Volume 04 - Heat Treating Part 8 pptx

2 Internally heated salt bath furnace with immersed electrodes and ceramic tiles Immersed-electrode furnaces, however, are not as energy efficient as submerged electrode furnaces.. As ex

Design and Operating Factors. In the design of fuel-fired furnaces, ample space must be provided for combustion so that the flame will not impinge on the pot If flame impingement is unavoidable, the pot should be rotated slightly at least once a week Rotating the pot and/or using a sleeve reduces local deterioration in the region of flame impingement and prolongs the service life of the pot The combustion-chamber atmosphere also has important effects on pot life A system with a control range from high-fire to low-fire is preferable to an on-off system because the latter allows air to enter the combustion chamber during the "off" portion of the cycle, thereby increasing the rate of sealing of the outer surfaces of the pot

Electrical-resistance-heated furnaces should be equipped with a second pyrometer controller whose thermocouple is located within the heating chamber This will prevent overheating of the resistance elements, particularly during meltdown, when the thermocouple that controls the temperature of the main bath is insulated by unmelted salt Because heating elements and refractories are severely attacked by salt, all salt must be kept out of the combustion chamber For this purpose, a high-temperature refractory fiber rope may be used to seal joints where the pot flange rests on the retaining ring at the top of the furnace

Externally heated pots should be started on low fire (low heat input) regardless of the method of heating Once the salt

appears to melt around the top, heat can be gradually increased to high fire to complete meltdown Caution: Excessive

heating of the sidewalls or pot bottom during startup may create pressures sufficient to expel salt violently from the pot

For added safety, the pot should be covered during meltdown with either a cover or an unfastened steel plate

The waste heat of flue gases may be fed to an adjacent chamber and used to preheat work Flue gases should always be visible to the operator The appearance of bluish-white or white fumes at the vent indicates the presence of salts within the combustion chamber; prompt action is required

Advantages and Disadvantages. Because of the ease with which they can be restarted, externally heated furnaces are well suited to intermittent operations Another advantage of furnaces of this type is that a single furnace can be used for a variety of applications by simply changing the pot for one containing the proper salt composition

Externally heated furnaces do have several characteristics, however, that limit their usefulness in certain operations They are less adaptable to close and uniform temperature control because the furnace dissipates heat by convection, creating temperature gradients in the bath Also, the temperature lag of the thermocouple and the recovery time of the furnace may result in overshooting or undershooting the desired temperature by 15 °C (25 °F) In addition to requiring an exhaust system for generated flue gases, externally heated furnaces may overheat at the bottom and sidewalls in restarting, which creates a pressure buildup in the thermally expanding molten salt and may cause an eruption Finally, externally heated furnaces are seldom practical for continuous high-volume production because of the limitations of pots with respect to size and maximum operating temperature High maintenance cost is also a factor

Immersed-Electrode Furnaces

Ceramic-lined furnaces with immersed (over-the-side) electrodes (Fig 2), when compared to externally heated pot furnaces, have greatly extended the useful range and capacity of molten salt equipment The most important of these technical advances are:

• The electrodes can be replaced without bailing out the furnace

• Immersed electrodes allow more power capacity to be put into the furnace, thus increasing production

• Immersed electrodes permit easy startup when the bath is solid A simple gas torch is used to melt a liquid path between the two electrodes, thus allowing the electrodes to pass current through the salt to obtain operating temperatures

Fig 2 Internally heated salt bath furnace with immersed electrodes and ceramic tiles

Immersed-electrode furnaces, however, are not as energy efficient as submerged electrode furnaces The area in which the immersed electrodes enter the salt bath allows additional heat loss through increased surface area As exhibited in Table

1, the surface area of the salt bath (A) in the submerged-electrode furnace is smaller than the surface area plus the immersed electrodes (A + B) in the immersed-electrode furnace However, a good cast ceramic and fiber-insulated cover placed over the bath and electrodes will reduce surface radiation losses up to 60%

Table 1 Service life of electrodes and refractories

Operating temperature Service life, years

1010-1285 1850-2350 1

4

-12

1010-1285 1850-2350 1

4

-12

735-955 1350-1750 1-2 2-3

955-1175 1750-2150 1

2-1

(a) 1-2

1010-1285 1850-2350 1

4

-12

(a) Hot leg only

Super-duty fireclay brick lines the immersed-electrode furnace Approximately 130 mm (5 in.) of castable and insulating brick then surrounds the fireclay brick on five sides Figure 2 is a schematic drawing of an immersed-electrode furnace with interlocking tiles and removable electrodes The removable electrodes enter the furnace from the top, and a seal tile

is located in the front of the electrodes to protect them from exposure to air at the air-bath interface This protection helps prolong electrode life Table 1 compares service life of electrodes and refractories for some basic furnace designs

Over-the-top (or over-the-side) electrodes are usually built with laminated cold legs, and water cooling is always required

A typical life expectancy for electrodes operating in such a furnace at 840 °C (1550 °F) is approximately 6 mo to 2 y for over-the-top electrodes, compared to 4 to 8 y for submerged electrodes

The salt is heated by passing alternating current through it with immersed electrodes As a result of the resistance built up

to passage of current through salt, heat is generated within the salt itself This heat is quickly dissipated by a downward stirring action created by the electrodes The electrodes are attached by copper connectors to a transformer that converts the line voltage of the plant to a much lower secondary voltage (approximately 4 to 30 V) across the electrodes Temperature is measured and automatically controlled by a system containing a thermocouple, pyrometer, relay, and magnetic contactor

The energy required by an immersed-electrode furnace is a function of:

• Furnace size necessary to hold the load and electrode well

• The energy (Qw) needed to heat the load to the desired temperature (The value of Qw is a function of load mass, the specific heat of the load, and bath temperature)

• Energy losses and safety factors

Once energy requirements are determined, then electrode number, size, and spacing can be determined

Microcomputers are used to calculate the rate of heat generation per unit length of the electrode to ensure that the current

is uniform from the top and bottom of the electrodes, taking into account the complexity of the current paths between the electrodes, the electromagnetic forces, and the circulation (influenced by the viscosity of the salt)

The electrode spacing is usually selected between 25 and 100 mm (1 and 4 in.); the height of the electrode should be smaller than the depth of the pot, the difference depending on electrode spacing The electrode width is usually 50 to 75

mm (2 to 3 in.) and rarely exceeds 125 mm (5 in.) Transformer voltages usually range from 4 to 30 V, with the ratio of maximum to minimum voltage of a given transformer approximately 4.5 (Ref 1)

Steel-Pot Furnaces. Some metal-treating processes are performed in salt compounds that cannot be contained in a ceramic liner For these applications, furnace manufacturers make use of a welded steel pot with immersed electrodes This type of furnace is suitable for special applications such as case hardening in straight cyanide baths, tempering, and marquenching

The steel pot often has a sloped back wall, which produces a bottom heating effect resulting in better circulation and uniform temperature This is accomplished by sloping the electrodes shown in Fig 3 and 4 As the current passes through the salt between the electrodes, the salt is heated, decreasing its density and causing it to rise toward the bath surface Control of the rate of rise of the salt is effectively gained by decreasing the distance from the electrodes to the steel pot

At the lower extremity of the electrode, the current enters the metal pot upon leaving the electrode to follow a shorter path

to the other electrode This arrangement ensures current flow through the salt along the entire electrode length Due to the close proximity of the lower portion of the electrode to the pot, most of the heating is done in the lower part of the bath This is the desired method of heating any liquid

Typical standard sizes

Working dimensions Temperature range

(A) Length (B) Width (C) Depth

Heating capacity

°C °F mm in mm in mm in

Fig 3 Metal pot, immersed-electrode salt bath furnace for ferrous tempering and isothermal annealing

Typical standard sizes

Working dimensions Temperature range

(A) Length (B) Width (C) Depth

Heating capacity

°C °F mm in mm in mm in

955-650 1750-1200 455 18 610 24 610 24 75 159 350

Fig 4 Metal pot, immersed-electrode salt bath furnace for liquid carburizing, cyaniding, and carbonate baths

The metal pots are made of either plain steel or hot-dipped aluminized steel, depending on the application Thicknesses range from 12 to 38 mm (1

Electrode Arrangement. Immersed electrodes are made of either mild steel or an alloy "hot" leg welded to a mild steel "cold" leg As previously mentioned, these are shaped to follow approximately the slope of the pot wall The portion

of the electrode that crosses over the top of the salt bath and is connected to the plant power source is referred to as the cold leg This is welded to the hot leg, the portion of the electrode that is immersed in the bath, with sufficient weld cross section to provide necessary current conductor capacity The shanks are drilled and tapped at the tinned terminal connection end for water cooling when necessary If the latter is not required, the electrical connection is water cooled Suitable clamping devices are used to facilitate electrode replacement

Electrode arrangements can vary as follows:

• Single-phase operation with metal or ceramic pots: Several electrode arrangements can be used,

depending on the size of the bath If only two electrodes are required, they are normally positioned on the sloped-wall side and at least 125 mm (5 in.) apart Three electrodes are usually placed so that the center electrode, equal in size to two of the other electrodes, is used as a common conductor with equal current paths to each of the outer electrodes More than three electrodes would be arranged in multiple groups

• Three-phase operation with metal pots: Three electrodes are used and spaced in a manner similar to the

spacing described above They are connected to three single-phase transformers that have Y-connected secondaries and delta-connected primaries The current flows from the electrodes to the metal pot, which is the neutral point Several variations of the three-phase connections are used, depending on the type of furnace and load requirements

All accessories, such as starting units, sludging tools, and secondary connectors, are the same for steel-pot electrode furnaces as for ceramic furnaces

immersed-Advantages and Disadvantages. Immersed-electrode furnaces do not require the use of iron-chromium-nickel alloy pots

These furnaces require minimum floor space and maintenance and can be used for all types of neutral salts Electrodes made of alloy steel should have an average service life equivalent to that indicated for steel pots in the section "Pot Service Life." Worn electrodes can be replaced while the furnace is in operation

Depending on the positioning of electrodes, control to within ±3 °C (±5 °F) is easily obtained with immersed-electrode furnaces Heat is generated within the bath, and overshooting is readily avoided These furnaces lend themselves to mechanization and are suitable for high-volume production in the range of 815 to 1300 °C (1500 to 2370 °F)

The depth of salt pots for immersed-electrode furnaces is not restricted for ceramic or ceramic-lined pots Metal pots may

be restricted to depths of about 0.6 m (2 ft) Pots may vary in length and width to suit requirements, and multiple pairs of electrodes can be installed to furnish the necessary heating capacity

The immersed-electrode furnace is not recommended for intermittent operation Depending on furnace size, reheating the salt charge may require a day or more Pots are not intended to be interchangeable Removal of the pot usually involves replacement of the surrounding insulation

Reference cited in this section

1 V Paschkis and J Persson, Industrial Electric Furnaces and Appliances, Interscience, 1960

Submerged-Electrode Furnaces

Submerged-electrode furnaces (Fig 5 and furnaces A and B in the figure to Table 1) have the electrodes placed beneath the working depth for bottom heating Many submerged-electrode furnaces are designed for specific production requirements and are equipped with patented features, which offer certain economical and technical advantages General characteristics of submerged-electrode furnaces include:

• Maximum work space with minimum bath area: The electrodes do not occupy any portion of the bath

surface, so that they only come in contact with the salt Bath size is consequently smaller, and electrode life increases many times over by incorporating unidirectional wear and eliminating excessive deterioration at the air-bath interface

• Circulation-convection currents: Bottom heating provides more uniform bath temperatures and bath

movement through the use of natural convection currents

• Triple-layer ceramic wall construction: The temperature gradients through the wall cause any salt

penetrating the wall to solidify before it can penetrate the cast refractory material that forms the center portion of the wall construction The design requires from 5 to 8% of the initial salt charge to fill the ceramic pot By comparison, in some designs 140 to 150% of the initial charge is needed to seal the ceramic walls of furnaces built with two layers of ceramic brick, backed up and supported by a steel plate Salt penetrates the ceramic walls of any furnace and distorts the geometry of the walls Reducing the amount of salt allowed to penetrate the ceramic walls aids in maintaining dimensions and in promoting a longer furnace life

• Electrode placement: Enclosing the electrode in a clear rectangular box, free of any protruding

obstructions, eliminates any potential hazards to operating personnel during cleaning Any sludge formed in the furnace is removed easily by operating personnel

Fig 5 Internally heated salt bath furnace with submerged electrodes This furnace has a modified brick lining

for use with carburizing salts

Frame Construction. A typical submerged-electrode furnace is made of brick and ceramic material reassembled, regardless of size, in a rigid, self-supporting welded steel frame (see, for example, Table 1) This frame consists of supporting channels or beams welded to the underside of a heavy steel plate that forms the frame base To this base are welded lengths of heavy angle iron around the outside and on top of the plate These pieces are notched to permit welding

of the heavy angle-iron posts to the plate and vertical sides of the base-plate angle iron Lengths of heavy angle iron are welded similarly to the top of the posts When required, additional vertical reinforcing members are welded between the bottom and top pieces of angle iron, and prestressed horizontal members also are used to ensure that the refractory material cannot move after the furnace has been brought to operating temperature

Brick Construction. Three types of refractory materials are commonly used in submerged-electrode furnaces A typical design is shown by furnace A in Table 1

Submerged-electrode furnace liners are constructed with 230 mm (9 in.) thick high-temperature burned bricks Consisting

of approximately 42% alumina and 52% silica, the brick material is used in standard brick sizes such as 60 by 115 by 230

mm (21

2 by 41

2 by 9 in.) and in various brick shapes, such as straights, flat backs, and splits The bricks are laid with a high-quality air-setting mortar that resists abrasion, erosion, and chemical attack by chloride, fluoride, and nitrate-nitrite salts The mortar offers sufficient wear and corrosion resistance to be economically used with some salts containing cyanide For straight cyanide or carbonate salts, a welded steel pot or a furnace with a modified brick lining (Fig 5) is used

The outer wall of the salt bath furnaces is made of a second-quality firebrick with the same dimensions as brick used for the liner The important qualities of this brick are the strength of the material and uniformity in size and shape

The inner castable wall is constructed with a maximum of refractory cement and aggregate that is poured between the liner and outer wall to form a 240 mm (9.5 in.) thick monolithic wall structure This dimension provides a temperature

gradient sufficient to cause the salt to freeze in the wall, thus making the wall self-sealing With this design, salt penetration into the wall amounts to less than 8% of the bath volume The maximum temperature of the outside wall during furnace operation is 60 °C (140 °F)

Electrode Construction. The electrodes used in submerged-electrode salt bath furnaces vary widely in size and shape, depending on the geometry of the furnace and the power requirements All of the electrodes are located near the bottom of the bath and are built into the wall (furnace A in Table 1) so only one face of the electrode is in contact with the salt This placement leaves the bath area free of obstruction for ease of cleaning and eliminates the possibility of touching the electrodes to the work

Alloy electrodes are made by welding a 1610 mm2 (2.50 in.2) alloy material to a mild steel backing, or by welding a 125

by 125 mm (5 by 5 in.) alloy material directly to the mild steel tank The spacing between electrode pairs is usually 65

mm (2.5 in.), or 190 mm (7.5 in.) The spacing is fixed and nonadjustable For this reason, computation of the secondary tap voltages is critical to the successful operation of the furnace throughout its lifetime

The durability of typical electrode and ceramic components of submerged-electrode furnaces is described in Table 1 Alloy electrodes can be replaced with graphite electrodes, which are renewed as they become consumed without disconnecting them (Fig 5) or shutting off the power

Startup and Shutdown. The submerged-electrode furnace can be started by adding molten salt from another furnace

or by using a gas-fired torch or electric starting coil to melt a pool of salt that will wet both electrodes and provide molten salt for the current path After the current path has been established in the molten salt between the electrodes, salt may be added to bring the bath up to working level Additional salt will be required to maintain this level because a small amount, approximately 5%, will seep into the brickwork and freeze

If the furnace must be shut down, the molten salt should be bailed from the furnace before it freezes However, if the salt

is allowed to remain in the furnace, a resistance-heated starting coil should be submerged in the bottom of the furnace while the salt is still molten This coil remains in the frozen salt and it is connected to the transformer leads to start up the furnace

Newer designs have one pair of electrodes close to the surface of the bath When the furnace cools, the surfaced electrode pair is exposed, thus simplifying startup

Advantages and Disadvantages. In common with the immersed-electrode furnaces, submerged-electrode furnaces require minimum floor space and maintenance and are highly adaptable to mechanization

Because the submerged-electrode furnace employs water to cool the electrodes and transformer, it may be operated at 50% overload without overheating the transformer, whereas the immersed-electrode furnace, being air-cooled, should not

be operated at an overload above 10%

Because a ceramic pot is used, unexpected pot failure is rare with submerged-electrode furnaces, and the furnaces can be rebuilt on a planned schedule during annual shutdowns In common with other electrical equipment, submerged-electrode furnaces are at a disadvantage where electric power rates are high, but this can be overcome to some extent by working the furnace in nonpeak periods when lower power rates are applicable

Because of the erosive effects on ceramic pots of water-soluble salts with high sodium carbonate or high sodium cyanide contents, submerged-electrode furnaces can be used with only low-cyanide, low-carbonate salts Baths with high cyanide

or carbonate salt require a modified basic brick The furnace with modified brick and submerged alloy electrodes provides many years of service in noncyanide and cyanide operations To increase furnace life, the furnace shown in Fig 5 is recommended This furnace has a modified basic brick lining for use with basic carburizing salts The alloy electrodes are replaced with continuing graphite electrodes The electrodes are renewed as they become consumed without disconnecting them or even shutting off the power

Air-Quality Assurance

Salt bath furnaces that operate at temperatures above 650 °C (1200 °F) will fume An open furnace containing a 50-50% NaCl/KCl mix, operating at 870 °C (1600 °F) at sea level, will fume at a rate of 0.2 kg/m2 per h (0.04 lb/ft2 per h)

Sodium chloride and potassium chloride are both edible; however, in large quantities they can be a nuisance The best way to overcome this nuisance is to capture it at the source

Figure 6 illustrates two ways of capturing fumes from a salt bath furnace The 380 mm (15 in.) location of a capture hood (Fig 6a) requires treatment of 200 m3/min (7120 ft3/min) of air and fumes, whereas a canopy hood (Fig 6b) at 305 mm (120 in.) requires treatment of over 900 m3/min (32 000 ft3/min) of fumes and air When the basket and parts are lifted from the salt bath, fumes are greatly increased, probably in proportion to the total surface area of the basket and parts exposed to air (plus the bath surface fumes) It is important to remember that the fumes coming off a salt bath are hotter and have more energy than fumes at standard temperature and pressure To calculate the type and amount of ventilation required, consult Ref 2

Fig 6 Ventilation of a salt bath furnace with (a) a capture hood and (b) a canopy hood The capture hood in (a)

requires a ventilation rate of 200 m 3 /min (7120 ft 3 /min), whereas the canopy hood in (b) requires a larger ventilation rate of 905 m 3 /min (32 000 ft 3 /min) All dimensions given in inches

Reference cited in this section

2 Industrial Ventilation, 20th ed., American Conference of Governmental Industrial Hygienists, 1988

Isothermal Quenching Furnaces for Austempering or Martempering

Isothermal quenching furnaces are pot-type furnaces with salt agitation, cooling, and chloride-elimination features As little as 10% chloride salt will cause the quench rate of a salt quench to be reduced by 50% Isothermal quenching furnace systems were designed to eliminate the occurrence of chloride carryover from the austenitizing bath to the quench bath, through salt separation and uniform vertical lamellar flow agitation The three most common approaches to alleviating the salt concentration are chemical, temperature, and gravity separation

Chemical Precipitation. Chemical agents have been used to attempt to lower the solubility of the chloride salts so that they will precipitate in the quenching salt When the salts settle to the bottom of the quench tank, they are removed as sludge This method offers little success because the precipitate that forms is fine textured and buoyant and therefore tends to remain in suspension rather than to precipitate out

Temperature Precipitation. The elimination of carryover salts has also been attempted by continuously pumping salt through a small auxiliary chamber whose temperature is maintained at a lower level than the main chamber As the salt is processed through the auxiliary chamber, chlorides are continuously precipitated out

Although this method appears practical, a fundamental error exists in its application The salt is cooled by air blown through a space between the pot and the outer shell of the precipitation chamber Air is blown through this space to maintain the temperature levels of the main chamber and precipitation chambers The moving air cools the pot walls below the salt-precipitation point so that the salt freezes and cakes to the sides Salt buildup continues until the bath is

unusable Consequently, depending on the level of salt concentration, the bath would have to be shut down, possibly after only a few weeks of operation, to remove the remaining molten salt and chip away the caked salt

Gravity Separation. This system of carryover salt removal also uses a two-chamber design The caking problem is eliminated by heavily insulating the pot walls at all points and using an internal air-water heat exchanger Because the pot walls and the salt are at the same temperature, there is no caking action The chloride salts settle into an easily removable shallow pan at the bottom of the precipitation chamber, or, if they are fine textured and buoyant, the salts float to the top

of the tanks and are easily skimmed off

The main advantages of two-chamber gravity-separation equipment include:

• Easily removable variable-speed propeller-type agitator with suitable baffling to provide vertical lamellar flow within the quench area, therefore ensuring maximum quench power and minimum distortion

• Separate chloride precipitation chamber with adjustable weirs to maintain a low chloride level and subsequently high quenching power

• Easily removable internal heat exchanger to maintain quench temperature and precipitate chlorides

• Easily removable settling pan to ensure maximum efficiency in removal of chlorides

• Heavily insulated pot and precipitation chamber to eliminate salt caking on walls

Furnace Heating. Generally either gas or electricity may be used to heat isothermal quenching furnaces When gas

heating is desired, immersion tubes are recommended because they are usually made of mild steel and provide long service life

Further, if the pot should develop a leak, the insulation and outer shell will contain the salt Caution: If a furnace with an

externally heated pot were to develop a leak, the nitrate-nitrite salt would drip on the hot refractory and may cause a fire hazard One or more immersion tubes normally are used, depending on bath size Generally, they will have nozzle-mix

sealed-in burners and will be available to Factory Mutual or Factory Insurance Association specification

Electric heating may be by one of the following methods, depending on the maximum operating temperature:

• Sheathed resistance strip heaters are mounted externally to the side walls near the bottom Maximum

operating temperature is 425 °C (800 °F) They are easily removable through the insulated plug-type door Protection against overshooting is achieved by locating a sensing device close to the heaters The sensors operate directly on line voltage

• Sheathed resistance immersion heaters have a maximum operating temperature of 425 °C (800 °F)

They can operate without a transformer but are susceptible to premature burnout due to the sludge accumulation or operator tampering and abuse

• Immersed-electrode heaters operate in the same manner as electrode pot furnaces for carburizing and

tempering

Furnace Construction. The pot is fabricated from firebox-quality steel plate, double welded inside and out and properly supported to maintain its shape Steel plate offers adequate resistance to chemical attack by the standard alkaline nitrate-nitrite salts at normal austempering and martempering temperatures The pot is insulated with 100 to 150 mm (4 to

6 in.) of slab-type mineral insulation to prevent the chloride-saturated nitrate salt from freezing to the side walls or the bottom The insulation is externally contained by a continuously welded outer steel shell The shell is reinforced to ensure retention of the original shape and dimensions throughout its designed operating temperature range

Automatic and Semiautomatic Lines

The use of automated hoists makes possible the combination of austempering, martempering, and tempering or carburizing in one line One or more hoists travel back and forth, automatically advancing the fixture carriers of work through the required stations

The hoist movement is controlled by a solid-state programmable control with functions that would normally require hundreds of relays, counters, switches, and extensive wiring Once programmed, the controller performs the desired commands and functions Time cycles, sequences, drills, and skips are easily entered or changed to meet metallurgical requirements For instance, parts can be programmed to be carburized, air cooled, washed, rinsed, and returned for unloading A push-button command then returns the program to standard processing

Parts suitable for fully automatic or semi-automatic installations are those that can be fixtured by wiring, racking, or placing in baskets and that do not present problems in either buoyancy or drainage

Fluidized-Bed Heat-Treating Equipment

Revised by Robert F Sagon-King, Can-Eng Ltd

Introduction

FLUIDIZED-BED TECHNIQUES are not new to the metalworking industry A 19th century American patent describes the roasting of minerals under fluidized-bed conditions Other established applications include potter's clay and miner's hydraulic slurries Systems of fluidized solid particles, such as quicksand, occur in nature

Early attempts to use fluidized beds in the heat treatment of metals were limited in the temperatures that could be employed Electrically heated furnaces capable of maintaining fluidized beds at temperatures up to 500 °C (930 °F) could

be produced commercially, but difficulties were encountered when attempts were made to attain higher temperatures A principal problem was the high rate at which refractory distributors, which distribute the hot fluidizing gases, were consumed

In early gas-fired fluidized-bed furnace design, gas entered the base of the container after being mixed with air to make it ignitable at the point of entry With newer designs, the mixtures are introduced separately and thus cannot be ignited accidentally This design eliminates the danger of explosion at the point of entry The surface of the bed is heated first, and the heating of surface particles causes progressive ignition downward through the container until the entire contents

of the bed achieves uniform heat-treating temperature Newer furnace designs extend fluidized-bed technology into the higher temperature ranges (540 to 1040 °C, or 1000 to 1900 °F) required for most common heat treatments

Principles of Fluidized-Bed Heat Treating

In fluidization, a bed of dry, finely divided particles, typically aluminum oxide in the heat-treating context, is made to behave like a liquid by a moving gas fed upward through a diffusor or distributor into the bed A gas-fluidized bed is considered a dense-phase fluidized bed when it exhibits a clearly defined upper limit or surface At a sufficiently high fluid-flow rate, however, the terminal velocity of the solids is exceeded, the bed goes into motion, and the upper surface

of the bed disappears This state constitutes a disperse, dilute, or lean-phase fluidized bed with pneumatic transport of solids The general phases or stages of fluidization are shown in Fig 1 Usually the aggregative or bubbling-type stage is used for heat-treatment processes

Fig 1 Various types of contacting in fluidized beds

Although the properties of solid and fluid alone determine the quality of fluidization (that is, whether smooth or bubbling fluidization occurs), many factors influence the rate of solid mixing, the side of the bubbles, and the extent of heterogeneity in the bed These factors include bed geometry, gas-flow rate, type of gas distributor, and internal-vessel features such as screens, baffles, and heat exchangers

Determination of Fluidization Velocity. In determining the quality of fluidization, a diagram of pressure drop (∆p)

versus velocity (μ0) is useful as a rough indication when visual observation is not possible A well-fluidized bed will behave as shown in the diagram in Fig 2, which has two distinct zones In the first, at relatively low flow rates in a packed bed, the pressure drop is approximately proportional to the gas velocity and usually reaches a maximum value

(∆pmax) slightly higher than the static pressure of the bed With an increase in gas velocity, the packed bed suddenly

"unlocks" and becomes fluidlike

Fig 2 Pressure drop versus gas velocity for a bed of uniform-sized particles Mmf, minimum fluidization velocity Source: Ref 1

When gas velocity increases beyond minimum fluidization (μmf), the bed expands and gas bubbles rise, resulting in a heterogeneous bed This is the second zone, in which, despite a rise in gas flow, the pressure drop remains practically unchanged The dense gas-solid phase is well aerated and can deform easily without appreciable resistance In its hydrodynamic behavior, the dense phase can be likened to a liquid If a gas is introduced into the bottom of a tank containing a liquid of low viscosity, the pressure required for injection is roughly the static pressure of the liquid and is independent of the flow rate of the gas The constancy in pressure drop in both the bubbling liquid and the bubbling fluidized bed may be taken intuitively to be analogous

The diagrams in Fig 3 show poorly fluidized beds The large pressure fluctuations in Fig 3(a) suggest a slugging bed In Fig 3(b), the absence of the characteristic sharp change in slope at minimum fluidization and the abnormally low pressure drop suggest incomplete contacting, with particles only partly fluidized

Fig 3 Pressure drop diagrams for poorly fluidized beds Source: Ref 1

One of the most important factors influencing the quality of fluidization is the uniformity of gas flow across a constant pressure drop Figure 4 illustrates this schematically

Fig 4 Quality of fluidization as influenced by type of gas distributor Source: Ref 1

Temperature Effect on Minimum Fluidization Velocity. One of the most important parameters of a fluidized bed

is the minimum fluidization velocity In simplified terms, minimum fluidization velocity (μmf) approximates to a function

of the square of the particle diameter (d) and a linear function of particle mass (p) as:

In the design of heat-treating furnaces, the effect of temperature must be considered Figure 5 shows that the flow of gas required for fluidization decreases rapidly with increases in temperature

Fig 5 Effect of temperature on the flow corresponding to minimum fluidization for particles 0.1 mm (0.004 in.)

in diameter having an apparent density of 2

Defluidization. One of the common concerns about fluidized beds is that, because of their principle of operation, they

are not well suited for large, solid parts with horizontal surfaces that remain stationary in the bed This a result of the incorrect belief that fluidization occurs only in a vertical direction With parts of this type, a cap of nonfluidized particles collects on the horizontal surfaces, forming a thermal screen The higher the temperature of operation, however, the greater the energy and agitation of the bed and the smaller the likelihood that the bed will collapse Moreover, various methods can be used to overcome this apparent disadvantage, and these are designed into most fluidized beds These methods are:

• Movement of the part being treated

• Introduction of additional agitation in the zone of fluidization around the part, either by localized injection of fluidizing gas or by careful design of the outline of the basket that holds the parts

• Increased fluidizing velocity

• A more favorable orientation of the part

Selective Heat Treatment. Bed collapse can be turned to advantage for special heat treatments in which one area of the path must be hard and tough and the remainder must be soft and more ductile, as in the case of the engineered parts of the shape described above In this case, after uniform heating, the part is removed from a hot fluidized bed and partially submerged in a fluidized quenching bed, with the part to be hardened facing down The top horizontal surface becomes covered with a cap of particles that form a thermal screen, which retards the vigorous cooling caused by the fluidized bed

Reference cited in this section

1 R.W Reynoldson, Controlled Atmosphere Fluidized Beds for the Heat Treatment of Metals, Heat Treatment

of Metals, University of Aston in Birmingham, 1976

Heat Transfer in Fluidized Beds

An important characteristic of fluidized beds is high-efficiency heat transfer The turbulent motion and rapid circulation

of the particles in the fluid furnace provide a heat-transfer efficiency comparable to that of conventional salt bath or lead bath equipment

The heat transfer coefficient of a fluidized bed is typically between 120 and 1200 W/m2 · °C (21 and 210 Btu/ft2 · h · °F) The turbulent motion and rapid circulation rate of the particles and the extremely high solid-gas interfacial area account for this feature The following factors are important in heat transfer

Particle Diameter. Of all the parameters that affect the heat transfer coefficient in fluidized beds, particle diameter exerts the greatest influence Particle diameter is generally a compromise between conserving fluidized gas flows and avoiding entrainment or carry-out Normally a sieve size of 80 to 100 grit is used

Bed Material. The governing physical property of any bed material is its density There appears to be an optimum density for bed materials: about 1280 to 1600 kg/m3 (80 to 100 lb/ft3) High-density materials tend to produce lower heat transfer coefficients and in addition require more power for fluidization Carry-out problems occur with low-density materials Other properties, such as thermal conductivity and specific heat, are less important

Fluidization Velocity of Gas. It is essential to use the optimum flow rate, that is, one that provides the maximum heat transfer rate for a particular particle density and diameter Generally, this flow rate is considered to be between two and three times the minimum fluidization velocity Too high a velocity leads to particle entrainment, high consumption of fluidizing gas, and poor heat transfer; too low a velocity leads to poor heat transfer and lack of uniformity in processing

Heating Rates. Relative heating rates of a 16 mm (0.6 in.) steel bar in salt, in lead, in a fluidized bed, and in a conventional furnace are illustrated in Fig 6(a); relative cooling rates for air, oil, water, and a fluidized bed are shown in Fig 6(b) Figure 7 presents heating and recovery rates for a fluidized bed Results of both hardening and isothermal quenching of type D3 tool steel with salt baths and with fluidized beds are given in Table 1 The difference between the two installations in total time for final heating and holding resulted from a difference in preheating conditions

Table 1 Comparison of the effects of hardening and isothermal quenching of type D3 tool steel in salt baths and in fluidized beds

Diameter

of test pieces

Preheating temperature

Hardness, HRC Heating or cooling

At surface

At center

Fig 6 Relative heat transfer rates (a) Heating rates for 16 mm (0.6 in.) diam steel bars in lead, in salt, in a

fluidized-bed furnace, and in a conventional furnace (b) Quenching rates for 16 mm (0.6 in.) diam steel bars in air, in oil, in water, and in a fluidized-bed furnace Source: Ref 1

Fig 7 Recovery rates for 25 mm (1 in.) diam steel parts in a 0.3 m3 (10 ft 3 ) fluidized bed furnace

Reference cited in this section

1 R.W Reynoldson, Controlled Atmosphere Fluidized Beds for the Heat Treatment of Metals, Heat Treatment

of Metals, University of Aston in Birmingham, 1976

Control of Atmospheres

A full range of atmospheres can be used within the work zones of fluidized beds The volume of gas used is clearly dictated by particle size, temperature of operation, and optimum fluidization velocity However, it can be shown that, with careful design and the use of low-cost carrier gases such as nitrogen, even low-temperature surface treatments can be both effective and economical In addition, one of the major advantages of a fluidized bed is that expensive gas need not be consumed while there is no work in the bed Atmosphere conditioning is rapid: within about 30 to 60 s after an inert gas is introduced into the bed, the purity of the atmosphere is equivalent to that of the gas supply In fluidized beds, various types of atmospheres can be obtained, as discussed below

Reducing or Oxidizing Atmosphere. Adjustment of a gas-air mixture to the bed so that it is either gas-rich or

oxidizing causes some decarburization or oxidation reactions in the materials being processed (the gas-rich mixture produces somewhat less severe reactions) However, these are time-dependent reactions, and, because of the rapid heating rates of parts being processed and the subsequent short immersion times needed to obtain the correct structure and through hardness, little surface effect other than discoloration and slight scaling is exhibited in section sizes up to 25 mm (1 in.) For larger sizes, the user must be aware of surface reactions that can occur, particularly as the processing temperature increases Figure 8 shows the relative decarburization bands for steels held in a fluidized bed

Fig 8 Representative decarburization bands for steel held in a fluidized bed Steels used: type O1 and type D3

tool steels and 0.75% C plain carbon steel Source: Ref 1

Neutral Hardening and Carburizing. Atmospheres for the neutral hardening of tool steels or the carburizing of carbon steels can be used for bed flotation This practice allows oxygen-free heating of tool steels However, care must be taken during the transport of workpieces to the quench tank to prevent decarburization or oxidation

low-Reference cited in this section

1 R.W Reynoldson, Controlled Atmosphere Fluidized Beds for the Heat Treatment of Metals, Heat Treatment

of Metals, University of Aston in Birmingham, 1976

Surface Treatments

Fluidized beds, using atmospheres composed of ammonia, natural gas, nitrogen, and air, or similar combinations, are capable of performing low-temperature nitrocarburizing treatments equivalent to conventional salt bath processes or other atmosphere processes High-speed steel tools oxynitrided in a fluidized bed are comparable to similar tools treated by the more conventional gaseous process Carburizing and carbonitriding in a fluidized bed can yield results similar to those achieved in conventional atmosphere furnaces

Mixtures of propane and air produced the results shown in Fig 9, which compares the case depths obtained on SAE 8620 steel bearing rings carburized in a fluidized bed and by the conventional atmosphere process An effective case depth of 1

mm (0.04 in.) was achieved in 1.5 h using the fluidized-bed technique Developmental work on this process is still being performed, but sufficient knowledge exists to compare the mechanisms of conventional gas carburizing and the fluidized-bed process

Fig 9 Comparison of hardness profiles obtained by fluidized-bed and conventional gas carburizing SAE 8620

steel, rehardened from 820 °C (1510 °F) Source: Ref 1

Conventional Gas Carburizing. Carburizing occurs through the catalytic decomposition of CO according to:

C3H8 →C ↓+ 2CH4 (Eq 4)

The amount of carbon precipitated is proportional to the number of carbon atoms in the hydrocarbon fuel gas; that is, propane forms more carbon than does methane In addition, the purity of propane is important, especially with respect to unsaturated hydrocarbon content, which increases its carbon-forming capability

The precipitated carbon reacts instantaneously with the oxidizing products of combustion:

Reference cited in this section

1 R.W Reynoldson, Controlled Atmosphere Fluidized Beds for the Heat Treatment of Metals, Heat Treatment

of Metals, University of Aston in Birmingham, 1976

Types of Furnaces for Heat Treating with Fluidized Beds

The type of fluidized bed most widely used for heat treatment is the dense-phase type, although units based on the dispersed-phase bed have been constructed, with particle circulation for the heat treatment of long, thin metal parts such

as shafts and plates In a typical dense-phase fluidized bed, the parts to be treated are submerged in a bed of fine, solid particles held in suspension, without any particle entrainment, by a flow of gas

Liberation of adequate quantities of heat within fluidized beds is a prime consideration in adapting them for metal processing Because transfer of heat from the bed to the workpiece is usually much more efficient than transfer of heat from the heat source to the fluidizing medium, the greatest difficulty is encountered in transferring suitable quantities of heat to the fluidizing medium In addition, the major part of the heat loss from any practical fluidized system is the heat content of the spent fluidizing gas In instances in which thermal efficiency is unduly influenced by this factor, recirculation of the fluidizing gas or installation of a recuperative system may be justified Each has been used in practical applications Heat input to a fluidized bed can be achieved by several different methods; the most accepted, however, are described in the paragraphs below

External-Resistance-Heated Fluidized Beds. A fluidized bed contained in a heat-resisting pot can be heated by

external resistance elements (Fig 10) Waste heat recovery can be used to increase thermal efficiency, and the fluidizing gas can be maintained at any desired composition Heat-up time from ambient to operating temperatures of 815 to 870 °C (1500 to 1600 °F) typically takes 3 to 4 h

Fig 10 Fluidized-bed furnace with external heating by electrical resistance elements

External-Combustion-Heated Fluidized Beds. A fluidized bed contained in a heat-resisting pot can be heated by external gas firing (Fig 11) In this arrangement, a fuel-air mixture is introduced through a standard commercial burner The burner can be controlled very accurately down to low temperatures for low-temperature tempering The products of combustion are then removed by flue in the normal fashion

Fig 11 Externally gas-fired fluidized-bed furnace

Submerged-Combustion Fluidized Beds. The technique of submerged combustion consists of passing the combustion products directly through the mass to be heated This method provides an excellent rate of heat transfer and is now well established for a wide range of liquid-heating applications, from the heating of swimming pools to the concentration of acid solutions The application of this method to the heating of a fluidized bed requires that the burner be used such that it provides strong agitation of the suspended particles, thereby achieving the desired properties of excellent heat transfer and uniformity of bed temperature

Equipment developed for this purpose consists essentially of a burner, two concentric tubes, and a particle separator A suitable gas mixture is fed through the burner into the central tube, where it is ignited The flame develops in the tube, and the combustion products escape at its lower end, where they impart heat to the suspended particles before moving up through the annular space between the two tubes As they rise, a quantity of particles is entrained These are separated from the gas stream by the deflector plate and fall back into the bed by virtue of gravity Figure 12 shows a system that incorporates submerged combustion with a controlled atmosphere for the low-temperature treatment of metals

Fig 12 Controlled-atmosphere fluidized-bed furnace heated by submerged combustion 1, burner; 2,

combustion tube; 3, tube through which combustion gases and particles rise; 4, particle separators; 5, heat exchanger; 6, gas recycle compressor for fluidization; 7, distributor plate; 8, parts to be treated

Internal-Combustion Gas-Fired Fluidized Beds. A major development in the heating of fluidized beds occurred when an air-gas mixture was used for fluidization and was ignited in the bed, generating heat by internal combustion Prior to this breakthrough, many technical difficulties prevented the use of this mode of fluidized-bed heating A typical furnace design incorporating this technique is shown in Fig 13

Fig 13 Gas-fired fluidized-bed furnace with internal combustion 1, insulating lagging; 2, refractory material;

3, air and gas distribution box; 4, fluidized bed; 5, parts to be treated

The advantage of this system is that the bed is fluidized by burning gases, and thus the heat is generated within the bed In gas-fired fluidized beds, the supporting gas or fluidizing medium is a near-stoichiometric mixture of gas and air This combustible mixture is ignited above the bed and quickly imparts its heat to the particles, which in turn heat the incoming gas further down the bed After a period, combustion takes place spontaneously within the bed and is complete within the first 25 mm (1 in.) of the diffuser once the spontaneous combustion temperature for the gas being used is reached This temperature commonly varies between 600 and 800 °C (1110 and 1470 °F) If the vessel is well insulated, the bed temperature can rise to a theoretical combustion temperature, and heat-up times from cold to 800 °C (1470 °F) are typically between 1 and 11

2 h However, problems inherent to the basic technique are:

• The bed is fluidized by burning gases To obtain good temperature control and optimum fluidizing conditions, however, it is desirable that the fuel input rate and fluidizing velocity be independently variable

• Combustion is somewhat unstable below the spontaneous combustion temperature

• Very high temperatures can occur in the immediate vicinity of the distributor/diffuser tile When the bed

is incorrectly fluidized so that this heat cannot be removed from the top of the distributor, theoretical flame temperatures are achieved with consequent deterioration of the distributor The thermal stresses of expansion and contraction on the distributor tile at these high temperatures tend, even with the best fixing techniques available, to cause failure of joints, which enhances the problem

Two-Stage, Internal-Combustion, Gas-Fired Fluidized Beds. The basic problem of separating the control of heat input from the control of fluidizing velocity has been overcome in two alternative designs (Fig 14) In both designs, the initial heat-up from cold to operating temperatures is carried out by two-stage internal combustion A noncombustible mixture of gas and air is introduced beneath the distributor tile Secondary air is added to make up a stoichiometric or slightly gas-rich mixture immediately above the tile by means of jet holes drilled into heat-resisting tubes This is done to reduce the possibility of explosion and to avoid high flame temperatures at the surface of the tile The technique has an

adverse effect on good fluidization, but this is unimportant during initial heat-up, in which the prime objective is to raise the temperature of the bed to operating temperature as quickly as possible Once this has been accomplished, the remaining objective is to isolate the heat-up control from the control of the fluidizing velocity This is achieved in two ways:

• Three-chamber design: In this design (Fig 14a), the heat control outer chambers are separated from the

treatment zone by a muffle The fluidizing velocity and atmosphere are independently controlled in the inner chamber, while the outer two zones are still supplying heat by internal combustion To achieve adequate heat input, fluidization levels in these outer chambers are above the optimum for heat transfer and surface reactions, but this is relatively unimportant

• Back-radiation design: When fuel-rich gases are permitted to burn by the injection of secondary air

immediately above the control chamber of the fluidized bed, a back-radiation effect causes a rise in bed temperature This design (shown operating in the heating/controlling and cooling modes in Fig 14b and c) makes use of this effect and at the same time utilizes heat that is normally dissipated when gases are burned outside the furnace It therefore uses fuel more economically In principle, the gas-rich mixture

is supplied to the central chamber, and extra air is added to produce stoichiometric conditions during initial heating of the bed When cold work is loaded for treatment, the extra air is injected above the bed

to produce a radiating flame and recover bed temperature If bed temperature exceeds set temperature, the extra air is switched to the outside of the furnace wall to provide cooling and finally is mixed with the rich gas/air to produce combustion at the top of the specially constructed hood

Fig 14 Two-stage, gas-fired, internal-combustion fluidized beds (a) Three-chamber design (b) Back-radiation

design in heating mode (c) Back-radiation design in cooling mode

Internal-resistance-heated fluidized beds are not accepted by users The elements and work load will make contact if insufficient care is taken

Applications of Fluidized-Bed Furnaces

The potential applications of fluidized-bed technology to heat treating are many Figure 15 specifies those applications in which fluidized beds can compete with conventional furnaces

Fig 15 Fluidized-bed applications; decision model Source: Ref 1

Applications of fluidized-bed furnaces to the heat treatment of metals include continuous units for all types of wire and strip processing (patenting, austenitizing, annealing, tempering, quenching, and so on) and all configurations of batch-type units for general heat-treating applications A typical batch-type unit with an output of approximately 150 kg/h (330 lb/h) is available as a standard furnace Using mechanical handling equipment, it can be automated into a continuous heat-treatment line The following example describes one firm's decision to install fluidized-bed furnaces for heat treatment

Example 1: Improved Turnaround Time with Fluidized-Bed Treatment

A company specializing in the design and production of aluminum extrusion dies had relied on sub-contract treatment facilities for the hardening of dies The decision to install in-house facilities came as a result of difficulties in meeting the 7- to 14-day turnaround of dies required by customers Previously, hardening, case hardening, and tempering had been done by salt bath immersion After studying alternatives, the firm decided to employ the latest fluidized-bed technology Approximately one year later, the firm installed a second fluidized-bed furnace and made available its surplus capacity to other firms on a subcontract basis

heat-Carburizing, Nitriding, and Carbonitriding. In recent years, design innovation has led to the use of fluidized-bed furnaces as a practical tool for carburizing, carbonitriding, nitriding, and nitrocarburizing processes In this technique, 80 mesh or 180 μm aluminum oxide particles produce a fluidizing effect so that the bed behaves like a liquid When gas or electricity is used as the heat source, the bed provides a faster heat transfer medium This is provided with quench and tempering furnaces

Previously, gas-fired internal-combustion units or submerged combustion units were used successfully to provide both heat source and fluidizing/carburizing medium Recently, more attention has been directed toward the use of externally heated fluidized beds, which is claimed to allow greater control over the carburizing process as a result of separate

heating and fluidizing functions (Ref 2, 3) The advantages of the fluidized-bed process include:

• High rates of heating and flow cause the utilization of higher treatment temperatures, which, in turn, provide rapid carburizing

• Temperature uniformity with low capital cost and flexibility is ensured

• A fluid bed furnace is very tight; the upward pressure of the gases minimizes air leakage

• The process produces parts with very uniform finish

References cited in this section

1 R.W Reynoldson, Controlled Atmosphere Fluidized Beds for the Heat Treatment of Metals, Heat Treatment

of Metals, University of Aston in Birmingham, 1976

2 A.J Hicks, Met Mater Technol., Vol 15 (No 7), 1983, p 325-330

3 K Boiko, Heat Treat., Vol 18 (No 4), 1986, p 65, 66

Cleaning Operations

Fluidized solids are nonabrasive and non-corrosive and do not wet immersed objects There is some drag-out loss of the aluminum oxide, however, because some particles accumulate on flat surfaces as work loads are removed from the fluidized bed These particles can be removed in part by agitation, bouncing, or blowing with an air pipe Particles can be reused by being dried, sieved, and returned to the bed When parts already scaled or preoxidized are placed in a fluidized

bed, particles tend to adhere to the scale to a greater degree than if the workpieces were clean These particles can be removed by water spraying

Heat Treating in Vacuum Furnaces and Auxiliary Equipment

Revised by the ASM Committee on Vacuum Heat Treating*

Introduction

VACUUM HEAT TREATING consists of thermally treating metals in heated enclosures that are evacuated to partial pressures compatible with the specific metals and processes Vacuum is substituted for the more commonly used protective gas atmospheres during part or all of the heat treatment Furnace equipment used in vacuum heat treatment differs widely in size, shape, construction, and method of loading

Although originally developed for the processing of electron tube materials and refractory metals for aerospace applications, vacuum furnaces are now employed in brazing, sintering, heat treating, and the diffusion bonding of metals Vacuum furnaces also are used for annealing, nitriding, carburizing, ion carburizing, heating and quenching, tempering, and stress relieving Furnaces for vacuum heat treating are equipped for workloads ranging from several pounds to 90 Mg (100 tons), and heated working chambers range in size from 0.03 m3 (1 ft3) to hundreds of cubic feet Although most vacuum furnaces are batch-type installations, continuous vacuum furnaces with multiple zones for purging, preheating, high-temperature processing, and cooling by gas or liquid quenching also are used Vacuum heat-treating furnaces also:

• Prevent surface reactions, such as oxidation or decarburization, on workpieces, thus retaining a clean surface intact

• Remove surface contaminants such as oxide films and residual traces of lubricants resulting from fabricating operations

• Add a substance to the surface layers of the work (through carburization, for example)

• Remove dissolved contaminating substances from metals by means of the degassing effect of a vacuum (removal of H2 from titanium, for example)

• Remove O2 diffused on metal surfaces by means of vacuum erosion techniques

• Join metals by brazing or diffusion bonding

The standard absolute pressure of the atmosphere is the reference or 0 gage pressure for a normal pressure gage Hence, gage pressure is negative for a vacuum condition For some technologies other than vacuum furnaces, a degree of vacuum

is measured by pressure below gage pressure It is important to know how degrees of vacuum are expressed in the various technologies

Most vacuum furnace pressure levels are expressed in terms of absolute pressure rather than gage pressure Normally the units of measure used are torr, mm Hg, or μm Hg When vacuum furnaces are pressurized above atmospheric pressure, such as for gas quenching, the pressure is expressed in terms of bars One bar is slightly less than one standard atmosphere of absolute pressure A bar is equal to 14.50 psia, 29.53 in Hg, 750 torr or mm Hg, or 750,000 μm Hg

The vacuum or pressure value of Hg refers to the height of a mercury column sustained by the differential between standard atmospheric pressure and an attained level of vacuum (or, more accurately, partial pressure) or pressure level (above standard atmospheric pressure) being measured

Table 1 compares vacuum and pressure to standard atmospheric pressure The normal pressure range of vacuum heat treating should be noted

Table 1 Pressure ranges required for selected vacuum furnace operations relative to standard atmospheric (0 gage) pressure

Equivalent pressures Gage

pressure

classification

Furnace application

Vacuum classification

177.17 87.02 72.32 5.92 6

147.65 72.52 57.82 4.93 5

118.12 58.02 43.32 3.95 4 High gas

88.59 43.51 28.81 2.96 3 Pressure

Rough

1.3×103 10 10 104

130 1 1 103

13 0.1 0.1 100

1.3 0.01 0.01 10

Normal range

1.3×10-6 10-8 10-8 10-5

(a) Equal to 133.322387415 Pa, it differs from torr by one part in 7 × 106

(b) psia = psig + 14.7 psi

Comparison of Vacuum and Atmosphere Furnace Processing

In most heat-treating processes, when materials are heated, they react with normal atmospheric gases, which consist of approximately (by volume) 21% O2, 77% N2, 1% H2O vapor, and 1% other gases If this reaction is undesirable, the work must be heated in the presence of some gas or gas mixture other than normal air This is done in normal atmosphere furnace processing

The gas or gas mixture may be varied to cause desirable reactions with the material being processed or it may be adjusted

so that no reactions occur At different temperatures, different reactions may occur with the work and furnace atmosphere

In most atmosphere furnaces it is not possible to change the atmosphere composition rapidly enough for optimum reactions or to control the atmosphere composition with the degree of precision required for some heat-treating processes Vacuum furnaces allow gas changes to be made quite rapidly because they contain gases of low weight

Vacuum furnace technology removes most of the components associated with normal atmospheric air before and during the heating of the work An analysis of the residual atmosphere in a leakproof vacuum furnace at a vacuum of about 0.1

Pa (10-3 torr) indicates that less than 0.1% of the original air remains The residual gases primarily consist of water vapor, with the remainder largely comprised of organic vapors from the seals, vacuum greases, and vacuum oils The oxygen content at 0.1 Pa (10-3 torr) is less than 1 ppm If all of the residual gas in the vacuum furnace were converted to water vapor, the water vapor content would be approximately 1.5 ppm, or equal to that of a gas with a dew point of about -80

°C (-110 °F) At a vacuum level of 10 Pa (10-4 torr), the equivalent dew point of gas is estimated to be approximately -90

°C (-130 °F) or less

These low dew point equivalents compare favorably with the driest inert gases available from highly efficient gas dehydration equipment With suitable vacuum pumping systems, the concentration of oxygen and water vapor can be reduced to lower levels than those achieved in inert or reducing-gas atmospheres

After a vacuum heat-treating furnace has been evacuated, gaseous reactions such as those encountered with atmosphere heat treatment are virtually eliminated Moreover, the vacuum extracts many gases, surface contaminants, and processing lubricants that would be difficult and costly to remove by any other method Gases drawn from the metal surface into the vacuum surrounding the charge are trapped by the vacuum pumps and exhausted from the system as the work is being processed This advantage of a vacuum system is of greater significance when parts with complex shapes, blind holes, or deep recesses are heat treated A complete purging of such parts in a protective atmosphere requires an extended purging period Even long-time purging, however, may not ensure the complete removal of entrapped air, other contaminants, or contaminants generated by reactions with the atmosphere

When more thorough purging is required, the furnace can be evacuated with a simple vacuum system, and the enclosure

or retort can be backfilled with the desired protective or reactive atmosphere (see the article "Furnace Atmospheres" in this Volume) This method markedly reduces the amount of protective atmosphere and time required to produce satisfactory results

Volatilization and Dissociation

In a vacuum furnace, materials can be pressed at temperatures and pressures at which the vapor pressure of the materials becomes an important consideration Vapor pressure, which is the gas pressure exerted when a substance is in equilibrium with its own vapor, increases rapidly with temperature because the amplitude of molecular vibration increases with temperature Some molecules in the outer surface of the solid material have higher energies than others, and they escape

as free molecules or vapor If a solid substance is contained in an enclosure devoid of any other material, molecules will continue to escape from the solid surface until their rate of escape is exactly balanced by the rate of condensation or recapture of the gaseous molecules The equilibrium pressure developed is the vapor pressure of the substance at that temperature The vapor pressure of a metal is dependent on temperature and pressure only but the effect is time dependent

It is normally desirable to use a vacuum-temperature combination that accelerates the desorption of gases without producing the vaporization of more volatile alloy constituents Alloys with high concentrations of volatile elements, such

as brass, are not heat treated in vacuum furnaces

If brass is heated in a vacuum at a temperature of 540 °C (1000 °F) and a vacuum level on the order of 13 mPa (0.1 μm Hg), the zinc component will vaporize (volatilize) and the brass will eventually be converted to copper sponge The zinc will deposit in the cold section of the furnace and can revolatilize on subsequent runs at higher temperatures, causing unwanted pitting or other surface reactions on the work load

Metals such as lead, zinc, and magnesium have relatively high vapor pressures; if heated above a temperature at which the vapor pressure of the element exceeds the pressure in the furnace, they will evaporate or sublime rapidly Thus, high-vacuum heat treatment is not applicable to some metals and alloys To handle certain metals and alloys properly, either the pressure must be limited to the soft (fine) vacuum range (Table 1) or a backfill to a higher vacuum pressure level must

be employed

Alloys with lower concentrations of volatile elements can be processed in vacuum by using the backfill pressure of an inert gas such as nitrogen or argon that exceeds the sublimation pressure of the element at the temperature involved A backfill pressure of a few hundred μm Hg at temperatures of about 980 °C (1800 °F) precludes the vaporization of elements such as chromium, copper, or manganese from steels processed at these temperatures

For example, if pure manganese were heated to approximately 790 °C (1455 °F) at a pressure of 13 mPa (10-4 torr), it would vaporize If the material were held at a higher temperature or lower pressure for an adequate period of time, the metal would become depleted and would eventually disappear, and the vapors would condense on the colder areas of the furnace and/or pumping system Backfill or higher pressures greatly slow the rate of evaporation or volatilization The vapor pressures of carbon and selected pure metals, as related to temperature, are shown in Fig 1 and Table 2

Table 2 Vapor pressures of various elements

Vapor pressure at Element

0.013 Pa 0.13 Pa 1.3 Pa 13 Pa 1.0 × 10 5 Pa

10 mm Hg

0.1 μm

10 mm Hg 1.0 μm

at a pressure of 130 mPa (10-3 mm Hg)

Fig 1 Vapor pressure versus temperature for carbon and various pure metals

Alloy Vapor Pressures. The vapor pressures of pure metals are constant, well-established values The vapor pressure

of a given alloy varies according to conditions The vapor pressure of an alloy is governed in part by a law analogous to Dalton's law of partial pressures: The total vapor pressure of an alloy, under ideal conditions, is equal to the sum of the partial vapor pressures of its constituents However, the partial pressure of each element in the alloy is lower than its normal vapor pressure and is proportional to its concentration

In processing at temperatures where the vapor pressures of more volatile minor constituents are still in the micron range, alloys behave in accordance with Dalton's law For example, if pure manganese is heated to 790 °C (1455 °F), its vapor pressure will reach 13 mPa (0.1 μm Hg), making it impossible to evacuate to lower pressures without evaporating all of the manganese However, when manganese is alloyed with other elements, as a solid solution in iron, for example, its effective vapor pressure is lowered The total vapor pressure for the alloy is the sum of vapor pressures of the individual elements multiplied by their concentrations in the alloy The vapor pressure of manganese in a 1% Mn alloy at 790 °C (1455 °F) is about 0.13 mPa (10-6 mm Hg) When alloys such as stainless steel are processed at high vacuum levels (theoretically exceeding the vapor pressures of some of its pure metal components), the volatilization is only a few molecules thick This volatilization tends to draw the stable elements with it in a complex molecular destabilization that results in a surface chemistry similar to that of the core material It is this molecular surface activity that can remove thin film oxides even though their theoretical combined vapor pressure has not been exceeded

Many metals form compounds by reaction with oxygen, hydrogen, and nitrogen These reactions are usually exothermic, and the possibility for dissociation of the resulting compound increases with higher temperatures Some oxides, such as water, vaporize at temperatures so low that dissociation occurs only in the vapor phase For an oxide, nitride, or hydride that remains a solid over a wide range of temperatures, a dissociation pressure exists at any temperature that represents an equilibrium between the compound, gas, and the metal

All metallic compounds decompose into constituent elements when heated to sufficiently high temperatures However, many of the metal oxides are quite stable, requiring low pressures at high temperatures to effect dissociation It is impractical to dissociate many of these compounds because of the combination of vacuum level and temperature required When a metal oxide dissociates, the metal remains and the oxygen is evacuated

For example, chromium oxide will dissociate in a 1.3 mPa (10-5 torr) vacuum at 1300 °C (2370 °F) The dissociation of a metal oxide usually depends more on temperature than on pressure Most oxides can be dissociated under normal operating vacuum levels at approximately their reduction temperature in a highly reducing hydrogen atmosphere

The nitrides and hydrides often have higher dissociation pressures, making many of them unstable when heated in a vacuum For this reason, vacuum heat treating can be used both to dissociate these compounds and to remove the evolved gas without disturbing the base metal

It is believed that when oxidized surfaces brighten during vacuum heat treating, the mechanism involved is not simply thermal dissociation of the oxide Bright surfaces do not discolor, or become brighter, when they are exposed to a vacuum atmosphere that is theoretically oxidizing A metal surface can be maintained almost free of visible oxidation at a partial pressure several decades higher than that suggested by theoretical calculations The following theories have been proposed to explain this apparent anomaly:

• The solution and diffusion rate for oxygen exceeds its surface absorption rate

• Oxide nucleation occurs at discrete sites rather than as a continuous film

• The effective concentration of oxygen is reduced by carbon and hydrogen in the solid metal and by the vacuum atmosphere

Heat Treating in Vacuum Furnaces and Auxiliary Equipment

Revised by the ASM Committee on Vacuum Heat Treating*

Vacuum Furnace Design

Although conventional atmosphere furnaces can be adapted for vacuum heat treating by adding a vacuum-tight retort connected to a suitable pumping system, furnace equipment developed especially for vacuum heat treating is generally used There are two distinctly different types of vacuum furnaces: hot wall (no water cooling of the exterior walls) and cold wall (water-cooled walls)

Vacuum furnaces can be grouped into one of three basic designs:

• Top-loading, or pit, furnaces

• Bottom-loading, or bell, furnaces

• Horizontal-loading, or box, furnaces

Furnace designs can be varied to fit a wide variety of processing requirements by changing the chamber length or by adding internal doors, circulating fans, recirculating gas systems, and/or internal quenching systems

Every vacuum furnace, regardless of its end use and basic hot- or cold-wall design, requires:

• Heating elements controlled to generate proper processing temperatures and cooling rates

• Suitable vacuum enclosures with access openings

• Vacuum pumping system

• Instrumentation to monitor and display critical processing data

Production furnaces may be single-chamber units, batch-type units, or multichamber, semicontinuous units

Hot-Wall Vacuum Furnaces

Vacuum furnaces are classified according to the location of the heating and insulating components Hot-wall furnaces were the first type to be designed Because of the demand of the heat-treating industry for higher temperatures, lower pressures, rapid heating and cooling capabilities, and higher production rates, hot-wall vacuum furnaces have become essentially obsolete with the exception of low-pressure chemical vapor deposition (LPCVD) and ion-nitriding processes and have largely been replaced by cold-wall vacuum furnaces

The entire vacuum vessel is heated by external heating elements in the hot-wall construction The heat is contained by insulation materials similar to the materials used in electrically heated heat-treating furnaces Hot-wall furnaces have limited use because of slow heating and cooling capabilities They are also limited in temperature because the strength of materials is reduced at elevated temperature However, hot-wall equipment is readily adaptable to low-temperature operations not exceeding 980 °C (1800 °F), with moderate-sized chambers

The double-pump modification of the hot-wall furnace permits the construction of larger vessels and the use of operating temperatures approaching 1150 °C (2100 °F) This system incorporates a second vacuum vessel outside the vacuum retort

to maintain a roughing vacuum during the heating cycle This removes the stress of the atmospheric pressure on the heated retort or vacuum vessel

Bell-Type Furnace. A bell-type hot-wall furnace is shown in Fig 2 The workload is placed on an elevated refractory metal hearth that rests on an insulated base clad with an alloy plate material A water-cooled circumferential flange and vacuum gasket are located on the vacuum-tight base cover adjacent to the heated zone but in an unheated area A retort made with a heavy-walled heat-resisting alloy covers the work load A flange at the bottom of the retort fits on top of the base gasket to provide a vacuum-tight enclosure The bell-shaped furnace equipped with internal electrical heating elements is lowered into position over the retort by a vertical hoist The vacuum pumping system is connected through the insulated base

Fig 2 Bell-type hot-wall vacuum furnace

Because this furnace cannot be heated or cooled rapidly, even when the bell-shaped vessel is removed, production rates and the number of thermal cycles within a given time period are limited Moreover, because the hot retort must support the entire pressure of the external atmosphere, its wall must be quite heavy Practical operating temperatures for a furnace

of this type are generally limited to approximately 925 °C (1700 °F)

Pit-Type Furnace. Figure 3 shows a pit-type hot-wall furnace The work load is placed in a top-loading muffle or retort made from a heat-resisting alloy The upper end of the retort is provided with a water-cooled flange and vacuum gasket that interlock with a flange on the upper part of the furnace above the heated zone The muffle is lowered into the furnace

by an overhead hoist, providing vacuum connections for the furnace and retort