Transformation Temperature notations in Iron and Steel (Heating)

i. Acm – In hypereutectoid steel, the temperature at which the solution of Cementite in             Austenite is completed during heating.
ii. Ac1 – Temperature at which Austenite begins to form during heating (around 723°C).
iii. Ac2 – Temperature at which α-iron changes to non-magnetic β-iron (around 768°C).
iv. Ac3 -- Temperature at which transformation of α-iron to Austenite is completed.
v. Ac4 -- Temperature at which Austenite transforms to δ-ferrite during heating               (around 1492°C).

[A- arrest, c- heating]