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Dr. Dmitri Kopeliovich
Alloying is changing chemical composition of steel by adding elements with purpose to improve its properties as compared to the plane carbon steel.
The elements, having the same crystal structure as that of austenite (cubic face centered – FCC), raise the A4 point (the temperature of formation of austenite from liquid phase) and decrease the A3 temperature.
These elements are nickel (Ni), manganese (Mn), cobalt (Co) and copper (Cu).
Examples of austenitic steels: austenitic stainless steels, Hadfield steel (1%C, 13%Mn, 1.2%Cr).
The elements, having the same crystal structure as that of ferrite (cubic body centered – BCC), lower the A4 point and increase the A3 temperature.
These elements lower the solubility of carbon in austenite, causing increase of amount of carbides in the steel.
The following elements have ferrite stabilizing effect: chromium (Cr), tungsten (W), Molybdenum (Mo), vanadium (V), aluminum (Al) and silicon (Si).
Examples of ferritic steels:transformer sheets steel (3%Si), F-Cr alloys.
The elements like chromium (Cr), tungsten (W), molybdenum (Mo), vanadium (V), titanium (Ti), niobium (Nb), tantalum (Ta), zirconium (Zr) form hard (often complex) carbides, increasing steel hardness and strength.
Examples of steels containing relatively high concentration of carbides: hot work tool steels, high speed steels.
Carbide forming elements also form nitrides reacting with Nitrogen in steels.
The following elements have graphitizing effect: silicon (Si), nickel (Ni), cobalt (Co), aluminum (Al).
The following elements lower eutectoid concentration of carbon: titanium (Ti), molybdenum (Mo), tungsten (W), silicon (Si), chromium (Cr), nickel (Ni).
Aluminum (Al), silicon (Si), and chromium (Cr) form thin an strong oxide film on the steel surface, protecting it from chemical attacks.
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Manganese (Mn) – improves hardenability, ductility and wear resistance. Mn eliminates formation of harmful iron sulfides, increasing strength at high temperatures.
Nickel (Ni) – increases strength, impact strength and toughness, impart corrosion resistance in combination with other elements.
Chromium (Cr) – improves hardenability, strength and wear resistance, sharply increases corrosion resistance at high concentrations (> 12%).
Tungsten (W) – increases hardness particularly at elevated temperatures due to stable carbides, refines grain size.
Vanadium (V) – increases strength, hardness, creep resistance and impact resistance due to formation of hard vanadium carbides, limits grain size.
Molybdenum (Mo) – increases hardenability and strength particularly at high temperatures and under dynamic conditions.
Silicon (Si) – improves strength, elasticity, acid resistance and promotes large grain sizes, which cause increasing magnetic permeability.
Titanium (Ti) – improves strength and corrosion resistance, limits austenite grain size.
Cobalt (Co) – improves strength at high temperatures and magnetic permeability.
Zirconium (Zr) – increases strength and limits grain sizes.
Boron (B) – highly effective hardenability agent, improves deformability and machinability.
Copper (Cu) – improves corrosion resistance.
Aluminum (Al) – deoxidizer, limits austenite grains growth.
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