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Heat Treatments and the Japanese Sword

June 2, 2017

As we develop our product line here at Koryu Crafts, we wanted to touch on one of the most distinguishing characteristics of many Japanese blades: the results produced by heat treatment.

 

For example, our Koryu Crafts Signature Shinken Line is made of differentially heat treated 1095 steel; although this is a more modern, mono-steel  construction (the blade is made of one type of steel as opposed to different types laminated together), we insisted that each blade receive a traditional differential heat treatment. This treatment ensures that the steel of the "ha", or cutting edge of the blade, is significantly harder than the "mune", or spine of the blade. This differential treatment helps the blade maintain flexibility thanks to the softer steel of the mune while better retaining a razor sharp cutting edge.

 

This ideal balance between flexibility and edge retention is achieved by the quenching process utilized while forging the sword. Prior to bringing the blade up to the appropriate quenching temperature in the forge, the smith will coat the ha and mune in different thicknesses of refractory clay; the different thicknesses allow the ha and mune to cool at different rates, changing the molecular structure of the steel in the process - hence the term "differential heat treatment".

 

Once plunged into the water, the ha will cool and harden first, forming a type of steel generally called "martensite" (depending on the type of steel used), The mune will cool at a slower rate and will remain a mixture of softer ferrite and pearlite. This quenching process is also where the blade will receive its signature "sori" or curve. If we could watch as the blade is quenched, we would see that as the smith plunges the glowing red blade into the water, the kissaki dips down, bending towards the bottom of the tank;this is due to the rapid quenching and constriction of the steel on the ha. We would then see the blade gradually curve back upwards towards the smith, taking it's final shape as the thicker mune is cooled at a slower rate.  If the smith is not careful, small surface cracks, warps, and critical failures can result during this extremely stressful process. It is undoubtedly one of the most difficult and rewarding moments during the construction of the traditional Japanese sword.

 

Out of the controlled chaos of the quench comes one of the swords' most beautiful and aesthetically pleasing qualities - the "hamon" or temper line. The hamon, which indicates the section of the blade that is now the hardened Martensite, is unique to each blade, and comes in many different shapes and styles that can be controlled by the smith through careful application of clay prior to the quenching process.

 

For the seasoned practitioner, blades that are differentially heat treated are ideal; their qualities, both aesthetic and functional, lend themselves well to both practice and appreciation. However, for those not well-versed in cutting with a Japanese-style sword, these blades can be challenging; the flexible spine tends to be unforgiving, and an improper hasuji (blade angle when cutting) could result in a bent blade. However, the ability of an edged weapon to bend without breaking is ideal; bends can be fixed easily enough by experienced practitioners.  We can see where, on the battlefield, it would be easier to fix a bent blade than to replace a broken one!


For the beginning cutter, however, we will be offering an alternative, through-hardened blade that is much more forgiving as you develop your technique. These blades do not undergo the differential heat treatment processes; instead, they are hardened uniformly all of the way through.  This is a more modern way of heat treating that results in a sharp, wear resistant sword that is still reasonably flexible but less likely to bend. Because of the different tempering process, these blades will lack the iconic hamon found on Koryu Crafts’ other offerings, but they will be strong, sharp, and excellent cutting katana.  Depending on your skill level and needs, this upcoming through-hardened Shoshin line of blades may perfectly match your needs.  More information will be coming soon!