Martensite Fraction (Koistinen-Marburger)
Computes the fraction of austenite already transformed into martensite when quenching stops at a given temperature, using the Koistinen-Marburger equation, f = 1 − e^(−0.011·(Ms − Tq)), where Ms is the martensite start temperature and Tq the temperature at which the part stopped cooling. The result is the percentage of martensite formed — whatever is missing from 100 % stays as retained austenite, which is soft, dimensionally unstable and able to transform later in service, distorting the part. Because the exponent is linear in the temperature difference, 63 °C below Ms already converts half the austenite, but 209 °C are needed to reach 90 % and the end of the transformation is asymptotic, never exact — which is precisely why precision parts get a cryogenic treatment after quenching. Enter the steel Ms temperature and the quench stop temperature.
Result
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Martensite Fraction with the Koistinen-Marburger Equation
Anyone quenching steel settles two questions the moment the part leaves the furnace: which medium to cool in, and how far down to cool. The second one governs how much austenite survives. A shaft quenched in oil that stops at 60 °C has not finished transforming, and the leftover austenite is soft, swells when it finally converts, and can shift the part dimensions months later in service. This calculator answers how much of the austenite became martensite when cooling stopped at Tq — the number that decides whether the process needs a sub-zero or cryogenic step.
The Koistinen-Marburger equation states that the transformed fraction grows as f = 1 − e^(−0.011·(Ms − Tq)): all that counts is how far below Ms the part went, never the time or the cooling rate. The 0.011 per kelvin constant sets the scale — 63 °C below Ms transform half the austenite, 209 °C transform 90 % and 419 °C transform 99 %. With the defaults, Ms = 320 °C and Tq = 20 °C, the exponent works out to −3.300, e^(−3.300) = 0.0369 and the result reads 96.31 % martensite, leaving 3.69 % retained austenite. Two checks: typing Tq equal to Ms returns exactly 0.00 %, and the predicted 209 °C gap for 90 % matches the classic rule that Mf sits roughly 215 °C below Ms in plain carbon steels.
The 0.011 K⁻¹ constant is an average fit for Fe-C alloys between 0.37 and 1.1 % carbon; heavily alloyed grades transform more slowly and need smaller constants, so the displayed fraction comes out optimistic. The model is athermal and assumes 100 % austenite at the start — if part of the structure already turned into pearlite or bainite above Ms, real martensite in the whole volume falls below the figure shown. Austenite stabilisation by an isothermal hold, strain-induced martensite, prior austenite grain size and carbon partitioning during tempering all sit outside the equation. And the value never reaches 100 %: the curve is asymptotic, which is exactly why cryogenic treatment exists.
Frequently asked questions
Why does the result never reach 100 % martensite?
What happens if I enter Tq above Ms?
Does the tool report retained austenite?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.