Martempering

Step hardening is a process where you bring the steel in a hot bath to a temperature just above the martensite start temperature and hold it there. This equalizes the temperatures in the part and minimizes stresses because no martensite forms yet. If you skip this or cool too fast, you get cracks and distortion. It's important that you know the temperature for the respective material and give the workpiece enough time in the bath so it becomes uniformly warm everywhere.