U.S. patent application number 13/699032 was filed with the patent office on 2013-05-30 for nitrided sintered steels.
This patent application is currently assigned to HOGANAS AB (PUBL). The applicant listed for this patent is Sigurd Berg, Senad Dizdar, Ulf Engstrom, Ola Litstrom, Eckart Schneider. Invention is credited to Sigurd Berg, Senad Dizdar, Ulf Engstrom, Ola Litstrom, Eckart Schneider.
Application Number | 20130136646 13/699032 |
Document ID | / |
Family ID | 45066971 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130136646 |
Kind Code |
A1 |
Berg; Sigurd ; et
al. |
May 30, 2013 |
NITRIDED SINTERED STEELS
Abstract
The present invention concerns a method of producing sintered
components, and sintered components by the method. The method
provides a cost effective production of sintered steel parts with
wear resistance properties comparable to those of components made
from chilled cast iron.
Inventors: |
Berg; Sigurd; (Hoganas,
SE) ; Dizdar; Senad; (Hoganas, SE) ; Engstrom;
Ulf; (Hoganas, SE) ; Litstrom; Ola; (Jonstorp,
SE) ; Schneider; Eckart; (Nyhamnslage, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Berg; Sigurd
Dizdar; Senad
Engstrom; Ulf
Litstrom; Ola
Schneider; Eckart |
Hoganas
Hoganas
Hoganas
Jonstorp
Nyhamnslage |
|
SE
SE
SE
SE
SE |
|
|
Assignee: |
HOGANAS AB (PUBL)
HOGANAS
SE
|
Family ID: |
45066971 |
Appl. No.: |
13/699032 |
Filed: |
May 24, 2011 |
PCT Filed: |
May 24, 2011 |
PCT NO: |
PCT/SE2011/050645 |
371 Date: |
November 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61351363 |
Jun 4, 2010 |
|
|
|
Current U.S.
Class: |
419/11 ;
75/244 |
Current CPC
Class: |
B22F 2999/00 20130101;
C22C 38/04 20130101; B22F 2998/10 20130101; B22F 3/02 20130101;
B22F 3/10 20130101; C22C 33/0207 20130101; B22F 2003/241 20130101;
B22F 2003/241 20130101; B22F 1/0059 20130101; C22C 33/0207
20130101; C22C 33/0264 20130101; B22F 2999/00 20130101; B22F
2998/10 20130101; B22F 2201/02 20130101; B22F 3/26 20130101 |
Class at
Publication: |
419/11 ;
75/244 |
International
Class: |
B22F 3/26 20060101
B22F003/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 4, 2010 |
SE |
1050576-6 |
Claims
1. A method of producing sintered components by single press/single
sintering comprising the steps of: a) providing pre-alloyed
iron-based steel powder comprising less than 0.3% by weight of Mn
and at least one of Cr in an amount between 0.2-3.5% by weight, Mo
in an amount between 0.05-1.20% by weight and V in an amount
between 0.05-0.4% by weight, and maximum 0.5% incidental
impurities, the balance being iron, b) mixing said pre-alloyed
iron-based steel powder with lubricant and graphite, and optionally
machining enhancing agent(s) and other conventional sintering
additives, c) subjecting the mixed composition of step b) to
compaction at pressures of 400-2000 MPa, thereby providing a
compact, d) sintering said compact from step c) in a reducing
atmosphere at a temperature between 1000-1400.degree. C., thereby
providing a sintered component, e) nitriding said sintered
component of step d) in a nitrogen containing atmosphere, at a
temperature of 400-600.degree. C., with a soaking time of less than
3 hours,
2. A method according to claim 1, wherein the lubricant consists of
composite lubricant particles comprising a core of 10-60% by weight
of at least one primary fatty acid amide having more than 18 and
not more than 24 carbon atoms and 40-90% by weight of at least one
fatty acid bisamide, said lubricant particles also comprising
nanoparticles of at least one metal oxide adhered on the core.
3. A method according to claim 1, wherein the compact is not steam
treated before nitriding in step e).
4. A method according to claim 1, wherein in step c) the compact is
compacted to a green density of at least 7.10 g/cm.sup.3.
5. A method according to claim 1, wherein in step d) the sintered
component is sintered to a density between 7.1-7.6 g/cm.sup.3.
6. A method according to claim 1, wherein the pre-alloyed
iron-based steel powder further comprises between 0.1-1.0% by
weight of Ni.
7. A method according to claim 1, wherein the pre-alloyed
iron-based steel powder is essentially free from Ni.
8. A method according to claim 1, wherein the pre-alloyed
iron-based steel powder further comprises between 0.05% and 0.50%
by weight of one or more of element(s) selected from the group of
tungsten (W), titanium (Ti), niobium (Nb) and aluminium (Al).
9. A method according to claim 1, wherein the pre-alloyed
iron-based steel powder consists of, in percentage by weight: Fe:
Bal. Mn: 0.09-0.3 Cr: 1.3-1.6 Mo: 0.15-0.3 and max 0.3 incidental
impurities.
10. A method according to any one of claims 1 5, claim 1, wherein
the pre-alloyed iron-based steel powder consists of, in percentage
by weight: Fe: Bal. Mn: 0.09-0.3 Cr: 1.5-1.9 Mo: max 0.1 and max
0.3 incidental impurities.
11. A method according to claim 1, wherein the pre-alloyed
iron-based steel powder consists of, in percentage by weight: Fe:
Bal. Mn: 0.09-0.3 Cr: 2.8-3.2 Mo: 0.4-0.6 and max 0.3 incidental
impurities.
12. A method according to claim 1, wherein the pre-alloyed
iron-based steel powder consists of, in percentage by weight: Fe:
Bal. Mn: 0.09-0.3 V: 0.05-0.4 Mo: max 0.1 and max 0.3 incidental
impurities.
13. A nitrated, sintered component, produced according to claim 1,
and having a wear resistance in lubricating sliding contact that
provides safe wear for hertzian pressures up to at least 800 MPa
when tested at a sliding velocity of 2.5 m/s during 100
seconds.
14. A nitrated, sintered component, produced according to claim 1,
and having a wear resistance in lubricating sliding contact that
provides safe wear for hertzian pressures up to at least 900 MPa,
when tested at a sliding velocity of 2.5 m/s during 100
seconds.
15. A nitrated, sintered component, produced according to claim 1,
and having a wear resistance in lubricating sliding contact that
provides safe wear for hertzian pressures up to at least 1000 MPa,
when tested at a sliding velocity of 2.5 m/s during 100 seconds.
Description
FIELD OF THE INVENTION
[0001] The present invention concerns a method of producing
sintered components by single press and single sintering, and
sintered components produced by the method. The method provides a
cost effective production of sintered steel components having wear
resistance properties comparable to components made from chilled
cast iron.
BACKGROUND OF THE INVENTION
[0002] In industries the use of metal products manufacturing by
compaction and sintering metal powder compositions is becoming
increasingly widespread. A number of different products of varying
shape and thickness are being produced and the quality requirements
are continuously raised at the same time as it is desired to reduce
the cost. As net shape components, or near net shape components
requiring a minimum of machining in order to reach finished shape,
are obtained by press and sintering of iron based powder
compositions in combination with a high degree of material
utilisation, this technique has a great advantage over conventional
techniques for forming metal parts such as moulding or machining
from bar stock, casting or forgings.
[0003] It is desirable to increase the performance of sintered
parts so that more parts can be substituted to this cost effective
technique. Various industrial steel components, for instance in the
automotive industry, have successfully been produced by the press
and sintering technique. Automotive parts are manufactured in high
volume for applications having strict performance, design and
durability requirements. The single press and single sintering
technique is therefore very suitable for production of such parts,
provided that the overall quality requirements can be met.
[0004] For certain power train and valve train components in the
automotive industry, such as cam lobes, requirements on wear
resistance have so far made it very difficult to convert to
conventionally sintered products. The predominant production
techniques for such components are today machining from bar stocks,
or casting with chilled cast iron (CCI). In the case of cam lobes
for small cars, where the wear resistance requirements are somewhat
lower; the parts have been produced successfully using double
press/double sintering. So far however, no manufacturing technique
involving single pressing and single sintering has proven to
provide wear properties comparable to those of components
manufactured using CCI.
[0005] WO2006/045000 concerns carburized sintered alloys for cam
lobes and other high wear articles fabricated from iron-based
powder metal mixtures consisting of 0.5-3.0% Mo, 1-6.5% Cr, 1-5% V,
and the balance Fe and impurities. However the wear resistance does
not reach the same levels as that of CCI components.
SUMMARY OF THE INVENTION
[0006] It has surprisingly been found that by using certain
iron-based powder alloy compositions in combination with warm die
compaction and a short nitriding process, components with wear
resistance comparable to that of components made with CCI can be
manufactured.
[0007] More specifically this can be achieved by a method of
producing sintered components by single press and single sintering
comprising the following steps: [0008] a) Providing pre-alloyed
iron-based steel powder comprising less than 0.3% by weight of Mn
and at least one of Cr in an amount between 0.2-3.5% by weight, Mo
in an amount between 0.05-1.20% by weight and V in an amount
between 0.05-0.4% by weight, and maximum 0.5% incidental
impurities, the balance being iron, [0009] b) Mixing said
pre-alloyed iron-based steel powder with lubricant and graphite,
and optionally machining enhancing agent(s) and other conventional
sintering additives, [0010] c) Subjecting the mixed composition of
step b) to compaction at pressures of 400-2000 MPa, thereby
providing a compact, [0011] d) Sintering said compact from step c)
in a reducing atmosphere at a temperature between 1000-1400.degree.
C., thereby providing a sintered component, [0012] e) Nitriding
said sintered component of step d) in a nitrogen containing
atmosphere, at a temperature of 400-600.degree. C., with a soaking
time of less than 3 hours
[0013] Components produced according to the method demonstrate wear
resistance properties similar to those of CCI-components. The
components have a hard case with softer core, and are thus not
through-hardened. A through-hardened component can make assembly
more difficult compared to a case hardened component having a
softer core.
[0014] The method is particularly suitable for automotive
components working in oil lubricated environments, where the
working temperature is below 250.degree. C., and which components
have functions that rely on sliding movements. For instance cam
lobes, sprockets, CVT, and other power train, valve train and
engine components. Of course the method can also be suitable to
produce components for other applications where good wear
properties are desirable.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Preparation of the Iron-Based Alloyed Steel Powder.
[0016] The prealloyed iron-based steel powder provided in step a)
of the method is preferably produced by water atomization of an
iron melt including the alloying elements. The atomized powder can
further be subjected to a reduction annealing process. The particle
size of the prealloyed powder alloy could be any size as long as it
is compatible with the press and sintering processes. Examples of
typical particle size is the particle size of the known powder
ASC100.29 available from Hoganas AB, Sweden, having maximum 2.0% by
weight above 180 .mu.M and 15-30% by weight below 45 .mu.m.
However, coarser as well as finer grained powders may be used.
[0017] The use of coarse iron-based steel powders is increasingly
popular in the field of powder metallurgy. Example of such powders
are iron-based powders having an average particle size between 75
and 300 .mu.m, wherein less than 10% of the powder particles have a
size below 45 .mu.m and the amount of particles above 212 .mu.m is
above 20%.
[0018] Finer iron-based steel powders could also be used. When
using fine powders, it is preferred that they are bonded with
binding agent(s) and/or flow agent(s), in order to provide better
powder properties and compressibility. Such powders could e.g. have
a average particle size in the range of 20-60 .mu.m.
[0019] Contents of the Prealloyed Steel Powder
[0020] The prealloyed steel powder provided in step a) of the
method is iron-based and comprises Mn and at least one element
selected from the group of Cr, Mo and V. The prealloyed steel
powder may optionally further comprise Ni and/or additional strong
nitride forming element(s), such as tungsten, titanium, niobium
and/or aluminium.
[0021] Manganese, Mn, is present in amounts between 0.02-0.3% by
weight. In practice, it is very hard to achieve contents below
0.02% by weight when using recycled scrap unless a specific
treatment for the reduction during the course of the steel
manufacturing is carried out, which increases costs. Furthermore,
Manganese increases strength, hardness, and hardenability of the
steel powder and it is therefore preferred to have a manganese
content above 0.05% by weigh, ore preferably above 0.9% by weight.
A Mn content above 0.3% by weight will increase the formation of
manganese containing inclusions in the steel powder and will also
have a negative effect on the compressibility due to solid solution
hardening and increased ferrite hardness. Therefore the Mn content
should not exceed 0.3% by weight. The most preferred range for Mn
is 0.1-0.3% by weight.
[0022] Chromium, Cr, as an alloying element serves to strengthen
the matrix by solid solution hardening. Chromium also increases
hardenability and abrasion resistance of a sintered body.
Furthermore, Cr is a very strong nitride former and thus promotes
nitriding. If chromium is added, it should be added in an amount of
at least 0.2% by weight to have desired impact on the properties of
the sintered component, preferably at least 0.4% by weight, and
more preferably at least 1.3% by weight. However, with increasing
addition of chromium the requirements of controlled atmospheres
during sintering increase, making components more costly to
manufacture. Therefore, if chromium is added it should be at most
3.5% by weight of Cr, preferably at most 3.2% by weight. In a
preferred embodiment the chromium content is 0.4-2.0% by weight,
more preferably 1.3-1.9% by weight. In another preferred embodiment
the chromium content is 2.8-3.2% by weight.
[0023] Molybdenum, Mo, stabilizes ferrite after sintering. If
molybdenum is added, it should be added in an amount of at least
0.1% by weight to have desired impact on the properties of the
sintered component, preferably in an amount of at least 0.15% by
weight. It is not desired to have a too high Mo-content as it will
not contribute enough to the performance. Therefore, if molybdenum
is added it should be at most 1.2% by weight of Mo, preferably at
most 0.6% by weight. In some embodiments the steel may be
essentially free from Mo, having contents of Mo below 0.1% by
weight, preferably below 0.05% by weight.
[0024] Vanadium, V, increases the strength by precipitation
hardening. Vanadium has also a grain size refining effect and is a
strong nitride forming element. If vanadium is added, it should be
added in an amount of at least 0.05% by weight to have desired
impact on the properties of the sintered component, preferably in
an amount of at least 0.1% by weight, more preferably in an amount
of at least 0.25% by weight. However, high vanadium contents
facilitate oxygen pickup, thereby increasing the oxygen level in a
component produced by the powder, which is not desired in too high
amounts. Therefore the vanadium content should be at most 0.4% by
weight, preferably at most 0.35% by weight.
[0025] The prealloyed steel powder may optionally further comprise
additional strong nitride forming element(s) as known in the art,
such as one or more of element(s) selected from the group of
tungsten (W), titanium (Ti), niobium (Nb) and aluminium (Al). If
added, the total amount of said optional additional strong nitride
forming element(s) should be between 0.05% and 0.50% by weight,
preferably between 0.1% and 0.4%, and more preferably 0.15% to
0.30% by weight.
[0026] Nickel, Ni, increases strength and hardness while providing
good ductility properties. However, nickel is an expensive element
and is avoided if possible. If added, contents are kept low. The
prealloyed steel powder may optionally comprise Ni in an amount of
0.1-1.0% by weight, preferably 0.1-0.5% by weight. In a preferred
embodiment the prealloyed steel powder is essentially free from
nickel, and thus contains below 0.1% by weight, preferably below
0.05% by weight.
[0027] Oxygen, O, is at most 0.25% by weight. Too high content of
oxygen impairs strength of the sintered component, and impairs the
compressibility of the powder. For these reasons, O is preferably
at most 0.18% by weight. In practice, when using water atomization
techniques, it is difficult to reach oxygen contents below 0.1% by
weight. The oxygen content in water atomized and annealed powders
are therefore normally in the range of 0.10-0.18% by weight.
[0028] Carbon, C, in the steel powder should be at most 0.1% by
weight, preferably less than 0.05% by weight, more preferably less
than 0.02% by weight, and nitrogen, N, should be at most 0.1% by
weight, preferably less than 0.05% by weight, more preferably less
than 0.02% by weight. Higher contents of carbon and nitrogen will
unacceptably reduce the compressibility of the powder.
[0029] The amount of each incidental impurity element such as any
element selected from the group consisting of copper (Cu),
phosphorous (P), silicon (Si), sulphur (S), and any other element
not intentionally added to the alloy, should be less than 0.15%,
preferably less than 0.10%, more preferably less than 0.05%, and
most preferably less than 0.03% by weight of each element, in order
not to deteriorate the compressibility of the steel powder or act
as formers of detrimental inclusions. The total sum of all
incidental impurities should be less than 0.5% by weight,
preferably less than 0.3% by weight, more preferably less than 0.2%
by weight.
Preferred Embodiments of the Prealloyed Steel Powder
[0030] In a preferred embodiment the prealloyed steel powder
according to the invention consists of (in % by weight):
[0031] Fe: Bal.
[0032] Mn: 0.09-0.3
[0033] Cr: 1.3-1.9
[0034] Mo: 0-0.3
[0035] and max 0.3 incidental impurities
[0036] In another preferred embodiment the prealloyed steel powder
according to the invention consists of (in % by weight):
[0037] Fe: Bal.
[0038] Mn: 0.09-0.3
[0039] Cr: 1.3-1.6
[0040] Mo: 0.15-0.3
[0041] and max 0.3 incidental impurities
[0042] In yet another preferred embodiment the prealloyed steel
powder according to the invention consists of (in % by weight):
[0043] Fe: Bal.
[0044] Mn: 0.09-0.3
[0045] Cr: 1.5-1.9
[0046] Mo: 0-0.1
[0047] and max 0.3 incidental impurities.
[0048] In yet another preferred embodiment the prealloyed steel
powder according to the invention consists of (in % by weight):
[0049] Fe: Bal.
[0050] Mn: 0.09-0.3
[0051] Cr: 2.8-3.2
[0052] Mo: 0.4-0.6
[0053] and max 0.3 incidental impurities
[0054] In yet another preferred embodiement prealloyed steel powder
according to the invention consists of (in % by weight):
[0055] Fe: Bal.
[0056] Mn: 0.09-0.3
[0057] V: 0.05-0.4
[0058] Mo: 0-0.1
[0059] and max incidental 0.3 impurities
[0060] Powder Composition
[0061] Before compaction, the prealloyed steel powder is mixed with
lubricants, graphite, optionally one or more machining enhancing
agent(s) and optionally other conventional additives, such as hard
phase materials.
[0062] In order to enhance strength and hardness of the sintered
component carbon is introduced in the matrix. Carbon is added as
graphite to the composition in amount between 0.15-1.0% by weight
of the composition. An amount less than 0.15% by weight will result
in a too low strength and an amount above 1.0% by weight will
result in an excessive formation of carbides, affecting the nitride
formation properties negatively. Preferably, graphite is added in
an amount between 0.20-0.80% by weight, and more preferably in an
amount of 0.30-0.60% by weight.
[0063] Lubricants are added to the composition in order to
facilitate the compaction and ejection of the compacted component.
The addition of less than 0.05% by weight of the composition of
lubricants will have insignificant effect and the addition of above
2% by weight of the composition will result in a too low density of
the compacted body. Preferably, the amount of added lubricant is
between 0.3-0.8% by weight of the composition, more preferably
0.4-0.6% by weight of the composition. Any type of lubricant
suitable for compaction may be used. Lubricants may be chosen from
the group of metal stearates, waxes, fatty acids and derivates
thereof, oligomers, polymers and other organic substances having
lubricating effect.
[0064] In one embodiment composite lubricant particles suitable for
compacting with a heated die are chosen, such as composite
lubricant particles comprising a core of 10-60% by weight of at
least one primary fatty acid amide having more than 18 and not more
than 24 carbon atoms and 40-90% by weight of at least one fatty
acid bisamide, said lubricant particles also comprising
nanoparticles of at least one metal oxide adhered on the core.
[0065] In a preferred embodiment the composite lubricant particles
suitable for compacting with a heated die comprise 10-30% by weight
of the at least one primary fatty acid amide and 70-90% by weight
of the at least one fatty acid bisamide. The at least one fatty
acid bisamide is preferably selected from the group consisting of
methylene bisoleamide, methylene bisstearamide, ethylene
bisoleamide, hexylene bisstearamide and ethylene bisstearamide. The
nanoparticles of the at least one metal oxide are preferably
selected from the group consisting of TiO2, Al2O3, SnO2, SiO2, CeO2
and indium titanium oxide.
[0066] Copper, Cu, is a commonly used alloying element in the
powder metallurgical technique. Cu will enhance the strength and
hardness through solid solution hardening. Cu, will also facilitate
the formation of sintering necks during sintering as copper melts
before the sintering temperature is reached providing so called
liquid phase sintering. The powder may optionally be admixed with
Cu, preferably in an amount of 0.2-3% by weight Cu. In a preferred
embodiment no copper is admixed to the composition.
[0067] Nickel, Ni, increases strength and hardness while providing
good ductility properties. However, contents above 1.5% by weight
will tend to form Ni-rich austenite during heat treatment
conditions, which will lower the strength of the material. The
powder may optionally be admixed with Ni in an amount of 0.1-1.5%
by weight. In a preferred embodiment no nickel is admixed to the
composition.
[0068] Machinability enhancing agent(s) can optionally be admixed
to the composition in an amount of 0.1-1.0% by weight of the
composition. Below 0.1% the effect is not good enough and above
1.0% no additional improvement is added. Preferably, if admixed,
the machinability enhancing agent(s) is in an amount of 0.2-0.8% by
weight of the composition, more preferably 0.3-0.7% by weight of
the composition. The machinability enhancing agent(s) are
preferably selected from the group consisting of MnS, MoS.sub.2,
CaF.sub.2, and/or phyllosilicates, such as kaolinites, smectites,
bentonites, and micas (such as muscovite or phlogopite). In working
conditions said machinability enhancing agent(s) also work as solid
lubricants and thus help to increase the wear resistance of the
components.
[0069] Other conventional sintering additives, such as hard phase
materials, may optionally be admixed to the composition.
[0070] Compaction
[0071] The iron-based powder composition is transferred into a
press mould and subjected to a compaction pressure of between
400-2000 MPa, preferably 500-1200MPa. In a preferred embodiment the
die in the press is heated to a temperature of 40-100.degree. C.,
preferably 50-80.degree. C., before and during compaction. This
technique is referred to as "warm die compaction" or "heated die
compaction". The component is preferably compacted to a green
density of at least 7.10 g/cm.sup.3, preferably at least 7.15
g/cm.sup.3, more preferably at least 7.20 g/cm.sup.3.
[0072] Thanks to the choice of lubricant and compaction process,
high green densities can be reached, ensuring high sintered
densities without excessive dimensional changes. This provides good
tolerances and closed porosity of the sintered component.
[0073] Sintering
[0074] The obtained green component is further subjected to
sintering in a reducing atmosphere at a temperature of about
1000-1400.degree. C. In a preferred embodiment the component is
sintered at regular sintering temperatures, in the range of at
1000-1200.degree. C., preferably 1050-1180.degree. C., most
preferably 1080-1160.degree. C. However, depending on requirements,
the component could also be sintered at higher temperatures, e.g.
in the range of 1200-1400.degree. C., preferably 1200-1300.degree.
C., and most preferably 1220-1280.degree. C.
[0075] The component is sintered to a density in the range of 7.1
to 7.6 g/cm.sup.3, preferably 7.15 to 7.50 g/cm.sup.3, more
preferably 7.20 to 7.45 g/cm.sup.3. However it is also possible to
sinter to higher densities than 7.6 g/cm.sup.3.
[0076] Post Sintering Treatments
[0077] The sintered component is then subjected to a nitriding
process, for obtaining the desired microstructure. The nitriding
process is performed in a nitrogen containing atmosphere in
temperatures around 500.degree. C. In a preferred embodiment, the
nitriding process is performed in a mixture of nitrogen and
hydrogen gas at a temperature of 400-600.degree. C., preferably
470.degree. -580.degree. C., with a soaking time of less than 3
hours, preferably less than 2 hours time, more preferably less than
1 hour. However, the soaking time during nitriding is preferably at
least 10 minutes, more preferably at least 20 minutes.
[0078] Optionally, other common types of nitriding process can be
used, such as (but not limited to) carbonitriding and
nitrocarburizing.
[0079] Usually, when gas nitriding sintered components, the
sintered components need to be steam-treated first in order to
close the pores and enable control of nitrogen penetration, since
an excessive nitrogen penetration into the component may lead to
brittle structure. However, this step is not necessary when
providing components according to the invention since the achieved
sintered density is high enough to ensure a closed porosity. The
components can thus be case nitrided in a controlled manner without
the prior step of steam-treatment.
[0080] Using the inventive method the surface of the component
comprises a nitride rich so-called white layer or compound layer of
1 to 20 .mu.m, preferably 5 to 15 .mu.m in thickness and a nitride
enriched hardened zone down to approx. 1-6 mm in depth, preferably
1-4mm.
[0081] Properties of the Finished Component Components manufactured
according to the invention achieve high wear resistance in sliding
lubricated contact. The wear resistance achieved is comparable to
components made with chilled cast iron.
[0082] The sintered components have closed porosity directly after
sintering, eliminating the need of steam treatment prior to gas
nitriding.
[0083] Furthermore, the components made by the claimed method
includes a deeper surface porosity in comparison with
CCI-components, which during working conditions, without being
bound to any specific theory, seems to provide a lubricating effect
as lubricating oil and the machining enhancing agent become present
inside these pores.
[0084] In a preferred embodiment the nitrided finished component
has a hardness of more than double that of the core at 0.5 to 1 mm
depth, preferably above 600 MHV.sub.0.05, more preferably above 700
MHV.sub.0.05 when the core hardness is around 300 MHV.sub.0.05 or
above 700 MHV.sub.0.05, preferably above 800 MHV.sub.0.05 when the
core hardness is around 350 MHV.sub.0.05. The total case depth
should be between 0.5-4.0 mm, preferably 1.0-3.0 mm, more
preferably 1.5-2.5 mm.
[0085] The term core hardness is to be interpreted as the hardness
value in the center of the component before nitriding. The term
total case depth is to be interpreted as the distance from the
surface of the component, where the hardness value is the same as
the core hardness value.
[0086] According to the test method described in the example
section, the finished component should demonstrate a good wear
resistance in lubricating sliding contact. When tested at a sliding
velocity of 2.5 m/s during 100 seconds, the component should show
safe wear for herzian pressures up to at least 800 MPa, preferably
up to at least 900 MPa, and more preferably up to at least 1000
MPa.
EXAMPLES
[0087] Testing Method
[0088] A general characterization of wear in lubricated sliding
contacts was done by researchers at international plane joined in
informal IRG-WOEM group supported by OECD in 1980'. The several
co-coordinated investigations gave a severity of valuable results
of which the IRG-wear transitions diagram may be the most important
one, see FIG. 1.
[0089] The IRG wear transitions diagram (FIG. 1) shows three main
wear regions, mild (safe) wear, limited wear and scuffing (severe
adhesive wear). The wear depends mainly on relative sliding
velocity between the contact surfaces but also on other factors
such as lubrication mode, lubricant chemistry, surface
roughness--topography, surface metallurgy and geometry of the
contacting bodies. Different alloys will have similar curves at
different pressures and FIG. 1 is only shown as an illustrative
example.
[0090] Automotive cam lobe to cam follower sliding contact is a
good example of a component subjected to sliding velocities of
about 0.1 m/s over 3 m/s when in use. In 1988, Chatterley [T. C.
Chatterley, "Cam and Cam Follower Reliability", SAE Paper No.
885033, 1988] summarized MIRA engine test bench testing of a number
of chilled cast iron (CCI) cam lobes to CCI, coated, boronized and
ceramic followers. A Hertzian level of 800 MPa was failure-free for
a majority of test runs, while 1000 MPa level passed only CCI to
SiN ceramic test combination.
[0091] Based on the above, wear testing in the investigation was
performed at three sliding velocities, 0.1, 0.5 and 2.5 m/s, having
standard engine oil (see table 1 for specification) at 90.degree.
C. as lubricant. At 2.5 m/s, testing was performed by stepwise
increasing Hertzian pressure until scuffing occurred.
[0092] Wear testing was done by using a commercial tribometer, a
multipurpose friction and wear measuring machine with crossed
cylinders test set-up (FIG. 2). The tribometer applies normal load
on the cylinder specimen holder by dead weights/load arm while an
AC thyristor controlled motor drives the counter ring. The counter
ring is immersed in an oil bath with approx. 25 ml oil and option
for heating up to 150.degree. C. A PC controls the test and logs
linear displacement in the contact, wear, friction force and oil
temperature. The linear displacement acquired is about three times
larger than the linear wear over the wear track, since the
displacement transducer is placed not over the test cylinder but on
the load arm lever. Hertzian pressure is proportional to the linear
wear h of the cylinder sample, which in turn is proportional to the
length a of the wear track. The length a and can be visually
determined by using a light optical microscope, as indicated by
FIG. 3.
[0093] Table 1 lists the properties of the lubricating oil used
during wear testing.
TABLE-US-00001 TABLE 1 Lubricating oil used in wear testing SAE
class/API grade 10W40/API SJ Oil base Semi-synthetic AW additive
ZnDDP Density in g/ml at 15.degree. C. (ASTM D4052) 0.875 Kin.
viscosity in mm2/s (ASTM D445) 40.degree. C. 88 100.degree. C. 13.5
Viscosity index (ASTM D2270) 150
[0094] Table 2 lists the prealloyed steel powders used in the
testing
TABLE-US-00002 TABLE 2 Prealloyed steel powders used ID Name Fe Mn
% Cr % Mo % Ni % R0 Distaloy .TM. DC-1 Bal. <0.3 -- 1.4 2
[Reference sample] A Astaloy .TM. CrL Bal. 0.12 1.5 0.2 -- B
Astaloy .TM. 85 Mo Bal. 0.11 <0.1 0.9 -- C -- Bal. 0.13 1.8
<0.1 --
[0095] Distaloy.TM. DC-1, Astaloy.TM. CrL and Astaloy.TM. 85 Mo are
well known powder metallurgy prealloyed steel powders available
from Hoganas AB (www.hoganas.com). Powder C is produced in the same
manner as Astaloy.TM. 85 Mo and Astaloy.TM. CrL.
[0096] Test specimens for this investigation were sintered test
specimens and reference cast iron specimens as overviewed in table
3 and 4.
TABLE-US-00003 TABLE 3 Reference specimens ID Type Manufacturing
method R1 DIN GJL-350 Chilled cast iron,
(Fe--3C--2Si--0.5Mn--0.3Cr--0.6Cu) ground, nitrited R2 DIN GJL-350
Chilled cast iron, ground (Fe--3C--2Si--0.5Mn--0.3Cr--0.6Cu)
TABLE-US-00004 TABLE 4 Specimens manufactured by powder metallurgy
ID Composition* Manufacturing method C-R Powder R + 0.65% C- Double
press/Double sinter [reference] UF4 + 0.5% MnS + 1.sup.st pressing
at 800 MPa followed by 0.6% Kenolube .TM. sintering to
approximately 7.1 g/cm.sup.3 density. 2.sup.nd pressing at 1000 MPa
followed by sintering to approximately 7.5 g/cm.sup.3 density. C-A
Powder A + 0.45% C- Single press/Single sinter + Nitriding UF4 +
0.5% lubricant Compaction with heated die, suitable for followed by
sintering at 1120.degree. C. for compacting with a 30 mins in 90%
N2/10% H2 heated die atmosphere, to a sintered density of 7.25
g/cm.sup.3. Gas nitriding at 510.degree. C., in
75NH.sub.3/25N.sub.2 atmosphere, with soaking time of 1 h. C-B
Powder B + 0.45% C- Single press/Single sinter + Nitriding UF4 +
0.5% lubricant Compaction with heated die, suitable for followed by
sintering at 1120.degree. C. for compacting with a 30 mins in 90%
N2/10% H2 heated die atmosphere, to a sintered density of 7.25
g/cm.sup.3. Gas nitriding at 510.degree. C., in
75NH.sub.3/25N.sub.2 atmosphere, with soaking time of 1 h. C-C
Powder C + 0.45% C- Single press/Single sinter + Nitriding UF4 +
0.5% lubricant Compaction with heated die, suitable for followed by
sintering at 1120.degree. C. for compacting with a 30 mins in 90%
N2/10% H2 heated die atmosphere, to a sintered density of 7.25
g/cm.sup.3. Gas nitriding at 510.degree. C., in
75NH.sub.3/25N.sub.2 atmosphere, with soaking time of 1 h.
[0097] *) MnS is a machining agent available from Hoganas AB
(www.hoganas.com), Kenolube.TM. is a compaction lubricant available
from Hoganas AB, and C-UF4 is a graphite product available from
Graphit Kropfmuhl AG (www.graphite.de).
[0098] FIG. 4 represents the results from the evaluation of the
test specimens at 2.5 m/s. It can be seen that all specimens
produced according to the invention surprisingly reach a level
comparable to that of the reference R1 and R2, i.e. the chilled
cast iron references. When comparing the reference C-R to C-A, C-B
and C-C of the invention, it becomes clear how efficient the new
method of producing sintered components by single press/single
sintering really is.
[0099] Moreover, a comparison was made for composition C-A, before
and after the nitriding step at three velocities. The results can
be seen in table 5.
TABLE-US-00005 TABLE 5 Results of wear testing for C-A Hertzian
Velocity/ pressure Time (MPa) As-sintered Nitrided 2.5 m/s 1100
Severe wear/scuff. for 100 1000 Safe/mild wear sec. 900 Safe/mild
wear 600 Severe wear 500 Severe wear 380 Severe wear 320 Severe
wear Linear wear/Wear coefficient k h (.mu.m)/(mm.sup.3/[Nm]) 0.5
m/s 800 50/71 7/10 for 23 h 500 36/20 5/2 0.1 m/s 800 19/2 6/2 for
23 h 500 14/2 4/3
[0100] It can be seen in table 5 that the nitriding step is
essential for the properties of the material. Already at a Hertzian
level of 320 MPa the component, which had only been subjected to
step a)-d) of the claimed method and not to the nitriding step e)
showed severe wear. The component subjected to step a)- to e) on
the other hand firstly showed severe wear on a Hertzian level of
1100 MPa, i.e. considerably better. The results of table 5 are
illustrated in FIG. 5.
[0101] FIG. 6 shows a metallographic image of nitrided specimen
C-A. The white nitride enriched layer can be seen at the sintered
surface, which provides high adhesive wear resistance as seen in
the results above.
[0102] FIG. 7 shows the hardness profile as measured in Vickers
(according to ISO 4498:2005 and ISO 4507:2000) of the specimen C-A.
As can be seen in this figure the hardness is above 700
MHV.sub.0.05 at 1 mm depth, and thus a case has been formed with
hardness more than double that of the core.
* * * * *