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dence has shown that, when the lattice disregistry is small at the melting point of the melt, the solid phase can nucleate the other solid.1-5

In the case of steel, this means a proposed

nucleation compound would assist the nucleation of either δ-ferrite or austenite.

Several authors have reported a reduction in grain size due to the addition of rare earth (RE) elements.6-12

work employing RE additions has been in austenitic stain- less steels.8,9,11 steels.7,10

In terms of actual mechanical properties due to the refinement, minimal information exists.

J.J. Moore made the earliest reference to RE additions re- ducing grain size.6

The focus of his work was to desulfurize

sand cast steels to improve impact toughness. He produced a 0.10%C, 1.25%Mn steel which was poured into cylindri- cal resin-bonded sand molds. Both misch metal and RE sili- cide additions were examined. The types of inclusions were characterized. The author reported complex RE-Al-Mn-O-S oxysulphides as the predominant inclusion type. The sulfur level in the steel matrix was reduced by 90%. The sulfur was found to be tied up in the complex oxysulphides.6


noticed a reduction in grain size, but attributed the improve- ment in properties to a reduction in sulfur content in the steel matrix.6

Li et al. conducted two series of experiments on 1045 steel with RE additions.7

The first set of experiments measured

the cooling curve of solidifying ingots. Rare earth addi- powder was theorized to be capable of

acting as a heterogeneous nuclei. Rare earth addition lev- els of 0.10%, 0.16%, and 0.40% were examined. Li et al. measured a reduction in primary dendrite arm spacing and secondary dendrite arm spacing (SDAS) with RE addi- tions placed in the ingot mold. Rare earth additions added in the furnace had no effect on the microstructure of the steel.7

tions were accomplished by adding RE silicide and CeO2 powder. The CeO2

had no effect on primary dendrite arm spacing or SDAS. The second series of experiments focused on the degree of undercooling required for nucleation and surface tension change. This second series consisted of a levitation drop experiment on 1045 steel with the same additions as the ingot experiment. The addition of both CeO2

The authors further reported the addition of CeO2 and RE de-

creased the undercooling required to initiate solidification. Li et al. also measured a reduction in the surface tension in the droplets. Moreover, they observed a single crystal rim on the solidified droplets. The cause of this single crystal rim was attributed to the high cooling rate and steep tem- perature gradient at the exterior of the droplet. They con- cluded RE silicide additions reduced the grain size and un- dercooling for solidification; however, evidence to prove that CeO2

assisted with nucleation was insufficient. The au-

thors theory was cerium redistribution during solidification impeded dendrite growth, which resulted in the observed SDAS and undercooling decrease.

52 Much of the Some research has been done in plain carbon

Eijk and Walmsey experimented with RE additions in su- per austenitic stainless steel.8

A heat of the super austen-

excellent alignment with the surrounding austenitic matrix. The authors concluded these particles served as heteroge- neous nuclei since their lattice parameter was only 3.82% larger than that of the surrounding austenite.

Suito and co-workers examined the role Ce2

and MgO particles played in determining the structure of steels.9

O3 , ZrO2 , Several 70 g, 10%Ni steel heats were melted in an

induction furnace. Either titanium, zirconium, or cerium was added to deoxidize the melt and form the desired ox- ide inclusions. A series of 0.15 to 0.50%C steels were created in 70 g heat sizes to determine the role of oxide particles in affecting the grain size of these alloys. For each carbon level experimented with, either aluminum, titanium, zirconium, or cerium was added with either cal- cium or magnesium to deoxidize the melt and create spe- cific oxide inclusions. After melting the 70 g sample in an alumina crucible, the melt was cooled to 1400ºC before being quenched to room temperature. The authors stated that for the 10%Ni samples, the area fraction of equiaxed grain increased in accordance with the lattice disregistry of the primary oxides formed.

were present. However, not all of these particles have a low lattice disregistry with austenite or δ-ferrite. Suito et al. theorized that Zener pinning was the dominant mechanism. Zener pinning occurs when an inclusion retards the growth of the solidifying phase during grain growth as the metal cools to room temperature. The authors thought Zener pin- ning was a more appropriate explanation since the decrease in grain size corresponded to the presence of a large number of inclusions in those samples.9

In the case of the 0.15% to 0.50%C alloys, there was a re- duction in grain size when Ce2

O3 , ZrO2 , and MgO particles

In work on a 0.20%C, 0.02%P steel, Guo et al. examined the role of Ce2

In a second set of experiments, sulfur was added to the same steel by a Fe-36%S alloy addition. Cerium metal also was added to form CeS particles. The sample then was cooled from 1873K to 1673K at a cooling rate of 1.02 K/s. After so- lidifying, 0.3 g of the sample was dissolved away to isolate

induction melted under an argon or argon-7 vol% H2 sphere. Cerium metal was added to create Ce2

O3 O3 International Journal of Metalcasting/Spring 2012 and CeS particles.10

A 70 g sample was atmo-


itic stainless steel was melted at an industrial steel mill. An iron-chromium-cerium master alloy was added at a rate of 3.5 kg per ton into a ladle. The ladle was then poured into a cast iron ingot mold to solidify. Next, the ingot was sectioned and metallographically examined. Eijk and Walmsey observed the SDAS dramatically decreased in the heat containing the iron-chromium-cerium addition. The authors stated that several different cerium and alu- minum oxides were observed in the cerium treated ingot. Transmission electron microscopy examination of the ceri- um treated ingot found several AlCeO3

inclusions that had

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