Japanese Swordsmithing - Traditional Methods - Construction - Heat Treating

Heat Treating

Having a single edge also has certain advantages, one of the most important being that the entire rest of the sword can be used to reinforce and support the edge, and the Japanese style of sword making takes full advantage of this. When finished, the steel is not quenched or tempered in the conventional European fashion i.e. uniformly throughout the blade. Steel’s exact flex and strength vary dramatically with heat treating. If steel cools quickly it becomes martensite, which is very hard but brittle. Slower and it becomes pearlite, which bends easily and does not hold an edge. To maximize both the cutting edge and the resilience of the sword spine, a technique which originated in China during the first century BC is used. In this process which is referred to as differential hardening or differential quenching the sword is heated and painted with layers of clay—the mixture being closely guarded trade secrets of the various smiths, but generally containing clay and coal ash as the primary ingredients—with a thin layer or none at all on the edge of the sword ensuring quick cooling to maximize the hardening for the edge, while a thicker layer of clay on the rest of the blade causing slower cooling and softer, more resilient steel to allow the blade to absorb shock without breaking. This process is sometimes erroneously called differential tempering but this is actually an entirely different form of heat treatment.

When steel with a carbon content of 0.7 percent is heated beyond 750 degrees C it enters the "austenite phase". When austenite is cooled very suddenly by quenching in water the structure changes into "martensite", which is an extremely hard form of steel. When austenite is allowed to cool slowly its structure changes into a mixture of ferrite and pearlite which is softer than martensite. By carefully controlling the heating and cooling of the blade being forged Japanese swordsmiths were able to produce a blade that had a softer body and a hard edge creating a superior weapon. This process also has two side effects that have come to characterize Japanese swords—first, it makes the edge of the blade, which cools quickly and forms evenly dispersed cementite particles embedded within a ferrite matrix (typical of tempered martensite), which will actually cause the edge part of the blade to expand while the sword spine remains hot and pliable for several seconds, which aids the smith in establishing the curvature of the blade. Second, the differentiated heat treatment and the materials with which the steel comes into contact creates different coloration in the steel, resulting in the Hamon 刃紋 (frequently translated as "tempering line" but really better translated as "tempering pattern"), that is used as a factor to judge both the quality and beauty of the finished blade. The differentiated Hamon patterns resulting from the manner in which the clay is applied can also act as an indicator of the style of sword making, and sometimes also as a signature for the individual smith.