Editing Differential heat treatment (section)

====Quenching====
[[File:Katana diagram of bending during quenching.JPG|thumb|The curving of a katana during quenching first begins with a downward bend as the edge cools, followed by an upward bend as the rest of the sword cools.]]
Once the coating has dried, the blade is heated slowly and evenly, to prevent the coating from cracking or falling off. After the blade is heated to the proper temperature, which is usually judged by the cherry-red glow ([[blackbody radiation]]) of the blade, it will change into a [[phase (matter)|phase]] called [[austenite]]. Both to help prevent cracking and to produce uniformity in the hardness of each area, the smith will need to ensure that the temperature is even, lacking any hot spots from sitting next to the coals. To prevent this, the blade is usually kept in motion while heating, to distribute the heat more evenly. Quenching is often done in low-light conditions, to help accurately judge the color of the glow. Typically, the smith will also try to avoid overheating the blade to prevent the metallic crystals from growing too large. At this time the blade will usually be plunged into a vat of water or oil, to quickly remove the heat from the edge. The clay, in turn, insulates the back of the blade, causing it to cool slower than the edge.<ref name="autogenerated114"/>

When the edge cools fast a [[diffusionless transformation]] occurs, turning the austenite into very hard martensite. This requires a temperature drop from around 750&nbsp;°C (cherry-red) to 450&nbsp;°C (at which point the transformation is complete) in less than a second to prevent the formation of soft [[pearlite]]. Because the rest of the blade cools slowly, the carbon in the austenite has time to [[precipitation (chemistry)|precipitate]], becoming pearlite. The diffusionless transformation causes the edge to "freeze" suddenly in a [[thermal expansion|thermally expanded]] state, but allows the back to contract as it cools slower. This typically causes the blade to bend or curve during quenching, as the back contracts more than the edge. This gives swords like [[katana]] and [[wakizashi]] their characteristic curved shapes. The blade is usually straight when heated but then bows as it cools; first curving toward the edge as it contracts, and then away from the edge as the spine contracts more. With slashing-type swords, this curvature helps to facilitate cutting, but increases the chances of cracking during the procedure. Up to one third of all swords are ruined during the quenching process.<ref name="pbs2">{{cite web|url=https://www.pbs.org/wgbh/nova/ancient/secrets-samurai-sword.html|title=NOVA &#124; Secrets of the Samurai Sword|publisher=pbs.org|access-date=27 May 2014}}</ref> However, when the sword does not crack, the internal stresses created help increase the toughness of the blade, similar to the increased toughness in [[tempered glass]].<ref name="jsme">{{cite web|url=http://www.jsme.or.jp/tsd/ICBTT/conference02/TatsuoINOUE.html|title=International Conference|publisher=jsme.or.jp|access-date=27 May 2014}}</ref> The sword may need further shaping after quenching and tempering, to achieve the desired curvature.<ref name="pbs"/>

Care must be taken to plunge the sword quickly and vertically (edge first), for if one side enters the quenching fluid before the other the cooling may be asymmetric and cause the blade to bend sideways (warp). Because quenching in water tends to cause a sudden loss of surface carbon, the sword will usually be quenched before the edge is beveled and sharpened. After quenching and tempering, the blade was traditionally given a rough shape with a metal-cutting [[draw knife]] (''sen'') before sending to a polisher for sharpening,<ref name="Smith, pp. 49–52">Smith, pp. 49–52</ref> although in modern times an electric [[belt sander]] is often used instead.