The next question to be asked is, "How do we cool after normalizing?" The generally accepted method of cooling is to “cool in still air and free from drafts.” While this is a general statement and a generally accepted practice, it does not ensure a repeatable and consistent final metallurgy.

One method of achieving a consistent and repeatable metallurgy is to follow a procedure that will ensure a consistent cooling rate. This procedure, in order, is austenitize (normalize), quench and temper.

The austenitizing of the steel will produce a uniform fine-grain structure (providing, of course, that the steel is not oversoaked at austenitizing temperature). The quenching operation will produce the refinement of the prior austenite grains and a reduction in the primary ferrite grains. The faster cooling (quenching) will produce a much finer ferrite grain with less primary ferrite present. The resulting as-quenched hardness will be considerably higher than one would expect after an annealing process, which is a slow-cooling process.

The rapid cooling is generally accomplished by an oil quench (depending on the steel composition). Once the steel has been quenched, followed by a wash (to remove residual oil from the steel surface), it is then tempered at an elevated tempering temperature that is still in the ferrite region of the Iron-Carbon equilibrium diagram. This will produce a consistent and almost-repeatable microstructure that will be conducive to reduced distortion at the final heat-treatment process.

Density of the normalized furnace load should be considered to ensure a uniform rate of cooling for all of the pieces being normalized in that particular furnace load. Air cooling, on the other hand, may influence the formation of the pro-eutectoid ferrite and pearlite. The following statement can be made regarding the normalizing of the higher-carbon steels: The carbon content will influence the formation of carbides and carbide networks as well as tend to produce finer-grained steel.

Remember, if the practice of air cooling is adopted, airflow around the normalized part should be uniform with no drafts. This will also apply to variations in cross-sectional dimension changes, which will tend to produce differential induced stresses, non-uniform metallurgy and varying mechanical properties.

 

Part 2 of this blog can be read here.