Wrought iron

Wrought iron is the simplest form of iron, and is almost pure iron (typically less than 0.15% carbon). It usually contains some slag. Its uses are almost entirely obsolete, and it is no longer commercially produced. Wrought iron is very poor in fires. It is ductile, malleable and tough. It does not corrode as easily as steel.

Cast iron

Cast iron is a brittle form of iron which is weaker in tension than in compression. It has a relatively low melting point, good fluidity, castability, excellent machinability and wear resistance. Though almost entirely replaced by steel in building structures, cast irons have become an engineering material with a wide range of applications, including pipes, machine and car parts. Cast iron retains high strength in fires, despite its low melting point. It is usually around 95% iron, with between 2.1-4% carbon and between 1-3% silicon. It does not corrode as easily as steel.

Steel

The 630 foot (192 m) high, stainless-clad (type 304) Gateway Arch in Saint Louis, Missouri

Steel is a iron alloy with between 0.2 and 1.7% carbon. Steel is used extremely widely in all types of structures, due to its relatively low cost, high strength to weight ratio and speed of construction. Steel is a ductile material, which will behave elastically until it reaches yield (point 2 on the stress-strain curve), when it becomes plastic and will fail in a ductile manner (large strains, or extensions, before fracture at point 3 on the curve). Steel is equally strong in tension and compression.

Steel is weak in fires, and must be protected in most buildings. Because of its high strength to weight ratio, steel buildings typically have low thermal mass, and require more energy to heat (or cool) than similar concrete buildings. The elastic modulus of steel is approximately 205 GPa. Steel is very prone to corrosion (rust).