austenite (gamma-iron)
/ AW-steh-nite /
Heat steel red-hot and its iron atoms rearrange into a face-centered-cubic packing called austenite. All at once it can dissolve a lot of carbon, it turns non-magnetic, and it becomes soft and easy to shape. This is the hot-working and the just-before-quenching state of steel.
More precisely, austenite (gamma-iron) is the FCC solid solution of carbon in iron, stable between about 727 and 1394 degrees C in plain steels. Its FCC structure has larger interstitial holes than BCC ferrite, so it dissolves far more carbon, up to 2.14 wt% at 1147 C versus ferrite's 0.022%. It is soft, ductile, and non-magnetic (paramagnetic). The phase is named after William Roberts-Austen.
Nearly all steel heat treatment starts by austenitizing, meaning heating up into the gamma field to dissolve the carbon uniformly. What happens next depends entirely on how you cool: slow cooling gives ferrite plus pearlite (equilibrium), while fast quenching traps the carbon and gives hard martensite (non-equilibrium, off the diagram). Some stainless steels are austenitic even at room temperature, stabilized by nickel, which is why they are non-magnetic.
You austenitize a steel around 850 degrees C before quenching; 304 stainless is austenitic and non-magnetic at room temperature.
Austenite's high carbon solubility, lost on cooling, is what drives hardening.
Austenite is normally a high-temperature phase, but nickel (as in 304 stainless) can stabilize it down to room temperature; its high carbon solubility, lost on cooling, is what makes hardening possible.