These are the critical transformation temperatures on heating, determined from an equilibrium phase diagram (for slow heating).
Ac1 (Lower Critical): The temperature at which austenite begins to form from ferrite/pearlite on heating. For 42CrMo, it's approximately 730-750°C.
Ac3 (Upper Critical): The temperature at which the transformation to austenite is complete (all ferrite has dissolved). For 42CrMo, it's approximately 780-800°C.
Importance for Heat Treatment:
For Annealing/Normalizing: Heating just above Ac3 ensures a fully austenitic, homogeneous starting structure before controlled cooling.
For Hardening (Quenching): The austenitizing temperature is set significantly above Ac3 (typically 840-880°C) to ensure complete austenitization and dissolution of carbides, which is necessary to achieve full hardenability and uniform properties upon quenching. Heating below Ac1 will not form austenite and thus no hardening is possible.