Hardness of lower bainite (tempered martensite) is about

Answer: Option C

Lower bainite is a microstructure in steel that forms at temperatures slightly above the martensite start temperature but lower than the range for upper bainite formation.

It is considered a form of tempered martensite because it consists of fine ferrite plates with dispersed carbide particles, which provide a good balance of strength and toughness.

When martensite is tempered, its extreme hardness is reduced to improve toughness, but lower bainite retains a relatively high hardness compared to other tempered structures.

On the Rockwell C (RC) scale, lower bainite typically measures around RC 57, making it harder than RC 48 but softer than untempered martensite at RC 65 or above.

This hardness level is ideal for applications needing both high strength and improved ductility.