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dynamically stable in the melt; be wetted by the melt; and have a similar crystallographic structure. Thermodynamic data can be utilized to determine if a solid will be stable in liquid steel. Typically, a computer program using the CALPHAD method is employed to establish stability of a possible nucleation phase. Wetting data is often unavailable when initially examining a possible phase. It is also chal- lenging to accurately determine wetting angles at molten metal temperatures, especially steelmaking temperatures. Crystallographic data has generally been used with thermo- dynamic data to assist with selecting possible heterogeneous nucleation phases for metals. It has been found that, when the difference in lattice parameters between the solid metal and foreign particle is small, the foreign particle can act as a heterogeneous nuclei.6


is called the disregistry (δ). Disregistry should be below 12% for effective nucelation.2-6


The difference in lattice parameters Disregistry can be calculated be-


tween any set of crystallographic planes in different crystal structures using the modified Turnbull-Vonnegut equation (See Equation 1).


Equation 1


Eijk and Walmsley conducted experiments on rare earth grain refinement additions to super austenitic stainless steels at a steel mill.9


A heat of super austenitic stainless steel was where (hkl)s


clei, and s denotes a factor involving the solid. The modi- fied Turnbull-Vonnegut equation is applicable for any set of crystal structures.


interatomic spacing along [uvw]s the [uvw]s


a low-index direction in (hkl)s the nucleated solid, [uvw]n d[uvw]n


is the interatomic spacing along [uvw]n and [uvw]n


Jackson conducted the earliest research on grain refining stainless steels.7


He melted 13.6 kg (30lbs) of 316 stain-


graphite, 13wt.% Mn steel, stainless steel chips, and boric acid powders. The most effective refiner was a FeCr powder. However, this FeCr powder was contaminated with another substance which Jackson was unable to identify.7


O3 , Co2 O3


less steel in a high frequency induction furnace. The melt was poured directly from the furnace into molds made from either refractory sleeves or sand. During pouring, grain re- finer powder was injected into the pouring stream with an argon or nitrogen carrier gas. Jackson examined additions of Ni, Ce, W, Al, Cr, Cu, electrolytic Fe, FeCr, FeMo, FeMn, FeTi, oxidized FeCr, CaSiMg, CaSiMn, SiC, Al2


, Several


heats with elevated nitrogen levels had finer structures than the baseline material. These high nitrogen heats also had bet- ter mechanical properties.7


In work by Roberts et al., additions of B, Ti, rare earths, and Al in CF-8M and CF-3M were utilized in an attempt to grain refine these stainless steels.8


Each heat was produced 28


is a low-index plane of the substrate, [uvw]s , (hkl)n


is


is a low-index direction in (hkl)n , d[uvw]s


is a low-index plane in ,


is the , θ is the angle between , n denotes a factor involving the nu-


austenite indicated excellent crystallographic alignment of the two structures. Therefore, the authors theorized that the Al- CeO3


. Electron diffraction of the AlCeO3 inclusions acted as heterogeneous nuclei.9


Recently, work has been done on 409 ferritic stainless steel by Ferry and coworkers.10


These researchers conducted a


series of experiments with titanium levels from 0.016 to 0.15%. A unique experimental apparatus consisting of a rotating paddle inserted into an induction furnace was used to simulate the casting conditions of a twin-roll caster. The paddle had a small region made from electrolytic tough pitch copper which provided a cooling rate similar to a twin-roll caster. Samples were created in the experimental apparatus and then examined via optical and electron microscopy. Electron backscattered diffraction (EBSD) on a scanning electron microscopy (SEM) was employed to characterize the texture developed under these experimental conditions. Furry and coworkers observed that the grain size decreased


International Journal of Metalcasting/Winter 2012


melted in an electric arc furnace. An iron-chromium-cerium master alloy was added to the liquid stainless steel at a rate of 3.5 kg per ton during tapping into a ladle. The steel was then poured into an ingot mold. After solidifying, the ingot was sectioned in several places to characterize the microstruc- ture. Optical microscopy of the samples found a significant reduction in primary and secondary dendrite arm spacing with the iron-chromium-cerium addition. Transmission elec- tron microscopy (TEM) observed cerium-aluminum oxides in the material. These cerium-aluminum oxides had a chemical composition and crystallographic structure consistent with AlCeO3


and surrounding


in a 272 kg (600lb) magnesia lined induction furnace un- der an air atmosphere. Once the target tap temperature was reached, the liquid stainless steel was poured into an alumina lined ladle. A step block casting was produced from each ex- perimental heat. Pouring temperatures were between 1480- 1650C. The castings were sectioned and polished for macro and microstructural examination. The titanium additions produced the finest structures.8


Titanium carbonitrides were


observed in the titanium containing castings. Only CF-3M was refined. The authors ascribed this difference in response to the dissimilar solidification routes for the two alloys. CF- 3M solidified directly from the melt as delta ferrite which could be nucleated by the observed titanium carbonitrides. The delta ferrite then underwent a solid state transformation to the room temperature austenitic structure. CF-8M initially solidified as delta ferrite also, but it underwent the peritectic reaction during solidification. This resulted in a consider- able amount of austenite forming while the casting solidified which could not heterogeneously nucleate on the titanium carbonnitrides.8


Roberts et al. theorized that CF-8M could be


refined if a combination of delta ferrite and austenite nuclei could be added. They were not able to find a suitable austen- ite grain refiner.8


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