TYPES OF HEAT TREATMENT OF STEELS

. purpose of heat treatment

. purpose of annealing

. difference between the processess of annealing and normalising

The properties of steel depend upon its composition and its structure. The properties of steel and its structure can be changed by heating it to a particular temparature and then, allowing it to cool at a definite rate. The process of heating and cooling for changing the structure of steel,and thus obtaining the required properties is called 'heat treatment of steel'.

If steel is heated to a suitable temaprature, and then, slowly cooled, the steel will be soft, weak and ductile.

If steel is heated to a suitable temaprature, and then, rapidly cooled (quenched) the steel will be hard and brittle.

Classification of heat treatments

The treatments that produce equilibrium conditions are annealing and normalising.

Treatments that produce non-equilibrium conditions are hardening and tempering (usually done in conjunction with each other)

Annealing

In this process, steel is heated to a suitable temparature depending upon its carbon content, and is held at that temparature for sufficient time, and then slowly cooled to room temparature.

The heating, soaking (holding the room temparature )and slow cooling cause the grains to become large, and so produce softness and ductility.

For annealing, hypoeutectiod steel is heated to 30° to  50°c above the upper critical temparature,and it is 50°c above the lower critical temparature for hypereutectoid steel.

Soaking time at temparature is 5mts/10 mm of thickness for carbon steel.

The cooling rate for carbon steel is 100 to 150°C/hour.

The cooling is done in the furnace itself by switching off the furnace or the steel is covered either in sand or dry lime  and dry ash.

ANNEALING TEMAPRATURE

carbon content %         temaprature °c

< 0.12                                 875 to 925

0.12 to 0.25                       840 to 970

0.25 to 0.50                       815 to 840

0.50 to 0.90                       780 to 810

0.90 to 1.3                         760 to 780

Purpose of annealing

Annealing is done

- to obtain softness

-to improve machinability

-to increase ductility

-to relieve internal stresses

-to reduce or eliminate structural inhomogenity

-to refine the grain size

-to prepare the steel for subsequent heat treament process.

Normalising

Due to continuous hammering or uneven cooling, strains and stresses are formed in the internal structure of steel. These should be removed from forgings or castins. Otherwise, they may fail at any time while in use.

Normalising is done to produce a fine grain for uniformly of structure and for improved machanical properties.

The normalising process

In this process, steel is heated to a suitable temaprature depending upon its carbon content, and held at that temparature, and then cooled freely in air.

Normalising is usually done, before machining and before hardening, to put the steel in the best condition for these operations.

The steel is heated to a temaprature (30 to 40° c above the upper critical temparature )at which all the austenite is present even in  the case of high carbon steel. This is because this process is the first step towards producing the final properties, and it is necessary to start with austenite to ensure uniformly.

PRECAUTION

The heated piece for normalising should not be kept at any wet place, in wet air or kept in forced air as they will induce some hardness