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during the experiments demonstrated a filling pattern very close to the computer simulations. The only improvement to the gating system would be the use of a filter. A filter would certainly ensure that any oxides or slag from the pouring ladle would not enter the casting. However, it would also have a tendency to remove the La2

pouring. Since the author was testing the addition of these powders into the steel, it would have negated one aspect of the research being conducted.


Microstructural examination of the castings found a lower inclusion content than the author has observed in industrial castings, a direct result of the gating system employed. Also, oxides were not usually observed with the shrinkage poros- ity in the fracture surfaces. The addition of rare earth ele- ments should have actually made the number and severity of oxides within the castings with RE additions worse than the baseline material. As such, one would expect that all of the mechanical properties were worse than the baseline. That was not observed in the data.

There was a clear difference in the final microstructures for some of the castings due to the addition of RE elements. Electron microscopy investigation also found a reason why RE additions worked in some cases and not in others. There- fore, much can be learned about the nucleation events in steel castings through this study.

Reviewer: The author should take account of the new theo- ries of the design of molds and casting systems. Using inap- propriate designs, such as the use of a conical pouring basin (assumed since no design was given), a well at the base of the sprue, together with the 1:4:4 gating ratio is not recom- mended. The damage to the castings is confirmed, by among other things, the poor elongation results and their extreme scatter. Furthermore, there appears to be no discernible ef- fect of the addition of the rare earth oxide powder. Any minor effect is swamped by the scatter, putting the results of the research in question.

Author: As stated, the gating system used in the test cast- ing represents good steel casting practice of the industry. It is common to use a sprue well and a gating ratio of 1:2:2 in steels. The 1:4:4 ratio was chosen to provide a slower fill rate to reduce oxide entrainment and re-oxidation of the

powder added during

steel. Newer theories on gating system design are far from universally accepted. The use of computer simulations pro- vides a better tool for evaluating the overall effectiveness of gating systems. Regarding elongation, significant scatter in elongation data can be caused by the testing method used. The company that conducted the tensile test uses extensome- ter data for calculating percent elongation. If the extensom- eter slips during the tensile test, this can lead to significant errors in the elongation measurement during fracture. It is better practice to measure elongation using gauge marks on the specimens. Unfortunately, the company providing the testing does not have the appropriate indenting fixture. It seems to the author that it is far more likely that the errors in measuring the elongation are causing the variation ob- served in the data than the presence of oxides or bi-films.

Reviewer: Table two seems to be shows a lot of variability in TRE content when the same type and amount of additions are made. This variability could explain some of the erratic results.

Author: Rare earth recovery varied significantly during the tests. Researchers actually repeated several of the ex- perimental treatments in an attempt to obtain the same TRE values. Discussions with a set of European researchers that have used rare earth oxide additions in stainless steel have indicated that they have also observed the same problem. It appears the in-stream addition during tapping results in significant and variable rare earth losses. The most likely cause are differences in how the steel stream impacts the ladle during filling and exactly how the rare earth contain- ing pieces hit the stream. The author currently has work ex- amining how to eliminate this variability by changing the addition method. This variation in TRE content is why yield strength was plotted as a function of actual TRE content to assist in examining the actual factors involved.

As also pointed out in the paper, many of the RE oxides in the poorly performing RE added steels had a reoxidation coating. This coating would prevent the RE oxides from acting as nuclei regardless of the TRE content of the alloy. Without direct contact between the steel and RE oxides there is no way for the RE oxides to assist any phase reactions. This factor and the variable rare earth recovery likely ex- plain the lack of improvement for these alloys.

International Journal of Metalcasting/Spring 2012


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