Diffrential Hardening Process
Differential hardening is a process that is used to provide sharpness and flexibility to the sword blade. Clay tempered katana swords are hard and strong. The differential hardened sword has to go through extreme stress. It can cause bending, corkscrewing and ripping. It depends upon the way the clay is placed onto the sword blade. It depends upon the evenness of the clay layer, the symmetry and the overall distribution of the clay. The hardened steel provides a better shape for the sword blade. It gives the real curvature of the sword.
Many cheap swords are through hardened. This means that the hardness of the entire blade is same. This is the best method as you have to keep the temperature of the sword same for hardening the blade from all the sides. It is also not treated with clay and the blade will have to bear the similar stress at all the sides.
The curve is provided when the steel is soft and it would not change its shape during the quenching process. Differential heating is also called as selective heat treatment. It is a technique that is used during the heat treatment given to the sword blade to harden or soften certain areas of the steel. It creates a difference in the hardness of the sword.
Differential hardening can be done in two ways. In one method, the metal can be heated evenly to a very hot temperature and then cool it at different rates. Another method consists of heating only a part of the metal quickly to a very high temperature and rapidly cooling it which is also called as quenching. This makes only a part of a hard substance and the rest of the part remains unchanged. Thus, differential heating involves either the heating of the entire metal and the cooling it at the same tie changing the complete blade or heating only a part of it and cooling it slowly leaving the other part unchanged. The choice entirely depends upon the wordsmith. Some warriors like to choose a sword that is made up of clay tempered steel in which the entire sword is heated and then cooled.