Austenite


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Related to Austenite: pearlite, cementite

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.
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If the hydrogen is not fully removed by baking, then upon austenitization the thermal energy will be sufficient to release the hydrogen back into solution near the pore with austenite having a higher solubility for hydrogen.
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The girth weld from the X70A pipe has elongated prior austenite grains that are highly variable in size.
The temperature transformation points of the austenite and martensite phases of the samples were determined and analyzed on the table and the thermal properties of the shape memory alloys were obtained.
Khan, Effect of repeated thermal cycling on the formation of retained austenite in 18% Ni 350 grade Maraging steel Materials Transaction JIM 39, 995 (1998).
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The quantity of the carbon concentrated in the primary austenite will be suppressed, being affected by the welding heat input, when the bainite ferrite is generated during the bainite transformation.
This is due to the fact that the austenitic phase is loaded elastically up to a "yield" stress where a stress-induced transformation from austenite to martensite takes place.
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Manganese improves mechanical properties of steel through its effect on solid solution strengthening and retained austenite stabilization.
One of the key factors influencing dimensional stability is the retained austenite content of the AISI 52100 steel [2].