Editing Differential heat treatment (section)

==Introduction==
Differential heat treatment is a method used to alter the properties of various parts of a steel object differently, producing areas that are harder or softer than others. This creates greater [[toughness]] in the parts of the object where it is needed, such as the tang or spine of a sword, but produces greater hardness at the edge or other areas where greater [[impact resistance]], [[wear resistance]], and [[strength of materials|strength]] is needed. Differential heat treatment can often make certain areas harder than could be allowed if the steel was uniformly treated, or "through treated". There are several techniques used to differentially heat treat steel, but they can usually be divided into ''differential hardening'' and ''differential tempering'' methods.

During [[heat treating]], when red-hot steel (usually between {{convert|1500|F|C}} and {{convert|1600|F|C}}) is quenched, it becomes very hard. However, it will be too hard, becoming very brittle like glass. Quenched-steel is usually heated again, slowly and evenly (usually between {{convert|400|F|C}} and {{convert|650|F|C}}) in a process called tempering, to soften the metal, thereby increasing the toughness. However, although this softening of the metal makes the blade less prone to breaking, it makes the edge more susceptible to deformation such as dulling, peening, or curling.<ref name="autogenerated115">''Knife Talk II: The High Performance Blade'' By Ed Fowler – Krause Publications 2003 p. 115</ref>

''Differential hardening'' is a method used in [[heat treating]] [[sword]]s and [[knife|knives]] to increase the hardness of the edge without making the whole blade [[brittle]]. To achieve this, the edge is cooled faster than the spine by adding a heat insulator to the spine before [[quenching]]. [[Clay]] or another material is used for insulation. To prevent cracking and loss of surface carbon, quenching is usually performed before beveling, shaping, and sharpening the edge.
<ref>{{cite book|author=James Drewe |title=Tàijí Jiàn 32-Posture Sword Form |url=https://books.google.com/books?id=U6UdWcIx95YC&pg=PT11 |access-date=11 November 2012 |date=28 February 2009 |publisher=Singing Dragon |isbn=978-1-84819-011-5 |pages=11–}}</ref><ref>{{cite book|author=Richard Cohen |title=By the Sword: A History of Gladiators, Musketeers, Samurai, Swashbucklers, and Olympic Champions |url=https://books.google.com/books?id=inND87EFFF4C&pg=PA124 |access-date=11 November 2012 |date=5 August 2003 |publisher=Random House Digital, Inc. |isbn=978-0-8129-6966-5 |pages=124–}}</ref> It can also be achieved by carefully pouring water (perhaps already heated) onto the edge of a blade as is the case with the manufacture of some [[kukri]]. Differential hardening technology originated in China and later spread to Korea and Japan. This technique is mainly used in later Chinese [[jian]], Chinese [[dao]], and the [[katana]], the traditional [[Japan]]ese sword, and the [[khukuri]], the traditional [[Nepal]]ese knife. Most blades made with this technique have visible temper lines. Earlier Chinese [[jian]] from the ancient era (eg. [[Warring States]] to [[Han dynasty]]) used [[tempering (metallurgy)|tempering]] rather than differential heat treatment. This method is sometimes called differential tempering, but this term more accurately refers to a different technique, which originated with the broadswords of Europe.

Modern versions of differential hardening were developed when sources of rapidly heating the metal were devised, such as an [[oxy-acetylene torch]] or [[induction heating]]. With flame hardening and [[induction hardening]] techniques, the steel is quickly heated to red-hot in a localized area and then quenched. This hardens only part of the object, but leaves the rest unaltered.

''Differential tempering'' was more commonly used to make cutting tools, although it was sometimes used on knives and swords as well. Differential tempering is obtained by quenching the sword uniformly, then tempering one part of it, such as the spine or the center portion of double edged blades. This is usually done with a torch or some other directed heat source. The heated portion of the metal is softened by this process, leaving the edge at the higher hardness.<ref>[http://www.primitiveways.com/pt-knives-1.html How to Make Your Own Steel Knifes (sic)]. Primitiveways.com. Retrieved on 2012-11-11.</ref>