Martensite Start
Martensite start (Ms) is the temperature at which austenite first begins transforming into martensite during cooling, marking the onset of a diffusionless, shear-dominant phase transformation in steel. It is the highest temperature where the martensitic reaction can begin; the amount formed depends on how far below Ms the steel is cooled, not on holding time.
On a heat-treat or CNC floor, Ms matters when a part is quenched after austenitizing; final hardness, residual stress, and distortion depend on how much martensite forms before room temperature. For tool steels, gears, shafts, dies, knives, and wear components, the Ms value indicates whether the quench can drive the part far enough below Ms to build the intended structure or whether retained austenite will remain. In a CNC cell, parts machined soft and then hardened grow or warp according to the transformation range between Ms and Mf, so rough-stock allowances and finishing strategy are planned around it. For millwork-adjacent tools such as cutters, knives, and saw blades, lower Ms means more alloying, more retained austenite risk, and tighter quench control. Because martensite formation is athermal, holding near Ms does not complete the reaction; only further cooling continues it, so quench path and section thickness matter more than soak time.
- Quench stops too warm: The part exits the quench before enough undercooling below Ms, leaving retained austenite and lower hardness. Thick sections and highly alloyed steels with depressed Ms/Mf values are especially vulnerable.
- Composition assumptions are wrong: Shop travelers treat the same hardness grade as the same Ms, but carbon, manganese, nickel, chromium, and molybdenum shift Ms significantly. A recipe copied from one grade can produce a different microstructure in another.
- Distortion is underestimated: Martensite formation generates transformation strain and volume change, causing internal stress, bow, or twist. If fixtures, quench severity, or roughing stock ignore Ms-driven movement, finish machining can lose tolerance stack-up.
Is the martensite start temperature the same point where hardening begins in quenched steel?
Yes, in metallurgical terms. Ms marks the start of austenite transforming into martensite, which is the key hardening reaction in quenched steel.
Does holding a part at the martensite start temperature continue to form martensite over time?
No. The martensitic reaction is athermal; it is not time-driven at constant temperature. Further transformation requires additional cooling below Ms. Holding near Ms will not finish the transformation.
Why is alloy content so important for the martensite start temperature?
Alloying elements change the free-energy balance between austenite and martensite and alter austenite stability. This shifts the temperature at which martensite can first nucleate, which is why different grades harden differently under the same quench.