Ac1 temperature is the temperature at which steel begins transforming into austenite during heating.
Ac3 temperature is the temperature at which the transformation to austenite is completed.
These temperatures vary depending on the carbon content and composition of the steel.
In this article:
What are Ac1 and Ac3 temperatures of steel?
Ac1 and Ac3 are important critical temperatures of steel used in heat treatment processes such as annealing, normalizing, hardening, and quenching. These temperatures indicate when structural transformations begin and end during heating of steel.
The notation uses:
- A → arrest or transformation temperature
- c → heating (chauffage, from French, meaning heating)
For cooling transformations, symbols like Ar1 and Ar3 are used.
These temperatures are very important because heat treatment results depend on heating steel to the correct range.
1. Ac1 temperature (Lower critical temperature)
Ac1 is the temperature at which austenite begins to form during heating.
Below Ac1:
Steel consists mainly of structures such as:
- Ferrite
- Pearlite
- Cementite (depending on carbon content)
At Ac1:
Transformation starts.
Part of the structure begins changing into austenite.
Definition
Ac1 is the temperature at which austenite formation starts during heating.
For plain carbon steel:
Ac1 is approximately 723°C (may vary slightly with composition).
What happens at Ac1?
When steel reaches Ac1:
- Pearlite begins transforming into austenite
- Structural changes start
- Atomic rearrangement begins
Transformation is not yet complete.
2. Ac3 temperature (Upper critical temperature)
Ac3 is the temperature at which transformation into austenite becomes complete during heating for hypoeutectoid steels (less than about 0.8% carbon).
Definition
Ac3 is the temperature at which ferrite completely transforms into austenite during heating.
Above Ac3:
Steel structure becomes fully austenitic.
What happens at Ac3?
During heating:
- Ferrite gradually disappears
- Austenite increases
- At Ac3 transformation is complete
Result:
100% austenite structure.
Structural changes between Ac1 and Ac3
For hypoeutectoid steel:
Below Ac1
Structure:
Ferrite + Pearlite
Between Ac1 and Ac3
Structure:
Ferrite + Austenite
Transformation is occurring.
Above Ac3
Structure:
100% Austenite
Transformation complete.
Temperature values depend on carbon content
Ac3 is not constant.
It changes with steel composition.
General trend:
- Increasing carbon lowers Ac3 temperature.
Typical examples:
| Carbon content | Approximate Ac1 | Approximate Ac3 |
|---|---|---|
| 0.2% carbon steel | ~723°C | ~900°C |
| 0.4% carbon steel | ~723°C | ~820°C |
| 0.8% carbon steel | ~723°C | ~723°C |
At eutectoid composition (~0.8% carbon):
Ac1 and Ac3 meet.
Importance of Ac1 and Ac3 in heat treatment
These temperatures determine correct heating temperatures.
Annealing
Steel is heated above Ac3 or above Ac1 depending on steel type.
Purpose:
- Soften material
- Refine structure
Normalizing
Heating is usually:
Ac3 + additional temperature margin
Purpose:
- Produce uniform structure
Hardening
Steel is heated above critical temperatures before quenching.
Purpose:
- Obtain complete austenitic transformation
Tempering
Usually performed below critical temperatures.
Importance in industrial practice
Correct selection of temperature:
- Prevents incomplete transformation
- Avoids overheating
- Improves hardness and toughness
- Controls grain size
- Improves final properties
If steel is heated below Ac3 during hardening:
- Austenite formation may be incomplete
- Hardness may decrease
If overheated:
- Grain growth may occur
- Mechanical properties may worsen
Relation with iron–carbon diagram
Ac1 and Ac3 are represented on the Iron–Carbon Phase Diagram.
- Ac1 corresponds to lower transformation line
- Ac3 corresponds to upper transformation boundary for hypoeutectoid steels
These critical lines guide heat treatment temperature selection.
Simple example
Consider a steel containing 0.4% carbon:
- Below 723°C → ferrite + pearlite
- At 723°C (Ac1) → austenite starts forming
- Around 820°C (Ac3) → fully austenitic
- Above Ac3 → suitable for quenching
Conclusion:
Ac1 is the temperature where austenite starts forming during heating, whereas Ac3 is the temperature where transformation into austenite is completed.
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