Silicon Carbide Crucible, Silicon Carbide Graphite Crucible for Sale

Chapter 2:

How to Choose the Best Crucible for Melting

2.1, The capacity and dimension of the furnace

There are different furnaces, so also there are different crucibles that match every furnace distinctively. When you know the type of your furnace, this will guide you in choosing the best crucible that matches your furnace. Likewise, the dimension of the space of the furnace will dictate the choice of crucible.

These crucibles can be charged while outside or when installed inside the furnace, but they are removed from the furnace when extracting molten metal. The shape of the silicon carbide graphite crucible allows them to be lifted with tongs designed to properly support the crucible.

When smelting large quantities of non ferrous metals and alloys, the silicon carbide graphite crucible is the ideal choice, as you can smelt up to 30kg of metal at once. The price of silicon carbide graphite crucible is more affordable compared with other types of crucibles. Apart from the large SiC graphite crucible sizes, there are also sizes below the 30kg metal holding capacity.

However, when smelting pure gold of 1 to 30 kg sizes, a silicon carbide graphite crucible or a graphite crucible is the best choice, this ensures that your gold remains pure even after melting.

2.2, The type of furnace

The different types of furnaces are; fuel fired furnace, electric resistance furnace and induction furnace. All these types of furnaces are designed to use removable silicon carbide graphite crucibles.

Propane torch or fuel fired furnaces: The fuel fired furnaces are powered by coke, gas, oil or propane. This type of melting furnace is not entirely safe for the environment because of the emissions that would occur when it is fired.

When using a fuel fired furnace, it is best to use a crucible that is 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 the crucible to heat up evenly.

The crucible material must also be able to resist oxidation damage from frequent exposure to flame and accommodate the rate of thermal change the crucible will experience. Good thermal conductivity 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 such as the silicon carbide graphite crucible offer high thermal conductivity in gas fired furnaces.

Electric resistance furnace: The electric resistance furnace works in such a way that heat is generated by conductors that offer resistance to the passage of a current through them. Resistance furnaces are widely used in heat treatment for heating prior to shaping and to dry or melt metals.

Electric resistance furnaces provide even and all round heating to a crucible; and they are ideally suited to precise temperature control in metal melting. An energy efficient crucible with high graphite content is best for the electric resistance furnace because it has a high thermal conductivity for faster melting. The silicon carbide graphite crucible fits into this description and it is best used for the electric resistance furnace.

Induction furnace: Crucibles for induction furnaces require a more careful consideration when compared to other types of furnace. The reason is because there are induction furnaces whose crucibles are designed to be heated in the furnace’s inductive field and are used to melt the charge. In other induction furnaces, 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 the melting application; this however prevents crucible overheating.

The silicon carbide graphite crucible is applicable for melting in an induction melting furnace, any other crucible without a great percentage of graphite material cannot be induced to generate heat in the induction furnace.

2.3, The type of metals you want to smelt

Knowing what type of metals or alloys you melt or hold will influence the choice of crucible. The silicon carbide graphite crucible is mostly used to melt metals such as brass, copper, nickel, chromium and their alloys, etc. The first thing to always have in mind when selecting a crucible is that the crucible must have a higher melting temperature than the metals you wish to melt. Silicon carbide graphite crucible can be used at 1600 degree celsius. silicon carbide crucibles can also be used to melt and hold aluminium and aluminium alloys, aluminium-bronze, copper and copper based alloys, nickel-bronze alloys, precious metals, zinc and zinc oxide. The silicon carbide graphite crucible performs higher in melting these metals because it has a higher shock resistance and higher density that prevents erosion.

To help guide further about choosing the right crucible for your application, consider answering the questions below:

What is the capacity of my melting?

What type of metals are my melting?

What type of furnace am I using?

What are the melting temperatures of the metals I want to melt?

Am I ready to keep changing crucibles every now and then or I want a long lasting crucible?

How much do I have to spend on a crucible?

Even though you are not sure of the crucible to choose despite having the knowledge of furnace capacity and dimension, you do not have to worry, our service team is always available to guide you in making a choice of crucible that best matches your furnace.

2.4, How to ensure your silicon carbide graphite crucible lasts longer

We are not only interested in guiding you through how to purchase the most suitable crucible that meets your needs, we are also interested in how you are able to maintain your crucible products. After purchasing your silicon carbide graphite crucible, it is also required that you take proper care of them to ensure that they last a very long time even after repeated use. The following practices will ensure you have a long lasting silicon carbide graphite crucibles:

• Select only the crucible that matches your application, this is the first and easiest step to ensuring a long lasting crucible use experience. You must consider the things mentioned above; such as type of furnace, capacity and type of metal, temperature changes, fluxs and additives used, etc. This contributes to a long lasting crucible.
• Maintain a careful log of crucible use and inspect the crucible often. Keeping an accurate log of crucible use is essential to the effective use and safe operations of crucibles. This practice allows you to create a history that will reveal what silicon carbide graphite cruciblelife you should anticipate in normal operations. With this information, you can set up a program of routine crucible replacement that will minimize downtime.
• Control dross build up by cleaning regularly. Dross or slag build up is bad in every way as it can affect future metals that will be melted in the silicon carbide graphite crucible. Dross has a low thermal conductivity that requires higher furnace temperatures. Dross absorbs fluxes and accelerates chemical attack on the crucible. The high expansion rate causes internal pressure on the crucible and can easily be damaged. The best thing to do is to frequently and thoroughly remove dross and slag, using a scraping tool that matches the curve of the crucible interior surface to prevent indentation of theSiC graphite crucible.
• In a fuel-fired furnace, operating the burner with excess air and direct flame impingement on the crucible surface will produce damaging oxidation of the SiC graphite crucible.In all furnaces, placing ceramic fiber between the crucible and its base or wedging ceramic fiber against the top edge of the crucible also will cause oxidation and cracking in those areas. Also, allowing a metal bath to be held at a minimal temperature for a lengthy period of time will accelerate crucible oxidation.
• Guide against chemical attack by minimizing the use of flux, flux should only be added after metal is fully molten. All crucibles, even those designed to resist chemical attack, can be damaged by flux additives. The most effective way to minimize this damage is to add flux only after the bath is fully molten and able to dilute the material. Adding flux with the solid charge allows the concentrated flux to attack the crucible surface during the melting process. It is important also to use the minimum amount of flux that is required.

• Avoid physical damage by charging crucibles correctly. Dropping a heavy casting or ingot into a SiC cruciblemay chip or crack the crucible, therefore, cutting their lifespan short. Follow proper charging practice of first loading small charge materials and then carefully lowering heavier materials on top of cushioning base. It is important also not to pack charge materials tightly into the furnace. When wedged material is heated, it expands and can crack the crucible.
• Every crucible has a maximum temperature limit. If you exceed that limit the crucible will be damaged and may fail. Keep close control of crucible temperatures. Allowing molten metal to solidify in the crucible also may damage the crucible when that metal is reheated for pouring.
• Preheating SiC graphite crucible prevents thermal shock. While some types of crucibles are designed to resist damage from rapid heating, all crucibles require careful preheating if allowed to cool between melts. Thermal shock causes cracking in crucibles heated too quickly.
• While installing a crucible is quick and easy, care must be taken to follow the instructions regarding base size, clearances between the crucible and the furnace and the initial preheating of the crucible. Too small a base may fail to provide sufficient support, and the wrong base height in a fuel-fired furnace may cause the burner flame to create an overheated and oxidized area on the crucible. Insufficient clearances between the crucible and the furnace sides and top may cause cracking when the heated crucible expands. Also, insufficient initial heating may result in crucible failure on first use.
• SiC graphite crucibles should be cleaned and stored properly after each use. Avoid stacking the crucibles into one another, this practice will damage the crucible.

2.5, Analysis common problems found in crucibles

PROBLEM

DESCRIPTION

CAUSES

A. LONGITUDINAL CRACK1. When the crucible is in use for a very long time, longitudinal and thin cracks appear on the crucible wall.1. When the crucible has completed its service life, the crucible wall becomes thinner and unable to bear excessive heat.2. A brand new crucible or first time use of crucible having cracks that run from the bottom of the crucible to the lower edge.2. Placing the cool crucible in a high temperature heat or heating of the bottom of the crucible too fast. This damage is usually accompanied by a glaze peeling.3. Longitudinal crack extending from the top edge of the crucible.

3. Caused by heating the crucible too fast, especially when the button heat is faster than the heat reaching the top.

Wedging at the top of the crucible is also likely to cause this problem.

Improper crucible tongs or knocking on the upper edge will also cause hard damage and damage to the edge of the crucible neck.

4. Longitudinal crack on the surface of the crucible (in this case, the crack does not extend to the bottom of the crucible.4. This problem is caused by internal pressure. For instance, if the cooled wedge shape casting material is put into crucible horizontally, the wedge-shaped casting material will get damaged when heated.B. TRANSVERSE CRACK1. Appears near the bottom of the crucible and may cause the bottom of the crucible to fall off.

1. It may be caused by indiscriminately throwing ingots into the crucible or hitting the bottom of the crucibles with hard objects such as iron rods.

Thermal expansion can also lead to transverse crack.

2. In the middle position.

2. It could also be caused by placing the crucible on an unsuitable crucible base.

If the crucible tongs and force is too high when removing the crucible, it will lead to a crack on the surface of the crucible.

3. When using a SA series crucible, there is a transverse crack at the bottom of the crucible mouth.

3. Caused by not installing the crucible properly.

If the refractory soil is tightly squeezed under the crucible mouth when installing the crucible; when the crucible cools and shrinks during use, the stress points will be solely on the crucible mouth, which then leads to cracks.

C. STAR CRACK1. Appears at the bottom of the crucible.1. Caused by thermal expansion.2. Appears on the side of the crucible.2. Usually caused by the expansion of the wedge after heating.D. HOLE/BREAK1. Large holes appear on the wall that are not thinned out through very long use.1. Mostly caused by throwing in ingots into crucibles indiscriminately.2. Small holes2. Usually caused by cracks.E. CORROSION1. Corrosion of the metal liquid level inside the crucible.

1. It is caused by additives and metal oxide floating on the metal surface.

Avoid excessive use of additives.

2. Corrosion appears in many places inside the crucible.2. Usually caused by corrosive substances such as additives not properly added to molten metal. Additives should be used according to stated regulations.3. Corrosion appears on the bottom or bottom edge of the crucible.3. Caused by the use of low quality slag or excessive heating temperature.4. The depression on the surface of the crucible.4. The additives infiltrate and erode the crucible outer wall through the inner wall of the crucible.F. COMPREHENSIVE PROBLEMS1. Mesh cracks on the outside1. Net-like cracks like crocodile skin, these are already too old and reached the end of its service life.2. Melting speed becomes slower than usual.2. This happens because the crucible is too old and has reached the end of its service life.3. The glaze comes off.

3. This is caused by heating a cool crucible directly in a hot furnace.

It may also be caused by heating the crucible too fast.

4. The bottom protrudes upwards and then cracks.

4. This could occur as a result of coke or slag sticking to the base of the crucible.

Also, if the crucible is placed directly on hard ground, the crucible protrudes upwards and then cracks.

5. Crack at the bottom of the crucible.5. Caused by slag expansion at the bottom of the crucible.6. The surface of the crucible appears greenish and begins to get soft.6. This is caused by long time melting above 1460 degree celsius.7. The bottom or bottom edge of a new crucible becomes detached.7. This is caused by not preheating a new crucible.8. Crucible deformation.

8. Different parts of the crucible become expanded unevenly as a result of uneven heating.

Heat up the crucible rapidly and evenly.

9. Oxidation.9. The crucible is oxidized in an oxidizing atmosphere between 600 and 900 degree celsius.10. The crucible wall becomes thinner.10. When the crucible has used its stipulated service life, stop using it.