When certain special properties are required, some alloying elements are added to carbon steels. The alloying element may be Ni, Cr, Mn, Va ,W etc. The steel thus obtained are called alloy steels. The purposes of alloying elements are to improve the following properties:
- Hardness, toughness, and tensile strength
- Wear resistance
- Corrosion resistance
- Ability to retain shape at high temperature
- Resistance to distortion at high temperature
- Fine grain size to steel
- Case hardening
- Cutting ability
Nickel – Nickel is added to steel to:
- Increase toughness
- Increase tensile strength, yield strength
- Improve response to heat treatment
- Improve corrosion resistance
- Improve forming(shaping) properties of stainless steel
Chromium – Chromium is added to steel for following purposes.
- Increases tensile strength, hardness and heat resistance
- Provide forming properties of stainless steel
- Decreases malleability of steel
- Improves corrosion resistance (about 12% addition)
Tungsten – Tungsten is added to steel or steel alloys to produce high speed steels with special heat resisting qualities. Tungsten is a carbide forming element and quantities of 0.5% may be used for steels employed in production of dies, but the quantity added may be as high as 20% in the case of special alloy, i.e. high speed steels. Magnet steel may be produced, from steels containing 5 to 7% Tungsten, but cobalt steel is more efficient for the purpose. Tungsten has the highest melting point of all metals, i.e. 6098ºF. When alloyed with steel causes a very dense and fine structure, increase hardenability, stabilizes the martensite and imparts the property of red hardness. Tungsten steel retains cutting edge even at 1300ºF.
Carbon – It increases tensile strength and hardness. It decreases ductility and weldability. It affects the melting point of the material.
Columbium – When added to steel, it minimizes the intergranular corrosion. It is added specially in stainless steel.
Lead - When added to steel, it imparts machineability.
Lithium – It is a powerful deoxidizer. It increases the fluidity of stainless steel.
Sulphur – It increases the machineability, but weakens the steel if the content is more than 0.06 percent and the steel becomes “red short”. Sulphur should not be present in steel above 0.06 percent, or the steel will become brittle at forging temperatures.
Phosphorus - Phosphorus is not desired in steel as it causes cold short, i.e. it is the condition of iron or steel in which it cannot be worked by hammering or rolling at or below dull red heat. But this impurity cannot be completely removed, hence the percentage is regulated to 0.06%. Small amount of phosphorous however increases the strength and also it helps in resisting corrosion.
Nitrogen - It forms hard nitrides with Aluminium. Therefore, nitrogen hardening is used for nitralloys.
Uranium - It is a good deoxidiser and increases the elastic limit and strength of the steel. It possesses good strength and a high corrosion resistance, poor thermal conductivity and a high coefficient of expansion.
Zirconium – It is a powerful deoxidiser and desulphuriser. Steels can be made without manganese by the use of it. It reduces fatigue in steels.