Beta ferrite
Encyclopedia
Beta ferrite and beta iron (β-Fe) are obsolete
terms for the paramagnetic form of ferrite
(α-Fe). The primary phase
of low-carbon or mild steel and most cast iron
s at room temperature is ferromagnetic ferrite (α-Fe). As iron
or ferritic steel
is heated above the critical temperature A2 or Curie temperature of 771°C (1044K or 1420°F), the random thermal agitation of the atoms exceeds the oriented magnetic moment of the unpaired electron
spins in the 3d shell. The A2 forms the low-temperature boundary of the beta iron field in the phase diagram
in Figure 1. Beta ferrite is crystallographically
identical to alpha ferrite, except for magnetic domains and the expanded body-centered cubic lattice parameter as a function of temperature, and is therefore of only minor importance in steel heat treating. For this reason, the beta “phase” is not usually considered a distinct phase but merely the high-temperature end of the alpha phase field. Similarly, the A2 is of only minor importance compared to the A1 (eutectoid), A3 and Acm critical temperatures. The Acm, where austenite
is in equilibrium with cementite
+ γ-Fe, is beyond the right edge in Fig. 1. The α + γ phase field is, technically, the β + γ field above the A2. The beta designation maintains continuity of the Greek-letter progression of phases in iron and steel: ferrite (α-Fe), beta ferrite (β-Fe), austenite
(γ-Fe), high-temperature delta ferrite (δ-Fe) and high-pressure hexaferrum
(ε-Fe).
of steel, such as for surface-hardening heat treatments. Steel is typically austenitized at 900-1000°C before it is quenched and tempered
. The high-frequency alternating magnetic field of induction heating heats the steel by two mechanisms below the Curie temperature: resistance or Joule (I2R) heating and ferromagnetic hysteresis losses. Above the A2, the hysteresis mechanism disappears and the required amount of energy per degree of temperature increase is substantially larger than below A2. Load-matching circuits may be needed to vary the impedance
in the induction power source to compensate for the change.
observed x-ray diffraction (XRD) evidence of a high-pressure, high-temperature phase they designated as beta iron (β-Fe). A foil of 99.9% pure α-Fe was compressed in a diamond anvil to 35-40 GPa to form the standard high-pressure phase, hexagonal close-packed (HCP) ε-Fe
. The ε-Fe was heated with a laser
to a temperature near 1500 K, scanned via XRD, quenched and re-scanned. The β-Fe phase was modeled as a four-layer HCP superlattice
, and was proposed as a possible phase in Earth’s iron core
. Subsequent studies were unable to reproduce β-Fe or a similar orthorhombic phase, and these phases came to be regarded as either metastable or incorrectly identified.
Obsolescence
Obsolescence is the state of being which occurs when an object, service or practice is no longer wanted even though it may still be in good working order. Obsolescence frequently occurs because a replacement has become available that is superior in one or more aspects. Obsolete refers to something...
terms for the paramagnetic form of ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
(α-Fe). The primary phase
Phase
-In physics:*Phase , a physically distinctive form of a substance, such as the solid, liquid, and gaseous states of ordinary matter**Phase transition is the transformation of a thermodynamic system from one phase to another*Phase...
of low-carbon or mild steel and most cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
s at room temperature is ferromagnetic ferrite (α-Fe). As iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
or ferritic steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
is heated above the critical temperature A2 or Curie temperature of 771°C (1044K or 1420°F), the random thermal agitation of the atoms exceeds the oriented magnetic moment of the unpaired electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
spins in the 3d shell. The A2 forms the low-temperature boundary of the beta iron field in the phase diagram
Phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
in Figure 1. Beta ferrite is crystallographically
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
identical to alpha ferrite, except for magnetic domains and the expanded body-centered cubic lattice parameter as a function of temperature, and is therefore of only minor importance in steel heat treating. For this reason, the beta “phase” is not usually considered a distinct phase but merely the high-temperature end of the alpha phase field. Similarly, the A2 is of only minor importance compared to the A1 (eutectoid), A3 and Acm critical temperatures. The Acm, where austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
is in equilibrium with cementite
Cementite
Cementite, also known as iron carbide, is a chemical compound of iron and carbon, with the formula Fe3C . By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more...
+ γ-Fe, is beyond the right edge in Fig. 1. The α + γ phase field is, technically, the β + γ field above the A2. The beta designation maintains continuity of the Greek-letter progression of phases in iron and steel: ferrite (α-Fe), beta ferrite (β-Fe), austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
(γ-Fe), high-temperature delta ferrite (δ-Fe) and high-pressure hexaferrum
Hexaferrum
Hexaferrum and epsilon iron are synonyms for the hexagonal close-packed phase of iron that is stable only at extremely high pressure. Takahashi and Bassett at the University of Rochester mixed 99.8% pure alpha iron powder with sodium chloride, and pressed a 0.5-mm diameter pellet between the...
(ε-Fe).
A2 critical temperature and induction heating
Beta ferrite and the A2 critical temperature are very important in induction heatingInduction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...
of steel, such as for surface-hardening heat treatments. Steel is typically austenitized at 900-1000°C before it is quenched and tempered
Tempering
Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...
. The high-frequency alternating magnetic field of induction heating heats the steel by two mechanisms below the Curie temperature: resistance or Joule (I2R) heating and ferromagnetic hysteresis losses. Above the A2, the hysteresis mechanism disappears and the required amount of energy per degree of temperature increase is substantially larger than below A2. Load-matching circuits may be needed to vary the impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
in the induction power source to compensate for the change.
Beta iron in geology
Saxena, Dubrovinsky et al. of Uppsala UniversityUppsala University
Uppsala University is a research university in Uppsala, Sweden, and is the oldest university in Scandinavia, founded in 1477. It consistently ranks among the best universities in Northern Europe in international rankings and is generally considered one of the most prestigious institutions of...
observed x-ray diffraction (XRD) evidence of a high-pressure, high-temperature phase they designated as beta iron (β-Fe). A foil of 99.9% pure α-Fe was compressed in a diamond anvil to 35-40 GPa to form the standard high-pressure phase, hexagonal close-packed (HCP) ε-Fe
Hexaferrum
Hexaferrum and epsilon iron are synonyms for the hexagonal close-packed phase of iron that is stable only at extremely high pressure. Takahashi and Bassett at the University of Rochester mixed 99.8% pure alpha iron powder with sodium chloride, and pressed a 0.5-mm diameter pellet between the...
. The ε-Fe was heated with a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
to a temperature near 1500 K, scanned via XRD, quenched and re-scanned. The β-Fe phase was modeled as a four-layer HCP superlattice
Superlattice
Superlattice is a periodic structure of layers of two materials. Typically, the thickness of one layer is several nanometers.- Discovery :Superlattices were discovered early in the 20th century through their special X-ray diffraction patterns....
, and was proposed as a possible phase in Earth’s iron core
Inner core
The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about in radius, or about 70% that of the Moon...
. Subsequent studies were unable to reproduce β-Fe or a similar orthorhombic phase, and these phases came to be regarded as either metastable or incorrectly identified.