Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2003-09-30
2004-09-21
Mai, Ngoclan T. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C419S011000, C075S243000, C075S246000
Reexamination Certificate
active
06793876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a valve seat, which is a structural member of an internal combustion engine such as a diesel engine or a gasoline engine, and relates particularly to an Fe-based sintered alloy valve seat (hereafter simply described as a valve seat) that exhibits excellent wear resistance under conditions of high surface pressure application.
2. Description of the Related Art
The cylinder heads of internal combustion engines such as diesel engines or gasoline engines are provided with valve seats for the exhaust valve and the intake valve.
Conventionally, the valve seats utilize an Fe-based sintered alloy which has an overall composition comprising, in terms of weight percentage (hereafter all % values relating to compositions refer to % by weight),
C: 0.7 to 1.4%,
Si: 0.2 to 0.9%,
Co: 15.1 to 26%,
Mo: 6.1 to 11%,
Cr: 2.6 to 4.7%,
Ni: 0.5 to 1.2%,
Nb: 0.2 to 0.7%,
and a balance of Fe and inevitable impurities, wherein
a substrate formed from an Fe-based sintered alloy, comprising a composition in which a hard dispersion phase formed from Co—Mo—Cr alloy is distributed in an Fe-based alloy matrix, and
having a porosity of 5 to 15%,
is infiltrated with copper or copper alloy to form the valve seat (for example, refer to the patent reference 1).
Furthermore, it is known that the valve seat described above can be produced using, as the raw material powder for forming the matrix, an Fe-based alloy powder with an average particle size of 75 to 107 &mgr;m, and comprising:
C: 0.8 to 2.1%,
Ni: 0.6 to 1.7%,
Cr: 1.2 to 3.6%,
Nb: 0.3 to 0.9%,
Co: 4.3 to 13%,
Mo: 1.4 to 4.2%,
and a balance of Fe and inevitable impurities, and using, as the raw material powder for forming the hard dispersion phase, a Co-based alloy powder with an average particle size of 68 to 102 &mgr;m, and comprising:
Mo: 20 to 35%,
Cr: 5 to 10%,
Si: 1 to 4%,
and a balance of Co and inevitable impurities,
by conducting solid phase sintering, in an ammonia cracked gas atmosphere, of a pressed compact formed from a mixed powder generated by mixing the Co-based alloy powder into the Fe-based alloy powder in sufficient quantity to account for 25 to 35% by weight of the combined weight with the Fe-based alloy powder, thereby forming an Fe-based sintered alloy substrate,
and then infiltrating this Fe-based sintered alloy substrate with copper or a copper alloy (refer to patent reference 1).
Patent Reference 1
Japanese Unexamined Patent Application, First Publication No. Hei 11-209855 A
On the other hand, the increase in the size and output of internal combustion engines in recent years has been remarkable, and accompanying these trends, the spring constant of the valve springs have tended to increase with the aim of preventing gas leakage of the combustion gases. As a result, the seat load applied to the valve contact surface of the valve seat increases even further, meaning operation of the valve seat under conditions of high surface pressure application is unavoidable, but when a conventional valve seat such as that described above, or any of a variety of other valve seats, is used under conditions of high surface pressure application, wear of the valve seat is accelerated considerably, meaning the valve seat reaches the end of its life in a comparatively short time.
SUMMARY OF THE INVENTION
Taking the above circumstances into consideration, the inventors of the present invention conducted research into developing a valve seat that exhibits excellent wear resistance, even when used under conditions of high surface pressure application, and made the following discoveries (a) to (c).
(a) The reason that the conventional valve seat described above displays inadequate wear resistance under conditions of high surface pressure application is that because the adhesion of the hard dispersion phase to the matrix is unsatisfactory, the hard dispersion phase readily separates from the matrix under conditions of high surface pressure application, causing an acceleration of the wearing process.
(b) The Fe-based sintered alloy substrate used to form the conventional valve seat described above is produced, as described above, using an Fe-based alloy powder for forming the matrix, and a Co-based alloy powder for forming the hard dispersion phase, both with an average particle size of 68 to 107 &mgr;m, by conducting sintering in an ammonia cracked gas atmosphere, and as a result, in the Fe-based sintered alloy substrate generated following sintering, the matrix has essentially the same composition as that of the Fe-based alloy powder used for forming the matrix, and similarly, the hard dispersion phase has essentially the same composition as that of the Co-based alloy powder used for forming the hard dispersion phase, whereas if the sintering atmosphere is altered to a vacuum atmosphere (a reduced pressure atmosphere), and the particle sizes of the raw material powder for forming the matrix and the raw material powder for forming the hard dispersion phase are reduced to average particle sizes within a range from 20 to 50 &mgr;m, and in addition, if the raw material powder for forming the matrix utilizes an Fe-based alloy powder comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 3 to 8%,
Cr: 0.2 to 3%,
and a balance of Fe and inevitable impurities, and the raw material powder for forming the hard dispersion phase utilizes a Co-based alloy powder comprising:
Mo: 20 to 32%,
Cr: 5 to 10%,
Si: 0.5 to 4%,
and a balance of Co and inevitable impurities, then during sintering, the Co, Cr and Si components of the Co-based alloy powder diffuse and migrate into the matrix, and the Fe component of the Fe-based alloy powder diffuses and migrates concurrently into the gaps in the Co-based alloy powder left by the migration of the Co, Cr and Si components, thereby generating a mutual diffusion and migration phenomenon of the alloy components.
(c) The Fe-based sintered alloy substrate generated during the sintering described in (b) above, in which the alloy components have undergone mutual dispersion and migration between the matrix and the hard dispersion phase, is formed from an Fe-based sintered alloy with a porosity of 10 to 20%, and comprising, according to measurements performed using an X-ray microanalyzer (EPMA), an Fe—Co alloy matrix comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 13 to 22%,
Cr: 1 to 5%,
Si: 0.1 to 1%,
and a balance of Fe and inevitable impurities, in which is uniformly distributed a hard dispersion phase of a Mo—Fe—Co alloy, having a composition comprising:
Fe: 20 to 30%,
Co: 13 to 22%,
Cr: 1 to 5%,
Si: 0.3 to 3%,
and a balance of Mo and inevitable impurities, and having a 2 phase mixed system of an Fe—Co alloy phase and a Mo—Co alloy phase, and from these findings it is evident that as a result of this mutual diffusion and migration of large quantities of the alloy components between the matrix and the hard dispersion phase, the adhesion of the hard dispersion phase to the matrix improves markedly, and moreover, the matrix displays excellent high temperature corrosion resistance in the fuel combustion gas atmosphere, and the hard dispersion phase has superior high temperature hardness and displays excellent high temperature corrosion resistance, and consequently, the Fe-based sintered alloy substrate described above exhibits excellent wear resistance as a valve seat, even under high surface pressure application conditions, and if the substrate is infiltrated with copper or a copper alloy then the thermal conductivity and the strength of the substrate can be further improved.
The findings in (a) to (c) above summarize the results of the research conducted by the inventors.
The present invention is based on the research results described above, and provides a process for producing a valve seat comprising the steps of:
(a) using, as a raw material powder for forming a matrix, an Fe-based alloy powder comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 3 to 8%,
Cr: 0.2 to 3%,
and a balance of Fe and inevitable impurities, and havin
Hanata Kunio
Hoshino Kazuyuki
Darby & Darby
Mai Ngoclan T.
Mitsubishi Materials Corporation
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