Retained austenite content
Retained austenite content refers to the volume fraction of the high-temperature face-centered cubic phase that persists in the microstructure at room temperature following incomplete martensitic or bainitic transformation. This fraction influences the deformation behavior and dimensional stability of the component during service.
Retained austenite is austenite that has not transformed after welding or heat treatment and remains at room temperature. If you have too much of it in the microstructure, the component can continue to deform under load and lose strength. You usually don't want that. To get it under control, you look at the microstructure under a microscope. If the proportion is too high, you have to heat-treat the part specifically; a normal tempering often isn't enough. Then the retained austenite transforms and the microstructure becomes more stable again.