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state phase change to another crystal structure. Scheil cal- culations using the assumption that carbon diffuses quick- ly through the solid were conducted by the author using Thermo-Calc©

. The experimental alloys in this work form

δ-ferrite initially. For 1010 steel, the δ-ferrite phase was stable until approximately the last 3-5% of the solidification range, where it transformed to austenite. In the case of 1030 steel, the δ-ferrite was predicted to be the only phase formed for 50% of the freezing range. The remaining portion of the alloy solidified as austenite with the previous δ-ferrite trans- forming to austenite. This results in an inconsistency with the theories by other authors that RE oxides nucleate austen- ite in carbon steels.6,7,19

The microstructural observations in this work correlate with a heterogeneous-based grain refinement mechanism. In both alloys, heats with high RE content inclusions not completely surrounded by other oxides resulted in a finer overall struc- ture. These finer grained alloys had higher yield strengths than the baseline material. If RE content primarily caused the reduction in observed grain size through a dendrite growth rate reducing mechanism, then the yield strength would be a direct function of TRE content. Since yield strength was not necessarily a function of TRE for both alloys, Li et. al’s theory on dendrite growth inhibition due to RE element re- distribution during solidification is not supported by the ex- perimental observations in this research.7

Despite the evidence that rare earth particles play a sig- nificant role in reducing grain size, the theory that RE2

particles play the major role in heterogeneous nucleation is inconsistent with nucleation theory. The difference in lat- tice parameters between the crystal structure of these oxides and δ-ferrite are on the order of 20%, far higher than any observed heterogeneous nuclei.1-5

O3 oxides played the

In either mechanism’s case, the RE oxides would need an excellent crystallographic match to act as a nuclei. The alu- minum-iron oxide coatings found in some of the samples with large grain sizes would prevent the RE oxides from be- ing effective nuclei. This means both proposed mechanisms are consistent with the experimental observations from this research.

Future Work

A considerable amount of work remains to understand the role of heterogeneous nucleation in steels. A TEM study of the high RE content oxides would provide the neces- sary crystallographic information to determine their struc- ture and confirm they act as heterogeneous nuclei. Further experimentation is necessary to determine how to ensure these RE oxides are not coated by another oxide. Such a development would improve the potency of RE additions and the mechanical properties of plain carbon steels.

The role of RE additions in stainless steels also would be of interest. Many stainless steels are not heat treatable, so an effective grain refining technique would improve their properties. Despite the work already done, the role of these additions are unclear. Additional experimentation on the structure-property relation in stainless steels with a RE ad- dition would provide a dramatic improvement in the current understanding of their role in strengthening.


austenite. Based on the evidence in this paper, the complex RE oxides found might have a structure capable of acting as a nuclei for δ-ferrite.


dominant role in assisting nucleation within these samples, then a composition closer to 85% TRE would be expected. A complicated oxide would be anticipated to have a different structure than the simple RE2

oxides theorized to nucleate

work indicated a TRE around 50-60% with 2-10% aluminum for the high RE content oxides. If RE2

The EDS results from this O3

A second possible mechanism is the high RE content ox- ides did not play a major role in refining the as-cast structure but assisted the nucleation of austenite during the solid state transformation from δ-ferrite. The strong crystallographic matching between RE oxides and austenite would mean they could have acted as a solid state nuclei and assisted in the growth of austenite in the solid. This would result in increas- ing the number of austenite grains and create a finer grain size. Once the steel underwent the eutectoid reaction, the α-ferrite would have nucleated on the prior austenite grain boundaries, and a finer structure would be evident.


Rare earth additions were found to improve the yield strength of plain carbon steels. In several cases, the per- cent elongation of the 1010 samples with a RE addition also improved. The average yield strength of the 1010 samples with RE additions was 180 MPa compared to the 170 MPa average strength of the baseline 1010 samples. One set of samples had an average yield strength of 198 MPa. The samples with the highest strength had a finer grain size than the baseline 1010. This indicates the RE addition acted as a grain refiner. In the 1030 samples, the RE containing materials had an average yield strength of 288 MPa, significantly higher than the 262 MPa average yield strength for the 1030 baseline material. Again, the samples showing a finer microstructure had the highest yield strength.

Complex RE oxides were observed in the 1010 and 1030 materials. Some of the samples had RE oxides coated with an aluminum-iron oxide, which appears to prevent contact between the RE oxide and steel. The poisoning effect of this coating resulted in a grain size similar to the baseline material and, therefore, similar mechanical properties. This indicates the complex RE oxides acted as heterogeneous nuclei at some point in the microstructure development of the steels.

International Journal of Metalcasting/Spring 2012

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