Martempering

Step hardening is a multi-stage quenching process where you don't cool the workpiece all at once, but in several steps. This relieves stresses more slowly and prevents the part from becoming too brittle or cracking. The trick is to choose the cooling rate so that you don't slam into the martensitic range, where the part becomes hard but also brittle and prone to fracture. Cooling too slowly makes the structure coarse and soft again. You need to find the right balance. In practice, this means: you heat the part to hardening temperature and then let it cool in stages, holding the temperature briefly between stages. You do this until the part has the desired hardness. It's important to develop a feel for the material and not just follow a rigid procedure.