US2992147A - Method of heat treatment using dual atmospheres - Google Patents

Description

C. I. HAYES July 11, 1961 METHOD OF HEAT TREATMENT USING DUAL ATMOSPHERES Filed April 29, 1957 INVENTOR. [a r/ [flayas BY I United States Patent v 2,992,147 METHOD OF HEAT TREATMENT USING DUAL ATMOSPHERES Carl I. Hayes, 196 Wentworth Ava, Cranston, RI. Filed Apr. 29, 1957, Ser. No. 655,686 1 Claim. (Cl. 148-16) The instant invention pertains generally to heat treatment apparatus and more particularly to industrial heat treatment furnaces of the type utilized in connection with the heat treatment of high-speed steels, stainless steel, and the like.

A primary object of this invention is the provision of industrial heat treatment apparatus capable of treating high-speed steels, stainless steel, and the like at tem peratures upwards of 900 F. and which may be operated at a relatively low cost.

Another important object of my invention is the provision of a heat treatment furnace utilizing a controlled treatment atmosphere, said atmosphere comprising primary and secondary sources of supply.

A further object of the instant invention is the provision of novel and improved means for more economically operating a heat treatment furnace of the type having an elevated heating chamber.

Another object of my invention is the provision of a heat treatment furnace which operates on a relatively small percentage of pure heat treatment atmosphere, but which, nevertheless, is just as effective as furnaces presently in use wherein 100 percent pure heat treatment atmosphere is utilized.

Still another object of the present invention is the provision of a novel and improved dual-atmosphere heat treatment furnace, the novelty of which may be readily and easily incorporated in existing treatment furnaces having an elevated heating chamber.

Other objects, features and advantages of the invention will become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.

In the drawings which illustrate the best mode presently contemplated by me for carrying out my invention:

FIG. 1 is a side elevation of a humpback-type furnace embodying the instant invention; and

FIG. 2 is a side elevation of an elevator-type heat treatment furnace embodying the instant invention.

Since approximately 1928, industrial heat treatment operations have been carried on by using a controlled heat treatment atmosphere. The purpose of using such a controlled atmosphere is to prevent contamination of the work load when it is subjected to the relatively high heat treatment temperatures, which contamination would take place if air or any other impure or non-humidified atmosphere were present in the heating chamber. Accordingly, it is standard practice in the operation of industrial heat treatment furnaces to maintain a steady flow of a carefully controlled and dried atmosphere through the furnace, said flow serving to prevent the ingress of air or other impure atmospheres into the furnace, or at least, into the heating chamber thereof, where contamination of the work load is most likely to take place.

Depending upon the size and construction of the particular heat treatment furnace involved, the necessary rate of flow of the controlled heat treatment atmosphere is determined, it being obvious that for any given furnace a certain minimum rate of flow would be required to maintain proper control. The heat treatment atmosphere may be any one of a number of pure gases, a pure hydrogen atmosphere perhaps being one of the most effective and common in use. Other pure gases, such ,as

2,992,147 Patented July 11, 1961 helium or argon, may also be used, it having been found that these pure gases will have no deleterious effect on the work load being heat treated, even at the extremely high treatment temperatures of 900 F. and upwards, which extremely high temperatures are specifically necessitated in the heat treatment of alloy steels, particularly the stainless types, and other materials requiring heat treatment. As afore indicated, however, in order to be completely effective, even the treatment atmosphere must be carefully controlled by removing therefrom all moisture and any undesirable chemicals, such as sulphur, etc., before introduction of the atmosphere to the furnace. The only disadvantage which exists in the use of these pure gas atmospheres, such as pure hydrogen, helium, argon, etc., which for the purpose of this invention shall be referred to as primary heat treatment atmospheres, resides in the relatively high expense involved, particularly in view of the fact that a constant flow of the atmosphere is required.

'In order to overcome this high expense factor, other types of heat treatment atmospheres have frequently and commonly been used. Reference is made to atmospheres such as dissociated ammonia, endothermic combustive gas atmosphere, and exothermic combustive gas atmosphere. These latter three atmospheres are indicative of a class which for the purposes of this invention are referred to as secondary heat treatment atmospheres, it being apparent that these secondary atmospheres differ from the above described primary atmospheres in that the former are not pure gases. The difiiculty or disadvantage which exists in the use of these so-called secondary atmospheres is that at certain elevated temperatures, they cause a reaction to take place on the surface of the steel being heat treated. For example, dissociated ammonia is comprised of 25 percent nitrogen, which at temperatures over 900 F. causes a nitriding effect or casing to form on the surface of the work load. Similarly, endothermic and exothermic combustive gas atmospheres cause the same general type reaction at temperatures of about 600 or 700 F. Thus, it will be apparent that where the particular heat treatment operation being performed necessitates temperatures of 900 F. or over, a pure heat treatment atmosphere, such as hydrogen, must be used, with the resultant high cost of operation afore noted.

It has now been found that where the heat treatment operation is being performed in a furnace of the type wherein the heating chamber is elevated, such as the socalled humpback or elevator type furnace, a combination of primary and secondary heat treatment atmospheres may simultaneously be employed with effective results. Thus, the basic concept of the instant invention is the provision of a heat treatment furnace utilizing a primary atmosphere supply located at or adjacent to the elevated heating chamber and secondary atmosphere supplies located adjacent the work entrance and exit openings.

Referring now to the drawings, there is shown generally at 10 in FIG. 1 a heat treatment furnace of the inclined or so-called humpback type. In most respects, the furnace 10 is of conventional and standard construction, comprising an inlet ascending passageway 12 merging with a throat or preheating portion 14, which in turn connects with a central heating chamber 16. As will be noted, heating chamber 16, on its opposite side from throat 14, connects with a horizontally disposed cooling chamber 18, which in turn merges with a descending outlet passageway 20. Pivotally mounted closure flaps 22 and 24 are provided at the entrance and exit openings, respectively, said flaps being normally maintained in their closed position by gravity. A conveyor belt 26 driven by any desirable means (not shown) may be provided for carrying the work load through the furnace.

In order to maintain the furnace 18 free from a con taminating atmosphere, and particularly to impede the ingress of air at the entrance and exit openings, a predetermined flow of controlled heat treatment atmosphere is introduced. More specifically, a supply of primary or pure heat treatment atmosphere, such as pure hydrogen, for example, is introduced adjacent the upper portion of the furnace as at inlet conduit 28. Although the con duit 28 is shown as leading into cooling chamber 18, it will be understood that this source of primary heat treatment atmosphere may be introduced at virtually any point along the furnace upper portion. Thus, conduit 28 could lead directly to the heating chamber 16, if desired, or even to throat portion 14, the important thing being that this primary atmosphere be introduced at a level higher than the point of introduction of the secondary atmosphere supply, now to be described. It will also be understood that more than one inlet conduit 28 may be employed for simultaneously feeding the primary atmosphere to a plurality of points along the furnace upper level.

Connected to the ascending passageway 12 and the descending passageway are a pair of additional inlet conduits 30 for introducing a supply of secondary heat treatment atmosphere, such as dissociated ammonia, for example. As will be noted, the conduits 30 are preferably located adjacent the entrance and exit openings, and each is provided with a flow meter 32 for measuring the rate of atmosphere flow therethrough. At the same time, a similar flow meter is connected with conduit 28. If desired, the pure hydrogen atmosphere being introduced through conduit 23 may be preheated by means not shown.

In operation and use, the necessary rate of flow of heat treatment atmosphere required to properly control and envelop the entire furnace is first determined. Once this necessary rate of flow has been determined, it has been found in accordance with the instant invention that only approximately one-third of the total flow need be primary heat treatment atmosphere, while the remain ing two-thirds may comprise secondary atmosphere. Thus, for purposes of illustration, if the dimensions of the furnace 10 are such that it has been determined that a rate of flow of approximately 600 cubic feet per hour of heat treatment atmosphere is required, then approxiately 200 cubic feet per hour of primary atmosphere should be introduced through conduit 28, while each of the conduits 33 would simultaneously introduce a flow of approximately 200 cubic feet per hour of secondary atmosphere. Since the primary or pure heat treatment atmosphere introduced through conduit 28 is considerably lighter in weight than the secondary atmosphere introduced through the conduits 33, there will be very little tendency for the heavier secondary atmosphere to move upwardly in the direction of the heating chamber 16. At the same time, the constant flow of primary atmosphere through conduit 28 will result in a steady flow of said atmosphere downwardly through the passageways 12 and 20, said downward flow, in effect, combating and blocking any upward drifting of secondary atmosphere that may exist. At the same time, the secondary atmosphere will form what amounts to a pressurized curtain at the furnace entrance and exit openings, which curtain will function to prevent undesirable ingress of contaminating air at those points, all in a well-known fashion.

The success and efiiciency of this dual-atmosphere heat treatment apparatus resides in the fact that the secondary atmosphere is introduced only to the cooler portions of the furnace, namely, to the portion thereof in advance of the heating chamber and the portion after the cooling chamber. Thus, while as afore noted, the secondary atmosphere may have deleterious effects on the surface Cir of the work if subjected to temperatures upward of 600 F. or 900 F. ii dissociated ammonia is being used, these temperatures will not be reached in those portions of the furnace to which the secondary atmosphere is supplied. On the other hand, in the heating chamber where extremely high temperatures may be necessitated, the treatment atmosphere is a pure gas which will not react at such high temperatures to adversely aifect the surface of the work being treated.

Referring now to FIG. 2, my invention is illustrated in connection with an elevator-type furnace generally shown at 34. The furnace 34 comprises a loading chamber 36 having an inlet opening 38 for loading the Work to be heat treated on an elevator (not shown) when the latter is in its lowermost position. After the work has been carried upwardly, it is conveyed horizontally to a heating chamber 40, then to cooling chamber 42, and finally to unloading chamber 44. It will be understood that the unloading chamber 44 also houses an elevator (not shown) for lowering the work so that it may be removed through outlet 46. Conventional drive means 48 may be utilized for conveying the work load through the heating and cooling chambers.

Heat treatment atmosphere is supplied to the furnace 34 in a manner very similar to that hereinbefore described in connection with furnace ill. More specifically, a supply of primary or pure treatment atmosphere is introduced by conduit 50 to the upper portion of the furnace at a point adjacent the heating chamber 4%. At the same time, a supply of secondary treatment atmosphere is introduced to loading and unloading chambers 36, 44, by means of conduits 52 and 54, respectively. It will be noted that the conduits 52, 54 connect to their respective chambers at points slightly above the inlet and outlet openings, here again the critical factor being that the points of introduction of the secondary atmosphere are at a lower level than the point of introduction of the primary atmosphere. Flow meters 56 are employed to measure the rate of flow through each of these conduits.

Since the use and operation of the furnace 34 is identical in all respects to that of the furnace 10 as hereinbefore described, no additional description is deemed necessary. Suifice to state that in this form of my invention the same ratio of primary atmosphere to secondary atmosphere holds true, namely, a ratio of approximately one to two. Thus, once the necessary rate of iiow of heat treatment atmosphere for protecting the furnace has been determined, approximately one-third of this amount will be primary atmosphere passing through the conduit 50, while the other two-thirds will be secondary atmosphere equally divided between conduits 52 and 54. 'It is here again repeated that the specific location of the conduits 5t), 52 and 5'4 is not critical, as long as the primary atmosphere is introduced to the upper reaches of the furnace, while the secondary atmosphere is introduced at lower points adjacent the furnace inlet and outlet openings.

In both forms of my invention, the basic advantage derived resides in the fact that operating costs are tremendously reduced since the expense of secondary atmosphere is substantially less than that of primary atmosphere. Since only one-third as much primary atmosphere is used as has been the case heretofore where pure gases were needed for heat treatment, it follows that a substantial saving is effected.

While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except in so far as indicated by the scope of the appended claim.

I claim:

The method of heat treating a work load in a furnace having inlet and outlet openings, a heating chamber disposed between said inlet and outlet openings and elevated with respect thereto, an inlet passageway extending between said inlet opening and said chamber, and an outlet passageway extending from said chamber to said outlet opening, comprising the steps of introducing a supply of relatively light primary heat treatment atmosphere into said heating chamber, simultaneously introducing a supply of relatively heavy secondary heat treatment atmosphere into said inlet and outlet passageways at points of entry substantially lower than the point of entry of said primary atmosphere, whereby said heavier secondary atmosphere saturates said inlet and outlet pas- 6 sageways, while the lighter primary atmosphere saturates the elevated heating chamber, and moving the work load through said inlet opening and passageway to said chamber, and then outwardly through said outlet passageway and opening.

References Cited in the file of this patent UNITED STATES PATENTS 890,314 Thompson June 9, 1908 1,643,775 Kelleher Sept. 27, 1927 1,905,809 Cowan Apr. 25, 1933 2,061,910 Kingston Nov. 24, 1936 2,701,712 Gilbert Feb. 8, 1955 2,892,744 Myers June 30, 1959

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