Issue 71

N.E. Tenaglia et alii, Fracture and Structural Integrity, 71 (2025) 80-90; DOI: 10.3221/IGF-ESIS.71.07

observed after extensive examination, suggesting that, if present, its amount is very low. Under these heat treatment conditions, the bainitic transformation stop has been achieved (see Fig. 3 or Tab. 1). Additionally, it can be observed that Nital 2% etching highlights differences in the solidification structure (compare Figs. 5a and 5b). However, no differences in the bainitic structure were found when observed by SEM (compare Figs. 5e vs 5f). It could be concluded that the segregation difference does not affect the quantity and morphology of phases present in the microstructure (bainitic ferrite and retained austenite) for treatment times longer than the stop of bainitic transformation. Both the FTF and LTF zones of the thinner and thicker casting samples showed a similar structure, although the distribution of LTF zones is finer in the thinner casting samples.

200 µm

200 µm

(a)

(b)

10 µm

10 µm

(c)

(d)

2 µm

2 µm

(e) (f) Figure 5: Micrographs corresponding to samples austempered at 280 °C for 360 minutes. (a), (c) and (e) thinner casting sample; (b), (d) and (f) thicker casting sample. LOM and SEM, Nital 2%. Fig. 6 shows the microstructure obtained after austempering treatments at 330°C for 360 minutes. Again, microstructures are composed of bainitic ferrite and retained austenite, and no martensite was identified. Under these heat treatment conditions, the bainitic transformation is also complete. From the comparison to the Fig. 5, a thicker microstructure with larger bainitic ferrite plates and a higher amount of retained austenite is observed. Additionally, no significant differences

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