Much has been written about the procedure of normalizing. However, it still puzzles me as to how little industrial engineers know about the subject and what creates the steel metallurgy and mechanical properties associated with the process.
The only significant difference between the process of full annealing and normalizing is simply the rate of cooling. Annealing is generally associated with a slow cool whereas normalizing is associated with a faster cool rate.
Why is this? The reason is to make the steel “normal” in terms of: grain size; grain flow after forging; and general mechanical properties such as impact strength, notch toughness, tensile strength, etc.
Normalizing is not an exact term other than to accomplish the above. Where it is necessary to achieve specific mechanical conditions, a more controlled normalizing process is necessary.
So, what is normalizing? The process necessitates the creation of austenite. This means an elevated temperature selection that is above the upper critical line of the Iron-Carbon Equilibrium diagram by a maximum of approximately 30°C (50°F).
This is then followed by cooling in air. This is where some misunderstandings begin to creep in. Room air temperature is not always the same and can vary from location to location. Another misconception lies in regard to the cooling rate.
If the components are being normalized to achieve a specific mechanical property range, then the cooling rate should be both constant and repeatable. If the cooling rate varies, then so will the resulting mechanical properties.
If the components are being processed in a continuous furnace for normalizing and are being collected in a hopper as they discharge from the furnace, then the components that are first dropped into the collection hopper will not be cooling at the same rate as the last components. The first components into the collection hopper may well start to cool at a specific rate, but subsequent components will begin to transfer their heat loss back into the first components, which will continue as more components are dropped into the collection hopper. Uniform and repeatable mechanical properties cannot be accomplished.
The only way around this problem if repeatable mechanical properties are necessary is to: ·
- Normalize above the upper critical line of the iron-carbon equilibrium line
- Quench into oil or water to create martensite (the amount of martensite will of course depend on the carbon content of the steel being normalized)
- Followed by tempering at an elevated temperature at say 570-650°C (1060-1200°F)