Localized low hardness on a quenched part. Common causes are vapor pockets in blind holes and recesses, scale or dirt insulating the surface, parts touching in the basket, contaminated quenchant and local decarb. Soft spots are found by Rockwell or file checks at several locations, and they are the classic sign of poor racking or a tired quench.
Every gear has a soft spot right where it sat against the next one, open up the racking.
Load blind holes facing up so steam can escape; trapped vapor is the commonest cause of soft spots in bores and pockets.
Also heard as
- soft spots
Related terms
Vapor blanket
The film of vapor that wraps red-hot steel at the start of a liquid quench and slows its cooling.
Quenchant
The liquid or gas used to quench parts, chosen for its cooling speed and the distortion it produces.
Washer
The parts washer that strips oil and chips before heat treat and quench oil after it.
File hard
Hard enough that a sharp file skates across the surface without biting, roughly 58 HRC and up.