GB2045408A

GB2045408A - Furnace system

Info

Publication number: GB2045408A
Application number: GB7910830A
Authority: GB
Original assignee: Holcroft LLC
Current assignee: Holcroft LLC
Priority date: 1979-03-28
Filing date: 1979-03-28
Publication date: 1980-10-29

Abstract

A method and apparatus are disclosed for heat-treating workpieces in a furnace system which uses a minimum total amount of energy. A self-contained, continuous heat- treating system 20 includes a carburizing furnace 24, a tempering furnace 30, and a cooler 32. Energy transfers between these and other components operable at different temperatures and/or energy requirements are used to maximize thermal efficiency of the system. Furnace components are provided and interconnected so that combustion air for radiant tube heaters 34 supplying thermal energy to the furnace 24 is preheated by exchanging heat in the cooler 32 and by recuperation of the carburizing furnace exhaust gases, combustion products from the carburizing furnace 24 supply energy for reheating the workpieces in the furnace 30, after they have been quenched and washed in units 26, 28. Exhaust heat from the furnace 30 is used for preheating the workpieces prior to their entry into the furnace 24, and energy transferred by the workpieces to the quench medium may be used to heat water for the subsequent washing of the workpieces. <IMAGE>

Description

SPECIFICATION Furnace system This invention relates to heat-treating furnaces such as furnace systems for continuous carburizing or forging of metal parts.

Heat-treating furnace systems such as continuous carburizing plants typically include components for performing three prirnar processing steps: 1) a hardening step wherein the workpieces such as ferrous metal parts are heated under a controlled atmosphere (e.g. a carbon-enriched atmosphere in the case of carburizing); 2) a quench step to rapidly decrease the temperature of the parts; and 3) a reheat step wherein the parts are generally reheated to a lower temperature than employed in the hardening step in order to stress relieve the parts (e.g. tempering). In addition to these primary steps, a heat-treating system usually includes a wash unit in which residual quench media are removed from the parts prior to reheating the parts.

Furnace systems for performing these processing steps use large amounts of energy, and in conventional arrangements of furnace components considerable energy is wasted due to flue losses, wall losses, and losses in transporting parts between components. Part of the energy normally wasted in flue gas exhaust may be recovered by providing recuperators such as are shown in our U.S. Patent No. 4,1 13,009. With the exception of recuperators, however, which may be conveniently added to furnace systems as retrofit equipment, energy saving devices for heattreating furnaces have heretofore been complex, inefficient, and difficult to integrate with existing furnaces. Yet in view of sharply escalating energy costs, furnace systems which provide significant further reductions in flue losses and in the other energy losses of heat-treating equipment are of considerable benefit.

Accordingly, it is an object of the present invention to provide a furnace system for, and a method of, heat-treating workpieces, which are operable at high thermal efficiences.

According to this invention, a furnace system for heat-treating workpieces comprises a first furnace; a preheater for heating the workpieces prior to entry of the workpieces into the first furnace; heater means for heating the workpieces to a predetermined temperature in the first furnace; a quench unit connected to an end of said first furnace from which the workpieces are discharged, the quench unit containing a quench medium for rapidly lowering the temperature of the workpieces received from the first furnace; a second furnace for heating the workpieces to a lower temperature than the predetermined temperature following passage of the workpieces through the quench unit; a cooler attached to an end of the second furnace from which the workpieces are discharged for cooling workpieces received from the second furnace and for heating air for use as combustion air in the heater means; means for directing heated air from the cooler to th heater means; means for directing products of combustion of the heater means from the first furnace and through the second furnace for heating the workpieces in the second furnace; means for directing the products of combustion exhausted from the second furnace through the preheater for heating workpieces therein; and transport means for moving the workpieces successively through the preheater, the first furnace, the quench unit, the second furnace and the cooler.

The invention also consists according to another of its aspects, in a method of heat-treating workpieces comprising in sequence the steps of:-- preheating the workpieces using exhaust gases from a tempering furnace; heating the workpieces in a carburizing furnace to a temperature in the range of from 1 5000F to 18000 F; quenching the workpieces by means of a quench medium; reheating the workpieces in the tempering furnace to a temperature in the range of from 3000F to 1 4000F using as at least a major portion of the thermal energy in said reheating step exhaust heat from the carburizing furnace; and, air cooling the workpieces, the air-cooling step also preheating air which is used as combustion air for combustion which heats the workpieces in the carburizing furnace.

In a preferred embodiment of the system in saccordance with the invention, there is a washer containing a cleansing fluid for washing the workpieces subsequent to their passage through the quench unit, the transport means being operative to transport the workpieces from the quench unit through the washer and thence to the second furnace; and means for circulating the cleansing fluid between the washer and the quench unit and for extracting energy from the quench medium to heat the cleansing fluid.

There are also preferably means for controlling the temperatures of the furnaces, such as an auxiliary burner and means for recirculating air as a heating medium within the second furnace.

Recuperators may be connected to the heating means which are preferably radiant tube heaters employed for heating the workpieces in the first, or high temperature, furnace. The recuperators transfer additional heat from the exhaust of the radiant tube heaters to the combustion air following initial preheating thereof in the cooler.

One particular system of interest is a compact multi-level arrangement wherein the first or carburizing furnace and a preheater are located at a lower level and the second, or tempering furnace is mounted on top of the carburizing furnace. The upper level of this system also includes a washer and a cooler. Another embodiment of the system is a single level furnace system having energy utilization features similar to those of the multi level system.

Some examples of systems and of methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block flow diagram of the furnace system showing diagrammatically the flow of parts being treated and the flow of energy through one example of the system; Figure 2 is a side perspective view of the first example of the system; Figure 3 is a plan view of the system of Figure 2; Figure 4 is a longitudinal section taken along the line 4--4 of Figure 3; Figure 5 is a cross-section taken along the line 5-5 of Figure 3; Figure 6 is a plan view of a second example which is a single level furnace system; and, Figure 7 is a fragmentary cross-section of a third example, which is a high temperature furnace, showing details of a recuperator suitable for use in the system.

In Figure 1 there are shown major components of a thermally efficient furnace system 20 for heat-treating workpieces such as metal parts.

Solid flow lines in this block diagram indicate the flow of parts through system components during continuous processing of parts, and broken lines indicate the flow of combustion products, air, and other fluids through specified components. As shown in this simplified diagram of the invention, parts to be processed are transported in succession through a preheater 22, a high temperature furnace 24, a quench unit 26, a washer 28, a low temperature furnace 30, and a part cooler 32.

Parts to be processed in the furnace system 20 are first loaded into the preheater 22. The parts are preheated by exchanging heat with the exhaust of the low temperature furnace 30 which is directed through the preheater 22 by means of a blower 33. This preheating step, though not essential to heat-treating processes to which the present invention is directed, permits recovery of a substantial portion of the energy normally wasted due to effluent losses. After passing through the preheater 22, parts are fed into a high temperature furnace 24, typically a furnace for carrying out a process such as carburizing in which parts are heated indirectly by radiant tube heaters 34 in the presence of a carbon-(or carbon/ammonia-) enriched atmosphere.

Combustion air for the heaters 34 is preheated in a two-step process wherein air is first drawn through the part cooler 32 by a blower 35 to exchange heat with parts during cooling thereof and then extracts additional heat from the exhaust of the high temperature furnace 24 in passing through a plurality of recuperators 36.

The quench unit 26, which may include a tank of oil or molten salt or any other suitable quench medium, is arranged to receive the parts following their treatment in the high temperature furnace 24 and to quickly lower their temperature. A washer 28 removes oil or salt residues from the parts following the quench. This facilitates further treatment of the parts in the low temperature furnace 30 and the part cooler 32 and minimizes burn-off fumes which would otherwise occur in the low temperature furnace 30. Preferably the water used in the washer 28 is heated by transferring thereto some of the energy normally dissipated in the quench medium instead of separately heating the water.This is achieved by circulating the water through a heat exchanger in the quench unit 26 or, if the temperature differential between the quench medium and the washer 28 precludes this, by circulating a suitable heat exchange medium through the washer 28 and the quench unit 26.

After the parts pass through the washer 28, they are fed to a furnace 30, where they are reheated to a lower temperature than the maximum temperature of the furnace 24 to relieve stresses in the parts (e.g. the furnace 30 may be utilized for tempering or annealing the parts). As is suggested in FIG. 1, a primary source of energy is the exhaust from the furnace 24 from which energy is recovered to successively preheat combustion air in the recuperators 36, heat parts in the furnace 30, and preheat parts in the preheater 22.

The final stage in the furnace system 20 is a part cooler 32, which, like the preheater 22 and the washer 28, does not produce fundamental metallurgical changes in the parts but is desirable since it permits cooling of the parts at a controlled rate and since air drawn through the cooler 32 and ducted to the recuperators 36 allows utilization of thermal energy remaining in the parts after their discharge from the furnace 30 for preheating combustion air for the radiant tube heaters 34.

FIG. 2 shows a multi-level carburizing furnace system 38 according to a preferred embodiment of the invention. (In the interest of clarity, there have been omitted from this three-dimensional illustration the ductwork for carrying preheated air from the part cooler 32 to the recuperators 36 and fuel input lines to the recuperators 36. Also, means for transporting parts through the system are illustrated schematically as rails 39.) With the exception of the quench unit 26, whose quench tank is preferably located below all other components, the components of the furnace system 38 are arranged on two levels. The preheater 22 and the high temperature furnace 24 are positioned adjacent and parallel to one another on a lower level, while the low temperature furnace 30 is mounted on top of the furnace 24 on an upper level which also contains the washer 28 and the cooler 32. This multi-level arrangement of components, although requiring one or more vertical moving devices such as elevators 41 and 42 shown in phantom at one end of furnace system 38 to move parts between levels, provides a compact furnace configuration.

Details of the structure and operation of the system illustrated in FIG. 2, shown in more detail In FIGS. 3-5, may be readily understood from a description of its components in the order in which parts are transported therethrough during heat processing. Workpieces such as ferrous metal parts start a processing cycle in a load/unload area 43 where they are loaded as from a bin 44 into trays 45. The trays are placed on a transport means schematically illustrated as rails 39 in FIG.

2 but which may be a table with rollers, a belt, or any other suitable transport means. The parts are directed along the transport means 39 to the preheater 22, whose part inlet end 46 is open to permit entry of parts and discharge of exhaust gases therefrom. The preheater 22 has an outlet door 47 at the opposite end which opens and closes at desired intervals to discharge parts.

Within the preheater 22, parts are preheated by heat exchange with the exhaust from the furnace 30. This exhaust is directed to the preheater 22 through a duct 48, enters the preheater 22 through a gas inlet 50 to circulate over the parts in counterflow to part movement, and then is discharged as the final exhaust of the furnace system 38 through the part inlet 46. The temperature to which parts are preheated depends on the temperatures of the carburizing furnace 24 and the low temperature furnace 30, the rate of flow of parts through the system, and other variables.In one representative heat-treating process wherein the temperature of the carburizing furnace 24 is 1 65O0F and that of the low temperature furnace 30 is 7000F, and with a part flow rate of 3000 Ibs/hour, parts would be heated from an ambient temperature of about 8O0F to about 3500F in the preheater 22.

Adjacent to the preheater 22 near the part discharge end thereof is a charge chamber 56 (FIG. 3) where gases which leave the preheater 22 with the parts are burned and the charge chamber 56 is filled with a gaseous atmosphere which prevents scaling or decarburization. To charge trays into the furnace 24, a carburizer inlet door 58 near the charge chamber 56 is apened and a loading mechanism such as a carburizer puller 60 loads at least one tray of parts into the carburizing furnace 24. It should be understood that instead of the puller 60 and the pullers referred to hereinafter, other devices may be used to move parts from one component of the furnace system 38 to another -- e.g. a pusher such as pusher 40 shown in FIG. 1 of U.S. Patent No. 3,662,996 "Multi-Chamber Carburizing Apparatus", issued to D. J. Schwalm and E. C.Bayer, the disclosure of which is incorporated herein by reference.

The furnace 24 includes transport means 62 (FIG.4)) which may be refractory skid rails, a table with rollers, or any other suitable transport system which in cooperation with a pusher 64 permits trays of parts to be transported through the furnace 24. The transport means 62 and the furnace 24 may accommodate a single row or multiple rows of trays, e.g. two rows as indicated in FIG. 3. The interior of the furnace 24 (FIG. 5) is defined by refractory sidewalls 66 and 68, a base 70, and a roof 72. As shown in FIG. 4, arches 74, 76, and 78 separate the interior of the furnace 24 into a heating zone 80, a carburizing zone 82, a diffusing zone 84, and a discharge zone 86. Within each of the zones one or more radiant tube heaters 34 extends transversely between the side walls 66 and 68 above the transport means 62.In one or more zones such as heating zone 80, additional radiant tube heaters 34 may be provided below the transport means 62.

As shown in FIG. 7, a typical heater 34 includes a U-shaped radiant tube 94 and a burner 95 which is connected to the inlet 96 of the radiant tube.

The outlet 97 of radiant tube 94 is located adjacent to the burner 95 and extends through one of the refractory sidewalls of the furnace 24 - e.g. the sidewall 66. During processing of parts, a suitable liquid or gaseous fuel is fed to the burner 95, and the radiant tube 94 radiates energy to heat parts in the furnace 24 to desired temperatures, preferably in the range of 1 5000 F to 18000F.

The furnace 24 also is provided with means (not shown) for supplying an appropriate gas atmosphere to each of the zones 80, 82, 84 and 86 for carburizing the parts, these supply means being well known and forming no part of the present invention. Also included within the furnace 24, but not illustrated, are fans for circulating these gases uniformly around the parts.

Suitable fans for this purpose are described in U.S.

Patent No. 4,093,195, "Carburizing Furnace", issued to Donald J. Schwalm, whose disclosure is incorporated herein by reference.

As noted above, the air supplied for combustion in burners such as a burner 95 is preheated initially in the part cooler 32 to recover energy from parts during cooling thereof and a second time in the recuperators 36 to extract energy from the exhaust of the carburizing furnace 24. A recuperator 36 suitable for use in the furnace system 38 is shown in FIG. 7 and comprises three concentric cylinders providing a double pass air flow pattern. Air from the part cooler 32 and the blower 35 enters the recuperator 36 through an inlet 98 near the base of an outer cylinder 99. The air flows upward between the outer cylinder 99 and a middle cylinder 100, down between the middle cylinder 100 and an inner cylinder 101, and then leaves the recuperator 36 through an outlet 102 near the base of the middle cylinder 100 for passage to the radiant tube heater 34.The inner cylinder 101 serves as a flue for the upward passage of combustion products exhausted from the heater 34 and transfers heat to the combustion air by a combination of radiation and convection. The blower 35 may circulate air to all of the recuperators 36 through appropriate ducting, or may comprise one of a plurality of blowers each circulating air to an individual recuperator.

To permit the discharge of carburized parts from the furnace 24, the sidewall 66 includes an outlet door 103 (FIG. 3) adjacent to the discharge zone 86, and a puller 104 is provided to remove trays from the furnace 24 through the door 103 at appropriate intervals. Parts discharged from the furnace 24 are lowered into and fed through the quench unit 26 by a conventional elevator means so that by immersion in a bath of oil, molten salt, or other quench medium the temperature of the parts is rapidly reduced, for example, to a temperature of about 3500F in the heat-treating process described above wherein the furnace 24 is operable at 1 65O0F.-The parts are then loaded onto an elevator 42 for transport to the upper level of the furnace system 38 and are directed through the washer 28 wherein oil or salt residues are removed from the parts by spraying them with water or water plus detergent. Preferably washing of the parts is performed at a temperature somewhat above ambient, for example at about 1800 F, and the wash water is heated to this temperature by circulating it through pipes 105 and 106 connected between the washer 28 and the quench unit 26 and through a suitable heat exchanger (not shown) in contact with the quench medium within the quench unit 26.

In line with the washer 28 on the upper level of the furnace system 38 above the carburizing furnace 24 is a low temperature furnace 30 for reheating parts to a temperature lower than the maximum temperature of the carburizing furnace 24, (e.g. in the range 3000to 14000 F) primarily to relieve stresses in the parts. The furnace 30 has a part inlet door 108 which, when open, admits trays of parts from the washer 28 into the furnace.

Also provided are transport means 110 which may be a chain driven rail system or any other suitable conveyor means, and a part outlet door 112 near the end of the furnace 30 opposite the inlet door 58 for permitting discharge of parts.

Energy for heating parts within the furnace 30 is obtained from the exhaust of the radiant tube heaters 34. As is best shown in FIG. 2, the heaters 34 are connected through the recuperators 36 and pipes 113 to a manifold 114 adjacent to one side of the low temperature furnace 30. A duct 11 7 near one end of the manifold 114 channels exhaust gases from the manifold 114 to the furnace 30 for entry through an inlet 118 into the furnace 30.As shown in FIG. 4, the interior of the furnace 30 is partitioned by a divider 120 into an upper zone 11 9, which includes features for controlling the temperature and, to a lesser extent, the pressure of the atmosphere within the furnace 30, and a lower zone 121 for reheating parts entering the furnace 30 to a specified temperature and then holding them at this temperature for a desired time interval as they are transported through the lower zone 121 towards the part outlet door 112. A fan or blower 122 is mounted within the upper zone 11 9 near the end of the surface 30 above the part inlet door 108 for circulating the furnace atmosphere in a generally clockwise direction as indicated by the arrows in FIG. 4.Flow from the duct 117 is directed through the inlet 118 into the upper zone 11 9, passes through the fan 122, down into and then along the lower zone 121 in the direction of part flow, and through a passage 123 formed between the divider 120 and a baffle 124 attached to a wall of the furnace 30 near the downstream end thereof.

The gas flow then splits so that a portion thereof is discharged from the furnace 30 through the exhaust duct 48 and the remainder passes above and along the divider 120 towards the fan 122 for mixing and recirculation with flow entering the inlet 118.

An auxiliary heater such as a gas-fired burner 127 may also be included within the upper zone 119 on the low pressure side of the fan 122 for supplying additional heat as required for temperature control within the furnace 30. The burner 1 27 (or suitably placed electric heaters) provides additional energy during start-up and process completion when the burners of the radiant tube heaters 34 are not firing or during steady-state operation to insure precise temperature control within the lower zone 121.

Additional control of the atmosphere within the furnace 30 is provided by a damper-regulated air intake 130 in the duct 117 and a damperregulated air intake 132 in the exhaust duct 48. A booster blower 33 in the duct 48 directs exhaust from the furnace 30 into and through the preheater 22 wherein energy is extracted from the exhaust to preheat parts prior to a final exhaust of gases from the furnace system 38 through the part inlet end 46 of the preheater.

Adjacent and parallel to the low temperature furnace 30 on the upper level of furnace system 38 is a cooler 32 which is operable to cool parts received from the furnace 30 and to extract thermal energy from the parts to preheat combustion air for the burners for the radiant tube heaters 34. As shown in FIGS. 2 and 5, the part discharge end of the cooler 32 is open to admit air for cooling the parts. A blower 35, within a duct 134 connected to an air outlet 138 near the part inlet end of the cooler 32, draws air through the cooler and directs this preheated air through the duct 1 34 to the recuperators 36 for further preheating.

To move the parts though the cooler 32 during operation of furnace system 38, a puller 1 50 removes trays from the furnace 30 through the outlet door 112 into a cooler vestibule 1 52. At appropriate intervals a pusher 1 54 then pushes the trays through a cooler inlet door 1 56 along the conveyor means 158 through the cooler 32. Trays move to the end of the conveyor 158, onto an elevator 41 for transport to the lower level, and are directed by a pusher 164 along conveyor means 1 66 to the load/unload area 43.

FIG 6 is a plan view, with portions broken away to expose certain details, of a single level furnace system 1 70 with components and energy transfer features similar to those of the multi-level system 38 of FIGFS. 2-5. In the embodiment of the invention shown in FIG. 6, a carburizing furnace 1 72 and a tempering furnace 174 are arranged in parallel on the same level as all other components except a quench tank 1 75 which is preferably located below the common level. This single level arrangement avoids the need for elevators except a conventionally employed elevator mechanism to lower parts into and raise them froin the quench medium in the tank 1 75. The tempering furnace 1 74is part of an essentially straight line configuration which also includes a washer 178, a rinse unit 180, and a part cooler 1 82. A part preheater 1 84 is also provided between the furnaces 1 72 and 174.

For efficient energy transfer among the components of the single level furnace system 170, air preheated in the part cooler 182 is drawn therefrom through a duct 185 by a fan or blower 186 and is directed to a manifold 188. Pipes 190 leading from the manifold 188 channel the air to recuperators 1 92 for additional preheating and then the twice-preheated air is supplied as combustion air to burners connected to U-shaped tubes 1 94 which radiate heat to the interior of the furnace 172.Dual manifolds 196 and 198 on opposite sides of and above the furnace 172 collect the exhaust of the tubes 1 94 after passage thereof through the recuperators 1 92. A duct 200 connected to the manifolds 1 96 and 198 near the part discharge end of the carburizing furnace 172 directs the collected exhaust to the tempering furnace 1 74 to furnish most or all of the thermal energy needed for tempering or annealing of parts therein. The exhaust of the tempering furnace 1 74 is drawn by a blower 201 through a duct 202 extending from the discharge end of the furnace 1 74 and is then directed through the preheater 1 84 prior to final exhaust from the system 1 70.

Also, water used in washing parts after quenching thereof is heated by circulating it through pipes 206 and 208 connecting the washer 178 to a suitable heat exchanger in contact with the quench medium within the quench tank 1 75.

While certain preferred embodiments of the invention have been shown and described, it will be apparent that various changes may be made without departing from the scope and spirit of the invention, and thus other embodiments are within the following claims.

Claims

1. A furnace system for heat-treating workpieces comprising:- a first furnace; a preheater for heating the workpieces prior to entry of the workpieces into the first furnace; heater means for heating the workpieces to a predetermined temperature in the first furnace; a quench unit connected to an end of said first furnace from which the workpieces are discharged, the quench unit containing a quench medium for rapidly lowering the temperature of the workpieces received from the first furnace; a second furnace for heating the workpieces to a lower temperature than the predetermined temperature following passage of the workpieces through the quench unit;; a cooler attached to an end of the second furnace from which the workpieces are discharged for cooling workpieces received from the second furnace and for heating air for use as combustion air in the heater means; means for directing heated air from the cooler to the heater means; means for directing products of combustion of the heater means from the first furnace and through the second furnace for heating the workpieces in the second furnace; means for directing the products of combustion exhausted from the second furnace through the preheater for heating workpieces therein; and transport means for moving the workpieces successively through the preheater, the first furnace, the quench unit, the second furnace, and the cooler.

2. A furnace system according to claim 1, further comprising.

a washer containing a cleansing fluid for washing the workpieces subsequent to their passage through the quench unit, the transport means being operative to transport the workpieces from the quench unit through the washer and thence to the second furnace; and means for circulating the cleansing fluid between the washer and the quench unit and for extracting energy from the quench medium to heat the cleansing fluid.

3. A furnace system according to claim 2, wherein the heater means comprises a plurality of radiant tube heaters each including a burner and a U-shaped tube connected to said burner and extending across the first furnace, and the system further including recuperator means connected between the cooler and the radiant tube heaters for effecting heat exchange between the products of combustion of the radiant tube heaters and heated air from the cooler.

4. A furnace system according to claim 3, wherein the recuperator means comprises a plurality of cylindrical recuperators, each recuperator being connected to one of the radiant tube heaters and including an inner cylindrical flue in communication with an outlet end of the radiant tube heater for receiving and passing therethrough the products of combustion exhausted from the radiant tube heater, and a wall surrounding the flue and having an inlet for receiving heated air from said cooler and an outlet for discharging air to the inlet end of the burner of the radiant tube heater ,our use therein as combustion air, the wall defining a passage for the flow of the heated air from the inlet to the outlet such that the thermal energy from the combustion products passing through the flue is transferred to and further increases the temperature of the heated air in the passage.

5. A furnace system according to claim 3 or claim 4, wherein the first furnace forms a carburizing furnace having a heating zone, a carburization zone, a diffusion zone, and a discharge zone, and said second furnace forms a tempering furnace.

6. Afurnace system according to any one of claims 3 to 5, wherein the means for directing the products of combustion from the radiant tube heaters through the second furnace includes a manifold for collecting said products of combustion and a duct connected between the manifold and an upper portion of the second surface.

7. A furnace system according to claim 6, wherein the second furnace includes divider means in the interior thereof dividing the furnace into a lower zone for heating workpieces transported therethrough and an upper zone having an inlet for admitting the products of combustion from the radiant tube heaters to form the atmosphere of the second furnace and the second zone also having fan means for circulating the atmosphere through the lower zone of the second furnace.

8. A furnace system according to claim 7, wherein the second furnace includes an outlet for discharging a first portion of the atmosphere after the atmosphere has passed through the lower zone and a baffle attached to a wall of the second furnace near the part discharge end thereof and defining with the divider means a passage for admitting a second portion of the atmosphere to the upper zone for recirculation by the fan means to the lower zone.

9. A furnace system according to claim 8, wherein the second furnace includes an auxiliary heater for supplying additional thermal energy to the second furnace.

10. A furnace system according to claim 9, wherein the auxiliary heater comprises a gas burner mounted in the upper zone of the second furnace.

11. A furnace system according to claim 3, wherein the first furnace is positioned at a lower level and the second furnace is mounted at an upper level above, and parallel to, the first furnace.

12. A furnace system according to claim 11, wherein the preheater is positioned at the lower level and the washer and the cooler are positioned at the upper level.

13. A furnace system according to any one of claims 3 to 12, in which the radiant tube heaters are arranged to heat the workpieces in the first furnace to a temperature in the range of from 1 50O0F to 18000 F; the preheater is connected to an end of the first furnace at which the workpieces enter the first furnace; the quench medium is oil or molten salt; and the second furnace is adapted to reheat the workpieces to a temperature in the range of from 30O0F to 14000F.

14. A method ot heat-retreating workpieces comprising in sequence the steps of: preheating the workpieces using exhaust gases from a tempering furnace; heating the workpieces in a carburizing furnace to a temperature in the range of from 1 5000F to 18000 F; quenching the workpieces by means of a quench medium; reheating the workpieces in the tempering furnace to a temperature in the range of from 300'OF to 14000F using as at least a major portion of the thermal energy in said reheating step exhaust heat from the carburizing furnace; and, air-cooling the workpieces, the air-cooling step also preheating air which is used as combustion air for combustion which heats the workpieces in the carburizing furnace.

15. A method according to claim 14, further including, between the steps of quenching and reheating the workpieces, washing the workpieces with a fluid heated by extraction of energy from the quench medium.

16. A method according to claim 15, wherein the combustion air used in heating the workpieces in the carburizing furnace is further preheated by extraction of energy from exhaust gases from the carburizing furnace.

1 7. A furnace system according to claim 1, substantially as described with reference to Figures 1 to 5, or Figure 6, or Figure 7, of the accompanying drawings.

1 8. A method according to claim 14, substantially as described with reference to Figures 1 to 5, or Figure 6, or Figure 7, of the accompanying drawings.