The present invention relates to a cooking vessel comprising a bottom made of a multilayer material and a side wall and to an article of multilayer material, the said vessel being more particularly intended for cooking food by induction.
The cooking of food by induction is carried out by means of an inductor generally placed beneath a glass-ceramic plate which is transparent to electromagnetic fields, on which the cooking vessel whose contents it is desired to heat is placed. The flow of the high-frequency current in the inductor creates a magnetic field inducing eddy currents in the vessel which therefore heats up by the Joule effect.
There are also specific appliances for cooking without glass-ceramic plates, for which the materials, forming the subject of this patent, should allow the manufacture of vessels.
To achieve a high energy efficiency, the metal bases used for these vessels have a high electrical conductivity and a high amplification of the magnetic field at the operating frequencies used, which are between 20 and 50 kHz. An alloy which is ferromagnetic in the working temperature range of the vessel is therefore generally used.
These vessels must have a high corrosion resistance for the face in contact with the food, but also to a lesser extent for this base which must not deteriorate with washing.
They must also be mechanically stable so that the vessel retains its geometry, in particular the flatness of the base in contact with the top of the inductor. In fact, when a vessel is heated, its base tends to expand. The side wall of the vessel, also called the skirt, is raised to a lower temperature than the base and therefore expands less, thereby exerting a radial compressive force on this base. The latter can then expand only by bowing, thereby reducing the energy efficiency of the assembly and making it less pleasant for the user because of the noise and the annoyance that it causes. This effect is reversible the first few times the vessel is used, but may result in irreversible degradation after a large number of these thermal cycles by structural transformation of the materials of the base. This phenomenon is particularly sensitive when only the base of the vessel contains a highly conducting material (aluminium or copper, for example).
In the case of multilayer materials, the various layers of which generally have very different expansion coefficients, this difference in coefficient introduces a bimetallic strip effect tending both to deform the base of the container and to the make adhesion of the various layers deteriorate irreversibly towards localized debonding, and therefore a significant loss of efficiency of the vessel, in the case of induction hetaing as well as by any other heating process (IR, halogen, etc. . . . )
To produce these vessels, it is common practice to use, for the ferromagnetic part, ferritic stainless steels such as 17% Cr—Fe or three-layer symmetrical materials, namely austenitic stainless steel/ferritic stainless steel/austenitic stainless steel. These materials have the drawback of having a Curie temperature above 600° C., which means that the bases of these vessels may themselves also reach this temperature, which may result in the loss and burning of food and the degradation of the vessel, this being so, even well below this temperature of 600° C.
To remedy this problem, it has been proposed in FR 2 453 627 to manufacture a vessel base from a three-layer material comprising an alloy whose Curie point is between 60 and 200° C. As long as the temperature of the vessel is below the Curie point, the alloy is ferromagnetic and can generate losses by induced currents, thereby heating the vessel. As soon as the temperature of the vessel exceeds the Curie point, the alloy is no longer ferromagnetic and the heating stops, resuming again only when the temperature of the vessel drops below the Curie point. Thus, thermal regulation of the vessel is therefore obtained. However, such a material is not suitable for cooking or for frying food, which needs to be able to reach temperatures ranging from 220 to 320° C. In addition, nothing was proposed in that patent to ensure good geometrical stability of the vessel and good corrosion resistance of both faces of the vessel.
The same principle is adopted in FR 2 689 748, in which it is proposed to manufacture vessels from a three-layer material comprising an alloy such as 64Fe-36Ni, the Curie point of which is 250° C. However, this type of alloy has a very mediocre corrosion resistance and a very low expansion coefficient. Yet this alloy is pressed against a metal layer having a markedly higher expansion coefficient, which results in deformation of the base of the container owing to the bimetallic strip effect when it is heated, which deformation may sometimes become irreversible. Furthermore, degradation in the bonding between the layers may also be observed, this being due to the phenomenon of creep under a cyclic stress and temperature.