US3575398A

US3575398A - Apparatus for minimizing atmosphere upset in a furnace for heat treating articles

Info

Publication number: US3575398A
Application number: US775459A
Authority: US
Inventors: Joseph A Lincoln; Orville E Cullen
Original assignee: Midland Ross Corp
Current assignee: Grimes Aerospace Co
Priority date: 1968-11-13
Filing date: 1968-11-13
Publication date: 1971-04-20
Anticipated expiration: 1988-04-20
Also published as: SE360111B; JPS4814523B1; GB1286986A; CA918418A

Abstract

This invention relates to the prevention of atmosphere upset and to the rapid recovery from such an upset which is caused by contaminating gases entering through the workpiece discharge opening of a controlled atmosphere furnace or hot chamber. The discharge opening is protected by a flame screen directed across its outer face and by a jet stream curtain of gases directed outwardly across the opening. Atmosphere is constantly supplied to the furnace and, additionally, a restoring gas is supplied intermittently to the furnace.

Description

United States Patent Joseph A. Lincoln;

Orville E. Cullen, Toledo, Ohio 775,459

Nov. 13, 1968 Apr. 20, 1971 Midland-Ross Corporation Toledo, Ohio Inventors Appl. No. Filed Patented Assignee APPARATUS FOR MINIMIZING ATMOSPHERE UPSET IN A FURNACE FOR HEAT TREATING ARTICLES 12 Claims, .10 Drawing Figs.

[52] US. Cl 263/36, 263/50, 98/36 [51] Int. Cl F27b 7/00 [50] Field of Search 263/36, 50; 98/36; -1 10/179; 148/165 [56] References Cited UNITED STATES PATENTS 549,235 ll/l895 Gatcke 110/179X 1,357,790 1 H1920 Marx 263/50 1,851,831 3/1932 Hayes 263/50 3,039,407 6/1962 Daman l 10/ l 79 3,142,272 7/1964 Phillips..... 1 10/179 3,270,655 9/1966 Guirl et al. 263/50X 3,356,541 12/1967 Cullen 263/36X 3,387,600 6/1968 Terzian l 10/179X Primary Examiner-Carroll B. Dority, Jr. Attorneys-Harold F. Mensing and Peter Vrahotes ABSTRACT: This invention relates to the prevention of atmosphere upset and to the rapid recovery from such an upset which is caused by contaminating gases entering through the workpiece discharge opening of a controlled atmosphere furnace or hot chamber. The discharge opening is protected by a flame screen directed across its outer face and by a jet stream curtain of gases directed outwardly across the opening. Atmosphere is constantly supplied to the furnace and, additionally, a restoring gas is supplied intermittently to the furnace.

Patented A ril 20, .1911 3,515,398

3 Sheets-Sheet 1 INVENTORS dos 5P A. IA/COL oer/.6- Cuuew ATTOENEV Patented April 20, 1971 5 Sheets-Sheet 2 INVENTORS .22 .Z' 4 JOSJPH 14- lnvcaov A TTO/t E APPARATUS POP MIINIIIWIZING ATMOSPHERE UPSET IN A FURNACE FOR IIIIEAT TREATING ARTIIEIAES BACKGROUND OF THE INVENTION The art of heat processing materials in protective or reactive atmospheres is well known and widely used. Examples of such uses are carburizing, nitriding, bright annealing. normalizing, hardening, and the like. Up to the present time, it has been a continuous problem to control or maintain the analysis of the protective or reactive atmosphere in a heat treating furnace as desired. The problem is particularly acute in atmosphere furnaces which require frequent opening of a discharge door for the removal of workpieces. A specific example of this is a carburizing furnace for treating gears or the like which must be press quenched. In such an operation the gears and separating fixtures are removed one at a time from the furnace. A typical cycle for the removal of one gear and its attendant separator is as follows:

Door is opened for 20 seconds for removal of a gear.

Door is closed for 30 seconds while gear is placed on press fixture and then into quench tank.

Door is opened for 10 seconds while separator is removed.

Door is closed for 9 minutes until gear is fully quenched and removed from quench tank and press fixture.

The atmosphere of a regularly equipped prior art carburizing furnace would be upset with the first door opening and if the above cycle were repeated, the atmosphere would never recover, but would get progressively worse with each cycle. In an effort to reduce the influx of contaminating cold air and gases through the discharge door opening of prior art carburizing furnaces, a protective flame screen or curtain was directed across the face of door opening by means of an elongated burner disposed along the top of the door or door opening. This device reduced the amount of upset, but the reduction was insufficient to prevent decarburization. Even with such a device, the surface carbon content of the workpieces was sometimes reduced as much as 50 points of carbon. A reduction of this amount usually rendered the workpieces unacceptable.

It is also common to provide such furnaces with a slightly positive internal pressure so that if any leakage occurs. it will be of the atmosphere out of the furnace rather than of contaminating air into the furnace. Conceivably the atmosphere could be supplied to the furnace at a rate which would be capable of maintaining at least a slight outward flow across the entire door opening when the discharge door was opened. The outflow rate required to accomplish this for a standard door opening of l" 34" has been computed to be about l8,000 standard cubic feet per hour (SCFH). The costs and problems created by such a high outflow of atmosphere prohibit its use.

SUMMARY OF INVENTION It is an object of this invention. to produce a simple, efficient. effective, and economical means for reducing or minimizing an atmosphere upset caused by the influx of contaminating air or gases through the discharge opening of an atmosphere furnace. Another object of this invention is to reduce the duration of the atmosphere upset. It is still another object to provide a press quench holding chamber of a carburizing furnace with a means for eliminating or preventing appreciable decarburization vof carburized workpieces normally caused by a prolonged and severe atmosphere upset occasioned by the opening of the discharge door. It is still another object of this invention to provide a method and apparatus for heat treating workpieces in an atmosphere furnace whereby the amount of heat treatment may be more precisely and readily controlled.

These objectives are accomplished by a combination of elements including a dynamic curtain or barrier formed by a high velocity wind stream of gases issuing from a slotted or apertured pipe located along at least one side of the discharge opening. This wind or jet stream curtain of gases is turned on in response to the opening of the discharge door and is directed across the entire discharge opening and at least slightly outwardly with respect to the interior of the furnace or hot vestibule. Preferably, the curtain is directed outwardly at an angle which is sufficient to cause eduction of some atmosphere from the furnace and to prevent back eddying of barrier curtain gases into the interior of the furnace. The jet stream curtain may impinge upon or brush over an opposite side of the discharge opening. Generally, atmosphere is supplied continuously to the furnace or hot vestibule during operation. In addition to this, a supplementary gas, such as a restoring or enriching gas, may be intermittently injected into the input atmosphere. The addition of this gas tends to reduce the duration of any upset and if its injection is timed to begin at or near the commencement of an upset, it tends also to reduce the amount of the upset. In carburizing furnaces which have atmospheres containing combustibles, the dynamic barrier comprises a flame screen which is directed across the face of the discharge opening so as to ignite the combustibles that flow out of the opening. The flame screen may be supplanted with a nonflammablc jet stream curtain in furnaces which do not contain readily combustible atmospheres.

The above mentioned objects and other objects and advantages and the manner of attaining them are described below with reference to the embodiments of this invention shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view in section showing an end portion of a controlled atmosphere furnace with a separate discharge vestibule embodying the jet stream barrier and supplementary gas surge means of this invention.

FIG. 2 is an enlarged clevational view of the discharge vestibule of FIG. I.

FIG. 3 is an elevational view of a rotary hearth furnace embodying the principles of this invention.

FIG. 4 is an enlarged front view of the discharge door assembly and door opening shown in FIGS. 1, 2, and 3 with parts broken away.

FIG. 5 is a fragmentary side elevational view in section of the discharge door assembly shown in FIGS. l4, with the discharge door in an open position and with the dynamic barrier in operation.

FIG. 6 is a fragmentary side elevational view in section of a discharge door assembly similar to that shown in FIGS. 1-4, but with the flame screen burner pipe directed upwardly and attached to the top of the vertically reciprocablc door.

FIG. 7 is a fragmentary side clevational view in section of a discharge door assembly similar to that shown in FIGS. 1-4, but with two flame screens directed across the discharge door opening from opposite sides.

FIG. 8 is a fragmentary side elevational view in section of a discharge door assembly similar to that shown in FIGS. I4, but with an additional jet stream curtain.

FIG. 9 is a fragmentary side elevational view in section of a discharge door assembly similar to that shown in FIGS. l4, but with a jet stream curtain directed upwardly and outwardly from the top of the open discharge door.

FIG. 10 is a fragmentary side elevational view in section of a discharge door assembly similar to that shown in FIGS. l-4,

but with a jet stream curtain directed upwardly and outwardly from the inner edge of thethreshold of the door opening.

DETAILED DESCRIPTION OF THE DRAWINGS FIGS. 1 and 3 show two types of heat treating atmosphere furnaces I0 and 20, respectively, embodying principles of this invention. The furnace 10 is a tunnel type furnace having a hot vestibule 22 or holding chamber attached alongside the heat treating section 24 of the furnace adjacent its discharge end. Radiant heating. means 25 and 27, such as fuel fired radiant tubes, are provided within the vestibule 22to maintain the temperature within the vestibule at the desired level, for

example, about l,SO F. in a holding chamber vestibule of a carburizing furnace. Workpieces which have been completely or partially heat treated are transferred from the heat treating section 24 directly into the vestibule 22 through an entrance opening. Subsequently, the workpieces are removed from the vestibule through a discharge opening. Preferably, these openings are mechanically sealed by an entrance door 26 and discharge door 28, respectively. These

doors

26 and 28 may be opened, other than simultaneously, for the input or removal of workpieces. Thus, the discharge end of the heat treating section 24 is not directly exposed to ambient air and therefore the amount of atmosphere upset in this section, due to the transfer of workpieces therefrom into the vestibule, is minimized or practically eliminated. It is particularly advantageous to provide large furnaces with such a vestibule or holding chamber in view of the discovery that recovery from an atmosphere upset can be accomplished more rapidly in a small chamber than in a large chamber. The relative internal size of the heat treating section 24 as compared to the vestibule 22 may be well in excess of 2 to l. ln one specific example of a continuous carburizing furnace, the volume of the vestibule was approximately 64 cubic feet and the volume of the heating section was about 30 times that of the vestibule. Vestibules or holding chambers having a volume of about 75 cubic feet or less are sufficiently small to permit a rapid recovery from an atmosphere upset.

The furnace 20 in FIG. 3 is a rotary hearth furnace having a heattreating chamber 30 with a volume less than 70 cubic feet which is heated by fuel fired radiant tubes 32. Although a separate hot vestibule or holding chamber could be provided, it is generally not needed because the chamber 30 is sufficiently small to permit rapid recovery from an atmosphere upset. On larger rotary hearth furnaces it is desirable to provide a discharge vestibule or hot chamber similar to that shown in FIG. 1.

Furnace 20 and hot vestibule 22 of furnace are supplied with similar discharge door assemblies 40. A door assembly 40 comprises a liner 42 fitted inside of the rectangular discharge door opening extending through a vertical wall of the furnace or hot vestibule. The height of the opening is kept to a minimum, for example about l0" or less, to reduce the stack effect created by hot atmosphere gases seeking to escape through the opening into the colder ambient air. The width of the opening is less critical and may be up to about four times the height without causing a severely detrimental effect. The rectangular liner 42 may be a casting made of heat resistant metal and have a relatively wide bottom side or threshold 44 which extends inwardly beyond the vestibule wall. A watercooled peripheral door frame 46 may be attached adjacent the outer edge of the liner 42. Preferably, the discharge opening is sealed by means of a vertically reciprocable sliding door 28 actuated by a hydraulic cylinder and piston means 48.

The dynamic barrier associated with the discharge door opening comprises at least two

jet stream curtains

50 and 52 directed across the opening at an angle towards each other. This angle and the location of the jet pipes from which these jet stream curtains emanate are such that the curtains intersect each other in close proximity to the face of the door opening and cause at least a slight outflow of atmosphere from the furnace or vestibule 22. When the furnace atmosphere contains combustibles, the jet stream curtain 50 may be a high velocity curtain of flames which is directed substantially vertically across the face of the opening by means of a slotted or apertured burner pipe 54. The burner pipe 54 may be located along top edge of the discharge opening or along top edge of the door 28. If exceptionally wide workpieces are being processed, a burner pipe 54 may he provided at both locations with theirjet streams directed towards each other at an angle at least slightly outward (see FIG. 7). Thus, the passage ofa wide workpiece through the flame curtain will not cause a break in the flame curtain barrier, such as would occur if only a unidirectional flame curtain barrier were used. A nonflammable jet stream curtain may be substituted for one or both of the flame curtains 50 when a nonflammable atmosphere is used in the furnace.

The other jet stream curtain 52 may be a high velocity curtain of gases ofa type which is not adversely contaminating to the furnace atmosphere, such as an inert or nonoxidizing gas. Preferably, the jet stream curtain emanates from an apertured or slotted pressure pipe 56 rotatably affixed along the top inside edge of the discharge opening. It is directed outwardly and downwardly towards the front edge of the threshold 44 so as to impinge upon or brush over the outer surface portion of the threshold. An angle of incidence of about was found to produce good results when used in combination with the vertically directed flame curtain, as shown in FIGS. 2, 3, 4, 5, and 7. The jet stream curtain 52 was also found to be effectual, in the combination, when it emanated from a

pressure pipe

57 or 58 at the lower inner edge of the threshold 44 or along the top of the door 28, respectively, and was directed upwardly and outwardly (see FIGS. 9 and 10). In a carburizing furnace having a hot vestibule and a discharge opening with the above mentioned dimensions, a

PA inch pipe

56, 57, or 58 with a row of 37 020 MTD size holes spaced 1 inch apart provided a satisfactory jet stream curtain 52 when supplied with gas at the rate of 600 SCFH under a pressure equivalent to [3 inch of water column. The

jet stream curtains

50 and 52 may be turned on simultaneously in response to the opening of the discharge door 28 and may be turned off simultaneously by its closing switch 59 located adjacent the top of the furnace door 28 and associated with a solenoid valve means 61 (see FIG. 4) to control the flow of gases into

jet curtain pipes

54 and 56. When the door 28 is opened the switch lever falls to its downwardly position as shown, closing the switch and energizing the valve means open, thus allowing gas to flow into

pipes

54 and 56. When the door is closed it forces the switch lever into its upward or open position, thereby deenergizing the valve means and shutting off the flow of gas to

pipes

54 and 56.

During furnace operation, atmosphere or an atmosphere carrier gas is injected into the furnace 20 or hot vestibule 22 through a pipe 60 at a rate which is sufficient to maintain a slightly positive pressure therewithin. This rate is normally about 5 to 10 times the volume of the furnace or vestibule per hour. An intermittent supply or surge of restoring gas is injected by means of pipe 62 into the atmosphere in response to or in anticipation of an atmosphere upset. The duration and timing of this gas surge may be governed by a timer 64 and the movement of the discharge door 28 (see FIG. 2) so that the surge of restoring or enriching gas begins when the door is moved to its open position thereby causing the door limit switch lever, incorporated with the timer, to be depressed to its closed position, as shown in dashed lines in FIG. 2. When the switch lever is in this position, the timer 64 is actuated and the solenoid valve 65 on the restoring gas supply pipe 62 is energized open. The timer is preset to hold valve 65 open for a set period of time sufficient to insure recovery from any atmosphere upset that may occur within the furnace or furnace vestibule. In the embodiment shown in HO. 3 the restoring or enriching gas surge is controlled by an atmosphere sensing device 66 which operates a solenoid valve 67 in a restoring gas supply line 68 communicating with the atmosphere supply line 60. When an atmosphere upset is sensed, a relay incorporated with the sensing device 66 energizes the solenoid valve 67 open and maintains it in this position until the sensing device senses that the atmosphere has been restored. Thus, the gas flow may be controlled so as to begin prior to or at the beginning of an atmosphere upset and continue until recovery of the desired atmosphere is attained. The flow rate of the restoring gas and the total amount required are dependent upon processing conditions, particularly the size of the furnace or vestibule and the extent of the atmosphere upset. Normally, this rate would be less than about one volume of the furnace or vestibule per hour. As, for example, in a 65 cubic foot pressquench holding chamber or vestibule of a carburizing furnace, methane was supplied at the rate of 45 SCFH from the time the discharge door was opened until 2 minutes after it was closed. The restoring gas surge was used in combination with the dynamic barrier and a continuous supply of atmosphere which was supplied at the rate of 500 SCFH. Using this combination, the carbon content of the workpieces in the holding chamber was maintained even though the aforementioned typical cycle was repeated continuously.

While there are described above the observed principles of this invention in connection with specific embodiments, it is to be clearly understood that there may be many unobserved side effects which contribute to the effectiveness of this device, and that this description is made only by way of example and not as a limitation of the scope of this invention.

We claim:

1. A controlled atmosphere furnace comprising: walls defining a heat processing chamber having a discharge opening associated therewith, a jet stream curtain barrier means directed across said opening for producing an unbalanced outward component of force in said opening, said barrier including two jet stream curtains directed at an angle towards each other with at least one of said curtains being directed outwardly with respect to said opening, means for continuously supplying atmosphere to said chamber means for automatically supplying a surge of enriching gas to said chamber in response to a predetermined event and when said chamber is open to ambient atmosphere, said surge supply means having an inlet which is separate from said jet stream curtains.

2. A furnace according to claim 1 wherein at least one jet stream curtain emanates from a side of said opening and is directed so as to impinge upon an opposite side surface of said opening.

3. A furnace according to claim 1 wherein said jet stream includes two jet stream curtains directed at an angle with respect to each other and one of said jet stream curtains is a curtain of flame.

4. A furnace according to claim 1 wherein one of said jet stream curtains is a curtain of nonoxidizing gas.

' 5. A controlled atmosphere furnace comprising: a heat processing chamber. a hot vestibule adjacent said processing chamber, an entry door means between said chamber and said hot vestibule for the admission of heat treated workpieces into said hot vestibule from said processing chamber, a discharge opening in said vestibule having a door for the removal of said workpieces from said hot vestibule, jet stream means for directing a barrier curtain of gases outwardly across the discharge door opening from within said hot vestibule and a flame curtain directed across the outside face of said opening whereby contaminating gases are prevented from entering said hot vestibule through said discharge door opening.

6. A furnace according to claim 5 wherein said discharge door means has a threshold and said curtain is directed from the top of said discharge door opening downwardly towards said threshold and impinges thereon at an angle of incidence of about 60.

7. A furnace according to claim 5 further including means for actuating said barrier in response to the opening of said discharge door.

8. A furnace according to claim 5 further including means for supplying a quantity of atmosphere restoring gas to said hot vestibule intermittently in response to an atmosphere upset.

9. A furnace according to claim 8 further including means for initiating the flow of said atmosphere restoring gas in response to the opening of said discharge door and discontinuing said flow when the atmosphere in the vestibule is restored.

' 10. A furnace according to claim 9 further including means for constantly supplying an atmosphere carrier gas to said hot vestibule at a rate at least 5 times the rate at which the restoring gas is supplied to said vestibule. I

ll. A furnace according to claim 9 further including means for constantly supplying an atmosphere carrier gas to said hot vestibule at the rate of approximately 5 to l0 times the volume of the chamber per hour.

12. A controlled atmosphere furnace comprising: a heat processing chamber, a subsequent chamber adjacent said processing chamber, an entry door means between said chambers for the admission of heat treated workpieces into said subsequent chamber from said processing chamber, a discharge door means in said subsequent chamber for removing workpieces therefrom, jet stream means for directing a high velocity curtain barrier outwardly across the discharge door opening from within said subsequent chamber, means for intermittently supplying a restoring gas to said subsequent chamber in response to an atmosphere upset, means for directing a curtain of flame across the outside face of said opening whereby the upset of the furnace atmosphere caused by opening of the discharge door is minimized and the desired atmosphere is rapidly restored.

@2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3.575.398 Dated April 20. 1971 Inventor(s) Josenh A. Lincoln et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 21, change "020" to #20 Between

lines

27 and 28 insert such as by means of a door actuated limit Signed and sealed this 10th day of August 1971 (SEAL) Attest EDWARD NLFLETCHERJR WILLIAM E SCHUYLER, JR Attesting Officer Commissioner of Patents

Claims

5. A controlled atmosphere furnace comprising: a heat processing chamber, a hot vestibule adjacent said processing chamber, an entry door means between said chamber and said hot vestibule for the admission of heat treated workpieces into said hot vestibule from said processing chamber, a discharge opening in said vestibule having a door for the removal of said workpieces from said hot vestibule, jet stream means for directing a barrier curtain of gases outwardly across the discharge door opening from within said hot vestibule and a flame curtain directed across the outside face of said opening whereby contaminating gases are prevented from entering said hot vestibule through said discharge door opening.

6. A furnace according to claim 5 wherein said discharge door means has a threshold and said curtain is directed from the top of said discharge door opening downwardly towards said threshold and impinges thereon at an angle of incidence of about 60*.

10. A furnace according to claim 9 further including means for constantly supplying an atmosphere carrier gas to said hot vestibule at a rate at least 5 times the rate at which the restoring gas is supplied to said vestibule.

11. A furnace according to claim 9 further including means for constantly supplying an atmosphere carrier gas to said hot vestibule at the rate of approximately 5 to 10 times the volume of the chamber per hour.