US3383098A - Reheat furnace having skid rails - Google Patents

Description

May 14, 1968 J, O'REILLY ET REHEAT FURNACE HAVING SKID RAILS Filed May 23, 1966 NN1\ \N N N \N x x Ni 0 Q IJVI/E'NTOR5. ALBERT L. PEN/ EX J45 E OWE/LL) JflH/V L. FREE MAN I 4TraZFWr United States Patent 3,383,098 REHEAT FURNACE HAVING SKID RAILS Jack E. OReilly, 1630 Evergreen Road, Homewood, Ill.

60430; John L. Freeman, 200 Mount Lebanon Blvd.

15234; and Albert L. Renkey, 629 Broughton Road 15102, both of Pittsburgh, Pa.

Filed May 23, 1966, Ser. No. 552,240 7 Claims. (Cl. 263-6) ABSTRACT OF THE DISCLOSURE A reheating furnace consisting of a furnace hearth having skid rails mounted thereon in spaced relation, the skid rails being fabricated from fusion cast, non-basic refractory shapes, the furnace bottom between the skid rails being fabricated from preformed phosphate bonded, unburned, high alumina refractory shapes.

Reheating furnaces are used to raise the temperature of steel ingots, billets, slabs, and the like, in the course of processing until they are sufficiently hot to be plastic enough for economic reduction by rolling or forging to the desired section. These furnaces may be divided into two general classes; namely, a batch type in which the charged material remains in a fixed position on the hearth until heated to rolling temperature, and a continuous type in which the charged material moves through the furnace and is heated to rolling temperature as it progresses through the furnace. In the latter type the shapes to be heated can be charged either from the end or through a side door. In either case, the shapes are moved through the furnace by pushing the last piece charged with a pusher at the charging end. As each cold piece is pushed into the furnace against the continuous line of material, a heated piece is removed. The heated piece is discharged either through an end door, by gravity, upon a roller table which feeds the mill or it is pushed through a side door to the mill table by suitable mechanical or manual means.

One type of a continuous reheat furnace uses'water cooled skid rails for-a portion of its length and a refractory skid rail hearth combination for the balance. Heat is applied above and below the ware being pushed through the furnace on the skid rails. Another type of reheating furnace employs a refractory hearth for a larger portion of its length. By refractory hearth, we mean one in which the skid rails are disposed within the refractory bottom of the furnace and are composed of a refractory material. In this case, heat is applied above the ware being pushed through this furnace. The present invention is primarily concerned with the refractory skid rails and hearth combination in either type of furnace.

In processing metal shapes or slabs in reheating furnaces, the shapes enter the furnace coated with an outer crust of mill scale or slag. Further scale development may come about in the furnace. As the slabs pass over the skid rails, the rails are subjected to thermal shock and abrasion. Further, the mill scale and slag fall off the slabs and a portion tends to adhere to the skid rails while the balance is deposited on the refractory structure .supporting the skid rails. The slag tends to permeate the supporting refractory structure thus causing ultimate failure thereof and complete collapse of the structural integrity of the hearth itself.

Accordingly, it is desirable that a combination of refractory shapes be employed in the composite hearth that would render a balanced service life for the entire structure in the most economical manner. The different types of refractories employed should wear uniformly and require replacement at about the same time. This necessitates that both types of refractories have a relatively high degree of abrasion resistance, with the skid rail refractories generally requiring a higher degree of abrasion resistance since they usually project above the surface of the hearth bottom and are in direct contact with the slabs being processed.

Both types of refractories should also have good resistance to thermal shock and resistance to slag and mill scale penetration. Fusion cast refractories which are usually employed to fabricate skid rails, inherently have good resistance to slag penetration; however, it would be economically unfeasible to use such refractories in the entire hearth.

Accordingly, it is an object of this invention to provide improved reheating furnaces of the type having a composite hearth installation.

Another object of the invention is to provide a composite hearth installation for reheating furnaces consisting of refractory skid rails and bottom having relatively good thermal shock and abrasion resistancefand a relatively high resistance to penetration by molten mill scale and slags.

Other objects of the invention will, in part, become apparent hereinafter.

In order to more fully understand the nature and scope of the present invention, reference should be had to the following description and drawings in which:

FIG. 1 is an elevation view in cross section of a reheat furnace relating to the invention, and

FIG. 2 is an elevation view, partly in cross section, of section A-A in FIG. 1.

In the broadest aspect of the invention, there is provided a metallurgical vessel, particularly a reheating furnace, consisting of a furnace chamber having an intake and discharge end, and a composite hearth in at least a portion thereof. The hearth contains preformed refractory shapes of at least two different types. One type is preformed fusion cast, non basic refractory shapes and another type is phosphate bonded, unburned, high alumina shapes.

In a preferred embodiment of this invention the preformed, fusion cast shapes are high alumina shapes substantially free of interstitial glass. That is, the shapes are petrographically characterized as having about 99% of alpha alumina and beta alumina crystals. The shapes analyze, on an oxide basis, at least about A1 0 with no more than about 1% SiO by weight, since SiO is a glass former. The presence of interstitial glass is undesirable since at elevated temperatures the glass tends to exude from the refractory and reacts with iron oxide from the slag and scale to form undesirable low melting fayalite.

In Table I below, there is given the typical chemical and petrographic analysis and physical properties of two particularly suitable fusion cast compositions, A and M, for use as skid rails. These compositions are proprietary products of the Harbison-Carborundu-m Corp., a subsidiary of the present assignee.

TABLE I Typical Chemical Analysis; Percent:

Composition A may be prepared by heating calcined 3 alumina (99+% A1 to a temperature of about 3800 F. After melting of the alumina, it is poured into molds and solidified. Composition M is prepared similarly with the exception that a small amount of Na O is mixed with the calcined alumina.

The above table shows the shapes to have a high bulk density, a relatively high modulus of rupture and resistance to load.

Modulus of rupture is a standard test in refractory studies. It is determined with simple apparatus, exhibits a good precision, and gives an excellent measure of strength. Therefore, its determination is often made in lieu of abrasion testing which requires much more elaborate equipment. Our experience has confirmed this relation of transverse strength to abrasion resistance and we prefer to rely upon the results learned through modulus of rupture measurements to indicate the degree of abrasion resistance, due to the excellent reproducibility of the modulus of rupture test. Hence, our experience has shown that where the strength of a brick increases, its resistance to abrasion is improved and, because modulus of rupture is a standard test which is recognized for its precision, we prefer to construe an imperical degree of abrasion resistance from this test.

High alumina refractories are generally classified by their A1 0 content in groups having approximately 50 to 90% A1 0 and 9099% A1 0 by analysis. Those containing 50-90% of A1 0 are made by blending various high alumina refractory materials, while those of 99% content are made from high purity alumina. The common high alumina refractory materials and their typical A1 0 contents are calcined alumina, 99%; calcined South American bauxite, 88%; calcined Alabama bauxite, 74%; calcined diaspore, 76%; burley diaspore, 48 and 58%; and kyanite, 56%. All of these materials are chemically compatible and accordingly, they can be blended to provide almost any desired resultant alumina content. Further adjustment is sometimes accomplished by including minor amounts of clay or silica.

While a preformed, phosphate bonded, unburned high alumina refractory shape having any alumina content of the above description, may be employed in the present invention, it is preferred that the alumina content range between about 80 and 90%, by Weight. Such a shape is readily :made by combining predetermined proportions of calcined South American bauxite, calcined alumina and Jackson ball clay and tempering with a suitable amount of phosphoric acid (i.e., to provide a P 0 content equivalent to that provided by about 2 to about of 85% phosphoric acid). A typical chemical analysis of the refractory ingredients set forth above is given in Table 11 below.

TABLE II.CALCINED AND TABULAR ALUMINA Percent Al O 99.4 SiOg 0.3 F6203 0.2 Alkaline earth oxides 0.1

As an example, a plurality of refractory shapes were made from a mixture of refractory raw materials shown in Table III. The batch was tempered with sufficient phosphoric acid of concentration to provide about a 6% P 0 content based upon the total weight of the batch. The mixture was power pressed at about 4000 p.s.i. into standard 9 inch straights. The resulting brick were dried and tested for density, modulus of rupture, panel spalling loss and linear subsidence.

TABLE 111 Mix, percent:

Calcined S.A. bauxite, -4 mesh 52 Calcined S.A. bauxite '(BMF) 28 Calcined alumina 15 Jackson ball clay 5 Density, p.c.f. 182 Modulusof Rupture, p.s.i 2540 After heating to 2000 F., p.s.i 3200 Reheat 2000 F.:

Linear change 0.0 Volume change 0.0 Loss in panel spalling test with preheat at 3000 F. (1650 C.) Load test, 25 p.s.i.:

Linear Subsidence at 2640 F., percent 1.1

The test results indicate that the brick had gOOd density and strength and a relatively high abrasion resistance as evidenced by the modulus of rupture at 2000 F.

The shapes were then tested for slag resistance. A cylindrical cup about 2 inches in diameter and about inch deep was cut in the face of each brick. The cup was filled with about grams of ground slag. Table IV below shows the approximate chemical analysis of the two different types of slags employed in this test.

TABLE IV Percent Percent Silica (SiOQ) 12. 9 24. 0 Alumina. (AlzO3) 4. l 23. 0 Titania (TiO2) 0. l 1. 5 Iron Oxide (Ft-3203).--- 37.9 1. 8 Iron (FeO) 43.2 Lime (CaO) 0.25 40. 0 Magnesia (MgO 0. 51 2. 2 Soda (NazO).. 0. OS 0.5 Potash (Kz0) 0.12 0.5 Lithia (LizO) 0.10 Manganese Oxide (MnO) 0.3 2.0

The test specimens charged with slag were heated to between 2700 and 2800 F. and held at this temperature for about five hours. After cooling, the brick was broken in two for inspection. Test results showed the brick to be in excellent condition and almost unaffected by either slag.

In view of the results above, and service data, it appears that the combination of unburned, phosphate bonded, high alumina shapes and fusion cast high alumina shapes substantially free of interstitial glass provides the best combination of properties in a composite hearth installation for reheating furnaces in terms of increased service life and in overall balance in wear and deterioration from use, and is the best mode for practicing the invention.

Referring now to the drawings, in FIG. 1, a counter current fired, continuous, reheat furnace is schematically indicated to contain an outer metal shell 10 lined with a refractory material 12. The furnace has a furnace chamber 14 having an intake door 16 and a discharge door 18. A burner 20 heats the interior of the furnace. Located at the bottom of the furnace is a composite hearth 22 which consists of preformed refractory shapes 24 and skid rails 26 which extend a substantial portion of the distance between the intake door 16 and the discharge door 18.

As is shown more clearly in FIG. 2, a pair of substantially parallel skid rails 26 is provided in the furnace to facilitate passage of the slabs therethrough. The skid rails 26 are fabricated of fusion cast refractory shapes in accordance with the present invention. The preformed shapes adjacent the skid rails, at least therebetween, is composed of phosphate bonded, high alumina, unburned refractory shapes. Other portions of the composite hearth may be composed of this material if desired.

The drawing shows that the shapes 24 are seated flush against the skid rail shapes 26. This may be accomplished if desired. However, in order to avoid cutting of the shapes and fitting them around the skid rails, they may be laid as closely as possible to the rails and the space therebetween may be filled with a plastic ramming mix of similar composition.

Also, while FIG. 2 shows the skid rails to project above the remainder of the hearth bottom, they may actually be flush with the shapes 24 as some installations desire. In addition, while the drawing shows a furnace chamber having an only pair of skid rails, it should be understood that the furnace may contain additional pairs of skid rails or may be divided into more than one chamber containing one or more pairs of skid rails.

While the invention has :been described with regard to particular embodiments and examples, it should be understood that modifications, substitutions and the like may be made therein without departing from its scope.

Having thus described the invention in detail and with sufficient particularity as to enable those skilled in the art to practice it, what is desired to have protected by Letters Patent is set forth in the following claims.

We claim:

1. A reheating furnace consisting of a furnace chamber having an intake and discharge end, at least one pair of substantially parallel skid rails extending at least a portion of the "distance between said ends and supported within a furnace bottom, said skid rails being fabricated from preformed, fusion cast, non-basic refractory shapes, said furnace bottom, at least between the skid rails, being fabricated from preformed phosphate bonded, unburned,

high alumina refractory shapes, said latter shapes being substantially contiguous said fused cast shapes.

2. A furnace according to claim 1, in which said fusion cast refractory shapes are substantially free of interstitial glass.

3. A furnace according to claim 1, in which said fusion cast refractory shapes analyze, on an oxide basis, at least about 95% A1 0 with no more than about 1% SiO by weight.

4. A furnace according to claim 1, in which said fusion cast refractory blocks petrographically analyze about 99% of alpha alumina crystals and beta alumina crystals.

5. A furnace according to claim 1, in which said unburned refractory shapes analyze, on an oxide basis, between about 80 and 90% A1 0 6. A furnace consisting of a furnace chamber having an intake and discharge end, and a composite hearth, said hearth consisting of preformed refractory shapes of at least two different types, one type being fusion cast, nonbasic refractory shapes and another type being phosphate bonded, unburned high alumina refractory shapes.

7. The furnace of claim 6, in which the fusion cast shapes are composed of high alumina material and are substantially free of interstitial glass.

References Cited UNITED STATES PATENTS 2,295,474 9/ 1942 Horn 263-6 2,618,671 11/1952 Van der Pyl z 263-6 X 2,984,474 5/1961 Emerson 263-6 3,258,255 6/1966 Tippmann 263-6 FOREIGN PATENTS 925,950 4/ 1955 Germany.

FREDERICK L. MATTESON, JR., Primary Examiner. JOHN J. CAMBY, Examiner.

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