Martensite Transformation and Retained Austenite (One Minute Mentor)

[Shade Tree Welder]

I have been getting these emails from ASM International.

June 2, 2006

The martensite start temperature (Ms), the temperature at which austenite begins to transform to martensite, decreases significantly with increasing carbon content in Fe-C alloys and carbon steels. The martensite finish temperature (Mf), or temperature at which the martensite transformation is complete in a given alloy, also is a function of carbon content, and drops below room temperature in alloys containing more than about 0.3% carbon. Therefore, significant amounts of untransformed austenite, especially in high-carbon steels, may be present with martensite at room temperature. Even in alloys containing only 0.3 to 0.4% carbon, some small amount of austenite is retained. Alloying elements that stabilize austenite increase the amount of retained austenite at any given carbon level and temperature.

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