Quenching is a hardening process used for hardening of ferrous alloys. The process consists of the following;
Heating (heating a steel above the critical temperature)
The steel is heated above the critical temperature. A3 for hypo-eutectoid steels and ACM for hyper-eutectoid steels. This stage is generally termed the austenizing phase.
Once the steel has been heated sufficiently it is then cooled rapidly in a cooling medium. The cooling medium can vary according to the required hardness, steel composition and the size of the steel to be quenched.
Cooling mediums can include water, brine, oil and forced air. Upon cooling most of or all of the austenite is transformed into martensite. In this state the steel is exceptionally hard and brittle and therefore difficult to be put to practical use. A tempering process is required.
- Increase in strength
- Increase hardness (wear resistance)
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= Hard, brittle
A very simplified drawing to show the way in which martensite propagates. When the steel is quenched and it passes the start of martensite, martensite grains will generate and continue to grow as the cooling continues.
The graph shows that when quenching a steel it passes through the MS and MF (start and finish phase for martensite formation). Martensite is a brittle and hard element. In this state the steel can not be put to real practical use.