Editing Differential heat treatment (section)

===Bladesmithing===
[[File:Differentially tempered sword.jpg|thumb|300px|A differentially tempered sword. Made of 5160 carbon steel and balanced right at the end of the fullers, the edge has been tempered slightly harder than a hammer, whereas the center, hilt and handle are tempered to a spring hardness.]]

Eventually, this process was applied to swords and knives, to produce mechanical effects that were similar to differential hardening, but with some important differences. To differentially temper a blade, it is first quenched to harden the entire blade evenly. The blade is then heated in a localized area, allowing the heat to flow toward the edge. With single-edged blades, the blade may be tempered with fire or a torch. The blade is heated along the spine and tang only, allowing the heat to conduct to the edge. The heat will need to be applied evenly, allowing the colors to spread evenly across the blade. However, with double-edged blades, the heat source will usually need to be more precisely localized because the heat must be applied evenly along the center of the blade, allowing it to conduct to both edges. Often, a red or yellow-hot bar is used to supply the heat, placing it along the center of the blade, typically fitted into a [[fuller (weapon)|fuller]].<ref>''The Art of Blacksmithing'' by Dan W. Bealer -- Castle Books 1969 Page 359</ref> Modern gas torches often have the ability to produce very precise flames. To prevent too much heat loss in the blade, it may be preheated, partially insulated, or sandwiched between two red-hot bars. When the proper color reaches the edge, it is immersed in water to stop the process.<ref>''The Wonder of Knifemaking'' By Wayne Goddard – Krause Publications 2011 pp. 47–48</ref>

====Guiding the heat====
Differential tempering can be made more difficult by the shape of the blade. When tempering a double-edged sword with a taper along its length, the tip may reach the proper temperature before the shank does. The smith may need to control the temperature by using methods like pouring water along certain parts of the edge, or cooling it with ice, causing the proper temperature to reach the entire edge at the same time. In this way, although it is less time-consuming than differential hardening with clay, once the process starts the smith must be vigilant, carefully guiding the heat. This leaves little room for error, and mistakes in shaping the hardened zone cannot easily be corrected. This is made even more difficult if the knife or sword has a curve, an odd shape, or a sharply tapered tip. Swords tempered in this manner, especially double-edged swords, will generally need to be rather wide, allowing room for a gradient to form. However differential tempering does not alter the blade's shape.<ref name="autogenerated2007">''The Medieval Sword in the Modern World'' By Michael 'Tinker' Pearce – 2007 pp. 39, 112</ref>

====Metallurgy====
When a sword, knife or tool is evenly quenched, the entire object turns into martensite, which is extremely hard, without the formation of soft pearlite. Tempering reduces the hardness in the steel by gradually changing the martensite into a microstructure of various [[carbide]]s, such as [[cementite]], and softer [[Allotropes of iron|ferrite (iron)]], forming a microstructure called "[[Tempering (metallurgy)#Physical processes|tempered martensite]]". When tempering high-carbon steel in the blacksmith method, the color provides a general indication of the final hardness, although some trial-and-error is usually required to match the right color to the type of steel to achieve the exact hardness, because the carbon content, the heating speed, and even the type of heat source will affect the outcome. Without the formation of pearlite, the steel can be incrementally tempered to achieve the proper hardness in each area, ensuring that no area is too soft.<ref>''Advanced materials & processes, Volume 149'' By the American Society of Metals – ASM International p. 114</ref> In arming swords, for instance, because the blade is typically rather wide and thin, a blade can be prone to bending during combat. If the center of the blade is too soft, this bending may likely be permanent. However, if the sword is tempered to a springy hardness, it will be more likely to return to its original shape.<ref name="autogenerated2007"/>

====Benefits and drawbacks====
A sword tempered this way cannot usually have an edge as hard as a differentially-hardened sword, like a katana, because there is no softer metal directly underneath the edge to back-up the harder metal. This makes the edge more likely to chip away in larger pieces. Therefore, such an extremely hard edge is not always desirable, as greater hardness makes the edge more brittle and less resistant to impacts, such as cutting through bones, shafts of pole-arms, hitting shields or blocking and parrying. The sword will often be tempered to slightly higher temperatures to increase the impact resistance at a cost in the ability to hold a sharp edge when cutting. The edge may need to be tempered to dark-straw or brown to achieve this, and the center tempered to a blue or purple color. This may leave very little difference between the edge and the center, and the benefits of this method, over tempering the sword evenly at a point somewhere in the middle, may not be very substantial. When a sword tempered in this way is resharpened the hardness will decrease with each sharpening, although the reduction in hardness will usually not be noticeable until a large amount of steel has been removed.<ref name="autogenerated2007"/>