Iron is one of the most important building materials in the world, allowing the rapid growth of humankind. We know about the common alloys with carbon to make steel, but what different types of cast Iron are there? We’ll explain all about them in this blog post.

What is Iron?

Comprising around 35% of the earth’s mass, Iron is an abundant element found on most continents around the globe. To extract, iron ore is mined in large open cast mines, then crushed and processed to refine the ore. From there it can be smelted using large furnaces to melt the iron, once melted the impurities are removed and the iron moved on for further processing.

What is the difference between Wrought and Cast Iron?

Wrought Iron

‘Wrought’ comes from the Germanic word ‘werken’ meaning ‘to-work’, which is exactly how it’s made. Slabs of iron are hammered, rolled and formed whilst hot, to both create the shape needed and to increase the material’s strength.

The material produced is therefore malleable and with a high tensile strength, it can also be reheated and formed making modification and repairs relatively simple.

Chemical properties of Wrought Iron

Containing around 0.1% carbon, 0.25% impurities (sulfur, phosphorus, silicone) and 2% slag, which is a mix of metal and silicone oxides created during the smelting process. Whilst controlled, the chemical composition of wrought iron was generally not as stringently monitored as with modern steel alloys, leading to a slightly more variable material. Ferrous silicate present in wrought iron helps prevent excessive corrosion, meaning wrought iron products left outside form a patina that does not ordinarily require painting.

Physical properties of Wrought Iron

Due to the carbon content and working of the material, which is akin to forging, wrought iron has a very high tensile strength, around 300MPa, which is comparable to modern structural steels.

Cast Iron

Cast iron can be split into two distinct categories: Grey and Ductile, the former being the first and original version, and the latter being an updated, more versatile variant.

Most cast iron has similar properties to wrought, but contains more carbon, around 2-4%. This high carbon content gives it its characteristic properties, hardness. Cast iron is made by smelting iron ore as with wrought, but instead of physical shaping, the molten metal is poured into molds. Iron is not the first material to be cast, but lends itself well to the process, enabling the production of intricately shaped parts that have a reasonably high strength, but with a very low cost.

Chemical properties of Cast Iron

Grey or Cast Iron generally contains around 2-4% carbon, with 1-3% silicone and manganese around 0.5%. Many gray irons also contain around 0.3% Copper. Cast iron is not as resistant to corrosion as wrought iron, meaning it needs some form of coating to prevent rust. The most common type is paint, which bonds chemically to the surface of the part, preventing corrosion.

Physical properties of Cast Iron

The headline features of cast iron are hardness, abrasion resistance and ease of manufacture. Manufacturing, however, generally only pertains to initial production, as welding of cast iron is a difficult task to execute successfully. Its hardness is slightly higher than commercial low-carbon steel, being in line with stainless at around 86 rockwell.

Advantages of Cast and Wrought Iron

As with any material, both cast and wrought iron have their uses, with some applications being suitable for either.

Advantages of Cast Iron

• Cheaper: Cast iron is a cheap way to make complex parts, with the raw materials being lower than steel or wrought iron
• Complex shapes: The casting process allows repeatable, complicated shapes to be formed from molds
• Hardness: Cast iron has a high hardness and abrasion resistance level, making it perfect for using in abrasive environments
• Vibration damping – due to its molecular structure cast iron has excellent vibration damping effects
• Heat retention – great heat retention comes from the materials high density

Advantages of Wrought Iron

• Strength: the manufacturing process and composition make parts made from wrought iron suitable for structural applications such as bridges
• Re-working: Wrought iron can be heated and reworked, unlike cast iron
• Corrosion resistance: wrought iron has a natural resistance to rust, meaning for less maintenance issues and cost for finishing

Applications of Cast Iron

Cast iron has hundreds of uses across industry, and is still widely used today:

• Valve bodies – the ability to form complex shapes makes cast iron great for pipework components
• Gearbox casings on industrial equipment – the stiffness and vibration damping effect of cast iron is perfect for gear trains.
• Engine blocks – good machinability alongside heat retention and casting flexibility makes cast iron engine blocks extremely economical
• Brake rotors – the high strength and low thermal expansion allows for brake rotors that resist warping
• Cookware – heat retention makes cast iron pans and dishes perfect for baking and frying

Applications of Wrought Iron

Wrought iron has extensive uses but has fallen out of favor to mild steel, common uses that still remain are:

• Furniture – such as garden tables and chairs
• Gates and railings – ornamental railings made from wrought iron were very common in 19th and 20th century domestic architecture
• Balconies – framework and handrailing in cities such as Paris and Rome are traditionally wrought iron