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Gamma Iron

Definition - What does Gamma Iron mean?

Gamma iron is an allotropic form of iron existing between the temperature 1670°F and 2550°F (910°C and 1400°C) and having a face-centered cubic lattice.

It is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. Gamma iron containing carbon or other elements in solution is known as austenite, which is named after Sir William Chandler Roberts-Austen (1843-1902).

This gamma form of iron is exhibited by the most commonly used type of stainless steel for making hospital and food-service equipment.

Corrosionpedia explains Gamma Iron

Gamma iron is used in the manufacture of austenite. At temperatures above 1335°F (725°C) the atomic stacking of iron changes to a cube with an extra iron atom at the middle of each face of the cube. This face centered cubic (FCC) phase is known as gamma iron, and it has many more spaces for carbon to rest between the irons than room-temperature alpha iron.

Gamma iron is soft and ductile, and is capable of holding much more carbon in solution and begins to accept carbon into the iron atomic matrix. Holding higher amounts of carbon in solution in the FCC configuration causes austenite to be unstable at temperatures below 1670°F (910°C).

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 destabilize austenite, raising the eutectoid temperature.

As austenite or gamma iron cools, it often transforms into a mixture of ferrite and cementite as the carbon diffuses. Depending on alloy composition and rate of cooling, pearlite may form. If the rate of cooling is very swift, the alloy may experience a large lattice distortion. It is a very important case in industry, as the carbon is not able to diffuse due to the cooling speed, which results in the formation of hard martensite. The rate of cooling determines the mechanical properties of the resulting steel, such as hardness and tensile strength.

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