The difference between iron and steel is simply that iron is an element and steel, in its most basic form, is an alloy of iron and carbon. Some may believe that “wrought iron” is, in some manner, also referring to steel since “wrought” means forged. The terms go back to the origins of ironwork, even before it was done on an industrial scale. The initial refining of iron from ore was, by today’s standards, a crude process, resulting in cast material that was full of defects and inclusions. Making “wrought iron”, the thermomechanical process of forging the cast iron, was the means to further refine and improve iron, making it more useful as a structural material.
History of Iron Production
As the production of iron turned from artisan craft to industrial process, new names were established for the end products of the smelting process, -“smelting” being the process of heating iron-bearing ore to extract the element and melt it. Once separated and molten, liquid iron was poured into molds called ingots, also termed “sows”, producing the initial form, “crude iron”. Sows were broken up into smaller pieces for further processing. From “sows” came smaller “pigs” – where the term “pig iron” originates.
A blacksmith would heat small ingots at the forge and hammer them to refine the “pig iron” into the more useful material, wrought iron, crushing the voids and dispersing the impurities. While the impurities might not be removed, the forging process redistributed large contaminant clusters to smaller sizes that had less propensity to weaken the structure of the elemental metal.
Creation of Steel
With the addition of very small amounts of carbon, added to the molten iron, the alloy known as steel was created. Dispersed carbon atoms disrupt and distort the crystal lattice of the iron which increases the mechanical properties. Subsequent thermomechanical processing such as forging was, and still is, an essential step in assuring the cast structure of the initial ingot is transformed, assuring consistent mechanical properties by dispersing clusters of impurities or alloying elements and crushing voids that would weaken the final product. The hot work of forging also drives the recrystallization of the alloy, producing a “fine grain” microstructure. This maximizes steel toughness and fatigue properties.
Steel Vs. Iron Properties
Steel is stronger than iron (yield and ultimate tensile strength) and tougher than many types of iron as well (often measured as fracture toughness). The most common types of steel have additions of less than .5% carbon by weight. Higher percent carbon additions, while increasing strength, will cause the steel to become brittle. Other elements commonly found in steel are manganese, silicon, phosphorus, and sulfur. The class of steels called “alloy steel” may also have additions of nickel, chromium, molybdenum, and vanadium. Stainless steel is a popular low carbon content steel that contains chromium. Each added element, even in small amounts, contributes additional desirable properties to the steel alloy. When the recipe for the steel alloy is controlled and the material is processed correctly, an alloy consisting mostly of iron becomes one of the most useful materials ever invented.