Carbon in bainite

It is simple to establish that martensitic transformation is diffusionless, by measuring the local compositions before and after transformation. Bainite forms at somewhat higher temperatures where the carbon can escape out of the plate within a fraction of a second. Its original composition cannot therefore be measured directly.

There are three possibilities. The carbon may partition during growth so that the ferrite may never contain any excess carbon. The growth may on the other hand be diffusionless with carbon being trapped by the advancing interface. Finally, there is an intermediate case in which some carbon may diffuse with the remainder being trapped to leave the ferrite partially supersaturated. It is therefore much more difficult to determine the precise role of carbon during the growth of bainitic ferrite than in martensite.

Diffusionless growth requires that transformation occurs at a temperature below T₀ , when the free energy of bainite becomes less than that of austenite of the same composition. A locus of the T₀ temperature as a function of the carbon concentration is called the T₀ curve, an example of which is plotted on the Fe-C phase diagram in Figure. Growth without diffusion can only occur if the carbon concentration of the austenite lies to the left of the T₀ curve.

An illustration of the T-zero construction on the Fe-C phase diagram. Austenite with a carbon concentration less than that given by the T-zero curve can in principle transform without diffusion. But diffusionless transformation is not possible even in principle, if the austenite has more carbon than given by the T-zero curve. Alpha refers to ferrite and Gamma to austenite.

Suppose that the plate of bainite forms without diffusion, but that any excess carbon is soon afterwards rejected into the residual austenite. The next plate of bainite then has to grow from carbon-enriched austenite ( Figure a). This process must cease when the austenite carbon concentration reaches the T₀ curve. The reaction is said to be incomplete, since the austenite has not achieved its equilibrium composition (given by the Ae3 curve) at the point the reaction stops. If on the other hand, the ferrite grows with an equilibrium carbon concentration then the transformation should cease when the austenite carbon concentration reaches the Ae3 curve.

(a) The incomplete reaction phenomenon. If bainite grows without diffusion, but with the carbon escaping from the plate immediately after growth ceases, then the next plate must grow from enriched-austenite. By this mechanism the reaction must stop at the T-zero curve. (b) Actual experimental data confirming the T-zero curve.

It is found experimentally that the transformation to bainite does indeed stop at the T₀ boundary (Figure b). The balance of the evidence is that the growth of bainite below the B_S temperature involves the successive nucleation and martensitic growth of sub-units, followed in upper bainite by the diffusion of carbon into the surrounding austenite. The possibility that a small fraction of the carbon is nevertheless partitioned during growth cannot entirely be ruled out, but there is little doubt that the bainite is at first substantially supersaturated with carbon.

These conclusions are not significantly modified when the strain energy of transformation is included in the analysis.

There are two important features of bainite which can be shown by a variety of techniques, e.g. dilatometry, electrical resistivity, magnetic measurements and by metallography. Firstly, there is a well defined temperature B_S above which no bainite will form, which has been confirmed for a wide range of alloy steels. The amount of bainite that forms increases as the transformation temperature is reduced below the B_S temperature. The fraction increases during isothermal transformation as a sigmoidal function of time, reaching an asymptotic limit which does not change on prolonged heat treatment even when substantial quantities of austenite remain untransformed. Transformation in fact ceases before the austenite achieves its equilibrium composition, so that the effect is dubbed the "incomplete-reaction phenomenon".

These observations are understood when it is realised that growth must cease if the carbon concentration in the austenite reaches the T₀ curve of the phase diagram. Since this condition is met at ever increasing carbon concentrations when the transformation temperature is reduced, more bainite can form with greater undercoolings below B_S. But the T₀ restriction means that equilibrium, when the austenite has a composition given by the Ae3 phase boundary, can never be reached, as observed experimentally. A bainite-finish temperature B_F is sometimes defined, but this clearly cannot have any fundamental significance.