Internal-combustion engines – Combustion chamber – Having coating or liner
Reexamination Certificate
1998-11-12
2001-01-16
Wolfe, Willis R. (Department: 3747)
Internal-combustion engines
Combustion chamber
Having coating or liner
Reexamination Certificate
active
06173702
ABSTRACT:
The present invention relates to a movable wall member in the form of an exhaust valve spindle or a piston in an internal combustion engine, particularly a two-stroke crosshead engine, the side of the wall member facing a combustion chamber being provided with a hot-corrosion-resistant material made from a particulate starting material of a nickel and chromium containing alloy which by a HIP process has been unified to a coherent material substantially without melting the starting material.
A hot-corrosion-resistant material in the present context means a material which is resistant to corrosion in the environment existing in the combustion chamber of an internal combustion engine at an operating temperature ranging from 550° C. to 850° C.
From practical construction of large two-stroke diesel engines of the make MAN B&W Diesel, an exhaust valve spindle of the compound type is known in which the lower surface of the valve disc and the seat area of a spindle base are provided by a HIP process with a layer of hot-corrosion-resistant material of the alloy Nimonic 80A, which contains 18-21% chromium and approximately 75% nickel. In addition to its corrosion resistance, this alloy is of such a hardness, approximately 400 HV20, that it is suitable as valve seat material. Conventionally, valve seats have to have a high hardness to counteract the formation of dent marks in the sealing surfaces when residual particles from the combustion process are squeezed tight between the seat surfaces at the valve closure.
EP-A 0 521 821 describes the use of the alloy Inconel 671 as a hardfacing alloy in the valve seat area. This alloy contains 0-04-0.05% C, 47-49% Cr, 0.3-0.40% Ti and a balance of Ni. The valve seat area is located on the upper surface of the valve disc as a continuous annular facing. As mentioned above it is a condition for seat areas that the alloy has a high hardness. The EP publication mentions that Inconel 671 is supposed to have a poorer corrosion resistance than the alloy Inconel 625, which is also proposed as a hardfacing material.
The Applicant's international patent application published as WO96/18747 describes an exhaust valve spindle with a welded-on hardfacing alloy analyzed at 40-51% Cr, from 0 to 0.1% C, less than 1.0% Si, from 0 to 5.0% Mn, less than 1.0% Mo, from 0.05 to 0.5% B, from 0 to 1.0% Al, from 0 to 1.5% Ti, from 0 to 0.2% Zr, from 0.5 to 3.0% Nb, an aggregate content of Co and Fe of 5.0% at the most, at the most 0.2% O, at the most 0.3% N and a balance of Ni. After the welding a high hardness of, for example, 550 HV20, is imparted to this valve seat material by means of a heat treatment at a temperature exceeding 550° C.
It is generally presumed that hot-corrosion-resistant alloys containing chromium and nickel age-harden at temperatures ranging from 550° C. to 850° C., viz. the alloy becomes harder and more brittle. In the case of cast members, to achieve excellent hot corrosion resistance, particularly in environments containing sulphur and vanadium from heavy fuel oil combustion products, it is known to use an alloy of the type 50% Cr and 50% Ni or an alloy of the type IN 657 consisting of 48-52% Cr, 1.4-1.7% Nb, at the most 0.1% C, at the most 0.16% Ti, at the most 0.2% C+N, at the most 0.5% Si, at the most 1.0% Fe, at the most 0.3% Mg and a balance of Ni. After casting, the alloy comprises a nickel-rich &ggr;-phase and a chromium-rich &agr;-phase where both phases, depending on the accurate analysis of the alloy, may constitute the primary dendrite structure. It is known that these alloys age-harden at operating temperatures exceeding 600° C. This is because the alloy, when it cools off, does not solidify in its equilibrium state. When the alloy is subsequently at the operating temperature, precipitation of the under-represented phase proportion occurs by transformation of the over-represented phase proportion, which causes embrittlement characterized in a ductility of less than 4% at room temperature. Owing to these relatively poor strength properties, the alloys have been used exclusively for low-load cast members.
The technical article “Review of operating experience with current valve materials” published by The Institute of Marine Engineers, London, in 1990, provides an overview of applicable facing alloys for exhaust valves for diesel engines, and describes the problems of hot corrosion in diesel engines in detail. The article is especially aimed at conditions existing at the seating surfaces of the exhaust valve spindle.
At the lower surface of the valve spindle and at the upper surface of the piston the hot-corrosion-resistant material is to limit corrosive attacks so that the valve spindle and/or the piston achieve(s) an advantageously long life. The upper piston surface and the lower valve disc surface have large areas and are therefore exposed to considerable heat stresses when the engine load is changed, for example when the engine is started or stopped. The heat impact is heaviest at the middle of the areas, partly because the combustion gases have the highest temperature near the middle of the combustion chamber, partly because the piston and the valve spindle are cooled near the edges of the areas. The valve disc is cooled near the seat areas on the upper surface, which is in contact with the water-cooled stationary valve seat while the valve is closed, and as for the piston heat is conducted away to the water-cooled cylinder liner through the piston rings in addition to the oil cooling of the inner piston surface. The colder peripheral material prevents thermal expansion of the hotter central material, causing considerable heat stresses.
It is well-known that the slowly varying, but large heat stresses caused by said thermal influences can cause star cracking initiated at the middle of the lower surface of the valve disc. The star cracks may become so deep that the hot-corrosion-resistant material is penetrated so that the subjacent material is exposed to the corrosive impact and is eroded, leading to failure of the exhaust valve.
The object of the present invention is to provide an exhaust valve spindle or a piston having an advantageously long life for the hot-corrosion-resistant material.
In view of this the wall member stated in the introduction of claim
1
is characterized according to the invention in that in terms of percent by weight and apart from the common impurities and inevitable residual amounts of deoxidizing components the corrosion-resistant material comprises from 38 to 75% Cr and optionally from 0 to 0.15% C, from 0 to 1.5% Si, from 0 to 1.0% Mn, from 0 to 0.2% B, from 0 to 5.0% Fe, from 0 to 1.0% Mg, from 0 to 2.5% Al, from 0 to 2.0% Ti, from 0 to 8.0% Co, from 0 to 3.0% Nb as well as optional components of Ta, Zr, Hf, W and Mo, and a balance of Ni, the aggregate contents of Al and Ti amounting at the most to 4.0%, and the aggregate contents of Fe and Co amounting at the most to 8.0%, and the aggregate contents of Ni and Co amounting at the least to 25%, and that the corrosion-resistant material has a hardness of less than 310 HV measured at approximately 20° C. after the material has been heated to a temperature within the range of 550-850° C. for more than 400 hours.
Quite surprisingly it has proved that the material of this composition produced by the HIP process does not harden at the operating temperatures to which the movable wall member is exposed in an internal combustion engine, and it is thus possible to maintain an advantageous low hardness of less than 310 HV20 and associated suitable ductility of the hot-corrosion-resistant material on the side of the movable wall member facing the combustion chamber. The low hardness limits or prevents crack formation in the material, and the life of the wall member is thus not limited by fatigue failures in the material. The invention provides the further advantage that the material retains very fine mechanical properties even after a long-term heat influence. Thus the material retains a high tensile strength combined with high ductility, which is quite
Benton Jason
Man B&W Diesel A/S
Sughrue Mion Zinn Macpeak & Seas, PLLC
Wolfe Willis R.
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