Material largely from Industrial Thermal Processing Handbook; J.H. Greenburg, ASM International, PDF

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Draw batch furnace. Courtesy of Wisconsin Oven Corp. BOX FURNACE Box furnaces, the simplest, most basic furnace, consist of refractory lined walls, a stack, a door, hearth, and roof or arch. Hearth sizes
Draw batch furnace. Courtesy of Wisconsin Oven Corp. BOX FURNACE Box furnaces, the simplest, most basic furnace, consist of refractory lined walls, a stack, a door, hearth, and roof or arch. Hearth sizes range from a few square feet to over 100 ft 2 (9 m 2 ). Heat sources can be gas or electric resistance types. Furnaces can be side or roof fired, and some low-temperature types use a hot gas-recirculation system. Refractories can be hard burned or ceramic fiber types. Charging is by hand, lift truck, manipulator, or crane, depending on the weight of the charge. Operating temperatures range from 200 to 2200 F (95 to 1200 C). Aging Solution heat treating Box furnace with quench. Courtesy of L&L Special Furnace Co. Inc. Other Applications Preheating; i.e., for forging Investment casting bakeout Ceramic mold firing Removal of wax (lost-wax process) Debindering of parts Metal bonding Car-bottom furnace for annealing and stress relieving tubes. Courtesy of Can-Eng Furnaces Ltd. CAR-BOTTOM FURNACE This is a variation of the box furnace, with the exception of provisions for handling large, heavy loads. Charges are placed on refractory topped cars, which usually are mounted on rails. The car becomes the furnace hearth when it is in place and ready for heating in the furnace. The principal advantage is the ability load a furnace bottom while it is outside the furnace proper and accessible to overhead crane and mobile truck loading. An optional design provides for bot- PRIMER ON I n simple terms, industrial heat treating furnaces are insulated enclosures designed to deliver heat to workloads for thermal processing. Basic elements of construction include an outer shell, refractory walls, hearth, roof, a source of heat (electricity or combustible fuel), and a means of accepting a workload, moving it through the furnace (if necessary), and removing it. Generally, furnaces are classified by heat source (combustion of fuel, or by conversion of electric energy to heat) and by two broad categories: batch and continuous. Fuel-fired (combustion type) furnaces are most widely used, but electrically heated furnaces are used where they offer advantages that cannot always be measured in terms of fuel cost. In batch-type furnaces, loads are placed in the furnace, the furnace and its loads are brought up to temperature together, and depending on the process, the furnace may or may not be cooled before it is opened and the load removed, which generally is through a single charging and discharging door. Batch furnace configurations include box, car bottom, bell, elevator, pit, vacuum, and pot. Continuous furnaces move the work while it is being heated. They operate in uninterrupted cycles as the workpieces move through them. Material passes over a stationary hearth, or the hearth itself moves. Continuous-type furnaces can be classified as either straight-chamber furnaces or rotary-hearth furnaces. Straightchamber furnaces can be classified as pusher type; walking beam type; conveyor type such as roller hearth, and continuous belt; and furnaces with tumbling or inertia action of the parts for movement. Material largely from Industrial Thermal Processing Handbook; J.H. Greenburg, ASM International, Effective in processing small parts Diversity of heat-treating and heating applications Blowers usually not required High-speed processing and cooling Low initial cost Low yields inherent in batch processing Possible oxidation and decarburization of work (See page 60 for suppliers of box General purpose floor model box furnace. Courtesy of Lucifer Furnaces Inc. PROGRESS NOVEMBER/DECEMBER toms that can be shifted sideways, allowing the use of more than one bottom per furnace. Another design, the through-type furnace, can be loaded from one end and discharged from the opposite end. Longer furnaces have multiple heating and control zones. Operating temperatures range from 200 to 2400 F (95 to 1300 C). ; e.g., castings Aging Spheroidizing Homogenizing Reheating for forging Investment casting bakeout Carbon baking Ceramic mold firing Ideal where larger loads cannot be handled by forklift Temperature uniformity Similar to box furnace, but has removable car hearth assembly High speed heating with afterburner Furnaces may be small (e.g., ft, or m), or huge, occupying the space of an entire building Are cost effective only if units are operated on a regular basis Are nonatmospheric Lack flexibility (See page 60 for suppliers of car-bottom PIT FURNACE These furnaces are vertically loaded, and heat treated parts are contained in baskets or resting on fixtures. Sizes range from small, floor-mounted units to large pit-mounted types. Work often is shielded from heating units by baskets or fixtures, and recirculating fans are almost always required to ensure Ceramic fiber lined car-bottom furnace. Courtesy of L&L Special Furnace Co. Inc. Representative pit furnace. Courtesy of Seco/Warwick Corp. uniformity of heating. Heating methods include direct firing, radiant tube heaters, and electric resistance heaters. Operating temperatures range from 300 to 2400 F (150 to 1300 C)., including long-cycle annealing of ferrous and nonferrous parts Drawing Steam treating Homogenizing Bluing Nitriding Solution heat treating Uniform temperature Accommodates long, heavy loads Flexibility Overhead handling required for loading and unloading Effective hearth areas; about 300 lb/ft 2 (1,465 kg/m 2 ) Potential for decarburization and scaling Pit construction costs (See page 63 for suppliers of pit Bell furnace for annealing large coils of wire and rod. Courtesy of Consolidated Engineering Co. (CEC). BELL AND HOOD FURNACE These furnaces consist of refractory lined covers equipped with heating devices in the form of direct firing burners, radiant type burners, and electric resistance heating units. Covers can be lifted off stationary bases by overhead cranes and relocated to spare bases. One cover can be used with several bases, providing efficient use of covers while coils or long bars are being loaded, cooled and unloaded. Inner mufflers may be used for controlled atmosphere operations. Rapid cooling facilities also are available. The operating temperature range is 300 to 2200 F (150 to 1200 C). of both ferrous and nonferrous metals is a major application, including the processing of strip and wire coils. Tubular products, bars, and extruded shapes also are annealed or normalized. Nitriding Aging Bluing Solution heat treating Brazing Coating Can use atmospheres One cover can serve multiple bases May be heated using gas or electricity Pit and overhead cranes are required Long process cycle times (See page 59 for suppliers of bell and hood 28 PROGRESS NOVEMBER/DECEMBER 2008 Vacuum furnace using elevator hearth for bottom loading. Courtesy of Seco/Warwick Corp. ELEVATING HEARTH FURNACE This furnace is a reverse version of the bell and hood type. The enclosure is built on a platform and is stationary. Hearths are loaded or unloaded at floor level and elevated to fit into the bottom of the furnace, which is direct fuel fired, radiant tube heated, or electrically heated. This type of furnace design also is used in vacuum furnaces. In solution treating aluminum, for example, the hearth can be lowered rapidly into a quench tank for minimum loss of heat before quenching. For ease of loading or unloading, the hearth also can be mounted on rails for movement outside the furnace area. The furnace also has heating applications. It is ideal, for instance, in treating graphite components and rocket nose cones and nozzles. The operating temperature range is 600 to 2400 F (315 to 1315 C). Solution treating Aging Reheating for forging Brazing Sintering (firing) Excellent thermal uniformity and atmosphere control Rapid heating and quenching Low dew points Manual transfer of parts Limited effective hearth area Headroom requirements (See page 61 for suppliers of elevating hearth INTEGRAL QUENCH (IQ) In this system, quenching takes place within the furnace. A means of transferring work through the furnace and quench is built into the equipment. Construction often is similar to that of a box furnace, with a manually operated door or a conveyor or roller hearth and a quench tank at the discharge end. Work drops into the quench and is brought out by an inclined metal mesh belt conveyor. In a variation of this design, parts are placed into an alloy basket, which is moved into the furnace and heated. After the heating cycle, the basket may be lowered into the quench by an elevator, brought back up to the surface, and moved out of the furnace. Furnaces are gas or electrically heated and protective atmospheres are required in some applications. Atmospheres are used, for example, in case hardening, hardening oil quenching steels, and in annealing precious metals. The operating temperature range is 1000 to 2050 F (540 to 1120 C). Case hardening Neutral hardening Clean hardening Nitrocarburizing Austenitizing Sintering ferrous materials Flexibility Transfer of work to quench under protective atmospheres Allcase batch integral quench (BIQ) furnace line. Courtesy of Surface Combustion Inc. Flexcase integral quench batch system. Courtesy of Consolidated Engineering Co. Work not decarburized and scale free Low capital cost Relatively long downtime in changing processes Manual transfer of parts in low-volume production and testing application (See page 62 for suppliers of IQ TIP-UP FURNACE This design is a variation of the bell and hood or car-bottom furnaces. The furnace enclosure, high at one end or on one side, is provided with a hydraulic or mechanical elevating mechanism for lifting the unhinged end or side up to expose the base on which the work is placed. The base can be stationary and loaded using a fork lift truck, or movable and loaded using an overhead crane. The configuration of the furnace can be square, rectangular, or circular, and heating can be direct Rectangular tip-up furnace with traversing load manipualtor. Courtesy of Consolidated Engineering Co. PROGRESS NOVEMBER/DECEMBER fire, radiant tube, or electric resistance. The operating temperature range is 600 to 2400 F (315 to 1315 C). of wire, long bars, rod, pipe, etc. Spheroidizing Schematic of walking-beam mechanism for advancing slabs through a furnace. Source: ASM Handbook, Vol. 4, Heat Treating, ASM International, 1991, p 468. Reheating for further processing or heating for forging Absence of door opening reduces heat losses Excellent seal Atmosphere controlled Easy loading and unloading Gas or electric heating More efficient than car-bottom furnace Accommodates heavy loads Simple to control Not cost effective when loading is light (See page 64 for suppliers of Tip up Screw conveyor furnace for high volume, continuous production of long products. Courtesy of Can-Eng Furnaces Ltd. WALKING BEAM/SCREW CONVEYOR In the walking beam furnace, work is lifted, moved forward, and dropped back on the hearth by means of a walking beam mechanism. In all other respects, the furnace is similar in construction to that of other continuous furnaces. Operations may be at any operating temperature. The top of the beam mechanism is covered with refractories in high-temperature applications. Firing is with burners mounted either in side walls or roof. Recuperation can be added to improve efficiency. Operating temperature range is 300 to 2400 F (150 to 1315 C). The screw conveyor is an alternative to walking beam or chain conveyor furnaces. The conveying mechanism consists of two or more screws that extend longitudinally through the furnace. Work is moved as the screws rotate. The design is particularly suited for long slender parts that have to be carefully handled during heating. Operating temperature range is 300 to 1750 F (150 to 955 C). Heating of steel billets, slabs, and blooms (walking beam) Reheating for forming Continuous processing of heavy loads (walking beam) Continuous processing of long cylindrical parts (screw conveyor) Part size (See page 65 for suppliers of walking beam furnace) ROTARY HEARTH FURNACE This type of furnace is designed for continuous production and is unique in its construction; a ring-shaped configuration with fixed inside walls, outer walls, and roof. A movable hearth fits inside the walls and is indexed periodically, carrying the work with it. Work is placed on a hearth and Large rotary hearth furnace. Courtesy of Surface Combustion Inc. removed after it has completed its cycle. Furnaces can be roof or side fired or electrically heated. They have a variety of heat treating applications. Efficiencies vary with firing methods and construction. Furnace sizes range from around 6 to 100 ft (2 to 30 m) in diameter. Throughput depends on part size and the application. Parts within the size range include bearing, gears, saw blades, and billets for seamless tube production. The advantage of this design is that continuous motion is obtained within a minimum of floor space. Charging and discharging can be handled by one operator. Either hard-burned refractories or ceramic fiber linings are used. Automatic temperature, fuel-air ratio, and pressure controls are commonly part of the system. The operating temperature range is 600 to 2400 F (315 to 1315 C). Austempering Billets 30 PROGRESS NOVEMBER/DECEMBER 2008 Small rotary hearth furnace. Courtesy of Seco/ Warwick Corp. Preheating for forging Investment casting bakeout Ceramic firing Press quench and reheat Ring hearth furnace for gas carburizing. Courtesy of Aichelin GmbH. Work normally moves through furnace back to initial loading zone High speed heating Modest space requirements Repeatability Simple to control Continuous production of small parts Easy to automate Low operating and maintenance costs Uniformity of atmospheres and temperatures Automation required for loading and unloading (See page 64 for suppliers of rotary hearth PUSHER FURNACE This is the simplest type of continuous furnace available for both heating and heat treating applications. The furnace does not have a built-in conveyor. Work is loaded onto trays or fixtures and is moved by being indexed by pusher mechanism mounted outside Large continuous pusher tray furnace system. Courtesy of Surface Combustion Inc. PROGRESS NOVEMBER/DECEMBER the furnace. Skid rails or alloy rollers are mounted inside the furnace and provide the surface on which work containers are pushed. The furnace can be end charged and discharged, side charged and discharged, or bottom charged and discharged. Types of furnaces include pusher skid tray, pusher roller tray, pusher roller rail, and pusher dog beam. The design and method of handling favor package line systems made up of a heating/heat treating furnace, quench tank, washing, and draw furnace. The operating temperature range is about 300 to 1750 F (150 to 955 C). Clean hardening Ferritic nitrocarburizing Solution heat treating Spheroidizing Brazing Investment casting bakeout Ceramic firing Metallizing PM sintering Preheating parts Benefits inherent in continuous processing, including high volume production of similar parts Tailored to application Simple mechanics, easy to maintain Positive positioning of work Lack of flexibility tailored to an application Space requirements Not cost-effective in low volume production High capital cost (See page 63 for suppliers of pusher Pusher furnace installation. Courtesy of Ipsen International GmbH. Schematic for pusher furnace system using ring hearth furnace for gas carburizing. Courtesy of Aichelin GmbH. Various roller-hearth furnace installations from: (top) Surface Combustion Inc., (middle) LOI Thermprocess GmbH, and (bottom) Seco/Warwick Corp. ROLLER-HEARTH FURNACE The distinguishing feature of the roller hearth is its method of material handling; work is transported through the furnace on rollers. Rollers may be powered or work may be pushed over them by such means as air or hydraulic cylinders. Rollers are made of heat resistant alloys and often are water cooled, as are their bearings. Furnace size varies with application. The heating of large parts and plates is a frequent application, and the equipment also handles a number of heat treating operations. Construction and heating methods are similar to those of other continuous conveyor furnaces. Linings are constructed of fire brick or ceramic materials. Heating is electricity or fuel-fired burners. Electric heating requires the use of circulating fans. Controls include electric types for temperature and for 32 PROGRESS NOVEMBER/DECEMBER 2008 Two large mesh-belt conveyor furnace installations by (left) Surface Combustion Inc. and (right) Seco/Warwick Corp. fuel-air ratios. Zone control is used where controlled rates of heating and cooling are required. Rollers tend to bend when they are idle and must be kept in continuous motion while a furnace is being heated. The operating temperature range is 300 to 1800 F (150 to 980 C). Solution heat treating Bluing Spheroidizing, e.g., copper and brass tubing and coils including clean hardening Heating large parts and plates Heating for forming operations Brazing Sintering Benefits of continuous processing and automation Efficient heat transfer Easy to load and unload Trays and fixtures not always required High volume production capability Reliability Minimum contact of conveyor with work High capital cost Space requirements (See page 63 for suppliers of roller hearth CONVEYOR HEARTH FURNACE These furnaces have moving hearths. Work is moved through the furnace by means of cast alloy link chain, roller chain, metal mesh, or other types of conveyors. Conveyors are of typical construction, with drives and sprockets, or drums, on head shafts. As with all conveyors, a take-up device compensates for chain or mesh wear. Conveyors may be entirely contained in a furnace, or the return half may be on the outside. General furnace configuration is similar to that of the roller hearth. Many different designs and conveyor systems are available and what is suitable in a given application depends on the materials being handled and operating temperatures. Operating temperatures differ; for example, 300 to 2100 F (150 to 1150 C) for mesh belt type, 300 to 1750 F (150 to 955 C) for cast mesh belt type, 300 to 1600 F (150 to 870 C) for chain conveyor type. including clean hardening PRIMER ON, including bright annealing, of ferrous and nonferrous metals Austempering Spheroidizing Homogenizing Steam treating Brazing, including copper and silver brazing to ferrous and nonferrous metals Sintering, including under protective gas atmospheres Glass-to-metal hermetic sealing Curing Metal bonding Electronic or thick film hybrids and soldering Billet heating Benefit of continuous processing High volume production Flexibility in loading Bright surfaces produced without oxidation Pickling not required Many available combinations of belt width, working heights, chamber length, and product volume per given application Heavy loading of multiple configurations possible with cast chain belt type conveyors Mesh belt type conveyors provide continuous processing of lightweight parts With slot/flight type conveyors, conveying surface is solid and flat With chain conveyor type, no belt is required; chain support is at ends of parts (generic) Light loading Relatively short belt life (See page 61 for suppliers of conveyor hearth Heavy loads require high operating temperatures Tendency of rollers to bend when idle Relatively high maintenance cost Hump-back belt conveyor type furnace. Courtesy of Abbott furnace Co. Continued PROGRESS NOVEMBER/DECEMBER SHAKER HEARTH FURNACE The name, shaker hearth, refers to the method of moving work through a furnace. Generally, parts are small and are moved over the hearth by an electrically activated vibrating mechanism. The general configuration is tunnel-like, with a door at each end. Refractories are enclosed in a steel housing. Heat treating is a primary application. Furna
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