Metal is the material of our time. It enables architecture to become sculpture; it also expresses technological possibility as well as the time-honored characteristics of quality and permanence. This quotation is a categorical description of stainless steel. Superiority in mechanical properties, distinct characteristics, and variety of options make stainless steel a preeminent choice in material selection. Designers, architects, artisans and others alike have all found applications for this elemental composite. The discovery, as well as its discoverer, is as equally interesting.
Having left school at the age of twelve, Harry Brearley took up the job of a bottle washer in a Sheffield, England chemical laboratory. Through years of self-teaching, he quickly earned a reputation as an expert in the analysis of metallurgical problems. He later became known as the inventor of stainless steel. In 1912, Brown Firth Laboratories sought to develop a steel for the production of rifles in which the thin diameter of the barrel would not erode away. This erosion was in reaction to the extreme heat and discharged gasses created in firing.
Brearley was quickly employed and began to experiment with Chromium mixtures to reduce the atomic weight of steel so as to allow for a denser barrel. To determine the resistance to wear, he needed to etch samples of the new alloy with an acid to examine the grain structure. Finding a strong resistance to even nitric acid, Brearley realized the significance of his new mixture. Independent from his employers, he had cutlery knives formed from his rustless steel. A friend and local chef found that the new knives not only were rustproof, but also remained unstained by vinegar and dubbed them stainless steel. ) Since then, experiments with the amounts of Chromium added have expanded to numerous grades and applications.
The process of creating steels, stainless or any of its other forms begins with the basic material of Iron ore. Iron ore is a naturally occurring resource, comprised of iron, oxygen, sulfur and silicon. The iron is refined in a blast furnace, using coke (a porous form of carbon), limestone and heated air as a catalyst. In the refining process, the carbon in the coke absorbs the oxygen from the raw ore while the limestone separates the silicates. The ending product is an iron-iron carbide, also known as raw steel. )
Three distinct forms of iron are discussed in the manufacturing of steel products. These are ferrite, austenite and cementite and are essentially iron crystals formed during the cooling process. Cooled the quickest, cementite has the largest grain crystals and is the hardest, followed by austenite and the softest, ferrite. Manipulating the cooling process allows the creation of varying amounts of the different crystal types; this results in a wide array of different steel products and the properties unique to each. There are five main classifications of steel.
Closely related, the first three are low carbon, medium carbon, and high carbon. Low carbon steel, contains less the 0. 2% carbon by weight. This steel is available in rolled sheets and is often stamped into a desired shape, such as car bodies. The next class is medium carbon steel, with 0. 4% carbon content by weight. This form of steel has an equal composition of ferrite (soft) and cementite (hard) iron. A popular manifestation of this type of steel is pearlite, know for its iridescent finish. The austenite is formed in pearlite during a more rapid cooling process, making it a sub-category of medium steel.
High carbon steel, the third classification, contains nearly 0. 8% carbon. While extremely durable, it is also difficult to form and is often used it railroad rails and spikes. Manipulating the cooling process of medium carbon steel creates the fourth category of steel, marensite. The creation process is similar to that of pearlite, except that the austenite is cooled too rapidly to allow it to fully dissolve and become pearlite. The large crystal structure lends marensite extreme hardness, but this also means it is brittle and has very little impact resistance.
This disadvantage can be addressed by locally tempering the metal. Applying a heat source, such as a torch, and immersing in cool water repeatedly is often necessary for steel tools. A chisel tip or leading edge needs to be hard, but the remainder needs to retain impact resistance. Tempering has been a staple for blacksmiths for centuries, and allows marensite to be the predominate steel in producing metalworking tools. The final category of steel is the alloy steels. By adding other elements, steel can be manufactured to contain virtually any desired properties and results in the creation of countless patented subcategories.
Alloy steel must contain at maximum 1. 65% manganese, 0. 6% silicon, and 0. 6% copper by weight to be considered within this classification, though other elements may be added. U. S. Steel Corporation has recently developed Cor-Ten, comprised additionally of Cr-Si-Cu-Ni-P molecules to offer a highly durable and corrosive resistant structural steel. (1) Further manipulation of the elemental compounds result in a product custom designed for any environmental conditions, be it salt water, highly acidic atmospheres or cryogenic temperatures. The most recognized type of alloy steel is stainless steel.
Stainless steel is produced by the addition of 10-20% of both nickel and chromium in late stage refining. Due to the multitude of manipulative factors of the steel making procedure, including cooling process and elemental make-up, innumerable varieties of stainless steel are available. Three different systems are used in the nomenclature of these products. (12) The first system utilizes the metallurgical structure-referring to the dominate iron present-ferrite, austenite, cementite or duplex (equal ferrite and austenite). The second system, Grade, is the most commonly used.
This method creates a table listing the various stainless steels in order of elemental content by weight. For example, Grade 404 would have a higher content of carbon, manganese, silicon and copper (among others) than would Grade 403. The third system, Unified Numbering System (UNS) is similar to the grade method, but allows for the inclusion of products between the grade system. Grade 404 in UNS is s40400 and grade 403 is s40300, which allows for subtle variations between the two. The most common stainless steels used are those in the austenitic classification, particularly grades 304 and 316.
