The subject of quenching remains (to a large extent) a mystery to many product and process engineers. The reasons for quenching are numerous. It can be used for:
  • Transformation from austenite to martensite in steels
  • Solutionizing of PH-type stainless steels
  • Annealing by quenching for nonferrous materials such as copper and its derivatives
  • Heat-treatable aluminum alloys
Our focus will be steel related. If the steel has sufficient carbon present in the composition and it is heated to the appropriate austenitizing temperature, the iron carbides will be taken completely into solution.

The steel is now ready for the cooling procedure from the austenitizing temperature. The question is what cooling rate should be applied in order to make the appropriate phase transformation from austenite to the phase we desire.

The choice of phase can be:
  • Martensite
  • Upper or lower Bainite
  • Pearlite
  • Ferrite
Each of the above phases will be accomplished by the process temperature selection followed by the appropriate cooling rate.

The metallurgist/heat treater and engineer will need to determine what the appropriate phase structure should be. The selection will be determined by the desired metallurgical and mechanical properties.

The appropriate reference for determining what the cooling rate should be is the Time Temperature Transformation (TTT) curve. These curves will give a very strong indication as to what phase will result based on the austenitizing temperature and the cooling rate. The curves for each steel are unique. One may also hear them called “S” curves.

The curves show the critical cooling rate in relation to the steel chemistry. If the steel is cooled fast enough from the austenite phase to exceed the critical cooling rate and thereby miss the left-hand side of the “nose” of the curve, then (providing there is sufficient carbon present) a phase known as martensite will be formed. Martensite is an unstable phase, however, and if not followed by tempering (sometimes known as “drawing”), the likelihood of cracking is extremely high.

Conversely, if the cooling rate is too slow to form martensite, other phases will be formed that are less hard than the martensite.

The next discussion will compare the formed phases as a result of differing rates of cooling.