NORMALIZING | Heat treatment for steels

NORMALIZING


Normalizing is a process of heating steel to about 40 to 50℃ above the critical temperature A3 or Acm, then holding for proper time and then cooling in still air or slightly agitated air to room temperature.

● In special cases cooling rate can be controlled either by changing air temperature or air volume. After normalizing the resultant microstructure should be pearlitic. This is particularly important for some alloy steels which are air hardening for martensite forms on air cooling.

● Slower cooling rates are required for this process. Since the temperature involved in this process is more than that for annealing, the homogeneity of austenite increases and it results in better dispersion of ferrite and cementite in the final structure, which enhanced mechanical properties.

Microstructure during normalizing :

The grain size is finer in normalized structure than in annealing structure. Grain size of normalized steel is governed by section thickness. As cooling rate deffer considerably from case to core, there is variation in grain size of normalized steel over it's cross section. This process is used for rolled and forged steels, possessing coarse grains due to high temperature involved in this, which is subjected to normalizing treatment for grain refinement.

Application of Normalizing Process :

The important application of this process is it's adoption as post treatment after achieving a homogeneous structure. Generally heavy castings including ingots and rolled and forged steels.

Normalizing - Hardening - heat treatment for steels
Normalizing and Hardening

Process of Normalizing :

● Here steels is heated to a temperature which is higher than that recommend for normalizing. The casting is held for sufficient period of time so that chemical homogeneity is achieved by diffusion.

● Then followed by air cooling. In fact cooling rate are not of much important because it followed by second cycle of treatment. The aim of the second heat treatment cycle is to refine the coarse grained structure developed to high temperature heat treatment.

● Refining is done by normalizing steel at lower temperature. Some internal stresses are developed in theses heavy casting or ingots because of large section thickness which results in variation in cooling rates from case to core.

Properties after Normalizing :

Normalized steels are generally stronger and harder than fully annealed steels. Correspondingly the machinability of steels shows an improvement on normalizing. By normalizing an optimum combination of strength and soften is achieved, which is results in satisfactory level of machinability in steels. This methods of improving machinability is specially applicable to hypoeutectoid steel.

● It has been observed that carbide gets precipitated at grain boundaries and forms continuous network, particularly in hypeeeutectoid steel by annealing. This process is very effective to eliminate carbide network. Such a network is quite stable and not eliminated by annealing treatment since there is repreciation during cooling.

● Normalizing treatment is frequently applied to sterling order to achieve any one or more of the objectives like grain refinement, improvement in machinability and enhance mechanical properties such as hardness, strength and toughness.





References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.
Metallurgist.

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