What is the purpose of Ac1 and Ac3 Temperature Calculator for Steel Heat Treatment?

Ac1 and Ac3 Temperature Calculator for Steel Heat Treatment is a controlled thermal process used to modify the microstructure and mechanical properties of steel. The process parameters — temperature, time, and cooling rate — are determined from the iron-carbon phase diagram and TTT/CCT diagrams for the specific steel composition.

What temperature should I use?

Temperature selection depends on the steel's A1 and A3 critical temperatures, which can be calculated from composition using the Andrews (1965) empirical equations (see our Ac1/Ac3 Calculator). For hardening: heat to 30-50°C above A3 for hypoeutectoid steels. For annealing: just above A3 for full anneal, or 690-720°C for spheroidise annealing. Always verify with the steel supplier's datasheet.

How do I verify the process was successful?

Verification methods include: (1) Hardness testing — Vickers, Rockwell, or Brinell at multiple locations to confirm uniformity; (2) Microstructural examination by optical metallography — nital-etched sections at 200-500×; (3) Mechanical testing — tensile, Charpy CVN per the applicable standard (EN ISO 6892-1, EN ISO 148-1). See our Hardness Conversion Calculator for interpreting results across scales.

Ac1 and Ac3 Temperature Calculator for Steel Heat Treatment

The Ac1 (lower critical) and Ac3 (upper critical) temperatures define the austenitising range for steel heat treatment. Ac1 is the temperature at which austenite begins to form on heating; Ac3 is where the transformation to austenite is complete. Austenitising above Ac3 ensures complete dissolution of carbides into austenite before quenching. This calculator uses the Andrews (1965) empirical equations.

Ac1 / Ac3 Temperature Calculator

%C — Carbon

%Mn — Manganese

%Si — Silicon

%Cr — Chromium

%Ni — Nickel

%Mo — Molybdenum

%V — Vanadium

%Al — Aluminium

Ac1 (lower critical temperature)—

Ac3 (upper critical temperature)—

Recommended austenitising temperature—

PWHT range (below Ac1)—

Ac1 (°C) = 723 − 10.7Mn − 16.9Ni + 29.1Si + 16.9Cr + 290As + 6.38W
Ac3 (°C) = 910 − 203√C − 15.2Ni + 44.7Si + 31.5Mo + 13.1W − 30Mn + 11Cr + 65Nb + 400Ti
Source: Andrews (1965), JISI

Figure: Schematic diagram for Ac1 and Ac3 Temperature Calculator for Steel Heat Treat… — key process, structure, and property relationships. © metallurgyzone.com/