Tempering is a process done subsequent to quench hardening.
Quench-hardened parts are often too brittle. This brittleness is caused by a due to formation of Martensite.
This brittleness is removed by tempering. Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability.
In this process, the deformation over 50% of metastable austenite in done in metastable bay between the pearlite and bainite noses of the pertinent TTT diagram (i.e., in range 600-400°C) and is then quenched to martensite.
Temperature should be low as possible such that carbides should be dissolved without grain growth.
The cooling rate from austinizing temperature to deformation temperature should be very low as to form ferritie and to avoid formation of bainite.
Deformation temperature also should be low enough to stop recovery and recrystallization. It has high dislocation density which is desirable.
Normalization is an annealing process in which a metal is cooled in air after heating in order to relieve stress.
It can also be referred to as heating a ferrous alloy to a suitable temperature above the transformation temperature range and cooling in air to a temperature substantially below the transformation range.
This process is typically confined to hardenable steel.
It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel.
It involves heating the steel to just above its upper critical point. It is soaked for a short period then allowed to cool in air.
Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum ductility achieved by annealing.
It eliminates columnar grains and dendritic segregation that sometimes occurs during casting.
Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes.