area were fifty to two hundred percent longer than a standard normalizing heat treatment. While the reduction in area im- proved with these heat treatments, the yield strength and ulti- mate tensile strength decreased to a point that they were only marginally higher than non-grain refined steels.
Jackson conducted the earliest work on grain refining cast aus- tenitic stainless steels.9
O3 , Co2 O3 Ni, Ce, W, Al, Cr, Cu, electrolytic Fe,
, graphite, 13wt.% Mn steel, stainless steel chips, and boric acid powders were injected into steel melts and then poured into small ingots. None of these compounds proved effective as grain refiners. Grain refinement was found in sam- ples from a melt that was treated with a contaminated batch of FeCr. Analysis of the contaminate revealed that it contained SiO2 ZrO2
, Fe2
duce an artificial dust with either all or combinations of these compounds produced mixed and unpredictable results in later experiments. Jackson noticed that high nitrogen content heats resulted in a finer grain size. This led to a set of experiments to examine the effect of nitrogen content on grain size. A nitrogen content of 0.2 wt.% with 30°C of superheat and a FeCr powder injection produced a refined microstructure.9 superheat is low for foundry applications.
, PgO2 , CaF, CaCo3 , Fe3 O4 , Cr2 , MgCO3 , Al2 O3
, CaO, MgO, MnO, TiO, TiO2 , and NaO3
.9 This amount of
Roberts et al. conducted the most recent work on grain re- fining cast stainless steel.10
Experiments were performed
on cast austenitic stainless steel alloys 18-8, CF-8M, and CF-3M. Their work focused on determining how the fully austenitic structure developed in those alloys. Grain refine- ment was attempted on CF-3M and CF-8M. Three possible solidification routes produce a fully austenitic matrix: 1) the alloy initially solidifies as delta ferrite and then undergoes a solid state transformation to austenite; 2) the alloy solidifies with some primary delta ferrite dendrites and then forms pri- mary austenite dendrites (referred to as solidifying through the peritectic reaction); and 3) solidify as austenite directly from the melt.10
The first solidification route resulted in the
formation of delta ferrite dendrites in the melt. CF-8M was observed to solidify through the peritectic reaction (route 2), and CF-3M solidified as delta ferrite (route 1). Titanium ad- ditions were discovered to refine the delta ferrite structure in both stainless steels. Because austenite dendrites also grew during solidification of this alloy, refinement of the entire CF-8M microstructure was not possible.10
The titanium ad-
ditions were not able to refine these austenite dendrites. To fully refine CF-8M and similar alloys, a grain refiner with nuclei for both delta ferrite and austenite would be required, which Roberts et al. were not able to find.10
In 1982, J.J. Moore et al. noted that cerium additions pro- duced a small amount of grain refining.11
Moore and co-
workers were attempting to increase impact toughness in cast steels by incorporating rare earth additions. These rare earth additions were designed to form rare earth sulfides to improve impact toughness. A 0.10% C, 1.25% Mn steel was
18 O3 O3 , Attempts to pro-
FeCr, FeMo, FeMn, FeTi, oxidized FeCr, CaSiMg, CaSiMn, SiC, Al2
melted in an induction furnace. After deoxidation, various rare earth additions were made before the steel was poured into sand or cast iron molds. During the course of this work, a noticeable reduction in grain size occured.11
Their experiment consisted of melting five tons of stainless steel and processing it using an argon oxygen de- carburizing (AOD) unit. While tapping the AOD, a cerium master alloy was added to the ladle to achieve a target cerium content of 0.03%. A second five ton heat was also produced following their plant’s normal operating procedure of adding misch metal (a metal that is a combination of rare earth metals) in the ladle while tapping the AOD. Next, the heat was poured into a conventional ingot mold and then sectioned. Samples were examined using optical, scanning electron, and transmis- sion electron microscopy. Greater refinement in the ingot was produced with the cerium master alloy. Transmission electron microscopy (TEM) of the experimental heat found inclusions of CeAlO3
Van der Eijk and Walmsley sought to refine the structure of a fully austenitic stainless steel through additions of a cerium master alloy.12
had excellent alignment with the surrounding austenite crystal structure when the TEM was employed to produce an electron diffraction image of the inclusion/steel interface.12
within the austenite dendrites. CeAlO3
Suito et al. experimented with steels that had carbon con- tents from 0.15% to 0.50%.13
These authors were inter-
ested in determining the role deoxidation products had on grain size. The experiments consisted of induction melting 70g samples of steel in an alumina crucible. The melt was then deoxidized with ferrotitanium, ferrocerium, ferrozir- conium, or a nickel-magnesium master alloy. After cooling the melt to 1400°C, the samples were quenched to preserve the as-solidified microstructure. For the alloys that solidi- fied with primary austenite, ZrO2
grain size. For alloys that solidified with primary δ-ferrite dendrites, these same oxides were also reported to produce grain refinement. The authors postulated that the oxides sim- ply reduced austenite grain growth after solidification, called Zener pinning, producing the smaller grain size observed.13
, Ce2 O3
To better understand the role of nucleation in steel castings, this paper presents work examining industrial castings for nucleation sites. A series of production castings were ac- quired in the as-cast condition. These castings were then sec- tioned, examined metallographically for austenite dendrite structure, and possible nucleation sites were located within those dendrites. One phase, TiN, was found within the prior austenite dendrites that could behave as a nuclei.
Examination Procedure
Samples from three foundries were acquired for examination without undergoing any heat treatment. The foundries were selected in an attempt to gain a broad selection of alloys and processes. Sample 1 was a section of 8620 steel gating from a local investment casting company. The foundry melts us-
International Journal of Metalcasting/Summer 10 , and MgO reduced inclusions
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