NORMALIZING PROCESS – ALL YOU NEED TO KNOW

NORMALIZING PROCESS – ALL YOU NEED TO KNOW

Normalizing is a widely popular heat treatment process for metals to increase ductility and toughness. This heat treatment is also known as normalizing annealing as the normalizing heat treatment process is very similar to annealing treatment. In this article we will learn more about normalizing; its definition, purposes, process steps, applicability, normalizing vs annealing, etc.

Normalizing is defined as a heat treatment process where a material is heated to a predecided elevated temperature, hold at that temperature for a certain period of time (usually 10-20 minutes), and then allowed to cool freely in the air to reach room temperature. The normalizing process is usually applied to metals that have been subjected to thermal or mechanical hardening processes and require their microstructure to be normalized. After normalizing, the metal gets back its ductility and the hardness is reduced.

Normalizing is often performed because another process has intentionally or unintentionally decreased ductility and increased hardness. Normalizing is used because it causes microstructures to reform into more ductile structures. This is important because it makes the metal more formable, more machinable, and reduces residual stresses in the material that could lead to unexpected failure.

Normalizing Process:

There are three main stages to a normalizing process.

Recovery stage
Recrystallization stage
Grain growth stage

Recovery Stage

During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.

Recrystallization Stage

During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form.

Grain Growth Stage

During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool to room temperature via contact with air. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the normalizing process.

Purpose of Normalizing:

The main purposes of the normalizing heat treatment on metals are:

To remove structural irregularities or impurities and defects from the metal.
To improve ductility that has been lost in some metal processing.
To reduce the hardness that has been increased by mechanical or thermal hardening processes.
To increase the toughness of the metal.
To relieve internal stresses.
To get an improvement in machinability.

To improve machinability of low carbon steels
To improve dimensional stability
To reduce banding
To improve ductility and toughness
To provide a more consistent response when hardening or case hardening.
To remove macro structure created by irregular forming or by welding.

Application of normalizing

Normalizing is the most extensively used industrial process since it is more economical to normalize the steel as against annealing. In normalizing since the cooling takes place in air, the furnace is ready for next cycle as soon as heating and soaking is over as compared to annealing where furnace cooling after heating and soaking needs 8 to twenty hours depending upon the quantity of charge. Hence in many cases annealing is replaced by normalizing to reduce the cost of heat treatment. Normalizing is adopted if the properties requirements are not very critical.

Some typical examples of normalizing in commercial practice are as below.

Normalizing of gear blanks prior to machining so that during subsequent hardening or case hardening dimensional changes such as growth, shrinkage or warpage can be controlled better.
Homogenization of cast and wrought structures
Improvement of machinability and grain size refinement of cast structures of castings
Stress relieving of castings
Cast metals and alloys are characterized by segregated, cored and dendritic structures as well as non uniform properties. Similarly wrought metal and alloys after mechanical working such as forging, rolling extrusion etc. have non uniform structure and properties. These structures and properties are made homogeneous by normalizing.
In some few cases, when the steel is hot or cold worked, it is necessary to perform a normalizing heat treatment in order to recover its original mechanical properties.
In case of normalizing heat treatment on weld metal the original as welded metal fine grained microstructure is changed to a coarse equiaxed ferrite with ferrite-carbide aggregates and the yield and tensile strength properties are considerably reduced.
It is very rare for a forging to be used without some sort of thermal treatment due to the heavy mechanical stresses impressed on the part and the variations in the microstructure. Normalizing is one of the simplest heat-treatments that can address refining (or normalizing) the microstructure and equalizing the effects of the range of temperatures the material has been subjected to during the forging operations. Normalizing forgings is very beneficial to any subsequent hardening operations
Steels that have undergone plastic deformation consist of pearlite which is irregularly shaped and relatively large, but varying in size. Normalizing is a heat treatment used on steel so as to refine its crystal structure and produces a more uniform and desired grain size distribution.