Difference Between Iron and Steel
Depending on how it is made, relatively pure iron—almost always alloyed with carbon—can take three different forms. Wrought iron, which has a low carbon content, is the easiest to make and was used first. Wrought iron can be hammered and twisted—”wrought” means “worked”—but its melting point is too high for it to be used in liquid form. Cast iron, with a higher carbon content, melts at accessible temperatures, so it can be cast—poured into molds. Steel, intermediate in carbon content, is not as brittle as cast iron and not as soft as wrought iron. Early metallurgists did not know what caused these properties, only that different manufacturing processes led to different results.
While wrought and cast iron still have important applications, nearly all the iron used goes through further processing to make it into steel.
The Bessemer process, developed in the 1850s independently and almost simultaneously by Englishman Henry Bessemer and American William Kelly, utilizes a pear-shaped, tilting converter made of plate steel; it is lined with fire brick and clay. The converter is open at the top and has a double bottom, a “wind box” perforated by holes at the top of the lower chamber. It is tilted to receive an amount of molten pig iron. As it is set upright, air is blown at a high velocity through the holes in the wind box. The air both keeps the molten material from going through the holes and burns out impurities in the iron.
The impurities can be seen burning in the flame at the top of the converter. Manganese, the first to burn, appears as short, ruddy flames out of the top. The flames turn to yellow as the silicon burns, then to white as the carbon burns. The process is done in about 20 minutes, and the molten steel is poured into a ladle. The steel still contains some oxygen from the process. This is removed by adding an alloy of iron and manganese that combines with the oxygen and is removed with the slag.
The open hearth furnace, named for the hearth or floor that is exposed to the sweep of the flames, is a rectangular, completely enclosed brick structure operating at 2,900° F (1,600° C). Open hearths once accounted for 90 percent of the steel production in the United States. They are still used extensively to process vast quantities of scrap steel. One of two burners at either end of the hearth is ignited, sweeping flame down and across the hearth. Pig iron, coke, and limestone are added, as in a blast furnace. It takes about 12 hours to produce a heat, or batch, of molten steel. A plug at the back of the furnace is knocked out and the molten steel flows into a ladle where alloying substances are added. The open hearth furnace also is used in the smelting of lead.
The electric furnace is used for steel alloys that require a large percentage of alloying metals. In this process a large electric arc delivers the heat to melt the ore. Oxygen is not required, thus keeping the oxygen content of the steel alloy to a minimum and keeping the expensive alloying metals from oxidizing.
The oxygen (LD) process was developed by Linz and Donnewitz in Austria and was introduced in the United States in 1954. It produces the same amount if not more steel by volume as the open hearth process. In the LD process, a vessel shaped like and used much in the same way as a Bessemer converter is utilized. High-purity oxygen is injected into the molten mixture from above, causing the impurities to burn off. With the injection of pure oxygen rather than air, nitrogen that makes up 78 percent of the air will not have a chance to be absorbed by the steel. Nitrogen can make the steel excessively brittle.
In vacuum processing the heating takes place in a vacuum chamber, with heat furnished by electric means. A stream of molten steel is poured into the chamber. The vacuum causes the molten ore to form into droplets from which unwanted gases evaporate. The process also prevents the contamination of the molten material by gases as it is being charged, melted, and tapped. The result is a high-strength steel used in turbine shafts, ball and roller bearings, and aircraft and spacecraft where environmental conditions vary widely.
The important alloying metals with steel include aluminum, chromium, cobalt, copper, lead, molybdenum, nickel, tin, tungsten, vanadium, and zinc. Various alloying materials along with various processes are used to produce steels of different strengths and hardnesses. These processes include normalizing, heating to high temperature to modify the submicroscopic structure; annealing, a process designed to relieve the internal strains by heating the steel to a moderate temperature, and then allowing it to cool at a slow rate; quenching, cooling at a very fast rate to cause hardness; tempering, or drawing, reheating the metal after quenching to control hardness; and case hardening, changing the chemical composition of the steel’s surface by hardening it, leaving the interior at the original hardness.