Martensite and residual austenite

Is retained austenite a villain?

First, by heating iron above the transformation point temperature, it is converted into an austenite structure, allowing many carbon atoms to enter the iron atoms. Next, by rapidly cooling it, a very hard structure (martensite structure) is formed, in which many carbon atoms are trapped within the iron atoms in a cramped arrangement. This is called martensitic transformation, and the hardness after quenching increases with the amount of carbon. When cooling from austenite, the faster the cooling rate, the more martensite structure is formed. The temperature at which martensitic transformation occurs, known as the Ms point, decreases with an increasing amount of carbon, and the temperature at which the transformation ends, known as the Mf point, also decreases. The austenite structure that could not transform remains as "residual austenite." If left alone, residual austenite may gradually transform, potentially causing issues such as distortion or cracking. Therefore, it is necessary to forcibly transform it into martensite or stabilize the structure. However, since residual austenite has a lower hardness compared to martensite, in cases where wear resistance is required, such as on gear tooth surfaces, the sliding properties of residual austenite may also contribute positively.

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