Silicon Carbide – Clay Graphite or DIY – Steel Crucibles – Which Type Is Best?
The melting of metals is a serious undertaking for the home hobby foundry worker, your personal safety & well being could be jeopardised if you choose to use poorly made, thin walled mild steel crucibles.
Not everyone involved in the metal crafts, i.e. foundry work, will posses the necessary welding skills, which enable the fabrication of good quality reliable steel crucibles. The novice hobby foundry worker quite often chooses steel crucibles out of financial necessity, or simply because they may have seen pictures of makeshift crucibles made by others….
Steel crucibles can be OK if they are constructed and skilfully welded together, a good-sized melting pot can be quickly made in the home workshop for next to nothing.
But there are several shortcomings with steel crucibles.
The fierce heat & hostile environment of the gas-fired furnace promotes corrosion and eventual destruction of a mild steel crucible, the writer learnt this from hands on experience, having used a steel crucible during the start up phase of metal casting.
The steel melting pot was certainly an attractive proposition financially, and also due to the ease of which one can be quickly obtained. But, the shortcomings were apparent very early on.
The biggest drawback with mild steel crucibles is that of corrosion, and the constant flaking of rust layers that peel of the inside of the pot, this is a natural occurrence when steel is heated to a temperatures in the region of 600 to 700c Deg, during the post cooling stage, large flakes of rust or iron oxide will peel off both external & interior walls of the crucible every time it is placed in the furnace for a melting session.
Rust and scale can be easily removed from a cold crucible if it is hit sharply with a hammer or piece of steel rod, the scale will fall
away, but there is always a chance that some rust crud will remain, and it will most likely dislodge during the melting process, and end up becoming mixed in the melt.
If you have any machining experience you will realize the problems that rust (Iron Oxide) flakes embedded in the metal will cause to the tools!
Any hard foreign matter that remains in the molten metal, and subsequent castings will make machining operations almost impossible to carry out.
The time that any crucible sits idle between hobby melting sessions may be anywhere between one to four weeks duration, this
standing period is the time when heavy corrosion takes it’s toll on a steel melting pot.
Fire Coating For A Steel Crucible.
Graphite/zircon coatings can be applied to the inside of the crucible to help reduce corrosion, but it will continue to be a problem for the simple fact that corrosion is a natural oxidation reaction of iron & steel when exposed to heat and the atmosphere.
It doesn’t take very long before tiny pin holes begin to appear in the walls, or the base of the crucible (Especially if the base is made of thin steel plate), this pin hole corrosion action can be potentially dangerous to the user, a pot full of molten metal that suddenly starts to leak while transferring from the furnace to the pouring shank could easily spill onto your legs & feet inflicting serious personal injury.
This is not the kind of thing you want to have happen at any time.
Personal safety does not have a price, there are no compromises, always wear proper work boots, long heavy duty trousers, face shield and eye protection, long arm length leather (Welders) gloves, a leather apron is also a good investment.
If you are contemplating getting involved with metal casting as a long-term hobby, be prepared to spend a reasonable amount of money to purchase a high quality name brand silicon carbide, or at the very least, a clay graphite crucible.
Clay graphite crucibles are OK, but they are very susceptible to thermal shock, which means if they are heated too quickly they will crack, silicon carbide crucibles do not suffer from thermal shock.
A five to seven year service life from a high quality silicon carbide crucible is not unusual at hobby foundry level, as long as the crucible is properly cared for.
Crucible longevity will more than repay you with safe melting practices and an assurance that your foundry melting session wont be marred with accidents & interruptions due to faulty or improperly constructed equipment.
It is a difficult decision for the hobby foundry worker who is generally on a tight budget and is always anxious to get the new foundry or project up and running before all of the proper tools & equipment have been acquired.
Good general advice for any one keen to start hobby foundry work is not to be too impatient, take your time to build the best equipment, tools, etc that you can afford, and concentrate on getting everything set up, do some experimenting and practice before you choose to carry out a full melt session.
Hobby metal casting is an enormously rewarding metal craft, the simplicity of the art quite often just amazes onlookers as they watch an experienced operator convert chunks of worthless scrap into valuable metal items which are created by pouring molten metal into simple green sand moulds…
The things that can be created via the art of foundry are limited only by your acquired skills & imagination.