10. VACUUM FURNACES

10.1 IntroductionVacuum furnaces are finding increasing application in the metallurgical processing of high temperature materials where pick up of Oxygen and Nitrogen both during melting and casting as well as during subsequent heat treatment is minimum. The vacuum environment is a pure protective atmosphere than any inert gas. The products are processed in vacuum furnaces with minimum contamination/distortion.

High purity gas approximately contains a total impurity of 10 ppm = 10 in 106 1 in 105 at 750 torr (atm) 10ppm means 750 / 105 torr which is equivalent to a 7.5 x 10-3 Torr. Today vacuum furnaces having a conventional diffusion pump, mechanical pump can have a vacuum level of better than 1 x 10-3 torr.

10.2 Role of Leak Rates

When a typical vacuum furnace pumped down to 10-4 Torr using conventional diffusion pump/mechanical pump, the water vapour exists predominantly even after pumping for 4 hrs. This situation continues till the furnace is heated approximately to 6500C. Above this temperature the water vapour molecules start dissociating and react with the elements like Carbon (coming from residual Hydro carbons, Graphite heating elements and Graphite insulation). Thus the vacuum atmosphere can be treated as slight oxidizing till 6500C. This point has to be kept in mind while designing furnaces for treatment below 6500C. This will not be a problem during cooling cycles as the moisture inside the furnace is completely removed. Thus if a component is heat treated above 6500C and still get oxidized, it means the leak rates are not within acceptable limits.

Even though vacuum atmosphere is better than inert/reducing atmosphere, the total atmosphere purity is drastically altered by leaks and adulterations from the product.

In case of furnaces which involve heavy degassing a roots pump (increasing the effective pumping speed of roots-rotary combination) is essential for maintaining the required backing pressure of the diffusion pump for its efficiency.

In a vacuum furnace for leaks at 10-3 Torr ( > d) where molecular flow exists, leaking gas (O2) molecules will have a tendency to touch the job before being removed by the pump. This results in oxidised product after the vacuum brazing process. Thus leak rate should be very small in high vacuum furnaces for brazing/sealing applications.

Similarly during off period the vacuum furnaces should not be exposed to air as it results in trapped humidity, which will not be removed easily in the next run and contaminates the hot zone. Leak checking of vacuum furnace should be done under high vacuum conditions initially when job is not hot and then with job and finally with

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job heated so that trouble shooting is easier with out confusion. MSLD leak detectors are usually employed.

10.3 Design of Vacuum FurnacesVacuum furnaces are broadly classified into two types:

A. Cold wall type : The vacuum chamber is water cooled and kept at room temperature. The job is kept in a hot zone (resistance heating/induction heating) supported by thermally insulating structures. Here size of the hot zone is small and energy input is less. But degassing from the walls of the vacuum shell remains to be a problem to be solved.

B. Hot wall type : The vacuum chambers/muffle is externally heated (by resistive heating) and job located inside receives heat mainly by radiation. Degassing/water vapour problems are absent. It can handle large quantities of jobs to be processed.

Hot BoxChamber Walls Main Valve

RoughingChevron

ValveBaffle

Fore Line

Valve

Hold 1200 Pump

Diffusion Pump Roughing

Pump Work Support

The design of vacuum furnace depends on:

Materials being processed, size of the useful hot zone, melt capacity in case of melting & casting, Heating rate and temperature control accuracy and cooling schedule.

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Heating Elements

75

75

777

600

500

Shields (Mo/S.S)

10.1 SCHEMATIC OF TYPICAL VACUUM BRAZING FURNACE

Further Vacuum considerations such as pumpdown time, operating vacuum and tolerable leak rate also should be considered.

Design of vacuum chamber considerations for cold wall type :

Mechanical strength : The chamber should not collapse under vacuum. The thickness of the chamber should be optimised to stand the pressure difference (1 Kg/Cm2 + cooling water pressure in the outer jacket) and not to increase the mass and out gassing rate. Further it should be able to prevent in-leakages from outside atmosphere when it is pumped down to a low pressure and to permit the material to be processed without any damage to the chamber as well as to the product.

However thinner sheets (to create lower weight) can also be used by re-inforcing the strength of the chamber by welding stiffeners. For vacuum applications, argon arc welding is preferred as the contamination from flux is totally avoided. If complete argon arc welding is not possible, we can weld few tax and then remaining by brazing with compatible metals/flux. It is essential to have good surface finish in all the grooves meant for 'O' rings,Wilson seals for rotating devices and door seals.

Regarding shape cylindrical and rectangular chambers are very often used. The best method of cylindrical chamber fabrication is from a sheet cylindrically rolled up and dished heads on both ends welded throughout. Although this type of construction offers good mechanical strength, some designers select a rectangular cross section for efficient space utilization and use of low capacity vacuum pumps. However, rectangular chambers have to be designed for higher stresses and obviously require higher shell thickness.

With respect to orientation/operation the furnace chambers are classified as :1. Vertical type, bottom loaded for heavy jobs.2. Vertical type, top loaded where top dished end can be lifted by an over head hoist

and the job can be loaded by a crane.If necessary System can be designed in such a way that both the chamber and the top dome can be lifted for loading the job(horijantally by a forklift) to avoid striking of the jobs with the heating elements/shields .

3. Horizontal type, front loaded (by opening the door) for long light weight components/flat pieces etc.

For the purpose of external water cooling most of the manufacturers prefer double walled configuration as the cooling efficiency is maximum and so the furnace can be designed for very high temperatures (14000C). The basic disadvantage in this design is the repairing of any water leak developed in the inner chamber.

1. One can also have single walled configuration (advisable upto 11000C) with channels welded/copper tube soldered on the outer surface of the chamber.

2. This has lower cooling efficiency but repairing is quick.

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Materials for chamber fabrication: For the fabrication of chamber, either hot rolled mild steel or stainless steels of grade 304, 304L and 316 are used. Vacuum theorists do not prefer bare hot rolled mild steel for the chamber, as the material will be rusted during usage. This not only will affect the compressive strength of the material but may also increase the surface available for condensation of water vapour, whenever the chamber is exposed to atmosphere.However, economic considerations, mild steel may be used but with a certain special surface treatment. By coating the inner shell walls with epoxy (gas loads from such paints are minimal) mild steel vessels are widely used for many applications. Whenever there is any suspected water contamination, a hot argon scrub is to be given.

The arguments in favour of a polished stainless steel chamber are the availability of less surface area for condensation of water vapour and a low degassing rate of the polished surface. Some designers prefer a stainless steel inner chamber and mild steel outer chamber due to economic consideration. However, it is essential to take care of the quality of the weldments, as galvanic corrosion of welded joints can occur due to water circulation. Stainless steel 304 grade weldments are susceptible to pitting corrosion when the sediments present in the water accumulate near the weldments, especially when such welded joints are present at the bottom of the chamber.

Even though stainless steel grade 316 can overcome this problem of pitting corrosion, the associated welding problems and high cost prevent its widespread use.Stainless steel grade AISI 304L, with low carbon content, is the most suitable material for a vacuum furnace chamber for Indian condition.

If there is any possibility of heat dissipation to the 'O' rings, the flanges have to be water cooled. The various types of materials used for 'O' rings for operations upto 1 x 10-6 Torr are neoprene (upto 500C) silicone (upto 1000C), and viton (upto 2000C).

Chamber design for hot walled type:

Since hot walled furnaces are heated externally, the furnace shall be designed keeping in view the yield strength, modulus of elasticity and creep properties of the shell material at the rated temperature. This restricts the usage of hot walled furnaces above 9000C. However, with the development of better class of nickel base super alloys, presently hot walled furnaces are being designed for certain metallurgical applications upto 11000C for short duration operations. Basic design calculation procedures applicable to cold wall type are also applicable here. For hot walled furnaces, design of the cooling water circulation near the flanges needs careful consideration as those areas are more prone for thermal stresses. Generally, these designs have internal insulation near the flange end, which enables in decreasing the

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temperature near the flange much below the actual furnace shell temperature. For hot walled furnaces, cylindrical shapes are only used.

For most of the metallurgical processes, such as sintering, brazing annealing, and melting & casting vacuum of the order 1 x 10-6 Torr is adequate. Hence, the vacuum system consists of a combination of the following pumps:

i) Mechanical vane pump/piston pump.ii) Roots pump.iii) Oil diffusion pump.

The combination includes other sub-assemblies such as chevron baffles, valves and pipe line.Regardless of the efficiency of the pumps, the cross sectional area of the pumping line, valves, manifolds etc., must be equal to or more than the inlet diameter of mechanical pump itself used as the backing pump for the diffusion pump, the diameter of the pipe line upto the inlet of diffusion pump must correspond to the diameter of the diffusion pump while the dia. of pipe from the outlet of diffusion pump must correspond to the in let diameter of the mechanical pump.

Vacuum valves:The various types of valves used in the pumping line are :

i) Angle valvesii) Ball valvesiii) Butterfly valvesiv) Gate valves.

Gate valves are the best suited for a pumping line, as they offer minimum impedance to gas flow. For high vacuum line, angle valves fabricated out of stainless steel are also used. The other type of valves i.e., ball valves and butterfly valves, are used in the low vacuum mechanical pumping lines. Electromagnetic valves help in automatic closing of the valves in case of power failures.

Hot zone for cold walled furnaces:

Resistance type:The primary criterion for the hot zone of any furnace is that the entire volume of the work piece should experience the same temperature. The important parameters on which the thermal uniformity of the hot zone depends are size, shape and location of the heating elements, rate of heating, the type of insulation/shields used etc. In general, the hot zone of the furnace consists of:i) Suitable heating elements.ii) Thermal insulation/radiation shields to conserve the heat.iii) Ceramics for electrical and thermal insulation.

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Heating Elements:

The various heating elements and their physical characteristics are presented in Table 3. Kanthal is not recommended for temperatures above 9000C in vacuum as its electrical characteristics change in course of time due to chromium losses. Molybdenum provides the high power density required for vacuum furnaces. However above 14000C, the discharge voltage is 40V, and so the cross sectional area of the rod/strip has to be increased which inturn imposes constraints on the formability. Hence, for operations above 14000C molybdenum is not preferred. Besides, after a few heats, grain growth occurs which will be sufficient to cause the material to become very brittle, thereby seriously limiting its ability to withstand any thermal shock due to which frequent replacement is required. For temperatures between 1400-18000C tungsten/tantalum is preferred due to its capability to withstand thermal shock. However, fabrication difficulties and higher initial costs are significant points to be considered.

With the availability of high quality graphite, graphite elements have been widely used upto 20000C.

The advantages of graphite heating elements are :

1) Low initial cost.2) No change in resistance at high temperatures.3) High hot strength.4) Ability to withstand reasonable mechanical shocks and accidental high

temperature air breaks.5) Ease of fabrication of the hot zone due to its availability in the forms of thin

sheets and strips and cloth.

However, the user has to ensure that the heating elements do not come in contact with the job, as carbon can diffuse into base metal interstitially. If carbon contamination is not tolerable even in ppm level, these elements are not recommended.

Heating elements design:

Three types of heating element arrangements are available for heating element location in the hot zone.1. Locating the heating elements circularly along the length as well as on top &

bottom sides - This design is generally followed for circular hot zones. The uniformity of temperature on the job depends on the location of it with reference to hot zone size.

2. Locating the heating elements along the sides. Depending on the uniformity of temperature required the heating elements are located on either on two sides, four sides or six sides.

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3. Zone trimming - In this the heating elements are located on all six sides. However power coupling is adjusted depending on the size, shape and uniformly of temperature required on the Jon. Generally this configuration is followed in brazing furnaces. The heating elements are generally used either in rod form or strip from.

Insulation/Shields:

The efficiency of the furnace depends on the way in which the heat is conserved. This can be maximised by properly designing the furnace to reduce heat losses. The main mode of heat loss in any furnace is through radiation and hence designing radiation shields/insulation plays an important role in the overall design of the furnace.

The essential requirement is the use of highly polished metallic sheets as shields. Depending on operating temperature and the shield materials, the number of shields needed may vary. The common materials widely used for shields are stainless steel, molybdenum and tantalum. Of these molybdenum and tantalum have very low emissivities (Table 3) in comparison to stainless steel and hence minimize heat losses. Thus, for high temperature applications above 8000C, four polished stainless steel sheets are used. For temperatures upto 16000C, six shields, consisting of a combination of SS + Mo, are used.

Graphite felt can also be used as insulating material in place of shields where carbon contamination is not critical. The features in favour of graphite felt insulation are:

a) The thermal efficiency of the furnace increases by 33% and thereby, power and water requirements are less.

b) The initial cost is low (66% of the cost of metallic shields) and c) Frequent replacement is avoided.

The main disadvantage with graphite insulation is that it quickly absorbs moisture, from the atmosphere. Whenever the furnace is opened prolonged initial pumping is required. In the chamber, carbon monoxide may form and react with the job. By opening the furnace chamber at temperatures 120-1500C for loading and unloading the job, and always keeping the chamber under vacuum when furnace is not in operation can cut down the initial pumping time considerably.

Ceramics:High quality ceramics which can withstand thermal shocks are used for element sports/spacers below shields, ( to ensure the elements do not touch during heating). These ceramics are fabricated from 99% Al2O3 and the degassing rate of alumina is 7.5 x 10-15 Torr 1/s. Depending on the design of heating elements, the shape of the ceramic components is decided.

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Electrical feed-throughs for resistance heating:

These require both electrical insulation and vacuum tight seal. The type of feed through generally used for vacuum resistance furnace application upto 1 x 10-5 mm Torr is dismountable type with conventional 'O' ring seals. Since high current passes through the feed through, water cooling is essential to avoid excess heating. The copper feed-through assemblies are fabricated either as a single piece or in two parts brazed together.

Induction heating:

Induction heating is characterised by excellent control over alloy composition because of the stirring action generated by eddy currents. Hence it is preferred for alloy melting and refining processes. Induction stirring homogenises the melt as well as brings the reactants to the melt vacuum interface so that the reactions can proceed rapidly.

Induction is also used for applications like sintering, brazing, annealing etc, where temperature accuracy on the component need not be so accurate. For these applications a graphite scepter is used which in turn radiates the heat to the job.

With the development of medium frequency generators using thyristor techniques, it has become possible to design vacuum furnaces having induction coils located inside the cold walled furnace. The frequency of these generators is in the range of 0.5 to 10 KHz.

The induction coils are fabricated out of conventional copper tubes which are water cooled using softened water. If needed, magnetic shunts shall be used to prevent induction leakage.

Characteristics of Heating ElementsHeating Element Normal Operating Temp.

CElectrical Resistivity

(Microohm m) at operating temp.

Kanthal A - 1 900 1.52Molybdenum 1600 0.5

Tungsten 1800 0.59Tantalum 1800 0.81Graphite 2200 19

Comparision of hot walled and cold walled furnacesSL No

Hot-walled type Cold-walled type

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1. Furnace shell material Thick nickel base superalloys

Mild steel & stainless steel of relatively low thickness

2. Volume of the furnace chamber for a given hot zone size

About half of the volume required for a cold walled furnace

---------------

3. Pumping system capacity

Low High

4. Power requirements to heating elements

Conventional power supply Low voltage, high amperge supply

5. Heat losses High Low6. Heating elements Conventional heating

elements such as SiC, Kanthal etc.

Specialised heating elements such asMo, W, Ta etc.

7. Operating temperatures 10000C max As high as 24000C8. Maintainance/Running

costHigh Low

9. Cost of furnace for a given hot zone size and vacuum

Cheaper Costlier

Today, microprocessor based temperature programmers/controllers, wherein it is possible to preset heating rates, holding time, cooling rates etc., are available.

Temperature Recorder/Over temperature protection:

Where multiple thermocouples are used for recording the temperature at different locations of the job a multipoint recorder or scanner is incorporated. In order to protect the hot zone materials/job from damage due to distortion or melting in a vacuum furnace, it is mandatory to incorporate an over temperature protector.

Safety interlocks:

Safety interlocks are essential to ensure that proper sequential operations are followed. This helps in increasing the life and preventing frequent breakdown of the furnace and its subsystems.Important interlock combinations are:

1. Water flow and switch ON of the systems.2. Diffusion pump ON and foreline vacuum after attaining proper.3. Diffusion pump and thermal switch for water flow.4. Heating element ON only under high and vacuum valve open.5. Roughing pump OFF and DP should cut OFF.

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Indicating lights corresponding to different on conditions display the status of the furnace operation. In addition, alarms and fault indication lights are also to be provided in.

Thermocouple feed-throughs:

These (electrical feed through in the forms of flange) are also positioned on the chamber wall through an 'O' ring seal. Depending on the requirement, one or more thermocouple can be positioned in the feed through by using a suitable glass-to-metal seal. It is essential that all the joints on the thermocouple feed through are tested by helium mass spectrometer leak detector, at a standard leak rate of 1 x 10-8 std cm3 /s.

Water flow switches:

These are incorporated in every outlet water line of the furnace assembly which requires water cooling and interlocked with respective subsystem.

Vacuum Drying of Transformers

The manufacturing of good transformer involves drying of the solid insulation (paper, wood, epoxy etc) of transformer by heating (oil heat exchanger) the assembly in a vacuum vessel (auto clave) and then applying the vacuum to remove the moisture. After the drying is completed, the transformer is allowed to cool down, and oil is filled in the transformer under vacuum. Before filling the oil it is filtered, and dehydrated by a separate vacuum filteration and degassing plant.

For drying of power transformer it is essential to have a combination of mechanical booster and Rotary oil sealed Vacuum Pump having enough water vapour tolerance.As such the vacuum drying process is slow.1 cc of water = 1150 litres of vapours @ 1 torr.

= 11,500 litres of vapours @ 0.1 torr. = 1,15,000 litres of vapours @ 0.01 torr.

As the vacuum improves, the volume of vapour also increases and to remove small amount of water, the vacuum pump has to pump out lot of vapours and hence booster stage is employed.

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Hot Thermic Oil

Sensor

Oil

Radiator CoilGlass wool insulation

Condensate Collecting Tanks

Condenser

Insulation Valve

Collecting Tank

Exhaust

CondenserRotary Pump

Roots Pump

Chilled water

Vacuum Gauge

Air Release Valve

AutoclaveeTransformer

Fig.10.2 Vacuum Drying System

Thermocouples:

If two wires of unlike metals or alloys are joined together firmly at one end, and this junction is heated or cooled, a small voltage (emf) will appear at the open ends. This emf is a function of the temperature difference and of the kinds of metals used.

Thermocouples for use in vacuum devices are to be chosen on the basis of (a) useful temperature range, (b) sensitivity, and (c) speed of response. Metals and alloys with high vapor pressures should be avoided where elevated temperatures and bake-out are scheduled.

The range of emfs produced by most conventional couples in use is up to about 50 v/oC. The speed of response is largely determined by the size of wire used for the two metals, and is greater with finer wires than with larger sizes.

But for use in oxidizing or corrosive atmospheres at high temperatures, as in furnaces and ovens where speed of response is not an important factor, the thermocouple should be made of heavy-gauge, corrosion-resistant material.

Platinum and platinum-rhodium alloy couples are expensive but can be used at temperature upto 15000 C in air. Exposure to high temperatures (up to 16000 C) may change the calibration at lower temperature and should be checked after each such exposure.

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Chromel-Alumel thermocouples, are widely used in industrial and laboratory equipment because of their uniformity, corrosion resistance, sensitivity, and relative low cost (compared to platinum). Chromel-Alumel thermocouples can be used between -2000 and +12600C in air or in non-reducing environments. The alloys cannot be used at high temperatures in hydrogen, sulfur, or carbon monoxide atmospheres.

Copper-Constantan junctions can be used in the range -2600 to +3500C, the upper value being set by the rapid oxidation of copper at higher temperatures. The copper preferred for making these couples is OFHC which is very homogeneous. Constatan is a somewhat variable material, having the composition Cu 50-60%, Ni 50-53%.

Iron-Constantan thermocouples are very widely used in oxidizing atmosphere up to 7600C and in reducing atmospheres up to about 9800C.

Tungsten-tungsten 74%, rhenium 26%. This couple can be used up to 28000C in vacuum, hydrogen, and inert gases such as nitrogen, argon, and helium, but it cannot be used in oxidizing or hydrocarbon atmospheres at high temperatures.

Thermocouples can be made in several ways. For use in electron (vacuum) devices, an arc-, spot-, or butt-welding technique can be used, as illustrated in Fig. 10.3, (a), (b) and (c). Platinum thermocouples can be arc-welded without using any flux, or plain borax can be used in torch-welding to prevent the effects of a reducing flame which may embrittle the wires. In spot-welding, if copper electrodes are used, any copper contamination should be removed by appropriate chemical treatment after the weld is made.

For furnace and oven thermocouples, the wires may be twisted together for a short distance at the end, and the junction made by welding or brazing, as shown in Fig. 10.3(d) and (e). In the case of the brazed (or soldered) junction (e), the presence of braze material between the elements does not introduces an error so long as the material is homogeneous and the temperature along it is uniform.

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Fig.10.3. (a) (b) (c) (d) (e)

Heat Transfer in vacuum systems

Above approximately 10 Torr, the heat transfer through a gas inside a relatively small chamber is dominated by convection (bulk motion of gas). Between 1 to 10 -3 torr the heat transfer is through gas conduction which is a linear function of pressure (hence applied in thermal conductivity gauges).

In high vacuum conditions (Below 10-4 Torr) the heat transfer is mainly due to radiation and to a some extent by solid conduction through support structure. It is to be mentioned here, if two solid pieces are in just apparent contact in a vacuum chamber the heat transfer is very poor, as the real area of contact between two rigid bodies is usually only 0.1% of the apparent area. Heat is transferred only through a few touching high points between the plates. To increase the heat flow, one (or both) of the plates should be soft and high clamping forces must be used. Even then, the heat transfer by conduction will be limited to the small areas of contact created near the clamping bolts.In view of the above facts, we can calculate the temperature rise of the job in a vacuum furnace due to resistive heating from surrounding heating elements, based on radiation formula.

Q12 = 12 A1 (T14 - T2

4) where 12 = Cb/1/E1 + A1/A2 (1/E2 - 1)where Cb = Radiation coefficient of the black body i.e.5.7 x 10-11 KW/(m2 T4)

E1 ,E2 = Emissivities of 1st and 2nd shields A1,A2 = Areas of the 1st and 2nd shields T1, T2 = Temperatures of the 1st and 2nd shields.

As the radiation between two surfaces is proportional to difference in fourth power of temperature, the heat flux in the initial period to the job from a heating element (reached fast to the equilibrium, say 8000C, due to resistive heating) is very high. Hence if we want more or less linear slow increase of job temperature, it is very important to raise the temperature of the heating elements in steps and allow the job to reach that set limit.

For heating in several steps using suitable temperature controllers, we can apply radiation formula piece wise taking the necessary temperature limits. Once heat flux is known, the temperature raise can be calculated using the physical properties of the job namely specific heat (more or less a constant value in high temperature range) and mass.

For calculating heat flux due to support solid structures, one can use Fourier equation by taking the thermal conductivity value at the average temperature. However, in high temperature region for most of the metals thermal conductivity does not change much and can be taken as constant.Q = - A / L K(T) dT.

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