Austenite

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austenite

[¦ȯs·tə‚nīt]
(metallurgy)
Gamma iron with carbon in solution.

Austenite

 

one of the structural components of iron-carbon alloys, a solid solution of carbon (to 2 percent) and alloying elements in gamma iron. Austenite derives its name from the English scientist W. Roberts-Austen (1843–1902). The crystal lattice is a cube with centered facets. Austenite is nonmagnetic. Its density exceeds that of other structural components of steel. In carbon steels and cast irons, austenite resists temperatures exceeding 723° C. In the process of cooling steel, austenite is transformed into other structural components. In iron-carbon alloys containing nickel, manganese, and chromium in significant amounts, austenite can be completely preserved after cooling to room temperature (for example, stainless chromium-nickel steels). Depending on the composition of the steel and its relative cooling, austenite can be partially preserved in carbon or alloyed steels (so-called retained austenite).

The study of the transformations of austenite began with the discoveries of D. K. Chernov (1868). He first pointed out the connection between these transformations and the critical points of steel. During cooling below these points, phases are formed with different mutual arrangements of atoms in the crystal lattice and in some cases with a modification of chemical composition.

Three areas of austenite transformation have been identified. In the upper temperature regions (723°-550°C) austenite disintegrates to form pearlite, a eutectoid mixture composed of alternating layers of ferrite (carbon concentration by weight, 0.02 percent) and cementite (carbon concentration by weight, 6.7 percent). Pearlite transformation begins after some heating, and after sufficient time the austenite disintegrates completely. Below a particular temperature (Ml), which depends on the content of carbon (for steel with 0.8 percent carbon it is about 240° C), a martensite transformation of austenite occurs. This transformation consists of a regular rearrangement of the crystal lattice, during which atoms do not trade places. In the interval between 550° C and Ml, an intermediate (bainite) transformation of austenite occurs. This transformation, like that of pearlite, begins after an incubation period and can be suppressed by quick cooling; like the martensite transformation, it can be stopped by constant temperature (a certain amount of austenite remains untransformed) and is accompanied by the formation of a characteristic relief on the surface of a section. During intermediate transformation, the ordered displacements of metallic atoms are coupled with the diffusional redistribution of carbon atoms into austenite. As a result, a ferrito-cementite mixture—and frequently a retained austenite with a carbon content that differs from the average—is formed. During intermediate transformation, cementite can separate itself directly from austenite or from a ferrite saturated with carbon.

When austenite alloys containing more than 2 percent carbon are converted in the presence of primary formations of cementite or graphite, the resulting structures are unique.

The diagram, which shows the proportion of transformed austenite on temperature-time coordinates, gives a representation of the kinematics of the transformation of austenite. On the diagram of alloyed austenite the transformation regions of pearlite (640°-520° C) and intermediate transformations (480°-300° C) are clearly delineated, and there is a temperature zone of high durability of austenite (see Figure 1). In pearlite transformation of alloyed austenite, a mixture of ferrite and special carbides occurs in many cases.

Alloying elements, with the exception of cobalt, increase the duration of the incubation period of the pearlite transformation.

The laws of austenite transformation are utilized in the treatment of alloyed steels for various purposes of thermal and thermomechanical treatment. The diagrams of austenite transformations permit the establishment of methods of annealing steel, cooling items, isothermic hardening, and so on.

Figure 1. Diagram of the isothermal austenite transformation of steel containing 0.4 percent carbon, 2 percent manganese, and 0.1 percent vanadium

REFERENCES

Kurdiumov, G. V. lavleniia zakalki i otpuska stali. Moscow, 1960.
Entin, R. I. Prevrashcheniia austenita V stali. Moscow, 1960.
References in periodicals archive ?
An experimental AHS-type steel with a reduced Ms temperature and a carbon content of 0.4% and a manganese level of 2.5% was experimentally treated using the Q&P process with various times at the austenitizing temperature and various rates of cooling to the quenching temperature.
In some cases, particularly involving hardening temperatures over 2000F, multiple preheating steps may be used prior to reaching the final austenitizing temperature.
and Gadicherla, P.K., "Influence of Austenitizing Temperature on Fracture Toughness of a Low Manganese Austempered Ductile Iron (ADI) with Ferritic as Cast Structure," Materials Science and Engineering A 268(1-2):15-31, 1999, doi:10.1016/S0921-5093(99)00120-3.
Hardening treatment Hardening treatment Material Austenitizing Austenitizing Tempering Tempering ([degrees]C) (s) ([degrees]C) (s) Steel 1 1030 1800 610-610 7200 Steel 2 1050 1800 510-510 7200 Steel 3 1040 1800 520-520 7200 Table 3: Chemical composition (wt.%) and hardness of hot work tool steels.
Given the need to obtain high strength and impact energy in grade AISI 422, in present research, the effect of austenitizing temperature and tempering time on strength and impact as well as microstructure has been evaluated.
In order to prevent formation of fine new cementite lamellae, the austenitizing time was reduced to 15 seconds (H15-1).
Conventional heat treatment (CHT) consisted of the following steps: heating up to the austenitizing temperature of 1050[degrees]C in a vacuum furnace, holding at the temperature for 30 min, and nitrogen gas quenching (5 bar).
The heat treatment of St-2 was normalization (austenitizing at 870[degrees]C, temperature 30-50[degrees]C higher than austenitizing temperature and air cooling to room).
The diagram was calculated for austenitizing temperature of 950[degrees]C, grain size of 10 [micro]m and lowest volume fraction of the phase of 0.1%.
The specimens were firstly preheated to 1473 K at a rate of 5 K/s and held for 5 min at isothermal conditions for fully austenitizing. Then the specimens were cooling to the deformation temperature at the rate of 5K/s and held for 30 s for the homogenization of temperature and compressed at the selected conditions.
Two C70S6 crackable connecting rods have been applied austenitizing 800[degrees]C for 1 hour in the controlled heat treatment furnaces separately and soonly after it was taken to salt bath in 450[degrees]C for 3 hours, then quenched in still air as shown in Fig.
In the 1930s, a fine, feathery, needle-like microstructure unlike ferrite, pearlite and martensite was found in steels when they were cooled rapidly from austenitizing temperatures and held at intermittent temperatures.