Note: Descriptions are shown in the official language in which they were submitted.
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ALLOY STEEL POWDER FOR POWDER METALLURGY AND METHOD
OF PRODUCING IRON-BASED SINTERED BODY
TECHNICAL FIELD
[0001] This disclosure relates to an alloy steel powder for powder metallurgy
preferably used in powder metallurgical techniques, and particularly, it aims
at improving strength and toughness of a sintered material using such alloy
steel powder for powder metallurgy.
Further, this disclosure relates to a method of producing an iron-based
sintered body having excellent strength and toughness produced using the
above alloy steel powder for powder metallurgy.
BACKGROUND
[0002] Powder metallurgical techniques enable producing parts with
complicated shapes in shapes extremely close to product shapes (so-called
near net shapes) with high dimensional accuracy, and therefore machining
costs can be significantly reduced. For this reason, powder metallurgical
products arc used as various mechanical structures and parts thereof in many
fields.
Further, in recent years, to achieve miniaturization and reduced weight of
parts, an increase in the strength of powder metallurgical products is
strongly
requested. In particular, there is a strong request for strengthening
iron-based powder products (iron-based sintered bodies).
[0003] Generally, an iron-based powder green compact for powder metallurgy
which is a former stage of an iron-based sintered body is produced by adding
to an iron-based powder, an alloying powder such as copper powder and
graphite powder, and a lubricant such as stearic acid and zinc stearate to
obtain an iron-based mixed powder, injecting said powder into a die and
performing pressing. Based on the components, iron-based powders are
categorized into iron powder (e.g. pure iron powder and the like), alloy steel
powder, and the like. Further, when categorized by production method,
iron-based powders are categorized into atomized iron powder, reduced iron
powder, and the like. Within these categories, the term "iron powder" is
used with a broad meaning encompassing alloy steel powder.
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[0004] The density of an iron-based powder green compact for powder
metallurgy which is obtained in a general powder metallurgy process is
normally around 6.8 Mg/m3 to 7.3 Mg/m3. The obtained iron-based powder
green compact is then sintered to form an iron-based sintered body which in
turn is further subjected to optional sizing, cutting work or the like to form
a
powder metallurgical product. Further, when an even higher strength is
required, carburizing heat treatment or bright heat treatment may be
performed after sintering.
[00051 Conventionally known powders with an alloying element added
thereto at the stage of precursor powder include (1) mixed powder obtained by
adding each alloying element powder to pure iron powder, (2) pre-alloyed
steel powder obtained by completely alloying each element, (3) diffusionally
adhered alloy steel powder obtained by partially diffusing each alloying
element powder on the surface of pure iron powder or pre-alloyed steel
powder, and the like.
[0006] The mixed powder (1) obtained by adding each alloying element
powder to pure iron powder is advantageous in that high compressibility
equivalent to that of pure iron powder can be obtained. However, the large
segregation of each alloying element powder would cause a large variation in
characteristics. Further, since the alloying elements do not sufficiently
diffuse in Fe, the microstructure would remain non-uniform and the matrix
would not be strengthened efficiently.
Therefore, the mixed powder obtained by adding each alloying element
powder to pure iron powder could not cope with the recent requests for
stabilizing characteristics and increasing strength, and the usage amount
thereof is decreasing.
[0007] Further, the pre-alloyed steel powder (2) obtained by completely
alloying each element is produced by atomizing molten steel, and although the
matrix is strengthened by a uniform microstructure, a decrease in
compressibility is caused by the action of solid solution hardening.
[0008] Further, the diffusionally adhered alloy steel powder (3) is produced
by adding metal powders of each element to pure iron powder or pre-alloyed
steel powder, heating the resultant powder in a non-oxidizing or reducing
atmosphere, and partially diffusion bonding each metal powder on the
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surfaces of the pure iron powder or the pre-alloyed steel powder, and
advantages of the iron-based mixed powder (1) and the pre-alloyed steel
powder (2) can be combined.
Therefore, high compressibility equivalent to that of pure iron powder can be
obtained while preventing segregation of alloying elements. Further, since a
multi-phase where partially concentrated alloy phase is diffused is formed,
the
matrix may be strengthened. For these reasons, development is carried out
for diffusionally adhered alloy steel powder for high strength.
[0009] As described above, increasing the alloy content is one way of
enhancing strength and toughness of a powder metallurgical product.
However, such alloying hardens the alloy steel powder to be used as the
material, leading to the problem of decreased compressibility and increased
burden on the equipment for performing pressing. Further, the decrease in
compressibility of the alloy steel powder causes a decrease in density of the
sintered body, which ends up canceling the increase in strength. Therefore,
in order to increase the strength and toughness of powder metallurgical
products, a technique is required for increasing the strength of the sintered
body while minimizing the decrease in compressibility.
[0010] As a technique for increasing the strength of the sintered body while
maintaining compressibility such as mentioned above, a technique of adding
to the iron-based powder, alloying elements such as Ni, Cu, Mo and the like
which improve hardenability, is commonly used. As an element that is
effective for this purpose, for example, PTL1 (JPS6366362B) discloses a
technique of adding Mo as a pre-alloyed element to the iron powder in a range
that would not deteriorate compressibility (Mo: 0.1 mass% to 1.0 mass%), and
diffusionally adhering, to the particle surfaces of the resultant iron powder,
powders of Cu and Ni to achieve both compressibility during green
compacting and strength of members after sintering.
[0011] Further, PTL2 (JPS61130401A) proposes an alloy steel powder for
powder metallurgy for a high strength sintered body obtained by diffusionally
adhering, to the steel powder surface, two or more kinds of alloying elements,
in particular Mo and Ni, or Cu in addition to said elements.
With this technique, it is further proposed that, for each diffusionally
adhered
element, the diffusionally adhered density with respect to fine powders of
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particle sizes of 44 um or less is controlled within a range of 0.9 to 1.9
times
the diffusionally adhered density with respect to the total amount of the
steel
powder, and it is disclosed that with a limitation to such relatively broad
range, impact toughness of the sintered body is obtained.
[0012] On the other hand, Mo based alloy steel powder containing Mo as a
main alloying element and not containing Ni or Cu has been proposed. For
example, in PTL3 (JPH0689365B), an alloy steel powder containing Mo
which is a ferrite-stabilizing element as a pre-alloy in a range of 1.5 mass%
to
20 mass% is proposed to accelerate sintering by forming an a single phase of
Fe having a rapid self diffusion rate. It is disclosed that, with this alloy
steel
powder, a sintered body with high density is obtained by applying particle
size distribution and the like in the process referred to as pressure
sintering,
and a uniform and stable microstructure is obtained by not employing a
diffusionally adhered alloying element.
[0013] Similarly, PTL4 (JP2002146403A) discloses a technique regarding an
alloy steel powder for powder metallurgy containing Mo as a main alloying
element. This technique proposes an alloy steel powder obtained by
diffusionally adhering 0.2 mass% to 10.0 mass% of Mo on the surface of the
iron-based powder containing, as a pre-alloy, 1.0 mass% or less of Mn, or less
than 0.2 mass% of Mo. It is disclosed that, atomized iron powder or reduced
iron powder may be used as the iron-based powder, and that the mean particle
size is preferably 30 um to 120 um. Further, it is disclosed that the alloy
steel powder not only has excellent compressibility but also enables obtaining
sintered parts having high density and high strength.
CITATION LIST
Patent Literature
[0014] PTL 1: JPS6366362B
PTL 2: JPS61130401A
PTL 3: JPH0689365B
PTL 4: JP2002146403A
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SUMMARY
(Technical Problem)
[0015] However, while Ni is an essential additive component for the
techniques disclosed in PTL1 and PTL2, the diffusion of Ni progresses at a
slow rate during sintering, and therefore sintering needs to be performed for
a
long period of time for sufficiently diffusing Ni in iron powder or steel
powder.
[0016] Further, with the technique disclosed in PTL3, since Mo is added in a
relatively large amount of 1.8 mass% or more and the compressibility is low,
high forming density cannot be obtained. Therefore, when
a normal
sintering process (single sintering with no pressurization) is applied, only
sintered parts having low sintered density can be obtained, and sufficient
strength and toughness cannot be obtained.
[0017] Further, the technique disclosed in PTL4 is applied to a powder
metallurgy process comprising re-compression and re-sintering of the sintered
body. In other words, with a normal sintering method, the aforementioned
effect could not sufficiently be achieved.
As a result of our study, it was revealed that it is difficult to achieve both
high
strength and high toughness with a sintered body using any alloy steel powder
disclosed in the above PTLs 1 to 4.
[0018] It could therefore be helpful to provide an alloy steel powder for
powder metallurgy that can solve the aforementioned problem and achieve
both high strength and high toughness of the sintered body using the alloy
steel powder.
(Solution to Problem)
[0019] To achieve the above object, we made intensive studies regarding the
alloy components of the iron-based powder and the adding means thereof, and
discovered the following.
That is, we discovered that, by using a composite alloy steel powder obtained
by adhering an Mo-containing alloy powder to a surface of an iron-based
powder, wherein the composite alloy steel powder has a specific surface area
of 0.100 m2/g or more and Mo content of 0.2 mass% to 1.5 mass%, pores of
the sintered body are appropriately refined because of the excellent
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sinterability that the alloy steel powder for powder metallurgy obtained from
the composite alloy
steel powder offers during pressing and sintering, and accordingly the
strength of the sintered
body as well as the toughness of the sintered body are improved.
This disclosure has been made based on these discoveries.
[0020] We thus provide:
1. An alloy steel powder for powder metallurgy comprising: a composite alloy
steel
powder obtained by adhering an Mo-containing alloy powder to a surface of an
iron-based
powder; and graphite powder, wherein the composite alloy steel powder has a
specific surface
area of 0.100 m2/g or more and Mo content in a range of 0.2 mass% to 1.5
mass%, and the
graphite powder content with respect to 100 mass% of the alloy steel powder
for powder
metallurgy is in a range of 0.1 mass% to 1.0 mass%.
[0021] 2. The alloy steel powder for powder metallurgy according to aspect 1,
further containing
Cu powder in a range of 0.5 mass% to 4.0 mass% with respect to 100 mass% of
the alloy steel
powder for powder metallurgy.
[0022] 3. The alloy steel powder for powder metallurgy according to aspect 1
or 2, wherein the
iron-based powder contains a reduced iron powder, and a mean particle size of
the iron-based
powder is 80 um or less.
[0023] 4. The alloy steel powder for powder metallurgy according to any one of
aspects I to 3,
wherein oxygen content of the iron-based powder is 0.3 mass% or less.
[0024] 5. A method of producing an iron-based sintered body comprising: adding
and mixing a
lubricant into the alloy steel powder for powder metallurgy according to any
one of aspects l to 4;
and performing pressing and sintering to obtain an iron-based sintered body.
[0024a] The invention further provides a method of producing an iron-based
sintered body
comprising: adding and mixing a lubricant into an alloy steel powder for
powder metallurgy
comprising: a composite alloy steel powder obtained by adhering an Mo-
containing alloy powder
to a surface of an iron-based powder having a mean particle size of from 76 gm
to 96 gm; and
graphite powder, wherein the composite alloy steel powder has a specific
surface area of
0.100 m2/g or more and Mo content in a range of 0.2 mass% to 1.5 mass%, and
the graphite
powder content with respect to 100 mass% of the alloy steel powder for powder
metallurgy is in a
range of 0.1 mass% to 1.0 mass%; and performing pressing and sintering once to
obtain the
iron-based sintered body.
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(Advantageous Effect)
[0025] With the alloy steel powder for powder metallurgy described herein,
since Ni is not
required and compressibility is high, a sintered material having both high
strength and high
toughness can be obtained at a low cost, even with a normal sintering method.
DETAILED DESCRIPTION
[0026] Our methods and products will be described in detail below.
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The alloy steel powder for powder metallurgy described herein is obtained
using a composite alloy steel powder that is obtained by adhering an
Mo-containing alloy powder to the surface of an iron-based powder, wherein
the composite alloy steel powder has a specific surface area of 0.100 m2/g or
more and Mo content in a range of 0.2 mass% to 1.5 mass%.
Further, by mixing the above composite alloy steel powder with an
appropriate amount described below of graphite powder, the alloy steel
powder for powder metallurgy is obtained. Then, by pressing the alloy steel
powder for powder metallurgy into a green compact, and sintering said green
compact, pores of the sintered body are effectively refined and sintered parts
with improved strength and toughness can be obtained.
[0027] The mechanism by which pores of the sintered body can be effectively
refined and sintered parts with improved strength and toughness can be
obtained is understood as follows.
Generally, many pores exist in a sintered body, and therefore stress
concentrates in pore parts and tends to cause a decrease in strength or
toughness of the sintered body. However, with the alloy steel powder for
powder metallurgy described herein, by setting the specific surface area of
the
composite alloy steel powder to 0.100 m2/g or more, the pores in the sintered
body are refined, the degree of stress concentration is mitigated and the
sintered neck part is toughened.
[0028] In addition, by setting the Mo content of the composite alloy steel
powder in a range of 0.2 mass% to 1.5 mass%, Mo concentrates in the pore
surrounding part of the sintered body, and the sintered body is further
strengthened. Further, since Mo-containing alloy powder is adhered to the
surface of the iron-based powder for the alloy steel powder for powder
metallurgy described herein and Mo is not contained in the matrix part,
carbide is less likely generated compared to the sintered neck part, and
therefore a microstructure with high toughness is obtained.
In other words, it is believed that by controlling pore distribution and Mo
distribution of the sintered body, both high strength and high toughness of
the
sintered body were made achievable.
[0029] The reasons for the limitations of thc disclosure are described below.
First, the method of producing an alloy steel powder for powder metallurgy
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described herein will be explained.
In the disclosure, iron powders such as atomized iron powder and reduced iron
powder as the iron-based powder, and Mo material powder which is the
material of Mo-containing alloy powder are prepared.
[0030] The above iron-based powder is not particularly limited as long as it
is
an iron-based powder which is normally used for powder metallurgical
techniques. However, the so-called as-atomized powder, atomized iron
powder, or reduced iron powder is preferable. As the atomized iron-based
powder, either one of as-atomized powder obtained by atomizing molten steel
and then drying and classifying the resulting powder, or atomized iron powder
obtained by reducing as-atomized powder in a reductive atmosphere, may be
used.
[0031] Further, as reduced iron powder, it is preferable to use reduced iron
powder obtained by reducing mill scale generated during production of steel
materials or iron ore. The apparent density of the reduced iron powder may
be around 1.7 Mg/m3 to 3.0 Mg/m3. More preferably, it is 2.2 Mg/m3 to 2.8
Mg/m3. Here, the apparent density is measured by the test method of JIS Z
2504.
[0032] Meanwhile, as the Mo material powder, the desired Mo-containing
alloy powder itself may be used, or an Mo compound that can be reduced to
Mo-containing alloy powder can be used. The mean particle size of the Mo
material powder is 50 um or less, and preferably 20 um or less. Here, the
mean particle size refers to the volume-based median size (so-called d50).
[0033] As the Mo-containing alloy powder, Mo alloy powders including pure
metal powder of Mo, oxidized Mo powder, Fe-Mo (ferromolybdenum) powder
and the like are advantageously applied. On the other hand, examples of an
Mo compound include Mo carbide, Mo sulfide, Mo nitride and the like.
[0034] Then, the above iron-based powder and Mo material powder are mixed
in a predetermined ratio to obtain a mixed powder. This ratio is adjusted so
that the final Mo content of the composite alloy steel powder is in a range of
0.2 mass% to 1.5 mass%. For the mixing, the mixing method or mixing
facility is not particularly limited, and the powders may be mixed in
accordance with conventional methods using a Henschel mixer, a cone mixer
or the like.
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100351 Further, by maintaining the mixed powder at a high temperature,
diffusing and bonding Mo to steel in the contact surface of the iron-based
powder and the Mo material powder (diffusing-bonding treatment), the
composite alloy steel powder used herein is obtained.
As the atmosphere for the diffusion-bonding treatment, reductive atmosphere
or hydrogen containing atmosphere is preferable, and hydrogen containing
atmosphere is particularly suitable. The heat treatment may be performed
under vacuum. Further, a
preferred temperature for diffusion-bonding
treatment is in a range of 800 C to 1000 C.
[0036] When diffusion-bonding treatment is performed as mentioned above,
the iron-based powder and the Mo-containing alloy powder are normally in
the state where they are sintered and agglomerated. Therefore, they are
ground and classified into desired particle sizes. Further, annealing may
optionally be performed. The particle size of the composite alloy steel
powder is preferably 180 um or less.
[0037] In the disclosure, the Mo-containing alloy powder is preferably
adhered uniformly to the surface of the iron-based powder. If not adhered
uniformly, Mo-containing alloy powder tends to come off from the surface of
the iron-based powder in situations such as when grinding the composite alloy
steel powder after diffusion-bonding treatment, or during transportation
thereof, and therefore Mo-containing alloy powder in a free state increases
particularly easily. When pressing
the alloy steel powder for powder
metallurgy obtained from the composite alloy steel powder in such state to
obtain a green compact and sintering the green compact, there may be
segregation of the dispersed carbide. Therefore, to enhance the strength and
toughness of the sintered body, it is preferable to uniformly adhere the
Mo-containing alloy powder on the surface of the iron-based powder to reduce
the Mo-containing alloy powder in a free state resulting from coming off or
the like.
[0038] The content of Mo to be diffusionally adhered is in the range of 0.2
mass% to 1.5 mass% (included number) with respect to the total amount of the
composite alloy steel powder. This is because while if said content falls
under 0.2 mass%, both the hardenability improving effect and the strength
improving effect are reduced, if said content exceeds 1.5 %, the hardenability
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improving effect reaches a plateau, and causes an increase in the
non-uniformity of the microstructure of the sintered body, and high strength
and toughness cannot be obtained. Therefore, the content of Mo to be
diffusionally adhered is in a range of 0.2 mass% to 1.5 mass% with respect to
the total amount of the composite alloy steel powder. Preferred content is in
the range of 0.3 mass% to 1.0 mass%.
[0039] On the other hand, the specific surface area of the composite alloy
steel powder which is obtained by diffusing and adhering Mo thereon is limited
to 0.100 m2/g or more. The area is preferably 0.150 m2/g or more. This is
because a specific surface area of less than 0.100 m2/g provides coarse pores
or insufficient reactivity during sintering, or for both reasons, which fact
will
result in little progress in refinement of pores and decreased toughness.
Although the upper limit of the specific surface area is not particularly
limited,
if said area exceeds 0.5 m2/g, a large amount of fine powder will be included
and compressibility will decrease. Therefore, the specific surface area is
preferably 0.5 m2/g or less.
[0040] Further, since the specific surface area of the alloy steel powder will
be reduced by diffusionally adhering Mo to the surface of the iron-based
powder,
the specific surface area of the iron-based powder as the base is preferably
0.150 m2/g or more. The specific surface area used herein is measured by a
gas adsorption method (BET method).
[0041] In the disclosure, the balance of the composite alloy steel powder is
iron and incidental impurities. Examples of impurities contained in the
composite alloy steel powder include C, 0, N, and S. As long as the contents
of these components in the composite alloy steel powder are limited to C: 0.02
mass% or less, 0: 0.3 mass% or less, N: 0.004 mass% or less, and S: 0.03
mass% or less, there is no particular problem. 0 content is preferably 0.25
% or less. This is because if the amount of incidental impurities exceeds the
above ranges, the compressibility of the alloy steel powder for powder
metallurgy obtained from the composite alloy steel powder decreases, and it
becomes difficult to perform compression molding to form a preformed body
having a sufficient density.
[0042] For the alloy steel powder for powder metallurgy containing the above
composite alloy steel powder as the main component, it is important that
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graphite powder is added in a range of 0.1 mass% to 1.0 mass% in a ratio with
respect to the total amount of the alloy steel powder for powder metallurgy
(100 mass%). Further, in the disclosure, 0.5 mass% to 4.0 mass% of Cu
powder can be added in a ratio with respect to the total amount of the alloy
steel powder for powder metallurgy (100 mass%).
C, which is a main component of graphite powder, dissolves in iron during
sintering and enables achieving solid solution strengthening and hardenability
improvement, and therefore C is a useful element for enhancing the strength
of sintered parts. In a case where carburizing heat treatment or the like is
performed after sintering and the sintered body is carburized from the
outside,
the amount of graphite powder added may be small. However, if it is less
than 0.1 mass%, the above effect obtained by adding graphite powder is
limited. On the other hand, graphite powder will also be added when
carburizing heat treatment is not performed during sintering. However, if
the amount of graphite powder added exceeds 1.0 mass%, the sintered body
becomes hypereutectoid, and cementite is precipitated and causes a decrease
in strength. Therefore, the amount of graphite powder is limited to a range
of 0.1 mass% to 1.0 mass%. The mean particle size of graphite powder is
preferably 50 p.m or less.
[0043] On the other hand, Cu is a useful element that achieves solid solution
strengthening of the iron-based powder and has an improving effect in
hardenability of the iron-based powder, thereby enhancing the strength of
sintered parts. Cu melts into a liquid phase during sintering of iron-based
powder, and has an effect of fixing iron-based powder particles to one
another.
However, if the amount of Cu powder added is less than 0.5 mass%, the
addition effect is limited. On the other hand, if it exceeds 4.0 mass%, not
only does the strength improving effect of the sintered parts reach a plateau
but also leads to a decrease in cuttability. Therefore, the amount of Cu
powder is preferably in a range of 0.5 mass% to 4.0 mass%, and more
preferably in a range of 1.0 mass% to 3.0 mass%. The mean particle size of
Cu powder is preferably 50 p.m or less.
[0044] The iron-based powder used herein contains reduced iron powder and
the mean particle size of the iron-based powder is preferably 80 1..im or
less.
This is because if powder with a mean particle size of larger than 80 1.1m
i.e.
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powder with a large particle size is included, the driving force during
sintering weakens and coarse holes are formed around the coarse iron-based
powder. These coarse holes become the cause of reducing the strength and
toughness of the sintered body.
Here, the above mean particle size refers to the mass-based median size
(so-called d50). In detail, the iron-based powder is sieved using a sieve
defined by JIS Z 8801, the mass of the sample powder remaining on each
sieve was measured, and a particle size where the amounts of small particles
and large particles become equal was obtained and defined as the mean
particle size.
[0045] In this disclosure, additives for improving characteristics may be
added in accordance with the purpose. For example, Ni powder may be
added as necessary for the purpose of improving the strength of the sintered
body, and powders for improving machinability such as MnS may be added as
necessary for the purpose of improving cuttability of the sintered body. Ni
powder is preferably in a range of 0.5 mass% to 5 mass% in a ratio with
respect to the total amount of the alloy steel powder for powder metallurgy
(100 mass%).
[0046] On the other hand, the additive amount of powders for improving
machinability such as MnS may be a conventionally known additive amount
i.e. around 0.1 mass% to 1 mass% in a ratio with respect to the total amount
of
the alloy steel powder for powder metallurgy (100 mass%).
[0047] Further, preferable pressing conditions and sintering conditions for
producing a sintered body using the alloy steel powder for powder metallurgy
described herein will be explained.
When performing pressing using the alloy steel powder for powder metallurgy
described herein, a lubricant powder may also be mixed in. Further, pressing
may be performed by applying or adhering a lubricant to a die. In either case,
as the lubricant, metal soap such as zinc stearate and lithium stearate,
amide-based wax such as ethylenebisstearamide, and other well known
lubricants may all be used suitably. When mixing the lubricant, the amount
thereof is preferably around 0.1 parts by mass to 1.2 parts by mass
(externally
added) with respect to 100 parts by mass of the alloy steel powder for powder
metallurgy.
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[0048] Pressing of the alloy steel powder for powder metallurgy described
herein is preferably performed with a pressure of 400 MPa to 1000 MPa.
This is because if the pressure is less than 400 MPa, the density of the
obtained green compact lowers and leads to a decrease in characteristics of
the
sintered body, whereas if it exceeds 1000 MPa, life of the die shortens and
becomes economically disadvantageous. The pressing
temperature is
preferably in the range of room temperature (around 20 C) to around 160 C.
100491 Further, the alloy steel powder for powder metallurgy described herein
is sintered preferably in a temperature range of 1100 C to 1300 C. This is
because if the sintering temperature is lower than 1100 C, progressing of
sintering stops and leads to a decrease in characteristics of the sintered
body,
whereas if it exceeds 1300 C, life of the sintering furnace shortens and
becomes economically disadvantageous. The sintering time is preferably in
the range of 10 minutes to 180 minutes.
[0050] The obtained sintered body can optionally be subjected to
strengthening treatment such as carburizing-quenching, bright quenching,
induction hardening, and carburizing nitriding treatment. However, even if
strengthening treatment is not performed, the sintered body obtained using the
alloy steel powder for powder metallurgy described herein has improved
strength and toughness compared to conventional sintered bodies (which are
not subjected to strengthening treatment). Each strengthening treatment may
be performed in accordance with conventional methods.
EXAMPLES
[0051] Although the disclosure will be described below in further detail with
reference to examples, the disclosure is not intended to be limited in any way
to the following examples.
As iron-based powders, an as-atomized powder with an apparent density of
2.65 Mg/cm3, 2.80 Mg/m3, or 3.25 Mg/m3, a reduced iron powder with an
apparent density of 2.60 Mg/cm3 or 2.75 Mg/m3, and an atomized iron powder
with an apparent density of 2.60 Mg/m3, 2.80 Mg/m3, or 3.30 Mg/m3 were
used.
Oxidized Mo powder (mean particle size: 10 um) was added to these
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iron-based powders at a predetermined ratio, and the resultant powders were
mixed for 15 minutes in a V-shaped mixer to obtain a mixed powder, then the
mixed powder was subjected to heat treatment in a hydrogen atmosphere with
a drew point of 30 C (holding temperature: 880 C, holding time: 1h) to
diffuse and adhere Mo to surfaces of the iron-based powders to obtain a
composite alloy steel powder. Mo content with respect to the composite
alloy steel powder is shown in table 1.
Then, copper powder (mean particle size: 30 1.1m) and graphite powder (mean
particle size: 5 1..im) in the amounts shown in table 1 were added to the
composite alloy steel powder (Mo-diffusionally adhered alloy steel powder )
to obtain an alloy steel powder for powder metallurgy. Then, 0.6 parts by
mass of ethylenebisstearamide was added with respect to 100 parts by mass of
the alloy steel powder for powder metallurgy, and then the resulting powders
were mixed in a V-shaped mixer for 15 minutes. Subsequently, the powders
were pressed until the density of the resulting green compacts reached 7.0
Mg/m', and tablet shaped green compacts with length of 55 mm, width of 10
mm, and thickness of 10 mm were produced.
The tablet shaped green compacts were sintered to obtain sintered bodies.
Sintering was performed in propane converted gas atmosphere at a sintering
temperature of 1130 C, for a sintering time of 20 minutes.
Then, the obtained sintered bodies were processed into round bar tensile test
specimens, each having a parallel portion diameter of 5 mm, for a tensile test
specified in JIS Z 2241. For Charpy impact tests specified in JIS Z 2242, the
obtained sintered bodies with shapes as sintered which were subjected to gas
carburizing of carbon potential of 0.8 mass% (holding temperature: 870 C,
holding time: 60 minutes), then quenching (60 C, oil quenching) and
tempering (180 C, 60 minutes) were used.
The sintered bodies were subjected to tensile tests specified in JIS Z 2241,
and Charpy impact tests defined by JIS Z 2242 to measure the tensile strength
(MPa) and the impact value (J/cm2). The measurement results of each
sintered body are shown in Table 1.
P0142158-PCT-ZZ (14/18)
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[0052] [Table 111
1
St
'Apparent Density' Mean Particle 'Oxygen Content. . pecificArea Surface
Tensile Impact
Mo Cu Graphite
rength Value
Material ii9 Size *2 ./ *4 Remarks
Mg/m3 pm mass% mass% , mz/g mass% mass% MPa
3/em2 ,
As-Atomized
2.65 76 0.58 1.4 0.152 1_0 0.5 1220 15.4
Example 1
Powder
i I
As-Atomized i
2.65 =
80 0.60 1.0 0.145 0,5 0_3 1124 14.1
Example 2
Powder
As-Atomized
2.80 85 0_56 0.8 0.120 2.0 0.3 1150 15.2
Example 3
Powder 1
As-Atomized
2.80 84 0.55 0_6 0.118 3.0 0.5 1120
15.5 Example 4 .
Powder
r I
I
r r
Reduced Iron
2.60 79 0.25 0.4 0 124 4.0 0.7 1163 15.3
Example 5
Powder
1
'
Reduced Iron
2.60 78 0.27 0.2 0.121 2.0 0.5 1105 15.7
Example 6
, Powder
R
Reduced Irono
2.75 96 0.28 1.4 0.105 1.5 0.5 1174
15.9 Example 7 I,
Powder
iii
1,
' .
"
ry
Reduced Iron0
2.75 94 0_31 0.6 0.103 3.0 1.0 1200
14_9 Example 8 r
Powder
in
As-AtomizedIv
2.80 94 0.57 0.7 0.109 2.0 0.1 1006
14.0 Example 9 o
Powder
ai
. 1
r 1 r
1
Atomized Iron
2.60 77 0.12 1.2 0.103 - 0.5 1010 14.1
Example 10 2
Powder
I
1,
As-AtomizedComparative
1,
2.80 95 0.62 0.1 0.115 1.0 0.3 880 12.7
Powder , Example 1
As-AtomizedComparative
3.25 96 0,08 1.0 0.080 2.0 0.3 1103 12.9
Powder Example 2
Reduced IronComparative
2.75 106 0.35 0.6 0.090 1.5 1.1 940 12.8
Example 3
Powder
Atomized IronComparative
2.80 102 0.12 1.7 0.103 3.5 0.7 1040 12.4
Powder 1 Example 4
i .
Atomizedd Iron
2.80 95 0.13 0,8 0.105 4.5 1.0 1212 14.0
Example 11
Power .
. - =
Atomized IronComparative
330 104 0.15 0.4 0.098 0.5 0_5 1047 9.8
Powder Example 5
i
As-AtomizedConventional
-
2.80 96 0.56 *5 - 0.3 998 13.3
Powder Example
i
*1 Apparent density of iron-based powder
*2 Mean particle size of iron-based powder
*3 Oxygen content of iron-based powder
*4 Specific surface area of Mo-diffusionally adhered alloy steel powder
*5 4%Ni-1.5%Cu-0.5%Mo
P0142158-PCT-ZZ (15/18)
CA 02922018 2016-02-22
=
- 16 -
[0053] As shown in Table 1, when comparing the tensile strength and impact
value of our examples with comparative examples, our examples all showed
tensile strength of 1000 MPa or more and impact value of 14.0 J/cm2 or more,
and both high strength and high toughness were achieved, whereas the
comparative examples all showed impact values of less than 14.0 J/cm2 and
were poor in at least one of tensile strength and impact value compared to our
examples.
[0054] Table i also shows the results of a 4Ni material (4Ni-1.5Cu-0.5Mo,
maximum particle size of material powder: 180 i_tm) as the conventional
material. It can be seen that our examples exhibit better characteristics over
the conventional 4Ni material.
P0142158-PC1-LZ (16/18)
