In alloy steel, varying proportions of alloying elements are used, in order to achieve desired (improved) properties such as weldability, ductility, machinability, strength, hardenability and corrosion resistance etc.  Alloy steels have two categories.

Low Alloy Steels

Low alloy steels have alloying elements less than 5%.

High Alloy Steels

High alloy steels have alloying elements more than 5%

Alloying Elements

Some of the most used alloying elements and their effects are as follows :

1. Manganese – Increases strength and hardness, decreases ductility and weldability;
2. Silicon – Used as deoxidizers used in steel making process
3. Phosphorus – Increases strength and hardness and decreases ductility and notch impact toughness of steel.
4. Sulfur –Decreases ductility, notch impact toughness and weldability. Found in the form of sulfide inclusions.
5. Copper –improved corrosion resistance
6. Nickel – Increases hardenability and Impact strength of steels.
7. Molybdenum – Increases hardenability and enhances the creep resistance of low-alloy steels