US1416436A - Electrical heating element - Google Patents

Description

PERCY A. E. ARMSTRONG, OF LO'UDONVILLE, NEW YORK.

'ELECTRICAL HEATING- ELEMENT.

Specification of Iietters Patent.

No Drawing. Application filed May 1, 1919, Serial No.293,927. Renewed October 1, 1921. Serial No. 504,676.

To all whom it may concern Be it known that I, PERCY A. E. ARM- STRONG, a subject of the King of Great Brit- .loyed with iron and carbon.

ain, and a resident of Loudonville, county of Albany, State of New York, have invented certain new and useful Improvements in Electrical Heating Elements, of which the following is a specification.

My invention relates to electrical heating elements.

I have discovered that alloys of iron and chromium have high specific resistance and are practically non-oxidizing and non-corrosive up to about 1300 F. Higher temperatures can be used, but the properties of these alloys are not so desirable for electrical heat ing units at temperatures substantially higher than 1300 F.. say, from about 1600-1800 F., particularly when run continuously for many hours at a time.

The quantity of chromium used depends on the uses to which the heating element is put.

For electrical heating elements to be run at comparatively low temperatures, the proportion of chromium may be low, say from about 4% upwards. For higher tem eratures, the proportion of chromium shou d be greater, and from about 12% of chromium and upward for'red heat elements gives good results. At present relative costs, about 30% to 40% chromium content is about the limit for economical production but. if desired, more chromium can be used, pract cally u to 95% or even 100% chromium, though it is to be understood that I do not claim the use of chromium per se as an electrical heating element, but only chromium when al- Also the higher ChIOL lum material is more difiicultto work mechanically.

When iron-chromium alloys are worked inheated state, as by forging, for example, there is a considerable production of oxides, scale and the like. When electrically heated,

however, without working. these materials are substantially free from oxidation and corrosion and, having substantially high specific resistance, are well adapted f r use as electrical heating elements. The fact at they oxidize and scale when worked at high temperatures, together with the known susceptibility of iron itself to scaling and OK!- dizing at high temperatures,

been responsible for failure heretofore of has apparently investigators in this field to discover th adaptability of these alloys for use as electrical heating elements.

The carbon content preferably does not exceed a maximum of about 1% which is permissible in alloys containing about 12% of chromium or over, though lower carbon, say from about .10%.60% is better in such material. Silicon up to about 45% in the material having 12% or more of chromium and proportionately lower silicon in lower chromium material down to about 20% or less in 49;- chromium material does not materially affect the alloy when made use of as a heating element.

Other metals, such as nickel. molybdenum, tungsten. etc.. may be present in small quantities up to about 2-47 without marked effect on the qualities of the heating elemeutj Small quantities of various lmpurities such as phosphorus, copper, tin and'the like, may, of course. be present, but are preferably reduced to the minimum conformable with economical production of the material.

It will be seen that my new heating element containing as it does, a large proportion of iron, is of low cost as compared with the expensive alloys heretofore used for this purpose. I find, for example, that a heating element such as described and containing only about 14% of chromium and the principal part of the remainder iron has substantially the same qualities and usefulness as a heating element containing sev-- .6% of carbon and the principal part of the v remainder iron.

In testimony that I claim the foregoing I hereto set my hand, this 29th day of April, 1919.

. PERCY A. E. ARMSTRONG.