Mechanical Properties and Fractography
Based on age-hardening kinetics (Figure 8), peak-aged and over-aged heat treatment parameters were used for the ten- sile bars (Table 5). The results of tensile and Charpy tests with uncertainties calculated at the 95 percent confidence level are presented in Table 6.
Fracture surfaces of over-aged (4 hours at 538°C [1000°F]) tensile specimens were observed using Field Emission SEM. Cup-cone fractures were observed with radial cracks from the center of the test bar to the base of the shear lip (see Figure 9 [a]). The central region exhibited microvoid
coalescence. Inclusions were observed in the voids indi- cating the microvoid nucleation started at these inclusions (see Figure 9 [b]). The shear lip showed elongated micro- voids typical of a shear fracture (see Figure 9 (c)). In Heat 3, radial cracks were observed from the centre to the base of the shear lip (see Figure 10 [a]). The shear lip as well as central region exhibited a fibrous appearance. Figure 10 (b) and Figure 10 (c) depict the shear lip whereas Figure 10 (d), Figure 10 (e), and Figure 10 (f) are from the central region. Features ’a’, ‘b’,‘d’, and ‘g’ are Nb-rich carboni-
trides, ‘c’ is matrix whereas feature ‘e’ and ‘f’ are SiO2 inclusions. The compositions of the features labeled in Fig- ure 10 are given in Table 7.
(a)
(b)
(c)
Figure 9. Fracture surface for an over-aged tensile bar of the base heat: (a) radial cracks at the base of a shear lip, (b) micro-voids forming at inclusions, and (c) shear lip.
(a)
(b)
(c)
(d) 66
(e) Figure 10. SEM-chemical analysis of surface fracture of over-aged tensile bar of Heat 3. International Journal of Metalcasting/Spring 10
(f)
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