Editing Heat treating (section)

==Types of heat treatment==
[[File:Castings fresh from the heat treatment furnace.jpg|thumb|Steel castings after undergoing 12-hour {{convert|1200|C|abbr=on}} heat treatment.|300x300px]]

Complex heat treating schedules, or "cycles", are often devised by [[metallurgist]]s to optimize an alloy's mechanical properties. In the [[aerospace]] industry, a [[superalloy]] may undergo five or more different heat treating operations to develop the desired properties. {{citation needed|date=January 2021|reason=Need a reference for writing}} This can lead to quality problems depending on the accuracy of the furnace's temperature controls and timer. These operations can usually be divided into several basic techniques.

===Annealing===
{{main|Annealing (metallurgy)}}
Annealing consists of heating a metal to a specific temperature and then cooling at a rate that will produce a refined [[microstructure]], either fully or partially separating the constituents. The rate of cooling is generally slow. Annealing is most often used to soften a metal for cold working, to improve machinability, or to enhance properties like [[electrical conductivity]].

In ferrous alloys, annealing is usually accomplished by heating the metal beyond the upper critical temperature and then cooling very slowly, resulting in the formation of [[pearlite]]. In both pure metals and many alloys that cannot be heat treated, annealing is used to remove the hardness caused by cold working. The metal is heated to a temperature where [[recrystallization (metallurgy)|recrystallization]] can occur, thereby repairing the defects caused by plastic deformation. In these metals, the rate of cooling will usually have little effect. Most non-ferrous alloys that are heat-treatable are also annealed to relieve the hardness of cold working. These may be slowly cooled to allow full precipitation of the constituents and produce a refined microstructure.

Ferrous alloys are usually either "full annealed" or "process annealed". Full annealing requires very slow cooling rates, in order to form coarse pearlite. In process annealing, the cooling rate may be faster; up to, and including normalizing. The main goal of process annealing is to produce a uniform microstructure. Non-ferrous alloys are often subjected to a variety of annealing techniques, including "recrystallization annealing", "partial annealing", "full annealing", and "final annealing". Not all annealing techniques involve recrystallization, such as stress relieving.<ref name="Dossett, 2006, 2-6" >{{harvnb|Dossett|Boyer|2006|pages=2–6}}</ref>

===Normalizing===
Normalizing is a technique used to provide uniformity in grain size and composition ([[equiaxed crystals]]) throughout an alloy. The term is often used for ferrous alloys that have been [[Austenite#Austenitization|austenitized]] and then cooled in the open air.<ref name="Dossett, 2006, 2-6" /> Normalizing not only produces pearlite but also [[martensite]] and sometimes [[bainite]], which gives harder and stronger steel but with less ductility for the same composition than full annealing.

In the normalizing process the steel is heated to about 40 degrees Celsius above its upper critical temperature limit, held at this temperature for some time, and then cooled in air.

===Stress relieving===
Stress-relieving is a technique to remove or reduce the internal stresses created in metal. These stresses may be caused in a number of ways, ranging from cold working to non-uniform cooling. Stress-relieving is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly.<ref name="Dossett, 2006, 2-6" /> Stress relieving is commonly used on items like air tanks, boilers and other [[pressure vessel]]s, to remove a portion of the stresses created during the welding process.<ref>{{cite web|url=https://www.nationalboard.org/PrintPage.aspx?pageID=177|title=The National Board of Boiler and Pressure Vessel Inspectors|website=www.nationalboard.org|access-date=29 April 2018|url-status=live|archive-url=https://web.archive.org/web/20101220235320/https://nationalboard.org/PrintPage.aspx?pageID=177|archive-date=20 December 2010}}</ref>

===Aging===
{{main|Precipitation hardening}}
Some metals are classified as ''precipitation hardening metals''. When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of the solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age " naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, maybe easier with a softer part.

Examples of precipitation hardening alloys include 2000 series, 6000 series, and 7000 series [[aluminium alloy]], as well as some superalloys and some [[stainless steel]]s. Steels that harden by aging are typically referred to as [[maraging steel]]s, from a combination of the term "martensite aging".<ref name="Dossett, 2006, 2-6" />

===Quenching===
{{main|Quenching}}
Quenching is a process of cooling a metal at a rapid rate. This is most often done to produce a martensite transformation. In ferrous alloys, this will often produce a harder metal, while non-ferrous alloys will usually become softer than normal.

To harden by quenching, a metal (usually steel or cast iron) must be heated above the upper critical temperature (Steel: above 815~900 degrees Celsius<ref>{{Cite book |title=Aviation Maintenance Technician Handbook |publisher=Federal Aviation Administration |year=2018 |edition=FAA-H-8983-30A}}</ref>) and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced [[Earth's atmosphere|air]] or other [[gas]]es, (such as [[nitrogen]]). [[Liquid]]s may be used, due to their better [[thermal conductivity]], such as [[oil]], water, a [[polymer]] dissolved in water, or a [[brine]]. Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to [[martensite]], a hard, brittle crystalline structure. The quenched hardness of a metal depends on its chemical composition and quenching method. Cooling speeds, from fastest to slowest, go from brine, polymer (i.e. mixtures of water + glycol polymers), freshwater, oil, and forced air. However, quenching certain steel too fast can result in cracking, which is why high-tensile steels such as AISI 4140 should be quenched in oil, [[tool steel]]s such as [[ISO 1.2767]] or H13 hot work tool steel should be quenched in forced air, and low alloy or medium-tensile steels such as XK1320 or AISI 1040 should be quenched in brine.

Some Beta titanium based alloys have also shown similar trends of increased strength through rapid cooling.<ref>{{cite journal|title=Mechanical properties enhancement in Ti–29Nb–13Ta–4.6Zr alloy via heat treatment with no detrimental effect on its biocompatibility|journal=Materials & Design|date=1 February 2014|volume=54|pages=786–791|doi=10.1016/j.matdes.2013.09.007|issn=0261-3069|last1=Najdahmadi|first1=A.|last2=Zarei-Hanzaki|first2=A.|last3=Farghadani|first3=E.}}</ref> However, most non-ferrous metals, like alloys of [[copper]], [[aluminum]], or [[nickel]], and some high alloy steels such as austenitic stainless steel (304, 316), produce an opposite effect when these are quenched: they soften. Austenitic stainless steels must be quenched to become fully corrosion resistant, as they work-harden significantly.<ref name="Dossett, 2006, 2-6" />

===Tempering===
{{Main|Tempering (metallurgy)}}
Untempered martensitic steel, while very hard, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. Most applications require that quenched parts be tempered. Tempering consists of heating steel below the lower critical temperature, (often from 400˚F to 1105˚F or 205˚C to 595˚C, depending on the desired results), to impart some [[toughness]]. Higher tempering temperatures (maybe up to 1,300˚F or 700˚C, depending on the alloy and application) are sometimes used to impart further ductility, although some yield [[strength of materials|strength]] is lost.

Tempering may also be performed on normalized steels. Other methods of tempering consist of quenching to a specific temperature, which is above the martensite start temperature, and then holding it there until pure bainite can form or internal stresses can be relieved. These include [[austempering]] and [[martempering]].<ref name="Dossett, 2006, 2-6" />

====Tempering colors====
[[File:Tempering standards used in blacksmithing.JPG|thumb|Tempering colors of steel|300x300px]]
Steel that has been freshly ground or polished will form [[oxide]] layers when heated. At a very specific temperature, the [[iron oxide]] will form a layer with a very specific thickness, causing [[thin-film interference]]. This causes colors to appear on the surface of the steel. As the temperature is increased, the iron oxide layer grows in thickness, changing the color.<ref>''Light, its interaction with art and antiquities'' By Thomas B. Brill - Plenum Publishing 1980 Page 55</ref> These colors, called tempering colors, have been used for centuries to gauge the temperature of the metal.<ref name="New Edge of the Anvil,98-99">{{Cite book
  |title=New Edge of the Anvil: a resource book for the blacksmith
  |first=Jack  |last=Andrews 
  |year=1994 
  |pages=98–99
}}</ref>

* 350˚F (176˚C), light yellowish
* 400˚F (204˚C), light-straw
* 440˚F (226˚C), dark-straw
* 500˚F (260˚C), brown
* 540˚F (282˚C), purple
* 590˚F (310˚C), deep blue
* 640˚F (337˚C), light blue<ref name="New Edge of the Anvil,98-99"/>

The tempering colors can be used to judge the final properties of the tempered steel. Very hard tools are often tempered in the light to the dark straw range, whereas springs are often tempered to the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel. Higher-carbon [[tool steel]] will remain much harder after tempering than [[spring steel]] (of slightly less carbon) when tempered at the same temperature. The oxide film will also increase in thickness over time. Therefore, steel that has been held at 400˚F for a very long time may turn brown or purple, even though the temperature never exceeded that needed to produce a light straw color. Other factors affecting the final outcome are oil films on the surface and the type of heat source used.<ref name="New Edge of the Anvil,98-99" />

===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.

===Cold and cryogenic treating===
{{Main|Cryogenic treatment}}
Although quenching steel causes the austenite to transform into martensite, all of the austenite usually does not transform. Some austenite crystals will remain unchanged even after quenching below the martensite finish (M<sub>f</sub>) temperature. Further transformation of the austenite into martensite can be induced by slowly cooling the metal to extremely low temperatures. Cold treating generally consists of cooling the steel to around -115˚F (-81˚C), but does not eliminate all of the austenite. Cryogenic treating usually consists of cooling to much lower temperatures, often in the range of -315˚F (-192˚C), to transform most of the austenite into martensite.

Cold and cryogenic treatments are typically done immediately after quenching, before any tempering, and will increase the hardness, wear resistance, and reduce the internal stresses in the metal but, because it is really an extension of the quenching process, it may increase the chances of cracking during the procedure. The process is often used for tools, bearings, or other items that require good wear resistance. However, it is usually only effective in high-carbon or high-alloy steels in which more than 10% austenite is retained after quenching.<ref>''Heat treater's guide: practices and procedures for irons and steels'' By ASM International - ASM International 2007 Page 12-13</ref><ref>''Handbook of residual stress and deformation of steel'' by George E. Totten, Maurice A. H. Howes, Tatsuo Inoue - ASM International 2002 Page 331-337</ref>

===Decarburization===
The heating of steel is sometimes used as a method to alter the carbon content. When steel is heated in an oxidizing environment, the oxygen combines with the iron to form an iron-oxide layer, which protects the steel from decarburization. When the steel turns to austenite, however, the oxygen combines with iron to form a slag, which provides no protection from decarburization. The formation of slag and scale actually increases decarburization, because the iron oxide keeps oxygen in contact with the decarburization zone even after the steel is moved into an oxygen-free environment, such as the coals of a forge. Thus, the carbon atoms begin combining with the surrounding scale and slag to form both [[carbon monoxide]] and [[carbon dioxide]], which is released into the air.

Steel contains a relatively small percentage of carbon, which can migrate freely within the gamma iron. When austenitized steel is exposed to air for long periods of time, the carbon content in the steel can be lowered. This is the opposite from what happens when steel is heated in a [[reducing environment]], in which carbon slowly diffuses further into the metal. In an oxidizing environment, the carbon can readily diffuse outwardly, so austenitized steel is very susceptible to decarburization. This is often used for cast steel, where a high carbon-content is needed for casting, but a lower carbon-content is desired in the finished product. It is often used on cast-irons to produce [[malleable cast iron]], in a process called "white tempering". This tendency to decarburize is often a problem in other operations, such as blacksmithing, where it becomes more desirable to austenize the steel for the shortest amount of time possible to prevent too much decarburization.<ref>''Steel Heat Treatment: Metallurgy and Technologies'' By George E. Totten -- CRC press 2007 Page 306--308</ref>