Austenite Grain Growth

If you heat steel too much or hold it at high temperature for too long, the austenite grains grow. That happens by diffusion at the grain boundaries. A coarse grain makes the material brittle and low in toughness after cooling – you don't want that in the weld. So keep the heat input as low as possible and don't cool too slowly, then the structure stays fine. Especially with higher-strength steels, you have to be careful not to put too much heat into the weld, otherwise you'll have trouble with toughness.