Normalizing :-
Normalizing is a heat treatment process used to refine the structure of the steel to improve machinability, tensile strength, structure of weld and to remove cold working strains etc. Normalizing is often used for ferrous alloys that have been austenitized and then cooled in open air. Normalizing not only produces pearlite, but also bainite and sometimes martensite, which gives harder and stronger steel, but with less ductility than full annealing.
The hardness obtained after normalizing depends on the steel dimension analysis and the cooling speed used.
Objectives of Normalizing :-
- It produces a uniform structure
- It improves tensile, impact & yield strength
- It refines the internal structure to fine grains
- It removes the internal stress formed during previous operations
- It improves structures in welds
- It produces a harder and stronger steel then full annealing
- It eliminates the carbide network at the grain boundaries of hypereutectoid steels
Normalizing Process :-
In this process steels is heated to a temperature 40 degree C and 50 degree C above its upper critical point. It is held at that temperature for a short period of time (above 15 minutes). It is cooled down to room temperature in still air. The austenite is transformed in to ferrite plus pearlite for hypereutectoid steels. In case of alloy steels the final structure will consist of sorbite plus ferrite.
Normalising is preferred where the main aim of heat-treatment is to improve the mechanical properties. when the main purpose is to attain better machinability, softening and greater removal of internal stresses, annealing process is preferred. Normalized steels are harder and stronger than annealed steels. In the normalized condition, steel is much tougher than in any other structural condition. Parts subjected to impact and those that require maximum toughness with resistance to external stress are usually normalized.