Austenitizing

Austenitizing means: you heat the steel until its structure completely transforms into austenite. You do this before hardening or welding it, for example. Only when the structure is fully austenitic will the subsequent transformation be uniform. If the temperature is too low or the holding time too short, remnants of the old structure remain – that later causes hardness differences and stresses that can lead to cracks during welding or hardening. So: heat it up properly, give it enough time, then the grain will be fine.