Austenite

Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel.
Allotropes of iron; alpha iron and gamma iron

Austenite, 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 1,000 K (1,340 °F; 730 °C); other alloys of steel have different eutectoid temperatures. It is named after Sir William Chandler Roberts-Austen (1843–1902).[2]

Allotrope of iron

From 912 to 1,394 °C (1,674 to 2,541 °F) alpha iron undergoes a phase transition from body-centred cubic (BCC) to the face-centred 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 exhibited by the most commonly used type of stainless steel for making hospital and food-service equipment.

Austenitization

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] An incomplete initial austenitization can leave undissolved carbides in the matrix.[4]

For some irons, 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

Main article: Austempering

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

As austenite 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 known as martensitic transformation in which it transforms into a BCT-structure instead of into cubic latticed ferrite and cementite. In industry, this is a very important case, 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 relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness and tensile strength. Quenching (to induce martensitic transformation), followed by tempering 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.

Behavior in cast iron

Heating white hypereutectic cast iron above 727 °C (1,341 °F) causes the formation of austenite in crystals of primary cementite.[9] This austenisation of white iron occurs in primary cementite at the interphase boundary with ferrite.[9] When the grains of austenite form in cementite, they occur as lamellar clusters oriented along the cementite crystal layer surface.[9] Austenite is formed by withdrawal of carbon atoms from cementite into ferrite.[9][10]

Stabilization

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.

Austenite is only stable above 910 °C (1,670 °F) in bulk metal form. However, the use of a face-centered cubic (fcc) or diamond cubic substrate allows the epitaxial growth of fcc transition metals.[11] 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.[11] The determined critical thickness is in close agreement with theoretical prediction.[11]

Austenite transformation and Curie point

In many magnetic 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 and ferrite are strongly ferromagnetic.

Thermo-optical emission

During heat treating, a blacksmith causes phase changes in the iron-carbon system in order 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, and when cherry-red the glow is near its lowest intensity and may not be visible in ambient light. Therefore, blacksmiths usually austenize steel in low-light conditions, to help accurately judge the color of the glow.

Maximum carbon solubility in austenite is 2.03% C at 1,420 K (1,150 °C).

See also

References

  1. ↑ Reed-Hill R, Abbaschian R (1991). Physical Metallurgy Principles (3rd ed.). Boston: PWS-Kent Publishing. ISBN 0-534-92173-6.
  2. ↑ Gove PB, ed. (1963). Webster's Seventh New Collegiate Dictionary. Springfield, Massachusetts, USA: G & C Merriam Company. p. 58.
  3. ↑ Nichols R (Jul 2001). "Quenching and tempering of welded carbon steel tubulars".
  4. ↑ Lambers HG, Tschumak S, Maier HJ, Canadinc D (Apr 2009). "Role of Austenitization and Pre-Deformation on the Kinetics of the Isothermal Bainitic Transformation". Metal Mater Trans A. 40 (6): 1355. Bibcode:2009MMTA..tmp...74L. doi:10.1007/s11661-009-9827-z.
  5. ↑ "Austenitization".
  6. ↑ Kilicli V, Erdogan M (2008). "The Strain-Hardening Behavior of Partially Austenitized and the Austempered Ductile Irons with Dual Matrix Structures". J Mater Eng Perf. 17 (2): 240–9. Bibcode:2008JMEP...17..240K. doi:10.1007/s11665-007-9143-y.
  7. 1 2 Batra U, Ray S, Prabhakar SR (2003). "Effect of austenitization on austempering of copper alloyed ductile iron". J Mater Eng Perf. 12 (5): 597–601. doi:10.1361/105994903100277120.
  8. ↑ Chupatanakul S, Nash P (Aug 2006). "Dilatometric measurement of carbon enrichment in austenite during bainite transformation". J Mater Sci. 41 (15): 4965–9. Bibcode:2006JMatS..41.4965C. doi:10.1007/s10853-006-0127-3.
  9. 1 2 3 4 Ershov VM, Nekrasova LS (Jan 1982). "Transformation of cementite into austenite". Metal Sci Heat Treat. 24 (1): 9–11. doi:10.1007/BF00699307.
  10. ↑ Alvarenga HD, Van de Putte T, Van Steenberge N, Sietsma J, Terryn H (Apr 2009). "Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels". Metal Mater Trans A. Bibcode:2015MMTA...46..123A. doi:10.1007/s11661-014-2600-y.
  11. 1 2 3 Hoff HA, Waytena GL, Glesener JW, Harris VG, Pappas DP (Mar 1995). "Critical thickness of single crystal fcc iron on diamond". Surf Sci. 326 (3): 252–66. Bibcode:1995SurSc.326..252H. doi:10.1016/0039-6028(94)00787-X.
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