First stage of a liquid quench, when the quenchant boils on contact and surrounds the part with an insulating film of vapor. Cooling through this stage is slow, and if the film hangs on unevenly it produces soft spots and distortion. It collapses into the fast nucleate boiling stage, then slower convection. Agitation, the salt in brine and the additives in fast oils all aim to break the blanket early.
Bottom of the basket is soft, the vapor blanket is hanging on because nothing's flowing up through that grid.
Also heard as
- vapor phase
- film boiling
- Leidenfrost stage
Related terms
Agitation
Forced movement of quenchant around parts by pumps, propellers or nozzles to speed up and even out cooling.
Quenchant
The liquid or gas used to quench parts, chosen for its cooling speed and the distortion it produces.
Soft spot
A small area on a hardened surface that reads lower than the rest, usually from uneven quenching.
Quench severity
How fast a quenchant pulls heat out of steel, usually rated by the Grossmann H value.