US2913239A - Furnaces for heat treatment of articles - Google Patents

Description

Nov. 17, 1959 T B. GREENE mamas FOR HEAT TREATMENT 05 ARTICLES- Filed Kay '10, 1954 2 Sheets-Shoat 1 N06. 17, 1959 B. GREENE FURNACES FOR FEAT-"TREATMENT OF ARTICLES 2 Sheets-Sheet 2 Filed lay 10, 1954 III/I'll III United States Patent FURNACES FOR HEAT TREATMENT or ARTICLES Ben Greene, Berkhamsted, England Application May 10, 1954, Serial No. 428,683

Claims priority, application Great Britain May 3, 1954 4 Claims. (Cl. 266-5) This invention relates to furnaces for heat treatment of articles and of the kind having a porous refractory lining and particularly furnaces in which a protective atmosphere such as hydrogen may require to be used at elevated temperatures.

It is found that pourous refractory materials tend to absorb air which at a certain temperature combines with hydrogen and an explosion results with damaging effect. In order to avoid such damaging effects it is known to place the articles to be treated into separate metal containers within the furnace and to introduce the protective gases into those containers whilst undergoing heat treatment in the furnace. The disadvantages of such containers are: V

(a) As these containers or retorts have to be heated in the heat treatment process, the maximum temperature to which the heat treatment can be taken is limited by the maximum temperature that the container will stand before it loses its mechanical strength and collapses. This factor also determines the maximum size of container and therefore the overall size of the material to be treated in that the loss of mechanical strength rises with the overall size of container.

(b) At temperatures over 1000 degrees centigrade (1832. degrees Fahrenheit) there is a deterioration in the container which requires to be replaced at intervals even where the best quality heat resisting metals are employed and represents a considerable cost in the heat treatment process.

(0) Owing to the presence of the container it is impossible to keep the heat treatment process under visual control and it is necessary to rely on the complete accuracy of pyrometers.

One object of this invention is to provide a furnace capable of providing heat treatment in protective atmosphere without the use of separate containers for the articles and without limitations as to size or weight of articles to be heat treated.

A further object is to provide a furnace capable of reaching the highest temperatures permitted by its heating unit and by its refractory lining without damaging effect of explosions.

A further object is to provide a furnace in which the highest heat input is possible so as to reach the required temperature in the shortest time in relation only to the mass of material undergoing heat treatment.

A further object is to provide a visual inspection of materials in the heat zone during heat treatment, in protective atmosphere.

A further object is the reduction of the rate of depreciation in the furnace itself, when operating at its maximum temperature.

A further object is to provide a lighter furnace of cheaper construction and maintenance giving smaller overall dimensions in relation to the size of its heat zone.

According to this invention a furnace is characterised in that the porous nature of the refractory employed and its thickness and bulk or volume is so chosen in relation Patented Nov. 17, 1959 2 to the volume of the free space within the furnace that the quantity of oxygen which may be contained in the porous refractory does not promote a spontaneous explosion with the protective atmosphere.

In the case where an alumina based refractory is employed and the protective atmosphere comprises hydrogen the bulk or volume of the refractory should not exceed about of the volume of the free space within the furnace. This particularly applies where the porous refractory lining exceeds Sinches in thickness since the thicker the lining the more diflicult it is to purge the pores from oxygen during the initial stage when hydrogen is caused to flow through the furnace before it is heated.

One suitable refractory which may be used according to the invention is a material supplied by the Morgan Crucible Company Limited under the designation M.R.I.

which comprises silica, alumina, lime, potash, soda and magnesia. The approximate chemical analysis of M.R.I. is as follows:

Silica 52% to 53%. Alumina 43% to 44%. Iron oxide (Fe O Less than 1%. Magnesia Less than 1%. Lime Potash About 1 /2%.

Soda

Another suitable refractory is that supplied by Messrs. Gibbons (Dudley) Limited under the designation H.T.I. These and other refractories adapted for use in the invention have a pore capacity varying from about 60% to 75%. These refractories may contain titanium oxide.

I-I.T.I. comprises approximately 58% silica ($0,) and approximately 37% alumina (A1 0 If possible the bulk or volume of refractory employed to satisfy the above requirements and the thickness of the refractory walls should be such that the temperature of the outer face is not higher than 300 or 400 centigrade thus the temperature gradient between the hot and cold faces of the refractory is about 1000 degrees centigrade where the temperature of the hot face is 1300 C. The aforesaid refractory is enclosed within a metal shell and by ensuring a temperature gradient of the above order no intensive cooling of the shell may be required,

' nor need it be formed from expensive heat resisting steel.

In order that the shell does not become overheated the refractory may be arranged not to be in thermal contact with the shell, for example there may be a gap between them.

When the thickness of the refractory is more than 5 inches the volume of the refractory should be less than 5A2 or 40% of the volume of the free space within the furnace according to the maximum temperature required and the volume of material undergoing heat treatment at any one time.

The heating elements may be placed inside the furnace by being fixed to the inner or hot face of themsulating refractories. This ensures that when the heat zone is at its maximum heat, the shell is protected from this heat by the insulating refractories and cannot therefore lose its mechanical strength and any size of furnace can therefore become practical. The maximum volume of the refractory or 2 the total volume of the free space within the furnace is within the safety margin of an explosion caused by the combination of hydrogen atmosphere with any oxygen locked in the pores of the refractory. If the refractory is less than 5 inches in thickness there is less likelihood of oxygen being locked in the pores after suitable cold purging treatment has taken place. This proportion will also permit-the impurities or water vapour in the refractories to be removed in the purging process within a reasonable period of time where the insulating refractories are of the best quality.

The interior of the furnace requires to be completely sealed off from the outside atmosphere when protective gas is employed and this can be achieved by'the usual methods of designing the container that it can be lifted off an oil, water, or powder seal or gasket for the removal or insertion of the material to be heat treated or alternatively by removing the top roof, or one or more sides and being replaced in a sealed condition before the commencement of heat treatment operations. Provision is included for an inlet and an outlet for the protective atmosphere. A spyhole fitted with heat resisting glass can be inserted so that a direct view into the heat zone at temperature is possible by the removal of sutlicient refractory material. Also included is an aperture for a thermocouple insertion.

The furnace can be made any size or shape whether round, square, oval or oblong. It can also be made with open ends either in the form of a tunnel or any other suitable shape. In such a case where artificial atmosphere is used, there must be sufficient pressure inside the furnace of this atmosphere so as to exclude the possibility of ordinary air or other gases finding their way into the container.

Where the shell is not made of heat resisting steel and is liable to suffer from oxidation if left to cool in the air, the outside surface should be kept cool by means of an air flow or a liquid cooling medium such as water di-' rected evenly over its outside surface through an outside casing fitted to the container. Where a water jacket is employed it is helpful that the refractory shall not be in thermal contact with the inner wall of the jacket.

The following is a description of a number of furnaces according to the invention, reference being made to the accompanying drawing in which each figure is a diagrammatic cross section through the furnace or a part of the furnace.

Referring to the arrangement shown in Figure l, the furnace is of the bell type comprising an outer shell 10 which may either be formed from heat resisting steel or mild steel according to whether the heat treatment is required to take place at temperatures up to 1550 C., or whether the heat treatment is at temperatures below 1200" C. As will be seen, the shell is lined with porous heat resisting refractory 11 which as indicated earlier in the specification may be an alumina-based refractory such as is sold for example, by Morgan Crucible Company Limited under the designation M.R.I.,. or by Messrs. Gibbons (Dudley) Limited, under the designation H.T.I. The shell is provided at its upper extremity with lifting eyes or hooks 12 and when in use the bell furnace rests on a base 13 which is so formed as to provide a reservoir 9 for a liquid seal such as oil and water. A hearth 14 extends up into the furnace. As previously indicated when the thickness of the refractory exceeds inches the volume of the refractory requires to be less than about ,4 of the volume of the free space within the furnace. A reducing gas such as hydrogen is introduced through the top of the furnace through a valve controlled pipe 15, and is withdrawn through the hearth by a gas outlet pipe 16.

The thickness of the refractory around the side walls and roof is substantially uniform. In one example according to the invention a furnace having a free internal space of 16 cubic feet has refractory walls of uniform thickness of about 5 inches the total volume of the refractory being about 7 cu. ft. Electric heating elements 17 are mounted on the side walls of the furnace and also on the top of the hearth. A spyhole 18 comprising a tubular element containing fire resisting glass is arranged in the top wall of the furnace, as is also a thermocouple 19 for measuring the temperature. In operation, when the furnace has been lowered onto its base in a cold condition, the hydrogen gas is caused to flow through the oven displacing the air from within the furnace, a certain amount of the oxygen being washed out of the pores of the refractory but leaving a certain amount trapped according to the volume and thickness of the refractory. The heating elements are then energised and the flow of hydrogen continued and it is found in practice, that if the bulk or volume of the refractory is much more than %2 of the volume of the free space, and the thickness is more than 5 inches when a certain temperature is reached, conditions arise in which spontaneous combustion is liable to take place causing an explosion, thus, the above values should not be exceeded. It is also found that if this ratio of the volume of the refractory to the free space within the furnace is observed, the temperature drop through the walls of the furnace may be sufiicient to render it unnecessary to actively cool the outer surface, sufiicient cooling being provided by the outer atmosphere.

The furnace construction shown in Figure 2, is similar to that of Figure 1, with the exception that in place of a liquid seal being provided, the refractory base or hearth 14, is provided with a suitable gasket 20 formed from heat resisting material. In this construction, both the side walls 21 and the roof 22 may carry the electric heating elements 17. The furnace may be raised or lowered by a suitable block and tackle arrangement 23.

In the construction shown in Figure 3, the refractory base or hearth 14 and the refractory side walls 21, and the roof 22 are formed as a unit, together with one end wall 24. These parts are enclosed within a steel shell 10. The furnace at the other end is provided with a sliding door 25 having a refractory lining on the inner side and comprising a steel plate on the outer side, which door is raised and lowered by a block and tackle mechanism 23. A suitable seal is provided between the door and the end faces of the side walls and base of the furnace and means are provided for clamping the door against this seal.

In the construction shown in Figure 4, the refractory base or hearth 14 the side walls 21, and the end walls 24 are formed as a unit enclosed within a steel shell 10, while the roof 22 is suspended by block and tackle mechanism 23 so that it may be raised and lowered into position. In which case, the upper part of the side walls and the end walls are provided with a trough 25 for containing a powder or sealing liquid material into which a flange 26 fixed to the roof may dip.

As already indicated the volume and thickness of the refractory is preferably so chosen that the temperature gradient is such that the temperature of the cool faces is kept down to three or four hundred degrees Centigrade in which case intensive cooling of the steel shell is not required. Under certain circumstances higher furnace temperatures may be generated in which case the temperature of the outer shell also tends to increase and in order to prevent deterioration additional cooling may be required and for this purpose a water jacket may be provided as shown in Figures 5, 6 and 7. As previously indicated in order that excessive heat shall not be transmitted from the refractory it is preferablethat it shall not be in thermal contact with the surrounding metal shell.

The furnace shown in Figures 5 and 7 is similar to that of Figure 4, but provided with a water jacket 27, in which case the aforesaid trough 25 may be formed in the upper edge of the water jacket (see Figure 7), the gas outlet extends through the hearth 14 and through the jacket 27. A valve controlled Water inlet pipe 28 (see Figure 5) may be provided at the bottom of the jacket and the Water passes out from the top of the jacket through a conduit 29 into a collecting funnel 30. The refractory roof 22 (Figure 6) which is shown arched, may be provided with a separate water jacket 31, and water inlet and outlet pipes 28, 29, as shown in Figure 7, and the gas inlet 15, spyhole 18 and thermo couple 19 are a anged to extend both through the water jacket and through the refractory roof. The roof is provided with downwardly directed flanges 26 which are arranged to dip into the trough 25 when the roof is resting on top of the furnace. In Figure 7, the trough 25 is shown to project beyond the sides of the jacket instead of along the top edges as shown in Figure 5.

In the construction shown in Figure 8, the refractory base or hearth 14, the side walls 21 and the roof 22 are formed as a unitary tunnel like structure open at its ends and the gas is introduced through'the inlet 15 at the centre of the roof and is arranged to flow through the oven outwardly to the two open ends. If hydrogen is employed it becomes ignited as it leaves the furnace and forms a fire curtain which prevents the ingress of air.

Figure 9 is a cross section of thepart of the roof containing the spyhole which comprises a heat resistant steel tube 32 which extends through the refractory roof 22 and the steel casing 10 projecting above the latter and encircled by asbestos or fibre glass packing 33. Mounted at the top of the tube is a block of heat resisting glass 34 accommodated in a suitable socket 35 which engages the top of the tube.

Figure 10 shows a similar type of furnace to that of Figure 1, but in which the steel casing 10 is provided with a number of upwardly directed circumferential heat radiating air cooling fins 36. The upwardly directed fins may be employed for holding a coolant such as Dry Ice or water.

I claim:

1. A high temperature furnace comprising a metal shell, a porous refractory lining within said shell of at least 5 inches in thickness and the volume of which refractory is not substantially greater than of the volume of the free space within the furnace, the relationship between said thickness and said volume of said refractory being for the purpose of providing a margin of safety for preventing an explosion, heating means within the furnace for raising the temperature to at least 1200" C., means for introducing a protective atmosphere directly into the free space within the furnace and a hearth adapted to support articles to be heated within said free space.

2. A high temperature furnace comprising a metal shell, a porous refractory alumina based lining within said shell of at least 5 inches in thickness and having a volume not substantially greater than 40% of the volume of the free space within the furnace, heating means within the furnace adapted to raise the temperature up to about 1550 C., means for introducing a protective atmosphere directly into the free space within the furnace and a hearth 50 adapted to support articles to be heated in said free space, whereby a margin of safety is provided to prevent explosion by the combination of the protective atmosphere and oxygen locked in the pores of the refractory.

3. A high temperature furnace comprising a porous alumina based refractory lining within said shell of at least 5 inches in thickness and having a volume not substantially greater than 40% of the volume of the free space within the furnace and which will produce a temperature gradient of over 1000 C. when the furnace is operating at'a temperature of about 1300 C., heating means within the furnace for raising its temperature over 1300 C., means for introducing an inert or reducing atmosphere directly into the free space within the furnace and a hearth adapted to support articles to be heated in said free space, whereby a margin of safety is provided by the relationship of the thickness and volume of the refractory to prevent an explosion by the combination of the introduced atmosphere and oxygen locked in the pores of the refractory.

4. A high temperature furnace comprising a metal I for introducing a protective atmosphere directly into the free space within the furnace and a hearth adapted to support articles to be heated within said free space.

References Cited in the file of this patent UNITED STATES PATENTS 1,376,617 Griggs May 3, 1921 1,528,639 Tone Mar. 3, 1925 1,566,224 Massey Dec. 15, 1925 1,569,197 Maccallum Jan. 12, 1926 1,627,682 Wyzalek May 10, 1927 1,869,237 Bruhn July 26, 1932 2,044,817 Schroeder June 23, 1936 2,089,843 Wilson et a1. Aug. 10, 1937 2,146,760 Pearson Feb. 14, 1939 2,168,028 Harsch Aug. 1, 1939 2,267,041 Patterson Dec. 23, 1941 2,434,003 Morrison Ian. 6, 1948 2,467,889 Harter Apr. 19, 1949 2,494,541 Burr et a1. Ian. 7, 1950 2,495,561 Wilson Ian. 24, 1950 2,502,827 Cooper Apr. 4, 1950 2,543,708 Rice et a1. Feb. 27, 1951 OTHER REFERENCES Industrial Furnaces, vol. 1, page 5, (Fig. 5 relied on; John Wily and Son (1925), vol. 2, page 142 (Fig. 131) relied on.

Norton: Refiactories, 3rd ed., pages 206-207, 1949.

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