When assessing the quality of a steel, checking its chemical composition…
Heat Treatments: Quenching and Tempering
Quenching and tempering is one of the most important heat treatments for steels, consisting of quenching followed by one or more tempering steps, used to achieve an excellent balance between mechanical strength and toughness.
Quenching: Creating Martensite
The purpose of quenching is to obtain a martensitic structure, which is the hardest but also the most brittle constituent of steel. For quenching to be effective, it’s necessary to:
- Achieve complete austenitization: The heating must transform the steel entirely into the austenitic phase to ensure temperature homogeneity throughout the treated component’s cross-section.
- Exceed the critical cooling rate: The cooling must be rapid enough to prevent undesirable transformations.
Quenching media are chosen based on the steel type, the component’s geometry, and the characteristics required by the designer:
- Water
- Aqueous polymer solutions
- Oil
- Molten salts
- Pressurized gas (nitrogen, helium, argon)
- Air
Tempering: Hardness + Toughness
Tempering is the treatment that follows quenching and is performed at temperatures below the critical point A1, typically between 500 and 680 °C. Its goals are to:
- Relieve stress in the newly quenched material.
- Reduce the brittleness of martensite.
- Impart the mechanical properties defined during the design phase.
Tempering after quenching transforms martensite into an aggregate called sorbite, a structure that represents the best compromise between hardness and toughness.
Cooling after tempering can be slow (furnace or air) or fast (water or oil). Rapid cooling is recommended for steels containing Mn, Ni, and Cr to prevent temper embrittlement.
Some steels, such as hot-work and cold-work tool steels, require multiple tempering cycles, interspersed with slow cooling to room temperature. This is done to:
- Transform any residual austenite.
- Promote secondary hardening.
- Ensure complete microstructural and geometrical stability
Conclusion
Quenching and tempering is a key treatment to bring steel to its best mechanical performance, making it suitable for components subjected to high stresses. The correct execution of the quenching and tempering phases is fundamental to guarantee its durability and stability.
Bonomi Acciai, representatives in Italy of Swiss Steel Group, supports designers and heat treaters in choosing the most suitable steel and the most performing thermal cycle. Contact us for a dedicated consultation.
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