THE BEHAVIOUR OF TWO NEW TOOL STEELS

REGARDING DIMENSIONAL CHANGE

R. Pierer

Institut f¨ur Eisenh¨uttenkunde, Montanuniversit¨at Leoben, Franz – Josef – Straße 18, A –

8700 Leoben / ¨

Osterreich

R. Schneider

Unterausschuss für W¨armebehandlung und H¨artereitechnik der Eisenh¨utte ¨

Osterreich /

B¨ohler Edelstahl GmbH & CoKG Mariazellerstraße 25, A – 8605 Kapfenberg / ¨

Osterreich

H. Hiebler

Institut für Eisenhüttenkunde, Montanuniversität Leoben, Franz – Josef – Straße 18, A –

8700 Leoben / ¨

Osterreich

Abstract

This paper investigates the distortion behaviour of the cold work tool

steel B¨ohler K360 and the plastic mould steel B¨ohler M340. Samples of
these two steels were produced for this purpose and classified as samples
perpendicular and parallel to the rolling direction of the bar. The experimental
set-up was designed to investigate the influence of the following parameters:
austenitising temperature, quenching medium and tempering temperature.

First, important interactions influencing dimensional change and change in

shape during heat treatment will be described. The results of the experiments
will be presented and discussed.

The results for K360 show a higher dimensional change in the thickness

than in the width at all three austenitising temperatures. At a tempering
temperature of approximately 525

◦

Can increase in the thickness and width

can be observed. At a hardness of about 60 HRc the dimensional change of
K360 is similar to that for Böhler K340 and 1.2379.

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In M340 shrinkage occurs. Only with sub-zero cooling (-80

◦

C) and with

high tempering temperatures can a few positive dimensional changes be ob-
tained. Generally, the dimensional change of plastic mould steels is less than
that for ledeburitic 12% - Cr cold work tool steels. The dimensional change
of M340 is comparable to 1.2083.

Keywords:

Dimensional change, tool steels, thermal treatment

INTRODUCTION

This article describes investigations on the dimensional change of two

new tool steels during heat treatment. The two new tool steels are a cold
work tool steel (K360) and a plastic mould steel (M340). The following heat
treatment parameters were investigated: quenching medium, austenitising
temperature and tempering temperature

The influence of the rolling direction was also considered using two types

of samples. The terms "dimensional change", "change in shape" and "dis-
tortion" were no longer defined in the 1988 edition of DIN 17014. In the
following section, the definitions from DIN 17014 (1975) are presented.

"Dimensional change" refers to the change in dimension of a component

without change in shape. "Changes in shape" are characterized by changes
of curvature and angles. "Distortion" is defined as dimensional change and
change in shape [1]. EN 10052 defines distortion as every dimensional
change and change in shape of a component, compared to the initial size,
as a result of heat treatment [2]. According to H. Berns, the most important
aspects in the development of dimensional change are the following [3]:

residual stress may already be present before heat treatment;

during heat treatment (heating and cooling) thermal stress is caused
by the difference in temperature between surface and centre of the
component;

if thermal stress occurs in combination with microstructural transfor-
mation, the temporal succession plays an important role;

if the residual stress shows elastic behavior during heat treatment, no
distortion occurs;

if symmetrical stress relief takes place as a result of plastic deforma-
tion, only dimensional change occurs;

The Behaviour of two New Tool Steels Regarding Dimensional Change

613

Figure 1.

Interactions of temperatures – microstructure – stress/strains [4].

if asymmetrical plastic deformation takes place, the shape changes.

Figure 1 shows a detailed schematic diagram of the interactions be-

tween temperature, microstructure (phase transformation) and stress/strain
in quenched steel.

The temperature gradient of the component as a function of time deter-

mines the phase transformations during quenching. On the other hand, the
heat released during phase transformation influences the cooling process.
Due to the difference in temperature between surface and centre, thermal
stress is caused. In addition, stresses caused by transformation are added as
a result of the different physical properties of the emerging phase. Trans-
formations can also be induced by deformation and stress. The mechanical
work of deformation produces heat, which again influences the cooling pro-
cess. The temporal succession of transformation stress and thermal stress
has an important influence on plastic deformation [4]. This results in a
variety of possible differences in dimensional change and deformation.

EXPERIMENTAL PROCEDURE

The dimension of the samples was

65×65×12 mm and a distinction was

made between samples perpendicular and parallel to the rolling direction of
the bar. This difference in the samples is illustrated in Fig. 2.

The samples were produced with the aid of a milling machine and were

milled to 1 mm oversize. Afterwards, stresses were removed by stress relief

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 2.

Sampling – samples perpendicular (type 1) and parallel (type 2) to the rolling

direction.

Table 1.

Chemical composition

C

[%]

Si

[%]

Mn

[%]

Cr

[%]

Mo

[%]

V

[%]

other

1.2379

1.55

0.25

0.35

11.80

0.80

0.95

–

–

K340

1.10

0.90

0.40

8.30

2.10

0.50

+Al

+N b

K360

1.25

0.90

0.35

8.75

2.70

1.20

+Al

+N b

1.2083

0.41

0.70

0.45

14.30

0.60

0.20

–

–

M340

0.54

0.45

0.40

17.30

1.10

0.10

+N

–

annealing (650

◦

C ) in order to obtain a defined initial size, and finally all

sides were ground to obtain the finished size. The samples were measured
according to an exactly defined scheme before and after the heat treatment.
The thickness of the samples was measured with 13 measuring points; width
1 and width 2 with five measuring points each. An experimental set-up was
designed to investigate three austenitising temperatures and six tempering
temperatures for each case. K360 was mainly investigated using salt-bath
quenching (230

◦

C ) and M340 using oil quenching. Table 1 shows the

nominal chemical composition of both steels and the reference steels.

B ¨ohler K360 is a new, ledeburitic cold work tool steel with an excellent

combination of toughness, abrasive wear resistance, adhesive wear resis-
tance, hardness, and compressive yield strength. In comparison to the 12%
- chromium cold work tool steels (such as 1.2379) with very good abra-
sive wear properties but poor toughness, K360 combines this high abrasive
wear resistance with the very good toughness of the 8% - chromium cold
work tool steels. The very good combination of properties and the excellent

The Behaviour of two New Tool Steels Regarding Dimensional Change

615

toughness result from the very homogeneous microstructure of this steel.
Due to the excellent combination of high hardness and very good toughness
it is possible to reach an outstanding compressive yield strength. Applica-
tions of K360 are for cutting tools, stamping tools, thread rolling tools and
machining tools.

B ¨ohler M340 is a plastic mould steel with outstanding corrosion prop-

erties (especially in salt water and media containing chlorine), high wear
resistance, good through-hardenability and a high obtainable hardness after
hardening (53 – 58 HRc). To meet these requirements, M340 is alloyed with
nitrogen in addition to the usual alloying elements for plastic mould steels.
Nitrogen improves the corrosion resistance and also the wear resistance due
to a homogeneous distribution of fine precipitates. The fine, homogeneous,
crystalline structure is the reason for its excellent processing and perfor-
mance characteristics. Applications of M340 are for moulds, mould inserts,
cutting tools and screws.

RESULTS

INFLUENCE OF THE QUENCHING MEDIUM (K360)

The influence of the quenching medium was investigated on samples per-

pendicular to the rolling direction. The experiments were carried out using
salt-bath quenching (230

◦

C ), oil quenching (30

◦

C ) and gas quenching.

The gas quenching was carried out in four different vacuum furnaces; three
used nitrogen and one helium as the quenching medium. The austenitising
temperature was 1060

◦

C and there was no tempering after quenching.

Figure 3 shows the dimensional change in the thickness (left graph) as

a function of the quenching medium. The graphs in Fig.3 show a positive
dimensional change in the thickness and width. The dimensional change in
the thickness is higher than the dimensional change in the width for all three
quenching media. The influence of the quenching medium on the change in
the thickness can be described in the following way. Oil quenching causes
the greatest dimensional change. The dimensional change decreases from oil
to salt- bath quenching and the lowest dimensional change can be observed
in gas quenching. The values for gas quenching show a scatter band due
to the use of four different vacuum furnaces. There is no clear influence
of the quenching medium on the change of dimension in the width, but the
dimensional change is less than 0.1 % in all three cases.

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 3.

Influence of the quenching medium.

DIMENSIONAL CHANGE AS A FUNCTION OF
AUSTENITISING AND TEMPERING TEMPERATURES
(K360)

The dimensional change as a function of the austenitising temperature

was also investigated on samples perpendicular to the rolling direction. The
austenitising temperatures were 1040

◦

C , 1060

◦

C and 1080

◦

C . Figure 4

(1040

◦

C and 1080

◦

C ) and Fig. 5 (1060

◦

C , right graph) illustrate the results

of these investigations.

The dimensional change in the thickness is positive for all three austeni-

tising temperatures and higher than the dimensional change in the width.
Above a tempering temperature of approximately 500

◦

C , the dimensional

change increases. This behaviour was found for all three austenitising tem-
peratures. The dimensional change in the width is positive with austenitising
temperatures of 1040

◦

C and 1060

◦

C and at all tempering temperatures. Only

the highest austenitising temperature of 1080

◦

C combined with tempering

temperatures of 200

◦

C and 500

◦

C shows a negative dimensional change.

Similar to the behaviour in the thickness, the dimensional change in the
width increases at tempering temperatures of approximately 500

◦

C . By way

of conclusion it can be said that the dimensional change in the thickness and
the width generally increases with increasing austenitising temperature and
that there is a rise at tempering temperatures above approximately 500

◦

C .

The Behaviour of two New Tool Steels Regarding Dimensional Change

617

Figure 4.

Influence of austenitising and tempering temperatures on dimensional change.

INFLUENCE OF THE ROLLING DIRECTION (K360)

The samples were classified as perpendicular and parallel to the rolling

direction of the bar in order to investigate the influence of carbide stringers.
So far, samples perpendicular to the rolling direction have been described
(width 1 and width 2 are normal to the rolling direction). All samples
taken parallel to the rolling direction were investigated at a austenitising
temperature of 1060

◦

C in order to compare them with the results of the

samples taken perpendicular to the rolling direction.

Figure 5 (left graph) shows the results of the dimensional change in the

thickness, width 1 and width 2. For samples taken parallel to the rolling
direction, thickness and width 1 are normal to the rolling direction. Figure
5 shows the comparison of both types of sample. It can be observed that
the dimensional change in width 2 is higher than in width 1, i.e. the di-
mensional change in the rolling direction (width 2) is higher. The thickness
of the samples taken parallel to the rolling direction is also normal to the
rolling direction. Accordingly, the dimensional change in the thickness of
these samples is less than that in samples taken perpendicular to the rolling
direction. There is no difference in the dimensional change between width
1 (parallel to the rolling direction) and width 2 (perpendicular to the rolling
direction) for samples taken perpendicular to the rolling direction. This cor-
responds to the fact that both directions are normal to the direction of the
carbide stringers.

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 5.

Comparison of samples perpendicular (type 1) and parallel (type 2) to the rolling

direction.

Figure 6.

K360 compared to K340 and 1.2379.

K360 COMPARED TO 1.2379 AND K340

Figure 6 shows the comparison of K360, K340 and 1.2379 for salt-bath

quenching and gas quenching (vacuum furnace). The aim of this experiment
was to reach a hardness of approximately 60 HRc. The results for K340 are
taken from [5]. The experiment was carried out with samples taken per-
pendicular to the rolling direction. The following heat treatment parameters
were used:

The Behaviour of two New Tool Steels Regarding Dimensional Change

619

Figure 7.

Influence of the quenching medium.

K360 and K340.

austenitising temperature: 1060

◦

C / salt bath quenching

or gas / tempering temperature: 560

◦

C

(3 × 1h)

1.2379.

austenitising temperature: 1060

◦

C / salt bath quenching or gas /

tempering temperature: 525

◦

C

(3 × 1h)

K340 could not be considered in the gas quenched condition, because no

results for this condition are given in [5]. When the results for dimensional
change in the thickness of all three steels are compared, it is found that
salt–bath quenching causes a higher dimensional change than gas quenching.
Following salt–bath quenching the change in the thickness of K360 is greater
then that of K340 which in turn is greater then that of 1.2379. For gas
quenching the dimensional change of K360 is only marginally different to
1.2379.

As mentioned above the dimensional change in the width is substantially

smaller than in the thickness. In this case, the results also show a smaller
difference in the dimensional change following gas quenching than that
following salt-bath quenching.

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INFLUENCE OF THE QUENCHING MEDIUM (M340)

The investigation on M340 was carried out in the same way as that on

K360. Salt-bath quenching (230

◦

C ), oil quenching (30

◦

C ) and gas quench-

ing were investigated using an austenitising temperature of 1020

◦

C and there

was no tempering after quenching. Samples perpendicular to rolling direc-
tion were used. Figure 7 shows two graphs of the dimensional change as a
function of the quenching medium. The graph on the left shows the results
for thickness and the other one shows the results for width.

For salt-bath quenching, positive dimensional change can only be ob-

served in the thickness. Oil and gas quenching cause a negative dimensional
change in both directions. In the width, however, the negative dimensional
change is higher. In conclusion it can be stated that salt-bath quenching
causes a positive dimensional change in the thickness and a negative dimen-
sional change in the width. The dimensional change of oil and gas quenched
samples is negative in both directions, more markedly in the width.

DIMENSIONAL CHANGE AS A FUNCTION OF
AUSTENITISING AND TEMPERING TEMPERATURES
(M340)

The austenitising temperatures were 980

◦

C , 1000

◦

C and 1020

◦

C for these

investigations. Figure 8 (980

◦

C , 1000

◦

C ) and Fig. 9 (1020

◦

C , right graph)

show the dimensional change as a function of tempering temperature. Re-
sults are shown on samples taken perpendicular to the rolling direction and
for oil quenching.

The dimensional change in the thickness is positive at an austenitising

temperature of 980

◦

C and at all tempering temperatures. The maximum

value can be observed at a tempering temperature of 450

◦

C . At austenitis-

ing temperatures of 1000

◦

C and 1020

◦

C the thickness and the width show a

negative dimensional change. A positive dimensional change in the thick-
ness can only be found at higher tempering temperatures.

Generally, the dimensional change of M340 in both directions gradually

reaches negative values with increasing austenitising temperatures.

The Behaviour of two New Tool Steels Regarding Dimensional Change

621

Figure 8.

Influence of austenitising and tempering temperatures on dimensional change.

Figure 9.

Comparison of samples taken perpendicular and parallel to the rolling direction.

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 10.

M340 compared to 1.2083 and 1.2379 without sub zero cooling.

INFLUENCE OF THE ROLLING DIRECTION (M340)

Figure 9 shows the comparison of samples taken perpendicular and par-

allel to the rolling direction. The austenitising temperature was 1020

◦

C .

The differences in these two samples will be explained again as follows: in
samples taken perpendicular to the rolling direction, width 1 and width 2
are normal to the rolling direction. In samples taken parallel to the rolling
direction the thickness and width 1 are normal to the rolling direction.

When comparing width 1 and width 2 of samples taken parallel to the

rolling direction the left graph shows a higher negative dimensional change
in width 2 (rolling direction). Comparing the change in thickness of samples
taken perpendicular and parallel to the rolling direction no clear influence
can be observed.

M340 COMPARED TO 1.2083 AND 1.2379

The comparison of M340 to 1.2083 was carried out using the following

heat treatment:

austenitising temperature: 1020

◦

C / oil quenching / sub zero cooling:

-80

◦

C tempering temperature: 200

◦

C

(2 × 2h)

The results of the dimensional change in thickness and width from this

investigation are shown in Fig. 10. Before analysing the results it should be
noted that the dimensional change in plastic mould steels is smaller than in

The Behaviour of two New Tool Steels Regarding Dimensional Change

623

cold work tool steels. The bar for 1.2379 in Fig. 10 should explain this. The
dimensional change is positive in all cases caused by sub-zero cooling. The
dimensional change of M340 is higher in the thickness than in the width and
M340 is comparable to 1.2083.

DISCUSSION

The dimensional change during quenching is a result of the interactions

between change in shape due to thermal stress and the change of volume
due to transformation stress. If there is a martensitic transformation due to
very fast quenching, there is an increase of dimensional change and change
in shape of the component [5]. As the difference of temperatures from
surface to centre decreases from oil quenching to salt-bath quenching and
further to gas quenching, there is also a simultaneous decrease in the thermal
stress. This can be recognised in K360 in the direction of thickness with an
decreasing dimensional change from oil quenching to salt-bath quenching
and further to gas quenching. The higher dimensional change of M340 af-
ter salt-bath quenching compared to oil quenching can be explained in the
following way [7]: The increase in dimensional change due to the transfor-
mation stress will be compensated for by the decrease in dimensional change
due to thermal stress.

With increasing austenitising temperature the dissolution of carbides and

nitrides increases and with that the C and N – concentration of the austenite
increases too. The result is a more expanded martensite with a positive
dimensional change when compared to the initial state. At the same time
there is a decrease in the Ms – temperature and the residual austenite produces
negative dimensional change. This negative dimensional change, however
decreases due to the enlargement of the austenite lattice with increasing
austenitising temperature [8]. With increasing tempering temperatures the
values of the dimensional change decrease due to the relief of the martensite
(transformation of tetragonal martensite to cubic martensite). The following
increase in dimensional change can be explained by transformation of the
residual austenite to martensite [9].

The influence of the rolling direction can be explained, according to

Frehser, in the following way [10]. Due to rolling, the carbide spacing
is in the rolling direction. The carbides have a poor thermal expansion co-
efficient compared to the surrounding microstructure. For this reason the
carbides hinder the expansion of the surrounding matrix during heating. At

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6TH INTERNATIONAL TOOLING CONFERENCE

quenching, the difference in contraction between carbide and matrix can not
be compensated for, and the result is a higher dimensional change in the
direction of rolling.

The dimensional change of K360 is similar to that for K340 and with

gas quenching the K360 is comparably low to the 1.2379 distortion. The
influence of the quenching medium is clearly higher, regarding dimensional
change, than the selection of the materials. K360 shows a positive dimen-
sional change in all cases, and in the thickness a higher dimensional change
is obtained. The influence of the rolling direction is not as high.

The dimensional change of M340 is very small and can be compared with

that of 1.2083. In M340 shrinkage occurs. Only with sub-zero cooling and
with high tempering temperatures can a few positive dimensional changes
be obtained. The influence of the rolling direction is again not as high.

ACKNOWLEDGMENTS

This master theses was carried out as part of a project for the "Unterauss-

chuss f¨ur Wärmebehandlung und H¨artereitechnik der Eisenh¨utte ¨

Osterreich"

(heat treatment sub-committee of the Austrian Society for Metallurgy).

The authors would like to thank Dr. Herwig Altena (Aichelin GmbH),

Ing. Friedrich Decker (H¨arterei B ¨ohlerstahl GmbH), Erich Lingenh¨ole (Lin-
genh¨ole Techn. GmbH), DI G ¨unter R ¨ubig and DI Georg Reithofer (R ¨ubig
GmbH & CoKG).

REFERENCES

[1] DIN 17014 (1975)

[2] EN 10052: Begriffe der Wärmebehandlung von Eisenwerkstoffen. Beuth-Verlag

[3] H. BERNS, AWT-Seminar, Berlin 1983

[4] A.J. FLETCHER, Elsevier Science Publishers Ltd. Essex (1989)

[5] Schweizerische Gesellschaft für Wärmebehandlung, Microtechnik Scriptar, Lausanne

(1971)

[6] H. BERNS, Werkstofftechnik 8 (1977), p. 149

[7] J. FREHSER and O. LOWITZER, Stahl und Eisen 77 (1977), p. 1221

[8] H. BERNS and W. TROJAHN, HTM 38 (1983) 1, p. 18

[9] E. HABERLING and H.H. WEIGAND, Thyssen Edelst. techn. Bericht, 9. Band (1983)

2, p. 89

[10] J. FREHSER, Archiv für das Eisenhüttenwesen 24 (1953), p. 483