Characteristics and properties of steel
Steel has been used for over 3000 years and represents an alloy composed of iron and carbon (between 0.1 and 1.8%) and unlike cast iron it is much more malleable and workable both cold and hot .
Thanks to its extremely ductile and versatile properties, steel is used in various fields such as transport, construction, electronics, domestic sector, mechanical and manufacturing industry, chemical and petrochemical industry etc. As the carbon content increases, steel becomes more resistant to compression and traction. In this way it is possible to confer the desired characteristics to the steel so as to use it in various fields of application.
Steel production involves decarburization (decrease in carbon content) and the elimination of impurities in cast iron, with the subsequent addition of the quantity of metals necessary to obtain the type of steel desired. Two types of processes are basically used for the decarburization of cast iron: liquid charge and solid charge.
The liquid charge process treats the molten cast iron from the blast furnace through various types of converters, inside which the air is blown which reacts with the carbon of the molten cast iron by decarburizing it. The solid charge process leads to melting, through ovens, cast iron loaves and scrap iron which ensure the decarburization of the cast iron. To modify and improve the characteristics of steel, metals and elements such as nickel, chrome, vanadium, molybdenum, cobalt, silicon, sulfur, aluminum, titanium, lead, nitrogen and wolfram are added.
Carbon content in steel
Among the main heat treatments of steel there is the tempering that heats the metal up to 760-870 ° and then quickly cool it in water and oil. The rapid cooling gives hardness but at the same time creates deformations and internal tensions in the metal making it more fragile. This defect can be solved with the tempering technique that heats the steel again to a lower temperature and then cools it quickly. This process, called remediation, decreases the hardness and strength of the steel, increasing its ductility.
Annealing eliminates all the effects of hardening and gives steel a high malleability and homogeneity. In this case the metal is heated again to a lower temperature and then heated to room temperature more slowly.
Normalization involves heating the steel to a lower temperature and then letting it cool in the air. This process eliminates internal tensions and makes the structure of the casting jets more homogeneous.
Finally, with the cementation and nitriding the pieces of the metal are hardened on the surface, keeping the interior unchanged.
Thermal treatments of steel
In steel, the predominant material is iron with a carbon content generally less than 2%. We speak of low steel when there is no element above 5% and high alloy steel when there is at least one element above 5%. Based on the different percentages of carbon content, specific steels are obtained for certain applications.
For example, case-hardening steels have a carbon content of less than 0.30% and are particularly suitable for construction. They are intended for the surface hardening treatment of cementation which consists in the enrichment of carbon of the surface of the piece and subsequent hardening. This procedure confers a remarkable surface hardness and a great resistance to wear, while the low carbon content of the core guarantees high toughness values in the underlying mass.
Cr steels have an average carbon content of 1% and are particularly suitable for the construction of rollers, balls and rolling bearing rings.
Based on the quantity of carbon, various types of steel can be obtained such as hardening and tempering steels, hot-creep resistant steels, spring steels and tool steels.
Examples of steel processing to obtain finished products
Among the types of steel processing to obtain finished products, basically three can be identified: hot forging, cold forging and casting.
Hot stamping is an ancient technique for modeling metals by heating at high temperatures and plastic transformation of the raw pieces into special presses that exert pressure on a mold built to give the desired shape.
Cold stamping is a process that is applied to ferrous alloys, and in particular to stainless steels and is suitable for the production of small objects, original equipment items and automotive supplies.
Finally, there is the steel casting technique which involves moving from a solid state to a liquid state and depends on the value expressed by the various units of measurement. This process is used for the production of tubes, ingots, sheets, drawn by pins and nuts.