ANNEALING AND ITS TYPE -- HEAT TREATMENT PROCESS

MECHANICAL TIPS AND STUDY MATERIAL

ANNEALING OF STEEL

Heating of steel to high temperature (austeniting temperature) and henceforth cooling it slowly is called annealing. Austenite is carbon or ferric carbide in iron that is found in steel at high temperature.

Most important advantage of annealing is to improve the machinability and to enhance ductility. Some other advantage like removal of internal stress, decrease hardness and refinement of grain size is also possible with annealing.

In annealing  after heating, rate of cooling of work piece is slow, out of which alloy steels are cooled at lower rate as compared to C- steels. On an average cooling rate is of order 40 degree/hour. On cooling soft spheroidal carbide is obtained  which counts for good ductility and low hardness.

TYPES OF ANNEALING

  • Complete/Full annealing
  • Spheroidise annealing
  • Diffusion annealing
  • Process annealing

1. COMPLETE/FULL ANNEALING

For hypoeutectoid steels ---> Heating it at 70 degrees above upper critical temperature (UCT) and holding it or maintaining it at this temperature for a while so that complete body acquire same temperature and then allowing it to cool. During this time austenite is converted into ferrite and pearlite.

For hypereutectiod steels ---> Heating it at 70 degrees above the lower critical temperature (LCT) and maintaing this temperature for some time so that each and every part acquire same temperature (equilibrium temperature) and henceforth allowing it to cool at slow rate. Here austenite decomposes to form cementite and pearlite.

2. SPHEROIDISE ANNEALING.

Steel is heated above the lower critical temperature  and them slow cooling is actuated.
In all annealing process the basics remain same, the only difference is its application. This process is employed for components which are hard to machine like high carbon steels. Carbides (Spheroids of spherical shape)  are formed on the surface of steel during cooling session.

3. DIFFUSION ANNEALING

For steels having chemical inhomogeneity diffusion annealing is prompt option.Complete refinement of structure is possible in this process. This process is also called HOMOGENIZING
On heating the specimen to around 1160 degrees diffusion proceeds. Then it is held at this temperature for small interval followed by cooling the specimen at around 800 degrees and further cooled by air.

4. PROCESS ANNEALING

Heating the specimen below the lower critical temperature. Cold working effects are eliminated and softer  output is achieved. Ductility is also improved. Internal stresses is reduced and plasticity is increased due to recrystalisation. In recrystalisation the deformed grains are re-oriented.
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