GB2068117A

GB2068117A - Improvements in bi-metallic strip

Info

Publication number: GB2068117A
Application number: GB8002632A
Authority: GB
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1980-01-25
Filing date: 1980-01-25
Publication date: 1981-08-05

Abstract

A bi-metallic strip comprises two layers of metal bonded to one another, one of the layers consisting of substantially 36% by weight of nickel with the remainder being iron except for incidental impurities and the other layer being of aluminium or aluminium containing alloy additions.

Description

SPECIFICATION Improvements in bi-metallic strip This invention relates two bi-metallic strip that is to say strip comprising two bonded layers of metal of different composition. The invention is particularly though not exclusively directed to such bi-metallic strip intended for use as temperature sensitive elements in thermostats circuit breakers and like devices producing mechanical movement or to develop a force in response to temperature change.

According to one aspect of the present invention a bi-metallic strip comprises two layers of metal bonded to one another, one of the layers consisting of substantially 36% by weight of nickel with the remainder being iron except for incidental impurities and the other layer being of aluminium or aluminium containing alloy additions.

The nickel iron alloy preferably is one commercially produced under the trademark INVAR (RTM) and has the advantage of an extremely low coefficient of thermal expansion. Coupled with the bonded layerofaluminium or aluminium alloy which displays a relatively high coefficient of thermal expansion, the bi-metallic strip of the present invention will display a relatively large mechanical excursion for a given temperature change; when used in thermostats or like devices the bi-metallic strip of the invention will accordingly produce a device with a high sensitivity to temperature change.

To produce such good temperature sensitivity, the nickel containing layer should manifest the lowest possible thermal expansion for a given temperature change. Thus while a 36.0% nickel content has been found to yield the lowest coefficient and is available commercially, some variation in nickel concentration preferably within the range of 35.5% to 36.5% by weight can be tolerated without departing from the technical advantages displayed by or essence of, the invention.

Suitably the aluminium or the aluminium alloy used has a coefficient of thermal expansion lying within the range 20-24 x 1 Di6cc .

Preferably the other layer of the bi-metallic strip is of substantially wholly aluminium; howeverto provide the strip with a degree of stiffness the aluminium may contain alloying elements. Conve niently the aluminium contains manganese, magnesium, copper or silicon as alloying elements either singly or in combination and in concentrations by weight of up to 5.0% according to the mechanical characteristics required. Table 1 appended hereto identifies by way of example a number of specific aluminium alloys which are useable in the bimetallic strip of the present invention and details the range of concentration of the essential alloyele- ments silicon magnesium copper and manganese.

The table also details in addition to the essential alloy elements, the other constituents which may be present in commercially available aluminium alloys.

In the case where the bi-metallic strip of the invention is intended for use in a corrosive environment, the aluminium or aluminium alloy layer may be clad with a metal or other material displaying corrosion resistance to the appropriate environment.

Conveniently, the ratio of the thickness of the nickel containing layer to the aluminium containing layer is 1:1.5; to copensate for differences in their modulus of elasticity.

The accompanying drawing illustrates by way of example an embodiment of apparatus for producing bi-metallic strip of the invention.

In the embodiment which can be used to produce say invar-aluminium bi-metallic strip, coils of aluminium strip and invar-strip of the selected width and thickness are respectively supported on conventional uncoilers 2, 4. The aluminium strip 6 and the invar-strip 8 fed from the uncoilers at the selected line speed are each degreased in degreasers 10, 12 and subsequently have their surfaces cleaned for example by scratch brushes rotating at stations 14, 16 which also include debris removal means 8.

Strips 6 and 8 which are initially spaced are brought to close proximity at station 20 where they are heated within a furnace 22 effective to produce a strip temperature of up to 300"C. In the case where significant heating of the strip occurs during the brush cleaning operation, the heat input provided by furnace 22 may be very small or may be altogether be dispensed with. Heated strip 6 and 8 emerging from the furnace 22 is fed into the nip of bonding rolls 24 which apply sufficient pressure to produce bond ingofthealuminiumtothe invarto form the bimetallic strip of the invention. Adequate pressure applied by the rolls 24 which may be lubricated, is that producing a strip reduction within the range 30% to 60%.

After bonding the bi-metallic strip may as illustrated by the dotted line follow route 26 for winding directly on to a coiler 28. Bi-metallic strip coiled directly after bonding will in general provide strip having mechanical characteristics capable of mild forming only. The strip may however in addition be subsequently batch heat treated in a nitrogenhydrogen atmosphere containing upto 5% hydrogen and at a temperature of up to 400 in order to improve the bond between the layers. After thins batch heat treatment the strip maybe rolled to achieve the required gauge and mechanical characteristics.

In an alternative embodimentofthe invention also illustrated in the drawing the bi-metallic strip emerging from the bonding rolls 24 is passed through continuous heat treating furnace 30 effective to raise the temperature of the strip to up to 550"C. To prevent oxidation the atmosphere within the furnace again contains nitrogen with a small proportion of up to 5% of hydrogen. After heating the strip is cooled, conveniently by passing through a cooling zone incorporated in the furnace 30 and is again directly fed through to a coiler 32. The strip on coiler 32 may be subsequently cold rolled to produce the required gauge and mechanical properties.

In the case where a heat treatable aluminium alloy is required as the high expansion component, for example alloys HS15 HC15 or HS30, the bi-metallic strip will be passed directly from the furnace through a quenching bath 34 suitably containing water as the quenching medium before passing to the coiler 32.

In this event the cooling zone attached to the furnace 30 will not be required. In strip so quenched, the aluminium alloy layer will be relatively soft and can more easily be rolled to the required gauge or formed to whatever shape is required. The required mechanical properties in the aluminium alloy layer can then be restored by ageing at room temperature or at atemperature within the range 120-200"C.

It will be appreciated that while the apparatus has been described with reference to the production of invar-aluminium strip it may equally be used to produce anyotherstrip having layers of different metal composition.

It will also be appreciated that while the invention has been described with reference to strip applied to coilersthe strip on the coilers may be subject to any finishing process which may include further cold reduction and or heat treatment in suitable furnaces. TABLE 1

a) a) =0) TER/AL DES/GNA TION E o, =E,,, " E =E E = E . Eco.- & = a) = = - E 0) 0) BRITISH 0 = C E Es .s STANDARD ISO < - Co N sf h SIB ! B199.5 99.5 0.05 - 0.3 0.4 0.05 - - - 0.05 1 - | ~ 24 NS3 AiMni Rem. 0.1 0.1 0.6 0.7 0.8 0.2 - - 0.2 - 23 1.5 NS4 AIMS2 Rem. 0.10 1.71 0.5 0.5 0.5 0.2 0.25 0.5 - 0.2 24 2A NS5 AIMg3.5 | Rem. 0.10 31/ 0.5 0.5 0.5 0.2 0.25 0.5 - 0.2 23 3.9 NS8 AlMg4.5Mn | Rem. 0.10 44o9l 0.40 0.40 1 05l 0.2 0.25 - - 0.2 23 4.9 1.0 HS15 AlCu4SiMg ' Rem. 3.9/ 0.2/ 10.5, 0.7 0.41 0.2 0.10 - - 0.2 22 l 5.0 0.80.9 1.2 5.0 AlCu4SiMg Rem. 3.9/ 0.2/ f 0.5/ 0.8 0.4/- 0.9 0.10 ~ ~ 1.2 HC1SH I 0.2 0.10 0.2 22 HS3O AlSi1MgMn Rem. 0.10 1 0.71 0.5 1.0 0.2 0.25 - - 0.2 23

Claims

1. A bimetallic strip comprising two layers of metal bonded to one another, one of the layers consisting of substantially 36% by weight of nickel with the remainder being iron except for incidental impurities and the other layer being of aluminium or aluminium containing alloy additions.

2. A strip as claimed in claim 1 wherein the concentration of nickel within the one layer lies within the range 35.5% to 36.5% by weight.

3. Astripasclaimed in claim 1 or 2 wherein the one layer is of INVAR (RTM).

4. A strip as claimed in any preceding claim wherein the composition of the other layer is selected to produce a coefficient of thermal expan sion lying within the range 20-24 X.! D-6/"C.

5. A strip as claimed in any preceding claim wherein the other layer is substantially wholly of aluminium.

6. A strip as claimed in any one of claims 1 to 5 wherein the other layer embodies as alloy constituents one or more elements selected from the group consisting of manganese, magnesium, copper and silicon.

7. Astrip as claimed in claim 6 wherein the concentration of any alloy element in the other layer is up to 0.5% by weight

8. A strip as claimed in any preceding claim wherein the ratio of the thickness of the first layer to that of the other layer is 1 to 1.5.

9. A strip as claimed in any preceding claim wherein the other layer is clad with a metal displaying corrosion resistance to a selected environment.

10. A bimetallic strip substantially as hereinbe fore described with reference to any one of the examples.

11. A method for producing a bimetallic strip comprising two layers of metal bonded to one another, one of the layers consisting of substantially 36% by weight of nickel with the remainder being iron except for incidental impurities and the other layer being of aluminium or aluminium containing alloy additions, the method including heating a strip of the first layer and a strip of the other layer and feeding the heated strip into the nip of a compaction roll to produce bonding of the layers.

12. A method as claimed in claim 11 wherein each strip is heated to a temperature of up to 300"C.

13. Amethod as claimed in claim 11 or 12 wherein the surface of each strip is cleaned before heating.

14. A method as claimed in any one of claims 11, 12 or 13 wherein the bimetallic strip produced after bonding is heat treated in a reducing atmosphere.

15. A method as claimed in claim 14 wherein the heat treatment is a batch heat treatment at a temperature up to 400"C.

16. A method as claimed in claim 14 wherein the heat treatment is a continuous heat treatment at a temperature up to 550"C.

17 A method of producing a bimetallic strip sui- stantially as hereinbefore described with reference to the accompanying drawing.

18. A bimetallic strip produced by the method as claimed in any one of claims 1 to 17.