US1944743A - Method for annealing steel - Google Patents

Description

Jan. 23, 1934. F. c. KELLEY METHOD FOR ANNEALING STEEL.

Filed June 26, 1950 lnveri towma 4... K c. m mm A s m H Patented Jan. 23, 1934 METHOD FOR ANNEALIN STEEL Floyd 0. Kelley, Schenectady. N. Y., assignor to General Electric Company, a corporation of New York Application June 26, 1930. Serial ma athon 3 Claims. (cums-1s) The present invention relates to a method for annealing steel and more particularly to a method for continuously annealing readily oxidizable steel. Steel of this'character iscommonly known as stainless" or rustless steel and may contain either about 8 to 18% chromium, the remainder being iron except for minor impurities, or about 18% chromium and 8% nickel with the remainder iron except for minor o impurities.

Stainless steel is often employed to provide articles of manufacture which mustbe shaped and which require a bright finish. In. orderto shape the material easily it must be annealed. Such steel however oxidizes readily even when the annealing process is carried out in a hydrogen furnace due to the fact that in practically all such furnaces water vapor and oxygen are pres-' ent in small amounts due to diffusion of air back into the furnace and also due to condensed moisture which may be held either in the furnace insulation or in the cooling chamber of the furnace. Oxidation of the steel causes it to become discolored and this discoloration is ordinarily removed by brushing and polishing.

In my copending application Serial No. 432,226, filed February 28, 1930, and entitled Method for annealing steel, 1 have disclosed a method whereby stainless or rustless steel may be annealed in a closed container. While the method disclosed in the prior application is entirely satisfactory, it is not adapted for use in a continuous process,-i. e. for annealing a strip of material which is drawn continuously through an annealing furnace.- It is one of the objects .'comes out of the annealing furnace.

The novel features which are characteristic of the present invention are set forth with par-- ticularity in the appended claims. The invention itself however will best be understood from reference to the following specification when considered in connection with the accompanying drawing in which the single figure shows-diagrammatically apair of inter-connected hydrogen furnaces with which my. invention may be carried into effect. b 4

Referring more particularly to the drawing, I have indicated at 1 and 2 apair of hydrogen furnaces of well known construction which are similar in practically all details. Like reference characters are employed to designate like parts nace. The closure members 6 and 7 of furnace furnace 2 and is connected to the tube 14-ata of each furnace. Each furnace comprises a heating chamber 3, a cooling chamber 4 and an extension 5. The ends of the furnaces are closed by carbon plugs 6 and 7 each of! which is pro-, vided with openings therethrough for a purpose which presently will be disclosed. a

Furnace 1 is'employed to provide a source of substantially pure, dry hydrogen. An iron or other suitable metal pipe or tube 8 is substantially'filled with a powdered material 10, a part or all of which is, more activeto oxygen than hydrogen is active to oxygen, theactive portion of the powdered material being adapted toretain oxygen forming an oxide which is difilcult to reduce. Tube 8 is placed in the furnace so that it 79 extends entirely through the hot zone thereof and projects slightly into both the furnace extension 5 and the cooling chamber 4 of the fur-' nace. The hot end of the tube 8 is closed by iron cap or plug which is provided with an open' 1 ing therethrough for a pipe 11. The opposite or I cold end of the tube 8 is provided with a carbon I closure plug] 12 which has an openingthere.- through to accommodatea. pipe13-which extends through the closure member 6 and is connected to a suitable source of hydrogen gas supply. 'A satisfactory powdered material for use in the tube 8 may be obtained by mixing about 50% ferro-silicon with about 50% aluminum oxide.

A receptacle 14 which may be an iron pipe of tube is mounted in furnace 2 and'extends through the furnace from a point closely adjacent the closure member 6 in the cold end thereof to a point inthe extension 5 of the furnace. Tube 14 is formed with an iron closuregmember 15 at one end thereof and with a graphite plug 16 atv the opposite or cold end of the furnace. An abutment 1'! is interposed between plugs 8 and 16 in order to prevent any longitudinal movement of the tube 14 towardthe cold end of the fur- 06 2 as well as the closure members 15 and 16 of the tube 14 are each provided with an opening. These openings are in alignment with one another and tube 8 extends through a closure member 7 of point within the heating chamber of furnace 2.

' In operation, hydrogen gas is supplied to,fur-- nace 1 through an inlet pipe 17' and the furnace is heated to a temperature of about 900 At the same time hydrogen gas is also supplied .to tube 8v at a relatively slow rate, for example 116 about five cubic liters per hour. As the latter gas passes through the powdered material 10 within thetube 8 all water vapor or oxygen is removed from the hydrogen leaving it in a pure and very dry condition. The purified hydrogen flows through pipe 11 into tube 14 in furnace 2.

The metal to be annealed is in the form of a strip 18 and is passed through the aligned openings in closure members 7, 15, 16 and 6 respectively and then drawn between rubber covered rolls 19 and 20 which are driven through reduction gearing 21 from an electric motor 22. In small furnaces, such as illustrated on the present drawing and in which the hot zone is only about 8" in length the speed at which the metal ribbon is drawn through the furnace is approximately 13" per minute. This speed would of course vary with furnaces of different capacity.

The temperature of the hydrogen furnace 2 is about 1100 0. As the metal strip is drawn through the tube 14 it is in contact with dry purified hydrogen and there is no tendency for the strip to become oxidized or discolored. Tube 14 is substantially closed since the slots in the closure members are each substantially equal in area to the cross section of the metal strip which is being annealed. The cold end of the tube 14 is spaced from the closure member 6 of the furnaceto prevent binding of the metal strip while the abutment 17 is employed to prevent movement of tube 14as the strip to be annealed is drawn through the tube.

At points of relatively low temperature within the hydrogen furnace 2 such as the water cooled chamber 4 or extension 5, there is no tendency for the steep strip 12 to become oxidized or discolored. By employing a tube 14 of the-length indicated, the metal strip to be annealed is per mitted to enter the tube without being heated to any appreciable extent until it reaches the hot zone in the heating chamber 3. In this manner any oxidation of the strip within thefurnace and before the strip enters the tube 14, is avoid ed. In like manner, when the annealed strip comes out of the cold end of tube 14 and into the cooling chamber 4 of the furnace 2, the annealed strip has such a low temperature that there is no tendency for it to become oxidized.

The rate of hydrogen flow through the furnace tube 14 is relatively slow, for example, as indicated above, about five cubic liters per hour, although this amount would obviously vary with furnaces of different capacity. Under any circumstances however, the pressure of hydrogen within the tube 14 should be sufiicient to prevent the hydrogen within the furnace from entering tube 14. The low pressure of hydrogen employed is, of course, a very favorable circumstance in connection with the life of the powdered material employed in tube 8 in furnace 1.

' While I prefer to employ a 50-50 mixture of ferro-silicon and alumina, the silicon content of the ferro-silicon being about 50%, the invention is not limited either to this specific purifying material or to the proportions of material disclosed. Other suitable powdered materials may be employed if desired. For example, any powdered material which exhibits great activity towards oxygen may beemployed as the material 10. This material might consist either of substantially pure chromium, manganese, silicon or ferrosilicon, or it may consist of one or more of these materials mixed with an inert powdered material such as aluminum oxide. Although a- 50-50 mixture of ferro-silicon and alumina will give satisfactory results other proportions of materials may be employed if desired. For example, 15% ferro-siliconwith 85% aluminum.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination, a furnace comprising a cooled portion, an'extension and a heating chamber intermediate said extension and cooled portion, a substantially closed tube mounted in said heating chamber, one end of said tube being within said extension and the other end within the cooling chamber, said tube being provided with relatively small openings in opposite ends thereof through which a metal strip may be drawn, means for supplying a non-oxidizing gas to said heating chamber, and means for supplying a gas to said tube.

2. In combination, a furnace comprising a cooled portion, an extension and a heating chamber intermediate said extension and cooled portion, a substantially closed tube mounted in said heating chamber, one end of said tube being within said extension and the other end within said cooled portion, said tube having relatively small openings in opposite ends thereof through which a metal strip may be drawn, said openings having substantially the same area as the cross section of said metal strip, means for supplying a non-oxidizing gas to said heating chamber and means for supplying a gas to said tube at a point intermediate the ends of said tube and within the heating chamber.

3. The method of annealing a metal strip which comprises drawing said strip through a substantially closed tube, heating the intermediate portion of said tube to the annealing temperature while the end portions of said tube are maintained at relatively low temperatures, surrounding said tube with a reducing gas and supplying a reducing gas to said tube at a pressure sufficiently high to exclude the surrounding gas from said tube.

FLOYD C. KELLEY.

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