Any of the transformation points where steel changes phase, written as A with a number. Ac1 is where austenite starts to form on heating, Ac3 is where ferrite finishes dissolving, and Acm marks full carbide solution in high-carbon steel. The c and r stand for heating (chauffage) and cooling (refroidissement); the Ar points sit lower than the Ac points because transformation needs some overshoot or undercooling to get going. Most hypoeutectoid steels are hardened from 30 to 50 °C above Ac3.
That's 4140, Ac3 is up around 1,460, so nobody sets the box at 1,450 and calls it hardening.
Also heard as
- transformation temperature
- critical
- Ac1
- Ac3
- Acm
Related terms
Austenitizing
Heating steel above its upper critical temperature and holding it until the structure becomes austenite.
Iron-carbon diagram
The phase map of iron and carbon that shows which structures are stable at each temperature and carbon level.
Austenite
The face-centered cubic form of iron that dissolves carbon and must form before steel can be hardened.
Normalizing
Heating steel above its upper critical temperature and cooling it in still air to refine and even out grain.