classification of tempering process based on their behavior over a specific temperature range.

1. Classification of Tempering
Tempering, in general, has been classified in three categories
depending on the tempering temperature range and properties
developed in the hardened steel.

2. Low Temperature Tempering (1-2 Hours at a Temperature up to
250°C)
• It is done to reduce brittleness
• The tempered martensitic double-phase structure increases the strength
with some improvement in toughness, and reduction in internal stresses.
• Tempered plain carbon steels (0.6 to 1.3% C) have a hardness of Rc 58 to
63.
• This treatment is given to tools of plain carbon and low alloy steels, where
properties developed are high cutting-ability, wear and abrasion resistance
with some toughness.

3. • Increasing tempering temperature reduces the hardness slightly, increases
toughness with more relief from internal stresses.
• Low temperature tempering is done either in oil baths (up to 250°C-silicone
oil), or in salt bath, or in an air-circulated furnace (as below 500°C, heat
transmission through air is very slow).
• Low temperature tempering is applied to components, which undergo surface
hardening treatments and case hardening treatments, like carburising,
cyaniding, or carbonitriding

4. Temper Colours
• Temper-colours develop due to the formation of an extremely thin, transparent
(initially) film of iron- oxide.
• Interference of light occurs in this thin surface Film, which appears as temper
colours depending on the thickness of the film
.
Temperature Temper colour
220 Straw yellow
285 Purple
310 Light-Blue
325 Grey
350 Grey-purple
375 Grey-blue
400 Dull-grey

5. Medium Temperature Tempering (350 C to 500°C):
• Troostile microstructure is observed with development of high elastic limit with
good toughness and hardness in range of HRC 40-50.
• Endurance limit can be increased by water-quenching after tempering in range of
400-450°C which induces compressive stresses in the surface layers.
• Because of high elastic limit and endurance limit, these are mainly used for
springs of both types, coil and laminated, and also for dies.
• Care must be exercised to avoid 350°C embrittlement

6. High Temperature Tempering (500-650°C):
• Produces ultrafine grains in plain C and low alloy steels, resulting in increase in
strength and toughness in machine component.
• Intricate and heavy duty alloys are normally cooled in furnace after tempering
to prevent development of thermal stresses
• treatment for 1-2 hours is almost able to relieve completely the residual-
stresses developed during quenching