Effects of Alloy Elements on Steels

Purpose of Alloying
The main purposes of adding alloying elements to steels can be listed as follows.
a) To improve hardening ability,
b) To increase hardness, strength and toughness,
c) To improve mechanical properties at low and high temperatures,
d) To increase the wear resistance,
e) To increase corrosion resistance,
f) To improve Magnetic Properties.
The effects of alloying elements on steel structure can be defined as follows.
Carbon (C): Provides strength and hardening ability. It reduces formability and weldability.
Chromium (Cr): It provides hardening depth, thermal strength, resistance to corrosion. Chromium is the basic alloying element of stainless steels.
Nickel (Ni): It has positive effects on hardening depth, ductility and thermal expansion. Increases the impact toughness of nickel and strength in annealed steels. Nickel is the second most important alloying element of austenitic stainless steels after chromium. The nickel content in austenitic stainless steels is between 7-20%.
Manganese (Mn): Increases strength, improves hardening depth, weldability and ductility. By binding with sulfur (S) (MnS), it prevents the brittleness (fragile FeS compound) caused by sulfur.
Silicon (Si): Improves high temperature resistance and magnetic properties; improves tensile strength and elasticity. It is included in steel as a deoxidizer.
Molybdenum (Mo): It provides thermal strength, temper brittleness, corrosion and abrasion resistance.
Vanadium (V): It provides thermal strength, resistance to tempering. It has a grain refining and carbide forming effect; increases the strength. Improves hardening ability.
Tungsten (W): It provides thermal hardness, resistance to tempering and abrasion resistance. It maintains hardness in rising temperature. Therefore, speed steels etc. are used.
Cobalt (Co): It is an alloying element used in alloyed tool steels. It is used to maintain the hardness of tool steels in hot. Provides abrasion resistance.
Titanium (Ti): It has a grain-reducing effect like vanadium. However, this effect is higher than that of vanadium.
Aluminium (Al): It is used to remove oxygen. It increases the yield strength and impact toughness. In addition, aluminum has a grain-reducing effect, it is the basic alloying element of nitriding steels. It is also used as a micro alloying element forming nitride and carbonitride in some micro alloy steels.
Lead (Pb): It decreases the rolling ability. It causes ruptures during rolling, negatively affects the surface quality. It increases the machining ability of steels, so it is used as an alloying element in free cutting steels.
Nitrogen (N): It is an undesirable element. It causes nitrogen brittleness, making its bending properties very bad.
Copper (Cu): It increases the yield and tensile strength, decreases the percent elongation and formability. Increases Corrosion resistance.
Tin (Sn): It does not affect the yield and tensile strength much, but creates problems in hot rolling.
Sulfur (S): It has little or no effect on yield and tensile strength. However, it has a great effect on the percentage elongation and toughness of the material. Sulfur significantly reduces the toughness and ductility of the material. Sulfur is an element in steel that remains from the production of steel and is removed from the structure as much as possible due to the above-mentioned undesirable properties. The sulfur content is kept high only in die cutting steels suitable for machining.
Phosphorus (P): Phosphor increases the yield and tensile strength of steel, worsens the percent elongation and bending properties too much, creates cold brittleness, and increases chip forming capability. Phosphorus is an element in steel that remains from production processes and is removed from the structure as much as possible due to its undesirable properties.
    Si Mn* Mn** Cr Ni* Ni** Al W V Co Mo S P  
Hardness ↓↓↓ ↑↑ ↓↓ - -
Strength ↑↑ - -
Yield Point ↑↑ ↑↑ - -
Elongation ↑↑↑ ↑↑↑ -
Impact Resistance - ↑↑↑ - ↓↓↓
Elasticity ↑↑↑ - - - - - - - -
Heat Resistance - ↑↑↑ - ↑↑↑ ↑↑ ↑↑ ↑↑ - -
Cooling Speed ↓↓ ↓↓↓ ↓↓ ↓↓ - ↓↓ ↑↑ ↓↓ - -
Wear Resistance ↓↓↓ ↓↓ - ↓↓ - - ↑↑↑ ↑↑ ↑↑↑ ↑↑ - -
Forgability ↓↓↓ ↓↓↓ ↓↓ ↓↓ ↓↓↓ ↓↓↓
Machinability ↓↓↓ - ↓↓↓ - ↓↓ - ↑↑↑ ↓↓↓
Tendency to Oxidize ↓↓ ↓↓↓ ↓↓ ↓↓ ↓↓ ↑↑ - ↓↓
Corrossion Resistance - - - ↑↑↑ - ↑↑ - - - - ↑↑
↑ Increases   //   ↓ Decreases   //   ↔ Unchanges   //   - Has no effect or unknown effect   //
* Austenitic Steel   //   ** Perlithic Steel

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