CA1068130A - Iron base sintered alloy for valve seat - Google Patents

Abstract

ABSTRACT
Iron-base sintered alloy for the valve seats of intern-al combustion engines is an iron-base alloy powder containing, by weight, 6 - 20 % chromium, less than 2.0% nickel, and carbon 0.2 - 1.5 %, together with at least two additives selected from among 0.3 - 1.5 % manganese, 0.2 - 1.5 % sulfur, 0.5 - 8 %
molybdenum, and 0.5 - 2-5 % silicon, and molded to a sintered density of 6.2 - 6.8 g/cm3.

Description

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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 ,,j 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 , .
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 . .' .
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 , 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

, 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 , -- ~068~30 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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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 ;' ' . I .

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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.

: ' -?-~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).
''; ' ' , . 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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:: 25 Steel of about the same composition as in Example 1 WQS pr duced.

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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:
.. , 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 ::
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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-` ~06~3130 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) . . , ~ 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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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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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE, IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Iron-base sintered alloy for use in valve seats which comprises 0.2 - 1.5 % carbon and at least two additives selected from the group consisting of 0.3 - 1.5 % manganese, 0.2 - 1.5 % sulfur, 0.5 - 8 % molybdenum, and 0.5 - 2.5 % silicon, with the balance con-sisting essentially of an iron-base alloy powder containing 6-20 %
chromium and 0 - 2.0 % nickel, all percentages being given by weight, said alloy having a sintered density of 6.2 - 6.8 g/cm3.

2. Iron-base sintered alloy as claimed in Claim 1 in which 0.5 % carbon and at least two additives selected from the group con-sisting of 0.3 - 1.5 % manganese, 1 - 8 % molybdenum and 1.5 - 2.5 %
silicon are mixed with an iron-base alloy powder containing 15 %
chromium and 1 % nickel by weight and said alloy has a sintered den-sity of 6.5 g/cm3.

3. Iron-base sintered alloy as claimed in Claim 1 in which 0.2 % carbon, 1 % manganese and 1 % molybdenum are mixed with an iron-base alloy powder containing 6 % chromium and 1 % nickel by weight and said alloy has a sintered density of 6.2 - 6.8 g/cm3.

4. Iron-base sintered alloy as claimed in Claim 1 in which 1.5 % carbon, 0.2 - 1.5 % sulfur and 1.5 - 4.5 % molybdenum are mixed with an iron-base alloy powder containing 20 % chromium and 0.2 %
nickel by weight and said alloy has a sintered density of 6.2 - 6.8 g/cm3.

5. Iron-base sintered alloy as claimed in Claim 1, which has been sintered at a sintering temperature of 1120 - 1200°C.

6. Iron-base sintered alloy as claimed in Claim 1, where-in said iron-base alloy powder has a particle size of -100 mesh.

7. Iron-base sintered alloy as claimed in Claim 1, where-in said carbon is in the form of graphite.

8. Method of manufacturing an iron-base sintered alloy for use in valve seats which comprises the steps of adding 0.2 - 1.5%
carbon in the form of graphite powder, at least two additives select-ed from the group consisting of 0.3 - 1.5 % manganese, 0.2 - 1.5 %
sulfur, 0.5 - 8 % molybdenum, and 0.5 - 2.5 % silicon by weight to an iron-base alloy powder containing 6 - 20 % chromium and 0 - 2.0 %
nickel by weight, molding the resulting mixture to a required shape by applying pressure thereto, and then sintering the molding at a temperature of 1120 - 1200°C to yield an alloy having a sintered den-sity of 6.2 - 6.8 g/cm3.

9. Method as claimed in Claim 8, in which said sulfur is added in the form of sulfide.

10. Method as claimed in Claim 8, in which said manganese, silicon and molybdenum are added in the form of iron compounds.