Why there is a need for annealing
In the metal fabrication process, it is important to anneal metals particularly if cold-working is required. As the amount of cold-working increases, the ductility of metal decreases and metal has to be annealed in order to increase its ductility. During the process of cold-working, damage is incurred by metal on a microscopic level that can cause its yield strength to increase and its ductility to decrease until it can no longer be cold-worked without cracking. There is only a specific amount of cold work that metal can endure and this depends upon its composition. Heat treatment in the Annealing equipment will restore the pre-cold worked stage so that metal can continue to be subjected to cold working. Metallurgical changes will occur in metal during the annealing process that results in a reduction on its yield strength but in an increase in its ductility.
Temperature in the Annealing equipment must be above the re-crystallization temperature and will depend upon the metal’s composition. During re-crystallization, new grains will form on the metal with the final grain size dependent upon annealing temperature and time. In order to obtain the right grain size, it is important to control temperature and time in the Annealing equipment and the amount of cold working prior to annealing. This is very important if metal has to undergo significant forming processes like deep drawing so that the orange peel can be avoided.
What is normalizing?
Normalizing is an annealing process that intends to leave metal in its normal state meaning it is without internal tensions and distribution of carbon. In the process of normalizing, high temperature is maintained inside the Annealing equipment until the complete transformation of austenite through air cooling. Normalizing is usually done as a post treatment to forging and pre-treatment to quenching and tempering. Since the process of annealing and normalizing is almost the same, there is a great deal of confusion on the type of heat treatment that will be applied. In normalizing, the cooling rate is slower than quenching or tempering but faster than annealing. Through normalizing, parts will gain hardness and strength that is greater than when they are annealed but less when tempered and quenched. Slower cooling rate means that the normalized sections will not be as highly stressed than the quenched sections. A moderate increase in strength is achieved but without increasing stress.
Normalizing also avoids many heat treatment problems and it should be the choice whenever there is a need for dimensional stability during manufacturing operations that can impart significant amounts of stress. However, it is important to take into account that hard carbon steel when normalized will maintain higher levels of hardness than when annealed.