GB2072313A

GB2072313A - Muffle furnace

Info

Publication number: GB2072313A
Application number: GB8010171A
Authority: GB
Original assignee: FHD FURNACES Ltd
Current assignee: FHD FURNACES Ltd
Priority date: 1980-03-26
Filing date: 1980-03-26
Publication date: 1981-09-30
Also published as: GB2072313B

Abstract

To achieve an effectively small furnace volume, with ease of control of atmosphere, in a through flow furnace, with the use of muffles, it is proposed to use a refractory muffle 6 comprising a base or hearth 8, side wall 9 and a roof 10 each made up of discrete elements of silicon-nitride-bonded silicon carbide. In this way, although the muffle 6 is permeable and requires the use of a gas tight casing 1, it does improve atmospheric control in the furnace and is cheaper than a conventional muffle of high temperature resistant alloy. <IMAGE>

Description

SPECIFICATION Furnace This invention relates to through flow furnaces and more particularly, but not solely to furnaces for sintering metal powder components.

Conventionally, sintering furnaces are of open hearth or muffle types.

In a muffle type of furnace the components to be sintered are fed through a gas-tight muffle which confines the sintering atmosphere, which is thus capable of ciose control. However, the materials normally used are nickel chrome alloys which are expensive and have a comparatively short working life. Also, the alloys place a constraint upon the working temperatures which may be used.

In an open hearth furnace the components are fed along a hearth, normally formed by refractory tiles, and exposed to the heating elements, the whole furnace being enclosed in a gas-tight casing. With the larger volume of atmosphere, control of the atmosphere adjacent the components is less easy and also the products of combustion of lubricants used in the formation of the powder compacts has a deleterious effect on the refractory material and on the heating elements.

In order to combat these disadvantages, recourse has been had to a type of furnace incorporating a gas-tight case and a longitudinal hood of refractory material of some structural strength. In this connection reference may be made to our United Kingdom Patent Specification 1 452 652. This arrangement has been proved reasonably satisfactorily in service but the expensive refractory tiles forming the hearth have on occasions been unsatisfactory in use.

According to the present invention, there is provided a furnace for the heat-treatment of components in continuous or semi-continuous movement through the furnace, comprising a gastight case surrounding an elongate furnace cavity including heating means and, within the furnace cavity, a muffle of refractory material formed from discrete elements and comprising a base, a roof and side walls.

Preferably, use is made of silicon-nitridebonded silicon carbide as a material for the muffle.

It has been found that a base of this material provides an adequate support for a conveyor for the components.

The side walls may have moulded-in supports for gas feed tubes. The roof may be provided with one or more transverse baffle ribs to control gas flow.

The use of a muffle made of discrete component allow for greater flexibility of design using similar basic components which may be kept in stock.

The invention will be further described with reference to the accompanying drawings in which: Figure 1 is a transverse sectional view of a typical through flow furnace incorporating a muffle in accordance with the present invention; Figure 2 is a transverse sectional view of a modified form of muffle; and Figure 3 is a longitudinal section of a muffle.

The furnace illustrated is primarily for sintering of metal powder components, but may be used for other similar heat treatment. The version illustrated could for instance be 7 metres long and 2 metres wide. The furnace consists of a gas-tight case 1 mounted on a metallic frame 2. The casing has removable sections 3 of refractory material for access to the interior of a furnace cavity 4. These removable sections 3 have a piate 3a which bolt onto the furnace structure with interposition of a suitable gasket for gas tightness. The ends of the furnace are provided with appropriate unions for gas extractors controlling the exit of the atmosphere from the ends of the cavity 4 and for ingress and egress of an appropriate conveyor 7 carrying the components to be sintered.It is also possible for the components to be fed through by pushing wherein the components are contained on trays or in containers, boats and so forth as appropriate.

The casing includes support 5 for a built-up muffle indicated generally at 6 and including the conveyor 7. It will be seen that the muffle comprises a base 8, which replaces the conventional hearth formed of tiles of siliconcarbide. The muffle further includes side-walls 9 and a roof 10. All the components 8, 9 and 10 are built-up of slabs of silicon-nitride-bonded silicon carbide which is a refractory material having greater structural strength than the conventionally used silicon carbide. It will be seen from Figure 3 that the slabs are provided with rebates as indicated at 11 to provide for suitable overlapping joints. Since the muffle is made of somewhat porous material, the joints do not have to be absolutely gas-tight.

Figure 2 shows a modified form of muffle in which the side walls 9 are provided with mouldedin groove 12 which may be used to support gas pipes for supplying atmosphere gases to appropriate parts of the furnace. In addition, the base 8 shown in Figure 2 is also provided with a rebated joint 1 3 between two slabs. It is envisaged that in certain cases the base may be wider than the commercially available slabs of silicon-nitride-bonded silicon carbide and that therefore it may be necessary to provide a join in such cases. If such a joint is provided it will also be required in the roof 10.

It will also be seen in Figure 3 that one of the slabs forming the roof 10 is provided with a transverse rib 14 which forms a flow controlling baffle within the muffle.

It will be appreciated that although the muffle is not gas-tight, the ieakage of gas through it is comparatively small so that the effect of such leakage on the composition of the atmosphere within the muffle is limited, as also is the effect of the leaking gases, particularly products of combustion, on the heaters provided within the furnace cavity 4.

The flat top of the roof 10 may be utilised to support heating elements.

Various modifications may be made within the scope of the invention.

Claims

1. A furnace for the heat treatment of components in continuous or semi-continuous movement through the furnace, comprising a gastight case surrounding an elongate furnace cavity including heating means and, within the furnace cavity, a muffle of refractory material formed from discrete elements and comprising a base, a roof and side wails.

2. A furnace as claimed in claim 1, in which the elements comprising the muffle are formed of silicon-nitride-bonded silicon carbide.

3. A furnace as claimed in claim 1 or 2, in which the side walls of the furnace are provided with moulded-in supports for gas feed tubes.

4. A furnace as claimed in any of claims 1 to 3, in which the roof is provided with at least one transverse rib forming a baffle to control gas flow.

5. A furnace as claimed in any of the preceding claims, in which the components making up the roof side walls and base of the muffle are in the form of slabs provided with rebates to form overlapping or scarf joints with adjacent slabs.

6. A through flow furnace for the heat treatment of components, substantially as hereinbefore described with reference to the accompanying drawing.