Note: Descriptions are shown in the official language in which they were submitted.
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1068130
BACKGROUND OF THE INVENTION
The conventional materials used for making valve seats in-
clude cast iron, cast steel, heat-resistance steel, non-ferrous
alloys and sintered alloys. A wide variety of sintered alloys with
different characteristics have been developed. Use of these conven-
tional sintered alloys, however, yields unsatisfactory results in
most cases with lead-free gasoline, though good results are obtained
when the gasoline contains an adequate amount of such anti-knock
additives as tetraethyl lead.
Various organic leads added to the gasoline as anti-
knocking agents turn into lead oxides when the gasoline burns and,
when deposited on the valve and valve seat surface, they serve to
protect and lubricate the valve seat or absorb the energy of valve
impact, thereby preventing wear of the valve seat, but when lead-free
gasoline i8 used, the wear-preventing effect of lead is absent and
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accordingly the valve seat suffers heavy wear. During use of a high-
octane gasoline with much tetraethyl lead, great quantities of the
products of combustion are deposited on the valve seat surface and
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are likely to cause heavy oxidation and lead to corrosion on the
valve seat of conventional materials. At the same time, as the re-
sult of a temperature rise in the exhaust system of an internal com-
bustion engine provided with anti-emission equipment for the preven-
~ tion of air polution, the heat load of the exhaust gas on the valve
`~ seat increases and conventional materials which lack heat-and-wear
~ resistance cannot stand up under severe operating conditions of the
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engine. Thus the valve seat materials have come to be required to
possess higher resistances to wear, oxidation and lead corrosion and
be able to stand up under severe operating conditions.
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068~30
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Furthermore, a valve seat, which has been pressed
into a cast iron cylinder head in a conventional manner, is
liable to drop out when subjected to a heavy heat load. Thus
the valve seat material is required to have a lower coefficient
of thermal expansion.
BRIEF SUMMARY OF THE INVENTION
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The object of the present invention is to provide
an improved iron-base sintered alloy for the valve seats of
internal combustion engines which is characterized by excellent
resistances to oxidation, lead corrosion and wear as well as a
low coefficient of thermal expansion, and can perform satisfac-
torily when using either conventional leaded g~soline or lead-
free gasoline, even when the temperature in the exhaust system
is high.
Another object of the present invention is to provide
an iron-base sintered alloy for valve seats which has its coeffi-
cient of thermal expansion lowered sufficiently to eliminate
any risk of the valve seat dropping out, and wh~ich is accordingly
available for a wide range of applications.
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BRIEF DESCRIPTION OF THE DRAWINGS
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Figure 1 is a sectional view of an apparatus for
testing the dropout durability of a valve seat.
` Figure 2 is a sectional view of a pulling force
measuring device.
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Figure 3 is a diagram showing the relation between
the force required to separate the seat from the valve and
the coefficient of thermal expansion when the seat is made
or various iron-base sintered alloys. ~-;
DETAILED DESCRIPTION OF THE INVENTION
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The effects of different constituents contained in -
the iron-base sintered alloy of the present invention and the
reasons for limiting their contents will now be explained.
The feature of the iron-base sintered alloy according
to the present invention lies in the use of an iron-chromium-
nickel alloy powder as the base. The chromium content of this
base forms a carbide which con-tributes to improvement of the
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wear resistance as well as to the enhancement of the resistance
to oxidation and to lead corrosion. When the chromium content
is less than 6% by weight, it has little effect, but the addi-
tion of more than 20% is not so effective as might be expected,
since it lowers the strength of the alloy. For this reason, the
chromium content is limited to the range of 6 - 20%.
Nickel is useful for increasing the resistance to
oxidation and to lead corrosion. In an iron-chromium-nickel
system alloy an increased addition of nickel will enlarge the
austenite region in the matrix, thereby increasing the coefficient
of thermal expansion. For instance, when nickel is 2 - 20%, it -
would be difficult to hold the coefficient of thermal expansion
down to less than 13.5 x lO 6 in the range of 0 - 600C. Accord-
ingly the u*ility of the valve seat will be restricted when such
15 an alloy is employed. At the same time, a nickel content of less
than 2.0~ will make it easy to increase the hardness and strength
of the alloy. Thus the nickel content is limited to less than 2.0%.
Carbon forms a solid solution or a chromium carbide
in the matrix, thereby increasing the hardness and strength as
20 well as the wear resistance of the alloy. It will not be effec-
tive when the addition is less than 0.2%, but the addition of
more than 1.5% is likely to develop a liquid phase in sintering
and lower the resistance to oxidation. Thus the carbon content
is limited to the range of 0.2 - 1.5%.
Manganese and silicon which form a solid solution in
the matrix are effective elements for enhancing the resistance to
oxidation and increasing the strength of the alloy. There will
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1068130
be no effect when manganese is less than 0.3 % or silicon is less
than 0.5 %, but the all~y will be embrittled if the manganese is more
than 1.5 ~ or the silicon is more than 2.5 %. Thus the manganese
content and the silicon content are limited respectively to 0.3 - -
1.5 % and 0.5 - 2.5 %. Manganese and silicon may be added singly or
in the form of an alloy powder such as ferromanganese or ferrosilicon.
Sulfur, when added, reacts with the alloying elements in
sintering to form a sulfide, whose lubricating effect improves the
wear resistance of the alloy. However, the addition of less than
0.2 ~ is not effective, while the addition of more than 1.5 % de-
creases the strength and resistances to oxidation and to lead corro-
sion of the alloy and results in a poor yield. Thus the sulfur con-
tent is limited to 0.2 - 1.5 %. Sulfur may be added singly, but it
can also be added in the form of a sulfide such as MoS2, ZnS, FeS or
Cu2S.
Molybdenum is an element which enhances the strength of the
alloy at high temperatures. Its effect, however, will not appear at
; less than 0.5 %i, while at more than 8 % the wear resistance may be ;~
improved but no improvement will take place in the resistances to
oxidation and lead corrosion. Thus the molybdenum content is limited
to O.S - 8 ~i. Molybdenum may be added singly or in the form of alloy
... .
powder such as ferromolybdenum.
At a sintered density of less than 6.2 g/cm3, the strength
` of the alloy tends to be insufficient, while the resistances to oxi-
dation, lead corrosion and wear are likely to drop. If, however, the
density is greater than 6.8 g/cm~, not only will the wear resistance
fail to improve, but molding will become difficult and the molded
article is likely to crack and chip, resulting in a shortened life
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1068130
for the molded article. Thus the sintered density is limited to 6.2
- 6.8 g/cm3.
At a sintering temperature of less than 1120C the sinter-
ing is insufficient, resulting in an insufficient strength of the
alloy, while at a sintering temperature of more than 1200C a liquid
phase is liable to develop, resulting in instability of product qual-
ity. Thus it is desirable to sinter at 1120 - 1200C, one time.
The following examples specifically illustrate the present
invention.
Example 1.
The mass of -100 mesh base alloy powder composed of chromi-
um 15 %, nickel 1 % by weight and the balance iron, to which the
following have been added: flaky graphite -0.5 %, -250 mesh silicon
- 1.5 % (hereafter silicon of the same particle size is used) and
molybdenum of 3~ average size -1 % (hereafter molybdenum of the same
particle size is used), together with 0.5 % zinc stearate as a lubri-
~ cating agent, was blended for 30 minutes in a V-type mixer.
; Next, the same mass was pressure-molded to a density of 6.5
g/cm3 in a mechanical press and sintered for 40 minutes at 1150C in
a dry hydrogen atmosphere. Thus an iron-base sintered alloy accord-
ing to the present invention having the final composition Fe-15cr-
lNi-l.SSi-lMo-0.4C was produced.
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Example 2. -
0.5% graphite, 0.5% silicon, and 1.5% of -250 mesh
manganese (hereafter manganese of the same particle size is used),
were added to the iron-chromium-nickel alloy powder of Example 1.
Thereafter, in the same way as in Example 1, an iron-base sintered
alloy according to the present invention having the final composi- ;
tion Fe-15Cr-lNi-0.5Si-1.5Mn-0.4C was produced.
Example 3.
0.5% graphite, 2.5% silicon and 0.3% manganese
were added to the iron-chrome-nickel system alloy powder of
Example 1. Then an iron-base sintered alloy according to the
present invention with the final composition Fe-15Cr-lNi-2.5Si-
0.3Mn-0.4C was produced in the same way as in Example 1.
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Example 4.
0.2% graphite, 8% molybdenum and 1% manganese by
weight were added to -100 mesh base alloy powder composed of 6%
chromium, 2% nickel and the balance iron. Then an iron-base
sintered alloy according to the present invention with the final
composition Fe-6Cr-2Ni-8Mo-lMn-0.2 C was produced in the same
way as in Example 1.
Example 5.
1.5% graphite, 0.5% molybdenum, and 2% sulfur by
weight, with the sulfur having an average particle size of ~G~
(hereinafter, sulfur of the same particle size is used), were
added to -100 mesh base alloy powder composed of 20% chromium,
0.2% nickel and the balance iron. Then an iron-base sintered
alloy according to the present invention with the final composition
Fe-19Cr-0.2Ni-0.5Mo-O.lS-1.3C was produced in the same way as in
~ Example 1.
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~68130
Example 6.
1.5% graphite, 4.5% molybdenum, and 1.5% sulfur
were added to the iron-chrome-nickel base alloy powder of Example
5. Then an iron-base sintered alloy according to the present
invention with the final composition Fe-19Cr-0.2Ni-4.SMo-1.3S-1.3C
was produced in the same way as in Example 1.
,
To verify the effect of using the iron-chrome-nickel
system base alloy powder according to the present invention, an
alloy of the same composition as the invented alloy was produced
by adding respective elements without using the above-mentioned
base alloy powder (see Comparison 1), while a heat-resistant
steel of about the same composition as that in Example 1 was
produced (see Comparison 2).
Meanwhile, another alloy with only its nickel content
out of the limited range of element contents in the invented
.: iron-base sintered alloy was produced (see Comparison 3).
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. Comparison 1.
24% of a ferrochrome alloy powder (Fe-63Cr) of
-200 mesh, 1% of a carbonyl nickel powder of average particle
.20 size 5 , 1.5% silicon, 1% molybdenum, and 0.5% graphite were
blended together and, following the same process as in Example 1,
. a sintered alloy of the same composition as in Example 1 was
: obtained.
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Compariso _2. '~
:: 25 Steel of about the same composition as in Example 1
WQS pr duced.
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106813V
Comparison 3.
0.5% graphite, 1.5% silicon, and 1% molybdenum
were added to -100 mesh base alloy powder composed of 15% chrome ;
and 8% nickel, by weight, with the balance iron. Thereafter,
following the same process as in Example 1, a sintered alloy
comprising Fe-15Cr-8Ni-1.5Si-lMo-0.4C, the same composition as -
in Example 1, except for an increased nickel content, was
obtained.
The sintered alloys obtained in these examples and
comparisons were subjected to various tests.
Hardness was measured in terms of Vickers hardness,
Hv(10), at ambient temperature. Strength was measured in terms
of the maximum rupture stregnth of a pressure ring at ambient
temperature in a ring test. For oxidation, the test specimen
was heated at 800 C for 100 hours in the atmosphere, and the
weight of the layer of scale on the specimen surface is indicated
in terms of its ratio to the original weight of the specimen, as
a measure of anti-oxidation property. This ratio was calculated ;~
according to the following formula:
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Ratio of scale weight = Scale weight x 100 (%)
Original Weight
In the lead corrosion test, the specimen was buried
in lead monoxide powder and heated at 800 C for one hour, whereby
the speciment lost weight due to corrosion through contact with
lead monoxide in the solid state and the weight loss was indicated
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as a corrosion loss per unit surface area of the specimen before
testing. The following formula was used:
; Lead corrosion loss = Corrosion loss (g/dm /hr)
Original surface area
Wear resistance was estimated in terms of the width
of a worn mark in the Ogoshi type wear test.
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~068~30
The coefficient of thermal expansion was measured
using a Leitz thermal expansion measuring device in vacuum in
the temperature range of 0 - 600 C.
The results of tests are summarized in Table 1.
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As seen from Table l, the iron-base sintered alloys
according to the invention are nearly equivalent in resistances
to oxidation and lead corrosion to the heat-resistant steel of
comparison 2. The alloy in comparison 1 to which specified
elements have been arbitrarily added proved unsatisfactory. This
can be explained as follows: Whereas in the present invention
an iron-chrome-nickel system alloy powder is taken as the base and
the main elements are distributed in the matrix with relative
uniformity, in Comparison l a macroscopic variance in density
develops due to incomplete diffusion of the alloying elements
during sintering.
In the iron-base sintered alloy according to the
present invention, as seen from Table l the pressure ring strength
and the wear resistance are improved through appropriate selection
; 15 of the base alloy composition.
The coefficient of thermal expansion can be improved,
depending on the nickel content, as seen from Comparison 3 and
Example 1, the values being 16.8 x 10 and 12.8 x 10 respective-
~` ly. Thus the reason for limiting the nickel content is clear.
For this reason, the permissible limit of the coeffi-
cient of thermal expansion for the valve seat material has been
determined and the nickel content limitèd so that this coefficient
; will fall below the limit.
Referring to Fig. 1 illustrating a section of the
apparatus to be used for the dropout test of valve seat, the
~ test process will now be described.
; A test specimen 2 in the shape of a valve seat ring
is pressed into a cast iron or aluminum holder l. The cooling
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water 3 filling the holder 1 is kept at 100 C. At the same time
the seat side of the test specimen 2 is heated by propane gas
burner 4. The surface temperature is maintained at 600 C for
100 hours, using a thermocouple 5. The test specimen 2 after
;~ 5 the dropout test is pulled out of the holder 1 and the force
required to do so measured, using a device having the section
illustrated in Fig. 2.
With the split jig 6 for pull-load measurement applied
to the seat side of the test specimen 2, the Jig 7 is fitted and
pressed by the Instron type testing machine. The force required
to pull out the test specimen 2 is thereby measured and the seat
pulling force decline rate is estimated using the following
formula:
Seat pulling force decline rate (~) = B-A/B x 100
where A . . . pulling force after dropout test (kg)
B . . . pulling force before dropout test (kg)
(fresh test)
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~ The pulling force decline rates of different seats ~
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including seats made of the iron-base sintered alloys according
to the invention have been measured using an aluminum holder and
a cast iron holder, the results being summarized in Fig. 3.
As indicated in Fig. 3, when an aluminum holder `
(coefficient of thermal expansion: 21 x 10 ) is employed, the
pulling force decline rate is so low even at a coefficient of
thermal expansion equal to 18 x 10 that there is no hazard
~ of the seat dropping out. When a cast iron holder is used the
,- pulling force decline rate is high at the seat's coefficient
~ of thermal expansion, which is over 13.5 x 10 6, and there is
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106~130
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a substantial risk that the valve seat will drop out of the cast
. iron cylinder head. A seat insert for a cast iron cylinder head
: is therefore required to have a coefficient of thermal expansion
less than 13.5 x 10 6. For this reason the nickel content in the
present invention is limited to less than 2~ to hold the
coefficient of thermal expansion down to less than 13.5 x 10 6. ;~
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