WO1999051134A1

WO1999051134A1 - Cooking vessel for induction heating

Info

Publication number: WO1999051134A1
Application number: PCT/NO1999/000107
Authority: WO
Inventors: Svein Eide
Original assignee: Designor As
Priority date: 1998-04-06
Filing date: 1999-03-30
Publication date: 1999-10-14
Also published as: AU3176099A; NO981552D0; EP1069852A1; NO981552L; NO309505B1

Abstract

The invention relates to a cooking vessel of stainless steel, fit for heating by induction. It has an additional bottom of aluminium and a laminate. The laminate has a middle layer of mild steel and is coated on both sides with austenitic stainless steel in a thickness of 15 % to 25 % of the laminate thickness. The wall of the cooking vessel has a thickness which is greater than 1 depth of penetration at 20 KHz to 40 KHz.

Description

COOKING VESSEL FOR INDUCTION HEATING

This invention relates to cooking vessels, frying pans of austenitic and ferritic stainless steel fit for heating by high-frequency induction. It has an additional bottom of aluminium and an outer capsule-shaped magnetizable bottom layer of a rolled laminate of mild steel coated on both sides with austenitic stainless steel. The capsule bottom may be welded to the wall of the cooking vessel. The power absorption per square measure of the bottom is higher on heating by induction, and the stability of the bottom is better than that of previously known similar embodiments.

Norwegian patent No. 152483 discloses vessels of austenitic stainless steel with, among other things, an additional bottom of aluminium and an outer bottom layer of a laminate of mild steel coated with stainless steel on both sides. The patent has solved the problem associated with the tack between the metal layers. It does not mention anything about bending of the bottom, power absorption, corrosion of the laminate, wear, welding and melting of aluminium.

In 1982, when the Norwegian patent was filed, commercial three-ply laminates of this type had a coating thickness of stainless steel of 5 % and 10 % of the laminate thickness as a standard. It was produced by Klockner AG Hutte Bremen Germany as "Platinox", and Japan Steel works Ltd. Tokyo with "NEO PLY". From the American version of the patent US 4596.236 it appears that the laminate is intended to be comparatively thick. In section 5 it is said that the interfaces are from 0,05 mm, which is the soldering layer, to 0,5 mm, which is laminate coating of stainless steel. This involves that with a 10 % coating the laminate thickness will be as much as 5 mm.

In practice there were not used laminates that thick. It was found that convenient thicknesses were from 0,8 mm to 1,25 mm .

It is common opinion among those skilled in the art of boiling vessels that the most important criterion for efficient heating by induction is that the bottom has good magnetizing properties and that, therefore, it should contain comparatively much steel or cast iron.

This may also be considered supported by persons skilled in the art of producing induction cooking plates. In European standard EN 60335-26 : 1990/A51: 1993 are described a test cooking vessel in the approving of induction plates. It shall consist of mild steel of a maximum carbon content of 0,004 %. The cooking vessel should be cylindrical, have a wall thickness of 1 , 5 mm and a height of 2/3 the diameter.

It is a known problem that cooking vessels of stainless steel for heating by induction have a relatively large bimetallic inward bending of the bottom. This results in an increased power consumption and longer heating time than desirable, when such cooking vessels are used on other heating plates. The problem is particularly noticeable on cooker tops of ceramic glass. Scott Glaswerke, D-6500 Mainz, Germany is the largest manufacturer of ceramic glass for cooker tops. They recommend a maximum inward bending of 0,1 mm by heating to 200 °C.

It appears from the following examples that cooking vessels according to Norwegian patent No. 152483 with the laminate assumed do not meet this requirement. The bimetallic bending develops as a consequence of a great difference in the linear expansion coefficient of the layers on each side of the additional bottom of aluminium. According to the present invention the bending is also affected by the tensile strength and the strength of the layers, so the thickness is critical. With a comparatively thick laminate the bimetallic inward bending will be large when the cooking vessel is of austenitic stainless steel.

Example 1

A 20 cm saucepan of 18-10 stainless steel of ordinary thickness 0,7 mm with an additional bottom of 4 mm aluminium and an 0,7 mm laminate of mild steel coated on both sides with 10 % of 18-10 stainless steel. The movement of the bottom on heating was 0 , 54 mm.

It is known that the additional bottom of aluminium on cooking vessels of stainless steel may melt on heating and fall off on moving. This may lead to persons being injured and to fire.

The thermostat of cast iron plates cuts off the power at

450° to 550 °C, but then the temperature on the surface is more than 600 °C. This is therefore no guarantee against aluminium reaching its melting point. On the market there are also cooking plates without a thermostat. With the lid on an empty saucepan, the additional bottom may melt within 10 minutes. The situation is the same for most glass tops. The 4

thermostat is set according to recommendations from Scott at 540° ± 20 °C, but if cookware with a large inward bending of the bottom is used, the temperature may reach up to 700 °C. The melting point of aluminium is 658 °C.

In the production of cooking vessels the bottom laminate reaches a temperature of 550° to 600 °C and becomes severely oxidized. This is ground off in a subsequent operation. However, the layer of corrosion protective 18-10 stainless steel by a 10 % laminate is very thin, e.g. only 0,08 mm. It is therefore easy to grind through it, with the risk of rust, and because of the tension potential between 18-10 stainless steel and mild steel a galvanic corrosion will develop.

Spot welding or seam welding is not recommendable for 10 % laminate in the thicknesses in question here. The thin layer of stainless steel will partly be burned away and partly become alloyed with the middle layer of mild steel. In wet conditions which may contain salt and acids from food, corrosion will occur in the welding. A capsule-shaped outer bottom of a 10 % laminate my therefore, in practice, not be welded in this way to a body of austenitic stainless steel.

In literature the penetration by magnetic fields to the interior room of the cooking vessel is not described. However, it is common opinion among biologists that the effect of electromagnetic fields of low frequencies may create biological harmful changes. The frequency range of 15 KHz to 60 KHz which is topical here, is considered low in this context.

On this background it is not certain whether some articles of food, e.g. milk, subjected to electromagnetic radiation from induction plates, will be affected and changed in a possibly unfortunate and harmful way. Trials in connection with the invention have shown that the magnet field may penetrate the wall and into the food in cooking vessels of austenitic stainless steels of ordinary thicknesses. This is, when the bottom diameter of the cooking vessel is smaller than the diameter of the inductor coil, and for all sizes when the cooking/frying pan is placed offset above the coil.

However, the magnet field does not penetrate when the wall of the cooking vessel consists of ferritic stainless steels. This is associated with the skin effect or penetration effect. The current density of high-frequency induced current decreases from the surface inward, when the material is sufficiently thick. The depth of penetration is calculated according to an experimental law:

5 = 500 yfϊ μ f

δ = depth of penetration in m t = resistance in ohm/m /m μ = relative permeability f = frequency in hertz

For 18-10 stainless steel δ = 2,80 mm at 24 KHz For 17 % chrome steel δ = 0,18 mm at 24 KHz For mild steel δ = 0,10 mm at 24 KHz

Ferritic stainless steels of low content of carbon and nitrogen, with addition of small amounts of titanium and molybdenum, have better corrosion properties towards chlorides and thereby the common cause of pitting of cooking vessels, than 18-10 austenitic stainless steel. The deep- drawing properties of these relatively new types of steel are also improved, so that they are suitable for the production of cooking vessels as disclosed in French patent FR 2594144- A.

It is an object of the present invention to provide a cooking vessel of ferritic and austenitic stainless steels with additional bottom of aluminium and a rolled laminate of mild steel coated on both sides with austenite stainless steel.

This is achieved by a cooking vessel specified in the characterizing part of claim 1.

It is a feature of the invention that by induction heating the power absorption of the laminate bottom is increased, contrary to what is known, by relatively and totally reducing the amount of magnetizable mild steel in the laminate, while at the same time the amount of non-magnetizable steel is increased correspondingly. This involves that the permeability, which is normally considered the most important criterion for efficient heating, is reduced.

This is elucidated here by some examples. Here, mild steel, as prescribed in the European standard, is chosen.

Example 2

One side of a roundel of mild steel with a diameter of 177 mm and a thickness of 12,5 mm was coated by hard soldering with an 0,6 mm thick laminate of mild steel coated with 15 % of 18-10 stainless steel.

The power absorption on the steel side was measured to 2730 watts, and on the laminate side, to 2975 watts. Example 3

The heating on an ATAG 3000 induction cooker with power settings 6-12 power absorption in kw.

24 cm saucepan

18-10 6 7 8 9 10 11 12

with 0,6 mm laminate 20 % 0,74 1,04 1,43 1,91 2,39 2,90 3,02

with 0 , 6 mm laminate 15 % 0,59 0,87 1,23 1,65 2,07 2,80 2,99

Example 4

Saucepan with a bottom diameter of 145

with 0,6 mm laminate 15 % 0,56 0,80 1,18 1,63 1,81 2,11 2,47

with 0 , 7 mm laminate 10 % 0,50 0,93 1,19 1,33 1,44 1,68 1,94

It appears from the examples that the power absorption within the specified limits increases with reduction, in terms of percentage, of the amount of mild steel and at the same time reduction of laminate thickness.

The reason why the power absorption increases, is the following:

Austenitic stainless steels have a high specific electrical resistance. It is not magnetizable, but in this case it borrows permeability from the layer within, of mild steel. All together this causes the depth of penetration to be very small, and when the specific resistance at the same time is high, the surface resistance which is critical to the power absorption, is very high. The result is high inductance, high current density and great development of heat in the surface when the material gets into a high-frequency magnet field.

Whenever the coating thickness is increased, the average linear expansion coefficient of the laminate also increases, and the bimetallic tensions are reduced.

The total effect of reduced laminate thickness, reduced tensile strength because of higher external temperature than internal, and less difference in the linear expansion coefficients of each side of the aluminium layer, cause the bottom to remain relatively stable during heating.

Example 5

A 24 cm saucepan of 0,8 mm 18-10 stainless steel with an extra bottom of 6 mm aluminium and an 0,6 mm thick laminate of mild steel coated on both sides with a layer of 15 % of the total thickness of 18-10 stainless steel, gets an inward bending of 0,33 mm on heating from 20° to 200 °C.

A saucepan of the same type provided with the same type of aluminium bottom, but with laminate coated on both sides with a 20 % thick layer of 18-10 stainless steel, gets an inward bottom movement of 0,26 mm by the same heat treatment.

Example 6

A 20 cm saucepan of 18-10 stainless steel with 4 mm of aluminium and an 0,7 mm laminate of mild steel coated on both sides with 10 % of its thickness of 18-10 stainless steel, gets a bimetallic inward bend of 0.54 mm on heating to 200 °C. A saucepan of the same size with a 5 mm aluminium bottom and an 0,6 mm thick 15 % laminate gets a bimetallic inward bending of the bottom of 0,43 mm on heating from 20° to 200 °C.

This shows that a reduction of the amount of magnetizable steel and a relative increase of non-magnetizable steel reduce the bimetallic inward bending.

From US 3,445,630 is known that a flange of a bottom layer may be welded to the wall of a cooking vessel. But this does not involve a magnetizable laminate with middle layer of mild steel and the corrosion problems this may involve.

Welding according to the present invention is inexpensive and leaves no welding marks. Three or four lugs may be embossed on the inside of the folded upper edge of the capsule bottom.

A week mark on the inside of the wall of the cooking vessel is easily removed in the internal polishing operation. The lugs are welded together simultaneously in one operation, which may be automated by simple means.

Laminate of the thicknesses in question with 15 % to 25 % thick coatings of stainless steel, can stand lug welding without any burn-through.

In the following the invention will be described with reference to the drawing in Fig. 1.

Fig. 1 shows a section of the bottom part of a cooking vessel/frying pan of stainless steel with a side wall 2, an additional bottom 4 of aluminium which is attached to the cooking vessel 1 by a layer of soldering 3, externally on the aluminium bottom there is also attached through soldering 5 a capsule-shaped laminate 6 consisting of mild steel coated on 10

both sides with a 15 % to 25 % thick layer (8,9) of austenitic stainless steel. In addition to being soldered, the laminate capsule 6 is attached to the wall 2 of the cooking vessel through lug welding 10.

Claims

C L A I M S

1. A cooking vessel (1) of stainless steels, intended for heating by induction, with a hard soldered additional bottom of aluminium (4) and a magnetizable layer (6) of a laminate of mild steel coated on both sides with austenitic stainless steel (8 and 9), c ha r a c t e r i z e d i n that the laminate (6) has a coating (8 and 9) of austenitic stainless steel on each side, of a thickness between 15 % and 25 % of the thickness of the laminate.

2. A cooking vessel (1) according to claim 1, c h a r a c t e r i z e d i n that the laminate (6) reaches a distance up on the side (2) of the cooking vessel (1) and is lug welded or spot welded to the cooking vessel wall (2) .

3. A cooking vessel (1) according to claims 1 and 2, c h a r a c t e r i z e d i n that the wall (2) of the cooking vessel (1) has a thickness greater than 1 depth of penetration at 20 KHz to 40 KHz.