Heat Treatment Time Calculator
Estimate the minimum soak (hold) time for steel heat treatment operations including hardening (austenitizing), tempering, and stress relief annealing. The standard industry rule of thumb is 1 hour per inch (25.4 mm) of maximum section thickness, with a minimum hold time of 30 minutes. Results are guidance only; always verify against the applicable heat treatment specification for the specific alloy.
Heat treatment soak time formula
Section thickness (in) = Thickness (mm) / 25.4
Soak time (hr) = Section thickness (in) x Rate (hr/in)
Soak time (min) = max(Soak time x 60, Minimum soak time)
This formula is the standard rule of thumb from the ASM International Heat Treater's Guide for steel alloys. It assumes furnace-loaded parts in a standard atmosphere or controlled atmosphere furnace. Salt bath furnaces may allow shorter soak times due to better heat transfer.
Common steel heat treatment temperatures
- AISI 1045: Austenitize 820-870 degC; Temper 150-650 degC depending on target hardness
- AISI 4140: Austenitize 840-870 degC; Temper 200-650 degC
- AISI D2 tool steel: Austenitize 1,010-1,040 degC; Temper 150-540 degC
- H13 hot-work tool steel: Austenitize 1,010-1,040 degC; Temper 530-650 degC
- Stress relief anneal (all carbon steels): 540-600 degC
Heat treatment time: frequently asked questions
How is heat treatment soak time calculated?
A common rule of thumb for steel heat treatment is to allow 1 hour per inch (25.4 mm) of section thickness for austenitizing (hardening) soak, with a minimum of 30 minutes. For tempering, the rule is similar: 1 hour per inch of section. These rules are guidance from ASM International and must be verified against the specific alloy's heat treatment procedure.
What is the difference between soak time and total furnace time?
Total furnace time = Heating-up time + Soak time + Cooling time. This calculator estimates soak time only. Heating-up time depends on furnace loading, heating rate, and part mass. Cooling time depends on the cooling method (air, oil, water, controlled atmosphere).
Why does section thickness determine soak time?
The part must reach a uniform temperature throughout before being quenched or cooled. Heat penetrates from the surface inward by conduction. Thicker sections take longer to equalize temperature. The 1 hour/inch rule ensures even the core reaches the austenitizing or tempering temperature.
What are typical austenitizing temperatures for common steels?
AISI 1045 medium carbon steel: 820-870 degC (1,510-1,600 degF). AISI 4140 alloy steel: 840-870 degC (1,545-1,600 degF). AISI D2 tool steel: 1,010-1,040 degC (1,850-1,900 degF). H13 hot-work tool steel: 1,010-1,040 degC. Per ASM International Heat Treater's Guide.
Does this rule apply to non-ferrous alloys?
The 1 hour/inch rule was developed for ferrous (steel) heat treatment. Non-ferrous alloys (aluminum, titanium, nickel superalloys) have different thermal properties and treatment cycles. For aerospace aluminum (e.g., 7075-T6), soak times are typically 20-60 minutes regardless of thickness, per AMS specifications. Always refer to the alloy-specific specification.
Official sources
- ASM International: ASM International Heat Treater's Guide: Practices and Procedures for Irons and Steels.
- ASTM A991: ASTM A991 Standard Practice for Conducting Temperature Uniformity Surveys of Furnaces Used to Heat Treat Steel.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.