Steel

Steel fabricators, contractors and suppliers

Iron and Steel Manufacture
The differences between the various types of iron and steel are sometimes confusing because of the names used. Steel in general is an alloy of iron and carbon, often with other elements.

The earliest iron implements discovered by archaeologists in Egypt date from about 3000 BC, and iron ornaments were used even earlier. The comparatively advanced technique of hardening iron weapons by heat treatment was known to the Greeks about 1000 BC. The alloys produced until about the 14th century AD would be classified today as wrought iron. A mass of iron ore and charcoal heated in a forge or furnace was reduced to a sponge of iron filled with impurities. This sponge of iron was beaten with heavy hammers to drive out the slag and consolidate the iron.

After the 14th century smelting furnaces increased in size. The product of these furnaces was pig iron, an alloy that was further refined to make steel. Modern steelmaking employs blast furnaces. The process of refining molten iron with blasts of air was accomplished by British inventor Sir Henry Bessemer, who developed the Bessemer furnace in 1855.

The basic materials used for the manufacture of pig iron are iron ore, coal coke, and limestone. A typical blast furnace consists of a cylindrical steel shell lined with a nonmetallic substance such as firebrick. The lower portion of the furnace is equipped with tuyeres (tubular openings through which the air blast is forced). The molten pig iron flows out through a hole near the bottom when the furnace is tapped. Above this hole, but below the tuyeres, is another hole for draining the slag. The top contains vents for the escaping gases.

Blast furnaces operate continuously. The iron itself is drawn off or tapped about five times a day. The air used to supply the blast in a blast furnace is preheated. The output of many blast furnaces can be increased 25 percent by pressurizing, an important development introduced after World War II (1939-1945).

Although almost all the iron and steel manufactured in the world is produced by blast furnaces, other methods have been practiced to a limited extent. The so-called direct method produces iron and steel from iron ore and coke mixed in a revolving kiln and heated. The kiln produces so-called sponge iron of much higher purity than pig iron. Virtually pure iron is also produced by means of electrolysis. The open-hearth furnace produces higher temperatures than a blast furnace by preheating the fuel gas and air used for combustion.

In the basic oxygen process, the fuel is fed by a high-pressure stream of nearly pure oxygen blown into the furnace at supersonic speed. The oxygen combines with carbon and other unwanted elements and rapidly burns out impurities, converting the pig iron into steel. In some furnaces, electricity instead of fire supplies the heat. Because refining conditions in such a furnace can be strictly regulated, electric furnaces are particularly valuable for producing steels that must be made to exacting specifications.

The process of making the tough, flexible alloy known as wrought iron differs markedly from other forms of steelmaking. Wrought iron is no longer produced commercially because it can be replaced in nearly all applications by low-carbon steel, which is less expensive to produce and is typically of more uniform quality than wrought iron.

Steel is marketed in a wide variety of sizes and shapes. These are produced at steel mills by rolling and otherwise forming heated ingots to the required shape. In hot rolling the cast ingot is first heated to bright-red heat in a furnace and is then passed between a series of pairs of metal rollers that squeeze it to the desired size and shape. The distance between the rollers diminishes for each successive pair as the steel is elongated and reduced in thickness. German engineers have devised a continuous casting system that produces an endless steel slab less than 5 cm (2 in) thick.

Pipes are shaped by bending a flat strip of hot steel into cylindrical form and welding the edges or by piercing a solid rod of steel with a pointed metal bar that forms the inside diameter of the pipe. By far the most important coated product of the steel mill is plate for the manufacture of containers. Other processes of steel fabrication include forging, founding, and drawing the steel through dies.

The physical properties of various types of steel and of any given steel alloy at varying temperatures depend primarily on the amount of carbon present and on how it is distributed in the iron. Steels are grouped into five main classifications. More than 90 percent of all steels are carbon steels. Alloy steels have a specified composition, containing certain percentages of other elements. High-strength low-alloy steels cost less than the regular alloy steels because they contain only small amounts of the expensive alloying elements. They have been specially processed, however, to have much more strength than carbon steels of the same weight.

Stainless steels contain chromium, nickel, and other alloying elements that keep them bright and rust resistant. Some stainless steels are very hard; some have unusual strength and will retain that strength for long periods at extremely high and low temperatures. Tool steels are fabricated into many types of tools, or into the cutting and shaping parts of power-driven machinery for manufacturing. They contain alloying elements that give them extra strength, hardness, and resistance to wear.

The basic process of hardening steel by heat treatment consists of heating the metal to about 760¡ to 870¡ C (about 1400¡ to 1600¡ F) and then cooling it rapidly, creating large internal strains in the metal. These strains are relieved by tempering, or annealing, (reheating the steel to a lower temperature). Tempering decreases hardness and strength, but increases ductility and toughness. Other methods of heat treating harden the steel with carbon or nitrogen compounds; charcoal, coke, or carbonaceous gases; molten cyanide salt; or ammonia gas.