US3409963A - Method and apparatus for descaling billets - Google Patents

Description

1968 o. A. EDGECOMBE ETAL 3, 6

METHOD AND APPARATUS FOR DESCALING BILLETS I Filed May 27, 1966 EXTRUSION PRESS DING FURNACE R S AM n l n United States Patent 3,409,963 METHOD AND APPARATUS FOR DESCALIN G BILLETS David A. Edgecombe and James S. Trees, Beaver Falls,

and Vincent C. Giolfre, Koppel, Pa., assignors to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Continuation-impart of application Ser. No. 331,024,

Dec. 16, 1963. This application May 27, 1966, Ser.

7 Claims. (CI. 29-81) ABSTRACT OF THE DISCLOSURE A clean, ferrous metal billet is prepared for hot extrusion by heating it in a fossil fuel furnace to a temperature not exceeding 1900 F. It is withdrawn from the furnace and coated with a scale-fluxing protective material of glass and salt and then heated throughout its entire volume to an extrusion temperature by an electric induction heating coil to form a slushy layer thereon. The slushy layer is then removed from the billet by high pressure liquid jets without appreciably lowering the surface temperature of the billet.

This application is a continuation-in-part of our prior application Ser. No. 331,024, filed Dec. 16, 1963 and now abandoned.

The present invention relates to the heating and descaling of ferrous metal workpieces, and more particularly to the preparation of ferrous metal billets for hot extrusion.

The general object of this invention is the provision of an economical and highly effective ferrous metal billet heating and descaling process adapted for use with all grades of carbon, low and medium alloy steels, and some high alloy steels, and more particularly a billet heating and descaling process especially designed for use with ferrous metal billets intended for hot extrusion into high quality hollow and solid sections by an extrusion press.

Billets intended for high quality extrusion products must be completely free of surface scale or any other harmful foreign material in order to prevent damage to the toolage of the extrusion press and/or to prevent a flaw in the surface of the extruded product. To accomplish such results it is usual to prepare the billets with predetermined dimensions and substantially clean surface characteristics prior to heating such billets. Ordinarily, medium and high alloy steels, such as stainless steels, will have the surface of the billets machined prior to heating, whereas with other types of steel the billets will be in an as rolled pickled condition prior to heating.

For this purpose, the billets must either be heated so as to completely avoid the formation of scale during heating or the heated oxidized billets must be completely descaled after heating. Scale-free billet heating can be carried out in a controlled atmosphere furnace or by the use of a salt bath furnace. Both of these billet heating methods are very expensive compared to convection heating in a conventional gas fired furnace. There is also danger in a controlled atmosphere furnace of damaging the surface of stainless steel billets by carburization. Salt bath furnaces are not only expensive to build and maintain but are also expensive to operate and represent a hazard to the operators in the event of a leak of molten salt through a furnace wall.

Post heating descaling of the billet, i.e. descaling the billet after heating has been completed, is difficult to accomplish with 100% scale removal. Mechanical methods have been used to break up and remove the scale on 3,409,963 Patented Nov. 12, 1968 "Ice heated billets, but these methods seldom result in complete scale removal and frequently cause upsetting of the billet or a significant loss of metal from the billet, resulting in errors in the dimensions of the extruded product. In either case the change in billet size or shape may be damaging to the product. For quality extrusion the billet must be delivered to the press with a precise size and shape.

It is essential that the billet should be at the proper hot working temperature on reaching the extrusion press and that the temperature distribution in the billet should be as uniform as possible. This requires that the billet surfaces not be allowed to become appreciably chilled by excessive delays between the final heating of the billet and its introduction into the extrusion press or by the character of the descaling operation itself.

In accordance with our invention, a billet heating and descaling process is provided in which ferrous metal billets are heated in two successive stages, in the first of which substantially clean but uncoated billets are heated in a conventional fossil fuel fired rotary furnace to a temperature such that the formation of surface scale is minimized and without the use of a protective furnace atmosphere, withdrawn from the furnace and coated with a special protective scale-fluxing chemical compound while at a temperature substantially equal to their temperature when leaving the furnace.

It is economically and operationally important that the final heating of the coated billets to the desired working temperature be done under conditions which will insure a substantially uniform distribution of heat throughout the entire volume of the billets, and in a time period sufficient to effect a chemical combination of the scale-fluxing compound with the scale present on the circumferential and leading end surfaces of the billet, but, not long enough to cause the coating and and scale to form a solid layer tightly adhering to the billet surfaces which would be extremely diflicult to remove. Each coated billet is preferably moved through an electric induction heating furnace and rapidly heated throughout its entire volume to a temperature slightly above the desired working, i.e. extrusion, temperature, for example 2300 F., while in an unprotective furnace atmosphere. During this second stage high temperature heat treatment, which should be completed in substantially less than one hour, the coating compound fuses and spreads over the end and circumferential surfaces of the billet and chemically combines which the scale thereon forming a viscous slush-like layer.

After this short time rapid heating of the billet, the billet is immediately subjected to a special descaling operation in which the slush-like coating of scale-fluxing compound and scale is separated and removed by the action of a cone-shaped sheet of high pressure water impinging at an acute angle on the coated surfaces of the relatively moving billet, producing a completely descaled billet at a temperature suitable for extrusion. The rapid movement of the billet through the descaling device and small amount of descaling water consumed by the high pressure jets avoids any appreciable chilling of the contacted surfaces of the billet.

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which we have illustrated and described a preferred embodiment of apparatus for carrying out the invention.

The single figure illustrates diagrammatically the path of movement of a billet through a heating and descaling apparatus arranged in an extrusion plant according to the present invention.

As indicated in the drawing, substantially clean billets 10 of cylindrical shape are transferred from storage (not shown) to a rotary furnace 11 wherein they are progressively heated to a substantial temperature while moving between the furnace inlet and outlet points. In the embodiment shown, the rotary furnace is of conventional construction and heated by the combustion of a fossil fuel, such as oil or gas. The rotary furnace is operated under slightly reducing atmospheric conditions to minimize oxidation of the billet surfaces during heating. The rate of oxidation or scaling of ferrous metal billets increases with the billet temperature, with a relatively low rate occurring at temperatures below 1600 F. and a high rate at temperatures above 1900 F. It has also been found that the furnace heat input to heat such billets to a working temperature of the order of 2300 F.

is approximately twice the fuel requirements for heating the billets to 1900 F.

In accordance with our invention, the billet heating in the rotary furnace is advantageously limited to a billet temperature of approximately 1900 F., while the second stage of heating to a working temperature of the order of 2300 F. is carried out in heating equipment designed for a rapid, intense heating of individual billets. An electric induction heating furnace 13 is particularly adapted for effecting such heating at a much lower cost than in a rotary furnace. In contrast to a heating period of 1 /2-2 hours in the rotary to reach a metal temperature of 1900 F., each billet is heated to 2300 F. in a period substantially less than one hour, and preferably of but -15 minutes, in the induction heating furnace.

In order to minimize scale formation on the billets during the second stage of heating and to eliminate what scale has formed on the billets in the rotary furnace and after withdrawal therefrom, each heated billet at a temperature approximately 1900 F. as withdrawn from the rotary furnace is immediately passed to a coating device 12 in which the billet is given a coating of a special mixture of a scale-fluxing material and a protective glass. In the apparatus illustrated, each billet is rolled over a bed of a dry powdered mixture of these materials located between the rotary furnace 11 and the induction furnace 13.

While the coating composition may vary considerably both as to the materials used and the proportions of these materials, a preferred coating is formed of a mixture of one part of a finely divided salt, preferably barium chloride (BaCI and three parts of powdered dry glass. Window glass or a lubricating glass known in the extrusion art as 3KB glass. may be used. A typical 3KB glass may contain 47% SiO 28.3% B 0 12.1% CaO; 0.2% MgO; 3.8% A1 0 8.4% Na O; and 0.1% CuO. A typical window glass may contain 71% SiO 9 to 12% CaO; 12 to 14% Na O; 0.5 to 1.5% A1 0 and 2% MgO. Other salts may be used in place of Bacl e.g. sodium 'or potassium chloride, and the proportions of salt to glass can be changed from a preferred 1 to 3 ratio, to equal proportions, 1 to 1, and even to 2 parts of glass to 1 part of salt. Proportions beyond the ranges mentioned are not as effective and either will not completely descale a billet or may require an additional pass of the billet through the heating and descaling apparatus to attain the desired billet cleanness.

With the coating mixture in dry powder form, the hot billet circumferential surface at a temperature approaching the melting point of the mixture rolling over the bed picks up suflicient material to form a fairly uniform coating thereon. On further heating, the coating mixture becomes viscous and wets the circumferential surface of the billet. The coating in this state not only tends to prevent further oxidation of the billet surface by covering that surface, but also combines with the oxide scale thereon to form a slush-like layer of scale, flux, iron and glass loosely adherent to the heated billet.

On leaving the coating device, the billet becomes the last of a line of end contacting coated billets moving continuously through one or more electric coils forming the induction heating furnace 13. The electric currents induced in the line of billets cause them to be rapidly and uniformly heated throughout their length and crosssection to the desired working temperature of the order of 2300 F. It has been found that a period of 5-15 minutes is adequate to bring each billet to its desired final temperature and yet leave the viscous combined coating and scale layer in condition to be relatively easily separated and removed from the billet in a subsequent operation. The induction heating furnace is normally open to the ambient atmosphere at one or both ends, permitting oxidation of any exposed portion of the billet front end and circumferential surfaces. The opposite or rear end surface of each billet is in intimate contact with the front end of the next billet when passing through the induction heating furnace and thus requires no descaling treatment.

As each billet passes out of the induction furnace 13, it is immediately conveyed to the descaling apparatus 14, where the front end and circumferential surfaces are completely cleaned of the layer of coating compound and scale thereon, presenting an unsually clean appearance and without any appreciable loss of met-a1 from the billet or change in its size or dimensions. The descaling apparatus used is preferably of the high pressure water jet type, as disclosed in US. Patent No. 2,785,924, wherein a jet ring is arranged to discharge a converging sheet of high pressure water first against the front end of the approaching billet and then impinging at an acute angle to the longitudinal axis of the billet on the circumferential surface of the billet, undercutting the slush-like layer of coating compound and scale thereon and removing the separated material. For example, in operation a jet spray ring of 10" inside diameter, using a water pressure of 1850 psi. at a rate of 163 gal. per minute was entirely effective on 7%" diameter billets passing through the jet ring at the rate of 60 feet per minute. Under the described conditions approximately 4 gal. of water was used to descale a billet and the descaled billet temperature was approximately 2280 F., the cooling effect of the water on the surface of the billets being inconsequential.

The billets on leaving the descaling device 14 are in an exceptionally clean condition and at the desired hot working temperature for high quality extrusion. Where the extruded product is a solid section, the descaled heated billets are coated with a glass lubricant in accordance with the process disclosed in US. Patent 2,908,587 before delivery to the extrusion press 15. Where the extruded product is to be hollow, the billet is first passed through a piercing press 16 and then in addition to the coating of the peripheral surface, a powdered glass lubricant isalso supplied to the central billet passage into which the extrusion press mandrel will be introduced, as disclosed in US. Patent No. 2,908,384. A holding furnace 17 may be used to compensate for any loss of billet temperature in the piercing press and/or temporary delays in the extrusion cycle.

While in accordance with the provisions of the statutes we have illustrated and described herein the best form and mode of operation of the invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of preparing a billet of ferrous metal for hot extrusion which comprises heating a substantially clean billet of circular cross-section to a billet temperature not exceeding approximately 1900 F. by the combustion of a fossil fuel in an atmosphere permitting formation of surface scale on the billet, withdrawing the heated billet and coating the withdrawn billet with a layer of finely divided solid scale-fluxing protective material, rapidly heating said coated billet throughout its entire volume to an extrusion temperature by electrical induction heating of the billet under scale-forming atmospheric conditions while said coating material combines with the scale to form a slushy layer on the outer surface of the billet, and contacting said heated billet with a high pressure annular liquid jet arranged to impinge on the circumferential surface of the billet at an acute angle and remove the slushy layer without appreciable lowering of the surface temperature of the billet.

2. The method of preparing a billet of ferrous metal for hot extrusion according to claim 1, wherein said scalefiuxing protective material consists substantially of a mixture of glass and salt in a ratio of 25 to 50% salt and 50-75% glass.

3. The method of claim 2, wherein said glass is ordinary window glass.

4. The method of claim 3, wherein said salt is one or more of the group of barium chloride, sodium chloride and potassium chloride.

5. The method of preparing a billet of ferrous metal for hot extrusion according to claim 1, wherein the electric induction heating of said coated billet is limited to a maximum of approximately one hour.

6. The method of preparing a billet of ferrous metal for hot extrusion according to claim 1, wherein the electric induction heating of said coated billet occurs in a period of approximately 5 to minutes.

7. Apparatus for preparing a billet of ferrous metal for hot extrusion which comprises a furnace for heating a substantially clean billet of circular cross-section to a billet temperature not exceeding approximately 1900 F. by the combustion of a fossil fuel in an atmosphere permitting formation of surface scale on the billet, means for withdrawing the heated billet from said furnace, means for coating the withdrawn billet with a layer of finely divided solid scale-fluxing protective material, an electric induction heating coil for rapidly heating an encircled coated billet throughout its entire volume to an extrusion temperature under scale-forming atmospheric conditions while said coating material combines with the scale to form a slushy layer on the outer surface of the billet, means for discharging a high pressure annular liquid jet against the billets leaving said coil, said jet being arranged to impinge on the circumferential surface of the billet at an acute angle and remove the slushy layer without appreciable lowering of the surface temperature of the billet.

References Cited UNITED STATES PATENTS 2,374,634 4/1945 Wile 21913 2,477,411 7/1949 King 2191 2,785,924 3/1957 Kane 200l07 FOREIGN PATENTS 743,095 1/ 1946 Great Britain.

OTHER REFERENCES The Making, Shaping and Treating of Steel, 1957 edition, p. 774.

WILLIAM I. PRICE, Primary Examiner.

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