US2191133A - Apparatus for heat treating - Google Patents

Description

Feb. 20, 1940. PmsoN' 2,191,133 I AfPARATUS FOR HEAT TREATING Origina l Filed Dec. 5, 1 934 2 Sheets-Sheet 1 V raven/01 Jafim flirulom peans-o/w,

Feb. 20, 1940. J. L. PEARSON APPARATUS FOR HEAT TREATING Original Filed Dec. 5; 1934 2 Sheets-Sheet 2 m A I l 8 I z I I I 6 m 4 z 4 l PERCENT OXYGEN Patented Feb. 20, 1940 APPARATUS FOR HEAT TREATING John Lindon Pearson, Norton-on-Tees, England,

assignor to Imperial Chemical Industries Limited, a corporation of Great Britain Original application December 5, 1934, Serial No. 756,185. Divided and this application July 15,

1938, Serial No. 219,450.

December 8, 1933 In Great Britain 15 Claims.- (01. 268-) This invention relates generally to apparatus for heat treating and it relates more particularly to apparatus for heat treating metals in a neutral or non-oxidizing atmosphere to prevent scalrecent years to this art, and more particularly tothat branch known as bright annealing, which offers many advantages over the older methods of heat treatment in oxidizing atmospheres followed by pickling or descaling. The main factor which has prevented the wider adoption of the use of neutral or reducing atmospheres in contheir products has been the cost of the gas necessary to produce this atmosphere. This is especially the case with continuous heat treating or annealing furnaces and with relatively cheap metal products.

In continuous furnaces, using protective atmospheres in which the articles to be treated move through the furnace in a continuous stream, it is not possible to seal the ends against leakage of protective atmosphere and infiltration of air. Hitherto the former has been allowed to fescape from the ends at a rate suflicient to prevent air leaking into the furnace up to any point where the temperature is such that oxidation would take place. Consequently, the quantity of gas required becomes so great in the case of large annealing furnaces that in many cases the process is considered to be uneconomical with known methods.

In batch furnaces during the actual period of heating, there is generally no difliculty with air contamination, since they can be effectively sealed from the outside atmosphere. However, when a batch furnace is closed after being charged with articles to be heat treated, it con.- tains air which has entered during the charging operation and its oxygen has to be removed before heat treatment can be commenced. Up to the present this removal has been effected by sweeping it out into the outside atmosphere by a stream of protective gases or by a special gas such as carbon dioxide, which is in turn swept out by a stream of protective gases. Although a conen masse, mixing between the protective gas or special gas and the air takes place to a substantial extent, with the result that an amount of the protective gas or special gas many times that of nection with the heat treatment of metals andsiderable proportion of the air is thus removedthe air, is blown out into the outside temosphere and wasted before the furnace is sufliciently free from] oxygen for heat treatment to commence.

An object of the present invention is to reduce the consumption of protective atmosphere in both continuous and batch furnaces for the thermal treatment of metals and their products in a protective atmosphere and to provide a furnace in which heat treatment, particularly bright annealing. of metal articles may be conducted with maximum emciency and minimum expense.

Other objects and novel features and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings which illustrate one practical form of apparatus embodying the invention, it being understood that the description of this specific embodiment is merely illustrative and not restrictive and that various changes in arrangement and details of construction can be made without departing from the spirit and scope of the invention as set out in the appended claims.

According to the present invention, the consumption of protective atmosphere is decreased in the case of continuous furnaces by recovering at least in part the protective atmosphere leaving the heat treatment zone of the furnace, and utilizing the adventitious oxygen, present in the protective atmosphere through air infiltration from outside the furnace, for the production of further amounts of protective atmosphere by a combustion with combustible gas, the resulting protective atmosphere being recirculated to the furnace, and in the case ofbatch furnaces, by driving the air left in the furnace after charging with the articles to be heat treated, at least in part into the protective atmosphere circulating system, and utilizing it for the production of further quantities of protective atmosphere by combustion with combustible gas.

In the case of furnaces of the continuous type, the withdrawal of the gases is most conveniently eifected by means of one or more ports in the inlet and outlet ducts where the temperature has fallen sufficiently low that the materials being bright annealed may be safely exposed to an atmosphere containing oxygen.

The removal of the oxygen from the withdrawn gases is carried out in apparatus (hereinafter referred to as a burner) in which the gases withdrawn from the furnace are heated (in the presgen-containing gas, and for a portion, at least, of the period of heating, in the presence of a catalyst, whereby oxygen contained in the gases withdrawn from the furnace is removed.

- .The gases leaving the burner may be reduced to the temperature required in known forms of coolers, and usually it is desirable to provide means for separating water from the gases before they are returned to the furnace.

In order to obtain reasonably smooth operation of the burner, it is desirable to ensure that the gases which are sent to it have a fairly uniform oxygen content. This may be achieved in the case of continuous furnaces by providing a number of curtains of soft material such as asbestos cloth in'the inlet and discharge ducts which control the ingress of air and egress of reducing gases through the openings, even when varying sizes of articles are passing in and out of the furnace. capacity vessel in the pipe line leading from the furnace backto the regenerative burner. Furthermore the amount of oxygen in the protective gas from the furnace may not be sufficient when burnt with ammonia, natural gas, or the like, or their partial combustion products, to render the burner autothermal. In this case autothermicity may be obtained by adding further oxygen, e, g., from the outside atmosphere, to the mixture of contaminated protective gas with ammonia, natural gas or the like, ortheir partial combustion products, and burning it in the burner.

The gas or gases withdrawn from the furnace may contain objectionable constituents arising from the lubricants used in cold working as for example in tube drawing, or wire drawing, and this objectionable constituent may be removed either before or after the regenerative burner by means of suitable scrubbing or purification means. wire are degreased before bright annealing, the gas withdrawn from the furnace may contain hydrogen sulphide, and this can be removed either by passing the gas through the ordinary type of'iron oxide box or by scrubbing the gasin a small tower or washer by means of a solution of sodium carbonate or other liquor.

The application of the present invention to heat treating or annealing furnaces of the continuous type will now be described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic elevation of an annealing furnace showing the gas circulation system, and Figure 2 is a graph showing the effect of different kinds of material when used for screening'the inlet and outlet passages of the furnace.

Referring to Figure 1, the neutral or reducing gas, consisting for example of nitrogen .or nitrogen containing a small percentage of hydrogen. is fed into the tunnel-like annealing furnace I through one or more ports 2 situated in the wall of the furnace where the cooling chamber 3 adjoins the heating chamber I. The rate at which the gas is introduced is measured by means of the flowmeter Ii, and is sufficient to produce in the inlet and outlet ducts 9 and I for the articles being annealed a velocity such that ingress of oxygen from the outside atmosphere is prevented from reaching the metal articles before they are cooled to a safe temperature. v

A band conveyor 8 carries the metal objects to be heat treated, e. g., tubes, bars, or wire, through the furnace, and flexible curtains 9 and I0 are arranged to screen the inlet and outlet ducts at their outer ends. Gas exit ports H and It is also an advantage to provide a.

Thus for example, unless the tubes or 12 are provided in the gas ducts 8 and I. 0onnections for manometers l2 and II are provided to enable the pressure of the gas at these points to be measured with a view to controlling the flow of gas by means of valves I5 and I. The ends of the furnace are fitted with vertical sliding doors l1 and It, to which are attached inclined baflles l9 and 29.

The gas which is withdrawn from the furnace through ports II and I2 passes via pipes 2| and 22 to a common main 2! leading to a capacity vessel 24 and a suction fan 25. -The fan delivers pipe 3|. This additional gas is derived from the decomposition of ammonia which is introduced by pipe 32 into a coil 33 in the vessel 34 adjoining the catalyst chamber 30. The coil 39 is heated in a manner to be explained and the ammonia is partially or completely decomposed during its passage through the coil. From the coil the gas passes through pipe 35 to a burner 39 where it is mixed with air extracted from near the ends of the furnace I by means of the fan 31, the gas delivery rate of which is measured by flowmeter 48. This air contains a certain amount 01' valuable gas which would otherwise escape from .the furnace and which is recovered by the arrangement shown. However, this arrangement is optional and it is possible to operate the burner with ordinary air. The gases issuing from the burner are ignited and burn in the space 38a within coil 33, thus heating the latter and causing the decomposition of the ammonia passing through the same.

The combustion products pass through pipe 9| and mix with the gas issuing from pipe 29 as previously described. The mixed gases then pass over the catalyst gauzes 39, 99 and lll in succession and their oxygen content is substantially eliminated. The gauze 98 is made of an iron alloy containing 18% chromium, 8% nickel, 1% tungsten and 1% vanadium: the gauze 39 is made of copper and the gauze 40 is made of palladium.

The resulting gases pass via pipe ll into the heat exchanger 21 and are caused to sweep over the coil 28 by the baille 92 which prevents their passing straight through the space surrounded by the coil. The gases then pass by pipe 49 to a cooler l4 and thence to a drier l8 and a capacity vessel 48 before being returned to the furnace by pipe 41 leading to the port 2.

During the starting up period all the neutral or reducing gas may be allowed to pass out through the ends of the ducts 9 and 1 until'the' atmosphere in the furnace is substantially free of oxygen. The recirculating fan 25 is now started and the valves l5 and I9 adjusted until the rate, measured by the flowmeter 5| at which gas is withdrawn from the furnace, is about of the rate of introduction of gas into the furnace via the pipe 41. About 5% of the Has introduced into the furnace is passed out to atmosphere through each of the inlet and outlet ducts, and this escaping gas together with the curtains 9. III and bailles I9, 20, minimizes the entrance of oxygen due to diffusion and/ornatural draughts. By means of the sampling points 49, 59 the composition of the gases leaving and passing to the furnace may be tested and the percentage of gases withdrawn adjusted to give the correct atmosphere in the furnace. In the case of a small annealing furnace with inlet and outlet ductsof 0.44 square foot cross sectional area and a total furnace length of 10 feet, a gas mixture consisting of 95% nitrogen and 5% hydrogen by volume was fed into the furnace at the rate of 230 cubic feet per hour. The rate at which gas was withdrawn from the furnace by the recirculating fan was about 218 cubic feet per hour, and the rate at which make-up gas was supplied was 12 cubic feet per hour. With an oxygen content of 2 per cent by volume in the withdrawn gas, a total ammonia rate of 9.3 cubic feet per hour was required to consume this oxygen and also to produce a make-up gas containing 10% hydrogen by volume. The amount of air provided by the air blower for the-makeup gas was 12.5 cubic feet per hour. This air or mixture of air and gas was taken from points near the furnace ends external to the curtains 9, 10 in order to minimize the effect of eddy currents or draughts of air in causing contaminationof thefurnace gas with oxygen. p

Under these conditions and with inlet and outlet passages without curtains or inclined baffies fitted to the doors, the degree of oxygen contamination is indicated by the curve A in Figure 2. Curve B in Figure 2 shows the improvement obtained by fitting the inclined baflles to the doors. Curve 0 in Figure 2 shows, the effect of fitting divided curtains in addition to the inclined baflles. Curve D in Figure 2 shows the effect of substituting string curtains for the divided curtains, the inclined battles being retained. Objects of various sizes could be passed in and out of the annealing furnace and no variation in the degree of oxygen contamination could be detected. I

This application is a division of my copending application Serial No. 756,185 filed December 5, 1934, now Patent 2,146,760, dated Feb. 14, 1939.

What is claimed is:

1. Apparatus for the heat treatment of metals in a non-oxidizing protective atmosphere comv prising a furnace having a substantially unobstructed heat treatment chamber for articles to be treated, and a closed circulating system connected to said chamber and including means for supplying a non-oxidizing gas to said chamber, means for withdrawing, from said chamber, gas contaminated with free oxygen, means for uniting with said withdrawn gas a supply of reducing gas at least sufiicient to combine with the oxygen of the oxygen-carrying withdrawn gas, means outside the heat treatment chamber for heating the mixture thereby produced to bring about a chemical combination of the oxygen and the reducing gas to form a substantially oxygenfree non-oxidizing'gas, and means for circulating the resultant oxygen-free non-oxidizing gas back to said chamber. 7

2. Apparatus for the bright annealing of met als in a non-oxidizing atmosphere comprising a. furnace having a substantially unobstructed heat treatment chamber for metals to be treated and a closed circulating system connected to said chamber for circulating a non-oxidizing gas into, through and out of said chamber whereby contaminating oxygen in the chamber is carried out with the outgoing portion of said circulating gas, said circulating system including a catalytic combustion chamber, outside the heat treatment chamber, into which said oxygen-contaminated gas passes, means for introducing a supply of reducing gas into said combustion chamber to formtherein a mixture of reducing gas and 011- ygen-contaminated. gas, means for heating the mixtureto cause the chemical combustion of substantially all of the oxygen in it with reducing gas, and means for circulating the resultant oxygen-free non-oxidizing gas back to said heat treatment chamber, said circulating system including an unobstructed capacity vessel between the heat treatment chamber and said combustion chamber.

3. Apparatus for the bright annealing of metals in anon-oxidizing atmosphere comprising a substantially unobstructed heat treatment chamber for metals to be treated, said chamber having inlet and outlet ducts for the continuous unobstructed passage of metals into, through and out of said chamber, means for conveying metals through said chamber, and a closed circulating system connected to said chamber for circulating anon-oxidizing gas into, through and out of said chamber whereby contaminating oxygen in the chamber is carried out with the outgoing portion of said circulating gas, said circulating system. including means for supplying a nonoxidizing gas under pressure to said chamber intermediate said inlet and outlet ducts to create a flow of non-oxidizing gas outwardly through both of said inlet and outlet ducts to resist inflow of air into said chamber through said ducts, means for withdrawing gas contaminated with oxygenpfrom the inlet and outlet ducts of said chamber, means for uniting with said withdrawn gas a supply .of reducing gas at least sufflcient to combinewith the oxygen of the oxygen-carrying withdrawn gas, means outside the heat treatment chamber for heating the mixture to cause the chemical combination of substantially all of the oxygen in it with reducing gas, and means for circulating the resultant oxygen-free nonoxidizing gas back to said chamber.

' 4. Apparatus as set out in claim 3, in which flexible curtains are provided to screen the inlet and outlet ducts of said heat treatment cham-- ber.

5. Apparatus as set out in claim 3, in which the inlet and outlet ducts are provided with sliding doors carrying inclined baiiles adapted to restrict the entrance and escape of gases through said ducts.

6. Apparatus for. the bright thermal treatment of metals in a non-oxidizing protective atmosphere comprising a substantially unobstructed heat treatment chamber for metals to be treated, said chamber having inlet and outlet ducts for the continuous unobstructed passage of metals into, through and out of said chamber, means for conveying metals through said chamber, means a for supplying said chamber with a continuous stream of non-oxidizing gas forming said protective atmosphere which flows outwardly along both of said inlet and outlet ducts and is there subject ,to contamination with air, means for withdrawing gas contaminated with air from said inlet and outlet ducts, means for mixing the withdrawn gas with an amount of combustible gas at least sufliclent to combine with the entire oxygen content of the air-carrying withdrawn gas, means outside the heat treatment chamber for subjecting the mixture to a temperature sufficient to cause the chemical combination of substantially all of the oxygen and the combustible gas, and means for returning the resultant oxygen-free non-oxidizing gas to the chamber intermediate the inlet and outlet ducts.

7. Apparatus for the bright annealing of metals in a non-oxidizing protective atmosphere --comprising a substantially unobstructed heat subject to contamination with air, a combustion chamber, outside said heat treatment chamber, means, including an unobstructed capacity vessel, for withdrawing gas contaminated with air from said inlet and outlet ducts and introducing it into said combustion chamber, means for introducing a supply of combustible gas into said combustion chamber to form therein a mixture of combustible gas and oxygen-contaminated gas, means for combusting the oxygen content of said mixture to form an oxygen-free non-oxidizing gas, and means for returning the resultant oxygen-free non-oxidizing gas'to the heat treatment chamber intermediate the inlet and outlet ducts.

8. Apparatus for the bright annealing of metals in a non-oxidizing atmosphere comprising a substantially unobstructed heat treatment chamber for metals to be treated, said chamber having inlet and outlet ducts for the continuous unobstructed passage of metals into, through and out of said chamber, means for conveying metal through said chamber, and a closed gas circulating system connected to said chamber for circulating a non-oxidizing gas into, through and out of said chamber whereby contaminating oxygen in the chamber is carried out with the outgoing portion of said circulating gas, said circulating system including means for supplying a non-oxidizing gas under pressure to said chamber intermediate said inlet and outlet ducts to create a flow of non-oxidizing gas outwardly through both of said inlet and outlet ducts to resist inflow of air into said chamber through said ducts, a combustion chamber, outside said heat treatment chamber, means for withdrawing gas contaminated with oxygen from the inlet and outlet ducts of said heat treatment chamber and introducing it into said combustion chamber, a burner, means for supplying said burner with combustible gas, means for conducting hot combustion productsi'rom said burner into said combustion chamber to form therein a mixture of said combustion products with said oxygen-contaminated withdrawn gas and cause the chemical combination of substantially all of the oxygen in said mixture with the hot reducing constituents thereof, and means for re turning the resultant oxygen-free non-oxidizing as to the heat-treatment chamber intermediate its ends.

9. Apparatus as set out in claim 8, in which a catalyst is provided in said combustion chamber to promote chemical combination of the gases therein.

10. Apparatus as set out in claim 8, including a heat exchanger whereby the hot gases from said combustion chamber will heat the. oxygencontaminated withdrawn gas flowing to said combustion chamber. A

11. Apparatus as set out in claim 8; including means for withdrawing air from near the outer ends of the inlet and outlet ducts of said heat treatment chamber and for supplying said air,

containing a proportion of non-oxidizing gas escaping through said ducts from said heat treatment chamber; to said burner to provide limited combustion of the combustible gas supplied to said burner.

12. Apparatus for the bright annealing of metals in a non-oxidizing atmosphere comprising a heat treatment chamber for metals to be treated, said chamber having inlet and outlet ducts for the continuous passage oLmetals into, through and out of said chambergmeans for conveying metals through said chamber, and a closed gas circulating system connected to said chamber for circulating a non-oxidizing gas into, through and out of said chamber whereby contaminating oxygen in the chamber is carried out with the outgoing portion of said circulating gas, said cir-- culating system including'means for supplying a non-oxidizing gas to said chamber, a catalytic combustion chamber outside of said heat treatment chamber, means for withdrawing gas contaminated with oxygen from the inlet and outlet ducts of said heat treatment chamber and introducing it into said combustion chamber, a burner, means for supplying said burner with combustible gas, means for supplying said burner with a limited amount of air for partial combustion of said combustible gas, a burner chamber into which said burner discharges hot combustion products, said burner chamber being provided with heat-exchange means whereby said combustible gas is heated to decomposing temperature prior to passage .to said burner, means for conducting the hot combustion products from said burner chamber to said combustion chamber to form therein a mixture of said combustion products with said oxygen-contaminated withdrawn gas whereby the oxygen in said mixture will chemically combine with the reducing constituents thereof, and means for returning the resultant non-oxidizing gas to the heat treatment chamber intermediate its ends.

13. An apparatus for the bright annealing of metals comprising a furnace having inlet and outlet ducts at its ends, flexible curtains to screen said inlet and outlet ducts, for the articles to be treated, at least one gas exit port in each of the ducts, at least one gas inlet port in the furnace, an external purifier adapted to remove free oxygen from gas passed therethrough, and means including gas pipe lines connected to said ports for circulating protective gas through the furnace, through the purifier and back to the furnace.

14. An apparatus for the bright annealing of metals comprising a furnace having inlet and outlet ducts at its ends, for the articles to be treated, at least one gas exit port in each of the ducts, at least one gas inlet port in the furnace, an external purifier adapted to remove free oxygen from gas passed therethrough, and means including gas pipe lines connected to said ports for circulating protective gas through the furnace, through the purifier and back to the furnace, the ends of the furnace being fitted with sliding doors carrying inclined bailles adapted to restrictthe transfer of gas through the inlet and outlet passages.

15. Apparatus for the heat treatment of metals in a non-oxidizing protective atmosphere comprising a furnace having'a closed heat treatment chamber for articles to be treated in batches, and a closed circulating system connected to said chamber and including means for supplying a non-oxidising gas to said chamber, means for withdrawing. from said chamber, as contaminated with free oxygen, means for uniting with said withdrawn gas a supply of reducing gas at least sumcient to combine with the oxygen or the oxygen-carrying withdrawn gas, means outside the heat treatment chamber for heating the mixture thereby produced to bring about a chemical combination of the oxygen and the re.- duoing gas to form a substantially oxygen-free non-oxidizing gas, and means for circulating the resultant oxygen-free non-oxidizing gas back to said chamber. 1

JOHN LINDON PEARSON.

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