Aging

After quenching, the structure is hard and brittle. During aging, you let the part warm up in a controlled manner so the structure can settle. This takes time and you must keep the temperature clean, otherwise the hardness becomes uneven and the part can crack. If aging is too short or too weak, the material remains too soft. If it's too intense, it becomes brittle again. You have to hit the window, then the final hardness is right.