Editing Differential heat treatment (section)

===Bladesmithing===
[[File:Katana cross section diagram showing different zones of hardness.JPG|thumb|Diagram of a cross section of a katana, showing the typical arrangement of the harder and softer zones.]]
'''Differential hardening''' (also called differential quenching, selective quenching, selective hardening, or local hardening) is most commonly used in [[bladesmithing]] to increase the [[toughness]] of a blade while keeping very high [[hardness]] and [[strength of materials|strength]] at the edge. This helps to make the blade very resistant to breaking, by making the spine very soft and bendable, but allows greater hardness at the edge than would be possible if the blade was uniformly [[quenching|quenched]] and [[tempering (metallurgy)|tempered]]. This helps to create a tough blade that will maintain a very sharp, wear-resistant edge, even during rough use such as found in combat.

====Insulation coatings====
A differentially hardened blade will usually be coated with an insulating layer, like clay, but leaving the edge exposed. When it is heated to red-hot and quenched, the edge cools quickly, becoming very hard, but the rest cools slowly, becoming much softer.<ref name="autogenerated114">''Knife Talk II: The High Performance Blade'' By Ed Fowler – Krause Publications 2003 pp. 114–115</ref><ref name="pbs">[https://www.pbs.org/wgbh/nova/ancient/secrets-samurai-sword.html NOVA | Secrets of the Samurai Sword]. Pbs.org. Retrieved on 2012-11-11.</ref> The insulation layer is quite often a mixture of clays, ashes, polishing stone powder, and salts, which protects the back of the blade from cooling very quickly when quenched.<ref name="samuraisword1">[http://www.samuraisword.com/REFERENCE/making/japanse_swordmaking_process.htm Japanse Swordmaking Process ~ www.samuraisword.com] {{Webarchive|url=https://web.archive.org/web/20090923131554/http://www.samuraisword.com/REFERENCE/making/japanse_swordmaking_process.htm |date=2009-09-23 }}. Samuraisword.com. Retrieved on 2012-11-11.</ref><ref name="autogenerated1960">Smith, p. 49</ref> The clay is often applied by painting it on, coating the blade very thickly around the center and spine, but leaving the edge exposed. This allows the edge to cool very quickly, turning it into a very hard [[microstructure]] called [[martensite]], but causes the rest of the blade to cool slowly, turning it into a soft microstructure called [[pearlite]]. This produces an edge that is exceptionally hard and brittle, but is backed-up by softer, tougher metal. The edge, however, will usually be too hard, so after quenching the entire blade is usually tempered to around {{convert|400|F|C}} for a short time, to bring the hardness of the edge down to around HRc60 on the [[Rockwell hardness]] scale.<ref name="autogenerated114"/>

The exact composition of the clay mixture, the thickness of the coating, and even the temperature of the water were often closely guarded secrets of the various bladesmithing schools.<ref name="autogenerated1960"/> With the clay mixture, the main goal was to find a mixture that would withstand high temperatures and adhere to the blade without shrinking, cracking, or peeling as it dried. Sometimes the back of the blade was coated with clay, leaving the edge exposed. Other times the entire blade was coated and then the clay was cut away from the edge. Another method was to apply the clay thickly at the back but thinly at the edge, providing a lesser amount of insulation. By controlling the thickness of the edge-coating along with the temperature of the water, the cooling rate of each part of the blade can be controlled to produce the proper hardness upon quenching without the need for further tempering.<ref name="samuraisword1"/><ref name="autogenerated1960"/>

====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.

====Metallography====
Differential hardening will produce two different zones of hardness, which respond differently to grinding, sharpening, and polishing. The back and center of the blade will grind away much quicker than the edge, so the polisher will need to carefully control the angle of the edge, which will affect the geometry of the blade. An inexperienced polisher can quickly ruin a blade by applying too much pressure to the softened areas, rapidly altering the blade's shape without much change to the hardened zone.<ref name="autogenerated3">Smith</ref>

[[File:Katana hardened edge pic with inset of nioi.JPG|thumb|A katana, shown at a long angle to reveal the nioi, which is the bright line following the hamon. The inset shows a close up of the nioi, which appears as the speckled area between the bright hardened edge and the darker soft zone. The wood-grain appearance is from the folding techniques used during [[forging]].]]
Although both the pearlite and martensite can be polished to a mirror-like shine, only the back and spine are usually polished to such an extent. The hardened portion of the blade (yakiba) and the center portion (hira) are often given a matte finish instead, to make the differences in the hardness stand out. This causes the various microstructures to reflect light differently when viewed from different angles. The pearlite takes on longer, deeper scratches, and either appears shiny and bright, or sometimes dark depending on the viewing angle. The martensite is harder to scratch, so the microscopic abrasions are smaller. The martensite usually appears brighter yet flatter than the pearlite, and this is less dependent on the viewing angle.<ref name="autogenerated3"/> When polished or etched with acid to reveal these features, a distinct boundary is observed between the martensite portion of the blade and the pearlite. This boundary is often called the "temper line", or the commonly used Japanese term, the "[[hamon (swordsmithing)|hamon]]". Between the hardened edge and the hamon lies an intermediate zone, called the '"nioi" in Japanese, which is usually only visible at long angles. The nioi is about a millimeter or two wide, following the hamon, which is made up of individual martensite grains (niye) surrounded by pearlite. The nioi provides a very tough boundary between the yakiba and the hira.<ref name="Smith, pp. 49–52"/>

====Decorative hardening====
In Japan, from the legendary time of the famous smith [[Amakuni]], hamons were originally straight and parallel to the edge, but by the twelfth century AD, smiths such as [[Shintogo Kunimitsu]] began producing hamons with very irregular shapes, which provided both mechanical and decorative benefits. By the sixteenth century AD, the Japanese smiths often overheated their swords slightly before quenching, to produce rather large niye for aesthetic purposes, even though a larger grain size tended to weaken the sword a bit. During this time, great attention began to be paid in Japan to making decorative hamons, by carefully shaping the clay. It became very common during this era to find swords with wavy hamons, flowers or clovers depicted in the temper line, rat's feet, trees, or other shapes. By the eighteenth century, decorative hamons were often being combined with decorative folding techniques to produce entire landscapes, complete with specific islands, crashing waves, hills, mountains, rivers, and sometimes low spots were cut in the clay to produce niye far away from the hamon, creating effects such as birds in the sky.<ref>Smith, pp. 41, 50–51</ref>

====Benefits and drawbacks====
Although differential hardening produces a very hard edge, it also leaves the rest of the sword rather soft, which can make it prone to bending under heavy loads, such as parrying a hard blow. It can also make the edge more susceptible to chipping or cracking. Swords of this type can usually only be resharpened a few times before reaching the softer metal underneath the edge. However, if properly protected and maintained, these blades can usually hold an edge for long periods of time, even after slicing through bone and flesh, or heavily matted bamboo to simulate cutting through body parts, as is in [[iaido]].<ref>''The Medieval Sword in the Modern World'' By Michael 'Tinker' Pearce – 2007 p. 39</ref>