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Technical Review & Discussion


Effect of Rare Earth Additions on Grain Refinement of Plain Carbon Steels R. Tuttle, Department of Mechanical Engineering, Saginaw Valley State University, University Center, MI USA


Reviewer: It is also not clear that the effect of the additives is nucleation of delta ferrite, and not in their effect on subse- quent phase transformation. Please comment on this.


Author: While other researchers have concluded that rare earth (RE) oxides must assist the nucleation of delta ferrite due to their own observations of grain size reduction, there is a discrepancy in this theory. Crystallographic matching between delta ferrite and RE oxides is high enough that the author questions the validity of such a theory. The au- thor contemplated that perhaps the presence of RE oxides made the initiation of austenite dendrites easier because there was a nucleation site for them. A similar situation exists in cast irons where inoculants are added to assist the growth of graphite instead of cementite. However, no ex- perimental evidence from this work or his other work sup- ported such an idea so this explanation was not included in the text. Another possible route is that the RE oxides act as nuclei during the transformation from delta ferrite to austenite during the solid state transformation. Creating more austenite grains would cause a reduction in the aus- tenite grain size which would then result in a reduction in the alpha ferrite grain size in the final room temperature microstructure. The revised paper includes information re- garding this second mechanism and incorporates this new literature source.


Reviewer: The author inspects the room temperature grain size that may bear little relation to the as-solidified grain size that he appears to be targeting in the effort to produce grain refinement. The room temperature grain size has suffered at least one or two solid state phase changes when cooling from the freezing temperature.


Author: The reviewer is correct that there is some trouble with examining room temperature grain size as a proxy for the as-cast grain size. These steels go through one or two solid state transformations before the room temperature microstructure develops. However, the solid state transfor- mations initiate at the existing grain boundaries and move inwards as they progress. The result is that they initiate at the grain boundaries of the decomposing phase. Therefore, final grain size is dependent on the initial grain size as long as the steels being compared have similar thermal histories. This is why the castings were always allowed to cool in the mold until they were below the eutectoid temperature and no additional heat treatments were done. It was vital for any useful comparisons that every test casting had a similar cooling history.


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None of the samples from this series of experiments respond- ed to the picric acid etchant that was etchant to reveal the prior austenite grain boundaries. Etching for prior austenite grain boundaries can be difficult to successfully complete. The literature has several references to the random ineffec- tiveness of the various prior austenite etching procedures for some alloys. Other researchers have indicated that steels of the same grade can exhibit drastically different etching behavior when trying to determine prior austenite structure. Also, since these alloys should initially solidify as delta fer- rite such etching would still not give a precise picture of the as-cast structure.


Some researchers have used high phosphorous levels to determine the delta ferrite structure. This technique works because the phosphorous strongly segregates during so- lidification. On subsequent etching, the high phosphorous regions that correspond to the interdendritic liquid at the final stages of solidification etch differently than the other portions of the steel. This provides great contrast. The prob- lem with this approach is that the mechanical properties are detrimentally impacted by the high phosphorous levels. Comparisons to any commercial material would not be pos- sible by readers. The author thought that it was better to use a steel composition closer to commercial alloys to help industrial readers of his work interpret the commercial im- plications. In the next phase of this grain refining project, the author is strongly thinking about using this approach to better understand how the as-cast structure develops and is related to the final structure.


Reviewer: The test casting design appears lacking in that it will allow entrapment of oxides from the melt and oxides generated during pouring into the area of the casting from which the test bars are pulled. If oxides are present in the metal, is that reflected in the mechanical properties of the test specimens? Does the appearance of shrinkage porosity confirm the oxides in the metal and the poor ductility re- sults? The ductility properties are low especially for 1030; those that are higher are merely approaching the values to be expected (all values should be in the range 30 to 50% elongation). The large feeder on the casting was carefully checked by computer simulation and should have been suffi- cient to prevent shrinkage porosity. Is the observed porosity truly shrinkage? The oxides generated from the current fill- ing system could overwhelm all other aspects of this work. This should be addressed.


Author: The gating system for the casting follows standard filling system design and was also verified by computer simulation. A gating ratio of 1:4:4 was used to maintain low metal velocities. Simulation predicted a maximum ve- locity in the in-gate of 0.5 m/s. Metal velocities were far lower inside the part cavity. These lower velocities reduce the turbulence while filling the mold and prevent entrain- ment of re-oxidation products. Observation of the filling


International Journal of Metalcasting/Spring 2012


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