen because it is a typical temperature at the tip of a tool for

example when cutting non-ferrous-metals (e.g. aluminium).

The hot hardness is expressed by dynamic hardness values which cannot

easily be converted to the Vickers scale. But by testing fully heat-treated
steel grades this method has been proven as a new and powerful tool for
obtaining quantitative data for softening at high temperatures.

New B¨ohler Powder Metallurgy High Speed Steel with Excellent Hot Hardness

383

The new PM-HSS grade S290PM Microclean exhibits a higher hot hard-

ness level than Böhler S390PM and other highly alloyed PM-HSS with more
than 9 wt% cobalt. For a cutting tool during operation this means that a Böh-
ler S290PM-tool does not soften as fast, and this indicates that the life time
of the tool would probably be substantially higher. This was confirmed in
practical cutting tests carried out by tool-makers.

Wear resistance / compressive strength.

Wear behaviour and com-

pressive strength are very important criteria for the efficiency of alloyed
tool steels. They are characterised by parameters such as carbide hardness,
carbide percentage, carbide size and distribution, and matrix hardness.

Figure 5 compares the optical micrographs of Böhler S290PM and Böhler

S390PM. The samples were austenitised at 1210

◦

C and multiply tempered

at 560

◦

C .

Figure 5.

Polished and etched microstructure of Böhler S390PM Microclean (a) and the

new PM-HSS grade S290PM Microclean (b). Samples austenitised at 1210

◦

C

The total carbide volume in Böhler S290PM is significantly greater than

that of Böhler S390PM. Because of the higher amount of carbides in Böhler
S290PM and because of the higher alloyed matrix, Böhler S290PM is of
course better in its wear resistance and its compressive strength.

Despite this, tool makers have reported that Böhler S290PM is not more

difficult to machine than Böhler S390PM.

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6TH INTERNATIONAL TOOLING CONFERENCE

SUMMARY

The most modern Powder Metallurgy plant in the world has been installed

at Böhler Edelstahl GmbH & Co KG in Kapfenberg, Austria. This plant
makes it possible to produce highly alloyed steel grades.

A new steel grade, BÖHLER S290PM Microclean, has been developed

for use in cutting applications such as end mills, milling cutters, hobs, etc.
and it can also be successfully used for particular cold work applications
such as punching metal flats or pressing metal powder.

A very well balanced chemical analysis resulted in a high hardness of

about 70 HRc, however the high hardness is not only achieved by an extremly
high carbide content, but also by a specific influence on the matrix. This - in
combination with production via the brand new Powder Metallurgy method
at Böhler with its advantages of best cleanliness, finest powder-size, finest
carbide-size and highly homogeneous carbide-distribution – ensures that
BÖHLER S290PM Microclean shows best toughness and best hot hardness
and is easy to machine.

All of these extraordinary advantages make the new steel grade BÖHLER

S290PM Microclean a cost effective alternative to carbide cutting-tools and
to cemented carbides in cold-work applications, which may be too brittle or
to difficult to machine.

ACKNOWLEDGMENTS

The authors are grateful to the Institut für Metallkunde und Werkstoffprü-

fung at the Montanuniversität Leoben and to the Materials Center Leoben
for carrying out the hot hardness investigations.

REFERENCES

[1] P. HELLMAN, "High speed steels by powder metallurgy", Veitsch-Radex Rundschau

1/1999, p.16

[2] P. HELLMAN, D. FRANCHI and G. PANIALE, International Conference on Tooling

Materials, Interlaken, Sept. 1992, p.551–559.

[3] C. TORNBERG, A. FÖLZER, "Fully Dense PM Tool Steels and High Speed Steels

with 3rd Gereration Processing Technology", PM2001, p.121.

[4] DIN 50 602

[5] H. LEITNER, R. EBNER, B. MAJOR and G. PÖCKL, "Investigations on the ther-

modynamics and kinetics of secondary hardening carbides in high speed steels", 5th

New B¨ohler Powder Metallurgy High Speed Steel with Excellent Hot Hardness

385

International Conference on Tooling, Leoben 1999, p.369.