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MATERIALS FORUM VOLUME 30 - 2006 Edited by R. Wuhrer and M.
Cortie Institute of Materials Engineering Australasia Ltd
THE STUDY OF WEAR RESISTANCE OF A HOT FORGING DIE, HARDFACED BY
A COBALT-BASE SUPERALLOY
M. Farhani1, A. Amadeh1, H. Kashani1 and A. Saeed-Akbari2*
1Department of Metallurgy and Materials Engineering, Faculty of
Engineering, Tehran University.
2 Faculty of Georesources and Materials Engineering, RWTH Aachen
University, Germany *Corresponding Author: Alireza Saeed-Akbari,
Rudolfstrae 27, 52070, Aachen, Germany.
[email protected]
Tel: (+49241) 9976908 ABSTRACT During hot working processes, due
to the simultaneous presence of high temperature and high stress,
the relevant dies are under a variety of failure mechanisms. The
predominant mechanism depends on the process and its parameters;
however various wear mechanisms are known to be of the most
important die failure mechanisms. Surface engineering techniques
are used to combat wear. In the current study, the hardfacing of a
hot forging steel die (H11) by a Cobalt-based super alloy (Stellite
21), was used to study the improvement of wear resistance and the
lifetime of the die. Initially some testing blocks of the H11 steel
were prepared and then heat treated as of the considered dies. Then
the hardfacing process by the TIG method was performed on the
testing blocks. Finally, the testing blocks properties, before and
after the pin-on-disk wear experiments, was studied using the
optical microscopy and hardness testing. Wear tests were performed
at three different temperatures: room temperature, 400 and 550 C.
After evaluating of the experimental results, a sample die was
hardfaced and practiced in service and its dimensions were
regularly controlled during service. After a rather long working
time, this was brought out of service. The metallographic and
hardness testing samples were prepared from the sample die.
Comparing the results of the hardfaced and H11 dies and samples,
indicated that, the increasing of the high temperature hardness due
to the formation of a hard and resistant layer on the surface of
hard-faced die, results in the substantial improvement in its wear
resistance and lifetime. 1. INTRODUCTION Hot forging is one of the
oldest metal-forming processes used in the production of the
critical parts for various industrial purposes. As a process,
forging can be characterized by good mechanical properties of the
workpiece, a short production time, high productivity and optimal
material utilization. These advantages are achieved normally for
rather large production quantities, because of the high costs of
tooling as well as the long set-up times for production line [1].
The dies lifetime is a very important factor determining production
cost and rate [2, 3]. Thus, optimizing dies to achieve longer
lifetime and cheaper production cost is always desirable in these
industries. Hot working tools undergo severe thermal and mechanical
shocks during each blow. During the actual hot forging process, the
dies surface reaches temperature range of 700-800C [2].
Simultaneous presence of high temperature and high stress results
in various die failure mechanisms. Damage of die surface can arise
owing to wear, plastic deformation, thermal fatigue and mechanical
fatigue [2]. Among these, various wear mechanisms are involved in
warm and hot forging processes. It is reported that wear is
responsible of approximately 70% of die damage and failure [3-6].
However, the major wear mechanism differs from one situation to
another. Figure 1 shows the principal modes of die damages and also
indicates the positions in a tool cavity where each type of failure
is most likely to occur [1].
Figure 1. Modes of damage and their positions in die
cavity at which each mode is likely to occur [1]. However, there
is almost no single material which can encounter all the mentioned
wear mechanisms. Even if a material is selected which can withstand
more than one of the factors causing wear, making a tool by means
of this material is not necessarily economic. Therefore, the
preferred strategy is to choose a cheaper material and to cover its
critical sections with a material having superior properties. In
this regard, various surface engineering techniques are widely
utilized. Hard-facing is a weld diffusion process that produces
deposits that are metallurgically bonded to substrate. It is now
being used increasingly often as an inexpensive means for
depositing a hard layer on die surfaces. It
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also can be used to repair and dimensional restoration of dies
[7]. Cobalt-base super alloys are most common hardfacing alloys.
Many of them are derived from the Co-Cr-W and Co-Cr-Mo ternaries.
Following the success of Cobalt-base tool materials during the
World War I, they were then used in the form of weld overlays to
protect surfaces from wear since 1922. Low carbon cobalt-base super
alloys are employed to combat wear at high temperature services
[8]. These alloys have low stacking fault energies and therefore
high density of stacking faults and partial dislocations [9]. Solid
solution hardening by tungsten and chromium,
dislocation-dislocation interactions and impenetrable particle
hardening due to metal-carbides are responsible for noticeable
hardness in these alloys [10]. Among these alloys, Stellite 21
alloy has been successfully utilized for many years, since 1940s,
in the variety of applications, and is still in use, but
predominantly as a wear resistant alloy [8]. Carbides observed in
this alloy are mostly of the Chromium-rich M23C6 type [10, 11].
These carbides can be observed at above 500C and precipitate in
particular on deformation bands and stacking faults [9]. With
increasing temperature and deformation, the density of stacking
fault, dislocations and deformation as well as volume fraction of
carbides increases, thus this alloy exhibits good high temperature
hardness [9]. It is also well accepted that cobalt-base super
alloys are resistant to deformation at temperature range of
500-900C [10]. 2. EXPERIMENTAL Since the final purpose of this
study was the improvement of the wear resistance of a hot forging
die made from H11 hot working tool steel, two test block of the
same material were prepared. These test blocks were heat treated as
for the die and finally a tempered martensitic microstructure
achieved. Then, one of them was hardfaced through TIG welding with
Stellite 21 rods. The composition of test blocks and the weld rods
are shown in table 1. Table 2 shows hardfacing parameters. Then,
specimens for metallographic, hardness and wear tests were cut and
machined from the experimental blocks. Wear tests were performed
using a pin-on-disk method. The disks were made from a T2 high
speed steel (table2). These disks were heat treated to achieve a
hardness of 64 HRC. Then the surface of the pins and disks were
machined to reach a similar condition for all the experiments.
Table 1. Composition of materials used
Alloy Composition (wt %)
H11 C 0.38%, Cr 5%, Mo 1.5%, V .5%, Fe bal.
T2 C 0.9%, Cr 4.5%, W 18%, V 2%, Fe bal.
Stellite 21* C 0.25%, Cr 27%, Ni 2.5% Mo 5.5%, Co bal. * Weld
rod; AWS ERCoCr-E, 3.2 mm in diameter
Wear tests were performed at three temperatures; room
temperature, 400 and 550 C. Invariable parameters for each wear
test involved: sliding speed of 0.4 m/s, normal load of 48 N and
total sliding distance of 1000 m. Prior to and after each
experiment, pins were ultrasonically cleaned and weighed after
drying. Then the weight loss due to wear was measured. Then, the
pins were cut along their cylindrical axis and prepared for
metallographic and microhardness experiments. In the current study,
all of the microhardness tests were performed by means of a knoop
indenter and under a 200gr load.
Table 2. TIG hardfacing parameters
Voltage & Current
Pre-heat temp.
Welding velocity
Post--heat
temp.
Heat input
12 V, 100 A 370 C 1.2 m/s 560 C
400 J/mm
According to the results of these experiments, one practical die
was hardfaced and put in service. Table 3 shows working conditions
of the die.
Table 3. Working conditions of the die
Preheating temperature
Forging temperature Lubricant
250-320 C 1050 C Graphite-Oil Press type Press capacity
Workpiece material
Mechanical 620 tons EN3C During service, dimensions of the die
were controlled at some stages, like other ordinary dies. After a
considerably long period (about 16000 blows) the die was took out
of service and specimens were cut from it for metallographic and
microhardness experiments. 3. RESULTS AND DISCUSSION 3.1 Test
Blocks 3.1.1 Weld overlay microstructure Microstructure of
hardfacing weld overlay is shown in figure 2. Dendritic structure
and interdendritic carbides can be seen in this micrograph. EDS
analysis indicated that interdendritic regions mostly include M23C6
type carbides and a supersaturated content of Cr and Mo. These
carbides have formed during solidification of the weld layer. Since
solidification speed during welding is very high, the matrix is a
supersaturated solid solution of alloying elements (especially Cr
and Mo) in Co.
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Figure 2. Weld overlay microstructure before the wear
test. Microstructures of the wear test pins after the experiment
are shown in figure 3. As can be seen, there is not considerable
difference between these microstructures and that of weld layer
before the tests; it includes dendrites and primary carbides. It
seems that even at 550C the test duration (about 41 minutes) was
not enough for considerable precipitation of carbides. 3.1.2
Hardness The macrohardness of H11 test block after the heat
treatment was 530 HV. Hardness profile in the weld overlay of the
hardfaced test block (before wear tests) is shown in figure 4. The
hardness of the weld overlay was at the same level at every depth
from the surface. The hardness profiles after the wear tests are
shown in figure 5. The hardness of the weld overlay increases near
the surface. By increasing the wear test temperature, the hardness
increment increases. H11 (non-hardfaced) pin shows no considerable
change in hardness after room temperature wear test, but after wear
test at 550C, the macrohardness of the pin decreased to 460 HV.
(a)
(b)
Figure 3. Microstructure of hardfaced pins after the
wear test at: a) room temperature and b) 550C.
300
350
400
450
500
550
600
650
0 1 2 3 4 5
Distance from Surface (mm)
Mic
roha
rdne
ss (H
V)
Figure 4. Microhardness profile in weld overlay before the wear
tests.
300
350
400
450
500
550
600
650
0 0.1 0.2 0.3 0.4 0.5 0.6
Distance from Surface (mm)
Mic
roha
rdne
ss (H
V)
(a)
300
350
400
450
500
550
600
650
0 0.1 0.2 0.3 0.4 0.5 0.6
Distance from Surface (mm)
Mic
roha
rdne
ss (H
V)
(b) Figure 5. Microhardness profiles in hardfaced pins after the
wear test at: a) room temperature and b)
550C. 214
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3.1.3 Wear Test Wear test results are shown in figure 6. At room
temperature, H11 pin shows a better resistance (lower weight loss)
to wear than hardfaced pin. At 400C wear resistance of the H11 pin
decreases considerably while the wear resistance of hardfaced pin
had no considerable change. At 550C, the wear resistance of both
pins increases compared to 400C and the hardfaced pin shows a
better resistance. Comparing room temperature and 550C results, the
wear resistance of the H11 pin shows a rise by increasing
temperature, while that of the hardfaced one is more satisfying at
higher temperature.
0
5
10
15
20
25
25 400 550Wear Test Temperature (C)
Wei
ght L
oss
(mg)
H11 pinsHardfaced pins
Figure 6. Wear rate (weight loss) results.
3.2 Discussion on the Results of the Test Blocks In case of H11
pins, at 400C, the decrease in the wear resistance relative to room
temperature test can be due to the decrease in hardness and
strength at higher temperatures. Transformation of surface layers
to a more tempered structure causes a considerable decrease in the
hardness and wear resistance. Moreover, formation of localized
metallic oxides on the surface and their removal during the test,
result in a more weight loss and lower wear resistance in H11 pin.
It should be noted that localized and scattered oxide spots act
with respect to a mechanism called oxidation-scarpe-reoxidation and
cause a decrease in the wear resistance. On the other hand,
continuous oxide layers can act as a ceramic coating on the surface
and can protect it against wear, providing that the sublayers have
enough strength. The localized oxide spots form in the hot spots of
the surface due to friction. At low ambient temperatures these
oxides are discontinuous and scattered, but at some higher ambient
temperatures these oxides can coalescence and form a continuous
coating. After the formation of this layer, the wear reaches a
steady state before which the wear resistance is relatively low.
The formation of this continuous oxide layer on the surface after a
while, leads into the increase of the wear resistance at 550C in
comparison with 400C. In case of hardfaced pins, no considerable
variation in hardness is observed at 400C in comparison with
room temperature. Furthermore, due to the oxidation resistant
nature of the cobalt-base superalloys, no oxidation can occur.
Thus, no considerable variation occurred in wear resistance. At
550C a surface layer with a high hardness of about 600 HV is
formed. Deformation of the surface layer and work hardening are
responsible for this increase in hardness. According to these
results, it seems that higher deformation on the surface and higher
temperature, can lead into a better wear resistance in hardfaced
specimens. Therefore, a single die was hardfaced and put in
service.
Figure 7. Micrograph from H11 die surface after service.
3.3 Dies 3.3.1 H11 die 3.3.1.1 Metallography Microstructure of a
H11 die after its service is shown in figure 7. White layer on the
surface (left) is a mixture of martensite and retained austenite.
The dark layer beneath, is a mixture of ferrite and carbides.
Figure 8 shows a micrograph of this layer at higher magnification.
As can be seen, it includes fine spherical carbides in ferrite
matrix. The final sublayer (at right) is the original tempered
martensite. According to this micrograph, it is evident that the
die surface reaches to a high temperature enough for the
austenitization of the surface layer. This austenite has been
quenched by the lubricant and has formed the mentioned martensite.
This transformation should be repeated at every blow. The heat
diffusion to the next layer was not enough for austenitization.
Nevertheless, during the total time of service, it was enough for
annealing this layer even to spherical carbides. Thus, a very hard
surface layer (probably fully martensitic at the surface) and a
very low-hardness layer just beneath it, has been formed in H11
die. Moreover, the formation of a brittle oxide layer on the
surface is possible.
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Figure 8. Microstructure of the sublayer beneath the
white surface layer (figure 7).
The die surface had too many cracks which have shown partly in
figure 9. These cracks are formed because of the thermal and
mechanical shocks as well as stresses due to the transformation.
Propagation of these cracks in soft (ductile) sublayers could be
the result of either thermal or mechanical fatigue. These cracks
join each other at the sublayers or propagate parallel to the
surface, and lead into the removal of large particles from the
surface.
(a)
(b)
(c)
Figure 9. Surface cracks in H11 die after service: a)
propagation; b) coalescence and c) propagation parallel
to the surface. 3.3.1.2 Hardness The hardness profile from the
surface to the depth of an H11 die is shown in figure 10. As can be
seen, a very hard layer has been formed at the surface, and just
beneath of this layer, the hardness falls into a very low level.
Retained austenite and annealed structure of the sublayers are
responsible for this low hardness. At more depths from the surface,
the hardness rises to its primal level.
Figure 10. Microhardness profile in the H11 die after
service
(a)
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(b)
Figure 11. Microstructure of the hard-faced die after service:
a) far from the surface and b) near the surface.
3.3.2 Hardfaced die 3.3.2.1 Metallography The microstructure of
two different regions of the weld overlay is shown in figure 11. As
can be seen, far from the surface, the structure contains
dendrites, primary carbides, some precipitated carbides on the
grain boundaries. But close to the die surface, a recrystallized
structure including the precipitated carbides in regular lines
inside the grains and grain boundaries, and primary carbides could
be distinguished. This indicates that the deformation at a
sufficiently high temperature for recrystallization has occurred.
Regular lines are the stacking faults or other planar defects in
the crystal structure which are decorated by the carbides
precipitation. The arrow on the micrograph shows a thermal twin.
Determining the defects type is beyond our discussions in the
current study. Surface cracks were observed in the hardfaced die,
although with a lower frequency as of the H11 die. Figure 12 shows
a crack propagating along interdendritic regions.
Figure 12- A surface crack in the hardfaced die after the
service.
3.3.2.2 Hardness The hardness profile for the hardfaced die
after the service is shown in figure 13. The hardness of the
surface layer is high because of the mentioned recrystallized
structure and carbides precipitation on the grains internal
defects. The main difference with that of the H11 die is the
absence of the soft sublayer beneath the hard surface layer. 3.3.3
Comparison of the dies performance Dimensions of both dies were
controlled during the service. Results of these controls are shown
in figure 14. The hardfaced die has lost only about 0.25 mm of its
dimensions after about 16000 blows, while the H11 die has lost
about 2.25 mm of its dimensions after about 4000 blows.
100
200
300
400
500
600
0 2 4 6
Distance from Surface (mm)
Mic
roha
rdne
ss (H
V)
8
Figure 13. Microhardness profile in the hardfaced die after the
service.
0
0.5
1
1.5
2
2.5
0 3000 6000 9000 12000 15000 18000
Number of blows
Dim
ensi
on L
oss
(mm
)
Hardfaced Die
H11 Die
Figure 14. The dies loss of dimension during the
service.
3.4 Discussion on the results of the dies According to the
results for the H11 die, the hard surface layer formed during its
work, breaks out, as a result of a weak support of the very soft
sublayer beneath it, and is then removed from the surface. The
formation repeating cycle of this hard layer and its break-out and
removal cause a severe mass removal and dimension loss in the H11
die. Additionally, the initiation and propagation of the cracks due
to the thermal and mechanical shocks as well as the
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transformation stresses result in the removal of relatively
large particles from the die surface. Furthermore, the oxidation of
the surface can occur and leads into a more mass removal; however,
the ultrasonic cleaning of the specimens in the current study made
the detection of the oxide particles almost impossible. In case of
the hardfaced die, oxidation resistant nature of the weld overlay
prevents the oxidation based wear mechanisms to be occurred. As a
result of deformation, work hardening, recrystallization of surface
layer, and precipitation of carbides on defects inside grains, a
hard surface layer forms. This layer has the strong support of a
tough sublayer and does not break out easily. Thus, this hard layer
can act as a protective coating against wear. The frequency the
surface cracks in the hardfaced die was very low in comparison with
the H11 die. This could be the result of superior fatigue
properties of the Stellite 21 in comparison with the H11 steel. 4.
CONCLUSIONS The results of this study can be summarized as
follow:
Wear is one of the most important failure mechanisms in the H11
hot forging dies. The formation cycle of a hard surface layer which
has a weak support of a soft sublayer, and its break out and
removal, leads into a severe wear and dimension loss in some of the
hot forging dies.
Another wear mechanism for these dies could be the cracks
propagation and their coalescence under the surface and thus,
removal of rather large particles.
In case of hardfacing with Stellite 21 superalloy, a hard
surface layer forms on the surface, as a result of deformation,
recrystallization and carbides precipitation on crystal defects
inside the grains. This hard surface layer has the good support of
a tough sublayer, and creates a protective coating against wear on
the die surface.
Due to absence of cyclic thermal or mechanical shocks in
pin-on-disk wear experiments, there is a difference between the
wear mechanisms during the pin-on-disk experiment and the real
industrial usage of the hot forging dies. Nevertheless, the wear
resistance trends in pin-on-disk wear tests demonstrate an
acceptable consistency with the industrial experiments.
Overall, the lifetime of the H11 hot forging dies could be
substantially increased via hardfacing by a Stellite 21cobalt-base
superalloy.
Acknowledgements The authors wish to thank MOHAM Industries for
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