Explain the effect of 'Cryogenic Treatment' on the dimensional stability of high-carbon steel rings.

Cryogenic treatment (cooling to $-300^{\circ}F$ using liquid nitrogen) is sometimes employed to ensure the complete transformation of retained austenite into martensite in high-carbon steels. Retained austenite is unstable and can transform over time at room temperature, causing a slight volume expansion. For precision spiral rings in high-accuracy optical or navigation systems, this dimensional drift can be problematic. Cryogenic processing ensures a stable microstructure, maximizing the hardness and ensuring that the ring's free diameter $D_s$ remains constant over the tool's lifecycle.