Vacuum Induction Melting Furnaces

Abstract

Vacuum induction furnaces comprising a crucible having a lining layer of bricks of sintered fusible lime, and surrounded by an insulated inductive heating coil present a problem of preventing metal break-out during use. This problem is overcome according to the invention by the provision of a crucible comprising three layers, namely progressing inwardly a heat-insulating ceramic layer, a rammed oxide layer and the said lining layer, the thickness of the said layers being selected so that in use the rammed oxide layer sinters in a zone adjacent the lining layer, the unsintered zone thus taking up movement due to contraction and expansion effects of the crucible in use.

Description

This invention relates to vacuum induction melting furnaces, of the type comprising a crucible having a lining layer of bricks of sintered fusible lime and surrounded by an insulated inductive heating coil.

Fused lime in the form of a rammed mass or bricks has previously been proposed for lining metallurgical furnaces, particularly melting furnaces for the production of high-alloy steels and non-ferrous metal alloys. Such a lining is produced by melting a conventional burned lime which after cooling is size reduced by crushing, granulating or grinding. It has also been proposed to use an at least 98% pure lime for applications in which high resistance to hydration is essential. The fused lime may also contain zirconia and possibly magnesite in specified quantities.

Due to the extremely low oxygen partial pressure of lime, conventional linings made of fused lime also provide major advantages for melting high-alloy steels and alloys in a vacuum. Thus the use of fused lime has been described for lining crucibles used for vacuum melting. However the lining of crucibles of vacuum furnaces with bricks made of fused lime is difficult and cannot readily be achieved. The lined crucibles of vacuum furnaces do not have a long life because the melt runs out through the joints of the lining and thereby damages the induction coil. It is the object of the present invention relating to vacuum induction melting furnaces to improve the durability of the lining of the crucible of such furnaces, which lining comprises a layer of fused lime bricks.

The invention provides a vacuum induction melting furnace which includes a crucible having a lining layer of bricks of sintered fusible lime, an insulated inductive heating coil surrounding the crucible, a heat-insulating ceramic layer interposed between the heating coil and the crucible adapted to provide a barrier against metal break-out, and a rammed oxide layer interposed between the heat-insulating ceramic layer and the crucible lining, the oxide layer being a layer of metal oxide which does not form a relatively low-melting point eutectic with lime, the thicknesses of the heat-insulating ceramic layer, the rammed oxide layer and the lining layer being selected so that, in operation, the rammed layer becomes sintered in a zone adjacent the lining layer.

The inductive heating coil needs to be insulated due to the high electrical potentials used in large furnaces. Thus the crucible used according to the invention comprises three layers inside the induction heating coil. From the outside inwards the layers consist of a ceramic layer which in preferred embodiments of the invention consists of a mass of plastics ceramic material, or a wall of fused oxide bricks; a rammed layer consisting of the oxidic material, and finally an inner lining for the crucible in the form of a layer of sintered fused lime bricks. The outer ceramic layer which covers the inside of the induction heating coil is required firstly to protect the coil from break-out of metal and secondly to provide heat insulation, the latter feature being contrary to an open induction furnace in which the transfer of heat to the coil is desired to be a maximum. The purpose of providing heat insulation by the outer layer is to impede the heat flowing from inside the crucible to the outside and thereby to cause that part of the rammed layer to sinter which is adjacent the layer of lime bricks. However, the layer of rammed material which consists of fused oxides should not sinter completely through the layer thereof.

According to the invention the thicknesses of the several layers are balanced to ensure that the inner zone of the rammed layer of material sinters but not the outer zone. To achieve this effect the ceramic layer covering the heating coil should not be so thick as to allow the heat flux to become too high, but it must be sufficiently thick to permit the inner zone of the rammed layer to sinter. The thickness of the layer of lime bricks which has good thermal insulating properties must be selected so that the adjoining inner zone of the rammed layer will sinter. The thickness of the rammed layer must be sufficient to prevent the entire layer from sintering through at the existing thermal gradient. The partial sintering of the rammed layer serves the purpose of permitting the unsintered zone of the rammed layer slidably to move without causing the sintered zone which is intended to stop liquid metal that may have penetrated the joints of the bricks from penetrating further.

In order further to improve the sliding movement when the lining expands and contacts in use, another useful feature of the crucible lining according to the invention consists in interposing a web of spun glass fibers between the ceramic layer covering the inside surface of the heating coil and the rammed layer. The glass silk web firstly has the effect of acting as a separating layer between the ceramic and the rammed layers, and secondly of providing a useful sliding surface for co-operation with the unsintered zone of the rammed layer when the crucible lining expands and shrinks.

According to the invention the floor of the crucible may consist of an outer layer of oxide ceramic refractory bricks and a covering layer of fused lime in the form of a rammed composition or of bricks.

One embodiment of the invention is hereinafter described and illustrated in the accompanying drawing which is a schematic longitudinal elevation of part of a vacuum induction melting furnace according to the invention.

Referring to the drawing, an electrical heating coil 1, comprising coils 2 provided with an electrical insulation 3, is covered on the inner side with a layer 4 of a plastics ceramic mass or of bricks. Preferably a web 5 of spun glass fibers covers the inside surface of the said ceramic layer 4. The floor of the crucible is preferably constructed of bricks 6. Bricks 7 of sintered fused lime form the lining of the crucible and are also disposed on the said crucible floor 6. A rammed oxide ceramic layer 8 is interposed between the layer of the said lime bricks 7 and the spun glass fibers 5. The inside zone 9 of the rammed layer 8 adjoining the layer 7 of lime bricks is sintered by heat penetrating outwardly through the layer during the melting down of a charge in the crucible, the outer zone of the rammed layer 8 remaining in an unsintered condition, enabling the said rammed layer slidably to move when the crucible expands or contracts in operation. If the lining of the crucible consisted of a completely sintered material the lining would be fractured during such expansion and the contraction resulting in molten metal breaking out through the lining from the interior to the outside.

The feature of a lining of the crucible of a vacuum induction furnace comprising an inner zone of sintered fusible lime provides the conditions that enable high-alloy steels and alloys to be vacuum melted and the life of the crucible to be prolonged. High-alloy steels, some of which have a high affinity for oxygen and alloys, for example high tensile steels and super alloys based on nickel and cobalt that harden with a martensitic structure, can be melted without reducing the crucible material as occurs in the case of conventional crucible materials, and without thus causing an increase both in oxygen concentration in the melt and the concentration of metal liberated by the reduction. Moreover, the alloying and refining elements zirconium, titanium, aluminum, beryllium, niobium, tantalum, boron and the rare earths, which partly also serve for binding and neutralizing non-metallic impurities and trace elements, can more effectively be prevented from slagging in a vacuum melting furnace lined with fusible lime according to the invention than in crucibles having conventional rammed linings possessing higher oxygen partial pressures.

Since there is no risk of the lime lining being reduced, the vacuum may be raised to values of the order of 10.sup.-.sup.4 torrs at which level a substantially more intense and rapid degassing and evaporation of the trace elements and a reduction of the carbon/oxygen equilibrium can be achieved, resulting in shorter melting times.

Claims

What is claimed is:-

1. A vacuum induction melting furnace comprising

a. a crucible having a lining layer of bricks of sintered fusible lime,

b. an insulated inductive heating coil surrounding the said crucible,

c. a heat-insulating ceramic layer interposed between the said heating coil and the said crucible adapted to provide a barrier against metal break-out, and

d. a rammed oxide layer interposed between the said heat-insulating ceramic layer and the said crucible lining, the said oxide layer being a layer of metal oxide which does not form a relatively low-melting point eutectic with lime,

the thicknesses of the said heat-insulating ceramic layer, the said rammed oxide layer and the said lining layer being selected so that in operation the said rammed layer becomes sintered in a zone adjacent the said lining layer while a zone adjacent the said heat-insulating ceramic layer remains unsintered to permit slidable relative movement as the said lining layer expands and contracts in use.

2. A furnace according to claim 1, wherein the said heat-insulating ceramic layer comprises a mass of plastics ceramic material.

3. A furnace according to claim 1, wherein the said heat-insulating layer comprises a wall of fused oxide bricks.

4. A furnace according to claim 1, wherein a web of spun glass fibers is provided between the said heat-insulating ceramic layer and the said rammed oxide layer.

5. A furnace according to claim 1, wherein the bottom of the crucible thereof comprises an outer layer of oxide ceramic refractory bricks supporting a layer of fused lime.

6. A furnace according to claim 5, wherein the said layer of fused lime is a rammed layer.

7. A furnace according to claim 5, wherein the said layer of fused lime is a brick layer.

8. A vacuum induction melting furnace comprising

a. a crucible having a lining layer of bricks of sintered fusible lime,

b. an insulated inductive heating coil surrounding the said crucible,

c. a heat-insulating ceramic layer interposed between the said heating coil and the said crucible adapted to provide a barrier against metal break-out,

d. a rammed oxide layer interposed between the said heat-insulating ceramic layer and the said crucible lining, the said oxide layer being a layer of metal oxide which does not form a relatively low-melting point eutectic with lime, and

e. a web of spun-glass fibers interposed between the said heat-insulating ceramic layer and the said rammed oxide layer,

the thicknesses of the said heat-insulating ceramic layer, the said rammed oxide layer and the said lining layer being selected so that in operation the said rammed layer becomes sintered in a zone adjacent the said lining layer, while a zone adjacent the said heat-insulating ceramic layer remains unsintered to permit slidable relative movement in combination with the said web of spun-glass fibers as the said lining layer expands and contracts in use.