US1569355A - Method of annealing sheet steel - Google Patents

Description

Patented Jan. 12, 1926.

GUERNEY H. COLE, OF FOREST HILLS BOROUGH,

INGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENN- SYLVANIA.

METHOD OF ANNEALING SHEET STEEL.

' No Drawing.

T (1 whom it may concern:

Be it known that I, GUERNEY H. COLE, a citizen of the Vnited States, and a resident of F orest Hills Borough, in the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in l\[ethods ol' Annealing Sheet Steel, of which the following is a specification.

T his invention relates to annealing metals, such as steel, silicon steel, etc, it being especially directed to an improved method of re-annealing electrical sheet metal, such as is ordinarily used in making transformer cores.

Steel sheets for transformer cores are generally annealed to a point where molecular rearrangement takes place, and the material so treated is then punched or cut to the desired form, after which the metal is reannealed in order to remove the strains set ,up during the punching or cutting process and to reduce the core loss of the metal, it being among the objects of this invention to devise a method for re-annealing such sheet material which is simple and economical.

Hitherto, it has been the custom to reanneal material, such as sheet silicon steel, in a receptacle of suitable kind, such as a box or pot sealed from the atmosphere by a cover or other suitable means, and the material was heated to a temperature of 785 to 1000 C. for a considerable period of time. I Usually, the sheet metal to be annealed was stacked in a large box on a car and a heavy cast iron or steel cover placed thereon. All cracks were carefully sealed with sand and clay to prevent access of air. The material, was then placed in a furnace where it was heated to about 800-S50 C., the heating requiring 36 to 48 hours. It was held at this temperature for about 8 hours, after which it was allowed to cool to room temperature which required several days. It is apparent that the heat losses were large because of the necessity of penclrating the heavy cast iron cover, a large amount of labor was necessary to prepare-the steel for annealing, and much time was consumed in the process.

It was generally considered that the higher the temperature of annealing the greater would be the improvement in the annealed material, provided oxidation thereof could be kept reasonably low. Precautions were taken to prevent access of air,

Application filed December 21, 1920. Serial Nb. 432,336.

and the temperature used was as high as was economically possible. Many processes utilized a temperature as high as 1100 C. or even higher in re-annealing. It will be seen that such methods had serious disadvantages in that the material was heated to a high temperature, necessitating the use of large amounts of fuel and requiring constant and careful supervision over the annealing process.

My invention obviates these disadvantages in a novel manner. I have discovered that a great improvement in core loss may be attained without the use of covers or other means for preventing access of air and I have also found that the use of a low temperature, very much lower than any which had hitherto been considercdfavorable for re-annealing, produced remarkable results in improving the core loss of electrical sheet steel. I have found that heating the material with free access of air at a temperature about 100 C. less than the minimum temperature hitherto used, produced double the improvement in core loss of electrical steel, compared to that produced by the old methods, excellent results having been obtained at temperatures below 645 C.

The analysis of the atmosphere surrounding the material being annealed has shown that the ratio of oxygen to nitrogen in the fninace was nearly as great as that usually found in normal air. It was also found that the percentage of water vapor in the furnace atmosphere was very high, but I am inclined to believe that the water vapor present contributes but little to the improvement produced by my new method.

In the present case, there is no detrimental effect of carbon monoxide or carbon dioxide upon the material being annealed on account of the low temperature used, such temperature being too low to allow of sub-' stantial ionization of these gases and, therefore, prevents combination of the same with the material being annealed. Therefore, in my method, it is not necessary to exclude the products of combustion from the material and, if a mufile furnace is used in the annealing, leakage of products of combustion into the muflle on account of cracked or burnt-out casings is not detrimental to the material. It is seen, therefore, that perfect and delicate control of the various factors is unnecessary with my method.

PENNSYLVANIA, ASSIGNOR TO WEST- Although I prefer to use atmospheric air in my method, I may substitute therefor a sufficient amount of pure oxygen or a substance capable of furnishing the desired amount of oxygen to the atmosphere surrounding the sheet material. I attempt to avoid a strong current of air through the material being annealed, since too much oxygen may cause excessive oxidation of the surface of the annealed metal and thus lower the electrical efliciency thereof. However, I have found that a' thin film of reddish oxide does not materially decrease the efiiciency of the steel for electrical purposes.

My method, as practiced with the use of a continuous furnace of the muflle type, is as follows: The furnace, which may be of considerable length, having a double muiile with tracts to allow cars to move in each of the compartments in opposite directions, is heated by any suitable means, such as gas or oil, to a temperature of approximately 700 C. Sheet steel is stacked on cars and slowly passed through the furnace, the end doors of which may be partly open in order to permit free access of air. ,If'it is found that -too much air is being circulated, the doors of the furnace may be more or less completely closed until it is found that sufficient air is entering to properly complete the annealing, and such adjustment will be permanent for any one set of conditions involved. In a large furnace it is preferable to keep the doors completely closed. The sheet material is allowed to pass through the furnace at such a rate that it reaches a temperature of about 700 in about 7 to 13 hours, such temperature being held for about 3 to 6 hours.

I have described above the use of a mufile furnace with my new method of annealing, but it is to be understood that my method is equally applicable to furnaces of other types and, although I have designated 700 C. as the preferable temperature at which optimum results are obtained, it is to be understood that other temperatures, even as high as 7 50, give a marked improvement over the results obtained by former methods. Lower temperatures are also beneficial and I have found that a temperature even as low as 600 C. will give excellent results. At the temperature of 700 C., which I prefer to use, the improvement in the core loss by re-annealing ranges from 15 to 25% even if the best commercial method was followed in the original annealing, whereas, by utilizing former methods, the best improvement that was ordinarily obtained was 642%. A teme rature of 645 C. gave an improvement 0 about 15%.

It. will be seen that, by the use of my new method, lower temperatures may be used with better improvement in the steel 'for electrical purposes; not only is there great economy of fuel and labor effected because of the lower temperature and shorter time necessary, but the wear on the furnace is more than proportionately decreased and its capacity increased. Since the presence of air is beneficial, it is unnecessary to take care that the furnace is air-tight nor to expend time and labor in making the annealing boxes air-tight. The expense of furnishing and replacing boxes and covers is eliminated.

I Having thus fully described my said in-.

vention, what I claim as new and desire to secure by Letters Patent is:

1. The' process of heat treating sheets of magnetic silicon alloy steel which consists in heating said sheets to a temperature not exceeding 700 C. in an atmosphere containing substantial amounts of oxygen and then cooling.

2. The process of heat treating sheets of magnetic silicon alloy steel which consists in heating said sheets to a temperature not exceeding 700 C. in an atmosphere containing oxygen in approximately the same ratio to nitrogen'asexists in air and then cooling.

3. The process of heat treating sheets of magnetic silicon alloy steel which consists in heating said sheets to a, temperature of about 700 C. in an atmosphere contain ng substantial cooling.

4. The process of heat treating sheets of magnetic silicon alloy steel which consists in heating said sheets to a temperature of about 700 C. in an atmosphere containing oxygen in approximately the same ratio to nitrogen as exists in air and then cooling.

5. The method of heat treating sheets of magnetic silicon alloy steel which consists in heating said sheets in an atmosphere containing substantial amounts of oxygen and water vapor and then cooling.

6. The method of heat treating sheets of. magnetic silicon alloy steel which consists in heating said sheets to a temperature of about 700C. in an atmosphere containing substantial amounts of oxygen, maintaining said sheets at said temperature for a period of time and then cooling.

7. The method of heat treating sheets of magnetic silicon alloy steel which consists in heating said sheets to a temperature of about 700 C. in an atmosphere containing substantial amounts of oxygen and water vapor, maintaining said sheets at said temperature for a period of time and then coolmg.

In testimony whereof, I have hereunto subscribed my name this 13th day of Decem ber, 1920.

GUERNEY H. COLE.

amounts of oxygen and then-