Hardening can be performed by rapid cooling, which is also known as quenching. During quenching, there is insufficient time for the diffusion-controlled decomposition of austenite to ferrite and pearlite. Instead, a diffusionless transformation takes place, in which martensite is formed. Martensite is a very hard phase, therefore the steel becomes very hard and brittle. Quenching, therefore, is usually followed by a tempering treatment to regain some toughness and ductility in the steel.
A quenching medium, such as oil, water, polymer or salt bath can be used during quenching. Other media include brine solution, caustic solution, metals or water mist, some of which may be used only under certain conditions.
| Mixing | Severity of different quenching medias (H-value) | ||
|---|---|---|---|
| Oil | Water | Salt bath | |
| No mixing | 0.25 - 0.30 | 0.9 - 1.0 | 2.0 |
| Remote | 0.30 - 0.35 | 1.0 - 1.1 | 2.0 - 2.2 |
| Moderate | 0.35 - 0.40 | 1.2 - 1.3 | |
| Good | 0.4 - 0.5 | 1.4 - 1.5 | |
| Strong | 0.,5 - 0.8 | 1.6 - 2.0 | |
| Extreme | 0.8 - 1.1 | 4.0 | |
Learning Outcomes for this Section
After completing this section, you should be able to:
- Describe different hardening methods
- List and explain the main reactions, which take place during hardening
See Also
- Engineering Steels > Engineering Steels: Overview
- Mechanical Properties > Mechanical Properties of Steels: Overview
