Editing Heat treating (section)

==Effects of composition==
[[File:Iron carbon phase diagram.svg|thumb|Phase diagram of an iron-carbon alloying system. Phase changes occur at different temperatures (vertical axis) for different compositions (horizontal axis). The dotted lines mark the eutectoid (A) and eutectic (B) compositions.|400x400px]]The specific composition of an alloy system will usually have a great effect on the results of heat treating. If the percentage of each constituent is just right, the alloy will form a single, continuous microstructure upon cooling. Such a mixture is said to be [[Eutectic#Eutectoid|eutectoid]]. However, If the percentage of the solutes varies from the eutectoid mixture, two or more different microstructures will usually form simultaneously. A hypo eutectoid solution contains less of the solute than the eutectoid mix, while a hypereutectoid solution contains more.<ref name="Patra, 75-77" >{{Cite book
  |title=Engineering Chemistry I
  |author=B.B. Patra
  |author2=Biswajit Samantray 
  |publisher=Dorling Kindersley 
  |year=2011
  |pages=75–77
}}</ref>

===Eutectoid alloys===
A [[eutectoid]] ([[eutectic]]-like) [[alloy]] is similar in behavior to a [[eutectic alloy]]. A ''[[eutectic]]'' alloy is characterized by having a single [[melting point]]. This melting point is lower than that of any of the constituents, and no change in the mixture will lower the melting point any further. When a molten eutectic alloy is cooled, all of the constituents will crystallize into their respective phases at the same temperature.

A '''eutectoid alloy''' is similar, but the phase change occurs, not from a liquid, but from a [[solid solution]]. Upon cooling a eutectoid alloy from the solution temperature, the constituents will separate into different [[crystal phase]]s, forming a single [[microstructure]]. A eutectoid steel, for example, contains 0.77% [[carbon]]. Upon cooling slowly, the solution of [[iron]] and carbon (a single phase called [[austenite]]) will separate into [[platelets]] of the phases [[Allotropes of iron|ferrite]] and [[cementite]]. This forms a layered microstructure called [[pearlite]].

Since pearlite is harder than iron, the degree of softness achievable is typically limited to that produced by the pearlite. Similarly, the [[hardenability]] is limited by the continuous martensitic microstructure formed when cooled very fast.<ref>{{Cite book
  |title=Practical heat treating
  |first1=Jon L.  |last1=Dossett  |first2=Howard E.  |last2=Boyer 
  |publisher=ASM International 
  |year=2006 
  |pages=17–22
}}</ref>

===Hypoeutectoid alloys===
A hypoeutectic alloy has two separate melting points. Both are above the eutectic melting point for the system but are below the melting points of any constituent forming the system. Between these two melting points, the alloy will exist as part solid and part liquid. The constituent with the higher melting point will solidify first. When completely solidified, a hypoeutectic alloy will often be in a solid solution.

Similarly, a '''hypoeutectoid alloy''' has two critical temperatures, called "arrests". Between these two temperatures, the alloy will exist partly as the solution and partly as a separate crystallizing phase, called the "pro eutectoid phase". These two temperatures are called the upper (A<sub>3</sub>) and lower (A<sub>1</sub>) transformation temperatures. As the solution cools from the upper transformation temperature toward an insoluble state, the excess base metal will often be forced to "crystallize-out", becoming the pro eutectoid. This will occur until the remaining concentration of solutes reaches the eutectoid level, which will then crystallize as a separate microstructure.

For example, a hypoeutectoid steel contains less than 0.77% carbon. Upon cooling a hypoeutectoid steel from the austenite transformation temperature, small islands of proeutectoid-ferrite will form. These will continue to grow and the carbon will recede until the eutectoid concentration in the rest of the steel is reached. This eutectoid mixture will then crystallize as a microstructure of pearlite. Since ferrite is softer than pearlite, the two microstructures combine to increase the [[ductility]] of the alloy. Consequently, the hardenability of the alloy is lowered.<ref name="Dossett, 2006, 17-22" >{{harvnb|Dossett|Boyer|2006|pages=17–22}}</ref>

===Hypereutectoid alloys===
[[File:Ülieutektoidne teras.jpg|thumb|Hypereutectoid steel]]
A hypereutectic alloy also has different melting points. However, between these points, it is the constituent with the higher melting point that will be solid. Similarly, a '''hypereutectoid alloy''' has two critical temperatures. When cooling a hypereutectoid alloy from the upper transformation temperature, it will usually be the excess solutes that crystallize-out first, forming the pro-eutectoid. This continues until the concentration in the remaining alloy becomes eutectoid, which then crystallizes into a separate microstructure.

A hypereutectoid steel contains more than 0.77% carbon. When slowly cooling hypereutectoid steel, the cementite will begin to crystallize first. When the remaining steel becomes eutectoid in composition, it will crystallize into pearlite. Since cementite is much harder than pearlite, the alloy has greater hardenability at a cost in ductility.<ref name="Patra, 75-77" /><ref name="Dossett, 2006, 17-22"/>