Issue 62

N.E. Tenaglia et alii, Frattura ed Integrità Strutturale, 62 (2022) 212-224; DOI: 10.3221/IGF-ESIS.62.15

Figure 2: Geometries of cast parts. Dimensions expressed in mm.

Parameters Volume [m 3 ] Surface [m 2 ]

1-inch KB

Heavy KB

0.0019

0.0064

0.116 0.016

0.213 0.03 3106 0.029

Thermal modulus [m] Solidification time [s] Cooling rate 1 [°C/s] Cooling rate 2 [°C/s]

725 0.31

0.162 0.048 Table 2: Thermal parameters of both cast geometries used.

Sol idi f i cat ion structure charact erization and mechani cal propert ies Samples were extracted from castings, as shown in Fig. 2. Then the specimens were grounded and polished using conventional techniques and etched with several regents. The characterization of inclusions and Ti(C,N) was made on samples without etching, using light optical microscopy (LOM). The microstructure was revealed by nital etching (2%) and the dendritic pattern was revealed by Motz etching. This etchant is commonly used in cast iron to reveal the LFT and FTF zones [12-15] and it has been useful to reveal the dendritic structure in cast steels containing high-Si contents [16]. Color etching produces the deposition of thin oxide films on the metallic surface. The thickness of the films depends mainly on Si concentration, and because of the light interference phenomenon, different thicknesses produce different colors, allowing the identification of the Si segregation pattern. Further information about color etching can be found in references [12,16]. The degree of refinement of the solidification structure as a function of Ti concentration was determined by measuring the prior austenitic grain size (austenitic phase corresponding to field A in Fig. 1). The determination of the grain size was performed according to the ASTM E112 requirements and using the lineal intercept procedure. For this purpose, allotriomorphic and Widmanstätten ferrite were used as references for the location of grain boundaries, due to these phases nucleating and growing at the prior austenite grain boundaries. This procedure was only made in samples in which grain boundaries were clearly identified. In addition, the dendritic pattern was also characterized using the same method. In this case, the average size of the FTF zones (dendrite cores) was determined by means of counting the interception of LTF zones with random lines. Each reported value is a mean of 15 measurements.

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