Experimental Determination of Continuous Cooling Transformation Diagrams of Hot-Rolled Heat Treatable Steel Plates Using
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RESULTS AND DISCUSSION Microstructure and initial austenite grain size Figure 1 shows the microstructural characteristics and initial austenite grain size of the studied steel. In general, the microstructure is mainly composed by a ferritic matrix with pearlite (α-Fe + Fe3C) regions and some Fe3C particles mostly formed on the ferrite grain boundaries (see Figure 1a). As can be seen in Figure 1b, the average austenite grain size resulting from the applied heat treatment (austenitizing at 900 °C/s and quenched at 200 °C/s) was 29.6 m. The initial austenite grain size has a significant effect on the different phase transformation kinetics. If the grain size is coarsened, there is a reduction in grain boundary area, and therefore the nucleation capacity of ferrite and pearlite is reduced, increasing the steel hardenability [1].
Figure 1. SEM micrographs showing the microstructure and initial austenite grain size of the hot-rolled heat treatable steel studied. Dilatometric curves All experimental dilatation curves of length-change versus temperature (∆𝐿 𝑣𝑠 𝑇) obtained as a function of cooling rate are illustrated in Figure 2. As can be seen, the non-equilibrium decomposition of austenite depends on the cooling rate. In general, it is observed that the transformation temperature of austenite decreases as cooling rate increases. As is well known [1], the sequence of phase transformations in steels by continuous cooling from the austenite is ferrite, pearlite, bainite and martensite. Therefore, each slope variation detected in these dilatometric curves corresponds to a specific phase transformation [2]. On the other hand, it can be seen that during continuous cooling at rates between 25 and 200 °C/s, there is only one change in the slope of the dilation curves characterized by an expansion on cooling which is related to the austenite to martensite phase transformation (Figure 2a) [14, 15]. At slower cooling rates the additional changes manifested in the slope of the dilation curves (Figure 2b) are associated to ferrite, bainite and/or pearlite phase transformations [14]. 200
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Figure 2. Experimental dilation curves obtained as a function of cooling rate: a) 200-25 °C/s and b) 10-0.1 °C/s.
CCT diagrams, microstructures and hardness results The determination of the start and finish temperatures of the non-equilibrium decomposition of austenite from changes in the slope of ∆𝐿 𝑣𝑠 𝑇 plots was conducted considering that the Ts corresponds to the temperature at which, the lineal contraction during cooling is deviated and the sample starts to expand as a result of the ferrite-pearlite, bainite or martensite phase transformations; whereas Tf is the temperature
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