Supercooled austenite microstructure
A metastable state of the face-centered cubic iron crystal below its thermodynamic transformation temperature prior to the onset of phase transformation. It forms the baseline for hardening and tempering processes.
You get undercooled austenite when you cool the steel so fast that it doesn't have time to transform in time. The austenite then just remains below its transformation temperature and is unstable in that state. When welding, this is tricky: if you don't process the part properly afterwards, the structure can tip into undesirable hard or brittle phases during cooling. Too fast cooling gives you hard martensite, too slow cooling makes the grain coarsen and the material tough. In practice, you have to control the cooling rate. You want to hit a range where the austenite transforms cleanly without risking cracks or deteriorating the structure. That requires a feel for the material and the heat input.