How to use this calculator
- Find the governing section. The thickest cross-section of the part — or the densest spot of the basket for production loads.
- Pick the process. Austenitize, temper and anneal carry their customary floors; custom takes your shop or datasheet rule.
- Add heat-up honestly. The result is at-temperature soak; furnace recovery and through-heating come first (thermocouple if it matters).
- Check the datasheet. Tool steels, stainless grades and certified work follow supplier curves and pyrometry specs, not the per-inch rule.
How it works
Soak time answers one question: has the metallurgical event you paid for — carbide dissolution, tempering reactions, recrystallization — actually happened all the way through the section? Heat conduction sets the through-heating; reaction kinetics set the hold. The shop encoding of both is the per-thickness rule with a process floor:
soak = max(t_min, rate × thickness) (classically 1 h/in)
Thin parts are governed by the floor (a 10 mm tool still tempers 2 hours); heavy sections by the rule. The calculator reports which one governed so the reasoning stays visible.
Neighbors in the same workflow: the carburizing case depth calculator for the diffusion clock that runs after through-heating, the hardness conversion chart for reading the result, and the Larson-Miller calculator when the "soak" is years of service rather than hours of furnace.
Worked example
Verified against the live calculator
Hardening a 25 mm (1 in) thick die block: the rule gives
25 × 0.0394 ≈ 1.0 h at austenitizing temperature — above
the 30-minute floor, so the rule governs. Tempering the same block:
the rule still says 1.0 h, but the 2-hour temper floor
governs, and for a tool steel you run it twice. A thin 6 mm
blade flips entirely to floor-governed: 30 minutes to harden,
2 + 2 hours to temper — the tempering, not the hardening, owns the
schedule.
Frequently asked questions
What is the rule of thumb for heat treat soak time?
One hour per inch of the thickest section, counted from when the section reaches temperature, with per-process floors: about 30 minutes minimum for austenitizing, 2 hours for tempering, 1 hour for annealing. It is a starting point for plain-carbon and low-alloy steels — datasheets and pyrometry specs override it.
Does soak time start when I put the part in the furnace?
No — that is the most common error. Soak counts from when the SECTION is at temperature; heavy parts and dense loads can need hours of heat-up and furnace recovery first. Load thermocouples are the honest way to know; without them, add a heat-up allowance before the soak clock starts.
Is a longer austenitizing soak safer?
No. Once carbides are dissolved, extra time at austenitizing temperature mostly grows grain, which costs toughness. Soak enough to through-heat and dissolve, then quench — "longer to be sure" is a real metallurgical mistake at the hardening step (tempering is more forgiving).
Why do tool steels temper for at least 2 hours, twice?
The first temper conditions fresh martensite and transforms some retained austenite — which becomes NEW untempered martensite on cooling. The second 2-hour temper treats that. High-speed steels often go three times. The per-inch rule rides on top of those floors.
Method & assumptions
- Customary shop screening for plain-carbon and low-alloy steels in adequately powered furnaces; supplier datasheets (tool steels, stainless, superalloys) and adopted pyrometry specs (e.g. AMS2750) override it.
- Output is at-temperature soak; furnace recovery, load density and radiation shadowing are not modeled — thermocouple the load when it matters.
- Salt baths and fluidized beds heat several times faster than air furnaces; vacuum furnaces often slower. The rate input absorbs shop-specific practice.
- This screen does not select temperatures, quenchants or temper counts — those come from the steel grade's datasheet.