Steel Quenching

Enter a steel’s carbon and alloy content and the calculator estimates the critical temperatures Ac1 and Ac3, a recommended austenitizing (hardening) temperature, and the maximum as-quenched hardness of fully martensitic steel.

How the calculation works

The critical temperatures come from K. W. Andrews’ empirical equations (1965): Ac1 = 723 − 10.7·Mn − 16.9·Ni + 29.1·Si + 16.9·Cr and Ac3 = 910 − 203·√C − 15.2·Ni + 44.7·Si + 31.5·Mo, with all elements in weight percent. In this form of the Ac3 equation manganese and chromium do not appear; silicon and molybdenum raise the temperature, nickel lowers it.

The hardening temperature follows the textbook rule: for hypoeutectoid steel (C below 0.8%) heat to Ac3 + 30–50 °C for full hardening; for hypereutectoid steel heat to Ac1 + 30–50 °C, which keeps hard carbides and avoids grain growth.

Maximum hardness is the Hodge–Orehoski (1946) polynomial for 100% martensite, which depends on carbon only. Above roughly 0.7% C martensite hardness barely increases, so the calculator caps carbon there and the result tops out near 64.8 HRC.

Worked example

The default is a medium-carbon steel like AISI 1045 / C45: C 0.45%, Mn 0.65%, Si 0.27%. Ac1 = 723 − 6.96 + 7.86 ≈ 724 °C; Ac3 = 910 − 203·√0.45 + 12.07 ≈ 786 °C. Recommended hardening temperature: 816–836 °C (about 1500–1537 °F); maximum hardness ≈ 58.6 HRC before tempering. A 1.0% C tool steel (0.25% Mn and Si) gives Ac1 ≈ 728 °C, hardening at 758–778 °C and ≈ 64.8 HRC.

Things to keep in mind

• The hardness figure assumes the section transforms fully to martensite. Thick parts or slow quenches leave softer, partly non-martensitic cores — check hardenability (Jominy data) for your grade.
• The tool does not pick a quenchant. Plain-carbon steels are usually water- or brine-quenched, alloy steels oil-quenched; follow the grade’s datasheet.
• As-quenched steel is brittle and highly stressed. Temper promptly; tempering lowers hardness depending on temperature.
• Andrews’ equations were fitted to low-alloy steels. For high-alloy tool steels, stainless or high-speed steels use the manufacturer’s heat-treatment data.