# What material is used to hold molten iron in a furnace?

When iron is melted, I guess it has to be transported and contained. I think the container in which it is has to be able to withstand higher temperatures than what you want to melt.

According to this webiste, "Iron, Wrought" has a melting temperature of 1482 - 1593 °C. There are a couple of other metals which have higher melting points (e.g. Wolfram (tungsten) with over 3400 °C), but all I can think of are much more expensive. So what material is the oven / "bottle" / "basin" (or however you call it) made of?

(Side question: Iron has been melted for quite a while now. I guess this has changed over the years. Of which materials was it before?)

• The "basin" is often called a crucible. What are crucibles made of? You might look at ceramics for a starting point. – user_1818839 Feb 3 '16 at 13:30
• @BrianDrummond Thank you very much! This was what I was missing. The answer is now basically in en.wikipedia.org/wiki/Crucible (easy to find when you know how it is called, very hard when you don't). Do you want to post an answer where you sum en.wikipedia.org/wiki/Crucible up or should I make a community wiki answer? – Martin Thoma Feb 3 '16 at 13:40
• Go ahead and make a Community Wiki answer. And hold off accepting it for a few hours in case someone has a more clever answer. I merely commented to give you a starting point to save waiting. – user_1818839 Feb 3 '16 at 13:42
• Graphite was often used for crucibles too since it doesn't really melt. – grfrazee Feb 3 '16 at 14:02
• Notice the outside of a crucible doesn't glow red-hot, which would be a waste of energy and weaken the metal structure. So the lining must sustain not only high temperatures but a high temperature gradient with hopefully not too much heat transfer. – Kevin Kostlan Nov 14 '16 at 23:25

# Summary

Crucibles are lined with refractory materials. Steel processing makes use of graphite or a combination of chromite and magnesite for direct contact with the melt. Cast iron processing often uses engineered clays, also known as alumina-magnesia-silica mixtures. Graphite is harder to form than clay-type refractories. To be suitable as a refractory, a material must meet a number of property requirements to be both economical and safe.

# Refractory Materials

As you noted, iron has a high-end melting point of about 1,540 °C on the far left side of the $\textrm{Fe-C}$ phase diagram below, in the form of pure iron. There are two categories of materials with higher melting points, but only a few of those materials are both economical and safe. Generally, any material with a melting point high enough to withstand the melting points of commercially used metals such as iron, copper, and aluminum are called refractory materials.

Source: ispatguru.com

## Refractory Metals (Not Useful for Foundries)

The first category of high melting point materials, of which you noted one material, are called refractory metals. Note that these are not generally referred to as refractories or refractory materials in the foundry industry. They consist of niobium, molybdenum, tungsten, tantalum, and rhenium, (Nb, Mo, W, Ta, Re) and have melting points ranging from about 2,500 °C to 3,500 °C. While the melting points are high enough and they have sufficient strength as structural materials, and some impact toughness to boot, there are a number of factors limiting their use.

• High reactivity with oxygen
• High reactivity with other metals
• High cost per weight
• High density
• High heat capacity
• High heat conduction
• Difficult to shape (requires either carefully controlled melting in a vacuum or powder metallurgy)

## Refractory Ceramics (Useful for Foundries)

The second category of refractory materials are based on a variety of ceramics, and are called refractory ceramics, or more commonly just refractories. However, not just any ceramic is suitable. Ideally the ceramic would have extremely high atomic bond strength, or higher affinity for oxygen than the metal being melted. These would make the material relatively inert with respect to the molten metal. Such a ceramic must also be easily formable, have low heat capacity and heat conduction, and should be reasonably inexpensive.

Graphite is a reasonable choice for direct contact with steel and aluminum as the carbon-carbon bond strength is very high and it has a sufficiently high melting temperature, higher than its decomposition temperature in atmosphere. Graphite is somewhat more expensive to form than alternatives, though the crucibles tend to last longer. Graphite crucibles are high strength, though as with all ceramics low impact toughness. It has low density and lower heat capacity and heat conductivity than the refractory metals. Magnesite $\left(\textrm{MgCO}_3\right)$ and chromite $\left(\textrm{FeCr}_2\textrm{O}_4\right)$ are also common steel refractories.

Other choices are those systems below the $\textrm{Fe} + \textrm{O}_2 \rightleftharpoons \textrm{FeO}_2$ line on the Ellingham diagram below.

• Chromia $\left(\textrm{Cr}_2\textrm{O}_3\right)$ can be used for some materials, but has high density and high heat capacity, as well as somewhat higher cost.

• Silica $\left(\textrm{SiO}_2\right)$ is suitable for metals and alloys with lower melting temperatures, but has low thermal shock resistance. Pure silica (fused silica) has much higher thermal shock resistance but is very expensive and difficult to form. It is used in telescope mirrors.

• Alumina $\left(\textrm{Al}_2\textrm{O}3\right)$ and magnesia $\left(\textrm{MgO}\right)$ are commonly used in cast-iron applications, where graphite is preferred for low alloy steel. Engineered clays, which are effectively specific mixtures of alumina, magnesia, and silica, are also frequently used for large-scale cast iron applications as they are very inexpensive and very easy to form in-place in 100 ton and larger applications. Additionally, cast iron has a lower melting point than steel (see the vertical line at about 4.3% carbon on the $\textrm{Fe-C}$ phase diagram and follow it up to the liquid region) and thus somewhat less strict requirements on refractory durability and reactivity.

• Generally, lime $\left(\textrm{CaO}\right)$ is not used for structural materials as it is too brittle and tends to turn to powder quickly. It is however sometimes used as a binder additive, but the industry is moving away from this as calcium attacks other refractories reducing durability. See the Ellingham diagram below: lime is lowest on the diagram.

• Titania $\left(\textrm{TiO}2\right)$ and manganese oxide $\left(\textrm{MnO})\right)$ are not generally used, though I do not know why; likely some combination of heat capacity and mechanical properties.

# Ellingham Diagram (Selecting Stable Refractories)

The way to read an Ellingham Diagram, for our purposes, is that moving up on the graph means a decreasing affinity for oxygen, while moving down means an increasing affinity. The diagonal lines with chemical equations indicate the standard free energy of that reaction (vertical axis) at the given temperature (horizontal axis). If, at a given temperature, one reaction line is above another, the higher reaction will proceed toward pure metal plus oxygen (chemical reduction) while the lower reaction will proceed toward metal oxide (chemical oxidation). Therefore, refractory materials with higher affinity for oxygen than the molten metal will be chemically stable during melting. Note that additional diagrams exist or can be made for non-oxide materials using thermodynamic principles and some experimentation, and are a harder to come by on the internet.

Source: Cambridge Ellingham Diagram Tutorial

• Unable to find the originals in a reasonable time frame. I'm sure they're in our campus library somewhere, but it'd take a bit of searching to find them. Put up secondary sources instead. – wwarriner Feb 5 '16 at 0:51

Molten ferrous metals are often handled in steel ladles with a refractory lining.

It's only since about the 1860s that any ferrous metals other than cast iron (which has a significantly lower melting point than steel) were handled in a molten state in any sort of quantity. Before that, steel production generally involved carburisation of iron or decarburisation of cast iron in a furnace and wrought iron is not a castable material.

Historically, wrought iron was produced in bloomery furnaces. These are essentially stacks of alternating iron ore and charcoal sealed with a layer of clay on the outside which are allowed to burn over a long period with an air draught coming in through a hole near the bottom. This process produces a spongy mass of metallic iron mixed with silicate slag. The mass would be repeatedly hammered while hot (but not molten) to remove porosity and create an approximately homogeneous ingot, albeit with fine laminations of silica slag—this is 'wrought iron.' The laminar structure contributes significantly to the mechanical properties of wrought iron.

Later industrial processes such as 'puddling' decarburised cast iron by stirring it with long iron rods on a bed of sand with indirect heat in a reverberatory furnace. Bloomery furnaces are able to reduce the iron oxides in the ore to produce metal but aren't hot enough to melt it in bulk.

Cast iron is produced in a 'cupola furnace,' historically built from brick although modern ones tend to be steel with a refractory lining. Charges of iron ore and charcoal (or coke) are fed into the top of the stack and molten metal accumulates in a well a the the bottom where it can be 'tapped' by punching through a clay plug. In iron smelting (from ore) these furnaces would normally be tapped straight into sand ingot moulds producing 'pig iron' which would either be remelted into cast iron components or further processed to produce wrought iron or steel.

Cupola furnaces introduce a lot of carbon into the iron (around 5%) which reduces its melting point to a temperature where it is practical to cast and as such cast iron can be produced with forced air (as opposed to pure oxygen) and at temperatures within the scope of simple refractory materials like fireclay which, not being very mechanically strong are usually used as a liner for the actual structure of the furnace/ladles.

You could just about get away with using unlined steel ladles for cast iron but the lining greatly prolongs their life and reduces the rate of heat loss from the metal between the furnace and the moulds.

The furnaces used for both smelting iron ore into pig iron and remelting ast iron are essentially similar.

As Brian Drummond noted, the "basin" is called a crucible:

A crucible is a container that can withstand very high temperatures and is used for metal, glass, and pigment production as well as a number of modern laboratory processes. While crucibles historically were usually made from clay, they can be made from any material that withstands temperatures high enough to melt or otherwise alter its contents.

A detailed answer to the question can be found in the linked Wikipedia article. The short answers are:

• Iron age: clay
• Medieval period: introduction of new tempering material for the ceramic crucibles (Mullit)
• Post Medieval: graphite

Other recycled materials such as mill scale can be used in refractories as well.

Refractories

Refractory material is made by crushing dolomite and mixing it with a flux suspension liquid or paint. Mill scale can be used as flux material that is combined with the liquid binder and ultimate used to produce the refractory material. https://en.wikipedia.org/wiki/Mill_scale

• This does not answer the question of what materials can hold/contain molten metal. – Fred Feb 5 '19 at 3:21