Note: Descriptions are shown in the official language in which they were submitted.
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METALLURGICAL POWDER COMPOSITION AND METHOD OF
PRODUCTION
Field of the Invention
The present invention relates to an iron-based powder.
Especially the invention concerns a powder suitable for the production of
wear-resistant products such as valve seat inserts (VSI) as well as a
component made from the powder.
Background Art
Products having high wear-resistance are extensively used and
there is a constant need for less expensive products having the same or
better performance as/than existing products. Only valve seats inserts are
produced in an amount of more than 1 000 000 000 components annually.
The manufacture of products having high wear-resistance may
be based on e.g. powders, such as iron or iron-based powders, including
carbon in the form of carbides.
Carbides are very hard and have high melting points,
characteristics which give them a high wear resistance in many applications.
This wear resistance often makes carbides desirable as components in
steels, e.g. high speed steels (HSS), that require a high wear resistance,
such as steels for drills, lathes, valve seat inserts and the likes.
A VSI in a combustion engine is a ring that is inserted where the valve
comes in contact with the cylinder head during operation. The VSI is used to
limit the wear, caused by the valve, on the cylinder head. This is done by
using a material in the VSI that can resist wear better than the cylinder head
material, without wearing on the valve. The materials used for VSI are cast
materials or more commonly pressed and sintered PM materials.
Producing a valve seat insert with powder metallurgy offers a wide
flexibility in composition of the VSI and a very cost effective product. The
method of fabricating a PM valve seat insert starts with preparation of a mix
which includes all ingredients needed in the final component. The powder
mix most commonly includes an iron or low alloyed powder serving as matrix
in the final component, elemental alloying elements such as C, Cu, Ni, Co etc
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which should to a lower or higher extent diffuse into the matrix material and
enhance strength and hardness. Further hard phase materials containing
carbides and similar phases can be added to increase the wear resistance of
the alloy. It is also common to have machinability enhancers added to
decrease tool wear when machining the finished product, as well as solid
lubricants in order to assist the lubrication during service in the engine.
Further, in all press ready mixes evaporative lubricants are added to assist
compaction and ejection of the compacted component. A known VSI
material, produced by Powder Metallurgy, is based on high speed steel
powder as carbide containing matrix material. All powders used normally
have a particle size of less than 180 pm. The average particle size of the mix
is usually between 50 to 100 pm to allow the mix to flow and facilitate
production. The alloying and lubricant additives are in many cases finer in
particle size compared to the matrix powder to improve distribution of
alloying
elements in the powder mix and finished component.
The powder mix is then fed into a tool cavity with the shape of a VSI
ring. An axial pressure between 400-900 MPa is applied resulting in a near
net shape metallic VSI component having a density between 6.4-7.3 g/cm3.
In some instances dual compaction is used to decrease the use of expensive
alloying elements. In dual compaction two different powder mixes are used.
One more expensive with excellent wear properties creating the wear surface
of VSI facing the valve and one less costly to give the desired height of the
component. After the compaction the individual grains are only loosely
bonded through cold welding, and a subsequent sintering operation is
required to allow the individual particles to diffuse together and to
distribute
alloying elements. Sintering is usually performed at temperatures between
1120 C and 1150 C but temperatures up to 1300 C can be used, in a
reducing atmosphere usually based on Nitrogen and Hydrogen. During
sintering or after, copper can be infiltrated in the pores of the component to
increase hardness and strength as well as improve heat conductivity and
wear properties. In many cases subsequent heat treatments are performed
to reach final properties. In order to achive desired geometrical accuracy of
the VSI it is machined to desired size. The final machining is in many cases
done after VSI is mounted in the cylinder head. The final machining is done
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in order to give the VSI and inverted valve profile and to have small
dimensional variations.
Examples of conventional iron-based powders with high wear
resistance are disclosed in e.g. the US patent 6 679 932, relating to a powder
mixture including a tool steel powder with finely dispersed carbides, and the
US patent 5 856 625 relating to a stainless steel powder.
W, V, Mo, Ti and Nb are strong carbide forming elements which
make these elements especially interesting for the production of wear
resistant products. Cr is another carbide forming element. Most of these
conventional carbide forming metals are, however, expensive and result in an
inconveniently high priced product. Thus, there is a need within the powder
metallurgical industry for a less expensive iron-based powder, or high speed
steel, which is sufficiently wear resistant for applications such as for valve
seats or the like.
As chromium is a much cheaper and more readily available
carbide forming metal than other such metals used in conventional powders
and hard phases with high wear resistance, it would be desirable to be able
to use chromium as principal carbide forming metal. In that way the powder,
and thus the compacted product, can be more inexpensively produced.
The carbides of regular high speed steels are usually quite small,
but in accordance with the present invention it has now unexpectedly been
shown that powders having equally advantageous wear resistance, for e.g.
valve seat applications, may be obtained with chromium as the principal
carbide forming metal, provided that a sufficient amount of large carbides
exists, supported by a minor amount of finer and harder carbides.
Summary of the Invention
An objective of the present invention is thus to provide an
inexpensive iron-based powder for the manufacture of powder metallurgical
products having a high wear resistance.
This objective, as well as other objectives evident from the
discussion below, are according to the present invention achieved through an
annealed pre-alloyed water atomised iron-based powder, comprising from 10
to below 18 % by weight of Cr, 0.5-5% by weight of each of at least one of
Mo, W, V and Nb, 0.5-2%, preferably 0.7-2% and most preferably 1-2% by
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weight of C, wherein the iron-based powder has a matrix comprising less
than 10% by weight of Cr. Further, the iron-based powder comprises large
chromium carbides and finer and harder chromium carbides.
As high Cr amounts in the powder promote formation of large type
carbides e.g. of the type M23C6 -, then 18% by weight and above of Cr will
give a too low content of fine and hard chromium carbides.
In accordance with the present invention this new powder which
achieves the above objectives may be obtained through a method of
producing an iron-based powder comprising subjecting an iron-based melt
including 10- below 18% by weight of Cr, 0.5-5% by weight of each of at least
one of Mo, W, V and Nb and 0.5-2%, preferably 0.7-2% and most preferably
1-2% by weight of C to water atomisation in order to obtain iron-based
powder particles, and annealing the powder particles at a temperature, and
for a period of time, sufficient for obtaining the desired carbides within the
particles.
In preferred embodiments, it has been found that temperatures
in the range of 900-1100 C and annealing times in the range of 15-72 hours
are sufficient for obtaining the desired carbides within the particles.
Brief description of the drawings
Fig. 1 shows the microstructure of OB1 based test material.
Fig. 2 shows the microstructure of M3/2 based test material.
Detailed Description of Preferred Embodiments
The pre-alloyed powder of the invention contains chromium, 10-
below 18% by weight, at least one of molybdenum, tungsten, vanadium and
niobium, 0.5-5% by weight of each, and carbon, 0.5-2%, preferably 0.7-2%
and most preferably 1-2% by weight, the balance being iron, optional other
alloying elements and inevitable impurities.
The pre-alloyed powder may optionally include other alloying
elements, such as silicon, up to 2% by weight. Other alloying elements or
additives may also optionally be included.
It should specifically be noted that the very expensive carbide
forming metals niobium and titanium are not needed in the powder of the
present invention.
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The pre-alloyed powder preferably has an average particle size
in the range of 40-100 pm, preferably of about 80 pm.
In preferred embodiments the pre-alloyed powder comprises 12-
17% by weight of Cr, such as 15-17% by weight of Cr, e.g. 16% by weight of
5 Cr.
In preferred embodiments the pre-alloyed powder comprises 12-
below 18% by weight of Cr, 1-3 wt% of Mo, 1-3,5 wt% of W, 0.5-1.5 wt% of
V, 0.2-1 wt% of Si, 1-2 wt% of C and balance Fe.
In most preferred embodiments the pre-alloyed powder comprises 14-
below 18 weight of Cr, 1-2 wt% of Mo, 1-2 wt% of W, 0.5-1.5 wt% of V, 0.2-1
wt% of Si, 1-2 wt% of C and balance Fe.
In another most preferred embodiment the pre-alloyed powder
comprises 12-below 15 weight of Cr, 1-2 wt% of Mo, 2-3 wt% of W, 0.5-1.5
wt% of V, 0.2-1 wt% of Si, 1-2 wt% of C and balance Fe.
In preferred embodiments, the large chromium carbides are of
M23C6_type, (M = Cr, Fe, Mo, W,), i.e. besides Cr as the dominating carbide
forming element one or more of Fe, Mo and W may be present.
In preferred embodiments, the finer and harder chromium
carbides are of M7C3- type (M = Cr, Fe, V), i.e. besides chromium as the
dominating carbide forming element one or more of Fe and V may be
present. Both types of carbides may also contain other than the above
specified carbide forming elements in small amounts. The powder may
further comprise other than the above carbide types.
The large carbides of the inventive powder preferably have an average
size in the range of 8-45 pm, more preferably in the range of 8-30 pm, a
hardness of about 1100-1300 microvickers and preferably make up 10-30%
by volume of the total powder.
The M,C3 _ type smaller carbides of the inventive powder are smaller
and harder than the M23C6 _ type large carbides. The smaller carbides of the
inventive powder preferably have an average size below 8 pm, a hardness of
about 1400-1600 microvickers and preferably make up 3-10% by volume of
the total powder.
As the carbides have an irregular shape, "size" defines the
longest extension as measured in a microscope.
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In order to obtain these large carbides, the pre-alloyed powder is
subjected to prolonged annealing, preferably under vacuum. The annealing is
preferably performed in the range of 900-1100 C, most preferably at about
1000 C, at which temperature chromium of the pre-alloyed powder reacts
with carbon to form chromium carbides.
During the annealing, new carbides are formed and grow and
existing carbides continue to grow through reaction between chromium and
carbon. The annealing is preferably continued for 15-72 hours, more
preferably for more than 48 hours, in order to obtain carbides of desired
size.
The longer the duration of the annealing, the larger the carbide grains grow.
However, the annealing consumes lots of energy and might be a production
flow bottle neck if it continues for a long time. Thus, although an average
chromium carbide grain size of the large chromium carbides of about 20-30
pm may be optimal, it might, depending on priority, be more convenient from
an economic point of view to terminate the annealing earlier, when the
average chromium carbide grain size of the large chromium carbides is about
10 pm.
Very slow cooling, preferably more than 12 hours, from
annealing temperature is applied. Slow cooling will allow further growth of
carbides, as a larger amount of carbides is thermodynamically stable at lower
temperatures. Slow cooling will also assure that the matrix becomes ferritic,
which is important for the compressibility of the powder.
Annealing the powder also has other advantages besides the
growth of carbides.
During annealing also the matrix grains grow and the inherent
stresses of the powder particles, obtained as a result of the water
atomisation, are relaxed. These factors make the powder less hard and
easier to compact, e.g. gives the powder higher compressibility.
During annealing, the carbon and oxygen contents of the powder
may be adjusted. It is usually desirable to keep the oxygen content low.
During annealing carbon is reacted with oxygen to form gaseous carbon
oxide, which reduces the oxygen content of the powder. If there is not
enough carbon in the pre-alloyed powder itself, for both forming carbides and
reducing the oxygen content, additional carbon, in form of graphite powder,
may be provided for the annealing.
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As much of the chromium of the pre-alloyed powder migrates
from the matrix to the carbides during annealing, the matrix of the resulting
annealed powder has a content of dissolved chromium of less than 10% by
weight of the matrix, preferably less than 9% by weight and most preferably
less than 8% by weight, why the powder is not stainless.
The matrix composition of the powder is designed such that
ferrite transforms to austenite during sintering. Thereby, the austenite can
transform into martensite upon cooling after sintering. Large carbides in
combination with smaller and harder carbides in a martensitic matrix will give
good wear resistance of the pressed and sintered component.
The annealed powder of the invention may be mixed with other
powder components, such as other iron-based powders, graphite,
evaporative lubricants, solid lubricants, machinability enhancing agents etc,
before compaction and sintering to produce a product with high wear
resistance. One may e.g. mix the inventive powder with pure iron powder and
graphite powder, or with a stainless steel powder. A lubricant, such as a wax,
stearate, metal soap or the like, which facilitates the compaction and then
evaporates during sintering, may be added, as well as a solid lubricant, such
as MnS, CaF2, MoS2, which reduces friction during use of the sintered
product and which also may enhance the machinability of the same. Also
other machinability enhancing agents may be added, as well as other
conventional additives of the powder metallurgical field.
Due to its good compressibility the obtained mix is well suited for
compacting into near net shape VSI components having a chamfered
inverted valve profile.
Example 1
A melt of 16.0 wt% Cr, 1.5 wt% Mo, 1.5 wt% W, 1 wt% V, 0.5
wt% Si, 1.5 wt% C and balance Fe was water atomised to form a pre-alloyed
powder. The obtained powder was subsequently vacuum annealed at
1000 C for about 48 hours, the total annealing time being about 60 hours,
after which the powder particles contained about 20% by volume of M23C6-
type carbides of an average grain size of about 10 pm and about 5% by
volume of M,C3-type carbides of an average grain size of about 3 pm in a
ferritic matrix.
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The obtained powder (hereafter referred to as OB1) was mixed with
0.5 wt% graphite and 0.75 wt% of an evaporative lubricant. The mix was
compacted into test bars at a pressure of 700 MPa. The obtained samples
were sintered in an atmosphere of 90N2/10H2 at a temperature of 1120 C.
After sintering the samples were subjected to cryogenic cooling in liquid
nitrogen followed by tempering at 550 C.
A similar mix based on the known HSS powder M3/2, was prepared
and test bars were produced using the same process as the one described
above.
The test bars were subjected to hardness tests according to the
Vickers method. Hot hardness was tested at three different temperatures
(300/400/500 C). The results are summarised in the table below.
Powder Porosity HVO.025 HV5 Hot hardness (HV5)
in mix (%) 300 C 400 C 500 C
OB1 21 925 382 317 299 249
M3/2 17 836 415 363 326 267
The microstructure of the OB1 test material (see Figure 1) consists of
the desired mixture of large and small carbides in a martensitic matrix. The
reference material has similar microstructure (see Figure 2) but with smaller
carbides than the OB1 material.
The OB1 material has somewhat higher porosity than the M3/2
material, which explains why the OB1 hardness values (HV5) are lower than
those for M3/2 although the OB1 microhardness is higher than that for M3/2.
In the production of PM VSI components, the porosity is normally eliminated
by copper infiltration during sintering and such effects can therefore be
neglected. In the light of this, the hardness values of the OB1 material are
comparable to those of the reference M3/2 material, which gives good
indication that the materials should have comparable wear resistance.
Especially, maintaining hardness at elevated temperatures is important for
wear resistance in VSI applications. The hot hardness test results show that
the OB1 material meets these requirements.
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Example 2
A melt of 14,5 wt% Cr, 1.5 wt% Mo, 2.5 wt% W, 1 wt% V, 0.5 wt% Si,
1.5 wt% C and balance Fe was water atomised to form a pre-alloyed powder.
The obtained powder was subsequently vacuum annealed at 1000 C for
about 48 hours, the total annealing time being about 60 hours, after which
the powder particles contained about 20% by volume of M23C6-type carbides
of an average grain size of about 10 pm and about 5% by volume of M,C3-
type carbides of an average grain size of about 3 pm in a ferritic matrix.
Processing this powder, mixed with 0.5 wt% graphite and 0.75 wt% of
an evaporative lubricant, to produce test bars in the same way as in example
1, resulted in a microstructure very similar to that in Figure 1.
