US3045335A - Rustless and refractory bimetallic strip for high temperatures - Google Patents

Description

y 1962 A. G. CHERREAU ETAL 3,045,335 RUSTLESS AND REFRACTORY BIMETALLIC STRIP FOR HIGH TEMPERATURES Filed Nov. so, 1959 20 Fig. 1

i F' g 2 20 i Fig. 3

United States Patent 3,045,335 RUSTLESSS AND REFRACTORY BIMETALLEC ST lFGR HllGll-ll TEMPERATURES Andre Gaston Cherreau, Employ, Andre Rene Michel Girard, Nevers, and Guy Lucien Robert Maingault, Imphy, France, assignors to Societe Metallnrgique dlmphy, Paris, France, a company of France Filed Nov. 30, 1959, Ser. No. 856,111 Claims priority, application France Dec. 4, 1958 6 Claims. (Cl. 29-1955) Numerous Varieties of bimetallic strips are known. These strips are formed by the association of two strips closely joined to each other, whose dilatabilities are different enough to allow the strip to incurve under the effect of temperature changes.

The extent of the distortion of the bimetallic strip depends on its sensitiveness, which is defined by the relation:

V is the Villarceaus coefficient.

a and a the dilatation coefficients of the most dilatable and the least dilatable constituents.

The bimetallic strip effect enables a displacement to be obtained and a stress to be developed when there are temperature variations, it is frequently utilized in recording, regulating or safety devices.

The evolution of techniques entails more and more severe requirements, and sets new problems. It is necessary that bimetallic strips:

Can be utilized at relatively high temperatures,

Have a deflection proportional to the differences in temperature throughout the entire field of temperatures used. In case, the deflection-temperature curve is a straight line,

Should resist somewhat injurious atmospheres,

Should allow the most diverse parts to be made, thanks to a satisfactory malleahility and especially an excellent aptitude for bending, While retaining sufficient mechanical characteristics,

Should have a perfect physico-chemical stability.

The field in which known bimetallic strips can be employed is generally limited to temperatures in the region of 400 to 500 centigrade, and not one of them complies with the above-mentioned requirements.

The present invention completes the range of existing bimetallic strips.

It must be borne in mind that the mere knowledge of each of the two alloys forming a bimetallic strip does not enable the properties of the bimetallic strip to be prejudged. Actually, it is indispensable, to obtain perfect functioning, that the transition zone between the two constituents assembled hota zone whose chemical composition is mixedshould possess mechanical and dilatometiical properties which do not run counter to forecasts. For example, it would seem at first sight that by selecting iron or mild steel as the slightly dilatable constituent, and as the most dilatable constituent the well-known ferrous alloy of 18% Cr and 8% Ni, we can have a normally utilizable bimetallic strip. Now, this is not so, because the welding not gives rise, between the ferri-tic and austenitic constituents, to an intermediate martensitic zone which is unstable and brittle. Research work and experiments have alone been able to show that this disadvantage does not exist with the bimetallic strip forming the subject of the invention. This bimetallic strip is rustless, useful at high temperatures and has a proportional deflection. It can be used up to 650 centigrade and formed by a slightly dilatable element of a ferrous alloy essentially of copper-chrome, containing 18 to 25% 3,045,335 Patented July 24, 1962 CC V Cr and, 0.3 to 2.5% Cu, associated wtih a highly dilatable alloy element formed essentially of iron, nickel and chrome containing 40 to 50% Ni and 17 to 28% Cr. Moreover, the slightly dilatable alloy may contain a maximum of 0.8% C, 1% Si, 1% Ni, 1.5% Mn and the highly dilatable alloy, a maximum of 0.5% C and 1.5% Mn with 0.5 to 3% Si.

Furthermore, these two alloys have the usual contents of impurities.

FIGURE 1 attached gives the dilatation curves of the two alloys mentioned above.

FIGURE 2 reproduces in function of the temperature shown in abscissae, the deflection curve of the bimetallic strip obtained by the association of the above-mentioned alloys, this deflection being the displacement measured in millimetres of the free end of a bimetallic strip 100 mm. long and 1 mm. thick embedded in the other end.

The bimetallic strip according to the invention complies with the requirements demanded.

Its deflections are proportional to temperatures up to 650 centrigrade.

It possesses the following physical and mechanical properties:

Sensitiveness Villarceaus coefiicient: V=7 10- (which corresponds to a specific deflection of 0.035)

Resistivity: By way of information, microhms-cm.

Elasticity moduli centigrade):

At 20 kg./mrn. 19,500 At "kg/mm? 19,000 At 200 "kg/mm?" 18,400 At 300 kg./mm. 17,800 At 400 kg/mm?" 17,100 At 500 kg./mm. 16,500 At 600 "kg/mm? 12,100

Mechanical characteristics:

After annealing for half an hour at 650 C. and air c0oling- Elastic limit kg/mm?" 73 Breaking load kg./mm. 85 Elongation "percent" 9 After annealing for half an hour at 700 C. and air cooling- Elastic limit kg./mm. 60 Breaking load kg/mm?" 75 Elongation percent 14 Permissible fatigue rate (bimetallic strip annealed for /2 hr. at 650):

At the ambient temperature, S:

about kg./mm. 25 At 300, S=about "kg/mm? 10 At 500, S=about kg/mm? 1.5 At 600, S= about kg./mm. 0.7

Thus, the scope of the temperature employed may be extended up to 650 centigrade when no stress is required, and preferably up to 600 centigrade when the bimetallic element must exert a mechanical stress of appreciable value.

In actual practice, certain kinds of bimetallic strips for which the maximum working temperature is 500 centitures, because its sensitiveness remains constant up to its maximum working temperature.

The constituents are high temperature alloys resisting oxidation and sulphurous atmospheres.

After annealing at 650 to 700 C. for at least half an hour, the physico-chemical stabilization of the constituents is perfect and its mechanical characteristics are equal to or exceeding that of bimetallic strips that must be used after a low temperature stoving. In particular, the elongation is suflicient to allow of complicated shapes to be obtained, without the risk of surface cracks forming.

The properties stated above for the bimetallic strip according to the invention are most invaluable in certain cases:

As a first example, we would mention the making of discs stamped in the shape of a spherical cap which must be turned over when the temperature reaches a relatively high given value. It is known that the displacement of the summit of the spherical cap in relation to its initial position occurs by an affected hysteresis cycle, as shown in the example given in FIGURE 3, which gives, in function of the temperature, the displacement in millimetres of the summit of a spherical cap. If the kind of bimetallic strip, the thickness and diameter of the disc, are carefully chosen, it is only necessary to adjust the stamping depth so that the equilibrium of the tensions causes the abrupt turning over of the spherical cap for heating or cooling at the required temperature.

With known bimetallic strips, it is not possible to produce parts that turn over at a relatively high temperature and possessing a satisfactory difference between the turning over temperatures at heating or cooling, because:

Their field of employment is limited to temperatures in the region of 500 C.,

Their parasite tension cannot be eliminated by mere stoving,

Their high sensitiveness leads to too great a stamping depth.

The new bimetallic strip, which can be subjected to real annealing, enables the problem to be easily solved. The weakness of its Villarceaus coefiicient, compared with that of other metallic strips, becomes an interesting characteristic.

Another example in which the bimetallic strip according to the invention enables a distinct improvement to be obtained is that of regulating gas appliances of all kinds, in which the bimetallic strips are liable to be speedily deteriorated by flames.

Although the whole of the characteristics stated can be obtained by utilizing the entire field of the compositions that have been mentioned, it is generally preferable to employ the following compositions:

For the slightly dilatable alloy- Cr, 20 to 25% Cu, 0.3 to 2% For the highly dilatable alloy Ni, 43 to- 48% Cr, 20 to 26% Within these limits, the following examples are especially interesting:

For each of the above-mentioned examples, the bimetallic strip was annealed at a temperature of 650 to 700 for at least half an hour.

We claim:

1. Rustless, high temperature bimetallic strip with proportional deflection utilizable up to 650 C., formed by a slightly dilatable element combined with a highly dilatable element, both of a ferrous alloy, characterised by the fact that the slightly dilatable element consists essentially of 18 to 25% of chromium and 0.3 to 2.5% of copper and balance Fe, and that the highly dilatable element consists essentially of 40 to 50% of nickel and 17 to 28% of chromium and balance Fe.

2. Bimetallic strip according to claim 1, characterised by the fact that the slightly dilatable element includes a maximum of 0.8% of carbon, 1% of silicon 1% of nickel and 1.5% of manganese.

3. Bimetallic strip according to claim 1, characterised by the fact that the highly dilatable element includes a maximum of 0.5% of carbon, 1.5% of manganese and from 0.5% to 3% of silicon.

4. Bimetallic strip according to claim 1, characterised by the fact that the slightly dilatable element consists essentially of from 20 to 25% of chromium and from 0.3 to 2% of copper and balance Fe, and that the highly dilatable element consists essentially of from 43 to 48% of nickel and from 20 to 26% of chromium and balance Fe.

5. Rustless, high temperature bimetallic strip with a proportional deflection utilisable upto 650 C., formed of a slightly dilatable element combined with a highly dilatable element, both of a ferrous alloy, characterised by the fact that the slightly dilatable element consists essentially of 0.24% of carbon, 0.40% of silicon, 0.40% of manganese, a maximum of 0.40% of nickel, 21.5% of chromium and 1.2% of copper and balance Fe, and that the highly dilatable element consists essentially of a maximum of 0.10% of carbon, 2% of silicon, 1.2% of manganese, 46.5% of nickel and 23% of chromium and balance Fe.

6. Rustless, high temperature bimetallic strip with a proportional deflection utilizable up to 650 C., formed of a slightly dilatable element combined with a highly dilatable element, both of a ferrous alloy, characterised by the fact that the slightly dilatable element consists essentially of 0.10% of carbon, 0.20% of silicon, 0.30% of manganese, a maximum of 0.40% of nickel, 25 of chromium, and 0.4% of copper and balance Fe, and that the highly dilatable element consists essentially of a maximum of 0.10% of carbon, 1.7% of silicon, 0.8% of manganese, 46% of nickel and 21% of chromium and balance Fe.

References Cited in the file of this patent UNITED STATES PATENTS 1,991,438 Wohrman Feb. 19, 1935 2,461,518 Chace Feb. 15, 1949 2,770,870 Mooradian Nov. 20, 1956

Claims (1)

1. RUSTLESS, HIGH TEMPERATURE BIMETALLIC STRIP WITH PROPORTIONAL DEFLECTION UTILIZABLE UP TO 650*C., FORMED BY A SLIGHTLY DILATABLE ELEMENT COMBINED WITH A HIGHLY DILATABLE ELEMENT, BOTH OF A FERROUS ALLOY, CHARACTERISED BY

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