Steel Tempering

Use this calculator to plan a tempering cycle after hardening: how long to hold the part at temperature, which tempering range the cycle falls into and what Hollomon–Jaffe parameter describes it.

How the calculation works

Soak time follows the widely used heat-treating rule of thumb found in references such as the ASM Handbook, Vol. 4: one hour per inch (25 mm) of the thickest section, with a minimum of one hour. The clock starts once the whole cross-section has reached tempering temperature; furnace and load heat-up time is not included.

The Hollomon–Jaffe tempering parameter is P = T × (20 + log t), with T in kelvin, t in hours and 20 as the usual constant for carbon and low-alloy steels. Cycles with the same P give similar hardness, so the parameter lets you trade a longer, cooler temper for a shorter, hotter one. The tempering type is read from temperature: below 250 °C is low tempering (keeps hardness, for tools and case-hardened parts), 250–500 °C medium (elasticity, springs), 500 °C and above high tempering (quench-and-temper for maximum toughness).

Enter the thickest section in millimetres (1–500) and the tempering temperature in °C (150–720).

Worked example

A 50 mm shaft is tempered at 550 °C. Soak time: 50 / 25 = 2 hours (120 minutes). P = (550 + 273.15) × (20 + log 2) = 823.15 × 20.301 ≈ 16,711, a high-temperature temper. Holding P constant, the equivalent cycle would take about 8.7 hours at 525 °C or roughly 0.5 hours at 575 °C, which shows how strongly time depends on temperature.

Things to keep in mind

• Temper promptly after quenching; freshly hardened steel carries high residual stress and can crack if left waiting.
• For critical parts avoid roughly 250–400 °C, where tempered-martensite embrittlement occurs. Alloy steels prone to temper embrittlement are usually cooled quickly (oil or water) after high tempering.
• High-speed and high-alloy tool steels are normally tempered two or three times; the one-hour-per-inch rule does not replace the steel maker’s data sheet.
• The constant 20 is an average. For precise hardness targets use the tempering curves for your specific grade.