Heat treating
Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching.
[3] 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.
[4] For instance, steel that has been heated above the austenizing temperature (red to orange-hot, or around 1,500 °F (820 °C) to 1,600 °F (870 °C) depending on carbon content), and then cooled slowly, forms a laminated structure composed of alternating layers of ferrite and cementite, becoming soft pearlite.
[6] Many metals and non-metals exhibit a martensite transformation when cooled quickly (with external media like oil, polymer, water, etc.).
The trapped atoms prevent the crystal matrix from completely changing into its low-temperature allotrope, creating shearing stresses within the lattice.
Upon cooling a eutectoid alloy from the solution temperature, the constituents will separate into different crystal phases, forming a single microstructure.
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.
When austenite is cooled extremely slowly, it will form large ferrite crystals filled with spherical inclusions of cementite.
Moreover, the defects caused by plastic deformation tend to speed up precipitation, increasing the hardness beyond what is normal for the alloy.
[18][19][20] Complex heat treating schedules, or "cycles", are often devised by metallurgists to optimize an alloy's mechanical properties.
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.
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.
[21] 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.
These intermetallic particles will nucleate and fall out of the solution and act as a reinforcing phase, thereby increasing the strength of the alloy.
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.
Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gases, (such as nitrogen).
Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to martensite, a hard, brittle crystalline structure.
Some Beta titanium based alloys have also shown similar trends of increased strength through rapid cooling.
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.
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.
The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.
When the steel turns to austenite, however, the oxygen combines with iron to form a slag, which provides no protection from decarburization.
[31] It is a process to refine grain size, improve strength, remove residual stress, and affect the electromagnetic properties...
Pit furnaces are suited to heating long tubes, shafts, and rods by holding them in a vertical position.
Concerns about associated occupation health and safety, and expensive waste management and disposal due to their environmental effects have made the use of salt baths less attractive in recent years.
[34] A fluidised bed consists of a cylindrical retort made from high-temperature alloy, filled with sand-like aluminum oxide particulate.
Gas (air or nitrogen) is bubbled through the oxide and the sand moves in such a way that it exhibits fluid-like behavior, hence the term fluidized.
The solid-solid contact of the oxide gives very high thermal conductivity and excellent temperature uniformity throughout the furnace, comparable to those seen in a salt bath.
