What is Normalizing in Heat Treatment of Metals ?

When steel is cold - worked the crystal structure is distorted, and the metal may be brittle and unreliable. The internal structure of hot - worked forged part may also be forged part may also be be distorted owing to being worked at a very low temperature. It can also be seen that a variable finishing temperature will result in a variable structure for forgings of the same carbon content. Likewise, if a casting is poured at a somewhat indefinite temperature and cools at different rates in different parts, it may be unreliable.

Normalizing is, therefore, used particularly for the following purposes :

1. To eliminate coarse - grained structure.

2. To remove internal stresses that may have been caused by working.

3. To improve the mechanical properties of the steel.

In addition to all these purposes, it may be used to increase the strength of medium carbon steels to a certain extent ( in comparison with annealed steels), to improve the machinability of low carbon steels, to improve the structure of welds, etc.

Normalizing is frequently applied as a final heat treatment for items which are to operate at relatively high stresses.

The process of normalizing consists of :

1. Heating the metal to temperatures within the normalizing range which is usually within 40*C to 50*C

2. Holding this Temperature for a short time (about 15 minutes.)

3. Cooling in air.

This produces a homogenous structure consisting of ferrite and pearlite for hypo-eutectoid steel, only pearlite for eutectoid steel and pearlite and cementite for hyper eutectoid steels. The normalized structure of alloy steel consists of ferrite and sorbite.

Normalized steels have a higher yield points , tensile strength and impact strength then if they were annealed, but ductility and machinability obtained by normalizing will be somewhat lower.

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