Tempering is usually categorised into 4 stages.
Stage 1: Temp usually till 100-150°C: In this usually the stress relieving occurs, by some dislocations rearrangements, or C atoms shifting towrds the dislocation sites and sitting there.
Stage 2: Between 150-250°C:
This stage formation of Epsilon Carbides occurs, as T is increased C atoms trapped in martensitic sites, diffusion becomes easier and they start escaping; formation of these carbides are easier.
Stage 3: Between 250-350°C
In this stage , as the temperature increase martensites convert into ferrite and small cementite. Also, retained austenite convert into lower bainites in some cases.
Stage 4: between 350-700°C
Due to prolong heating cementite starts growing as more C diffuses and also reduction in surface energy causes it’s spheroidisation. so final we get equiaxed ferrites and coarse globules cementite
Bonus point:
In industrial tempering two important products can be observed:
*Toorsite- at temp 350-500°C: very fine globular pearlite.
*Sorbite- at temp 500-650°C- finer globular pearlites.
Note: The temperature ranges may differ depending on different sources.