Note: Descriptions are shown in the official language in which they were submitted.
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SINTERED METAL PARTS AND METHOD FOR THE MANUFACTURING
THEREOF
FIELD OF THE INVENTION
The invention relates to powder metal parts.
Specifically the invention concerns sintered metal parts
which have a densified surface and which are suitable for
demanding applications. The invention also includes a
method of preparing these metal parts.
BACKGROUND OF THE INVENTION
There are several advantages by using powder
metallurgical methods for producing structural parts
compared with conventional matching processes of full
dense steel. Thus the energy consumption is much lower
and the material utilisation is much higher. Another
important factor in favour of the powder metallurgical
route is that components with net shape or near net shape
can be produced directly after the sintering process
without costly shaping such as turning, milling, boring
or grinding. However, normally a full dense steel
material has superior mechanical properties compared with
PM components. Therefore, the strive has been to increase
the density of PM components in order to reach values as
close as possible to the density value of a full dense
steel.
One area of future growth in the utilization of powder
metal parts having high density is in the automotive
industry. Of special interest within this field is the
use of powder metal parts in more demanding applications,
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such as power transmission applications, for example,
gear wheels. Problems with gear wheels formed by the
powder metal process are that powder metal gear wheels
have reduced bending fatigue strength in the tooth root
region of the gear wheel, and low contact fatigue
strength on the tooth flank compared with gears machined
from bar stock or forgings. These problems may be reduced
or even eliminated by plastic deformation of the surface
of the tooth root and flank region through a process
commonly known as surface densification. Products which
can be used for these demanding applications are
described in e.g. the US patents 5 711 187, 5 540 883,
5 552 109, 5 729 822 and 6 171 546.
The US 5 711 187 (1990) is particularly concerned with
the degree of surface hardness, which is necessary in
order to produce gear wheels which are sufficiently wear
resistant for use in heavy duty applications. According
to this patent the surface hardness or densification
should be in the range of 90 to 100 percent of full
theoretical density to a depth of at least 380 microns
and up to 1,000 microns. No specific details are
disclosed concerning the production process but it is
stated that admixed powders are preferred as they have
the advantage of being more compressible, enabling higher
densities to be reached at the compaction stage. Fur-
thermore it is stated that the admixed powders should
include in addition to iron and 0.2% by weight of
graphite, 0.5% by weight of molybdenum, chromium and
manganese, respectively.
A method similar to that described in the US patent
5 711 187 is disclosed in the US 5 540 883 (1994).
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According to the US patent 5 540 883 bearing surfaces
from powder metal blanks are produced by blending carbon
and ferro alloys and lubricant with compressible ele-
mental iron powder, pressing the blending mixture to form
the powder metal blank, high temperature sintering the
blank in a reducing atmosphere, compressing the powder
metal blanks so as to produce a densified layer having a
bearing surface, and then heat treating the densified
layer. The sintered powder metal article should have a
composition, by weight percent, of 0.5 to 2.0% chromium,
0 and 1.0% molybdenum, 0.1 and 0.6% carbon, with a
balance of iron and trace impurities. Broad ranges as
regards compaction pressures are mentioned. Thus it is
stated that the compaction may be performed at pressures
between 25 and 50 ton per square inch (about 390-770
MPa).
The US 5 552 109 (1995) patent concerns a process of
forming a sintered article having high density. The
patent is particularly concerned with the production of
connecting rods. As in the US patent 5 711 187 no
specific details concerning the production process are
disclosed in the US patent 5 552 109 but it is stated
that the powder should be a pre-alloyed iron based
powder, that the compacting should be performed in a
single step, that the compaction pressures may vary
between 25 and 50 ton per square inch (390-770 MPa) to
green densities between 6.8 and 7.1 g/cm3 and that the
sintering should be performed at high temperature,
particularly between 1270 and 1350 C. It is stated that
sintered products having a density greater than 7.4 g/cm3
are obtained and it is thus obvious that the high
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sintered density is a result of the high temperature
sintering.
In the US 5 729 822 (1996) a powder metal gear wheel
having a core density of at least 7.3 g/cm3 and a
hardened carburized surface is disclosed. The powders
recommended are the same as in the US patents 5 711 187
and 5 540 883 i.e. mixtures obtained by blending carbon,
ferro alloys and lubricant with compressible a powder of
elemental iron. In order to obtain high sintered core
density the patent mentions warm pressing; double
pressing, double sintering; high density forming as
disclosed in the US patent 5 754 937; the use of die wall
lubrication, instead of admixed lubricants during powder
compaction; and rotary forming after sintering.
Compacting pressures of around 40 tons per square inch
(620 MPa) are typically employed.
The surface densification of sintered PM steels is dis-
cussed in e.g. the Technical Paper Series 820234, (Inter-
national Congress & Exposition, Detroit, Michigan, Febru-
ary 22-26, 1982). In this paper a study of surface roll-
ing of sintered gears is reported. Fe-Cu-C and Ni-Mo al-
loyed materials were used for the study. The paper re-
veals the results from basic research on the surface
rolling of sintered parts at a density of 6.6 and 7.1
g/cm3 and the application of it to sintered gears. The
basic studies includes surface rolling with different di-
ameters of the rolls, best results in terms of strength
were achieved with smaller roll diameter, lesser reduc-
tion per pass and large total reduction. As an example
for a Fe-Cu-C material a densification of 90% of theo-
retical density was achieved with a roll of 30 mm diame-
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ter to a depth of 1,1 mm. The same level of densification
was achieved to a depth of about 0.65 mm for a 7.5 mm
diameter roll. The small diameter roll however was able
to increase the densification to about full density at
5 the surface whereas the large diameter roll increased the
density to about 96% at the surface. The surface rolling
technique was applied to sintered oil-pumps gears and
sintered crankshaft gears. In an article in Modern Devel-
opments in Powder Metallurgy, Volume 16, p. 33-48 1984
(from the International PM Conference June 17-22, 1984,
Toronto Canada,) the authors have investigated the influ-
ence of shot-peening, carbonitriding and combinations
thereof on the endurance limit of sintered Fe+1.5% Cu and
Fe+2% Cu+2.5% Ni alloys. The density reported of these
alloys were 7.1 and 7.4 g/cm3. Both a theoretical evalua-
tion of the surface rolling process and a bending fatigue
testing of surface rolled parts is published in an arti-
cle in Horizon of Powder Metallurgy part I, p.403-406.
Proceedings of the 1986 (International Powder Metallurgy
Conference and Exhibition, Dusseldorf, 7-11 July 1986).
According to the prior art many different routes have
been suggested in order to reach high sintered density of
a powder metallurgical component. However, the suggested
processes all include steps adding additional costs. Thus
warm compaction and die wall lubrication promote high
green density. Double pressing and double sintering re-
sult in high sintered density and shrinkage as a result
of high temperature sintering also results in high sin-
tered density.
Furthermore, for high load applications such as gear
wheels, special precautions has to be taken in account
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regarding the pore size and pore morphology in order to
achieve sufficient fatigue properties. A simple and cost
effective method for the preparation of gear wheels and
similar products with a high sintered density and
mechanical strength, regardless the pore size and
morphology, would thus be attractive and the main object
of the present invention.
SUMMARY OF THE INVENTION
In brief it has now been found that powder metal parts in
more demanding applications, such as power transmission
applications, for example, gear wheels, can be obtained
by subjecting an iron or iron-based powder to unaxially
compaction at a pressure above 700 MPa to a density above
7.35 g/om3, sintering the obtained green product and
subjecting the sintered product to a densification
process. A characteristic feature of the core of the
metal part according to the invention is the pore
structure, which is distinguished by comparatively large
pores.
Specifically the invention concerns a sintered metal part
which has a densified surface and a core density of at
least 7.35, preferably at least 7.45 g/cm3 wherein the
core structure is distinguished by a pore matrix obtained
by single pressing, without applying die wall
lubrication, to at least 7.35 g/cm3, preferably at least
7.45 g/cm3, and single sintering of an iron-based powder
mixture having coarse iron or iron-based powder particles
as well as the method of producing such metal parts. The
pore structure was measured an evaluated by using image
analysis according to ASTM E 1245 giving the pore area
distribution related to pore size.
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The density levels above concerns products based on pure or
low-alloyed iron powder.
According to one aspect of the present invention, there is
provided a sintered metal part which has a densified surface
with a densification depth of at least 0.1 mm, a sintered
density of at least 7.35 g/cm3 and a core structure
distinguished by a pore structure, wherein at least 50 % of
the pore area in a cross section consists of pores having a
pore area of at least 100 m2 obtained by single pressing to
at least 7.35 g/cm3 and single sintering of a mixture of a
coarse iron or iron-based powder, wherein the iron-based
powder has a particle size such that at most 10 % of the
particles are less than 45 m, and optional additives.
According to another aspect of the present invention, there
is provided method for producing powder metal parts having a
densified surface, comprising the steps of: uniaxially
compacting an iron or iron-based powder having coarse
particles, wherein the iron-based powder has a particle size
such that at most 10 % of the particles are less than 45 m,
to a density above 7.35 g/cm3 in a single compaction step at
a compaction pressure of at least 700 MPa; subjecting the
green parts to sintering in a single step at a temperature
of at least 1100 C to a density of at least 7.35 g/m3; and
subjecting the sintered parts to a surface densifying
process.
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DETAILED DESCRIPTION OF THE INVENTION
Powder types
Suitable metal powders which can be used as starting
materials for the compaction process are powders prepared
from metals such as iron. Alloying elements such as
carbon, chromium, manganese, molybdenum, copper, nickel,
phosphorous, sulphur etc can be added as particles,
prealloyed or d.iffusion alloyed in order to modify the
properties of the final sintering product. The iron-based
powders can be selected from the group consisting of
substantially pure iron powders, pre-alloyed iron-based
particles, diffusion alloyed iron-based iron particles
and mixture of iron particles or iron-based particles and
alloying elements. As regards the particle shape it is
preferred that the particles have an irregular foz-m as is
obtained by water atomisation. Also sponge iron powders
having irregularly shaped particles may be of interest.
As regards PM parts for high demanding applications,
especially promising results have been obtained with pre
alloyed water atomised powders including low amounts such
as up to 5% of one or zinore of the alloying elements Mo.
and Cr. Examples ofsuch powders are powders having a
chemical composition corresponding to the chemi,cal.
composition of Astaloy Mo (1.5% Mo and Astaloy 85 Mo
(0.85s Mo) as well as Astaloy CrM (3 Cr,0.5 Mo)and
Astaloy CrL (1.5 Cr,0.2 Mo) from Hoganas AB, Sweden.
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A critical feature of the invention is that the powder
used have coarse particles i.e. the powder is essentially
without fine particles. The term "essentially without
fine particles" is intended to mean that less than about
10%, preferably less than 5% of the powder particles have
a size below 45 pm as measured by the method described in
SS-EN 24 497. The average particle diameter is typically
between 75 and 300 pm. The amount of particles above 212
pm is typically above 20%. The maximum particle size may
be about 2 mm.
The size of the iron-based particles normally used within
the PM industry is distributed according to a gaussian
distribution curve with a average particle diameter in
the region of 30 to 100 m and about 10-30% of the
particles are less than 45 pm. Thus the powders used ac-
cording to the present invention have a particle size
distribution deviating from that normally used. These
powders may be obtained by removing the finer fractions
of the powder or by manufacturing a powder having the
desired particle size distribution.
Thus for the powders mentioned above a suitable particle
size distribution for a powder having a chemical
composition corresponding to the chemical composition of
Astaloy 85 Mo could be that at most 5% of the particles
should be less than 45 ~im and the average particle
diameter is typically between 106 and 300 pm. The
corresponding values for a powder having a chemical
composition corresponding to Astaloy CrL are suitably
that less than 5% should be less than 45 pm and the
average particle diameter is typically between 106 and
212 pm.
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In order to obtain sintered metal parts having
satisfactory mechanical sintered properties according to
the present invention it may be necessary to add graphite
to the powder mixture to be compacted. Thus, graphite in
amounts between 0.1-1, preferably 0.2-1.0, more pre-
ferably 0.2-0.7% and most preferably 0.2-0.5% by weight
of the total mixture to be compacted could be added
before the compaction. However, for certain applications
graphite addition is not necessary.
The iron-base powder may also be combined with a
lubricant before it is transferred to the die (internal
lubrication). The lubricant is added in order to minimize
friction between the metal power particles and between
the particles and the die during a compaction, or
pressing, step. Examples of suitable lubricants are e.g.
stearates, waxes, fatty acids and derivatives thereof,
oligomers, polymers and other organic substances with
lubricating effect. The lubricants may be added in the
form of particles but may also be bonded and/or coated to
the particles.
Preferably a lubricating coating of a silane compound of
the type disclosed in WO 2004/037467 is included in the
powder mixture. Specifically the silane compound may be
an alkylakoxy or polyetheralkoxy silane, wherein the
alkyl group of the alkylalkoxy silane and the polyether
chain of the polyetheralkoxy silane include between 8 and
30 carbon atoms, and the alkoxi group includes 1-3 carbon
atoms. Examples of such compounds are octyl-tri-metoxy
silane, hexadecyl-tri-metoxy silane and
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polyethyleneether-trimetoxy silane with 10 ethylene ether
groups.
According to the present invention the amount of
5 lubricant added to the iron-based powder may vary between
0.05 and 0.6%, preferably between 0.1-0.5% by weight of
the mixture.
As optional additives hard phases, binding agents,
10 machinability enhancing agents and flow enhancing agents
may be added.
Compaction
Conventional compaction at high pressures, i.e. pressures
above 600 MPa with conventionally used powders including
finer particles, in admixture with low amounts of
lubricants (less than 0.6% by weight) is generally
considered unsuitable due to the high forces required in
order to eject the compacts from the die, the
accompanying high wear of the die and the fact that the
surfaces of the components tend to be less shiny or
deteriorated. By using the powders according to the
present invention it has unexpectedly been found that the
ejection force is reduced at high pressures and that
components having acceptable or even perfect surfaces may
be obtained also when die wall lubrication is not used.
The compaction may be performed with standard equipment,
which means that the new method may be performed without
expensive investments. The compaction is performed uni-
axially in a single step at ambient or elevated
temperature. Preferably the compaction pressures are
above about 700, more preferably above 800 and most
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preferably above 900 or even 1000 MPa. In order to reach
the advantages with the present invention the compaction
should preferably be performed to densities above 7.45
g / cm3 .
Sintering
Any conventional sintering furnace may be used and the
sintering times may vary between about 15 and 60 minutes.
The atmosphere of the sintering furnace may be an endogas
atmosphere, a mixture between hydrogen and nitrogen or in
vacuum. The sintering temperatures may vary between 1100
and 1350 C. With sintering temperatures above about
1250 C the best results are obtained. In comparison with
methods involving double pressing and double sintering
the method according to the present invention has the
advantage that one pressing step and one sintering step
are eliminated and still sintered densities above 7.64
g/cm3 can be obtained.
Structure
A distinguishing feature of the core of the high density
green and sintered metal part is the presence of large
pores. Thus, as an example, in a cross section of the
core of a sintered metal part according to the invention,
at least about 50% of the pore area consists of pores
having a pore area of at least 100 um2 , whereas, in a
cross section of a core prepared from a corresponding
normal powder (i.e. a powder including normal amounts of
fine particles which has to be double pressed and double
sintered in order to reach the same density), at least
about 50% of the pore area consists of pores having a
pore area of about 65 }zm2 .
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Surface densification
The surface densification may be performed by radial or
axial rolling, shoot peening, sizing etc. A preferred
method is radial rolling as this method provides short
cycle times in combination with great densification
depth. The powder metal parts will obtain better
mechanical properties with increasing densifying depth.
The densification depth is preferably at least 0.1 mm,
preferably at least 0.2 mm and most preferably at least
0.3 mm.
In this context is should be recalled that normally the
presence of large pores in sintered parts is regarded as
a drawback and different measures are taken in order to
make the pores smaller and rounder. According to the
present invention, however, it has surprisingly been
found that the negative effect of the comparatively high
amount of larger pores can be totally eliminated by a
surface densification process. Thus, when comparing the
effect of surface densification on the bending fatigue
strength of sintered samples containing larger pores in
the core with the effect on samples containing smaller
pores, it has been found that the surface densification
process increases the bending fatigue strength to a much
higher extent when the samples are produced from metal
powder with the particle size distribution discussed
above. After the surface densification process, the
bending fatigue strength of samples produced of these
powders will surprisingly reach the same level as that of
surface densified samples which are produced from
powders having a normal particle size distribution(given
the same chemical composition and the same sintered
density level). Accordingly, as high sintered density can
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be reached in a single pressing, single sintering
process, costly processes, such as double pressing-
double sintering, warm compaction, can be avoided by
utilising the method according to the present invention
for production of for example gear wheels.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 shows the bending fatigue strength before and after
the surface densification process of samples produced
from the mixes 1A and 1B according to example 1.
Fig 2 is a light optical micrograph of a cross section of
a surface densified sample prepared from mix 1A.
Fig 3 is a light optical micrograph of a cross section of
a surface densified sample prepared from mix 1B.
Fig 4 shows the bending fatigue strength before and after
surface densification process of samples produced from
the mixes 2C and 2D according to example 2.
Fig 5 is a light optical micrograph of a cross section of
a surface densified sample prepared from mix 2C.
Fig 6 is a light optical micrograph of a cross section of
a surface densified sample prepared from mix 2D.
The invention is further illustrated by the following
non-limiting examples.
The following iron-based powders were used;
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Powder A;
Astaloy 85 Mo, an atomised pre-alloyed iron base powder
with a Mo content of 0.80-0.95%, a carbon content of at
most 0.02% and an oxygen content of at most 0.20%.
The particle sized distribution of powder A is similar to
the particle size distribution for powder normally used
in powder metallurgy; about 0% greater than 250 m, about
15-25% between 150 and 250 m and about 15 to 30% less
than 45 m.
Powder B;
The same chemical composition as powder A but with a
coarser particle size distribution according to the table
below;
Particle size m % by weight
>500 0
425-500 1.9
300-425 20.6
212-300 27.2
150-212 20.2
106-150 13.8
75-106 6.2
45-75 5.9
<45 4.2
Powder C;
Astaloy CrL, an atomised Mo-, Cr- prealloyed iron based
powder with a Cr content of 1.35-1.65%, a Mo content of
0.17-0.27%, a carbon content of at most 0.010% and an
oxygen content of at most 0.25%.
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The particle sized distribution of powder C is similar to
the particle size distribution for powder normally used
in powder metallurgy; about 0% greater than 250 m, about
5 15-25% between 150 and 212 m and about 10 to 25% less
than 45 m.
Powder D;
The same chemical composition as powder C but with a
10 coarser particle size distribution according to the table
below;
Particle size m % by weight
>500 0
425-500 0.2
300-425 7.4
212-300 21.9
150-212 25.1
106-150 23.4
75-106 11.2
45-75 7.1
<45 3.7
Example 1
Two mixes, Mix 1A and Mix 1B were prepared by thoroughly
mixing before compaction.
Mix 1A was based on powder A with an addition of 0.2% by
weight of graphite and 0.8% by weight of H wax.
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Mix 1B was based on powder B with an addition of 0,2% by
weight of graphite and 0.2% by weight of hexadecyl trime-
toxy silane.
FS-strength test bars according to ISO 3928 were
compacted.
Test bars based on Mix 1A was compacted to a green
density of 7.1 g/cm3 and pre sintered at 780 C for 30 min-
utes in an atmosphere of 90% nitrogen and 10% hydrogen.
After sintering the samples were subjected to a second
compaction at a pressure of 1100 MPa and finally sintered
at 1280 C for 30 minutes in an atmosphere of 90% nitrogen
and 10% of hydrogen. The sintered density was measured to
7.61 g/cm3.
The sample prepared from mix 1B was compacted in a single
compaction process at 1100 MPa was subsequently sintered
at 1280 C for 30 minutes in an atmosphere of 90%
nitrogen and 10% of hydrogen. The sintered density was
7.67 g/cm3.
The results are summarized in table 1 below.
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Table 1
MIX POWDER Pressure Sintering Pressure Sintering SD
MPa/GD C MPa C g/cm3
1A Astaloy 7.1 780 1100 1280 7.61
0.80-0.95
Mo
standard
0.2
graphite
1B Astaloy 1100 1280 7.67
0.80-0.95
Mo
coarse
0.2
graphite
Half of the number of the obtained sintered bodies was
5 subjected to a surface densifaction process by shot
peening at 6 bars air pressure with steel spheres with a
diameter of 0.4 mm.
Both the surface densified samples and the samples not
10 subjected to a surface densification process were case
hardened at 920 C for 75 minutes at a carbon potential of
0.8% followed by a tempering operation at 200 C for 120
minutes.
15 Bending fatigue limit (BFL) was determined for all of the
samples.
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Figure 1 shows the bending fatigue limit for both the
surface densified samples and the samples which were not
subjected to surface densification.
From figure 1 it can be concluded that surface densifica-
tion of the samples produced with the coarser powder con-
tributes to a much higher increase in BFL compared with
the increase in BFL which was obtained by surface densi-
fication of the samples produced with a powder having a
conventional particle size distribution.
Figure 2 is a light optical micrograph showing a cross
section of a surface densified sample prepared from mix
lA and figure 3 is a similar micrograph from a surface
densified sample prepared from mix 1B.
Image analysis according to ASTM E 1245 of cross section
of surface densified samples produced from sample 1A
shows that about 50% of the total cross section pore area
consists of pores having a surface area of 65 Pm2 or
more, whereas the same measuring of surface densified
samples produced from mix 1B shows that about 50% of the
total cross section area consists of pores having a
surface area of 200 pm 2 or more.
Example 2
Two mixes, Mix 2C and Mix 2D were prepared by thoroughly
mixing before compaction.
Mix 2C was based on powder C with an addition of 0.7% of
nickel powder, 0.2% by weight of graphite and 0.8% by
weight of H wax,
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Mix 2D was based on powder D with an addition of 0.7% of
nickel powder 0.2% of graphite and 0.2% of hexadecyl
trimetoxy silane.
FS-strength test bars according to ISO 3928 were
prepared.
Test bars based on mix 2C, was compacted to a green
density of 7.1 g/cm3 and pre sintered at 780 C for 30 min-
utes in an atmosphere of 90% nitrogen and 10% hydrogen.
After sintering the samples were subjected to a second
compaction at a pressure of 1100 MPa and finally sintered
at 1280 C for 30 minutes in an atmosphere of 90% nitrogen
and 10% of hydrogen. The sintered density was measured to
7.63 g/cm3.
Test bars prepared from mix 2D was compacted in a single
compaction process at 1100 MPa followed by sintering
1280 C for 30 minutes in an atmosphere of 90% nitrogen
and 10% of hydrogen. The sintered density was measured to
7.64 g/cm3.
The results are summarized in table 3 below.
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Table 3
MZX POWDER Pressure Sintering Pressure Sintering SD
MPa/GD C MPa C g/cm3
2C CRL 7.1 780 1100 1280 7.63
Standard
1.35-1.65
Cr
0.17-0.27
Mo+0.7 0
Ni
2D CRL 1200 1280 7.64
Coarse
1.35-1.65
Cr
0.17-0.27
Mo+0.7%
Ni
5
Half of the number of the obtained sintered bodies were
subjected to a surface densification process by shot
peening at 6'bars air pressure with steel spheres with a
diameter of 0.4 mm.
Both the surface densified samples and the samples not
subjected to a surface densifaction process were case
hardened at 920 C for 75 minutes at a carbon potential of
0.8% followed by a tempering operation at 200 C for 120
minutes.
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Bending fatigue limit (BFL) were determined for all of
the samples.
Figure 4 shows the bending fatigue limit for both the.
surface densified samples and the samples which were not
subjected to surface densification.
From figure 4 it can be concluded that surface densifica.-
tion of the samples.produced with the coarser powder con-
tributes to a much higher increase in BFL compared with
the increase in BFL which wa,s obtained by surface densi-
fication of the sample,s produced with a powder having a
conventional particle size distribution.
Figure 6 is a light optical micrograph showing a cross
section of a surface densified sample prepared from mix
2C and figure 7 is a aimilar micrograph.from a surface
densified sample prepared from mixtuze 2D.
Image analysis according to ASTM E 1245 of cross section
of surface densified samples produced from sample 2C
shows that about 50% of the total cross section pore area
consists of pores having a surface area of 50 um:t:or
more, whereas the same measuring of surface densfied
samples produced from mix 2D shows that about 50% af the
total cross section area consists of pores 'having a
surface area of 110 puM2 or mo.re..
