Food or edible material: processes – compositions – and products – Processes – Heating above ambient temperature
Reexamination Certificate
1999-12-01
2001-04-10
Yeung, George C. (Department: 1761)
Food or edible material: processes, compositions, and products
Processes
Heating above ambient temperature
C126S390100, C219S621000, C220S912000
Reexamination Certificate
active
06214401
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cooking vessel which may be, and which may be specially intended and designed to be, induction heated. Also included are an alloy for producing such a vessel, and a method for making the vessel.
2. Discussion of the Background
Cooking vessels, such as saucepans, brazing pans or frying pans, are generally made of a food-compatible alloy, such as an austenitic stainless steel or an aluminum alloy. When they are intended to be induction heated, these cooking vessels include, in the external part of their bottom, an insert made of a ferromagnetic alloy. This is because food-compatible alloys are generally nonmagnetic and, consequently, cannot be induction heated given the frequency range (20 to 50 kilohertz) generally permitted. On the other hand, the insert made of a ferromagnetic alloy can be induction heated and constitutes a heating part. Various constructions exist. For example, when the vessel is made of an aluminum alloy, the heating part may be a grid inserted into the bottom of the vessel. When the vessel is made of an austenitic stainless steel, the heating part is a plate made of a ferromagnetic alloy placed on the external wall of the bottom and either separated from the wall by an aluminum layer intended to distribute the heat well or joined to the bottom and covered on its external face with an austenitic stainless steel layer intended to ensure good corrosion resistance.
In general, the ferromagnetic material used for the heating part is a ferritic stainless steel. This technique has the drawback of allowing induction heating up to temperatures that may be as high as 600° C. when the vessel is left on the induction-heating device. As a result, the food may be burnt and the vessels damaged.
In order to remedy this drawback, it has been proposed, particularly in French patent applications No. 2,527,916, No. 2,453,627 and No. 2,689,748, to use heating parts made of a ferromagnetic alloy having a Curie temperature that is not too high, for example between 60° C. and 350° C., so as to avoid overheating. The advantage of a heating part made of a ferromagnetic alloy whose Curie temperature is not too high is that, when the temperature of the heating element becomes close to the Curie temperature of the alloy, the heating part becomes gradually nonmagnetic, which stops the heating. This results in a thermostatic effect whose consequence is that the temperature of the bottom of the cooking vessel cannot exceed the Curie temperature of the alloy of which the heating part is composed. It should be noted however that the aforementioned patent applications either give only very vague information about the alloys that can be used or propose the use of alloys which are not suitable for the envisaged use. For example, French patent application 2,689,748 proposes the use of Fe—36Ni or of Fe—18Ni9Co5Mo or of Fe—80Ni5Mo. None of these solutions are satisfactory.
The alloy Fe—36Ni is well known. This is an alloy having a very low thermal expansion coefficient incompatible with the thermal expansion coefficient of an austenitic stainless steel or of an aluminum alloy. The use of this alloy would result in a very large bimetallic effect leading either to the bottom of the cooking vessel distorting or the heating element disbanding. In addition, this alloy is not corrosion resistant.
The alloy Fe—18Ni9Co5Mo is a well-known maraging steel but does not absolutely have the required properties. Its choice probably results from a confusion between the Curie temperature and the martensitic transformation start temperature Ms.
The alloy Fe—80Ni5Mo, also well known, but it is not suitable either, since its Curie temperature is approximately 450° C.
Moreover, cooking vessels intended for induction heating furthermore have the drawback of producing a disagreeable whistling noise when they are heated.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a process for producing a cooking vessel intended to be induction heated which can be left on an induction-heating appliance without the risk of overheating,
Another object is to provide a cooking vessel where induction heating of the vessel does not generate noise. Alloys for such vessels are also provided, as is a method of making such vessels.
In more detail, one subject of the invention is a cooking vessel intended to be induction heated, comprising a heating part made of a ferromagnetic alloy having a Curie temperature of between 150° C. and 370° C., including 175, 200, 225, 250, 275, 300, 325, and 350° C., and preferably less than 350° C., and a magnetostriction coefficient &lgr;
s
≦5×10
−6
in absolute value. Preferably, the alloy has a Curie temperature of between 275° C. and 340° C., and better still between 275° C. and 325° C., and a magnetostriction coefficient &lgr;
s
≦3×10
−6
in absolute value.
The ferromagnetic alloy of which the heating part comprises, consists essentially of, or consists of, is, for example, an alloy of the FeNiCuX type, X being one or more elements taken from Cr, Mo, Mn, Si, Al, W, Nb, V and Ti, the chemical composition of the alloy (in % by weight) being such that:
50%≦Ni≦85%
0%≦Cu≦50%
0%≦X≦12%
(35/25)×(75−Ni)≦Cu≦(50/35)×(85−Ni)
(7/20)×(Ni−65)≦X≦(12/35)×(Ni−50)
with: X=Cr+Mo+Mn+Si+Al+W+Nb+V+Ti, the balance comprising, consisting essentially of, or consisting of iron and impurities resulting from smelting.
Preferably, the chemical composition of this alloy is such that:
55%≦Ni≦65%
22%≦Cu≦36%
0%≦X≦3%.
Better still, the copper content is between 25% and 35%.
The heating part is not limited in terms of shape or size and may be composed of an insert incorporated into the bottom of the vessel, which insert may in particular be a plate.
The heating part may also constitute the wall of the vessel and be covered, at least on its internal part, with a layer of enamel.
REFERENCES:
patent: 4441940 (1984-04-01), Pfeifer et al.
patent: 5064055 (1991-11-01), Bessenbach et al.
patent: 5669989 (1997-09-01), Inoue et al.
patent: 1198477 (1998-11-01), None
patent: 2 453 627 (1980-11-01), None
patent: 2 527 916 (1983-12-01), None
patent: 2 689 748 (1993-10-01), None
Patent Abstracts of Japan; vol. 15, No. 114, Mar. 19, 1991 & JP 03 004479, Jan. 10, 1991.
Chemical Abstracts, vol. 115, No. 16, Oct. 21, 1991; Abstract No. 163973; “Thermal -spray coating of stainless steel pot bottom with copper alloy”; XP-002109861.
S.R. Lampman et al; “Metals Handbook”; vol. 2; 1990; ASM International, Metals Park, Ohio, US XP002109860; pp. 435-437.
Chaput Laurent
Coutu Lucien
Grosbety Yves
Leroy Marc
Imphy Ugine Precision
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yeung George C.
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