Isothermal annealing and quenching

Heat treatment of steel is heating it to a definite temperature, holding it there, and cooling at a chosen rate, to change the microstructure and obtain required properties (hardness, strength, ductility), without change of shape.

Isothermal annealing

Isothermal annealing is a variant of full annealing in which steel is heated above its upper critical temperature (austenitised), cooled rapidly to a temperature just below the lower critical point (about 600-700 C), held at this constant temperature until the austenite fully transforms to pearlite/ferrite, and then cooled in air.

 Temp
  ^
  |   ____ austenitise (A3 + 30-50 C)
  |  /    \
  |-/------\------------- A1 (723 C)
  | /       \_________
  |/    rapid cool  | hold (isothermal)
  |                 |_______ air cool
  +--------------------------> Time

It saves time compared with full annealing, gives uniform structure and good machinability, and is used for alloy steel forgings and castings.

Quenching

Quenching is the rapid cooling of steel heated above the upper critical temperature (30-50 C above), by dipping in water, brine or oil.

• Rapid cooling prevents formation of soft pearlite, and austenite changes into martensite, giving very high hardness and strength.
• The steel becomes brittle and has residual stress, so it is tempered afterwards.
• Cooling rate: brine > water > oil > air. High-carbon steel is hardened by quenching; low-carbon steel does not harden much.

 Temp
  ^
  |   ____  heat 30-50 C above A3
  |  /    |
  | /     | very rapid cooling
  |/      |  in water/oil
  |        \______
  +------------------------> Time
   Result: martensite (hard, brittle)