U.S. patent application number 10/470144 was filed with the patent office on 2004-06-17 for sintered ferrous material contaning copper.
Invention is credited to Maulik, Paritosh.
Application Number | 20040112173 10/470144 |
Document ID | / |
Family ID | 26245610 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040112173 |
Kind Code |
A1 |
Maulik, Paritosh |
June 17, 2004 |
Sintered ferrous material contaning copper
Abstract
A process for the manufacture of a ferrous-based sintered
article containing copper in the range from 12 to 26 weight % is
described, the process including the steps of: making a powder
mixture having a desired composition, at least a proportion of a
total content of iron and copper being provided by an iron powder
having copper indivisibly associated therewith for instance being
pre-alloyed or diffusion bonded; compacting said powder mixture to
form a green compact of an article to be produced and sintering
said green compact.
Inventors: |
Maulik, Paritosh; (Coventry,
GB) |
Correspondence
Address: |
Robert L Stearns
Howard & Howard Attorneys
The Pinehurst Office Center Suite 101
39400 Woodward Avenue
Bloomfield Hills
MI
48304-5151
US
|
Family ID: |
26245610 |
Appl. No.: |
10/470144 |
Filed: |
December 29, 2003 |
PCT Filed: |
January 17, 2002 |
PCT NO: |
PCT/GB02/00176 |
Current U.S.
Class: |
75/246 ;
419/38 |
Current CPC
Class: |
C22C 33/0278 20130101;
C22C 33/0207 20130101 |
Class at
Publication: |
075/246 ;
419/038 |
International
Class: |
B22F 003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2001 |
GB |
0101770.6 |
Aug 22, 2001 |
GB |
0120401.5 |
Claims
1. A process for the manufacture of a ferrous-based sintered
article containing copper in the range from 12 to 26 weight %, the
sintered article having a ferrous matrix comprising a martensitic
structure, the process including the steps of: making a powder
mixture having a desired composition, said powder mixture
composition including at least some iron powder and, separately,
martensite promoting material, at least a proportion of a total
content of iron and copper being provided by an iron copper
material selected from the group comprising: a diffusion bonded
iron-copper powder and pre-alloyed iron-copper powder; compacting
said powder mixture to form a green compact of an article to be
produced and sintering said green compact to produce an article
wherein the ferrous matrix has a martensitic structure.
2. A process according to claim 1 wherein the copper content lies
in the range from 15 to 20 weight %.
3. A process according either claim 1 or claim 2 wherein the powder
mixture also contains a steel powder.
4. A process according to claim 3 wherein the steel powder contains
chromium.
5. A process according to claim 3 wherein the steel powder contains
molybdenum.
6. A process according to claim 3 wherein the steel powder contains
nickel.
7. A process according to claim 3 wherein the steel powder is a
high-speed steel powder.
8. A process according to claim 7 wherein the steel powder is an
M3/2 steel powder.
9. A process according to claim 3 wherein the steel powder is a
stainless steel powder.
10. A process according to claim 9 wherein the stainless steel
powder is 316 steel.
11. A process according to any one preceding claim wherein the
powder mixture contains carbon powder.
12. A process according to any one preceding claim wherein the
iron-copper material has a composition in weight % of Fe-20 Cu.
13. A process according to any one preceding claim wherein the
powder mixture also includes elemental copper powder.
14. A process according to any one preceding claim wherein the
powder mixture includes an elemental powder which promotes the
formation of martensite.
15. A process according to claim 14 wherein the element is selected
from the group comprising: chromium, molybdenum and nickel.
16. A process according to any one preceding claim further
including the step of cryogenically treating the sintered
material.
17. A process according to any one preceding claim further
including the step of tempering the sintered material.
18. A process according to any one preceding claim further
including the step of providing molybdenum disulphide or tungsten
disulphide in the powder mixture.
19. A sintered ferrous-based material article when made by the
process of any one of preceding claims 1 to 18.
20. A sintered article according to claim 19, the article
comprising a valve seat insert for an internal combustion engine.
Description
[0001] The present invention relates to sintered ferrous materials,
articles made therefrom and to a method for their manufacture
particularly, ferrous materials containing copper.
[0002] The powder metallurgy route enables the design of metallic
materials which it is not possible to make by conventional casting
and ingot working processes. It is known to infiltrate sintered
ferrous powder metallurgical products with metals having lower
melting points such as lead and copper, for example. Lead is used
to improve machinability of sintered ferrous materials whilst
copper also has this effect but also has other desirable properties
which it confers on the sintered material. Lead is nowadays avoided
due to its harmful environmental properties. Copper improves
machinability and also improves the thermal conductivity of the
sintered article.
[0003] Copper infiltrated products are used extensively in the
automotive industry for applications such as valve seat inserts in
the cylinder heads of internal combustion engines, for example.
Such products have to perform under very arduous conditions
including repeated impact loading, marginal lubrication, elevated
service temperatures and hot corrosive gases. Properties to
withstand these conditions are achieved by the suitable design of
the ferrous matrix system. Such ferrous matrices are often highly
alloyed which adversely affects machinability. Machinability is
important to an engine builder in a production context as it
affects productivity. Copper infiltration provides improved
machinability whilst the copper itself provides improved thermal
conductivity which has the effect of lowering operating service
temperatures which helps to retain mechanical properties.
[0004] The infiltration process is effected by stacking a copper
alloy compact in contact with the ferrous component and passing the
stacked assembly of the two items through a sintering furnace at a
sintering temperature in the region of about 1100.degree. C. under
an inert or reducing gaseous atmosphere thus effecting sintering
and infiltration simultaneously. During this sintering process the
copper alloy compact melts and the molten alloy infiltrates and
fills the pores of the ferrous component by capillary action. Only
interconnected pores can be filled in this manner, isolated or
otherwise unconnected porosity cannot be so filled. The composition
of the copper alloy compact is so chosen that it is compatible with
the ferrous material and undesirable reactions or erosion thereof
is avoided as far as possible. The weight of the copper alloy
compact is chosen so as to be able to fill the majority of the
pores, however, as noted above there is inevitably some residual
porosity.
[0005] In a variation of the above process, the copper alloy
compact is stacked with a pre-sintered ferrous component and the
two passed through a sintering furnace to effect infiltration.
[0006] The infiltration process is an expensive process owing to
the extra process steps involved. The process requires the
additional steps of: making a separate copper alloy powder mixture;
pressing suitable compacts of the correct weight from the powder
mixture; stacking the compacts with the ferrous articles themselves
prior to passing through the sintering furnace; and, barrelling the
sintered and infiltrated articles after cooling to remove the
powdery deposit which inevitably forms on the articles during the
sintering process.
[0007] In conventional copper infiltrated ferrous products, the
level of copper content generally lies in the range from. 15 to 25
weight %. In non-infiltrated products it is common to add up to
about 5 weight % of copper powder in the pre-compacted powder
mixture. Such relatively small additions of copper to
non-infiltrated ferrous materials assist the sintering process due
to the liquid copper phase being present.
[0008] People have tried to add levels of copper achieved in the
infiltration process by means of additions of the appropriate
amount of elemental copper in the initial powder mixtures prior to
compaction and sintering. However, due to differences in, for
example, powder particle size, powder density and powder particle
morphology, segregation of the copper tends to occur during
handling of the powder mixtures. Such powder segregation causes
unacceptable variations in the resulting products. Where only small
amounts of elemental copper powder is present such as the case of
up to about 5 weight % noted above, segregation still occurs but
the effect in the resulting products is minimised and does not
cause a serious problem.
[0009] At one time components such as valve seat inserts for
engines having the most arduous service environment were made
entirely from highly alloyed steels such as M3/2 class steels for
example. Such steels contain relatively high quantities of
chromium, tungsten, molybdenum, vanadium and the like. Whilst
components made from such materials have excellent performance and
long service lives, they are inherently expensive to make and
process. They are expensive to make firstly because of the high
intrinsic material cost and secondly expensive to process because
of the difficulty in machining components having high contents of
hard carbide in the microstructure thereof. In the never ending
quest to lower costs, much work has been carried out to reduce
material cost by adding relatively high proportions of
substantially pure iron powder to the powder mixes and consequently
reducing processing costs by making the resulting sintered
materials easier to machine by reducing the amount of hard phases
and adding phases which assist machinability such as copper or
chip-breaking phases.
[0010] A disadvantage in terms of performance and longevity of life
of these newer materials such as may be exemplified in GB-A-2 188
062 for example is the retention in the cores of the iron grains,
formed by the sintering together of the original compacted iron
powder particles in the powder mixture, of soft ferrite phase which
can deleteriously affect the wear and strength properties thereof.
Such materials may initially comprise mixtures of about 50% of the
highly alloyed M3/2 material, for example, and about 50% of pure
iron powder and minor additions of carbon, die lubricating waxes
and the like. Even when fully sintered the iron grains have ferrite
cores with only some diffusion of chromium, from the M3/2 regions,
into the surface regions of the iron grains, where martensite may
be formed, after sintering. This structure still applies even when
the material is infiltrated or when up to about 5 weight % of
elemental copper has been added to the powder mixture.
[0011] It is an object of the present invention to provide a
process for making ferrous material articles having a high copper
content commensurate with that of infiltrated material but without
the disadvantage of the additional process steps required in the
prior art processes.
[0012] Other advantages will become apparent from the description
of the invention below.
[0013] According to a first aspect of the present invention, there
is provided a process for the manufacture of a ferrous-based
sintered article containing copper in the range from 12 to 26
weight %, the process including the steps of: making a powder
mixture having a desired composition, at least a proportion of a
total content of iron and copper being provided by an iron powder
having copper indivisibly associated therewith; compacting said
powder mixture to form a green compact of an article to be produced
and sintering said green compact.
[0014] The copper content is primarily intended to enhance the
thermal conductivity of articles produced, however, other important
benefits are also provided to articles made by the method of the
present invention. Below 12 weight % copper the required
enhancement in thermal conductivity is not achieved whilst above 26
weight % "bleeding" of molten copper from the material during
sintering is a problem.
[0015] Preferably, the copper content may lie in the range from 15
to 20 weight %.
[0016] In the process according to the present invention, the iron
powder indivisibly associated with copper is effectively a
pre-alloyed powder in that the individual powder particles comprise
both iron and copper and consequently significant segregation
between the iron and copper is not possible. The iron and copper
powder particles may be selected from two basic types of powder
stock: a pre-alloyed iron-copper powder; or, a diffusion bonded
iron-copper powder. The pre-alloyed iron-copper powder may be
produced by known techniques of melting the constituent materials
together and then atomising the molten melt by water or gas, for
example, to produce the required pre-alloyed powder. The diffusion
bonded iron-copper material is produced by making a mixture of
elemental iron and copper powders, for example, and passing the
mixture, uncompacted, through a furnace such that diffusion between
the particles occurs so as to bond them together. The "cake" so
formed is given a light crushing operation to break it up into
particles comprising both iron and copper adhered to each other.
Such a process causes diffusion of some copper into the outer
regions of each iron particle.
[0017] The method of the present invention obviates several of the
process steps of prior art processes in that a separate copper
alloy powder mixture and consequent compacts do not need to be
made, they do not need to be stacked with the ferrous material
compacts and the final sintered articles do not need to be treated
to remove the adherent deposit thereon as with prior art
infiltration processes.
[0018] A particular advantage conferred by the method of the
present invention relates to the processing of those ferrous
materials which comprise mixtures of an alloyed steel powder and a
low-alloy iron or substantially pure iron powder. It is known to
use such mixtures with additions of carbon powder, for example, and
to process them by compaction, sintering and post-sintering thermal
treatment into articles such as valve seat inserts for internal
combustion engines, for example. Such prior art materials may or
may not be infiltrated with a copper alloy by one of the
conventional processes described above. Such materials are
exemplified by those materials and production processes described
in GB-A-2 188 062 and EP-A-0 312 161, for example. These materials
may comprise a proportion, e.g. about 50 weight % of a highly
alloyed steel powder with about 50 weight % of a substantially pure
iron powder. The alloyed steel powder usually contains chromium
which under the prevailing sintering conditions of about
1100.degree. C. is one of the most mobile element atoms after
carbon, in terms of rate of diffusion, of those alloying elements
which promote the formation of martensite on cooling of the article
following sintering. Carbon atoms are the most mobile, moving into
the interstices of the iron atoms in the crystal structure.
However, since chromium is of a similar atomic size and weight to
iron it substitutes for iron and consequently has a similar
mobility to iron under the prevailing sintering conditions. The
presence of chromium promotes the formation of martensite in those
regions of the sintered material into which it diffuses, the
martensite being formed on cooling of the material at the end of
the sintering cycle. Sintering is frequently effected for such
articles in furnaces which have continuous moving means, such as a
belt or a walking-beam type mechanism, for transporting the
articles, generally supported on trays for example, through the
furnace. Generally, a first portion of the furnace raises the
temperature of the articles to the sintering temperature; a second
portion maintains the articles at the sintering temperature; and, a
third portion allows the articles to cool from the sintering
temperature to a temperature which will preclude significant
oxidation of the articles on exit from the sintering furnace. The
articles are generally sintered under a continuous protective gas
atmosphere flowing through the furnace which serves to provide
either a neutral or reducing atmosphere and preclude air (oxygen)
from entering the furnace. The atmosphere is at substantially
atmospheric pressure with only a slight positive pressure within
the furnace to prevent air from entering therein. Where the
sintered material contains a significant quantity of iron powder in
the original mix it is frequently found that the iron grains
resulting from the sintering of the compacted iron powder particles
possess a microstructure ranging from ferrite to pearlite and
mixtures of the two phases, depending upon the carbon content, in
the core of the iron-rich non-tool steel regions. The outer region
of the iron grains generally comprises martensite resulting from
chromium which has diffused in during the sintering operation but
the core remains essentially as ferrite or pearlite or a mixture of
ferrite and pearlite depending upon the added carbon level. In the
as-sintered condition, the iron-rich non-tool steel phase or grain
structure consists of mainly pearlite, though there may be some
ferrite, at the centre and the outer regions of the grains are a
mixture of martensite/bainite. If there is any retained austenite
in the sintered article it is generally transformed by cryogenic
treatment after sintering. During a tempering operation usually
carried out after cryogenic treatment, partial decomposition of the
pearlite phase occurs leading to the formation of ferrite areas
within the iron-rich grains or phase. This can result in the
material having inferior wear properties due to the presence of
ferrite and also lower strength due to the ferrite. The
post-sintering thermal treatments comprising cryogenic treatment to
transform any remaining .gamma.-phase (austenite) to martensite
followed by tempering treatments are to reduce the degree of
hardness and brittleness of the martensite phase rather than to
effect decomposition of the pearlite which is an undesirable side
effect of the tempering process. Since the tempering treatment is
carried out at a temperature in excess of the expected service
temperature, size stability of the article in its service
environment (e.g. a valve seat insert in the combustion chamber of
an internal combustion engine) is ensured. However, such treatments
do not affect the presence (other than to be responsible for
generating at least a proportion of the ferrite) of the ferrite
phase or its inherently poor wear and mechanical properties.
[0019] It has been found that with the method of the present
invention that there appears to be a synergistic effect of the
copper (either from the diffusion-bonded form or in the pre-alloyed
form with the iron) and chromium together in promoting the
diffusion of copper and chromium towards the centre of the iron
grains and, instead of the core of the iron grains remaining as
ferrite or pearlite or a mixture of these, the core of the iron
grains is found to transform to martensite during normal furnace
cooling. Sintered ferrous materials made according to the process
of the present invention using either pre-alloyed iron-copper or
diffusion bonded iron-copper powders reveal the presence of
martensite in the cores of the iron-rich grains due to the
diffusion of chromium or other martensite promoting elements into
the iron grains. The martensite is formed during the cooling of
austenite and any retained austenite is transformed by cryogenic
treatment following sintering. During the cooling process from the
sintering temperature some of the austenite can also transform to
bainite. The martensite may then be tempered to form a structure of
tempered martensite which is readily machinable. However, it is
important to note that the previously soft ferritic/pearlitic cores
of the iron grains now comprise material which is harder, stronger
and more wear resistant due to the process of the present
invention. It is believed that the processing used to form the
pre-alloyed and diffusion bonded iron-copper material causes at
least some diffusion of the copper phase into the iron constituent
and the presence of the copper assists in the diffusion of chromium
and other martensite promoting elements into the cores of the iron
grains formed on sintering thus, enabling martensite to be
formed.
[0020] Tests making materials according to the method of the
present invention and making substantially identical materials by
prior art infiltration processes, but using substantially identical
processing parameters of pressing pressure and sintering
temperature for example, have shown the beneficial effects of using
an iron-copper pre-alloy or diffusion bonded powder as described
hereinabove. Materials of largely identical composition except for
the copper content were made by 1) the method of the present
invention; 2) by the route of simultaneous sintering and
infiltration; and, 3) by adding 13 weight % elemental copper powder
to the initial powder mixture and sintering (i.e. without
infiltration and without the addition of pre-alloyed iron-copper
powder).
[0021] Materials made by conventional infiltration techniques under
the same processing conditions do not show the beneficial effect of
martensite formation in the iron grain core. Analysis by scanning
electron microscope has shown the presence of chromium in the
particle core in materials made by the method of the present
invention. It is to be emphasised that the processing conditions
used in the comparative tests are the same processing conditions
used for production of commercial prior art materials and thus
represent the current optimum processing conditions taking all
factors into account.
[0022] Materials made according to the method of the present
invention may also receive post-sintering thermal treatments such
as cryogenic treatment at -120.degree. C. or below to convert any
residual austenite phase to martensite, followed by tempering to
make the martensite softer, more dimensionally stable and make it
amenable to machining.
[0023] Thus, according to a feature of one embodiment of the
present invention, the powder mixture contains a powder component
comprising a relatively un-alloyed iron powder and a powder
component comprising a steel powder containing at least some
chromium or other martensite promoting element as an alloying
element in addition to the pre-alloyed or diffusion bonded
iron-copper powder. Alternatively or additionally the powder
mixture may contain addition(s) of elemental martensite promoting
material such as molybdenum and/or nickel for example.
[0024] Examples utilising M3/2 high speed steel powders are
described herein, however, any other suitable tool steel or high
speed steel, for example, chromium-containing steel powder may be
employed depending upon the application in which the article
produced therefrom is to be used.
[0025] An example of an alternative steel material is so-called 316
steel which is a stainless steel comprising in weight %: 17 Cr/ 2
Mo/ 13 Ni/ Bal Fe and which is substantially carbon free.
[0026] Thus, it appears that the manner in which copper is
introduced into the sintered ferrous material, i.e. by being
associated with the iron where there has been prior treatment
causing reaction therebetween, has an unexpected and synergistic
effect in aiding diffusion of chromium or other martensite
promoting elements through the iron matrix to assist in the
transformation to martensite on cooling after sintering or by
transformation of retained austenite by cryogenic treatment.
[0027] The composition of the iron-copper pre-alloyed or
diffusion-bonded material may be any desired, e.g. iron-20 copper.
Powder mixtures may be made up having powder components comprising:
iron; iron-copper; pre-alloyed steel powder; and, carbon powder,
for example. The amount of iron-copper pre-alloy powder will depend
upon the final required copper content in the article and on the
initial composition of the iron-copper pre-alloy powder.
[0028] The use of iron-copper pre-alloyed and/or diffusion bonded
material in a powder mixture together with an addition on elemental
copper powder is not precluded and in some circumstances may be
beneficial. The use of both pre-alloyed and diffusion bonded
iron-copper powder may also be employed in a powder mixture.
[0029] The pre-alloyed iron-copper material appears to be somewhat
more effective in promoting the formation of martensite in iron
grains than does diffusion bonded iron-copper material. Therefore,
the use of the pre-alloyed material is preferred, however, it is
pointed out that the diffusion bonded material produces martensite
after sintering and subsequent processing whereas prior art
infiltrated materials do not produce any martensite in the iron
grain cores, the cores comprising only mixtures of pearlite and
ferrite.
[0030] According to a second aspect of the present invention, there
is provided a sintered article produced by the first aspect of the
present invention.
[0031] In order that the present invention-may be more fully
understood, examples will now be described by way of illustration
only with reference to the accompanying drawings, of which:
[0032] FIG. 1 shows a histogram showing wear of valve seat inserts
in an engine test on material made according to the present
invention; and
[0033] FIG. 2 which shows a graph of tool wear vs number of parts
machined for materials made according to the present invention and
prior art material.
VALVE SEAT INSERT MATERIAL--EXAMPLE 1
[0034] Ferrous powder mixtures of a typical composition used in the
production of valve seat inserts for internal combustion engines
were prepared by various routes. The compositions of the powder
mixtures in terms of the actual constituent component powders used
to make them were as set out below in Table 1:
1TABLE 1 Constit- uent Graph- Elemental Fe-Cu Lub Fe wt % M 3/2 ite
MoS.sub.2 Cu powder Wax Powder Exam- 45 0.55 1 6 47.47 0.75 -- ple
1 Exam- 42.9 0.42 0.87 13 -- 0.75 42.9 ple 1a Exam- 49.75 0.5 --
Infil- -- 0.75 49.75 ple 1b trated
[0035] Example 1 was a material prepared by the method of the
present invention where all of the iron and a proportion of the
copper were added as pre-alloyed iron-20 copper powder. The
pre-alloy powder contributes about 9.5 weight % of copper to the
final material. A further 6 weight % of elemental copper powder was
added to the initial powder mixture to bring the total copper up to
15 weight %. The steel pre-alloy powder was a water atomised M3/2
powder having a nominal composition of: 1 C; 4 Cr; 5 Mo; 3 V; 5 W.
Since only 6 weight % of elemental copper powder was added,
segregation was minimised.
[0036] Example 1a is powder mixture wherein all of the iron powder
content is provided as pure iron powder and the copper content as
13 weight % of elemental copper powder. Whilst such material would
not normally be made with such a high content of elemental copper
powder for the reasons discussed hereinbefore, the material was
made to determine the effect of the copper content on the diffusion
characteristics of the chromium into the iron constituent.
[0037] Example 1b was made by the prior art process according to
GB-A-2 188 062 wherein the copper is supplied via a simultaneous
sintering and infiltration step.
[0038] All of the powders were blended according to established
principles in a Y-cone mixer. Compaction pressure was in the range
650-800 MPa in each case followed by sintering at around
1100.degree. C. in a conveyor furnace, all Examples being sintered
under the same conditions. Following sintering all Examples were
cryogenicaylly treated at -120.degree. C. to transform any
remaining austenite (.gamma.-phase) in the structure and then
tempered at 600.degree. C. for 2 hours to soften the martensite,
make it more dimensionally stable and enhance machinability
qualities.
[0039] Table 2 below gives the actual compositions in terms of the
constituent elements, the density of the sintered material and its
final hardness following cryogenic and tempering post-sintering
treatment.
2TABLE 2 Constit- uent Density Hardness wt % C Cr Cu Mo S V W Fe
Mgm.sup.-3 HRA Example 1 1.8 15.5 2.9 0.4 1.4 2.3 Bal 7.2 64-67 1
Example 0.9 1.7 13 2.7 0.3 1.3 2.1 Bal 7.0 59-64 1a Example 0.9 2.0
15 2.5 1.5 2.5 Bal 7.95 67-71 1b
[0040] The microstructure of samples made according to Example 1
showed a tempered martensite structure even in the cores of the
iron grains. The martensite was formed on cooling from the
sintering temperature. Cryogenic treatment was used to transform
any retained austenite in the M3/2 phase of the material to
martensite. The change from austenite to martensite is not easily
seen under the microscope, the change being evidenced by increased
hardness on the change from austenite to martensite.
[0041] Samples from Example 1a showed a microstructure comprising
some martensite formed on cooling from the sintering temperature
and retained austenite. Following cryogenic treatment, the retained
austenite transformed to martensite in the M3/2 regions and the
iron grains comprised mainly pearlite (a phase comprising a
lamellar structure of ferrite and cementite) and some ferrite. The
pearlite was formed by virtue of the carbon powder added as
graphite, however, owing to the absence of chromium in the iron
grain cores, no martensite was formed. On tempering, extensive
decomposition of pearlite took place and the volume fraction of
ferrite increased compared with that of the as-sintered state.
Thus, the wear resistance of Example 1a material is inferior and
the mechanical properties, as evidenced by the hardness figures,
are also inferior.
[0042] Samples from Example 1b demonstrated almost identical
structure and properties as did Example 1a. This material was made
according to the known process of GB-A-2 188 062. The hardness of
Example 1b was slightly higher than Example 1, this being
attributed to the higher density of the material following
infiltration. However, the material of Example 1b showed extensive
quantities of inherently weaker ferrite areas after tempering and
not the desirable tempered martensite structure shown by Example 1
made according to the process of the present invention.
[0043] FIG. 1 shows a histogram of valve seat insert wear of valve
seat inserts, made from the material of Example 1, in the exhaust
positions of a 1.81, 4-cylinder, 16-valve engine which was run for
180 hours at 6000 rev/min on unleaded gasoline, the engine having
Stellite (trade name) faced valves. The success criteria for this
test is that valve seat insert wear must not exceed 100 .mu.m. As
may be seen from FIG. 1 the maximum wear was at valve seat position
4 at 60 .mu.m, all other inserts having wear of about 30 .mu.m or
less.
[0044] Thus, it is clear from Examples 1, 1a and 1b that the only
substantive difference in the manufacture thereof was the manner in
which the copper was introduced into the sintered material. It is
believed that the improved structure and properties are directly
attributable to the use of the iron-copper pre-alloyed materials
wherein at least a proportion of the copper is indivisibly
associated with the iron and stem from the enhanced diffusion
promoted by this pre-alloyed material.
EXAMPLE 2
[0045] A powder mixture comprising 45 wt % M3/2 tool steel powder/
0.55C/ 1 MoS.sub.2 / 6 Cu/ 47.45 FeCu20 (diffusion bonded powder)/
0.75 lubricating wax was made. This mixture was compacted into
green compacts at 770 MPa to a green density 7.1 Mgm.sup.-3 and
sintered at about 1100.degree. C. under a continuous flowing
nitrogen-hydrogen gas atmosphere in a conveyor furnace. The
sintered articles were cryogenically treated at -120.degree. C. or
below to convert retained austenite to martensite and finally
tempered at 600.degree. C. Density of the sintered material was 7.0
Mgm.sup.-3. The hardness of the as sintered material was 61HRA;
that of the cryogenically treated material 65HRA; and that of the
cryogenically treated and tempered material 62-65 HRA.
[0046] The microstructure of the Example 2 material (made with
diffusion-bonded iron-copper powder) after tempering, (following
sintering and cryogenic treatment) showed some small occasional
areas of ferrite in the iron-rich non-tool steel phase. However,
this iron-rich phase comprised essentially pearlite rather than the
extensive regions of ferrite typified by the prior art material
made using the infiltration technique.
EXAMPLE 3
[0047] A mixture comprising in weight %: 75% pre-alloyed Fe-Cu20
powder/ 23% 316 stainless steel powder/ 0.75% MoS.sub.2 powder/ 1%
carbon powder was prepared; this material being coded N1. The
composition of the 316 stainless steel was 17 Cr/ 2 Mo/ 13 Ni/ bal
Fe. A comparative example coded N was made from the following
mixture in weight %: 70.9% unalloyed iron powder/ 27% 316 stainless
steel powder/ 0.9% MoS.sub.2 powder/ 1.2% carbon powder. Both
materials were compacted at 770 MPa. However, material N1 was
sintered only (as there was about 15 wt % Cu provided by the Fe--Cu
pre-alloy) and material N was simultaneously sintered and
infiltrated according to the known prior art process. The final
theoretical overall composition of both materials N1 and N in
weight % was: 1 C/3.9 Cr/15 Cu/0.9 Mo/3 Ni/S 0.3/bal Fe. The
sintering/infiltration steps were carried out at about 1100.degree.
C. under a flowing nitrogen/hydrogen atmosphere. Both materials
following sintering were cryogenically treated and tempered.
[0048] The N1 material showed a microstructure having no ferrite,
even in the cores of the grains which were predominantly iron. The
structure of this material showed essentially a tempered martensite
structure. The N material on the other hand showed extensive
ferrite in the iron grains with a pearlitic structure in the
transition zones between prior iron particles and 316 stainless
steel particles even though this material had slightly higher
carbon at 1.2%. Thus, again the influence of the copper being
indivisibly associated with the iron is shown in the resulting
structure after processing.
EXAMPLE 4
[0049] Further mixtures denoted as material FMCA and FMCD were made
according to the present invention. The blend compositions of these
materials in terms of the constituents in the powder mixtures are
given below in Table 3.
3 TABLE 3 FMCA FMCD Fe-20 Cu (pre-alloyed) 75 75 C 1.35 1.35 Mo 0.5
MoS.sub.2 1 Unalloyed Fe 23.15 22.65 Lubricating wax 0.75 0.75
[0050] The materials were compacted at 770 MPa and sintered at
about 1100.degree. C. under a continuous gaseous atmosphere as with
previous examples. The resulting densities and hardnesses of the
sintered materials are given below in Table 4. For these samples no
post-sintering heat treatment was carried out.
4 TABLE 4 FMCA FMCD Green density, Mgm.sup.-3 7.05 7.05 Sintered
density, Mgm.sup.-3 7.35-7.40 7.15-7.20 Hardness, HRB 99-101
95-98
[0051] In the FMCA material made according to the present invention
pre-alloyed Fe--Cu powder and 0.5% elemental Mo powder were used in
the initial powder mixture. The FMCA material showed extensive
Mo-rich zones and martensitic and bainitic areas associated with
these zones. The FMCA material also showed grain boundary carbides.
The microstructure of the FMCA material was somewhat similar to a
comparative material, coded FMC (unalloyed iron powder, 1.35% C,
0.5% Mo), wherein the copper content was provided by a simultaneous
sintering and infiltration process according to the prior art.
Apart from the infiltration step, the sintering conditions were the
same as those for the FMCA and FMCD materials. In the FMC material
grain boundary carbide was present, the matrix was pearlite and the
Mo-rich zones associated with the Mo particles were present but
very small compared with the FMCA material.
[0052] During sintering, the MoS.sub.2 in the FMCD material
undergoes partial decomposition and donates free Mo to the
structure which potentially is able to generate a localised
martensitic/bainitic structure associated with the Mo-rich zones.
Some of the sulphur from decomposed MoS.sub.2 reacts with iron and
copper to form metallic sulphides which are beneficial for
improving machinability. In the FMCD material no carbide networks
could be seen and the matrix was pearlitic.
[0053] FIG. 2 shows a graph of tool wear vs number of parts
machined for FMC, FMCA and FMCD materials. The Figure confirms that
the materials using pre-alloyed Fe-Cu powders which give rise to
extensive martensitic/bainitic areas do not have their
machinability impaired in spite of the stronger, more wear
resistant material structures so formed. Indeed, the machinability
of the both the FMCA and FMCD materials is superior to the FMC
material made by a prior art process.
* * * * *