Martensite Start Temperature Ms (Andrews)
Computes the Ms temperature, the point at which austenite starts transforming into martensite during quenching, using the linear Andrews equation: Ms(°C) = 539 − 423·C − 30.4·Mn − 17.7·Ni − 12.1·Cr − 7.5·Mo, with every content in mass percent. Nearly every element dissolved in austenite lowers Ms — cobalt and aluminium are the exceptions and raise it —, but carbon dominates by far: each 0.1 % of carbon drops Ms by 42 °C, nearly 14 times the effect of the same manganese content. Knowing Ms sets the martempering bath temperature, tells whether retained austenite will survive at room temperature, and predicts how severe the quenching stresses will be, because a low Ms makes the martensite expansion happen late, with the part already cold and rigid, and that is where cracks appear. The correlation is fitted to low-alloy steels with carbon up to roughly 0.6 %, and the page rejects compositions above 0.8 % carbon, where the extrapolation loses its footing. Enter the carbon, manganese, nickel, chromium and molybdenum contents.
Result
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Ms Martensite Start Temperature by the Andrews Equation
The Ms point never shows up on a steel data sheet, yet it drives three quenching decisions: the martempering bath temperature, which sits just above it; how much retained austenite to expect at room temperature; and the cracking risk, because a low Ms pushes the martensite expansion to the end of cooling, when the part has gone cold and stiff and cannot accommodate the strain. Measuring Ms takes a dilatometer. Estimating it takes only the chemical composition from the heat certificate, and that is what this calculator does.
The Andrews equation is a straight line in five contents: Ms(°C) = 539 − 423·C − 30.4·Mn − 17.7·Ni − 12.1·Cr − 7.5·Mo, everything in mass percent. Each coefficient tells how many degrees Ms drops per 1 % of that element dissolved in austenite, and carbon dominates: 0.1 % carbon pulls Ms down by 42.3 °C, nearly 14 times the effect of 0.1 % manganese. With the defaults, a typical 4340 composition (0.40 C / 0.70 Mn / 1.80 Ni / 0.80 Cr / 0.25 Mo), the arithmetic reads 539 − 169.20 − 21.28 − 31.86 − 9.68 − 1.88 = 305.1 °C. The independent Steven-Haynes correlation, built on different coefficients, returns 298.8 °C for the same steel, and the measured Ms of 4340 is quoted between 285 and 300 °C.
The fit covers low-alloy steels with carbon up to roughly 0.6 %; the page accepts up to 1.2 %, but in that range real Ms falls less than linearly and the estimate turns pessimistic. Only elements dissolved in austenite count: chromium and molybdenum locked in undissolved carbides, after a short or cold austenitising soak, leave Ms where it was, so the number lands too low. Cobalt and aluminium, which raise Ms, and silicon have no field in this five-term form. Austenitic stainless, Hadfield steel, maraging alloys and cast irons fall outside. And Ms marks the start, never the finish: to learn how much has already transformed at a given temperature, use the Koistinen-Marburger model.
Frequently asked questions
Why is there no field for silicon, cobalt or aluminium?
Which temperature should the martempering bath hold?
Does the result apply to stainless or tool steel?
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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.