Molten salt’s advantages
The most distinct advantage of salt quenching is its wide operating temperature range-150 to 595°C (300 to 1100°F) for a typical nitrate-based salt, which is unmatched by any other quench medium.
The quenching mechanism also is considerably different. In most liquid quenchants, heat extraction occurs throughout the three stages of vapor, boiling and convection. Salt quenching has no vapor phase, so problems with vapor phase barriers are avoided. Most of the heat extraction is by convection. As a result, distortion is minimized and the hardness achieved is more uniform and consistent.
Other important advantages of salt quenching include:
- Quench severity can be controlled by varying the temperature, agitation and water content of the salt bath.
- Productivity in salt quenching is high because parts rapidly attain temperature equalization.
- Due to excellent thermal and chemical stability, salt baths can provide consistent quenching performance for many years. Ordinarily, the only maintenance required is to replace salt that is dragged out.
- Salt can be easily washed off with water and recovered for reuse. Choosing to recover salt eliminates disposal and reduced operating costs.
- The same salt bath can be used for tempering.
Limitations: Salt has to be used above its melting point of about 150°C (300°F), and, because of its oxidizing nature, combustible or incompatible materials should definitely be kept out of the salt bath to avoid the possibility of violent reactions.
Salts used to present safety and environmental problems, but technology for dealing with them is now well developed and they are no longer considered deterrents to its use.
منبع:
http://heatbath.com/2012/06/the-basics-of-molten-salt-quenchants/