The Common Uses of Cast Iron

Cast iron is one of the oldest industrial materials and is still used for a huge number of applications. Although it has been around for a very long time – it was first made in China nearly 3000 years ago – cast iron is a widely misunderstood metal. A major reason for this is that its name, although familiar to many, is actually quite misleading.

Read this blog from Rapid Metals, one of the UK’s leading stockists and suppliers of all kinds of metal, including cast iron, to learn more about cast iron and the typical uses of cast iron.

What is Cast Iron?

Despite its name, cast iron is not pure iron that has been cast. What’s more, cast iron has less iron in it than steel!

Cast iron is actually an alloy – a mixture of a metallic element and another. In this case, it is iron alloyed with carbon, just like steel. While steel has a carbon content of less than 2.0 per cent, if the carbon content exceeds 2.0 per cent, the result is classified as cast iron.

In reality, most cast iron produced has a carbon content of between 2.0% and 3.5%, as higher carbon content makes the cast iron unusably soft and brittle.

Why is it Called Cast Iron?

So cast iron isn’t pure iron, and is less ‘iron’ than steel. Why is it called cast iron? It’s because of cast iron’s antiquity. It was the first iron alloy to be reliably manufactured, many thousands of years ago. Ancient wood- or charcoal-fuelled furnaces produced a lot of soot (carbon) and offered only limited control of the air supply and temperature.

This meant that iron ore processed in them inevitably had a high carbon content. Cast iron represented the form of processed iron with the lowest carbon content for millennia. That relatively low carbon content (when compared to other forms of processed iron) gave it greater fluidity, making it unsuitable for forging but ideal for casting. Hence why this particular proportion of iron and carbon came to be known as cast iron.

Later developments in metallurgy, fuels and furnace design allowed the carbon content of an iron/carbon alloy to be more carefully controlled below 2.0 per cent. These metals had such different properties from cast iron that they were given a new name – steel.

While both cast iron and steel are alloys and have the same basic constituents, just in slightly different proportions, their physical and mechanical properties are sufficiently different that they have always been classed as different materials, not just grades of the same alloy.

How is Cast Iron Made?

Given how early in human history cast iron was first produced, it’s not surprising that its manufacturing process is simple. It requires iron ore to be heated in a furnace to between 1200 and 1300 degrees Centigrade until the iron content becomes molten and separates from the other materials in the ore (which usually form slag). The molten iron is poured out of the furnace and allowed to cool and harden into ingots. This is the form that cast iron is stored and transported in.

When needed to make finished products, the cast iron ingots are reheated and melted again. The liquid metal is then poured and cast into the final moulds, forming whatever the required shape. In this second melting and pouring stage, other materials can be added to the molten cast iron to give it particular properties. For instance, graphite nodules will improve the resulting metal’s ductility, while adding graphite flakes improves machineability.

Modern processes have advanced on this traditional method, and cast iron is now usually produced as part of the steel-making process. Cast iron is now made in a blast furnace, where iron ore, coke and limestone are processed at temperatures of around 1700 degrees Centigrade. Most of the slag and impurities are either burnt, boiled or floated off the molten metal. This will result in an alloy with around 4 per cent carbon content. This molten metal is ‘tapped’ from the blast furnace. Most goes on to be further processed to make steel, but for cast iron, it is transferred to an electric cupola furnace for further heating and refining, to become cast iron ingots.

What are the Properties of Cast Iron?

Although no longer the premier metal alloy available, cast iron remains a widely used and commonly encountered material. Even within the array of modern alloys, cast iron has many desirable properties. These include:

• The castability that gives it its name
• Good fluidity
• Excellent machineability
• Good compressive strength
• Strong wear resistance
• Low cost in bulk

Limitations of cast iron include relatively low tensile strength and high brittleness.

Uses of Cast Iron in Construction

When mass production of high-quality cast iron became possible in the 18th century, it led to a boom in the use of cast iron in construction. Cast iron offered greater strength in compression than wood and was lighter for a given load capacity. It could be cast into specific, complex shapes at low cost.

Cast iron’s poor tensile strength means it doesn’t resist torsional, tension or bending loads well, which limits its suitability as a structural material. In the 19th century, there were several infamous collapses of buildings and bridges where cast iron was used incorrectly.

However, cast iron’s good compressive strength means it is well-suited to bearing steady, unidirectional loads in compression, such as in vertical pillars. Steel and concrete quickly replaced cast iron as a primary structural material when they were developed.

But because it can be easily formed into complex shapes, cast iron was still a popular choice for decorative architectural elements that did not handle significant loads, such as railings, fences, balconies, facades, crests and finials. It also remained a popular choice for making greenhouses and conservatories, since it could be cast into lightweight, graceful shapes and didn’t have to support heavy loads.

Common Uses of Cast Iron

One of the breakthroughs in modern cast iron came in the 18th century when Abraham Darby developed a way of making thin, lightweight pots and kettles from cast iron. Cookware is still a very common use of cast iron, with skillets and pans of cast iron being preferred by many professional and keen cooks because of their durability and the way they evenly distribute and retain heat.

Another use of cast iron that most of us will see every day is for manhole covers, which are still often made of cast iron because it can be cost-effectively cast and finished to the required strength, has good inherent corrosion resistance and can withstand the load of passing traffic.

The automotive industry remains a large user of cast iron. The material’s combination of strength and machineability with low cost makes it a typical choice for engine blocks and cylinder heads, as well as parts like brake discs where durability, the ability to be mass produced in volume at low cost and temperature resistance are required. Other casings, shafts, gears and chains may be made of cast iron.

Cast iron is still chosen for structural purposes, especially columns, to take advantage of its compressive strength. Decorative fittings and components are also still often made from cast iron, either in restoring historic properties or in contemporary forms. Another domestic use of cast iron is in stoves, fire grates and fireplace surrounds, because of its durability, ease of being formed into complex and attractive shapes and its heat retention properties.

Cast Iron from Rapid Metals

Rapid Metals can supply high-quality cast iron, as G250 (also classed as BS1452) grade continuous cast grey iron bar.