Stainless steel families

Steels are the iron alloys with 2% maximum carbon content that constitute over the 80% of the industrial alloysAlloy is the material that is composed from various chemical elements. When it is solid, it is characterized by the participation of all the elements into the crystal structure. That is, the material is crystalline and in this crystal the atoms of its constituents are arranged in space as if they are atoms of the same kind. The crystal structure reflects the geometrical construction of a chemical element. It is called “crystal” in the case of a solid material that exhibits regular geometrical arrangement of its structural constituents. Depending on the geometrical arrangement the atoms or other particles of the element form, the crystal structure is distinguished into seven lattice systems: triclinic, monoclinic, orthorhombic, rhombohedral, tetragonal, hexagonal and cubic.. The latter is attributable to their low cost of production and the relative ease of producing steel in large quantities with accurate standards - specifications. The following three factors play a determinant role in the widespread use of steel.

(a) the high global reserves of mineralMineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure (usually crystalline), and specific physical properties. By comparison, a rock is an aggregate of mineral and/or mineraloids and does not have a specific chemical composition. (the earth crust consists of around 4% iron FeIron the chemical element symbolized by Fe is a metal with atomic number 26, melting and boiling temperatures 1535°C and 2750°C respectively. It is the forth most abundant element on the earth’s crust after oxygen (O), silicon (Si), and aluminum (Al). Moreover, it is the metal with the widest use, especially in the form of its two most important alloys, that of steel and that of cast-iron (iron alloy with carbon °C > 2,1 %).) which can be easily converted into the metallic condition, together with the availability coming from the recycling of scrap.

(b) the iron’s melting point (1539°C) allows the thermal activation of processes at temperatures (Τ.>400°C) which can be achieved relatively easily and be controlled by industrial manufacturing.

(c) the allotropy of iron and the transformation of steel’s phases (i.e. martencitic transformation) allow the formation of a great variety of microstructures which leads in turn to a respectively large range of mechanical properties. Elements that are characterized by the condition of allotropy are those that appear with more than one natural forms, due to the fact that their atoms are matched with one another in various ways (i.e. graphite and diamond constitute manifestation for the allotropy of carbon).

All the above-mentioned, make steel as the most important and popular material in mechanical engineering. Much of this stems from the allotropy of iron. The formation of the structure and the properties shown by different types of steel are achieved through industrial procedures named as heat treatments (most known being that of annealing). The greatest variety of steel microstructures appear during the transformation of austenite while it is being cooled. So, according to the allotropy of iron, we have the phase of α-Fe BCC (body centered cubic crystal lattice) dominating up to 910°C, the phase of γ- Fe FCC (face centered cubic crystal lattice) between 910 and 1400°C and α-Fe reappearing from 1400°C and to the melting point.

Stainless steel families
1. By adding nickel, the crystallographic structure changes from body-centered cubic (little or no nickel) to face-centered cubic (at least 6% nickel – 300 series).

The solid solution α-Fe with carbon is named ferrite, while the corresponding solid solution γ-Fe with carbon, austenite. Main difference between the two phases is the solubility of carbon which is much higher in the austenite than the ferrite. For example, the austenite can dissolve 2% carbon while the ferrite just 0,02% by weight.

Even though the most important alloying element in steel is carbon, in most cases several other alloying elements are added for various reasons. Thus, in most types of steel we will observe the presence of Mn and Si as well as that of Cr, Ni and Mo. The role of an alloying element is compound. It influences the solid solubility of the other alloying elements, the thermodynamic stability of the phases and in general, the formation of the steel’s microstructure together with its physical and mechanical properties. More specifically, the elements are distinguished into two categories according to their tendency to promote either the austenitic or the ferritic microstructure. The most important impacts of the alloying elements are mentioned below in brief.

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