Editing Heat treating (section)

===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>