Editing Differential heat treatment (section)

===Flame hardening===
{{Main|Surface hardening}}
[[File:Flame hardened sprocket.jpg|thumb|A flame-hardened timing-sprocket. The discoloration around the teeth indicate the metal that was heated and quenched, with the heat-affected zone visible as a thin layer between it and the ring of tempering colors.]]

Flame hardening is often used to harden only a portion of an object, by quickly heating it with a very hot flame in a localized area, and then quenching the steel. This turns the heated portion into very hard martensite, but leaves the rest unchanged. Usually, an [[oxy-gas torch]] is used to provide such high temperatures. Flame hardening is a very common surface hardening technique, which is often used to provide a very wear-resistant surface. A common use is for hardening the surface of [[gear]]s, making the teeth more resistant to [[erosion]]. The gear will usually be quenched and tempered to a specific hardness first, making a majority of the gear tough, and then the teeth are quickly heated and immediately quenched, hardening only the surface. Afterward, it may or may not be tempered again to achieve the final differential hardness.<ref>Prabhudev, pp. 394–400</ref>

This process is often used for knife making, by heating only the edge of a previously quenched and tempered blade. When edge turns the proper [[color temperature]], it is quenched, hardening only the edge, but leaving most of the rest of the blade at the lower hardness. The knife is then tempered again to produce the final differential hardness.<ref>''Blades Guide to Making Knives'' By Joe Kertzman – Krause Publications 2005 p. 47</ref> However, unlike a blade that has been evenly heated and differentially quenched, flame hardening creates a [[heat-affected zone]]. Unlike the nioi, the boundary between the hot and cold metal formed by this heat-affected zone causes extremely rapid cooling when quenched. When combined with the stresses formed, this creates a very brittle zone between the hard and softer metal, which usually makes this method unsuitable for swords or tools that may be subjected to shear and impact stresses.<ref>''Steel Metallurgy for the Non-Metallurgist'' By John D. Verhoeven – ASM International 2007 p. 51</ref>