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18. - 20. 5. 2011, Brno, Czech Republic, EU
STRUCTURE AND PROPESTIES OF SUB-ZERO PROCESSED VANADIS 6
P/M LEDEBURITIC TOOL STEEL
Jana SOBOTOV a, Peter JURI a, Ji CEJP a, Petra SALABOV b, Otakar
PRIKNER b
a Czech Technical University in Prague, Faculty of Mechanical
Engineering, Karlovo nm. 13, 121 35 Prague 2, Czech Republic,
[email protected] b Prikner - tepeln zpracovn kov,
Martnkovice 279, 550 01, Czech Republic
Abstract
The P/M Vanadis 6 cold work steel was austenitized, quenched and
tempered at various combinations of processing parameters. For
selected sets of specimens, also sub-zero period, made at different
temperatures and processing dwell times was inserted between
quenching and tempering. The microstructure and mechanical
properties have been investigated as a function of austenitizing
temperature, parameters of sub-zero processing and tempering.
As-quenched structure is composed of martensite, retained austenite
and carbides. Sub-zero processing led to the decrease of the
retained austenite amount and increase of tetragonality of the
martensitic lattice. Therefore, the as-quenched hardness of
sub-zero processed samples was than that of no sub-zero processed
steel. On the other side, the use of sub-zero period results in a
slight hardness decrease after tempering. Furthermore it was found
that the three point bending strength, decreased markedly with
increased austenitizing temperature. The lowering of bending
strength with increasing austenitizing temperature can be
considered as natural because of the austenitic grain growth. The
situation for sub-zero treated steel seems to be more complex.
Bending strength for the material processed at -90C was lower while
the sub -zero processing at -196C led generally to increas e of the
bending strength.
Key words: P/M cold work steel, heat-treatment, sub-zero
treatment, bend strength, microstructure
1. INTRODUCTION
Vanadis 6 is a powder metallurgical cold work tool steel
offering a combination of very high wear resistance and good
toughness. High compressive strength and very good dimensional
stability during heat treatment are also typical properties of this
material. Based on these characteristics, Vanadis 6 is suitable for
long tooling of cold work materials (blanking, forming operations,
knives). Demanded tool life is given by means of heat treatment.
Suitable heat treatment depends on application. For optimum wear
resistance [1], recommended regime is a hardening from a
temperature of 1150C and 3x tempering at 560C, resulting in
hardness from the range 63-65 HRC. For optimum ductility is
directed heat treatment with lower temperatures (1000C, 2x 250C,
resulting hardness is stated 60-62 HRC). During the austenitizing,
eutectoid and a part of secondary carbides are dissolved in the
austenite and results in high hardness of the material after heat
treatment. Other part of carbides, which does not undergo the
dissolution, hinders the austenite grains coarsening and makes the
steels wear resistant. Blek et al. [2] reported that there are two
types of carbides in Vanadis after austenitizing. The M7C3 carbides
underwent intensive dissolution in the austenite and they were not
detected above the temperature of 1100C. On the other side, MC
carbides remained almost completely unaffected and symptoms of
dissolution were found only at the temperature of 1200C. The
saturation o f the austenite with carbon, chromium and partly also
vanadium results to an increased hardness of the as-quenched
material, with the maximum at the austenitizing temperature of
1025C. Preceding publ ication [3] states that higher austenitizing
temperature
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18. - 20. 5. 2011, Brno, Czech Republic, EU
results in hardness increase of Vanadis 6 and lowering of three
point bending strength, since increased austenitizing temperature
results in the grain coarsening. After the quenching, Cr-V
ledeburitic steels contain martensite, retained austenite and
undissolved carbides. For a more complete martensitic
transformation, sub-zero treatment can be inserted between
quenching and tempering. There are not doubtless opinions on the
effect of sub-zero processing on properties of ledeburitic steels.
In [1] it is reported a hardness increase of 1HRC after sub-zero
treatment (-70 and -80C/1-3 ho urs). Juri in [3] reported
increasing hardness for Vanadis 6 after sub-zero treatment (-90C/4
hours) compared to not sub-zero, but it is valid only before
tempering. As-tempered hardness of sub-zero material is by 2,5 -
3,5 HRC lower than that of non-sub-zero processed. This opinion is
in good agreement with [4]. Berns [5], on the other hand, reported
a significant hardness increase for the sub-zero processed X290Cr12
ledeburitic steel. Authors [3,4] reported a decrease of three point
bending with the application of sub-zero processing of Vanadis 6.
The nature of this effect is not clear yet and should by subjected
to further investigations. It is well known that the microstructure
is one of the most important parameters, which determines the wear
resistance of steel. Particularly for tool steels, the content and
the distance between primary and secondary carbides are the key
microstructural variables, controlling the tribological response
[6]. Stratton [7] reported for tool steels dramatically improved
wear resistance by deep cold treatment (-196C for minimum of 24
h).
2. EXPERIMENTAL
The experimental material was the PM ledeburitic steel Vanadis 6
with nominally 2,1 %C, 1,0 %Si, 0,4 %Mn, 6,8 %Cr, 1,5 %Mo, 5,4 %V
and Fe as balance. The hardness of as-received material was 284 HV
10. There were made two types of specimens. Three samples of
structural investigation and hardness measurement ( 17x10 mm) and 5
samples for the three point bend testing (10x10x100 mm) which were
fine ground to surface roughness of 0,2 0,3 m were heat treatment
in each set. Heat treatment was the following: vacuum austenitizing
at the temperatures from the range 1000 1075C for 30 min, nitrogen
gas quenchi ng at 5 bars pressure, sub-zero treatment at a
temperature -196C/4 or 10 hours and 2x tempering at 530C per two
hours. Three point bending tests have been carried out at following
parameters: the distance between supports was 80 mm, loading in the
central region and loading rate of 1 mm/min, up to the moment of
the fracture. Microstructure of the material was analysed by the
light microscopy.
3. RESULTS
It was stated recently [3] that three point bending strength
stated decreased with the increasing austenitizing temperature.
This reality is connected with grain coarsening with increasing of
austenitizing temperature. The effect of sub-zero processing on
three point strength depends on conditions. Figure 1 presents that
the three point strength after sub-zero treatment (-90C/4 hours)
and tempering 2x530C is lower and, afte r sub-zero treatment
(-196C/4 hours) is generally higher . Although there is a
difference in the bending strength for the given sub-zero treatment
regimes, the resulting hardness for the material processed by these
two regimes is practically the same, Fig. 2. In addition, the
hardness of sub-zero processed material is of about 2,5 HRC lower
than that of no sub-zero processed steel. Figure 3 brings a
comparison of the obtained three point bending strength results of
the material without sub-zero processing and sub-zero processed
steel at -196C for 4 and 10 hours, respectively. The hardn ess plot
for the material as a function of the austenitizing temperature and
sub-zero processing done at -196C is in Fig.4. Although there is a
slight difference in bending strength for monitored times of
sub-zero processing at -196C (lowering with longer soaking ti me),
the hardness practically does not depend on the time of the
sub-zero processing. These results are rather surprising, since one
can expect higher hardness after sub-zero processing compared to
that after no sub-zero treatment.
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Fig.1. The effect of sub-zero treatment on bending strength
(tempering 2x 530C)
Fig.2. The effect of sub-zero treatment on hardness (tempering
2x 530C)
Also, the three point bending strength can be expected to be
lower due to sub-zero treatment. But, the situation can be
considered to be more complex since various effects should be taken
into account in the assessment of the heat treatment response of
the examined material. There are at least three phenomena happened
during the tempering. Firstly, the tempering of the martensite
proceeds, which is always connected with the hardness decrease. The
transformation of the retained austenite during the cooling down
from the tempering temperature, corresponding to the temperature of
the secondary hardness peak, induces
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the increase of hardness. The last process is the precipitation
of carbides during the hold at the tempering temperature, which
also results in an increase of hardness.
Fig.3. The effect of the time of sub-zero processing on bending
strength (tempering 2x 530C)
Fig.4. The effect of the time of sub-zero processing on hardness
(tempering 2x 530C)
The amount of retained austenite was found to be three times
lower for the sub-zero processed steel than that of on sub-zero
processed [8]. In the same literature, there was an increased
tetragonality of the martensite in sub-zero processed material
established. These facts confirm that the microstructure of
sub-zero processed Vanadis 6 steel differ from that of no sub-zero
processed in many factors. In addition, Stratton et al. assumed
that a precipitation of nano-sized particles can happen during the
hold at a temperature of deep-cooling [7]. However, this effect can
only hardly be verified since the diffusion at such a
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18. - 20. 5. 2011, Brno, Czech Republic, EU
low temperature is practically impossible. Thus, the size of
these carbides can be expected well below the resolution of common
TEMs. But, based on the results presented here [8] and the
Strattons consideration, it should be noted that the contribution
of the retained austenite transformation to the final as-tempered
hardness can be expected to be much lower when the material was
sub-zero processed. Regarding to the martensite state after
quenching and/or sub-zero processing, there is no doubtless
interpretation of the effect of higher martensite supersaturation
(due to sub-zero period) on the as-tempered hardness. However, it
is known that the as-tempered hardness of plain carbon steels with
a near eutectoid carbon content (but slightly different) does not
differ significantly. Therefore, one can also assume that the
as-tempered hardness of the Vanadis 6 steels martensite developed
by the sub-zero process (with higher tetragonality) would also not
differ significantly from that generated by simple quenching. This
consideration seems to be a logical interpretation of lower
as-tempered hardness of sub-zero processed Vanadis 6 steel at the
current state of knowledge.
-196C/4 hours -196C/10 hours
Fig.5. The effect of the time of sub-zero processing -196C on
microstructure Vanadis steel (austenitizing 1000C, tempering 2x
530C).
-196C/4 hours -196C/10 hours
Fig.6. The effect of the time of sub-zero processing -196C on
microstructure Vanadis steel (austenitizing 1075C, tempering 2x
530C).
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The microstructures of the material quenched from 1000C and
1075C, sub-zero processing -196C/4 or 10 hours and tempering 2x
530C are show n in Fig.5.and 6. The material consists of the matrix
and carbides. The matrix contains mainly the tempered martensite,
the second phase is the retained austenite. Some portion of the
lower bainite could be also expected [3]. Carbides are very fine
and uniformly distributed in the structure. The material quenched
from 1000C contains more carbides than that quenc hed from 1075C,
since the carbides, mainly the M 7C3, are dissolved in the
austenite in a larger extent. This fact is in accordance with [2].
Nevertheless, possibility of magnification of light metallography,
are not enough for investigation changes of structure after
sub-zero processed Vanadis 6.
4. CONCLUSIONS
a) Three point bending strength of the Vanadis 6 steel is
generally higher for sub-zero processed samples at -196C for 4 h
than these of the materia l processed at -90C for 4 h or no
sub-zero processed. This becomes an importance with increasing
austenitizing temperature.
b) Prolonging of the sub-zero period to 10 h (at -196C) does not
bring any benefits with respect to t he three point bending
strength.
c) The hardness of sub-zero processed (-196C/4 and 10 hours) is
of about 2,5 HRC lower than that of no sub-zero processed Vanadis
6, and practically the same as that after deep cooling to -90
oC/4h, and does not depend on the sub-zero processing time.
d) The microstructure of sub-zero processed Vanadis 6 steel
probably differ from that of no sub-zero processed in many factors.
Nevertheless, they can not be evaluated with the light microscopy
and more detailed checking with TEM seems to be necessary.
ACKNOWLEDGEMENTS
Authors wish to thank the Ministry of Industry and Trade of the
Czech Republic for the financial support for the solution of the
Project TIP FR-TI1/003
REFERENCES
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Ledeburitic Tool Steels
Depending on Temperature Austenitization, Materials and
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