Knowledge of furnace technology is essential when working in a foundry. Metals usually melt at very high temperatures and there are different types of furnaces for different applications. Back in time, when humans were smelting ores of lead and tin they didn’t need much more heat than it would take to cook their food but as time went on, the need for something more than wood flame fires became apparent.
Let’s talk about a few types of furnaces.
These very tall furnaces mostly thought of for metalworking, are injected with pressurized gasses. They’re generally used for extracting iron and some other metals from their ores. The blast furnace can cast ingots of alloys for use in foundries that manufacture other products.
The foundries will use those metal alloys and additives when manufacturing-specific grades of cast metal. Cupola and crucible furnaces were the most common ways to forge metals for casting in the past and are still used today but electric arc and induction furnaces are more commonly used now.
A crucible is the most basic form of the metal furnace and can handle incredibly high temperatures. They are often made of ceramic and can be placed directly into the heat source/fire and filled with metal and additives. Jewelry makers and hobbyists still use crucible furnaces as well as some non-ferrous foundries, or those doing very small batch work.
These chimney-like furnaces which are filled with coal-coke and other additives. When the furnace is sufficiently hot, pig and scrap iron are added. This melting process adds carbon and other elements producing different grades of iron and steel. Electric arc and induction methods are more efficient and have replaced cupola furnaces for most applications. However, some foundries still keep with tradition and have cupola furnaces in operation.
Electric arc furnaces:
These furnaces became popular in the late 1800s. An electrical current runs through the metal inside the furnace by electrodes rather than adding external heat when melting high volumes at one time. These large furnaces can hold up to 400 tons when melting steel which is often made of heavy iron like slabs, pig iron, and recycling of things like car scraps. Once all the components are melted, the whole furnace is tilted discharging the liquid metal to a ladle which can sometimes be smaller furnaces that can keep the metal hot before casting.
These furnaces work with magnetic fields rather than with electrical arcs. Metal is charged into a crucible surrounded by a powerful electromagnet made of coiled copper. This copper coil creates a reversing magnetic field by the introduction of an alternating current. As the metal melts, the electromagnet creates eddies within the liquid causing the material to be more or less self-stirred. There’s no addition of oxygen or other gasses to the system so whatever goes into the crucible is what comes back out making it easier to control variables during melting. However, this means that an induction furnace can’t be used to refine steel. Induction furnaces are simple to operate and are commonly used for high-quality input, producing as much as 65 tons of steel at each charge.
Furnaces all have one common enemy which is steam. Even the slightest amount of water can create splashing or explosions. Everything must be dry before use, from the alloys to the tools being used. Foundry workers have to carefully check scrap metal as it could have closed areas where water could get in and get trapped. Many foundries have a drying oven to make sure that scrap and tools are free of all moisture and condensation and are bone dry before touching the casting furnace.
After the metal is melted, it will be poured into the mold. In ferrous manufacturing, ladles are often used to transfer smaller portions of the melted material from the main furnace. These many types of ladles are designed to protect foundry workers from flames, sparks or splashing while pouring.
Whatever type of alloys or types of furnaces a foundry utilizes, the basics are the same. Molten metal gets cast into the voids in the mold to create the desired shape. Someone’s concept, whether it’s a piece of jewelry or an auto part becomes a reality when the melted metal meets the mold.
That’s what we do here at Faircast Inc., one of America’s finest foundries