US2595991A

US2595991A - Annealing

Info

Publication number: US2595991A
Application number: US64301A
Authority: US
Inventors: Fred C Sweeney
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1948-12-09
Filing date: 1948-12-09
Publication date: 1952-05-06
Anticipated expiration: 1969-05-06

Description

May 6, 1952 F. c. SWEENEY ANNEALING Filed Dec. 9, 1948 y T. E mn VE m N w m S T D E R F Patented May 6, 1952 ANNEALING Fred C. Sweeney, Steubenville, Ohio, assignor to National Steel Corporation, a corporation of Delaware Application December 9, 1948, Serial No. 64,301

* 6 Claims.

1 The present invention relates toa novel process of annealing ferrous metal articles and is primarily concerned with inhibiting oxidation and decarburization of ferrous metal articles while annealing such articles in a box annealing furnace.

It is a customary practice to use a box annealing furnace to bright anneal ferrous metal articles such as, for example, sheets or coils of steel. A furnace of this type includes a base on which the articles are supported and an inner cover which cooperates with the base to form an enclosed chamber about the articles. A second outer cover is placed over the inner cover. The outer cover is lined with suitable refractory material, and heat is supplied to the space between the covers to heat the inner cover and the articles therein to the annealing temperature. A controlled atmosphere is maintained within the inner cover and about the ferrous metal articles in an attempt to prevent undesirable oxidation or decarburization of the ferrous metal articles. This controlled atmosphere is customarily maintained by flowing non-oxidizing gas through the chamber within the inner cover. The non-oxidizing gas may be an inert gas such as nitrogen that is non-reactive with ferrous metal at elevated temperatures or may be a reducing gas such as carbon monoxide or a reducing gas mixture. The pressure of the controlled atmosphere is maintained above atmospheric pressure and seals are provided between the lip of the inner cover and the base to prevent contamination of the controlled atmosphere with atmospheric air which would cause oxidation and discoloration of the articles. When the articles being annealed are of steel containing a relatively higher percentage of carbon, special care is taken to prevent contamination of the controlled atmosphere which in turn causes decarburization of the surface metal in addition to oxidation.

In practice, when annealing ferrous articles in a controlled atmosphere within such an annealing furnace, some discoloration or discoloration and decarburization of the steel takes place. The undesirable chemical reactions take place despite the care taken to maintain the exacting conditions of the controlled atmosphere that should prevent these reactions. Thus, the prior art methods of box annealing ferrous metal articles have not been entirely successful in eliminating discoloration or decarburization of the annealed articles.

Accordingly, the primary object of the present invention is to provide an improved method of box annealing which will be more successful than the prior art methods in preventing oxidation and decarburization of the articles. j

I have discovered that oxidation or oxidation and decarburization of ferrous metal articles in a box annealing furnace can be inhibited if the base is heated and the cover placed over the articles when the base is at a temperature above 212 F. While I do not want to be limited to any particular theory, it is believed that heating the base and placing the cover on the base when the base is at such a temperature eliminates a source of contamination of the controlled atmosphere within the cover maintained during the subsequent annealing operation. It is also believed that the base while exposed to the atmosphere absorbs water and possibly other constituents from the atmospheric air and that these constituents are subsequently driven off during annealing to contaminate the controlled atmosphere within the cover. At the elevated temperatures employed during annealing, water breaks down into oxygen and hydrogen which will in turn cause oxidation or oxidation and decarburization. By heating the base in accordance with the present invention, such oxidation or oxidation and decarburization can be eliminated or reduced. The heating of the base to prevent contamination of the controlled atmosphere is not part of the heating cycle to effect annealing of the articles but is a separate pretreatment,

These and other objects and advantages will become more readily apparent from the following detailed description taken with the accompanying drawings in which:

Figure 1 is an isometric view of a box annealing furnace having portions removed to more clearly illustrate the internal structure; and,

Figure 2 is a fragmentary view showing the the heating units of the apparatus of Figure 1.

Referring to the drawings and more particularly to Figure 1, the annealing furnace comprises a base l0, an inner cover II, and an outer cover l2. The base includes a relatively larg flat foundation portion l3 which may be formed of suitable brick work, steel members, or other elements. On the foundation I 3, there is a platform it formed of refractory bricks and which supports the ferrous metal articles being annealed. As shown in the drawings, coils [5 of strip are shown on the platform M in position to be annealed. To prevent contamination of the controlled atmosphere maintained within the inner cover ll, sand or other suitable means is used at It; to seal the joint between the edges of the inner cover It and the base 10. Sand or other suitable means is also used at IT to seal the joint between the edges of the outer cover l2 and the base [0. The cover [2 is lined with refractory brick as shown in part at I 9 and contains a series of radiant heaters 20. These heaters 20 are of the radiant heat type in which a combustible gaseous mixture is discharged into the tubes through nozzles 2|. The gaseous mixture burns within the tubes to heat the tubes and of contaminating the chamber within the cover' A pipe 23 is connected to a source of suitable non-oxidizing gas and has an outlet at 24 for discharging the gas into the chamber within the cover II. A pipe 2% communicates with the inside of the cover H and with a stack or other suitable discharge point so that non-oxidizing gas can be fed into the interior of the cover H through pipe 23 and then out through pipe 24 so as to maintain a continuous flow of non-oxidizing gas through the chamber within the cover. Preferably, a plurality of inlet and outlet pipes are provided, arranged to provide the best distribution of the currents of gas within the cover.

As described up to now, the annealing furnace is of customary construction. When operating such a furnace in the usual manner, the coils l5 are placed on the cold base l0. The covers H and i2 are placed in position, and the heat is turned on. The flow of non-oxidizing gas is also turned on to maintain the controlled atmosphere within the cover H. Within a relatively few hours, the temperature within the furnace will reach the annealing temperature which may be, for example, about 1350 F. Sufiicient heat will be continuously supplied to the furnace to maintain the furnace at the annealing temperature so that the heat will slowly soak into the coil of strip and raise the coils to the annealing temperature. After the coils have been brought to the annealing temperature, the heat is turned off and the outer cover I2 removed. Normally in a plant there will be a large number of bases l0 and a relatively smaller number of covers. Quite often, the outer cover 52, after its removal, will be placed on a different base to heat another charge while the charge on the first base is cooling. The time required to cool down the coils will vary and may take two days or longer. After the outer cover has been removed, the fiow of non-oxidizing gas may be turned off. Preferably, the how of gas is 'is turned off. At the time the inner cover is removed, the charge is at sufiicient low temperature so that it is not readily oxidized or decarburized. a V

. The uncovered coils are removed from the base if) and are then treated in the manner desired. At some future date, another charge is placed on the base, and, when the covers are available, they are placed on the base over the charge, and thecharge is then annealed. Nor

mally, a number of hours will elapse between lifts during which the base is exposed to atmospheric air so that the base cools down to or close to atmospherictemperaturebefore the next charge is annealed.

In accordance with the present invention, a pair of

heaters

26 and 21 are provided in the base It]. Each heater comprises a pair of U- shaped

resistant heater strips

28 and 29. One end of each

strip

28 and 29 of each heater is connected to a wire 30 which is in turn connected to one side of a suitable source of electric current (not shown). The other end of each strip is connected to a wire 3| which is also connected to the other side of the source of electric current. Thus, when the current is turned on, the

heaters

26 and 21 heat the base and particularly the platform portion M.

In accordance with the present invention, the heaters 26 and 2! are turned on and the base is heated before the cover ll is placed in'position on the base over the charge to be annealed. Preferably, the heaters are turned on before the base cools below 212' F. so that the base is heated and maintained at a temperature above 212 for the entire period it is exposed to the atmosphere. Where a relatively long time elapses between lifts or if for other reasons it is desirable to allow the base to cool down below 212 F., then'the heaters 25 and El are turned on and the base is heated to a temperature above 212 before the cover is placed in position. With either procedure, the uncovered base is heated while exposed to atmospheric air, and the cover I I is placed on the base over the charge while the base is at'a temperature above 212 F.

In operation, the non-oxidizingga-s may be, for example, a mixture having the following analysis: l

' Per cent CO2 2.8 02. 0.0 CO 13.96 Hz .162 CH4 0.0 N2 67.04

Where the cover is placed on the base when the base .is relatively cold and at a temperature below 212, then, when the base and charge are heated, the gas flowing through the outlet pipe 2 i when analyzed will show a higher, percentage content of carbon dioxide and hydrogen than the inlet gas and may contain a small 'the ferrous metal articles are steel containing a relatively high percentage of carbon, the exposed surfaces will be decarburized, which is undesirable.

" When the heaters 26 and 2? are used in accordance with the present invention to heat the "base so that the base is at a temperature above 2l 2when the cover is placedin position, analysis of the gas issuing from pipe 24 will bemore nearly the same as the analysis of the inlet gas discharged through pipe 23 into the chamber within the cover. In addition, discoloration and decarburization is eliminated or reduced. 'Thus, preheating the base inhibits discoloration and decarburization of the ferrous .metal articles during the annealing process and apparently this is efiected by preventing subsequent contamination of the controlled atmosphere maintained Within the cover ll.

Electrical heaters have been shown and described for heating the base iii, but the present invention is not limited to the electrical heaters, and any other suitable means may be used for heating the base. The heating of the base to prevent contamination of the controlled atmosphere is not part of the annealing cycle as it constitutes a pre-treatment. If desirable, the

heaters

26 and 27 may be left on during the heating cycle so as to add heat to the charge to aid the annealing operation. Electrical heat is relatively expensive, and it is preferred to turn the heaters off after or just before the time when the cover I I is placed over the charge on the base.

I claim:

1. In the process of annealing a ferrous metal article in an annealing furnace including a base for supporting the article and a removable cover for covering the article on the base while being annealed, the combination of steps comprising, heating the uncovered base while exposed to atmospheric air, arranging the furnace cover over the article on the base while the base is at a temperature above 212 F., and heating and annealing the covered article while maintaining a controlled atmosphere within the cover about the article, the cover being placed on the base while at said temperature to inhibit subsequent contamination of the controlled atmosphere within the cover.

2. In the process of annealing a ferrous metal article in an annealing furnace including a base for supporting the article and a removable cover for covering the article on the base while being annealed, the combination of steps comprising, heating and maintaining the uncovered base at a temperature above 212 F. while exposed to atmospheric air, arranging the furnace cover over the article on the base while the base is at a temperature above 212 F., and heating and annealing the covered article while maintaining a controlled atmosphere within the cover about the article, the uncovered base being heated and maintained at said temperature until the cover is placed on the base to inhibit subsequent contamination of the controlled atmosphere within the cover.

3. In the process of annealing a ferrous metal article in an annealing furnace including a base for supporting the article and a removable cover for covering the article on the base while being annealed, the combination of steps comprising, heating the uncovered base from a temperature below 212 F. to a temperature above 212 F. while exposed to atmospheric air, arranging the furnace cover over the article on the base while the base is at a temperature above 212 F., and heating and annealing the covered article while maintaining a controlled atmosphere within the cover about the article, the uncovered base being heated to said temperature before the cover is placed on the base to inhibit subsequent contamination of the controlled atmosphere within the cover.

4. In the process of successively annealing a series of ferrous metal articles in an annealing furnace including a base for supporting such an article and cover for covering the article on the base while being annealed in a controlled atmosphere, the combination of steps comprising, arranging the cover over one of the articles of such a series on the base while the base is at a temperature above 212 F., heating and annealing the covered article while maintaining a controlled atmosphere within the cover about the article, cooling the annealed article, removing the cover from the base and the cooled article and exposing the base to atmospheric air, removing the annealed article from the uncovered base, and heating the uncovered base while exposed to the atmospheric air and arranging the cover over the next article of the series while the base is at a temperature above 212' F. to inhibit subsequent contamination of the controlled atmosphere maintained within the cover when subsequently annealing the next article of the series.

5. In the process of successively annealing a series of ferrous metal articles in an annealing furnace including a base for supporting such an article and cover for covering the article on the base while being annealed in a controlled atmosphere, the combination of steps comprising, arranging the cover over one of the articles of such a series on the base while the base is at a temperature above 212 heating and annealing the covered article while maintaining a controlled atmosphere within the cover about the article, cooling the annealed article, removing the cover from the base and the cooled article and exposing the base to atmospheric air, removing the annealed article from the uncovered base, and heating and maintaining the uncovered base at a temperature above 212 F. while exposed to the atmospheric air and arranging the cover over the next article of the series while the base is at a temperature above 212 F. to inhibit subsequent contamination of the controlled atmosphere maintained within the cover when subsequently annealing the next article of the series.

6. In the process of successively annealing a series of ferrous metal articles in an annealing furnace including a base for supporting such an article and cover for covering the article on the base while being annealed in a controlled atmosphere, the combination of steps comprising, arranging the cover over one of the articles of such a series on the base while the base is at a temperature above 212 F., heating and annealing the covered article while maintaining a controlled atmosphere within the cover about the article, cooling the annealed article, removing the cover from the base and the cooled article and exposing the base to atmospheric air, removing the annealed article from the uncovered base, and

EEFFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,427,319 Peacock M Aug. 29, 1922 1,546,532 Baily July 21., 1925 1,678,875 Rohn July 3l, 1928 2,201,308 Edge May 21., 1940