US1991438A - Thermostatic metal - Google Patents

Description

Feb. 19, 1935. c. R. E. WOHRMAN 1,991,438

THERMOSTAT IC METAL Filed Dec. 9, 1931 (ALL. x1o'). 16 18 2o 22 24 26 2a COEFFICIENT 0F THERMALEXPANfiION 2 4 6 a 10 TEMPERATURE (oceness CENTIGRADEL,

Patented Feb. 19, .1935

. UNITED STATES THERMOSTATIC METAL Carl R. E. Wohrman, 'Attlcboro, Mass., assignor to General Plate Company, Attleboro, Mass., at corporation of Massachusetts Application December 9, 1931, Serial No. 579,922 7 Claims; (01. 297-15) This invention relates to thermostatic metal, and with regard to certain more specific features,

to thermostatic metal particularly adapted for.

use under corrosive conditions, and/or at high temperatures.

Among the several objects of the invention may be noted the provision of thermostatic metal of the class indicated which is capable of successfully withstanding the attack of corrosive agencies, both liquid and gaseous, especially at intermediate temperatures (300-900 F.) and high temperatures (900 F. upwards) the provision of thermostatic metal of the class described which is capable of successful operation at temperatures in excess of 1000 F.; and the provision of thermostatic. metal of the class described which is readily formed from inexpensive, readily available material. Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction and composition, and arrangements of parts, which will be exemplified in the structures and products hereinafter described, and the scope of the application of which will be indicated in the following claims.

The accompanying drawing is a graph illustrating the temperature-expansion characteristics of certain alloys.

While a great variety of thermostatic metals have been and now are in use in temperature con-'- trolled devices, up to the present time noneof the metals have been capable of successfully withstanding the attack of corrosive agencies generally. Attempts have been made to protect thermostatic metal elementsby electroplating them, or otherwise plating them, with brass or a similar corrosion-"resistant medium. However, the porosity of such plating is objectionable; also, the fact that each element must be individually plated and polished makes such procedures quite impracticable. v Welded or soldered plating ordinarily leaves the edges of the element unprotected, and even if the edges are protected, piercing of the element as for mounting exposes the region around the piercing to attack. Further, platings of the above characteristics prove a deterrent to proper functioning of the element, and objectionably decrease its sensitivity.

Further, there has been up to the present time no thermostatic metal which operates successfully and consistently at temperatures in excess of 1000 F., retaining its elasticity at such temperatures and being at the same time free from,destructive oxidation and scaling.

The present invention provides thermostatic metal which is adapted for use at the high temperatures in question, and which also successiully withstands the attack of such corrosive agencies,

thereby overcoming both of the above dlsadvan tages.

' While the present invention will be described in connection with the-more usual thermostatic bimetal, comprising a composite sheet having two layers, each layer having a different temperature coeflicient of expansion, it is to be understood that the invention is likewise applicable to trimetals,

or any multimetal or like forms. i

Proceeding with the description of the present invention as applied more particularly to thermostatic bimetal, the individual metal layers will be considered.

The high expansion metal layer according to the present invention comprises a 'nickel-chrd mium steel of the stainless or heat-resisting variety. The range of composition of such nickelchromium steel is approximately as follows:

' Per cent Ni 7-40 Cr 14-40 C 0-3 Mn 0-3 W 0-15 P below 0.05 S below 0. 05 Fe "balance Phosphorus and sulphur are unavoidably pres-. ent in small amounts as impurities.

Molybdenum, silicon, cobalt, and vanadium may be added in amounts up to the order of 10% of each to improve the quality of the metal for special in stances.

Nickel-chromium steels of the class indicated are characterized by their high resistanceto corrosion at ordinary as well as at elevated temperatures. This property permits the use of a thermostatic metal made from such nickel-chromium steels in corrosive media such as ammonia, sulphur-bearing fumes, acid and alkaline liquors, corrosive waters and oils, and sundry organic liquors and gases and the like. These nickelchromium steels have at the same time an abnormally high resistance to oxidation and scaling ,at high temperatures.

Nickel-chromium steels of the class described have a temperature coeflicient. of expansion of about 16 x 10- per degree C. forthe tempera-' ture interval 0 to 200 C., and of about 20 x 10- for the temperature interval 0 to 1000 C. (See shaded area A of the drawing). Thus the coeflicient of thermal expansion of such steels compares favorably with that'of the Monel metal now extensively used in thermostatic metals in the intermediate temperature field.

} These nickel-chromium steels are adapted particularly for use as the higher expansion member of the thermostatic metal, the particular analysis (and heat treatment) being varied for specific applications within the approximate limits indicated. As a non-limiting example, one suitable composition for use with low and inter- Another non-limiting example, being a suitable composition for high temperature (900 to 1500) thermostatic metal is as follows:

Per cent N1 Q Cr 17 Mn 0.6 Si s 3 W i .15 C below 0.15 S below 0.04 P below 0.04

Fe balance,

The nickel-chromium steels above described may be used with any suitable lower expansion member or members. For high temperature use, the stainless chromium irons hereinafter described are recommended. For low and intermediate temperatures, the currently employed nickel irons of the invar group containing some 36% to 48% nickel may be used, and, if necessary,

. of corrosion in question.

It will be understood that in many cases only one side of a thermostatic metal need be corrosion resistant, as in installations where the thermostat is mounted in the walls of the vessel and only one face is exposed to the corrosive medium. In such instances, the nickel-chromium steels described are desirable as the component metal coming in contact with the corrosive media, and it will beseen that any other suitable metal may be used as the other component of the thermostatic metal, it not being necessary for it to have, under such conditions, the quality of resisting corrosion.

The recommended metal for use as the lower expansion component in a high temperature thermostat hereinbefore indicated as chromium iron, comprises an alloy of that general description containing in excess of 14% chromium. The range of composition of such chromium iron is substantially as follows: I

Per cent lCr 14-30 Mn 0.3-0.7 C 02.5

W 0-15 P below 0.05 S below 0.05 Fe balance Molybdenum, silicon, cobalt, vanadium, and

um steel described, a suitable composition is as follows: I

Per cent Cr 1'? Mn 0.3 Si 0.8 C 0.06 P below 0.04 S below 0.04 Fe balance Another non-limiting example, adapted particularly for use .with the second nickel-chromium steel described, is as follows:

Chromium irons of the class indicated are characterized by their resistance to corrosion by a great variety of corrosive agencies at ordinary as well as at elevated temperatures and are able to stand occasional temperatures up to 1100 C. without defective oxidation or scaling. The chromium content is varied in accordance with the maximum temperature of which it is desired to use. A higher chromium content endows the alloy with a higher resistivity to corrosion at high temperatures. Conversely, a lower chromium content is sufficient in alloys which are to be used at lower temperatures.

Because the temperature coefiicient of expansion of chromium irons of the class described is about 10x10 for the interval 0-200 C. and about 12 x 10 for the temperature interval 01000 C. (See shaded area B of the drawing.) It will be seen that this coefiicient is considerably lower than that of the nickel chromium steel hereinbefore described, and it is thus a suitable metal to plate with the nickel chromium steel to form a thermostatic bimetal.

This temperature coefficient of expansion, however, is considerably higher than the temperature coeflicient of expansion of invar or 42% nickel steel now extensively used, in the low and intermediate temperature fields, but is, however, lower than the coefficient of these metals at temperatures above the order of 350 C. Line C of the drawing represents a typical 42% nickel steel. Chromium irons, accordingly, are preferable even to the invar or 42% nickel steel for thermostatic metals operating at 350C. or higher, even when the highly desirable corrosion resistivity characteristic is left out of consideration.

The higher expansivity of the chromium irons for temperatures up to 350 C. has the advantage of preventing the building up of unnecessary stresses in elements intended for use at, say, 500 C. and higher. In addition, there is sufiicient deflection at lower temperatures to make the metal useful .in elements adapted for such lower temperatures, especially in thermostats of a. devel-= opable character in which small deflections and stresses are desired in order to prolong the life of the element.

While the chromium irons above described are used primarily as the lower expansion member in conjunction with the nickel chromium steels hereinbefore described, they are likewise adapted for advantageous use with other high expansion members, such as Monel metal. nichrome, and

without departing from the scope of the invention, it is intended that. all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim:

l. Thermostatic metal comprising a plurality oi layers, the higher expansionof said layers having a composition substantially as follows:

Per cent Ni 8-9 Cr 18-19 n o6 Si 0.5 W--- 0.1 Mo--- 3 C below 0.05 S below 0.04 P below 0.0% F balance and the lower expansions? saicllayers having a composition substantially as follows:

Per cent Cr 1'? Mn 0.3 Si 0.3 Q 6.06 v P below 0.04

S. below 0.04 F balance 2. Thermostatic, metal comprising a plurality of lays the higher expansion of said layers havmg a composition substtially as follows:

Percent Ni 25 Cr 17 m 6.6 Si 3 W I 0.15 C below 0.15 8 4 belowiiili P below 9.04 F balance and the lower exsoisi i or said layers having a composition sritia v as follows:

Per t Cr "r so Mn 0.3 El--- :3

. 0---- 0.07 W 0.06 P I below 0.04 s below 0.0% F balance 3. Thermostatic metal comprising a plurality of layers, the higher coefiicient of expansion layer having a composition substantially as follows:

Per cent N 8-9 Cr 18-19 Fe "balance and the lower coemcient of expansion layer having a composition substantially as follows:

Percent Cr 1! F balance 4. Thermostatic metal comprising a plurality of layers, the higher coefficient or expansion layer having a composition substantially as follows:

Per cent Ni "1-40 Cr. 14-40 Fe balance and the lower coemcient of expansion layer having a composition substantially as iollowsz Fer cent Cr; 7 1 lei-30 Fe balance 5. Thermostatic metal as set forth in claim 4. in which the composition of the higher coemcient of expansion layer is substantially as follows:

Per cent. Ni 8-9 Cr 18-19 Mn- 0.6 Si 0.5 W- 0.1 Mol 3 c l... below one S- .4 below 9.0% P below cos Fe balance 6. Thermostatic metal as set forth in claim 4. in which the composition of the higher coeficient of expansion layer is substantie. as iollows:

. For cent Ni. 25 Cr 17% Mn- 0.6 S 3 W 0.15 G below $.15 S- below are P; below 0.0% F balance 7. 'i'lieostatic metal as set forth in. claim 4, in which the composition of the lower coefilcient oi espion layer is substantially as follows:

Per cent

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