Beta ferrite

Main article: Allotropes of iron

Beta ferrite (-Fe) and beta iron (-iron) are obsoleteterms for the paramagnetic form of alpha ferrite (-Fe).[1][2] The primary phase of low-carbon or mild steeland most cast irons at room temperature is ferromagneticferrite (-Fe). As iron or ferritic steel is heated abovethe critical temperature A2 or Curie temperature of 771C (1044K or 1420 F),[3] the random thermal agita-tion of the atoms exceeds the oriented magnetic mo-ment of the unpaired electron spins in the 3d shell.[4] TheA2 forms the low-temperature boundary of the beta ironeld in the phase diagram in Figure 1. Beta ferrite iscrystallographically identical to alpha ferrite, except formagnetic domains and the expanded body-centered cu-bic lattice parameter as a function of temperature, and istherefore of only minor importance in steel heat treating.For this reason, the beta phase is not usually considereda distinct phase but merely the high-temperature end ofthe alpha phase eld. Similarly, the A2 is of only minorimportance compared to the A1 (eutectoid), A3 and Acritical temperatures. TheA, where austenite is in equi-librium with cementite + -Fe, is beyond the right edge inFig. 1. The + phase eld is, technically, the + eldabove the A2. The beta designation maintains continuityof the Greek-letter progression of phases in iron and steel:alpha ferrite (-Fe), beta ferrite (-Fe), austenite (-Fe),high-temperature delta ferrite (-Fe) and high-pressurehexaferrum (-Fe).

Figure 1: The beta eld and A2 critical temperature on the iron-rich side of the iron-carbon phase diagram.[3]

1 A2 critical temperature and in-duction heating

Beta ferrite and the A2 critical temperature are very im-portant in induction heating of steel, such as for surface-hardening heat treatments. Steel is typically austenitizedat 9001000 C before it is quenched and tempered.The high-frequency alternating magnetic eld of induc-tion heating heats the steel by two mechanisms below theCurie temperature: resistance or Joule (I2R) heating andferromagnetic hysteresis losses. Above the A2, the hys-teresis mechanism disappears and the required amountof energy per degree of temperature increase is substan-tially larger than below A2. Load-matching circuits maybe needed to vary the impedance in the induction powersource to compensate for the change.[5]

2 Beta iron in geologySaxena, Dubrovinsky et al. of Uppsala University ob-served x-ray diraction (XRD) evidence of a high-pressure, high-temperature phase they designated as betairon (-Fe).[6] A foil of 99.9% pure -Fewas compressedin a diamond anvil to 3540 GPa to form the standardhigh-pressure phase, hexagonal close-packed (HCP) -Fe. The -Fe was heated with a laser to a tempera-ture near 1500 K, scanned via XRD, quenched and re-scanned. The apparent -Fe phase wasmodeled as a four-layer HCP superlattice,[7] and was proposed as a possiblephase in Earths iron core. Subsequent studies were un-able to reproduce -Fe or a similar orthorhombic phase,and these phases came to be regarded as either metastableor incorrectly identied.[8]

3 References[1] D. K. Bullens et al., Steel and Its Heat Treatment, Vol. I,

Fourth Ed., J. Wiley & Sons Inc., 1938, p. 86.

[2] S. H. Avner, Introduction to Physical Metallurgy, 2nd Ed.,McGraw-Hill, 1974, p. 225.

[3] ASM Handbook, Vol. 3: Alloy Phase Diagrams, ASM In-ternational, 1992, p. 2.210 and 4.9, ISBN 0-87170-381-5.

[4] B. D. Cullity & C. D. Graham, Introduction to MagneticMaterials, Second Ed., IEEE Inc., 2009, p. 91, ISBN 978-0-471-47741-9.

1

2 3 REFERENCES

[5] S. L. Semiatin and D. E. Stutz, Induction Heat Treat-ment of Steel, ASM International, 1986, p. 95-98, ISBN0-87170-211-8.

[6] S. K. Saxena, L. S. Dubrovinsky, P. Haggkvist, Y. Cere-nius, G. Shen & H. K. Mao, Synchrotron X-ray Studyof Iron at High Pressure and Temperature, Science, Vol.269, 22 Sep 1995, p. 1703-4.

[7] Powder Diraction File 00-050-1275, International Cen-tre for Diraction Data, 1998.

[8] Y. Ma, M. Somayazulu, G. Shen, H. Maob, J. Shub &R. J. Hemley, In situ X-ray diraction studies of iron toEarth-core conditions, Physics of the Earth and PlanetaryInteriors, Vol. 143144 (2004) p. 455467.

34 Text and image sources, contributors, and licenses4.1 Text

Beta ferrite Source: http://en.wikipedia.org/wiki/Beta%20ferrite?oldid=578196282 Contributors: Alan Liefting, Anomalocaris, Wiz-ard191, Headbomb, Belovedfreak, Bentogoa, JL-Bot, Anon lynx, SchreiberBike, His Manliness, Addbot, JackieBot, ArthurBot, Ebe123,ChrisGualtieri and Anonymous: 1

4.2 Images File:Beta_iron_carbon_pd.TIF Source: http://upload.wikimedia.org/wikipedia/commons/2/2f/Beta_iron_carbon_pd.TIF License: CC0

Contributors: Own work (Original caption: self-made) Original artist: His Manliness at en.wikipedia

4.3 Content license Creative Commons Attribution-Share Alike 3.0

A2 critical temperature and induction heating Beta iron in geology ReferencesText and image sources, contributors, and licensesTextImagesContent license