Putting the curve in a katana via quenching
When looking at the graceful curve of a katana, many imagine the smith hammering on a folded steel blank to create the final shape.
Outside of serious students of forging and the study of nihonto (Japanese swords), few people realize that this curve — known as “sori” in Japanese — isn’t the result of any mechanical manipulation of the steel.
It’s actually directly created by the quenching process, which you can see happening in the following video; the blade goes through a serious of deformations (shape changes) as it cools.
When getting near its finished forging state, the smith would coat the with a combination of clay and other elements; while the entire blade would be coated, with a thicker coat applied towards the back of the blade.
The blade would then be fired in the forge, the heat causing the carbon and other materials to fuse with the coated steel.
The heated blade would then be quenched, the liquid being used in quenching depending on the smith in question. The rapid cooling of the quenching would force the carbon-laced clay-coated steel to form the orderly, super-hard carbon-crystal material known as martensite.
The result would be a hard “skin” over most of the blade, but most particularly near the cutting edge, where the thinner coat of carbon-clay allow the edge to be brought to a higher temperature, causing almost all the steel to transform into martensite. This could then be polished into an extremely sharp and long-lasting cutting edge.
As an aside, the manner in which smiths applied the coat of clay near the cutting edge would also create the “hamon”; the distinctive beautiful temper patterns Japanese blades are known for. Thus, in the image to the right you see a smith applying the clay in patterns which will create a beautiful finished hamon when polished.
The border between the mostly martensite cutting edge and the softer steel of the rest of the blade is what creates this pattern.
Of course the interior of the blade, relatively untouched by the tempering clay, remains a “softer” form of steel, giving the blade flexibility and the ability to withstand impact without breaking.
In being fired, the thinner edge of the blade would become much hotter than the thicker and heavily coated back. During quenching the hotter steel near the edge cools at a different rate than the sides and back of the blade. As you might expect, as metal cools it tends to shrink slightly. The difference in cooling rate between edge and and spine govern the amount of shrinkage, and the difference between the shrinkage rates is what causes the blade to curve.
At first plunge the thinner edge gives up it’s heat quickly and more drastically, with shrinkage pulling the tip down. The formation of martensite crystals then quickly fixes the edge, preventing any further shrinkage. The back of the blade continues to cool and shrink even more, pulling the blade up in its characteristic curve after a few seconds.
A smith would carefully watch the quenching of a blade, pulling it out to stop the curving of the steel when it was precisely at the correct angle.
Researchers in Japan created a computer model to study this: The Japanese Sword: The Material, Manufacturing and Computer Simulation of Quenching Process. While a somewhat technically dense article, their model did show snapshots of this process.
In the first image, the researchers looked at how martensite crystals would form on a blade after firing and quenching, depending on the clay coating.
The third example (c), where the edge is lightly coated and rest of the blade more heavily covered, shows the creation of the desired structures along the edge (in red), with the majority of the blade remaining a softer form of steel. Both (a) and (b) examples result in blades which either have too a soft cutting edge, or too hard a body — meaning the blade would be fragile and prone to breaking.
In the second chart from the paper, the author’s computer model shows the steps in the quenching deformation process as the blade cools in the quenching tank over ten seconds. The blade starts at an even temperature of 850 degrees Celcius. It then enters the tank and beings to cool, the thinner and less covered edge first, then the thicker and more heavily covered back last.
Those knowledgeable about nihonto will realize that, at different periods, the style of katana or tachi changed. During certain periods the curve was preferred nearer the hilt; in others it was centered in the middle of the blade, in yet other periods placed nearer the cutting tip.
We begin the realize the level of skill of the smith in not only creating the steel of the blade, but in their control of the quenching process. To have the skill to not only create the steel, but to control the placement and amount of curvature simply through quench speaks volumes of their experience, knowledge and ability.
With thanks to The Japanese Sword‘s Facebook page, I’d like to credit the maker of the video listed above: sword smith Kokuten Komiya‘s page on Facebook, video link: https://www.facebook.com/video.php?v=245811752214385
The Japanese Sword: The Material, Manufacturing and Computer Simulation of Quenching Process: Inoue Tatsuo, Materials Science Research International, Vo1.3, No.4 pp. 193-203 (1997)