METALLAI › Free tools › Tempering parameter

Trade tempering time for temperature

“Can I run 2 hours at 550 °C instead of 6 at 500?” The Hollomon–Jaffe parameter answers the arithmetic. This page also answers the part that gets people into trouble: whether you are allowed to.

Current schedule

Proposed temperature

Common schedules

Equivalent time

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HJP, current schedule—
HJP, proposed schedule—
Time change—
Constant C in use—

Equal parameter is not equal steel. The parameter tracks softening. It says nothing about embrittlement, carbide coarsening, secondary hardening in hot-work and high-speed steels, or residual stress relief — all of which have their own kinetics. Use it to plan, then confirm with hardness.

The relation

HJP = T · (C + log10 t) / 1000 T tempering temperature in KELVIN (°C + 273.15) t holding time in HOURS C material constant, about 20 for steels Equal HJP → equivalent tempered condition: log10 t₂ = T₁(C + log10 t₁) / T₂ − C

Note where each variable sits. Temperature multiplies the whole bracket; time only enters through a logarithm. That asymmetry is the entire practical point: a 20–30 °C rise can halve the time, while doubling the time moves the parameter by barely 0.3 %.

The two limits the arithmetic cannot see

LimitRangeWhat happens
Ac1 ceiling ~700–740 °C for most low-alloy steels Above Ac1 the steel re-austenitises on heating and re-hardens on cooling. This is not a softer temper — it is a failed heat treatment.
Tempered martensite embrittlement 260–370 °C Also called 500 °F embrittlement. Toughness drops even though hardness behaves normally.
Reversible temper embrittlement 375–575 °C In steels carrying P, Sn, Sb or As. Worsened by slow cooling from the tempering temperature — quench out of this range.
Check your own Ac1 before trusting the ceiling. It moves with composition by more than 100 °C across common alloy steels — a 12 % Cr grade sits far above a plain carbon one. Compute Ac1 for your composition →

Questions

Can I temper hotter for less time?
Usually, and the trade favours temperature steeply. The limits are physical: stay below Ac1, and avoid the two embrittlement bands. Neither is visible in the parameter.
What value of C should I use?
20 is the usual default and what most published charts assume. Hollomon and Jaffe found it varies with carbon, roughly C = 21.3 − 5.8 × %C — about 19 for a 0.4 % carbon steel. It shifts the absolute parameter more than it shifts the equivalence between two nearby schedules.
Does the time include heating up?
The parameter assumes time at temperature. For a thick section the soak to reach temperature can be a large share of a short cycle, and it does contribute some tempering on the way up. For short schedules on heavy sections, measure rather than calculate.
Does this work for aluminium ageing?
No. Hollomon–Jaffe describes the softening of tempered martensite. Precipitation ageing in aluminium is a different mechanism with a peak — going hotter for less time can take you past peak strength into the over-aged condition rather than to an equivalent one.

Predict the hardness, not just the equivalence

METALLAI takes composition, quench and tempering schedule and returns yield, tensile, hardness and elongation — with the Ac1 check built in, so an impossible temper is refused rather than answered.

Run a prediction — free Hardenability calculator