Steel structures

Feb. 1, 2006
Steel, that special, engineered form of iron, is indispensable to industry, providing strength and function no other material can match. Produced since

Steel, that special, engineered form of iron, is indispensable to industry, providing strength and function no other material can match. Produced since Roman times, modern steel is now made by the technique that Henry Bessemer developed in 1855 to mass produce it in a form strong enough to replace wrought iron.

Steel is an alloy of iron and carbon, hardest when around 0.6% carbon. Other elements including chromium, copper, cobalt, manganese, vanadium, and tungsten are also added to steel to improve different characteristics, including wear resistance, toughness, corrosion resistance, and hardness.

Flavors of iron

The solid, liquid, and gas phases of material are familiar to everyone, but steel has more than just one “solid” form. The iron and carbon atoms can arrange themselves in several different ways, each permutation exhibiting different behavior. Because steel is so important, these different solid phases each have names and Greek letters assigned to them. For example, solid alpha iron — a magnetic form that is homogeneous and appears whitish — is called ferrite. Its nonmagnetic gamma form (very similar to martensite induced by quenching, for strength) is called austenite. When heated from 30° to 1,500° C, iron morphs through both to eventually settle into its delta ferrite form.

Two for one

Mixed-phase steel offers distinct benefits. Finer grains translate into higher strength with no loss of ductility, as well as easier formability. But how are multiple phases induced in one piece of steel? A process called eutectoid transformation (physically brought about by heating or annealing and quenching methods) turns one solid phase into two, both with structures and properties unique from the original. So cooled, austenite containing 0.8% carbon changes into low-carbon ferrite and cementite with 25% carbon; in effect the carbon atoms collect in the Fe3C cementite. Nuclei of small ferrite plates and cementite form at the austenite's grain boundaries and carbon diffusion takes place just ahead of the interface. Thus the pearlite plates grow to consume the austenite.

Politics of steel

Today the biggest producer of steel is China. In 2004, the country produced 272.5 million metric tons of steel, followed by Japan (at 112.7 tons) and the United States (with 98.9 tons.) China is also the biggest importer of steel, followed by the United States and Germany. In January 2002, the Bush Administration put short-lived tariffs — to 30% — on all steel imported to the U.S. Today, there are different moves to stabilize steel prices and other effects of demand fluctuation. Price regulation in India and less fragmentation of the industry (with large corporations now controlling the majority of steelmaking) are two examples. Depending on grade, one ton of steel sells for $300 to $650.

Statistics source: The International Iron and Steel Institute To comment on this article, e-mail the editor at [email protected].

American steelmaking

Pig iron — the high-carbon, less pure cousin of steel — was made in America even before the American Revolution. As one traded item not tariffed in Britain, it soon became a major export. In the 1830s, railroads and their steam-engine boilers and rail increased demand for better iron. With this jump start from the railroad industry, the drive of Andrew Carnegie plus the vast iron-ore deposits surrounding the Great Lakes came to make the U.S. world leader in steel production for more than a century. More efficient plants and lower wages abroad slowly overtook the industry in the 1970s.

Worth the effort

Plate-like pearlite is one of iron's strongest and most useful forms. A sizable phase-boundary area between its ferrite and cementite means a lot of energy goes into forming it. Still, making pearlite is efficient because the carbon atoms diffuse only one plate spacing, while the interface between the new phases and the austenite traverses whole grains — for added strength.

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