The hardened steel has low resistance, in addition to an high hardness. Therefore, the hardened steel would be too fragile for most uses. What should you do?
Tempering of hardened steel
After hardening, it is essential to use the tempering treatment, as it is fundamental to undo any tensions and fragility. How does this kind of treatment work?
- heat the steel at a temperature below AC1
- keep it at this temperature for a given period of time
- cool the steel in an appropriate way to room temperature.
What happens, specifically, during the tempering treatment? The latter results in the spreading of carbon atoms. Martensitic pattern slowly loses these atoms, taking the form of cementite (iron carbide). As a result, steel loses greatly in hardness and simultaneously increases toughness.
When should the tempering be run?
In order to avoid the cracks danger (just due to the high tension of the hardened parts) it is essential to carry out the tempering immediately after the hardening. How to choose the tempering temperature? In such a way as to obtain the best compromise between hardness and toughness. Higher temperatures, in fact, even the strength progressively increases. In addition, there will be an increase of elongation and contraction and, consequently, the decrease in hardness, tensile strength and, finally, yield strength.
What is the hardening treatment?
The steels hardening is a heat treatment that consists of various operations that require careful consideration of some fundamental conditions, including:
- type of the steel
- composition of the steel
- parts sizes
- tempering temperature and duration.
What is this heat treatment? In a hardening martensitic structure which is followed by tempering treatment. This is precisely done in order to achieve a compromise between toughness and hardness. But what determines the structure of hardened and tempered steel? Mainly depend on these two factors:
- 1ˢͭ factor: tempering temperature adopted
- 2ⁿᵈ factor: more or less rude structure of martensite hardening.
Therefore, it will be necessary to adopt a hardening temperature as low as possible. Martenisitc hardening, must also reach the heart of the piece from harden.
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