The decomposition of Austenite

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 

The higher solubility of carbon in y-iron than in a-iroii is because the face-ceiiued lattice can accommodate carbon atoms in slightly expanded octahedral holes, but the body-centred lattice can only accommodate a much smaller carbon concentration in specially located, distorted tetrahedral holes. It follows that the formation of fenite together with cementite by eutectoid composition of austenite, leads to an increase in volume of the metal with accompanying compressive stresses at die interface between these two phases. 

In the most frequently used steels, having less dran the eutectoid content of carbon (about 0.8 wt%), die vaiious forms in which the cementite phase can be produced in dispersion in fenite depend upon die rate of cooling to 

Coarse pearlite Fine pearlite Upper Bainite 

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 

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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... 

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. 

A modified solute drag model, taking into account the interfacial segregation of alloy elements, is applied to the decomposition of austenite into ferrite in the C-Fe-Mo alloys [1997Liu]. The ealeulations are performed with assessed thermodynamic and kinetic data stored in a database. The solute drag-like effeet suggested in the literature is discussed in comparison with the quantitative calculation by [1997Liu]. 

Figure 10.36 Possible transformations involving the decomposition of austenite. Solid arrows, transformations involving diffusion dashed arrow, diffusionless transformation.

Earlier chapters discussed a number of phenomena that occur in metals and alloys at elevated temperatures—for example, recrystallization and the decomposition of austenite. These are effective in altering the mechanical characteristics when appropriate heat treatments or thermal processes are used. In fact, the use of heat treatments on commercial alloys is an exceedingly common practice. Therefore, we consider next the details of some of these processes, including annealing procedures, the heat treating of steels, and precipitation hardening. 

Closely following the Avrami expression is an empirical relation introduced by Austin and Rickett, based on the experimental results for the decomposition of austenite steel.(51) The relation can be expressed as... 

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. 

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. 

A microanalysis study of the eutectoid decomposition of austenite into ferrite and M2C (to bainite) at the bay in Fe-0.24C-4Mo is reported by [2003Hacl]. It was concluded that alloy element partition between ferrite and alloy carbides at the reaction front is largely responsible for the slow kinetics in this and related alloys. A thermodynamic analysis showed that ferrite-carbide interfacial energy and nonequilibrium carbide compositions reduce the thermodynamic driving force for diffusion processes (Mo partition) by up to 20% further slowing the kinetics. 

Neh] Nehrenberg, A.E., Discussion of the Isofliermal Decomposition of Austenite in Fe-Mo-C Alloys , Metall. Trans., 2(8), 2283-2284 (1971) (Review, Phase Relations, 8)... 

The temperature-composition section at 6.3 mass% W was calculated by [1998Hac] during examination of a ternary C-Fe-W steel with the purpose of characterizing the carbide morphologies that arise during the diffusional decomposition of austenite. Phase stability at the melting point of a low-alloyed steel (1527°C) was estimated by [2002Bab]. 

Here T must be calculated at temperature of minimal stability for austenite, viz. T 500 °C. To investigate the mechanical response of the material following from welding and the induced phase transformations, mechanical materials models are required, in fact before the mechanical FE analysis. Such models allow for the decomposition of the strains by incorporating the phase evolution, e = e + eP + e + Here e -yM identifies... 

Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite. 

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