Click to enlargeWrought Iron, Steel and Cast Iron

Introduction
These three metals are all different, and material and names are not interchangeable. Wrought iron is a type of metal; today it also (unfortunately) refers to a style of metalwork. Most "wrought iron" fences, gates, railing, fireplace set, etc. that are made today are made from steel. Usually a mild or low carbon steel. So let's discuss each metal and it's uses.

Property

Steel

Wrought Iron

Cast Iron

Base Metal

Iron (Fe)

Iron (Fe)

Iron (Fe)

Carbon Content

Low to high

None

Very high

Malleability

Highly

Highly

Little to none

Corrosion resistance

Alloy dependent

Highly resistant

Little resistance

Usage

Ubiquitous

Rare

Special applications

Application

Almost everywhere

Restoration, period correct production

High temperature and abrasion

Best feature

Alloying gives it great versatility

Easy to weld, has a grain,

Heat and wear resistant

Worst feature

Oxidation

Very expensive

Brittle



Cast Iron
Most people are aware that the old wood and coal stoves were made from cast iron. Because the metal has a very open grain it is able to withstand the heat without melting or warping, even though it has a lower melting point than steel. Because cast iron is so hard, to the point of being brittle, it is also good for high abrasion environments. But because of its brittleness, it is very susceptible to breakage. Welding cast iron usually requires a pure nickel rod; it can not be forge welded. Repair can also be made by brazing.

Cast iron is made in a foundry where the metal is heated to a liquid state and poured into a mold. This differs from the blacksmith's work where the metal is heated to the plastic state, but never melted into a liquid.

Cast iron has a very high carbon content, is brittle like glass, can be very heat resistant and because it is so hard, is often used in high abrasion environments. It can not tolerate shock, which would cause cracking.

Ductile iron is similar to cast iron but because of a different grain structure is far less brittle.

Wrought Iron
Almost pure iron, the only other component is the slag imbedded in the iron. As the iron is gathered in the bottom of the smelting furnace it picks up bits of slag that become long and thin like strands as the metal is put though the rolling mill to make bar stock. As can be seen in the photograph of the anchor, wrought iron has grain, similar to wood. This grain structure must be respected, as any woodworker knows. A metal bar will split on the end just like boards will if you drive a nail too close to the end. Anchors like the one above still exist today because real wrought iron will rust 7 times slower than low carbon steel. This is partially due to the silica slag and partially due to the pure iron.

Steel
While steel has been around for a very long time, its use became widespread only after Samuel Bessemer created a way to make large quantities of steel very quickly and cheaply. He first showed his process in 1856, but it was after the industrial revolution was in full swing before its use became widespread. There were a few problems to overcome long the way. John Deere made a name by creating the turning plow. It was able to cut through the prairie grass because he added a cutter wheel and used a steel blade. Both modifications reduced the drag of the plow, making it possible for the horses to pull bigger plows. Acceptance was slow in some areas because the highly polished blade was thought to be bad for the soil.

A low carbon steel is very malleable, easy to work with and fairly responsive. High carbon steels are much harder and are often used for tools. Steels can be hardened, annealed and tempered.
• Harden: heat to bright red (exact temperature depends on the alloy) and quench (cool quickly)
• Anneal: heat to the same temperature and cool slowly, possibly over several days
• Temper: done after hardening, by reheating to a low temperature to remove brittleness and add toughness


Wrought Iron and Steel
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