Machine Design
What’s the Difference Between Cast and Wrought Iron?

What’s the Difference Between Cast and Wrought Iron?

Fig. 1People might think the term “cast iron” refers to all early iron work, or that early iron work was always “wrought,” or that both may be true. They would be wrong. Actually, the basic distinction between the two is simple:

Cast iron is iron that has been melted, poured into a mold, and allowed to cool.

Wrought iron is iron that has been heated and then worked with tools. In fact, the term “wrought” derived from the past participle of the word “worked.”

But let’s dig into the details.

Cast Iron

Fig. 2“Cast iron” is a generic term that refers to a range of iron alloys, but is typically associated with the most common, gray iron. While cast iron may sound like the cast form of pure iron, it's actually an alloy containing 2 to 4% carbon, plus smaller amounts of silicon and manganese. Other impurities, such as sulfur and phosphorus, are also common.

Cast iron is formed by smelting iron ore, or melting pig iron (the product of iron ore extraction), and mixing with scrap metals and other alloys. The liquid mixture is poured into molds and allowed to cool and solidify. Due to its higher carbon content, cast iron solidifies as a heterogeneous alloy, so it contains several materials in different phases within its microstructure, which affects its physical properties.
For example, cast iron has carbon particles within its microstructure which form elongated graphite flakes when the metal cools. Graphite is low in density and hardness but high in lubricity. So it offers little in terms of structural advantage, but it does compromise the surrounding iron matrix—creating internal stress points that can lead to fractures.

Fig. 3Compared to wrought iron or steel, cast iron is brittle, hard, and non-malleable. It can’t be bent, stretched, or hammered into shape. Its weak tensile strength means that it will fracture before it bends or distorts. It does, however, have good compression strength, and was used prominently in building construction before the advent of the steel industry in the early 20th Century.

Compared to steel, cast iron has a lower melting point, and is more fluid and less reactive with mold materials, making it well-suited for casting. This is a significantly less labor-intensive process than making product out of wrought iron, so it was a prominent form of production throughout the 18th and 19th Centuries. Cast iron has largely been replaced by steel in the construction industry, but it is still used for many industrial applications.

Wrought Iron

Wrought iron is composed primarily of iron with 1 to 2% of added slag, the byproduct of iron ore smelting—generally a mix of silicon, sulfur, phosphorous, and aluminum oxides. During manufacture, the iron is removed from heat and worked with a hammer while still hot to get it into its intended final form.

Fig. 4Wrought iron is often characterized by its fibrous appearance, but it’s also softer and more ductile than cast iron. Wrought iron is highly malleable, meaning it can be heated, and re-heated, and worked into various shapes. In fact, it gets stronger the more it’s worked.
Wrought iron has a much higher tensile strength than cast iron, making it more suitable for horizontal beams in construction. In general, it strongly resists fatigue. It deforms without failing unless overloaded well beyond capacity or distorted from exposure to intense heat (e.g.., from fire).

It was widely used throughout the 19th Century in building construction, but was replaced by steel in the 20th Century. Today, wrought iron is used primarily for decorative applications.

Corrosion

Cast and wrought iron are both susceptible to corrosion when bare surfaces are exposed to oxygen in the presence of moisture. Unlike other metals that form a protective oxidative coating, iron will—given enough time—rust and flake away entirely. This can be challenging for outdoor environments where exposure to precipitation and humidity can be ongoing.

To prevent rust, iron products should be coated to prevent exposure. Paint is commonly used to protect bare metal. Powder coatings are another method and well-suited for outdoor furnishings susceptible to wear in high-traffic areas. Powder coatings are highly durable and won’t fade, chip, or crack over extended periods of time.

Brad Done, Vice President
Reliance Foundry Co. Ltd., British Columbia, Canada

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