The Annealing/Normalization Process

Article by Christopher Lau / Motoyasu

There are many inexpensive replica swords on the mass market, often made in third world countries, mostly from what's marketed as "high carbon spring steel." The thing that cheap sword manufacturers often miss is heat treating. Improper heat treating can result in an inferior sword despite the use of good steel!

Motoyasu shares of his experience forging Japanese-style swords and his use of swords as a practitioner of Iaido. He is not only a Togi professional (traditional Japanese sword polishing) but holds the rank of Shodan in Iaido.

Annealing is the process of bringing a piece of steel up to its critical temperature, and then letting it cool very, very slowly (many hours to cool to room temperature). This causes the crystalline structure of the steel to become all pearlite/cementite/ferrite (depending on the carbon content). All these structures are extremely soft; the annealed condition represents about the softest state that steel can get without being heated to forging temperature.

Annealed steel really is very soft and can be cold forged somewhat. It grinds "like butter" (a length of bar stock could easily be bent in two along its narrow axis with your bare hands, and with a vise, it can also be quite easily bent along its widest dimension as well). This makes it very easy to form the steel to shape via grinding or forging.

After you grind or forge the bar to shape, you should perform a related process to annealing called "normalizing". There's not a lot of difference between annealing and normalizing. In normalizing, the goal is to relieve stresses and more importantly, to ensure a consistent and fine grain size rather than to soften the metal, so the slow cool isn't quite as critical for normalizing. The grinding and or heating/cooling and hammering performed during the shaping of the blade tends to induces stresses in the blade. Relieving work/heating induced stresses through normalization helps prevent or reduce the amount of warpage when you harden the blade, and refining the grain helps produce stronger blades that hold a better edge. When normalization is done, you can perform the hardening procedure - you heat the steel up to its critical temperature again, but this time, instead of letting it cool slowly, you quench it quickly in a quench medium (this could be water, brine, oil, or even air, depending on the type of steel you use) so that it cools very quickly. This causes the crystalline structure to re-arrange into a different form called martensite, which is an extremely hard form of steel capable of holding a keen edge.

Now although you relieved stresses prior to hardening, the quench procedure tends to induces new stresses of its own, so a subsequent tempering draw or the reheating of the blade to a relatively low temperature (well below red hot) results in stress relief and the decomposition of some of the martensite into another form called troosite which is slightly less hard, but much tougher. The blade thus becomes less brittle and gains more "spring". So you see that in addition to the usual heat-treating process of hardening and the subsequent drawing of temper, annealing and normalization can play an important role in the creation of a fine blade.

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