US2282163A

US2282163A - Treatment of silicon-iron alloys

Info

Publication number: US2282163A
Application number: US379481A
Authority: US
Inventors: Stephen L Burgwin
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1941-02-18
Filing date: 1941-02-18
Publication date: 1942-05-05
Anticipated expiration: 1959-05-05

Description

Patented May 5, 1942 TREATMENT OF SILICON-IRON ALLOYS Stephen L. Burgwin, Wilkinsburg, Pa., assignmto Westinghouse Electric & Manufacturing. Company, East Pittsburgh, Pa., a corporation oi Pennsylvania No Drawing.

6 Claims.

Application-February. 18, 1941, Serial No. 379,481

employed for heat treating the silicon-iron al- J loys and fabricating them into strip form suitable ior use as the magnetic cores of electrical apparatus, The known processes and methods have for their objective the development of the magnetic characteristicsof the alloy by reducing the impurities in the alloy, relieving working strains and/or imparting a preferred orientation to the grain structure of the alloy. The silicon-iron alloys now commercially produced in strip form have excellent magnetic characteristics.

In a number of commercial installations of electrical apparatus, such'as large power transformers which utilize the now commercially produced silicon-iron alloy in strip form as the magnetic core member, it has been found that undesirable and objectionable noise is developed when the transformer is energized. The noise developed in a transformer can be traced directly to the magnetostriction oi the silicon-iron alloy strips utilized as the core member of the transformer. The magnetostriction of magnetic material is recognized as being the expansion and contraction of the magnetic material when subjected to a magnetizing force. For the purpose of this invention, the kind of magnetostriction of particular interest and the quantity measured is or the change in length per unit length in the direction of magnetization under the magnetizing force applied.

An object of this invention is to provide for reducing the magnetostriction or silicon-iron alloys which have been fabricated into strip form and treated to develop the magnetic characteristics of the alloy.

A more specific object of this invention is to subject silicon-iron alloys which have been fabricated into strip form and treated to develop the magnetic characteristics of the alloys to a heat treatment suitable for placing the alloys under tension to effectively reduce the magnetostriction of the alloy strip.

Other objects of this invention will become veloping the magnetic characteristics of such alloys have been commercially practiced in the past. For example, silicon-iron alloys containing up to about l of silicon are commercially produced in strip form by subjecting them to different heat treatments including the step of cold rolling them to thermal stages and then subjecting the resulting strip to an anneal at a temperature'of 900 C. or higher to develop the magnetic characteristics of the alloy. Silicon-iron alloys containing higher contents of silicon up to about 6% are not being commercially produced by cold rolling processes, since the higher silicon content renders them somewhat brittle and it has, therefore, become cuss tomary to hot work such alloys to the final stages after which a final anneal is employed for relieving working strains.- Other methods are well-known and practiced for developing the magneticcharacteristics of the silicon-iron alloys containing up to 6% of silicon.

In order to reduce the magnetostriction oi the commercially produced silicon-iron alloys containing up to 6% of silicon, in accordance with this invention the commercially produced alloy strips are subjected'to a further heat treatment for permanently placing the alloyed components of the strip under tension to effectively reduce the magnetostrictionof the commercially produced alloys. The heat treatment of this invention consists in heating the commercially produced alloy strip at a preferred temperaturev between 700 C. and 1200" C. in the presence of a component which is capable of reacting with the silicon-iron alloy to form a thin adherent film on the surfaces of the strip, the thin film having a linear thermal coefflcient of expansion less than the linear thermal coeflicient of expansion oi the silicon-iron alloys so that in the cooledstate the adhering film is present in compression while the silicon-iron alloy is maintained under tension. Many reagents may be employed in forming the adhering film on the silicon-iron alloy strip, it being essential, however, that the reagent be selected to produce a resulting film having a linear thermal coeflicient of expansion of less than the coeiiicient of expansion of the silicon-iron strip on which it is formed.

apparent from the following description and the In a preferred method embodying the teachings of this invention, the treated alloy strip is subjected to a heat treatment at a temperature between 700 C. and 1200 C. in an oxidizing atmosphere, such as air, moist air or carbon dioxide atmosphere or a combination thereof for a period' of time of not more than 8 minutes, the oxidizing atmosphere reacting with the alloy to form a thin film of oxides which adhere to the alloy strip. The oxide film usually contains silicates. Any suitable furnace may be employed for heating the strips to the desired temperature as they are treatment at less than that time which will cause fiaking of the oxide when the strip is cooled.

In a modification of the method described hereinbeiore, tension'is applied to the strip dur ing theheat treatment for the purpose of insur ing that the alloying components will be so; strained that as. the strip is cooled, the film of oxides formed on the strip during the heat treat; ment will retain th alloying components under tension even afterthe external tension is removed. In applying tension during the heat tratment, it is desired, howevento maintain the.

The heat treatment given each of the samples in an atmosphere of air is indicated in the above table. In all cases thereis a definite improvement in the magnetostriction values obtained after the commercially produced strip has'been treated in accordance with this invention. From the results given, it is evident that a preferred treatment is to oxidize the commercially produced strips at a temperature between 925 C. and 1100C. vfor a length of time ranging between one and four minutes.

The films formed by the oxidizing treatment are of the order of 2 10- inches thick. The

* films formed have a definite effect inplacing the tension below theelastic limit of the. alloy strip, a

' it being foundthat tensions of the order of 500 to 2 000 lbs. per square inch during the heating cycle is s ufficient for cooperating with the oxide film for effectively reducing the .magnetostriction of the alloy strip.

It is not known exactly what change takes place inthe components or structure ofthe alloy when the adhering film is formed thereon and the strip is cooled. The results obtained, however, illustrate quite definitely that the combination of the thin film under compression with the silicon-iron alloy under tension effectively reduc es the magnetostriction of the commercially produced silicon-iron strip, l i In determining the effect of ,the thin films formed on the commercially produced strips, dif:

ferent methods have been :developedfor measur- I ing the ,magnetostrict-ion'of the alloy strips." The results obtained by the different methods are quite close and for this reason,-it is believed to be unnecessary to describe the different methods of measuring the-magnetostriction of the alloy strip when treated in accordance with this in-. vention. Two methods for measuring the mag;

netostriction are, however, described in my 00- pending application, Serial No; 385,759, filed March 28, 1941, the methods described thereirf being suitable for measuring the magnetostriction of the strips treated by the method of this inven-l tiOIl'. i

In order to illustrate the improvement in the: magnetostriction of commercially produced strips when subjected to the treatment describ'ed the followingtable may be referred to as givingfthe;

magnetostriction values,-that is, change in length; per length at a given magnetization of l silicon-iron alloy strips. after the commercial mill anneal and after it has been given'the oxidizing treatment for forming the thin adhering} films thereon in accordance with this invention.

silicon-iron alloy under tension and the stresses 7 the other. This has been accomplished by electrolytic etching the filmirom the strip with a 10% solution of sulphuric acid, measuring the bending-of :the strip "and then calculating the tensile stress'based on the bending of the strip;

Tensions of the order of 1200 lbs. per square inch have been found in stripstreated in accordance with this invention. v

In another embodiment of this invention, sodium silicate was employedfor applying a film during the heat treatment for effectively reducing the magnetostriction "of the commercially produced alloys; In employing the sodium silicate, the commercially produced alloy strip is coated with a thin film of sodium silicate solu- .tion and-then'dr-ied at about 200 C., after which the coated strip issubjected to the heat treatment at a temperature between 875 C. and 1000.C.'in a'ir fora period of time of about two minutes. When cooled,'it is iound that a very adherent film is formed on the treated strip and when the magnetostriction of the strip is measured, it is found that a definite improvement in the magnetostriction is obtained. In the following table, there is given the average magnetostriction values obtained on a number of commercially produced l silicon-iron alloy strips before and after the sodium-silicate treatment described.

Avcrngcmagnctosi rim ion 5X10 -minutes in air.

'l'cst number B'=l0000 B=l-l000 Mill Mill anneal Tl'LfitLd anneal Treated Each of tests58, 59 and 60 is the average of a number of tests on strips of the same composition which were subjected to the heat treatment at 900 C. for two minutes in air while strips upon which the results for test 61 are based were of a carbonate, sulphate or'silicate solution. The

coating formed of the solution will evolve oxygen during the heat treatment to effect an adjustment in the oxygen content of the annealing atmosphere and thereby control the formation of the adhering oxide film on the alloy strip. Strontium carbonate can be effectively employed in this manner for controlling the formation of the adhering oxide film on the strip. Further, by

'employing the superficial films formed from the solutions of carbonates, sulphates or silicates, it is possible to utilize hydrogen or nitrogen or combinations thereof as the annealing atmosphere, the oxygen evolved from the superficial films being sufficient to effect the formation of the adhering oxide films to establish stresses in tension in the alloystrip and reduce the magnetostriction of the strip. v

From the results given hereinbefore, it is quite evident that the method of treating the siliconiron alloy containing up to 6% of silicon which has been fabricated into strip form and treated to develop its magnetic characteristics at a temperature between 700 C. and 1200 C. in the presence of a component capable of reacting with the silicon-iron alloy to form a thin film on 5 the surface of the strip for placing the alloy under tension effectively reduces the magnetostriction of the alloy.

While this invention has been described with reference to particular embodiments thereof, it is, of course, not to be limited thereto except insofar as is necessitated by the prior art and the scope of the appended claims.

I claim as my invention:

1. In the method of reducing magnetostrict-ion in silicon-iron alloy containing up to 6% of silicon and which has been fabricated into strip form and treated to develop its magnetic characteristics, in combination, coating the treated alloy strip with sodium silicate, subjecting the coated strip to a heat treatment at a temperature between 900 C. and 1l00 C. in air for a period of time of not more than three minutes to effectively oxidize the strip and form a thin fused adhering film thereon, and cooling'the alloy strip with the adhering film of oxidized and fused material thereon, the adhering film having a linear thermal coefficient of expansion lessthan the linear thermal coefficient of expansion of the silicon-iron alloy to give a strip in which the silicon-iron alloy is under tension and the adhering film is in compression to effectively reduce the magnetostriction of the alloy strip.

2. In the method of reducing magnetostriction in silicon-iron alloy containing up to 6% of silicon and which has been fabricated into strip form and treated to develop its magnetic characteristics, in combination, heating the treated alloy strip to a temperature between 700 C. and 1100 C. in the presence of a reagent capable of m reacting with the silicon-iron alloy to form a thin film of oxidized material on the surfaces of the strip while applying tension to the strip, the tension applied being below the elastic limit of the alloy strip, and cooling the alloy strip ,5 with the thin film of oxidized material thereon as an adherent part thereof, the reagent being selected to form a thin film of oxidized material having a linear' thermal coefficient of expansion less than the linear thermal coefficient of :0 expansion of the silicon-iron alloy to give a strip in which the silicon-iron alloy is under tension and the adhering film of oxidized material is in compression to effectively reduce the magnetostriction of the alloy strip.

3. In the method of reducing magnetostriction in silicon-iron alloy containing up to 6% of silicon and which has been fabricated into strip form and treated to develop its magnetic characteristics, in combination, heating the treated allo strip at a temperature between 925 C. and 1100 C. in an oxidizing atmospherefor a period of time of not-more than five minutes to form a thin film of oxidized material on the surfaces of the strip while applying tension to the strip, the tension applied being below the'elastic limit of the alloy strip, and cooling the oxidized strip to retain the oxidized film as an adherent part thereof, the thin oxidized film having a linear coefficient of expansion less than the linear coefficient of expansion of the silicon-iron alloy to give a strip in which the adhering oxidized film maintains the silicon-iron alloy under tension to effectively reduce the magnetostriction of the alloy strip.

4. In the method of reducing magnetostriction in silicon-iron alloy containing up to 6% of silicon, and which has been fabricated into strip form and treated to develop its magnetic characteristics, in combination, heating the treated alloy strip to a temperature between 825 C. and 1100 C, in the presence of a reagent capable of reacting with the silicon-iron alloy to form a film of oxidized material containing silicates having a thickness of about 2 l0- inches thick on the surfaces of the strip while applying tension to the strip. the tension applied being below the elastic limit of the alloy strip, and cooling the alloy strip with the film of oxidized material containing silicates thereon as an adherent part thereof, the film of oxidized material containing the silicates having a linear thermal coefficient of expansion less than the linear coefficient of expansion of the silicon iron alloy to give a strip in which the silicon-iron alloy is under tension and the adhering film of oxidized material containing silicates is in compression to effectively reduce the magnetostriction of the alloy strip.

5. In the method of reducing magnetostriction in silicon-iron alloy containing up to 6% of silicon and which has been fabricated into strip form and treated to develop its magnetic characteristics, in combination, heating the treated alloy strip to a temperature between 825 C. and 1100 C. in the presence of a reagent capable of reacting with the silicon-iron alloy to form a thin film of oxidized material on the surfaces of the strip while applying tension to the strip, the tension applied being between 500 and 2000 pounds per square inch and maintained below the elastic limit of the alloy strip, and cooling the alloy strip with the film of oxidized material thereon as an adherent part thereof, the thin film of oxidized material having a linear coefficient of expansion less than the linear coefficient of expansion of the silicon-iron alloy, the tension applied during the heating so effecting the straining of the aloying components of the silicon-iron strip that the adherent film of oxidized material having the lower coefficient of expansion maintains the silicon-iron alloy under tension when the strip is cooled to effectively reduce the magnetostriction of the alloy strip.

6. In the method of reducing magnetostrictlon in silicon-iron alloy containing up to 6% of silicon and which has been fabricated into strip form and treated to develop its magneticcharacteristics, in combination, coating the treated alloy strip with sodium silicate, subjecting the coated strip to a heat treatment at a temperature between 900 C. and 1100" C. in air ior a period of time of not more than three'minutesto effectively oxidize the strip andform a thin fused adhering film thereon while applying tension to the strip, the tension applied being between 500 :and 2000 pounds per square inch and maintained below the elastic limit of the strip, and cooling the alloystrip with the adhering film of oxidized which the silicon-iron alloy is under tension and theadhering film is in-compression to eflectively reduce the magnetostriction of the alloy strip.

STEPHEN L. BURGWIN.