Editing Heat treating (section)

==Physical processes==
[[File:IronAlfa&IronGamma.svg|thumb|Allotropes of iron, showing the differences in lattice structures between alpha iron (low temperature) and gamma iron (high temperature). The alpha iron has no spaces for carbon atoms to reside, while the gamma iron is open to the free movement of small carbon atoms.|300x300px]]
[[file:Photomicrograph of annealed and quenched steel, from 1911 Britannica plates 11 and 14.jpg|thumb|Photomicrographs of steel. '''Top''': In [[annealing (metallurgy)|annealed]] (slowly cooled) steel, the carbon precipitates forming layers of ferrite (iron) and cementite (carbide). '''Bottom''': In [[quenched]] (quickly cooled) steel, the carbon remains trapped in the iron, creating great internal stresses in the needle or plate-like grains.]]

Metallic materials consist of a [[microstructure]] of small [[crystal]]s called "grains" or [[crystallite]]s. The nature of the grains (i.e. grain size and composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate of [[diffusion]] and the rate of cooling within the microstructure. Heat treating is often used to alter the mechanical properties of a metallic [[alloy]], manipulating properties such as the [[hardness]], [[strength of materials|strength]], [[toughness]], [[ductility]], and [[elasticity (physics)|elasticity]].<ref name="Abdul Wasy ZIA">{{cite journal |last1=ZIA |first1=Abdul Wasy |last2=Zhou |first2=Zhifeng |last3=Po-wan |first3=Shum. |last4=Lawrence Li |first4=Kwak Yan |date=24 January 2017 |title=The effect of two-step heat treatment on hardness, fracture toughness, and wear of different biased diamond-like carbon coatings |journal=Surface and Coatings Technology |volume=320 |pages=118–125 |doi=10.1016/j.surfcoat.2017.01.089}}</ref>

There are two mechanisms that may change an alloy's properties during heat treatment: the formation of [[martensite]] causes the crystals to [[deformation (engineering)|deform]] intrinsically, and the diffusion mechanism causes changes in the homogeneity of the alloy.<ref>{{Cite book
  |title=Solid state phase transformations
  |author=Shant P. Gupta 
  |publisher=Allied Publishers Private Limited 
  |year=2002 
  |ref={{harvid|Gupta|2002}}
  |pages=28–29
}}</ref>

The crystal structure consists of atoms that are grouped in a very specific arrangement, called a lattice. In most elements, this order will rearrange itself, depending on conditions like temperature and pressure. This rearrangement called [[allotropy]] or [[Polymorphism (materials science)|polymorphism]], may occur several times, at many different temperatures for a particular metal. In alloys, this rearrangement may cause an element that will not normally [[solvation|dissolve]] into the base metal to suddenly become [[solubility|soluble]], while a reversal of the allotropy will make the elements either partially or completely insoluble.<ref>{{Cite book
  |title=Physical Metallurgy
  |volume=2
  |editor1=Robert W. Cahn |editor2=Peter Haasen
  |publisher=Elsevier Science 
  |year=1996 
  |ref={{harvid|Physical Metallurgy|1996}}
  |pages=10–11
}}</ref>

When in the soluble state, the process of diffusion causes the atoms of the dissolved element to spread out, attempting to form a homogenous distribution within the crystals of the base metal. If the alloy is cooled to an insoluble state, the atoms of the dissolved constituents (solutes) may migrate out of the solution. This type of diffusion, called [[precipitation (chemistry)|precipitation]], leads to [[nucleation]], where the migrating atoms group together at the grain-boundaries.  This forms a microstructure generally consisting of two or more distinct [[phase (matter)|phases]].<ref name=Alvarenga>{{cite journal |last1=Alvarenga |first1=H. D. |last2=Van de Putte |first2=T. |last3=Van Steenberge |first3=N. |last4=Sietsma |first4=J. |last5=Terryn |first5=H. |title=Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels |journal=Metallurgical and Materials Transactions A |volume=46 |pages=123–133 |date=8 October 2014 |doi=10.1007/s11661-014-2600-y |s2cid=136871961 }}</ref> For instance, steel that has been heated above the [[austenizing]] temperature (red to orange-hot, or around {{convert|1500|F|C}} to {{convert|1600|F|C}} depending on carbon content), and then cooled slowly, forms a laminated structure composed of alternating layers of [[Allotropes of iron|ferrite]] and [[cementite]], becoming soft [[pearlite]].<ref>{{harvnb|Physical Metallurgy|1996|pages=136–198}}</ref>   After heating the steel to the [[austenite]] phase and then quenching it in water, the microstructure will be in the martensitic phase. This is due to the fact that the steel will change from the austenite phase to the martensite phase after quenching. Some pearlite or ferrite may be present if the quench did not rapidly cool off all the steel.<ref name=Alvarenga/>

Unlike iron-based alloys, most heat-treatable alloys do not experience a ferrite transformation. In these alloys, the nucleation at the grain-boundaries often reinforces the structure of the crystal matrix. These metals harden by precipitation. Typically a slow process, depending on temperature, this is often referred to as "age hardening".<ref>{{harvnb|Gupta|2002|pages=299–347}}</ref>

Many metals and non-metals exhibit a martensite transformation when cooled quickly (with external media like oil, polymer, water, etc.). When a metal is cooled very quickly, the insoluble atoms may not be able to migrate out of the solution in time. This is called a "[[diffusionless transformation]]." When the crystal matrix changes to its low-temperature arrangement, the atoms of the solute become trapped within the lattice. The trapped atoms prevent the crystal matrix from completely changing into its low-temperature allotrope, creating shearing stresses within the lattice. When some alloys are cooled quickly, such as steel, the martensite transformation hardens the metal, while in others, like aluminum, the alloy becomes softer.<ref>{{harvnb|Physical Metallurgy|1996|pages=1508–1543}}</ref><ref>{{harvnb|Gupta|2002|pages=501–518}}</ref>