HIGH-TEMPERATURE FURNACE FOR THE ANNEALING OF SHEET-METAL JOINTS

Abstract

A high-temperature furnace for the annealing of sheet-metal joints (4) is described, having an annealing base (2), having a support device (3), which forms a placement surface (17) for coaxially receiving a sheet-metal joint (4) with spacing above the annealing base (2), having a protective hood (6), which coaxially encloses the annealing base (2) with the support device (3), and is connected to a protective gas supply line (10) and a protective gas exhaust, made of a cylindrical jacket (7) and a cupola (16) which terminates the jacket (7) on top, having a peripheral seal (9) between the annealing base (2) and the protective hood (6), and having a heating hood (13) which encloses the protective hood (6) with spacing. In order to allow uniform annealed material heating, it is proposed that the axial jacket section of the protective hood (6) which is determined by the vertical spacing (h) of the placement surface (17) of the support device (3) from the annealing base (2) has a surface which makes up at least three-fourths of the cupola surface.

Description

1. FIELD OF THE INVENTION

The invention relates to a high-temperature furnace for the annealing of sheet-metal joints having an annealing base, having a support device which forms a placement surface for coaxially receiving a sheet-metal joint with spacing above the annealing base, having a protective hood, which coaxially encloses the annealing base with the support device and is connected to a protective gas supply line and a protective gas exhaust, made of a cylindrical jacket and a cupola which terminates the jacket on top, having a peripheral seal between the annealing base and the protective hood and having a heating hood which encloses the protective hood with spacing.

2. DESCRIPTION OF THE PRIOR ART

Transformer sheet-metal made of steel having a silicon component of 0.5-3.5 wt.-% is typically subjected to a high-temperature treatment in a hood furnace for technological reasons. The annealed material is heated in a protective hood under protective gas, in particular nitrogen and/or hydrogen, up to 1200° C., essentially by radiant heat. At these high temperatures, the intrinsic strength of the annealed material decreases greatly, so that the sheet-metal joints, which comprise coiled steel strips, are each supported per se on a support device, which rests in a load-dissipating manner on an annealing base or penetrating the annealing base on a foundation. The known high-temperature furnaces of this type have the disadvantage above all that during annealing of a single sheet-metal joint, the sheet-metal joint which is centrally supported above the annealing base on the support device is heated significantly less in its lower area than in its upper area. The annealing procedure must therefore be lengthened in order to achieve the magnetic properties of the sheet metal over the entire joint height.

If two sheet-metal joints are each supported coaxially one above another on a corresponding support device, it is to be noted that the heating of the upper joint or the upper joints occurs significantly more rapidly than that of the lower joint, which thus does not reach the desired final temperature and therefore has correspondingly impaired magnetic properties. The thermal-insulating construction of the annealing base made of refractory concrete slabs provides hardly any improvements in this regard. In addition, to exhaust the protective gas introduced centrally into the protective hood, the protective hood is sealed via a sand bed, which receives the lower edge of the protective hood, so that the protective gas escapes through the sand bed into the heating hood and is exhausted from there.

SUMMARY OF THE INVENTION

The invention is therefore based on the object of implementing a high-temperature furnace of the type described at the beginning for the annealing of sheet-metal joints so that the annealed material can be heated uniformly to high treatment temperatures in a protective gas atmosphere, in order to be able to ensure advantageous, uniform annealing even of sheet-metal joints for transformer sheet-metal made of steel having a silicon content up to 3.5 wt.-%.

The invention achieves the stated object in that the axial jacket spacing of the protective hood, which is determined by the vertical spacing of the placement surface of the support device, has a surface which makes up at least three-fourths of the cupola surface.

The invention proceeds from the finding that additional radiant heat is supplied to the annealed material in the upper area of the protective hood via its cupola, so that in protective hoods for the annealing of a single sheet-metal joint, the sheet-metal joint is heated more rapidly in its upper area than in the lower area. During the annealing of two or more sheet-metal joints provided coaxially one above another on the support device, the upper or uppermost has a greater quantity of radiant heat applied thereto. To compensate for this varying heat introduction in the protective hood, the protective hood is lengthened so that an additional radiant surface, which corresponds to at least three-fourths of the cupola surface, is provided between the annealing base and the placement surface of the support device for the (lower) sheet-metal joint by the lengthened jacket of the protective hood in this area, so that a sufficient amount of heat can also be supplied to the annealed material in the lower area of the protective hood, in order to at least partially compensate for the heat supplied in the upper area of the protective hood. Because of this measure, substantially uniform heating of the annealed material to the treatment temperature can thus be ensured.

In order that a radiant surface corresponding to the radiant surface of the cupola can be achieved in the area of the placement surface of the support device via the protective hood jacket, the area of the jacket section of the protective hood corresponding to the vertical spacing of the placement surface from the annealing base must at least correspond to the cupola surface. In this way, however, heat losses because of the heat dissipation via the annealing base are not yet taken into consideration. For this reason, implementing the surface of the jacket section corresponding to the vertical spacing of the placement surface from the annealing base as greater than the cupola surface suggests itself. An enlargement by 5 to 20% is generally sufficient to cover the heat losses through the annealing base. In order that the heat dissipation via the annealing base can be largely suppressed, the annealing base can be constructed from mineral wool, which has a significantly higher thermal insulation value than typical refractory concrete slabs. However, larger loads cannot be dissipated via an annealing base constructed from mineral wool. For this reason, the carrier device penetrates the annealing base constructed from mineral wool and is supported on the furnace foundation for load dissipation. In order to ensure advantageous thermal insulation by the annealing base, the height of the annealing base constructed from mineral wool is to correspond to at least one-third, preferably one-half of the diameter of the protective hood.

If the peripheral seal between the annealing base and the protective hood is implemented as gas-tight, the protective gas supply line having outlet openings distributed around the circumference of the protective hood and the protective gas exhaust having an exhaust gas line centrally penetrating the annealing base, particularly advantageous conditions result in regard to the protective gas guiding, because the protective gas is heated uniformly on the external envelope of the annealing base and the internal surface of the protective hood cylinder. This heated protective gas flows past the external hot sheet-metal joint areas and is supplied via the colder areas in the sheet-metal joint interior to the exhaust gas line guided centrally through the annealing base.

A further possibility in order to take varying heating of the annealed material over the height of the protective hood into consideration comprises applying heating power which decreases with height to the protective hood through the heating hood, so that because of the higher heat supply in the lower area of the protective hood, improved heating of the annealed material can be ensured. In heating hoods having an electrical heater, separate sections of the heating unit which are each activatable per se may be provided according to height for this purpose. In a gas-heated heating hood, the burners may preferably be situated in the area between the annealing base and the placement surface of the support device for the annealed material which is provided above the annealing base.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention is shown for exemplary purposes in the drawing. In the figures:

FIG. 1 shows a high-temperature furnace according to the invention having a gas-heated heating hood in a schematic longitudinal section and

FIG. 2 shows an illustration corresponding to FIG. 1 of a high-temperature furnace according to the invention having an electrically heated heating hood.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The high-temperature furnace according to FIGS. 1 and 2 has a furnace foundation 1 having an annealing base 2, which is penetrated by a support device 3 for the annealed material, which is supported on the furnace foundation 1. The support device 3 is designed according to the two exemplary embodiments for receiving two sheet-metal joints 4, which are mounted one above another on support disks 5. For the annealing of the sheet-metal joints 4, they are enclosed by a protective hood 6, whose jacket 7 is terminated gas-tight with the aid of a placement flange 8 and a ring seal 9 in relation to the furnace foundation 1 and the annealing base 2. The protective gas, typically nitrogen and/or hydrogen, is supplied via a protective gas supply line 10, which has outlet openings 11 distributed around the circumference of the protective hood 6, so that the protective gas flows between the jacket 7 and the annealing base 2 along the jacket 7 into the protective hood 6 and can be exhausted centrally via an exhaust gas line 12 penetrating the annealing base 2.

The heating of the annealed material occurs via a placed heating hood 13, which has burners 14 distributed around the circumference according to FIG. 1 and has an electrical heating unit made of individual heating sections 15 which are each activatable per se according to FIG. 2. The annealed material is thus heated via the particular heating unit of the heating hood 13, essentially by radiant heat which is emitted from the heating hood 6. The heating hood 6 not only offers radiant surfaces in its jacket 7, but rather also using its cupola 16, so that the joint heating also occurs via the cupola surface in the area of the upper sheet-metal joint 4. In order to compensate for the greater heat introduction in the upper area of the heating hood 6 thus caused, the jacket 7 is lengthened in the area between the annealing base 2 and the placement surface 17 of the support device 3 for the lower sheet-metal joint 4. The configuration is made so that the vertical spacing h of the placement surface 17 of the support device 3 from the annealing base 2 determines a jacket section of the protective hood 6, which has a surface corresponding to at least three-fourths of the cupola surface, preferably the entire cupola surface. In order to consider additional heat losses via the annealing base 2, this lateral surface section 5 can be up to 20% larger than the cupola surface. A radiant surface is thus available for the heating of the lower sheet-metal joint 4, which is adapted to the radiant surface for the upper sheet-metal joint 4, which allows uniform heating of both sheet-metal joints 4, with the advantage that the material properties caused by the annealing correspond. To suppress heat dissipation by the annealing base 2, it can be constructed from mineral wool having good thermal insulation properties, at a height which corresponds to at least one-third of the diameter of the jacket 7 of the protective hood 6. An annealing base 2 constructed from mineral wool is not capable, however, of dissipating the loads caused by the annealed material to the furnace foundation 1, so that the support device 3 for the sheet-metal joints 4 is supported per se on the furnace foundation 1.

To support the uniform heating of the sheet-metal joints 4, the protective hood 6 can have varying heating power applied over its height. For this purpose, the burners 14 for the gas heating in FIG. 1 are situated below the placement surface 17 for the lower sheet-metal joint 4. According to FIG. 2, the individual sections 15 of the electrical heating unit may be activated, so that varying heating power may be ensured as needed depending on height by turning off or regulating individual sections, for example.

The invention is not restricted to the exemplary embodiments shown, of course. Thus, the high-temperature furnace may also only be designed to heat a single sheet-metal joint 4 or more than two sheet-metal joints 4. In case of heating of a single sheet-metal joint 4, the vertical spacing h, which is selected as a function of the cupola surface, of the placement surface 17 of the support device 3 from the annealing base 2 prevents uneven heating of this sheet-metal joint 4 depending on height.

Claims

1. A high-temperature furnace for the annealing of sheet-metal joints (4) having an annealing base (2), having a support device (3), which forms a placement surface (17) for coaxially receiving a sheet-metal joint (4) with spacing above the annealing base (2), having a protective hood (6), which coaxially encloses the annealing base (2) with the support device (3), and is connected to a protective gas supply line (10) and a protective gas exhaust, made of a cylindrical jacket (7) and a cupola (16) which terminates the jacket (7) on top, having a peripheral seal (9) between the annealing base (2) and the protective hood (6), and having a heating hood (13) which encloses the protective hood (6) with spacing, wherein the axial jacket section of the protective hood (6) which is determined by the vertical spacing (h) of the placement surface (17) of the support device (3) from the annealing base (2) has a surface which makes up at least three-fourths of the cupola surface.

2. The high-temperature furnace according to claim 1, wherein the surface of the jacket section corresponding to the vertical spacing (h) of the placement surface (17) from the annealing base (2) at least corresponds to the cupola surface.

3. The high-temperature furnace according to claim 1, wherein the surface of the jacket section corresponding to the vertical spacing (h) of the placement surface (17) from the annealing base (2) is 5 to 20% greater than the cupola surface.

4. The high-temperature furnace according to claim 1, wherein the annealing base (2) is constructed from mineral wool, and the support device (3) penetrates the annealing base (2) and is supported on a furnace foundation (1).

5. The high-temperature furnace according to claim 4, wherein the height of the annealing base (2) constructed from mineral wool at least corresponds to one-third, preferably one-half, of the diameter of the protective hood (6).

6. The high-temperature furnace according to claim 1, wherein the peripheral seal (9) between the annealing base (2) and the protective hood (6) is implemented as gas-tight, and the protective gas supply line (10) has outlet openings (11) distributed around the circumference of the protective hood (6), and the protective gas exhaust has an exhaust gas line (12) which centrally penetrates the annealing base (2).

7. The high-temperature furnace according to claim 1, wherein the protective hood (6) can have a heating power, which decreases according to height, applied to it by the heating hood (13).