appeared to be associated with the sodium silicate used as the binder, had an average thickness of 0.002 in. (0.056 mm).
3
This study demonstrated and documented the casting skin effect on fatigue properties. The as-cast samples exhibited significant reduc- tion in fatigue strength compared to the machined samples (Fig. 3). The fatigue skin factor (reduction in the as-cast fatigue data compared to the machined data) was found to be 0.687 and 0.650 at a mean stress of 250 and 300 MPa. The study allowed the development of a linear equation to estimate the dependency of fatigue strength on mean stress, or the fatigue skin factor.
Results and Conclusions The average visual skin
thickness in the study was 0.003 in. (0.084 mm). The pearlitic rim, which
Fig. 3. Shown is the stress-strain diagram for the as-cast (AC) and machined (M) samples at a mean stress of 250 MPa. The as-cast samples exhibited a significant reduction in fatigue strength compared to the machined samples.
Cracks originated at the corner of the samples and increased stress. The fracture surface was found at the casting skin. The effect was similar to the way fracture surfaces typically arise in the crack propaga- tion zone. Line profiling using an electron
probe micro-analyzer was found to be a useful method of revealing the mag- nesium microsegregation in CGI. Te results showed the magnesium-rich areas coincided with the interdendritic areas, while the magnesium-depleted areas coincided with the dendritic areas. As casting skin exhibits higher amounts of dendrites than the bulk material, magnesium depletion is seen in the casting skin and is expected to be the cause of graphite degradation. Te magnesium distribution has a good correlation with the observed microstructure.
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48 | MODERN CASTING March 2012
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