Layer of iron oxides (wustite, magnetite and hematite) that grows on steel surfaces above about 570 °C in air and thickens fast as temperature rises. Scale eats material, insulates parts against the quench and causes soft spots, and has to be removed by shot blasting or pickling. Protective atmospheres, vacuum, salt, stainless foil wraps and anti-scale coatings prevent it.
Box furnace parts come out caked in scale, wrap the tool steel in foil next time.
Also heard as
- furnace scale
- oxide
- mill scale
Related terms
Decarburization
Loss of carbon from the surface of steel during heating, leaving a soft skin that will not fully harden.
Bright
Describes parts that come out of the furnace clean and unoxidized, with no scale or discoloration.
Soft spot
A small area on a hardened surface that reads lower than the rest, usually from uneven quenching.