with the titanium content of the metal. They also measured that within the chill zone the steels with titanium prefer- entially grew in the <001> direction.10
TiN particles were
found at the surface of the chill zone. The presence of TiN particles and nucleation on them was postulated by Furry and coworkers as the cause of the observed texturing. TiN and delta ferrite have a low lattice disregistry between the <011> direction in TiN and the <001> direction in delta fer- rite on their {001} planes.10
Ohno and Matsuura focused on the effects of titanium ad- ditions to S45C stainless steel.11
A 250g sample of S45C
was melted in an alumina crucible with a SiC electric resistance furnace under an argon atmosphere. Once the sample reached 2822F (1550C), it was poured into a steel mold with a rectangular ingot cavity. Titanium levels from 0 to 0.3% were examined. Phosphorous was pur- posely added to all the samples to assist with determin- ing the delta ferrite structure. The samples were sectioned and polished for metallographic examination. A minimum grain size was observed for samples containing approxi- mately 0.15% titanium.11
Samples with titanium contents
between 0.13% to 0.17% had the smallest columnar grain zone. The 0.15% sample had a fully equiaxed structure over the entire ingot. Ohno and Matsuura found a strong correlation between the columnar structures of the austen-
experimental Procedure
A 23 kg heat of either 304 or HK steel were melted in a 3kHz induction furnace with an aluminum crucible under
ite and the original delta ferrite structure in the samples with titanium levels between 0.13% and 0.17%.11
Accord-
ing to these authors, the titanium carbonnitrides detected in the microstructure assisted in nucleating delta ferrite. Then, it transformed to the austenite found in the room temperature structure. Without the titanium additions, the austenite was relatively coarse.11
Most of the current research in grain refinement of steels has focused on titanium or rare earth additions. There has been little attempt to use heterogeneous nucleation theory and current material databases to search for other possible nucleation phases. This paper presents work on two austen- itic stainless steels to conduct an initial survey of possible nucleation phases. These steels were melted in an induction furnace and poured into thermal analysis cups with powders of the candidate phases. The cooling curve and solidified steel were analyzed to determine if the powder acted as an effective heterogeneous nuclei. Several powders were found to reduce undercooling. Secondary dendrite arm spacing (SDAS) was not affected in the 304 alloy.
(a)
(b)
Figure 1. Thermal analysis setup used in this work. (A) The TA cup and test stand. (B) Overhead view of one of the TA cups.
Table 1. Average chemical composition of the Heats
International Journal of Metalcasting/Winter 2012
29
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