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Precipitation hardening, also referred to as age hardening, is a technique where heat is applied to a malleable material, such as a metal alloy, in order to strengthen it. The technique hardens the alloy by creating solid impurities, called precipitates, which stop the movement of dislocations in the crystal lattice structure. Dislocations are the primary cause of plasticity in a material; thus, the absence of dislocations increases the material's yield strength. Alloys typically used include alloys of aluminum, magnesium, nickel, and titanium.
Creating precipitation hardened materials starts with heating the material to a very high temperature in order to dissolve the precipitate. It takes anywhere from 1 hour to 20 hours for the precipitate to completely dissolve. The new mixture then becomes supersaturated and is ready to be treated further.
Supersaturation of the solution is achieved through quenching. Quenching can be completed in water, air, or some mixture of a air and water. As an important step in solid solution strengthening, it leaves the material softer and more prepared for the next phase of precipitation hardening.
After the initial phase and the quenching phase, the solution is aged. Artificial aging occurs by heating the solution to above room temperature and then allowing it to soak for 2 to 20 hours. The aging temperature and soaking time is determined by the desired strength of the final product. It is important to be very careful about both the temperature and time, because too high of a temperature and too much soaking can lead to less precipitates which decreases strength and increases ductility.
Natural aging is another option. If one decides to use this option, take note that naturally aging a solution takes much longer than the aforementioned method. It can take anywhere from a few days to several weeks.
Impurities that are produced through the precipitation hardening process disrupt the structure of the crystal lattice in stainless steel, aluminum, and other alloys. They inhibit dislocations in these structures which make it harder for loose particles to cut through the material. Inhibiting these disruptions is key to increasing the yield strength of the alloy over time.
Alloys manufactured by precipitation hardening have many practical uses in the manufacturing industry. They are usually utilized for making parts that must withstand high temperature situations, such as parts of a pressure vessel or a turbocharger. Many alloys have very important uses in industry. Silicon is used to manufacture semiconductor materials. Copper is used for a wide variety of purposes from shipbuilding to roofing.
Sometimes iron and steel are heated to a very high temperature to add carbon. This is called carburizing. When the metal is heated with a carbon based material, like charcoal or carbon monoxide, the carbon defuses into the metal and makes it harder.
Using time and temperature you can cause the carbon to infuse into the metal deeper and deeper. The metal is eventually cooled quickly, usually with a liquid like water, to seal the process. This makes the outer part of the metal harder while maintaining a purer metal in the core.
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