Sunday, September 11, 2011

Brief History of Steel.

To understand metals, start with iron. As a basic chemical element Fe, iron is the most abundant metal on Earth. For many centuries, iron furnaces have heated limestone and iron ore that was excavated from the ground. The intense heat melts the both the rock and iron ore, along with several chemical reactions and the lighter liquid rock rises to the top and the heavier liquid iron sinks, creating pig iron. This pig iron is an intermediate step on the way to a final product.

Historically, wrought iron was a building product made from this pig iron. The wrought iron was mostly pure iron (with some slag and small amounts of carbon added). Wrought iron was actually “wrought” (i.e. worked or hammered) into bars and has been used as a construction material for thousands of years. Wrought iron is tough and ductile, easy to weld. Lacking the carbon content for tempering, wrought iron is not hard enough to hold a good edge for a tool or weapon.

Other final products from the pig iron are alloys. An alloy is a combination of two or more elements, in which at least one is a metal. Most metals used for construction purposes are alloys. For example, steel is an alloy with iron and carbon being the primary elements. Generally, iron-carbon alloys with up to 2.1% carbon by weight are considered steel and iron-carbon alloys with greater amounts of carbon are cast iron.

Cast iron is made by re-melting pig iron in a blast furnace, removing undesirable elements like phosphorus and sulfur, adjusting carbon levels and adding other elements. The resulting alloy, commonly called grey cast iron, has a high corrosion resistance and strong compressive strength, but tends to be brittle and difficult to weld. Historically, cannons and cannon balls were made from grey cast iron, as well as some early bridges.

Steel is an alloy that finds tremendous number of uses in today’s construction world. Hot rolled steel shapes, most commonly found as steel beams and columns on construction projects, are created in steel mills by rolling the heat steel between large rollers, deforming the steel into the typical shapes: W, S, C, angles, tube sections, pipes, etc. Most hot formed steel is either 36,000 psi or 50,000 psi yield strength.

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