Tempering decreases hardness because it reduces internal stresses in hardened metal.
Heating during tempering changes the brittle structure into a tougher and more stable form.
This lowers hardness slightly while improving toughness and ductility.
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Why does tempering decrease hardness?
Tempering decreases hardness because it reduces the internal stresses and brittle structure created during quenching.
Here’s the process in simple terms:
- After quenching:
Steel is rapidly cooled, creating martensite, a very hard but brittle structure. Martensite traps carbon atoms in a distorted crystal structure, making the steel extremely hard. - During tempering:
The steel is reheated to a moderate temperature (below its critical temperature) and held there for some time. - What changes happen:
- Trapped carbon atoms begin to move out of the martensite.
- Internal stresses are relieved.
- Some martensite transforms into softer structures like tempered martensite with fine carbide particles.
- Result:
- Hardness decreases somewhat.
- Toughness and ductility increase.
- The steel becomes less brittle and more useful in real applications.
The trade-off is:
Higher hardness ↔ lower toughness
Tempering sacrifices some hardness to gain toughness and durability.
A common example: a knife blade or spring is tempered after hardening so it doesn’t chip or crack easily.
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