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What is Hardening in Heat treatment of metals ?

 The operation of hardening is applied to all tools and some important machine parts intended for especially heavy duty service as well as to all machine parts made of alloy steel. 

The Purposes of hardening with subsequent tempering are :

1. To develop high hardness to resist wear and to enable it to cut other metals.

2. To improve strength, elasticity, ductility, and toughness. 




The process consists of :

1. Heating the steel to a temperature above critical point.

2. Holding at this temperature for a considerable period.

3. Quenching (rapid changing) in water, oil or molten salt bath.

Hypo-eutectoid Steels are heated from 30*C to 50*C above high critical temperature while hyper eutectoid steels are heated about the same amount above lower critical temperature. In first case, ferrite + pearlite , and in the second, pearlite + cementite , are transformed into austenite upon heating. A considerable part of  the cementite is retained.

Rapid cooling should enable the austenite to be supercooled to the martensitic point. 

Alloy steels are high speed steels are heated for hardening to about 1100*C to 1300*C , and cooled in a current of air. The alloying elements increase the stability of austenite and retard the decomposition of austenite into martensite. This means that with less drastic speed of cooling the same harder (martensite) constituent can be obtained as would be developed in a carbon steel by a faster rate of cooling. 

Cooling treatment and Structures Obtained in the Iron :

Water Quench - Martensite

Oil Quench     - Very fine pearlite 

Air cool          - Fine Pearlite 

Furnace Cool  - Coarse pearlite 



REQUIREMENTS FOR HARDENING :

It will probably be advantageous to summarize at this time several of the more important requirements in the hardening of steel. They are given below :

1.Composition, carbon and alloy content. 

2. Homogeneity of the austenite.

3. Grain size of the austenite.

4. Heating rate and time.

5. Quenching medium.

6. Quenching rate.

7. The size (mass of the part).

8. Surface Condition.

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