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Melting and Pouring

From the casting furnace into the mold

April 16, 2018

Two electrodes at least 10” in diameter glow with heat in a metal melting furnace
High voltage electrical current is melting metal in this electric arc casting furnace.

The most dramatic images available from the foundry are those of melting and pouring. Huge furnaces, glowing with heat, transform chunks of metal into a flowing fiery liquid. When ready, their contents are poured into waiting ladles amid a shower of sparks. Workers guide the flow of metal from furnace to mold behind heat shields, guarding against the dangers of the temperature and materials. The foundry floor is where design becomes actual, in an extraordinary process that creates everyday objects.

Metal melting furnaces

The manufacture of cast metal relies on furnace technologies, since metals usually melt at very high temperatures. The first smelting of ores in human history were lead and tin, which can be melted in the heat of the cookfire. After that, metallurgy needed something more than wood flame. The first copper may have been smelted accidentally in a pottery kiln, which runs at least 200°C hotter than a campfire, but lack of written record makes it hard to be sure.

Blast furnaces, which are very tall furnaces injected with pressurized gasses, are commonly associated with metal working, but they are used for extracting iron and some other metals from their ores. Usually, a blast furnace casts only ingots of an intermediate alloy, which is then shipped to foundries involved in manufacturing.

Manufacturing foundries take metal alloys and additives and melt them to make specific grades of cast metal. Traditionally, cupola and crucible furnaces were the most common ways to forge metals for casting; in the modern day, electric arc and induction furnaces are common.

Two propane blowtorches heat a quarter cup sized crucible to melt precious metal.
A jeweler uses a crucible and blowtorches as a metal melting forge.

Crucible furnaces

are the most basic form of metal furnace. A crucible is a vessel made of material that can handle incredibly high temperatures, often made of ceramic or other refractory material. It is placed into the source of heat like a pot might sit in a fire. The crucible is filled, or charged, with metal and additives. In the modern era, crucible furnaces are still in use by jewelry makers, backyard hobbyists, some non-ferrous foundries, and foundries doing very small batch work. Crucibles can range from a very small cup where metals are melted by blowtorch, like might be done at a jeweler’s, to large vessels that contain 50lbs of material. Larger crucibles are often put inside a kiln-like furnace and can be lifted-out for pouring, or have material ladled off the top.

Cupola furnaces

are long, chimney-like furnaces which are filled with coal-coke and other additives. The fuel inside the cupola is lit, and when the furnace is sufficiently hot, pig and scrap iron is added directly. The process of melting the iron around the coke and additives adds carbon and other elements and produces different grades of iron and steel. Cupola furnaces are no longer usually used in production, as electric arc and induction methods are more efficient at producing the needed heat. However, there are some places where tradition keep the cupola furnaces running, like in this video of Da Shu Hua, where Chinese foundry workers throw molten iron against a wall to create dramatic sparks to welcome in the New Year.

Electric arc furnaces

(EAFs) came into use in the late 1800s. Electrodes run electrical current through the metal inside the furnace, which is more effective than adding external heat when melting high volumes at one time. A large EAF used in steel production can hold up to 400 tons. A “charge” of this steel is often made of heavy iron like slabs and beams, shredded scrap from cars and other recycling, and pig iron ingots from a smelter.

After the tank is filled, electrodes are placed into the metal, and an arc of electricity passes between them. As the metal begins to melt, the electrodes may be pushed farther into the mix or pulled apart to create a larger arc. Heat and oxygen might be added to speed the process. As molten metal starts to form, the voltage can be turned up, as the slag created on top of the metal acts like a protective blanket for the roof and other components of the EAF.

When everything is melted, the whole furnace is tilted, to discharge the liquid metal to a ladle below. Sometimes the ladles themselves can be smaller EAF furnaces, tasked with keeping the metal hot before pouring.

Induction 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. When the induction furnace is turned on, the coil creates a rapidly reversing magnetic field by the introduction of an alternating current. As the metal melts, the electromagnet creates eddies within the liquid that self-stir the material. The heat in an induction furnace is created by the excitation of the molecules in the iron itself, meaning that whatever goes into the crucible is exactly what comes out: there is no addition of oxygen or other gasses to the system. This means fewer variables to control during melting, but it also means that an induction furnace cannot be used to refine steel. What goes in, comes out. Like an EAF furnace, induction furnaces often discharge by tilting into ladles below.

Induction furnaces are very common and are simple to operate when given high quality input. Common models can produce 65 tons of steel at each charge.

Thin metal scraps protrude from a dirty induction metal furnace and are red with rust and heat
Like traditional metal forges, this induction furnace is open and can be charged with metal while hot.

All furnaces on the foundry floor face a fatal enemy: steam. Water, even in small amounts, can cause splashing or explosions, and so all scrap and ferroalloys, as well as every tool used in production, must be dry before use. Scrap metal must not have any closed areas in which water or vapor may have been trapped. Even the tools used by the foundry workers must be free of condensation or moisture. Many foundries have a drying oven to make sure that scrap and tools are bone dry before anything touches the casting furnace.

Casting ladles

After metal is melted, it must be introduced to the mold. In smaller foundries, this may all happen in one stage: a tilting or lift-out crucible may take metal from the furnace to the sand. However, this is impractical when a furnace holds many tons of metal. Typically, in ferrous manufacturing, ladles transfer smaller portions of the melt from the main furnace.

Two foundry workers in silver heat protective outfits pour metal into a mold using a tilting ladle
This ladle tilts to pour melted steel. The distance puts workers out of harm’s way.

In these systems, a ladle may bring metal straight to the mold. However, a transfer ladle might take the liquid to a holding tank or secondary furnace. Treatment ladles are another available type, used to break the melt into portions, like a baker might separate a basic dough to use it as the base for other recipes. For example, liquid cast iron may have agents added in the treatment ladle to make the carbon within it spherical in shape, rather than flaked, creating a more malleable metal called ductile iron.

Ladles can be very small and lifted by foundry workers or they can hold many tons of metal and need mechanical support. The largest ladles are moved through a foundry by ladle-car or by an overhead crane or track system.

Ladles of all sorts are designed to protect the worker from splash, flames, or sparks while pouring. Some ladles pour over the top lip, or a pour spout, and need to be tilted: these often have gears that allow the foundry worker to carefully control the rate of pouring. Other ladles have their pouring spout at the bottom of the bucket and the pour is controlled by removing and replacing a plug.

Mixing alloys

Metal alloys are made of mixtures of elements which are standardized by a formula that specifies the percentages of each type as well as the steps taken in its manufacture. The melting furnaces and treatment ladles of a foundry are where these alloy types are created for castings.

Foundries often specialize in either ferrous alloys, which contain iron, or specific non-ferrous alloys, like precious metals, copper-based, or aluminum-based alloys.

Ferrous alloys sort into iron and steelCast iron alloys include grey iron, which includes silicon, and ductile iron, which has a type of spherical carbon. Grades of cast steel are defined by percentages of carbon and other additives in the mix. Stainless steel is a steel that includes chromium, to help prevent rust through passivation.

Non-ferrous alloys include all other metals, so it is not surprising there is further specialization in non-ferrous foundries. Some places specialize in zinc, some in aluminum; others work primarily with copper-based alloys like brass and bronze. However, there is crossover. If a particular foundry works with both bronze and aluminum, for example, they will likely specialize in certain grades of each.

Whatever alloys a foundry works with, the premise of making molten metal and casting into voids to shape it is the same. An idea becomes actual the moment that metal flows into a mold.

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