Editing Heat treating (section)

===Selective heat treating===
{{Main|Differential heat treatment}}
Many heat treating methods have been developed to alter the properties of only a portion of an object. These tend to consist of either cooling different areas of an alloy at different rates, by quickly heating in a localized area and then quenching, by thermochemical diffusion, or by tempering different areas of an object at different temperatures, such as in [[differential tempering]]. {{citation needed|date=January 2021|reason=Need a reference for paragraph}}

====Differential hardening====
{{Main|Differential hardening}}
[[Image:Katana hardened edge pic with inset of nioi.JPG|thumb|A differentially hardened katana. The bright, wavy line following the [[hamon (swordsmithing)|hamon]], called the nioi, separates the martensitic edge from the pearlitic back. The inset shows a close-up of the nioi, which is made up of individual martensite grains (niye) surrounded by pearlite. The wood-grain appearance comes from layers of different compositions.|300x300px]]Some techniques allow different areas of a single object to receive different heat treatments. This is called [[differential hardening]]. It is common in high quality [[knife|knives]] and [[sword]]s. The Chinese [[jian]] is one of the earliest known examples of this, and the Japanese [[katana]] may be the most widely known. The Nepalese [[Khukuri]] is another example. This technique uses an insulating layer, like layers of clay, to cover the areas that are to remain soft. The areas to be hardened are left exposed, allowing only certain parts of the steel to fully harden when quenched. {{citation needed|date=January 2021|reason=Need a reference for paragraph}}

====Flame hardening====
{{Main|Surface hardening}}
Flame hardening is used to harden only a portion of the metal. Unlike differential hardening, where the entire piece is heated and then cooled at different rates, in flame hardening, only a portion of the metal is heated before quenching. This is usually easier than differential hardening, but often produces an extremely brittle zone between the heated metal and the unheated metal, as cooling at the edge of this [[heat-affected zone]] is extremely rapid. {{citation needed|date=January 2021|reason=Need a reference for paragraph}}

====Induction hardening====
{{Main|Induction hardening}}
Induction hardening is a [[surface hardening]] technique in which the surface of the metal is heated very quickly, using a no-contact method of [[induction heating]]. The alloy is then quenched, producing a martensite transformation at the surface while leaving the underlying metal unchanged. This creates a very hard, wear-resistant surface while maintaining the proper toughness in the majority of the object. [[Crankshaft]] journals are a good example of an induction hardened surface.<ref>''Surface hardening of steels: understanding the basics'' By Joseph R. Davis - ASM International 2002</ref>

====Case hardening====
{{Main|Case hardening}}
Case hardening is a thermochemical diffusion process in which an alloying element, most commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.<ref name="Dossett, 2006, 2-6" />

Laser surface engineering is a surface treatment with high versatility, selectivity and novel properties. Since the cooling rate is very high in laser treatment, metastable even metallic glass can be obtained by this method.