US1931134A - Bright annealing - Google Patents

Description

Oct. 17, 1933. lNZEL BRIGHT ANNEALING Filed June 26, 1929 MQQkRkMQQM ,4/v/vm 4 we 72/1/ 524 n/RE INVENTOR/ TTORNEY5.

Patented Oct. 17, 1933 BRIGHT ANNEALING Augustus B. Kinzel, Beechhurst, Long Island,

N. Y., assignor to Electra Metallurgical Company, a corporation of West Virginia Application June 26,1929. Serial No. 373,841

6 Claims.

This invention relates to the heat treatment and the annealing of metals, and especially to the method of annealing known as bright annealing in which metals and alloys free from coatings are annealed without producing discoloration or oxidation of the metal.

My process is primarily applicable to the annealing of steel and ferrous alloys and the description of the process will be made with reference to the treatment of such materials, although the process may be used in connection with the treatment of other metals and alloys.

Heretofore, steel has been bright annealed in atmospheres of pure nitrogen, pure hydrogen or pure carbon monoxide. The use of such atmospheres in connection with annealing has various disadvantages. The high degree of purity of the gas required to produce unoxidized clean surfaces is objectionable from the standpoint of the cost of the gases. Hydrogen and carbon monoxide are combustible and form explosive mixtures with air and their use in connection with high temperatures is hazardous. The poisonous nature of carbon monoxide makes its use a serious hazard. The presence of extremely small amounts of oxygen, water or carbon dioxide in pure nitrogen is known to produce blueing of steel at the temperatures of annealing. Attempts have been made to treat air by passing the air through heated carbon so as to reduce the oxygen and carbon dioxide to carbon monoxide and to use this gas as an atmosphere forbright annealing. The reduction of carbon dioxide is never complete. This gas contains an amount of carbon dioxide due to the equilibrium I which is sufficient to produce a blueing of the steel at annealing temperatures.

It is among the objects of my invention to provide methods of bright annealing that avoid toxic, fire and explosion hazards, that do not require costly materials and methods of manipulation and that are capable of producing an unoxidized uncolored product.

I have found that the blueing of steel will not occur when it is heat treated or annealed in an atmosphere of impure nitrogen containing about 95% or more of nitrogen and the remainder carbon monoxide with or without small amounts of carbon dioxide. (Barbon dioxide may be present in the gas if the amount of carbon dioxide does not exceed the equilibria, 20022 200 +02 which are produced by heating gases containing 5% or less of oxygen orcarbon dioxide at certain temperatures which are above the annealing temperatures. Due to the preponderance of carbon monoxide the residue of carbon dioxide in such a reduced gas does not dissociate at annealing temperatures so as to produce blueing 0 of the steel. The condition may be produced by passing a dry nitrogen containing gas through a layer of carbon, such as charcoal at temperatures above the predetermined annealing temperatures. Dry untreated gases are preferred, 5 but absolute freedom from'moisture is not necessary when the gas is reduced with carbon.

The untreated impure nitrogen may consist of about 95% of nitrogen and the remainder oxygen, carbon dioxide or either of these gases with 7 carbon monoxide. Various suitable sources of untreated gas may be found. For example, the untreated gas may be obtained by oxidizing the oxygen of air mainly to carbon dioxide and then removing the carbon dioxide by appropriate 7 means, so as to produce a nitrogen content of at least 95%. Another convenient and inexpensive untreated gas is a by-product of the process of making oxygen by the distillation of liquid air. In this process, a residue of nitrogen-containing gas having 5.0% or less of oxygen is produced.

The minimum difierence between the temperature of deoxidizing the impure 95% gas and the temperature of annealing is variable according to the temperature used for annealing. Lower annealing temperatures require a greater difference between the deoxidizing treatment and annealing temperatures than do higher annealing temperatures, but a difiference between the deoxidizing treatment and annealing temperatures that is greater than the minimum may be used. For example, for annealing at 600 C., the untreated gas may be deoxidized by heating in the presence of charcoal to about 700 C. to obtain the best results, and for annealing at 875 C. or lower the gas may be deoxidized at a temperature of about 900 C. It is essential that the gas is always deoxidized at a higher temperature than the temperature to which it is subjected in the presence of the metal.

The curve C shown in the sole figure of the drawing represents the approximate relationship between the temperatures of annealing and the minimum temperatures of deoxidizing the gas. The abscissa: of the curve C represent the annealing temperatures and ordinates of the curve represent deoxidizing temperatures. The point A represents a temperature of 700 C., the minimum deoxidizing temperature for 95% nitrogencontaining gas of the above described composition when the deoxidized gas is used as the atmosphere in an annealing box containing metal that is heat treated at 600 C. The point B represents a deoxidizing temperature of 900 C., the minimum deoxidizing temperature to be used when annealing at 875 C. From the curve it appears that smaller difierences between the deoxidizing and annealing temperatures may be used as higher annealing temperatures are used. The curve represents only the approximate relationship between the annealing temperature and the minimum temperature of deoxidizing the gas with charcoal since some departures from the exact relationship represented by the curve may be made depending somewhat upon the materials employed. Regardless of the temperature used in the annealing box, the gas is always deoxidized at temperatures above 600 C.

Higher deoxidiz ing temperatures than 900 C. will produce smaller proportions of carbon dioxide. Such a gas may be used at annealing temperatures far below the deoxidizing temperatures if desired. Gas which is deoxidized at the lower temperatures does not usually contain more than about 0.50% carbon dioxide. When the gas is deoxidized under the most favorable conditions, no carbon dioxide can be detected by the ordinary qualitative tests in small portions of the gas. The use of atmospheres consisting of at least 95% nitrogen and the remainder carbon monoxide is contemplated.

As illustrative of my invention, impure nitrogen consisting of about 95.5% nitrogen and oxygen is passed through a bed of charcoal which is heated to 900 C. so as to reduce the oxygen and produce a gas which is substantially in equilibrium with carbon at this temperature. The air in a carefully luted annealing box which contains bright unoxidized sheets of steel, is displaced by the deoxidized gas. The sheets are then annealed at 800 C. in the presence of the deoxidized gas. If heated deoxidized gas is used, care should be taken to avoid heating the metal above the annealing temperatures by the heated gas. A slow current of deoxidized gas may be passed into the annealing box during annealing and during the cooling of the annealing box to prevent influx of air.

The amount of carbon monoxide in the above described atmospheres is insufficient to support combustion or to form explosive mixtures with air. The liability of carbon monoxide poisoning from leakage of this gas is practically eliminated by reason of its low content of carbon monoxide and the diffusion of the discharged gas in the atmosphere.

While I have disclosed the use of nitrogen, which I prefer to use because of cheapness and availability, I. may if I so desire, replace the nitrogen, in whole or in part, with other essentially inert gases such, for example, as helium or argon.

I claim:

'1. The method of bright-annealing metal articles which comprises reducing a substantially dry gaseous atmosphere containing at least 95% nitrogen and the remainder oxygen containing by heating the atmosphere in the presence temperatures than the pregas of carbon to higher determined temperatures of annealing, and

heating the articles to said predetermined temperatures in the reduced atmosphere.

2. The method of bright-annealing metal articles which comprises reducing a gas consisting of at least 95% nitrogen and the remainder oxygen, by heating the gas in the presence of carbon to higher temperatures than the predetermined temperatures of annealing, and heat treating the articles at said predetermined temperatures in an atmosphere consisting of said reduced gas.

3. The method of bright-annealing metal articles which comprises reducing a gas containing at least 95% nitrogen and the remainder an amount of oxygen containing gas capable of discoloring said metal at predetermined annealing temperatures, by heating said gas above the predetermined temperatures in the presence of carbon, and heat treating the metals at said predetermined temperatures in an atmosphere consisting of said reduced gas.

4. The method of heat-treating metals which comprises the production of an atmosphere having substantially more carbon monoxide than required by the equilibrium, 2002 ZCO+O2, which may be produced by deoxidizing a nitrogen containing gas comprising at least 95.0% nitrogen and the remainder one or more than one of the gases of said equilibrium by heating the gas in the presence of carbon to predetermined temperatures higher than the heat treating ternperatures, and heat treating the metal in the presence of said atmosphere at the heat treating temperatures.

5. The method of annealing metals which comprises the production of an atmosphere having substantially more carbon monoxide than the equilibrium, 2CO2.; .2CO+O2, which may be produced by deoxidizing an impure nitrogen containing at least 95.0% of nitrogen and the remainder one or more than one of the gases of said equilibrium by heating the impure nitrogen in the presence of carbon to deoxidizing temperatures above 600 C. said deoxidizing temperatures bearing about the relation to annealing temperatures as those represented by a smooth curve having as rectangular coordinates deoxidizing temperatures and annealing temperatures, the position of the curve on the coordinates being 12' determined by the following deoxidizing temperatures: 700 C., 850 C., 1000 C., 1150 C., and by the following corresponding annealing temperatures: 600 C., 810 C., 990 C., 1140 C., respectively, and heating the metal in the presence of 12 said atmosphere to temperatures which do not exceed said corresponding annealing temperatures.

6. The method of bright-annealing metals which comprises producing an atmosphere hav- 12 ing more carbon monoxide than is required for the equilibrium, 2002:32004-02, so that the atmosphere will not discolor iron-containing alloys at annealing temperatures, and heating the metal in said atmosphere to said annealing tempera- 13 tures; the production of said atmosphere being effected by a method which comprises deoxidizing a gas containing at least 95.0% nitrogen, the remainder being one or -more of the gases of said equilibrium, by heating the gas in the pres- 1. ence of carbon to a deoxidizing temperature at least as high as that represented by the ordinate at the intersection of the abscissarepresenting the annealing temperature with a smooth curve having rectangular coordinates, the curve being 1 determined by the following deoxidizing temperatures as ordinates: 700" C., 350 C., 1000 C., 1159 C., and the following annealing temperatures as abscissa: 600 C., 810 C., 990 C., 1140" C.

AUGUSTUS B. KINZEL. 1

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