Austempering

Austempering is a heat treatment step where you hold the workpiece in a hot bath so that a tough, fine structure forms. This makes the steel less prone to cracking and improves the mechanical properties, especially toughness. If you don't hit the temperature or holding time cleanly, you won't get proper bainite, and the part becomes brittle. So: leave the workpiece in the hot bath until the transformation is complete, then cool in a controlled manner so that the structure forms uniformly.