Furnace

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Furnace is a heating device that is used for different heating process like calcination, roasting etc. The name derives from Latin word Fornax, which means oven.
There are different types of furnace available in the market on the basis of application, environment required for heating and methods of heating.
On the basis of heating method, we have these following types of furnace……

• Resistance Heating
• Induction Heating
• Microwave Heating

Resistance Heating Method- Resistance heating is based upon the Joule’s law of heating (I2R effect). This method of heating has wide applications such as heat treatment of metals (annealing, hardening etc.), drying and baking of potteries. Furthermore there are two types of resistance heating, one is Direct Resistance Heating and another is Indirect Resistance Heating.

In the case of direct resistance heating method, the material or charge to be heated is taken as resistance and current is passed through it. The charge may be in the form of powder, pieces or a liq-uid. Two electrodes are immersed in the charge and connected to the supply in case of availability of direct current or single phase ac supply and three electrodes are immersed in the charge and connected to supply in case of availability of 3-φ ac supply. When some pieces of metals are to be heated some highly resistive powder is sprinkled over the surface of pieces to avoid direct short circuit. The current flows through the charge and heat is produced. This method has high efficiency since heat is produced in the charge itself. As the current in this case is not easily variable, therefore, automatic temperature control is not possible. However, uniform and high temperature can be obtained. This method of heating is used in salt bath furnaces and in the electrode boiler for heating water.

In the method of indirect resistance heating, the current is passed through a wire or other high resistance material forming a heating element. The heat proportional to I2R loss produced in the heating element is delivered to the charge by one or more of the modes of transfer of heat viz. conduction, convection and radiation. If the heat transfer is by conduction the resistor must be in contact with the charge. An enclosure, known as heating chamber, is required for heat transfer by radiation and convection for the charge. For industrial purposes, where a large amount of charge is to be heated, the heating element is kept in a cylinder surrounded by jacket containing the charge. This arrangement provides a uniform temperature. Automatic temperature control can be provided in this case. This method of heating is used in room heaters, immersion water heaters and in various types of resistance ovens employed in domestic and commercial cooking, and salt bath furnaces.

Induction Heating Method – Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, where eddy currents (also called Foucault currents) are generated within the metal and resistance leads to Joule heating of the metal. Induction Heating is a form of non-contact heating for conductive materials, when alternating current flows in the induced coil, varying electromagnetic field is set up around the coil, circulating current(induced, current, eddy current) is generated in the work piece(conductive material), heat is produced as the eddy current flows against the resistivity of the material. It is a rapid, clean, nonpolluting heating form which can be used to heat metals or change the conductive material’s properties. The coil itself does not get hot and the heating effect is under controlled. The solid state transistor technology has made induction heating much easier, cost-effective heating for applications including soldering and induction brazing ,induction heat treating, induction melting, induction forging etc.

Microwave Heating Method – Microwave heating is a multi-physics phenomenon that involves electromagnetic waves and heat transfer; any material that is exposed to electromagnetic radiation will be heated up. The rapidly varying electric and magnetic fields lead to four sources of heating. Any electric field applied to a conductive material will cause current to flow. In addition, a time-varying electric field will cause dipolar molecules, such as water, to oscillate back and forth. A time-varying magnetic field applied to a conductive material will also induce current flow. There can also be hysteresis losses in certain types of magnetic materials.

Furthermore on the basis of application and chamber ambient we have different types of furnace like

• Oxidation Furnace
• Vacuum Furnace
• Hydrogen Furnace
• Conveyor Belt Type Furnace
• Muffle Furnace

Oxidation Furnace – Thermal oxidation is a simple route to cover a silicon substrate with oxide. In micro devices oxide may be used for a variety of purposes from chemical attachment and modifiable of surfaces to using oxide as a filler material in gaps etc.

Some of the advantages of the vacuum furnace are

• Temperature can be controlled within a small area.
• Low contamination of the product by carbon, oxygen and other gases.
• Quick cooling (quenching) of product.

Applications of Furnace

Furnace has a wide range of applications in various fields of Semiconductor R&D, Defence and Security and in automobile sector. On the basis of process we have divided application into following groups

• Heat Treatment
• Chemical Process
• Joining
• Powder Metallurgy/Ceramics
• Melting and other process like
• Activation, Calibration, Glass Bending, Sterilization, Surface Coating, Tempering etc.

Heat Treatment Applications

Annealing- It is a heat treatment that alters physical and chemical properties of a material by making it more workable. Annealing is done to improve materials ductility and reduce hardness. In many semiconductor processes annealing is done to increase the conductivity as well.

Borating- Process used as a Flux in Metallurgy.

Carbonisation- Conversion of organic substance into carbon or carbon containing residue through pyrolysis or destructive distillation.

Hardening- it is a Metalworking process used to increase the hardness of a metal.

Nitriding- It is a heat treating process that defuses nitrogen into the surface of metal to create a case hardens process.

Ferritic Nitrocarburizing- Ferritic nitrocarburizing, also known by the proprietary names Tennifer/Tenifer and Melonite, is a range of proprietary case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures during a salt bath. The processing temperature ranges from 525 °C (977 °F) to 625 °C (1,157 °F), but usually occurs at 565 °C (1,049 °F). At this temperature steels and other ferrous alloys are still in a ferritic phase, which is advantageous compared to other case hardening processes that occur in the austenitic phase. There are four main classes of ferritic nitrocarburizing: gaseous, salt bath, ion or plasma, and fluidized-bed.

Stress Relieving- It is carried out on metal products in order to minimise residual stresses in the structure thereby reducing the risk of dimensional changes during further manufacturing or final use of the component. Stress relieving is normally done after rough machining, but before final finishing. Stress relieving does not change the material’s structure and does not significantly affect its hardness.

Chemical Process Application

Ashing- It is a test to deduce the amount of ash forming material present in a petroleum product so as to decide its use in certain applications. Ash-forming materials are considered to be undesirable impurities or contaminants.

Calcination- the IUPAC defines calcination as “heating to high temperatures in air or oxygen”. However, calcination is also used to mean a thermal treatment process in the absence or limited supply of air or oxygen applied to ores and other solid materials to bring about a thermal decomposition. A Calciner is a steel cylinder that rotates inside a heated furnace and performs indirect high-temperature processing (550–1150 ° C or 1000–2100 ° F) within a controlled
atmosphere.

Oxidation- Thermal oxidation is a simple route to cover a silicon substrate with oxide. In micro devices oxide may be used for a variety of purposes from chemical attachment and modifiability of surfaces to using oxide as a filler material in gaps etc.

Pyrolysis- Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere. Produce smaller hydrocarbon molecules.

Thermal Recycling- At higher temperature metals and other materials can be converted into their different forms. Thermal recycling has wide range of applications in energy systems, where we can decompose wastes at particular temperature and convert them to some sources of energy.

Reduction- It is a process where we can make the metal oxygen free or we can say convert the metal from metal oxides to metal hydrides at higher temperature.

Joining Process Applications

Brazing- Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.

Shrink Fitting- Shrink-fitting is a technique in which interference fit is achieved by a relative size change after assembly. This is usually achieved by heating or cooling one component before assembly and allowing it to return to the ambient temperature after assembly, employing the phenomenon of thermal expansion to make a joint.

Soldering- Soldering is a process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering.

Melting Process Applications

Casting- Casting is a manufacturing process in which a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify.

Fusion- The process or result of joining two or more things together to form a single entity, for example making of Bronze, bronze contains copper and zinc.

Levitation Melting- Melting metal out of contact with a supporting material by using induced current provided by a high-frequency surrounding magnetic field to suspend melt. Levitation melting is an advance material preparation technology.

Precision Fine Casting- Precision casting, also called investment casting, is a lost wax casting process widely used for producing ferrous and nonferrous metal parts. Unlike other casting processes, precision casting produces net shape parts with excellent surface finish and dimensional accuracy.

Powder Metallurgy/Ceramics Process Application

Sintering- Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction. Sintering happens naturally in mineral deposits or as a manufacturing process used with metals, ceramics, plastics, and other materials.

Debinding- Debinding is one of the main production steps in the ceramic and powder-metallurgical industries. It refers to the thermal or catalytic removal of additives used in steps prior to production such as casting. The debinding process for ceramic materials commonly takes place in oxygen-containing atmospheres and results in complete combustion of the organic components. In contrast, in powder-metallurgy often inert atmosphere is required for the debinding process.

Dewaxing- Dewaxing of oil is a process of eliminating higher melting point constituents like waxes or triglycerides from oil by gradually cooling vegetable oils and separating saturated glycerides from the oil.