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Examination of the fracture surface in the 1010 bars found primarily dimple fracture. The samples contained large shrinkage pores within the gauge section (Figure 28). Many of the 1030 bars also had similar shrinkage defects (Figure 29). Even the bars with high elongation had pores on the same order of magnitude. The author cannot com- pletely explain why these pores were so large. The plate casting design was designed using solidification modeling software to eliminate shrinkage porosity. Shrinkage of this magnitude is usually accurately predicted by this type of software. In any case, the high elongation in some of the 1010 samples despite these large defects was unexpected. Shrinkage porosity has been well documented as a cause of low ductility in cast metals.14,18


The electron microscopy observations indicate why there was not clear trend with addition alloy or TRE content. Samples with aluminum-iron oxide coatings on the RE oxides did not have finer microstructures or improved mechanical properties. The cause of these aluminum-iron oxides was likely reoxidation during tapping. Flow of the liquid during tapping was turbulent. The RE additions were


added into the steel stream when it entered the ladle. These additions reacted with entrained oxygen to form RE oxides, but those same conditions appear to also randomly gener- ate aluminum-iron oxides that either form on the outside of the RE oxide particles or coat the RE oxides later. These aluminum-iron oxides likely did not form during mold fill- ing since computer mold filling simulations indicated a low velocity and laminar filling pattern. Also, the gating system controlled the in-gate velocity effectively. Less control had been exerted on the addition process because of human control of the furnace during tapping and human control of the exact point of RE additions. These variations could result in the inconsistent RE recovery observed and the seemingly random presence of the poisoning aluminum- iron oxide coating.


Strengthening Theory


Many of the authors working with RE additions have been working in austenitic stainless steels.8,9,11


The austenitic


steels experimented on were thought to solidify by forming primary austenite dendrites and never going through a solid


Figure 26. Electron micrograph of the 1030 0.1% MM sample.


Figure 28. SEM image of large shrinkage pore in a 1010 tensile bar.


Figure 27. Image of typical inclusions from the 1030 0.2% MM sample.


International Journal of Metalcasting/Spring 2012


Figure 29. Fracture image for a 1030 sample. 61


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