Austenite, decomposition

Since the rate of formation of cementite is determined by nucleation, and therefore proceeds more rapidly in fine-grained steels, it follows that the T-T-T diagram will show a more rapid onset of austenite decomposition than in steels of the same composition, but a coarser grain size. The shape of the T-T-T curve is also a function of the steel composition, and is altered by the presence of alloying elements at a low concenuation. This is because the common alloying elements such as manganese, nickel and clrromium decrease... 

I99OG0I] Goldenstein, H., Aaronson, H.I., Overall Reaction Kinetics and Morphology of Austenite Decomposition Between the Upper Nose and the Ms of a Hypoeutectoid Fe-C-Cr Alloy , Metall Trans. A, 21A(6), 1465-1478 (1990) (Morphology, Phase Relations, Experimental, Kinetics, 65)... 

Hacl] Hackenberg, R.E., Granada, D.G., Shiflet, G.J., Austenite Decomposition to Carbide rich Products in 0.30C-6.3W , Metall. Mater. Trans. A, 33A(12), 3619-3633 (2002) (Crys. Structure, Morphology, Phase Diagram, Phase Relations, Experimental, Kinetics, Meehan. Prop., 61)... 

Decomposition of Austenite. In heat-treating steels, the initial step is usually to heat the steel into the austenite region (>723° C) and then control the cooling process to produce the desired stmeture. The phase diagram (Fig. 2) shows that austenite decomposes into the two phases d and Fe C... 

When a component at an austenitizing temperature is placed in a quenchant, eg, water or oil, the surface cools faster than the center. The formation of martensite is more favored for the surface. A main function of alloying elements, eg, Ni, Cr, and Mo, in steels is to retard the rate of decomposition of austenite to the relatively soft products. Whereas use of less expensive plain carbon steels is preferred, alloy steels may be requited for deep hardening. 

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Eutectoid structures are like eutectic structures, but much finer in scale. The original solid decomposes into two others, both with compositions which differ from the original, and in the form (usually) of fine, parallel plates. To allow this, atoms of B must diffuse away from the A-rich plates and A atoms must diffuse in the opposite direction, as shown in Fig. A1.40. Taking the eutectoid decomposition of iron as an example, carbon must diffuse to the carbon-rich FejC plates, and away from the (carbon-poor) a-plates, just ahead of the interface. The colony of plates then grows to the right, consuming the austenite (y). The eutectoid structure in iron has a special name it is called pearlite (because it has a pearly look). The micrograph (Fig. A1.41) shows pearlite. 

In fundamental terms, the transformation temperature affects both the driving force for the decomposition of austenite and the diffusion rate of carbon. In effect, therefore, the transformation temperature alters both the rate of nucleation and the rate of growth. This in turn manifests itself in... 

Aurantin, molecular formula and structure, 5 91t Ausimont, 7 641 Austempering, 23 287 Austenite, 23 272, 273, 275 decomposition of, 76 197-198 grain size of, 23 276-277 in hardening of steels, 76 196-197 phase transformations in, 23 277 transformation rates of, 23 282—283 Austenite phase, in martensite... 

Pearlite The two-phase structure (aFe,C + Fe3C) originating from the eutectoidal decomposition of austenite (that is the C solid solution in Fe) and having the overall composition of 0.76 mass % C (3.46 at.%C). The stable Fe-graphite eutectoid has a composition of 2.97 at.% C. 

The importance of metastable phases which persist at ambient pressure and temperature need not be emphasized. Control of the eutectoidal decomposition of the metastable BCC iron-carbon phase, austenite, below 996 K is essential to the steel... 

BAINITE. A product of the decomposition of austenite that usually occurs at temperatures between those that produce pearlite and those that produce martensite. Its structure consists of finely divided carbide particles in a matrix of ferrite. See also Austenite. 

Specimen preparation involves wet grinding to remove the surface layer, which may be decarburized or otherwise nonrepresentative of the bulk of the specimen, followed by standard metallographic polishing and etching. This procedure ensures a flat, reproducible surface for the x-ray examination, and allows a preliminary examination of the specimen to be made with the microscope. In grinding and polishing, care should be taken not to produce excessive heat or plastic deformation which would cause partial decomposition of both the martensite and austenite. 

Chen, S., Gao, M., and Wei, R. P., Hydride Eormation and Decomposition in Electrolytically Charged Metastable Austenitic Stainless Steels, Metallurgical and Materials Transactions, 27A, 1 (1996), 29-40. 

---