Silicon Carbide Vs Graphite Crucible - What's The Difference

Introduction

Properties including density and heat resistance are compared between  Silicon Carbide vs Graphite Crucible. The density of graphite is about 1.8&#;2.1 g/cm³ and won&#;t be destroyed at temperatures of up to °C. Therefore, SiC crucibles have a density close to 3.1 g/cm³, and stand well at around °C. Both serve in hot furnaces. Find out what each of them serves in metal melting.

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What Is Graphite Crucible?

Graphite crucibles reach °C. That is hot! It melts Al and Cu. It is light, due to a density of 1.8 g/cm³. Its strong because it uses SiO&#; coating. This material has a heat flow of 200 W/m· K, excellent melting capability.

What Is Silicon Carbide Crucible?

Silicon carbide crucibles reach °C. But they are harder, rated 9 on the Mohs scale. The middle comparison, &#;Graphite Crucible vs Silicon Carbide Crucible,&#; shows this crucible is heavy at 3.1 g/cm³. It works in Fe and Pb foundries. This has 300 MPa strength.

Graphite Crucible vs Silicon Carbide Crucible &#; Key Differences!

· Melting Point

Graphite crucibles melt at 3,650°C, but SiC stops at 2,700°C. This requires change in the furnace. Graphite Crucible vs Silicon Carbide Crucible shows SiC heats quicker. Both melt metals differently.

High heat tasks but at different temperatures, they can run appropriately for different furnaces and alloys.

· Chemical Resistance

Strong acids are easily handled by SiC. Fluorine doesn&#;t hurt it. Here, Graphite doesn&#;t work that well. The Graphite Crucible vs Silicon Carbide Crucible comparison shows SiC resists chemicals better.

When molten salt is involved, this is useful. They can all fitting different metals or chemicals, depending what they do in furnaces.

· Material Porosity

The gases pass more easily, as the graphite has 10 percent more porosity. Gases can&#;t get in with SiC when melting metal. This helps metals stay pure. The Graphite Crucible vs Silicon Carbide Crucible comparison shows SiC&#;s porosity makes it better for purity. If graphite is used in such settings, then gases can weaken the process.

· Electrical Conductivity

SiC is electrically resistant and graphite is electrically conductive. SiC has 1.0e+06 Ω·cm, graphite 10&#; Ω·cm, and 105 S/m. In electric furnaces this is an issue. Electric arc melting depends on conductivity.

Electrical heating is better with graphite. It&#;s unsuitable as electric melting, but more than suitable for insulation. Jinsun Carbon graphite electrodes conduct electricity better for efficient arc furnace operation.

· Thermal Stability

Reaching fast heat changes up to 1,500°C, SiC is able to absorb and release extreme change in heat compared to conventional materials. When temperatures change it, graphite can crack more easily.

For this reason, SiC can operate stably under fast changes in heat. However, each material is suitable for other uses depending on temperatures in the furnace, with SiC providing best performance when there is quick change in furnace temperatures.

· Oxidation Resistance

Graphite oxidizes at 450°C, but SiC holds stronger, taking up to 1,000°C. SiC remains strong in oxygen. It has an advantage since it is in oxygen heavy environments. Graphite has to be protected from itself, otherwise it wears down faster. In oxidation resistance SiC wins again, remaining intact during hot, oxygen full processes.

· Heat Absorption

SiC absorbs less heat (at 1.23 J/g·K) than graphite (at 1.75 J/g·K), but it releases it faster. These changes melting rates. For example, furnace energy is affected by crucible heat absorption in the melting metals.

The way each material deals with heat is different. It is beneficial to know what crucible to use for a certain metal job. For high performance metal smelting, our graphite electrodes are exported to more than 30 countries.

Parameter Graphite Crucible Silicon Carbide Crucible Melting Point 3,600°C 2,800°C Chemical Resistance Moderate (Acids/Alkalis) High (Acids/Alkalis) Material Porosity Low Lower than graphite Electrical Conductivity High (Conductive) Moderate (Semi-conductive) Thermal Stability Excellent (Up to 2,500°C) Very high (Up to 2,200°C) Oxidation Resistance Low Higher (Better against air) Heat Absorption High Moderate

Table on Graphite Crucible vs Silicon Carbide Crucible &#; Key Differences!

Thermal Conductivity and Heat Resistance of Graphite Crucible vs Silicon Carbide Crucible!

· Heat Transfer Efficiency

Graphite moves heat faster at 700 W/m·K. SiC transfers heat at 360 W/m·K  Silicon Carbide vs Graphite Crucible shows that SiC melts iron (Fe) evenly. For faster heat changes, graphite is better. The different suits work on different kinds of metal like aluminum (Al).

· Maximum Operating Temperature

SiC can reach °C where graphite hits °C. As a result, Graphite is perfect for very hot tasks. Eventually they melt these things like steel. SiC handles common jobs. The difference in temperatures is key in the Graphite Crucible vs Silicon Carbide Crucible debate.

· Heat Retention

Graphite retains heat longer. SiC has 0.75 J/g·K while SiC has 0.7 J/g·K. That means SiC cools faster. How they behave towards metals like copper (Cu) is different. If heating takes longer, then graphite is an excellent help, so it is used in a large number of processes.

· Thermal Degradation

Graphite resists high heat. At °C SiC begins to breakdown. It resists up to °C. That difference makes Graphite last longer. SiC cracks in intense heat. And their lifespan depends on the heat.

· Temperature Range

Graphite is stable from room temperature to °C and working temperatures from -50°C to °C. The SiC temperature range is -20°C to °C. They do different jobs.

Durability and Mechanical Strength in Graphite and Silicon Carbide Crucibles!

· Fracture Toughness

Graphite crucible is strong at 4 MPa&#;m. It is tougher at 9 MPa&#;m with Silicon carbide (SiC) crucible. It can withstand hotter melts up to °C. Thicker walls stop the cracks. Also, this is good if pressure is psi.

· Compressive Strength

Heavy loads can be carried. Graphite crushes at 40 MPa. SiC is 300 MPa. Molten metal at over °C are protected by this. SiC crucibles are thicker. They last through melts. With kg/cm² pressure, it works.

· Wear Resistance

Wear resistance of crucibles is up to °C. Silicon carbide lasts longer. This makes it stronger than graphite crucibles. In 200 melts, SiC resists friction. Graphite Crucible vs Silicon Carbide Crucible shows SiC wears less. It achieves surface hardness of 25 GPa, high compared to graphite at 15 GPa.

· Crack Propagation

SiC crucibles are also slower growing cracks. Its thermal expansion has a value of 4.6 μm/m°C. At 7.4 μm/m°C, graphite expands. Under heat, the crucible is strong. Graphite Crucible vs Silicon Carbide Crucible tests show fewer cracks. This is good for molten metal safety, especially at °C.

· Impact Resistance

Crucibles resist impact well. Graphite absorbs 80 J energy. It absorbs 200 J. SiC crucibles are tougher due to that. When dropped, they won&#;t break easily. What&#;s more, they stay strong when temps change. At 2.1 g/cm³ weight, SiC is most effective.

Material Composition and Structure!

· Carbon Content

Graphite has 95% carbon. Its 70% is Silicon carbide (SiC). Their atoms stick tightly. Graphite has lighter C-C bonds. SiC adds silicon atoms. That makes it strong at °C. At +°C, graphite melts better. Both works differently! Graphite Crucible vs Silicon Carbide Crucible shows how carbon levels change metal heating speed.

· Grain Alignment

The SiC grains measure 12 microns. The graphite grains are bigger at 16 microns. This makes surfaces smoother because smaller grains. This helps metal not stick! In strong terms, fracture toughness for SiC is 50 MPa in contrast to graphite&#;s 30 MPa. The cracks are controlled by grain direction. That helps keep the crucibles working hard!

· Crystal Lattice

Lattice spacing of 3.35 Å of graphite. Its 7.48 Å wide and thus tougher. At °C SiC holds shape better. Silicon atoms in its bonds make it that way. Heat flows differently on a lattice structure. The design of Graphite Crucible vs Silicon Carbide Crucible shows this difference clearly.

· Molecular Bonds

This means SiC bonds have 452 kJ/mol energy. Graphite has 348 kJ/mol of carbon bond. However, the Si&#;C bonds are quite heat-crack resistant! According to delegates, graphite is good at bearing sudden heat changes. They are sturdy for the way that they react a but flexible. That&#;s both types are good for different heating jobs at high temperatures!

· Material Density

However, SiC is also denser (3.1 g/cm³). Graphite is only 1.9 g/cm³, so it&#;s obviously not very dense. That slow heat and handle more pressure. They are different than how they react to weight. Temperature at °C can be handled by the dense SiC. Graphite is lighter so it heats faster. Both materials work very hard to melt metal!

Differences in Performance in Specific Industrial Applications!

· Steel Foundries

Hot steel melts at 1,500°C. The SiC crucibles provide resistance to shocks caused by heat change. But they hold heat fast, 130 W/m·K. Both handle 15 tons of steel each day. It can survive 3,000 heating cycles. 200 kW power furnaces work better because of the crucible.

· Jewelry Casting

Gold melts at 1,064°C. Casting in SiC crucibles lasts 200 cycles. Contamination on less than 0.01% is prevented by graphite, and gold mixing has not affected the absorption of gold. 2 kw machines heat fast.

Graphite Crucible vs Silicon Carbide Crucible shows better performance in vacuum casting at 2 bar pressure. They make nice shiny rings too both.

· Aluminum Smelting

Aluminum melts at 660°C. Silicon Carbide vs Graphite Crucible helps speed the heating, up to 2°C/min. 1,200 cycles show the SiC type is more resistant to cracks. It is used in furnaces of 5 kW. The 10% more production melts aluminum a bit faster too. Crucibles hold 50 kg.

· Ceramic Manufacturing

To make ceramics, they reach 1,400°C. Fast heating measured to 3°C/s is handled by SiC crucibles. It keeps it clean, there is no metal contamination because the graphite type. They each work in kilns up to 50 liters. It survives 800 cycles at 1,200°C. That aids in making smooth ceramic.

· Chemical Processing

Reactors go up to 1,200°C. Graphite crucibles resist acids, lasting longer. SiC can also withstand high pressure &#; up to 2,500 PSI. This makes reactions faster. In 100-liter reactors, it heats up 90 W/m·K thermal flow. In 100 kW reactors, they are pretty solid at handling heat, too.

Which Crucible is Right for Your Process?

· Operating Temperature

Up to 3,000°C it can get hot, graphite. SiC itself remains at a cool 1,600°C. This affects heat flow. By managing temperature differently. SiC has thermal conductivity of 120 W/m·K, which quick cooling or heating helps. Your process&#;s speed depends upon choosing right. That&#;s how &#;Graphite Crucible vs Silicon Carbide Crucible&#; performs under heat. Each suits different needs.

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· Material Reactivity

Graphite reacts above 450°C. SiC stays safe till 1,600°C. Less reactions happen inside. The extraction rate of SiC is 0.1 µm/year. It keeps things clean. It prevents troubles of the furnace. Reactivity changes how &#;Graphite Crucible vs Silicon Carbide Crucible&#; behave with gases or chemicals. Keeping things pure means picking wisely.

· Process Duration

Graphite lasts 1,200 cycles. SiC can last 2,500 cycles. For SiC, it is a 0.5 mm/year wear rate. They stay strong for longer. Your process time is affected by each cycle. SiC is 9 on the Mohs scale for hardness. Better durability means longer process needs better durability. This makes your process smooth and economical.

· Metal Compatibility

Graphite melts steel. SiC melts copper, brass and aluminum. The porosity of SiC is 8%. Different metals work with them. Melting cleanly is about compatibility. It keeps metals pure. SiC&#;s thermal expansion is 4.3 µm/m°C. Do not contaminate with metal.

· Budget Constraints

SiC costs $100, although graphite is only $50. This impacts your money plan. They wear differently. The SiC lasts longer and in the end, it saves your money. The price of each crucible, however, dictates how it works. Save money paying for replacements. Pick wisely. The more you pay for something and the longer it takes to arrive, the more you will spend.

Conclusion

If you melt metal or hold a molten bath, chances are your operation is unique. Your particular combination of furnaces, alloys, working practices, metallurgical treatments, pouring arrangements and end products are not likely to be duplicated at any other facility. So choosing a crucible that will provide maximum performance for your operation is an individualised and complex task.

This article is designed to serve as a guide for selecting the optimal crucible for your operation. It explains the relationship between metal melting/holding operations and specific crucible characteristics. It provides support for but does not replace the need for metal melters and crucible suppliers to work closely together in the crucible selection process.

The modern crucible is a highly heterogeneous, graphite-based composite material, which relies on its material composition and control of the graphite&#;s structural alignment to achieve the performance required. Crucibles may be as small as teacups or may hold several tons of metal. They may be fixed in place within a furnace structure or may be designed to be removed from the furnace for pouring at the end of each melt. Crucibles are used in fuel&#;fired furnaces, in electric resistance furnaces, in induction furnaces or simply to transfer molten metal. They come with or without pouring spouts and in a wide variety of traditional and specialized shapes.

They also offer many different performance characteristics since each application presents a complex set of temperature, chemical and physical parameters which define the technical boundaries within which the crucible has to be designed to operate.

So how do you select the right crucible for your operation from the extensive range of crucible types and materials available to you?

The best approach is to begin with your own detailed assessment of your operations. You need to fully document and, where possible, quantify all aspects of your melting, holding and metal handling processes. These include:

• The capacity, dimensions and type of your furnace
• The specific alloy or range of alloys you melt
• The melting and/or holding temperatures you maintain
• The temperature change rate the crucible will experience
• How the crucible is charged
• The fluxes or additions used
• Degassing or refining processes
• How slag or dross is removed
• How the crucible is emptied.

These nine categories reflect the more common factors you must take into account when selecting a crucible to match your specific requirements. You also should consider any additional processes or requirements that might be specific to your operations. An example might be your ability to tolerate or your need to avoid alloy cross-contamination.

While you bring the detailed information on your own operations to the crucible selection process, your crucible supplier must contribute a high level of expertise on crucible materials, characteristics and performance. For the greatest selection, look for a crucible supplier able to offer overlapping crucible product lines suitable for each specific metal but offering different operational characteristics. Then, working together, you will be able to closely match a specific crucible to your specific requirements. Achieving this match is the key to crucible safety, performance and maximum service life.

Be aware, however, that on a practical level, there may not be a single crucible type that offers the highest level of every desirable characteristic for your application. Crucible performance characteristics often involve trade-offs. For example, the crucible with the best thermal conductivity may not also offer the best protection against thermal shock. Therefore, you should prioritise the list of crucible properties most important for your application and review those priorities with your crucible supplier.

Furnace Capacity, Dimensions and Type

The capacity, dimensions and type of furnace you use will establish most of the observable details about your crucible. For example, when you know the metal capacity your furnace was designed for, you will know what capacity your crucible should provide. Similarly, the dimensions of the space for the crucible in your furnace will dictate the dimensions and shape of your crucible. This also will determine if your crucible must include a pouring spout. But choosing a crucible to match your furnace type will give you many other less obvious factors to consider.

Fuel-fired furnaces

Fuel-fired furnaces include furnaces powered by gas, oil, propane or coke. Each of these fuels directly exposes the crucible to the heating source and each provides a different level of heat, normally measured in BTUs. Any crucible selected must be able to withstand the maximum BTUs the furnace fuel is able to apply to the crucible. In gas, oil and propane furnaces, the crucible must be able to withstand the effects of the burner flame at the base of the crucible and the crucible must be tapered to allow the flame to circulate around the crucible from bottom to top. This allows even heating of the crucible. The crucible material also 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 properties 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 offer high thermal conductivity for fast melting in gas-fired furnaces.

Electric resistance furnaces

Electric resistance furnaces provide even, all-around heating to a crucible and are ideally suited to precise temperature control in metal holding application. But they are slower than fuel-fired furnaces in melting applications. Consequently, energy efficient crucibles with high graphite content in the carbon binder are often selected to provide high thermal conductivity for faster melting in these furnaces.

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

Induction furnaces

Selecting crucibles for induction furnaces 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 crucible to the operating frequency of the furnace and to the melting application. For example, 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 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.

Removable crucible furnaces

All of the above furnace types can be designed to use removable crucibles. These crucibles can be charged while outside or when installed in the furnace, but they are removed from the furnace for pouring. Like crucibles used only for metal transfer, they are bilge-shaped or A-shaped to allow them to be lifted with tongs designed to properly support the crucible.

Furnace power limitations

A final factor to consider when documenting your crucible requirements based on your furnace&#;s specifications is power availability. In many locations, power for melting or holding might not be available at all times or might be prohibitively expensive at certain
times or at certain levels. If this is the case at your facility, it may be particularly important to select an energy efficient crucible.

Metals You Melt and/or Hold

Knowing what metals and alloys you melt or hold will tell you a lot about what characteristics you need in a crucible. Your detailed catalogue of the metals you intend to melt will help to establish the maximum temperature the crucible must support for melting and holding, will define how the metal will interact with the crucible material both chemically and physically and it will be a key factor in determining what characteristics your optimal crucible should offer. A case in point, 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 use in melting and holding aluminum and aluminum alloys, aluminum-bronze, copper and copper-based alloys, cupro-nickel and nickel-bronze alloys, precious metals, zinc and zinc oxide. Crucibles also are used in melting cast iron. Taken together as a group, these metals represent a temperature range from 400°C/750°F to °C/°F.

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

Melting and Holding Temperatures

Generally speaking, the metals and alloys you melt or hold will determine the temperature range within which your crucible must be able to operate. Crucibles must never be heated above their maximum temperature. This can lead to dangerous crucible failure. However, operating below the crucible&#;s lower temperature limit can also cause problems. For example, crucibles designed for the high temperature melting of copper-based alloys will oxidize if used at low temperatures for zinc melting.

Melting and holding practices involving metal temperatures also need to be taken into consideration in selecting crucibles. If your operations involve superheating, you will need to take the higher metal temperatures reached into account.

Rate of Temperature Change

The ability of a crucible to handle the rate of temperature change is as important as its minimum and maximum temperature limits. If your operational practices lead to frequent heating and cooling cycles for the crucible or otherwise subject it to rapid temperature changes, you will need to select a crucible that is resistant to thermal shock. Some crucible types are much better at handling rapid temperature change than others. For example, high carbon content of the graphite in a crucible imparts high thermal conductivity and non-wetability. And when that graphite forms a directionally oriented matrix, the crucible also provides high thermal shock resistance. This is critical to foundry applications where temperatures can change by several hundred degrees in seconds. Your crucible supplier can advise which crucibles provide the best resistance to thermal shock for your application.

How the Crucible Is Charged

If your furnace is always charged with molten metal, it probably does not need a crucible designed to be highly resistant to physical damage. However, if metal ingots or other heavy materials make up the bulk of your charge and they are not carefully lowered into the furnace via an automatic loading system, you may want 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.

You also will want a crucible with a durable protective glaze. Damage to the glaze from rough handling can lead to oxidation damage to the crucible. Extruded aluminum ingots often have sharp edges that cut deeply into a crucible&#;s body leading to damaging cracks.

Fluxes and Additives

All crucibles offer some level of resistance to corrosion and chemical attack. But most fluxes and other metal treatments used in melting aluminum 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 melting application involves the use of corrosive metal treatments, you certainly will want a crucible offering the appropriate level of protection against these agents.

Degassing and Refining

Degassing aluminum and aluminum alloys typically involves bubbling inert gas, usually nitrogen, through the molten bath while agitating the 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, often used along with fluxing agents, physically erodes the crucible and attacks it chemically as well. Therefore, a dense and mechanically strong crucible that is highly resistant to chemical attack is required. Silicon carbide crucibles provide excellent resistance to elevated temperature erosion and to chemical corrosion. Also, when isostatically pressed, crucibles form a random arrangement of the graphite in their structure. This contributes to creating denser products that can survive erosive and corrosive conditions more effectively.

Many refining and metal treatment processes used with other nonferrous metals also call for a mechanically strong and chemically resistant crucible.

In refining and melting precious metals, it is particularly important that the crucible you use provide clean metal by incorporating non-wetting properties. That means that the crucible must be well sealed against metal penetration. This characteristic is imparted by having a dense crucible material structure and a durable protective glaze.

Slag and Dross Removal

A dense, non-wetting crucible also will help reduce slag and dross accumulation and will make it easier to clean the crucible when it is empty.

Emptying the Furnace

Crucibles for melting and holding molten metal that is dipped out of the furnace need to be designed for easy access to the metal and with high thermal efficiency. This allows the furnace to hold the metal at the proper temperature with minimal fuel or power use.

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

Conclusion

With a full and detailed understanding of all aspects of your metal melting and/or holding operations, you and your crucible supplier will be well positioned to select a crucible product that meets your specific operational requirements and provides a consistently longer service life.

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