Isothermal martensite

We have mentioned above the tendency of atoms to preserve their coordination in solid state processes. This suggests that the diffusionless transformation tries to preserve close-packed planes and close-packed directions in both the parent and the martensite structure. For the example of the Bain-transformation this then means that 111) -> 011). (J = martensite) and <111> -. Obviously, the main question in this context is how to conduct the transformation (= advancement of the p/P boundary) and ensure that on a macroscopic scale the growth (habit) plane is undistorted (invariant). In addition, once nucleation has occurred, the observed high transformation velocity (nearly sound velocity) has to be explained. Isothermal martensitic transformations may well need a long time before significant volume fractions of P are transformed into / . This does not contradict the high interface velocity, but merely stresses the sluggish nucleation kinetics. The interface velocity is essentially temperature-independent since no thermal activation is necessary. 

Kak] Kakeshita, T., Sato, Y, Saburi, T., Shimizu, K., Matsuoka, Y, Kindo, K., Effects of Magnetic Field on Athermal and Isothermal Martensitic Transformations in Fe-Ni-Cr Alloys , Mater. Trans., JIM, 40(2), 100-106 (1999) (Experimental, Kinetics, Phase Relations, 13)... 

Austempering. Lower bainite is generally as strong as and somewhat more ductile than tempered martensite. Austempering, which is an isothermal heat treatment that results in lower bainite, offers an alternative heat treatment for obtaining optimum strength and ductility if the specimens are sufficiently small. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

Figure 5.30. Schematic drawing showing the construction of an isothermal transformation diagram from measurements of the progress of the transformation at various constant temperatures. This may be done, for instance, by metallographic examination of several specimens, quenched from the 7-field quickly enough to miss the nose of the C-curve and then isothermally annealed for various length of time. Notice that curves for the transformation of different samples may be shown on the same diagram and that more complex trends may be observed in real diagrams of specific alloys. In the example reported, Ms is the temperature at which the alloy will begin to show the martensitic transformation, Mf indicates the temperature below which no additional martensite forms.

E.S. Machlin and M. Cohen. Isothermal mode of the martensitic transformation. Trans. AIME, 194 489-500, 1952. 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

Bainite is a crystalline constituent that can be formed during heat treatment of steel by isothermal transformation or continuous cooling in the temperature range between those of perlite and martensite. Here, iron diffusion is no longer possible, while... 

E. J. Mittemeijer, Liu Cheng, P. J. van der Schaaf, C. M. Brakman, and B. M. Korevaar. Analysis of non-isothermal transformation kinetics tempering of iron-carbon and iron-nitrogen martensites. Metall. Trans. 19A, 925 (1988). 

Bee] Light microscopy, TEM, high speed dilatometry, isothermal dilatometry 800-400°C, Fe-5 mass% Cr-0.2 mass% C, Fe-10 mass% Cr-0.2 mass% C, y, a + y -H M23C6, M7C3, martensite, bainite... 

Numerous investigations have been done regarding the liquidus surface, die isothermal sections and the vertical sections in the stable and metastable systems. The other investigations on die ternary system concern the solubility measurements of carbon in the "y and liquid phases which go always widi activity measme-ments, the determination of the phase diagram under high pressures and die kinetics studies of die austenite transformation in martensite or bainite because these phases are important in die forecast of mechanical properties of steel. The main experimental investigations on crystal structure, phase equilibria and thermodynamics are gathered in Table 1. 

The optical microstructures and composition ranges of the martensites characteristic of ice-brine-quenched massive samples of T-V, T-Nb, T-Mo, and T-Fe are exemplified by Fig. 5.2. Composition-ally, T-Fe and T-Nb are extreme examples, their Mg curves bounding those for all other measured T-TM alloys and intersecting a 200 °C isothermal, for example, at 3.3 and 20.5 at.%, respectively [ZWI74, p. 174]. 

---