Hand check from the forge and knife shop: steel stops attracting a magnet at about 770 °C, close to the hardening range of simple carbon steels, because ferrite passes its Curie point and austenite is nonmagnetic. Smiths heat until the magnet no longer sticks, then go a little hotter before quenching. It works for plain carbon grades and says little about high-alloy tool steels or martensitic stainless, which harden from well above the Curie point.
No pyrometer on the forge, so do the magnet test and take it a shade past nonmagnetic before you quench.
Also heard as
- nonmagnetic check
- magnet check
Related terms
Cherry red
Color name for steel glowing at roughly 750 to 800 °C, near where most hardening heats begin.
Austenite
The face-centered cubic form of iron that dissolves carbon and must form before steel can be hardened.
Critical temperature
A temperature at which steel's structure changes phase on heating or cooling, such as Ac1 or Ac3.