ing processes. The position and amplitude of the interval is modified mainly by the alloy chemical composition and the solidification cooling rate. For instance, the presence of Si in DI diminishes the solubility of C in austenite, thereby promoting ferrite formation; this phenomenon leads to an increment of both critical temperatures, as qualitatively il- lustrated in Figure 1-b.10
Other elements also generate fur-
ther modifications over the intercritical interval: Mn leads to a decrease in the intercritical temperature range, while Cr reduces its amplitude increasing mainly the lower critical temperature.11
calculate the upper critical temperature, taking into account the influence of some common alloying elements9,10,12
The literature reports equations designed to . How-
ever, no equations estimating the lower critical temperature have been reported so far.
Thermal Cycles
Special thermal cycles have been developed in order to ob- tain dual phase ADI. The first paper concerning this subject matter was written by Aranzabal et al.13
The authors ob-
tained different variants of dual phase ADI by modifying the silicon level of the melt and heat treating ferritic DI at fixed austenitizing temperatures. During the austenitizing stage different amounts of austenite nucleate and grow as a function of the position (temperature) inside the intercritical interval of the melts, which vary according to the different silicon contents. The austenitizing stage is followed by an austempering step in order to produce the austenite to aus- ferrite reaction, thus obtaining a final microstructure com- posed of free (allotriomorphic) ferrite and ausferrite.
On the other hand, Wade et al.14 and Verdu et al.15 obtained
dual phase ADI microstructures by means of heat treat- ments based on quick and incomplete austenitizations in the austenitic field (over the upper critical temperature). The austenite, nucleated at high temperature, mainly surrounds graphite nodules and then is transformed into ausferrite as
a result of an austempering step. The relationship between the relative quantities of each phase is controlled by the aus- tenitizing time; the longer the time, the higher the amount of austenite and, therefore, the larger the amount of ausferrite in the final microstructure.
Basso et al.16, 17 and Kilicli et al.18 utilized an alternative
methodology to obtain dual phase ADI. It consists of sub- jecting a fully ferritic DI (with a fixed chemical compo- sition) at an incomplete austenitization stage at different temperatures within the intercritical interval followed by an austempering step to transform austenite into ausfer- rite. This heat treatment results in microstructures made up of different percentages of ausferrite and allotriomorphic ferrite (original matrix of the samples), depending on the austenitizing temperature. The amount of ferrite increases when the austenitization step is closer to the lower critical temperature. On the other hand, when using austenitizing temperatures close to the upper critical temperature, the amount of allotriomorphic ferrite diminishes and it is pres- ent as a dispersed microconstituent in an ausferritic matrix. Figure 2 shows these kinds of microstructures obtained for different samples of the same melt austenitized at different temperatures within the intercritical interval and then aus- tempered at 662F (350C).
Druschitz et al.19,20 and Valdés et al.21 employed the same
heat treatment as Basso et al. and Kilicli et al. did to ob- tain dual phase ADI microstructures. However they started from ferritic–pearlitic as-cast microstructures, avoiding the ferritizing stage. Obtaining dual phase ADI microstructures by means of the methodology applied by Basso et al. and Kilicli et al. is considered to offer an important advantage over the methodologies proposed in the other two studies mentioned. Microstructures with well controlled phase percentages are obtained. In this case, the amount of aus- tenite during the partially austenitizing step is as indicated by the phase diagram in thermodynamic equilibrium. It has
(a) Figure 1. a) Representation of Fe-C phase diagram (at 2.5%Si),9 8
(b) b) Influence of Si content on the intercritical interval position.10 International Journal of Metalcasting/Winter 2012
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