2.2 FerroAlloys in the development of steelsMetalist
As mentioned, old historical iron artifacts were made of almost pure iron containing only carbon as the alloying element. Properties like hardness, strength, and toughness were controlled by changing carbon content with the use of carburizing or decarburizing treatments, skills that had been mastered by blacksmiths. Historical “super steels” like “Damascus” steel and “Bulat” steel, which used Indian “Wootz” iron as a raw material, had amazing properties that are difficult to attain with modern technologies. Rather, in these steels the properties were based on complicated and sophisticated processing with a combination of high and low carbon source materials, which form a composite layered structure via forging-folding-welding tens and maybe hundreds of times (Reibold et al., 2006; Smith, 1988).
Famous Japanese katana swords were made of Tatara iron, which contained some titanium within iron sand (ilmenite FeO-TiO2) and was typically used as an iron source. Also, traditional Japanese sword masters used a folding/forging technique.
Generalized steel alloying was not possible until the 19th century. When understanding of basic chemistry, including analytical chemistry, progressed, it became possible to measure steel composition. At the same time, great advancements were attained in materials investigations and characterization.
An important tool in materials research was optical microscopy, which could reveal the microstructure of steel and find relations between steel composition, microstructure, and properties. A contemporary of Bessemer, H.C. Sorby in Sheffield, was one pioneer who examined steel microstructure (Tylecote, 1984). The value of metallography was appreciated in Europe by 1885. At that time a French research group at Creusot-Loire investigated relations between the microstructure and heat treatment of steels. The polymorphic forms of iron and different microstructures were identified (austenite, ferrite, cementite, pearlite, martensite, and, somewhat later, bainite in the 1920s). The invention of electron microscopy in the 1930s and its development as a common tool for materials scientists further improved knowledge about steel structure and phenomena influencing its structure and properties.
In most steels, carbon is the base element influencing the structure, but by adding different alloying elements, the formation of microstructures and their properties could be controlled. Further, new precipitated phases could be obtained, like carbides (with Cr, Mo, Nb, Ti, V, W, etc.), nitrides, or carbonitrides (Al, Nb, Ti, V). Edgar C. Bain, who in the early 1920s discovered the structure named bainite, and H.W. Paxton, gave a comprehensive summary of the behavior of different alloying elements in steels and their influences on microstructure and properties in the book Alloying Elements in Steel (Bain and Paxton, 1966). In the following 50 years, enormous progress was made in the development of new steels with improved performance properties. Improved knowledge of thermodynamics, kinetics, and mechanisms of precipitates formation and phase transformations in different steels has been the basis for creating new steel grades and manufacturing methods. The processes and science of heat treatments have strongly evolved. One example is the thermomechanical controlled rolling process, which can produce better mechanical properties in steels with less alloying but a cheaper and faster process route (DeArdo et al., 1990).
Because of great innovations in steelmaking processes, steels can be produced with high “purity” (a low content of impurities like S, P), a low content of interstitial atoms (N, H, and C in ultra-low-carbon grades), and ultimate cleanliness (low oxygen content, strict inclusion control, and the absence of harmful precipitates). Technological developments in processes also have given means to new evolutions in micro-alloyed and high-alloyed steels.
The requirements for the alloying materials depend not only on the need of alloying to attain the desired analysis limits, but depending on the process stage and circumstances, sometimes quite strict requirements can be set regarding the chemistry and physical state of the alloys. Consequently, a wide variety of alloy materials have become available with various compositions specified to certain purposes and in different forms (e.g., lumps, grains, powder, wire, cored wire).
Except for different carbon grades, the ferroalloys can be categorized for example, according to Al-, Ti-, Ca-, Mg-, P-, or S-contents as well as metallic impurities (Cr, Ni, Cu, Sn, As, Sb, etc.).