Editing Differential heat treatment (section)

===Blacksmithing===
The most common use for differential tempering was for heat treating cutting tools, such as [[axe]]s and [[chisel]]s, where an extremely hard edge is desired, but some malleability and springiness is needed in the rest of the tool. A chisel with a very hard edge can maintain that edge longer and cut harder materials, but, if the entire chisel was too hard, it would shatter under the hammer blows. Differential tempering was often used to provide a very hard cutting edge, but to soften parts of the tool that are subject to impact and shock loading.<ref>''The Complete Modern Blacksmith'' By Alexander Weygers – Ten Speed Press 1997</ref>

Before a tool is differentially tempered, it is first heated to red-hot and then quenched, hardening the entire tool. This makes the tool much too hard for normal use, so the tool is tempered to reduce the hardness to a more suitable point. However, unlike normal tempering, the tool is not heated evenly. Instead, the heat is applied to only a part of the tool, allowing the heat to [[thermal conductivity|thermally conduct]] toward the cooler cutting edge. The quenched-steel is first sanded or polished, to remove any residual [[iron oxide|oxidation]], revealing the bare metal underneath. The steel is then heated in a localized area, such as the hammering-end of a chisel or the handle-end of an axe. The smith then carefully gauges the temperature by watching the [[tempering (metallurgy)#Tempering colors|tempering colors]] of the steel. As the steel is heated, these colors will form, ranging from yellow to brown, purple, and blue, and many shades in between, and will indicate the temperature of the steel. As heat is applied, the colors will form near the heat source, and then slowly move across the tool, following the heat as it conducts toward the edge.<ref name="autogenerated4">''Blacksmithing Basics for the Homestead By Joe DeLaRonde – Gibbs Smith 2008 p. 101</ref>

Before the yellow or "light-straw" color reaches the edge, the smith removes the heat. The heat will continue to conduct, moving the colors toward the edge for a short time after the heat is removed. When the light-straw color reaches the edge, the smith will usually dip the steel in water, to stop the process. This will generally produce a very hard edge, around HRc58-60 on the Rockwell scale, but will leave the opposite end of the tool much softer. The hardness of the cutting edge is generally controlled by the chosen color, but will also be affected primarily by the carbon content in the steel, plus a variety of other factors. The exact hardness of the soft end depends on many factors, but the main one is the speed at which the steel was heated, or how far the colors spread out. The light-straw color is very hard, brittle steel, but the light-blue is softer and very springy. Beyond the blue color, when the steel turns grey, it is more likely to be very malleable, which is usually undesirable in a chisel. If the steel is too soft it can bend or mushroom, [[plastic deformation|plastically deforming]] under the force of the hammer.<ref name="autogenerated4"/>

====Grade of temper====
Unlike with differential hardening, in differential tempering there is no distinct boundary between the harder and softer metals, but the change from hard to soft is very gradual, forming a [[continuum (theory)|continuum]], or "grade" (gradient), of hardness. However, higher heating temperatures cause the colors to spread less, creating a much steeper grade, while lower temperatures can make the change more gradual, using a smaller portion of the entire continuum. The tempering colors only represent a fraction of the entire grade, because the metal turns grey above {{convert|650|F|C}}, making it difficult to judge the temperature, but the hardness will continue to decrease as the temperature rises.<ref>''Fracture Mechanics: Fourteenth Symposium – STP 791'' By the American Society for Testing and Materials – ASTM 1983 pp. 148–150</ref><ref>''Heat Treater's Guide: Practices and Procedures for Irons and Steels'' By Harry Chandler – ASM International pp. 98–99</ref>

====Guiding the heat====
[[File:Differentially tempered chisel.jpg|thumb|300px|A differentially tempered cold chisel]]Heating in just one area, like the flat end of a [[center punch]], will cause the grade to spread evenly down the length of the tool. Because having a continuous grade along the length of the entire tool is not always desired, methods of concentrating the change have been devised. A tool like a chisel may be heated quickly but evenly along the entire shaft, tempering it to a purple or blue color, but allowing the residual heat to quickly conduct a short distance to the edge. Another method is to hold the edge in water, keeping it cool as the rest of the tool is tempered. When the proper color is reached, the edge is removed from the water and allowed to temper from the residual heat, and the entire tool is plunged in the water when the edge turns the proper color. However, heating in localized areas with such low temperatures may be difficult with larger items, like an axe or a [[splitting maul]], because the steel may lose too much heat before it can conduct to the edge. Sometimes the steel is heated evenly to just below the desired temperature, and then differentially tempered, making it easier to control the temperature change. Another way is to partially embed the steel in an insulator, like sand or lime, preventing too much heat loss during tempering.