Alloy steels are steels whose properties mainly attributed to the presence of one or more elements other than carbon.

When the maximum range given for the content of alloying element exceed one or more of the following percentage limit,

Manganese: 1.65%

Silicon: 0.6%

Copper: 0.6%

or when Aluminum, Boron, Chromium is upto 3.99%; Cobalt, Columbium, Molybdenum, Nickel, Titanium, Tungsten, Vanadium, Zirconium, or any other alloying element are added, steel is called alloy steel.

The primary alloying elements added to steel in widely varying amounts either singly or in complex mixtures are nickel, chromium, manganese, molybdenum, vanadium, silicon and cobalt. Surface hardness of steel depends upon the carbon content but the depth of hardness depends upon the alloys present in the steel.

Alloy steel may be classified according to their chemical compositions, structural class and purpose.

Advantages and Disadvantages of Alloy Steel:

Advantages:

1. Greater hardenability
2. Less distortion and cracking
3. Greater ductility at high strength
4. Greater high temperature strength
5. Greater stress relief at given hardness
6. Better machinability at high hardness
7. High elastic ratio and endurance strength.

Disadvantages:

1. Tendency toward austenite retention
2. Cost
3. Special handling
4. Temper brittleness in certain grades.

Purpose of alloying:

1.  Strengthening of the ferrite
2.  Improved corrosion resistance
3.  Better hardenability
4.  Grain size control
5.  Greater strength
6.  Improved machinability
7.  Improved ductility
8.  Improved toughness
9.  Better wear resistance
10.  Improved cutting ability
11.  Improved case hardening properties etc.
12.  Improved high or low temperature stability.

Classification of alloy steels according to chemical composition:

Alloys steels are divided into three-component steels, containing one alloying element in addition to iron and carbon: four component steels, containing two alloying elements, etc

Classification of alloy steels according to structural class:

Alloys steels may be classified on the basis of the structure that is obtained when specimens of small cross section are cooled in air.

They are classified as:

1.  Pearlitic
2.  Martensitic
3.  Austentic
4.  Ferritic
5.  Carbidic

Classification of alloy steels according to purpose:

Alloys steels are further classified according to their use.

•  Structural Steels
•  Tools Steels
•  Steels with special physical properties
  • Quenched and Tempered Steels

  • Corrosion resistant steels

    • Stainless steels
  • Ultra-High Strength Steels
  • Heat Resisting Steels
  • Shock resisting Steels
  • Magnet Steels