Austenite grain
An individual crystallite grain of the high-temperature austenite phase prior to the onset of phase transformation, whose initial size strongly dictates the formation of the subsequent transformation microstructure. A fine austenite grain favors the creation of fine transformation products with enhanced toughness.
The austenite grain forms during heating in the high-temperature range, before the structure transforms during cooling. The finer this grain is kept, the tougher and more uniform the final structure after welding. If it becomes too coarse, you get an unfavorable transformation structure and the crack susceptibility increases. So you have to control the heat input and cooling so that the grain does not grow unnecessarily. Keep the dwell time in the critical temperature zone short and do not cool too slowly. The exact settings depend on the steel grade, but the principle remains: fine grain before transformation means better behavior in the component.