Not to be confused with Austinite
This article is about the alloy and iron allotrope. For Jane Austen fans, see Janeite
Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. Allotropes of iron; alpha iron and gamma ironAustenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element.[1] In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902).[2] It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.
Allotrope of iron
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From 912 to 1,394 °C (1,674 to 2,541 °F) alpha iron undergoes a phase transition from body-centered cubic (BCC) to the face-centered cubic (FCC) configuration of gamma iron, also called austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.03% by mass at 1,146 °C (2,095 °F)). This gamma form of iron is present in the most commonly used type of stainless steel[citation needed] for making hospital and food-service equipment.
Material
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Austenitization means to heat the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite.[3] The more-open structure of the austenite is then able to absorb carbon from the iron-carbides in carbon steel. An incomplete initial austenitization can leave undissolved carbides in the matrix.[4]
For some iron metals, iron-based metals, and steels, the presence of carbides may occur during the austenitization step. The term commonly used for this is two-phase austenitization.[5]
Austempering
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Austempering is a hardening process that is used on iron-based metals to promote better mechanical properties. The metal is heated into the austenite region of the iron-cementite phase diagram and then quenched in a salt bath or other heat extraction medium that is between temperatures of 300–375 °C (572–707 °F). The metal is annealed in this temperature range until the austenite turns to bainite or ausferrite (bainitic ferrite + high-carbon austenite).[6]
By changing the temperature for austenitization, the austempering process can yield different and desired microstructures.[7] A higher austenitization temperature can produce a higher carbon content in austenite, whereas a lower temperature produces a more uniform distribution of austempered structure.[7] The carbon content in austenite as a function of austempering time has been established.[8]
Behavior in plain carbon-steel
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Austenite microstructure at two different temperaturesAs austenite cools, the carbon diffuses out of the austenite and forms carbon-rich iron-carbide (cementite) and leaves behind carbon-poor ferrite. Depending on alloy composition, a layering of ferrite and cementite, called pearlite, may form. If the rate of cooling is very swift, the carbon does not have sufficient time to diffuse, and the alloy may experience a large lattice distortion known as martensitic transformation in which it transforms into martensite, a body centered tetragonal structure (BCT). The rate of cooling determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness and tensile strength.
A high cooling rate of thick sections will cause a steep thermal gradient in the material. The outer layers of the heat treated part will cool faster and shrink more, causing it to be under tension and thermal straining. At high cooling rates, the material will transform from austenite to martensite which is much harder and will generate cracks at much lower strains. The volume change (martensite is less dense than austenite)[9] can generate stresses as well. The difference in strain rates of the inner and outer portion of the part may cause cracks to develop in the outer portion, compelling the use of slower quenching rates to avoid this. By alloying the steel with tungsten, the carbon diffusion is slowed and the transformation to BCT allotrope occurs at lower temperatures, thereby avoiding the cracking. Such a material is said to have its hardenability increased. Tempering following quenching will transform some of the brittle martensite into tempered martensite. If a low-hardenability steel is quenched, a significant amount of austenite will be retained in the microstructure, leaving the steel with internal stresses that leave the product prone to sudden fracture.
Behavior in cast iron
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Heating white cast iron (containing iron carbide, i.e. cementite, but no uncombined carbon) above 727 °C (1,341 °F) causes the formation of austenite in crystals of primary cementite.[10] This austenisation of white iron occurs in primary cementite at the interphase boundary with ferrite.[10] When the grains of austenite form in cementite, they occur as lamellar clusters oriented along the cementite crystal layer surface.[10] Austenite is formed by diffusion of carbon atoms from cementite into ferrite.[10][11]
Stabilization at lower temperatures
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The addition of certain alloying elements, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature.
On the other hand, such elements as silicon, molybdenum, and chromium tend to de-stabilize austenite, raising the eutectoid temperature.
Thin films
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Austenite is only stable above 910 °C (1,670 °F) in bulk metal form. However, fcc transition metals can be grown on a face-centered cubic (fcc) or diamond cubic.[12] The epitaxial growth of austenite on the diamond (100) face is feasible because of the close lattice match and the symmetry of the diamond (100) face is fcc. More than a monolayer of γ-iron can be grown because the critical thickness for the strained multilayer is greater than a monolayer.[12] The determined critical thickness is in close agreement with theoretical prediction.[12]
Transformation and Curie point
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In many magnetic ferrous alloys, the Curie point, the temperature at which magnetic materials cease to behave magnetically, occurs at nearly the same temperature as the austenite transformation. This behavior is attributed to the paramagnetic nature of austenite, while both martensite[13] and ferrite[14][15] are strongly ferromagnetic.
Thermo-optical emission, colour indicates temperature
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During heat treating, a blacksmith causes phase changes in the iron-carbon system to control the material's mechanical properties, often using the annealing, quenching, and tempering processes. In this context, the color of light, or "blackbody radiation", emitted by the workpiece is an approximate gauge of temperature. Temperature is often gauged by watching the color temperature of the work, with the transition from a deep cherry-red to orange-red (815 °C (1,499 °F) to 871 °C (1,600 °F)) corresponding to the formation of austenite in medium and high-carbon steel. In the visible spectrum, this glow increases in brightness as temperature increases. When cherry-red, the glow is near its lowest intensity and may not be visible in ambient light. Hence blacksmiths usually austenitize steel in low-light conditions to accurately judge the color of the glow.
See also
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References
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If you are using stainless steel cookware, then you know that it is durable, easy to use, and easy to clean. But occasionally, you come across a conversation about nickel being used in stainless steel cookware, followed by some valid questions –Should I start looking for nickel free stainless steel cookware brands for kadhai, pressure cooker, and frypans? How to prevent the leaching of such metals into food? The key is to not panic and start replacing your stainless-steel cookware. All cookware is subject to wear and tear irrespective of what material it is made up of. When you compare different types of cookware, stainless-steel cookware is by far the least harmful compared to the rest. Good quality stainless steel (SS 304) has many elements in it including nickel. But it’s there for a good reason.
The 300 series stainless steel has chromium and nickel to increase its durability. Nickel and Chromium also help to give surgical stainless steel its corrosion resistance properties. Nickel as an element also adds life and versatility to your cookware because it has the ability to absorb energy without breaking.
According to this report of the National Library of Medicine, our body can tolerate up to 1000 µg of nickel. It has been observed during tests that 300 series stainless steel leaches only 88 µg of nickel. Just to give you an idea of just how often we come in contact with nickel apart from stainless-steel cookware, there’s more nickel to be found in vegetables, legumes, seafood, and even some fruits like raspberries and pineapples. A study showed that a cup of peanuts has 136 µg of nickel which is still more than what you might get out of stainless-steel cookware.
Stainless steel jewelry comes in direct contact with skin which is one of the inlets for nickel into your body.
Some premium clothing has stainless steel fasteners and zippers for increased durability. Touching such surfaces also results in nickel intake.
Coins usually contain copper and nickel. Handling loose change frequently is also another channel for nickel into your body.
Some of the inks have metallic salts including that of chromium, copper, and nickel.
To prevent oxidation and deposition of rust on the paper, paper clips have nickel plating.
A study showed that 80% of a key consisted of nickel to prevent corrosion.
Certain brands of metallic watches have stainless steel for increased durability. Hence, wearing such watches for a prolonged duration may result in rashes around the wrists.
Fishes like shrimps, crawfishes, and mussels are rich in nickel. According to a study, some types of fish contain 0.08 milligrams of nickel.
Legumes like chickpeas, lentils, and peanuts contain high levels of nickel.
Green vegetables like cabbage, kale, spinach, and lettuce contain around 0.11 milligrams of nickel.
There is a small percentage of the population with nickel allergy. How can you tell if a person is allergic to nickel? People allergic to nickel usually experience mild rashes, blisters, and skin irritation. If the allergy goes undiagnosed for a long time, there are painless medical treatments available for such conditions including oral antihistamines and creams. Either way, if someone has a nickel allergy, they are generally advised to avoid anything containing nickel. People allergic to nickel are also advised to avoid foods like oats, multi-grain bread, spinach, beans, and even chocolate among others apart from stainless steel cookware. Studies have found 12% - 15% women and only about 1%- 2% men are allergic to nickel.
Research has proven that stainless steel cookware is more likely to leach nickel if you are cooking with something highly acidic like curd or lemon juice for a long period of time. There is no nickel leaching while cooking liquids like coffee, tea, milk, or any other liquid with low acidity levels. Distilled water being neutral is also free from leaching. In the case of tap water, the PH levels are low. This means that even tap water is free from nickel leaching.
Yes, if your cookware is made of stainless steel, it would most likely have chromium and nickel in there.If there are cookware brands that claim to have nickel free cookware, they are most likely not using a durable version of stainless steel that is rust resistant.
18/8 proportion stainless steel has 2% lesser nickel than 18/10 proportion stainless steel. This also means that it is less durable and lesser corrosion resistant than 18/10 stainless steel. If you are certain that you are allergic to nickel, choose the 18/8 variety of cookware but in all other cases, it is best to use 18/10 variety.
Stainless steel cookware tends to leach nickel only under a specific set of circumstances. But with proper care and usage, the amount of nickel leached into the food can be reduced. Here’s how it’s done.
The non-stick prevents the contact between the acidic contents and the stainless-steel surface which prevents nickel leaching.
Research shows that nickel leaching reduces over time and use. Once your cookware has gone through more than six or seven cooking cycles, the amounts of nickel leached into the food lower over time.
Only when the stainless-steel cookware reaches a temperature over 200℃, it starts leaching nickel. The best way to prevent this is to switch to triply stainless-steel cookware. It is an excellent conductor of heat, so you don’t have to heat the cookware over 200℃.
Unless you’re allergic to nickel, using stainless-steel cookware is the last thing you need to worry about. In fact, some studies suggest that our body needs nickel in small amounts for proper functioning. If you come across any brand claiming to sell you nickel free stainless steel cookware, it is merely a marketing gimmick if not proven by data.
No matter which series of stainless steel you come across, nickel will always be present and is in fact playing an important role. If you happen to stumble upon stainless steel cookware which is completely free of nickel, it only means that it is not durable and will be more prone to corrosion. So next time you come across a conversation about nickel leaching, you will have an excellent opportunity to do some myth-busting.