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Austenitic Stainless Steel Group:

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Austenite is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of 1333˘XF (about 723˘XC). It is named after Sir William Chandler Roberts-Austen (1843-1902). Its face-centred cubic (FCC) structure allows it to hold a high proportion of carbon in solution.

As it cools, this structure either breaks down into a mixture of ferrite and cementite (often in special forms such as pearlite and bainite), or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Quenching (to induce martensitic transformation), followed by tempering (to break down some martensite and retained austenite), is the most common heat treatment for high-performance steels.

The addition of certain other metals, 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.

A Blacksmith causes phase changes in the iron-carbon system in order to control the material's mechanical properties, often using the quenching-and-tempering process described above. In this context, the color of light emitted by the workpiece is an accurate gauge of temperature, with the transition from red to orange corresponding to the formation of Austenite in medium- and high-carbon steel.

Maximum carbon solubility in austenite is 2.03% C at 1147˘XC.

Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition is 18% chromium and 10% nickel, commonly known as 18/10 stainless is often used in flatware. Similarly 18/0 and 18/8 is also available. ˇ§Superausteniticˇ¨ stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series. The higher alloy content of "Superaustenitic" steels means they are fearsomely expensive and similar performance can usually be achieved using duplex steels at much lower cost.

The source of this article is Wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.

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Type 304ˇXthe most common; the classic 18/8 stainless steel.
An economic balance of alloying materials. Excellent corrosion resistance in unpolluted and freshwater environments, though not recommended for seawater.

Type 316ˇXthe next most common; for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. Also known as "marine grade" stainless steel due to its increased ability to resist saltwater corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. The addition of 2 -3% Molybdenum in this grade gives increased corrosion resistance in offshore environments, however it does pit when immersed in seawater. A nickel content of 12% maintains the austenitic structure.

L GradesˇXMost austenitic grades can be provided as low carbon grades e.g. 316L where carbon is restricted to 0.03 to 0.035%. This reduces the tensile strength.
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