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Chapter 2:

Factors to Consider when Choosing a Crucible

To select a crucible for your operations, you need the right crucible from the wide range of crucible types and materials available to you. The following should be considered when choosing an induction heating graphite crucible:

2.1, Type of Smelting Furnace

The capacity, dimension and type of furnace you use will determine your choice of furnace crucible. If you know the capacity your furnace was designed for, you will know what capacity your crucible should provide. Likewise, the dimension of the space of the graphite crucible in your furnace will dictate the dimension and shape of your graphite crucible. This will also determine if your furnace crucible must include a pouring spout. However, choosing a crucible to match your furnace type will give you many other less obvious factors to consider. There are different furnace types and different crucibles peculiar to each.

Fuel Fired Furnace:

Fuel fired furnaces include furnaces powered by coke, propane, oil or gas. Each of these fuels directly exposes the furnace crucible to the heating source and each provides a different level of heat, normally measured in BTUs (British Thermal Unit). Any furnace crucible selected must be able to withstand the maximum BTUs the furnace fuel is able to apply to the graphite crucible. In gas, oil and propane furnaces, the graphite crucible must be able to withstand the effects of the burner flame at the base of the furnace and the crucible must be tapered to allow the flame to circulate around the graphite crucible from bottom to top. This allows even heating of the graphite crucible. The crucible material must be able to resist oxidation damage from the flame and accommodate the rate of thermal change the crucible will experience.

Good thermal conductivity and even heating are important crucible factors in transferring the heat from the interior of the furnace through the crucible to the metal charge. Crucibles with high graphite content in the carbon binder have high thermal conductivity for fast melting in gas fired furnaces.

Resistance Furnace:

Electric resistance furnaces provide even, all round heating to a graphite crucible and are ideally suited for even and precise temperature control in metal holding application. Energy efficient crucibles with high graphite content in the carbon binder are often selected to provide high thermal conductivity for faster melting in this type of furnace.

Crucibles designed for resistance furnaces are normally basin shaped and provide a uniform distance between the crucible and furnace heating elements.

Induction Furnace:

Selecting a crucible for an induction furnace is a more complex task. In some applications such as refining precious metals, crucibles designed to heat in the furnace&#;s inductive fields are used to melt the charge. In other applications, crucibles that allow the inductive field to pass through them and heat the metal charge directly are used. Therefore, it is important to match the electrical characteristics of the graphite crucible to the operating frequency of the furnace and to the melting application. For instance, in some designs, lower frequency induction furnaces require crucibles with high silicon carbide content and in other applications, higher frequency induction furnaces require crucibles with high clay content. Matching a crucible&#;s electrical resistivity to the induction furnace is key to preventing crucible overheating.

Most graphite crucibles designed for induction furnaces are cylindrical to provide a uniform distance between the crucible and the furnace coil. However, some small furnaces designed for removable crucibles feature a tapered coil to match the profile of bilge-shaped crucibles.

Vacuum Induction Melting Furnace:

Vacuum induction melting has high application potential in comparison to alternative melting techniques. Vacuum induction melting (VIM) is a commonly used melting and casting technique which involves melting an alloy under vacuum or an inert atmosphere by electromagnetic induction using coils. Vacuum induction melting can be hindered by lack of appropriate crucible. A suitable crucible must combine low with metal being melted at degree celsius, have thermodynamic stability and thermal shock resistance. Titanium and titanium alloys are widely used in a variety of fields including; the aerospace and automotive industries, manufacture of biomedical components and surgical instruments, chemical and petrochemical engineering, marine applications, etc These industries adopt the vacuum induction melting furnace for melting titanium.

Vacuum Arc Remelting Furnace:

Vacuum arc remelting is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. The furnace crucible commonly used in a vacuum arc remelting furnace is made of copper, which is usually surrounded by a water jacket used to cool the melt and control the solidification rate.

Electron Beam Melter: Electron E-beam crucibles are designed to offer E-beams users improved evaporation performance the bare hearth modes. Graphite crucibles used in electron beam furnaces act as an energy efficient thermal barrier between the molten evaporant and the water cooled copper hearth.

Plasma Arc Melting Furnace:

The plasma arc furnace is a device used to melt a substance by low-temperature plasma flow, typically created by an electric arc heater (plasmatron). The main field of application of the plasma furnace is electrometallurgy. There are three types of the plasma furnaces: plasma furnaces for melting in a ceramic crucible; plasma furnaces for melting in a crystallizer and plasma furnaces for melting in a scull. The ceramic crucible plasma furnaces are used mainly for melting steel, nickel-based alloys and waste metals with alloying additions. Plasma furnaces with a crystallizer are used mainly for the metal refining process. By contrast to electroslag, vacuum-arc and electron-beam refining processes, the main technological means of action on the liquid metal is the gas phase. The plasma furnaces for melting in a scull are designed to make steel castings, high-temperature alloys and refractory metals.

2.2, Furnace Power Limit

Induction furnace power can be categorized into three: Low power frequency (less than 400Hz), medium power frequency (400-Hz) and high power frequency (more than Hz). The higher the operating frequency, the greater the maximum amount of power that can be applied to a furnace of a given capacity and the lower the amount of turbulence induced. The power supply system has two functions, to provide power to the primary coil and the other to control the melting of the metal.

The principle of induction heating is that a high voltage electrical source from a primary coil induces a low voltage, high current in the metal, or secondary coil. Induction heating is simply a method of transferring heat energy. A good furnace crucible must be able to adapt to the heating principle of the induction furnace. Therefore, this will influence your choice of crucible.

2.3, The Type of Precious Metal Determines the Crucible

Understanding the metal type or alloy will let you know the characteristics you are looking for in a furnace crucible. Your detailed catalogue of the metals you intend to melt will help to establish the maximum temperature the crucible must support for melting or holding, will define how the metal will interact with the furnace crucible material both chemically and physically and it will be a key factor in determining what characteristics your optimal crucible should offer. For example, in melting copper based alloys in fuel-fired furnaces, roller formed silicon-carbide crucibles perform better due to higher thermal shock resistance. In other types of furnaces, crucibles are often selected because of their high density. Less dense and more porous crucibles may allow erosion.

Carbon-bonded and ceramic-bonded clay graphite and silicon carbide crucibles are widely used in melting aluminium and aluminium alloys, aluminium bronze, copper, copper based alloys, cupro-nickel, and nickel-bronze alloys, precious metals, zinc and zinc oxide. Crucibles are also used in melting cast iron. Taken together as a group, these metals represent a temperature range from 400 degree celsius to degree celsius.

While some crucible types support metal temperature encompassing a broad spectrum of metals, it is necessary to select furnace crucibles targeted to melt specific metals or alloys. Selecting such crucibles are often more advantageous because they offer permanence characteristics important to your operations. For instance, using a crucible that is able to melt metal from iron to zinc may not be as important to your aluminium alloy melting operation having a crucible limited to the temperature range you need but able to resist corrosion damage from your metal treatment fluxes.

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The most applicable crucibles for melting precious metals are the graphite crucible and the silicon carbide crucible. The melting temperature of a silicon carbide crucible is up to degree celsius. Clay graphite on the other hand has a melting temperature ranging from 850 degree celsius to degree celsius. Graphite crucibles are for melting precious metals such as gold, silver, platinum, palladium. The graphite crucible has extremely good thermal shock resistance allowing fast melting from cold.

However, metals that have melting points reaching up to or more than are melted using the quartz crucible. Platinum and palladium fall under this category.

Platinum has a melting point of degree celsius while palladium has a melting point of degree celsius.

Quartz crucible has fused silica as main material and its many desirable properties includes high chemical purity, high corrosion resistance, high melting point, extreme hardness, low coefficient of thermal expansion, high refractoriness, good thermal stability, high thermal shock resistance.

2.4, Melting and Holding Temperatures

The melting temperature of the metal and alloy you melt or hold will determine the temperature range within which your furnace crucible must be able to operate. Crucibles must never be heated above their maximum temperature, this can lead to crucible failure. Operating below the crucible temperature limit can also cause problems. For instance, crucibles designed for high temperature melting of copper based alloys will oxidize if used at low temperatures for zinc melting.

Melting and holding practices are important factors that need to be considered when selecting crucibles. If your operations involve superheating, you will need to take the higher metal temperatures into account.

2.5, Rate of Temperature Change

The ability of a furnace crucible to handle the rate if temperature change is as important as its minimum and maximum temperature limits. If your operations usually involve frequent heating and cooling cycles, which means it is subject for rapid temperature change, you will need to select a crucible that is resistant to thermal shock. For instance, a high carbon content of the graphite in a crucible imparts high thermal conductivity and non-wettability and provides high thermal shock resistance. This is critical to foundry applications which can change by several hundred degrees in seconds.

2.6, How the Crucible is Charged

If your crucible is charged with only molten metal, then there is no requirement for a crucible designed to be highly resistant to physical damage. In the case where heavy metals make up the bulk of your charge, you will need to select a crucible that is mechanically strong and able to survive physical shocks. Crucibles featuring high carbon content and a directionally oriented graphite structure provide excellent impact resistance.

2.7, Fluxes and Additives

All furnace crucibles offer some level of resistance to corrosion and chemical attack. Fluxes and other metal treatments used in melting aluminium and other nonferrous metals are highly corrosive and require a crucible that offers a high level of resistance to chemical attack. This resistance is best imparted by both a consistently dense crucible material structure and a durable protective glaze. If your metal application involves the use of corrosive metal treatments, you need a furnace crucible offering the appropriate level of protection against these agents.

2.8, Degassing and Refining

Degassing aluminium and aluminium alloys typically involves bubbling inert gas, usually nitrogen, through the molten bath with a rotor designed to break apart and disperse the gas bubbles. These small bubbles then pull the undesirable hydrogen and oxides out of the bath and carry it along with dross and inclusions to the surface where the gas escapes into the air and the solid material can be removed. This process is often used along with fluxing agents that physically erodes the crucible and attacks it chemically. Therefore, a dense and strong furnace crucible that is highly resistant to chemical attack is needed. Graphite crucibles provide excellent resistance to elevated temperature erosion and to chemical corrosion.

In refining and melting precious metals, it is important that the crucible you use provides clean metal by incorporating non-wetting properties.

2.9, Slag and Dross Removal

A dense and non-wetting crucible helps to reduce slag and dross accumulation and will make it easy to clean the crucible when empty.

2.10, Emptying the Furnace

Crucibles for melting need to be designed for easy access to the metal. This allows the furnace to hold the metal at the proper temperature that prevents wastage of power or fuel.

Crucibles for furnaces that are tilted for pouring often require integral pouring spouts that provide the reach and accuracy needed for the pour.

2.11, Atmosphere type

• Air/Oxidizing- Furnace or thermal processing system employing an air or oxidizing atmosphere. Furnace crucibles that are resistant to oxidation or burn-up at the end-use temperature should be chosen. Graphite or refractory metal crucibles may burn up if used at high temperature in an oxidizing atmosphere.
• Inert- Furnace or thermal processing system employing an inert gas such as argon as an atmosphere or shielding blanket.
• Reducing- Furnace or thermal processing system employing a reducing gas such as hydrogen atmosphere. Furnace crucibles should be chosen without constituents that can be reduced at the end-use temperature. Iron oxides or silica could be reduced to iron or silicon under high temperature and reducing conditions.
• Vacuum- Furnace or thermal processing system employing a vacuum as an atmosphere. Furnace crucibles should be chosen that do not consist of volatile or high vapor pressure components at the end-use temperature.

2.12, Capacity of melting

Melting capacities can be small, medium or large. Depending on the size of your production, you can choose the furnace crucible size that is only needed. There are 250g, 500g, 1kg and 2kg crucibles for small scale melting and 4kg to 6kg crucibles for medium scale melting and 8kg to 10kg crucibles for large melting.

2.13, Price of crucible

The cost of a furnace crucible is very important. As mentioned earlier, metals like tungsten and platinum are used as furnace crucibles, these crucibles can be very expensive to purchase. Graphite crucibles are less expensive crucibles and perform excellently for melting.

Understanding all the aspects of your melting furnace, the melting temperatures of your furnace and the size or capacity of your melting will help you to choose the best graphite crucible for your melting activities.

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