Transformation Point

When welding steel, you must know that the material changes its internal structure when heated. Above a certain temperature, the transformation point, the structure is re-formed. If you cool the part too quickly afterwards, it becomes hard and brittle, as an unfavorable structure like martensite forms. You rarely want that, because it can cause cracks. So don't leave the workpiece in a draft or on a cold stone, but cool it slowly and in a controlled manner. And don't hit it with a hammer while it's still hot.