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18. Thiem, S., Löser, W., “Reanalysis of Solidification Behavior from the Microstructure in Near-net-shape Casting of Steels,” Steel Research, vol. 63, no. 7, pp. 291-296 (1992).


Technical review & discussion


Thermal analysis study of heterogeneous nuclei in stainless steels R. Tuttle; Saginaw Valley State University, University Center, MI USA


Reviewer: My experience has found some unintended con- sequences like TiN affecting the fracture toughness. Some precoats also reacted with the melt to produce pinholes and rough surface in investment castings. Another difficulty is getting the material into the melt in a homogenous manner.


Author: The author agrees with the reviewer on this. Previ- ous work by the author on titanium additions in 1030 also found problems with TiN, TiC, or titanium carbonitride par- ticles. These particles appeared to be responsible for the ob- served grain refinement, but also reduced the elongation of the steel. In that system, the author theorizes that the titanium carbonitride particles formed at the end of solidification and actually acted as austenite growth inhibitors instead of het- erogeneous nuclei. Based on the thermodynamic calculations in that work, it appears these particles are unstable in liquid steel. The solidification rate of the TA cups seems to be fast enough that some of the TiN particles do not dissolve before solidification occurs. Unfortunately, the author knows from his work in plain carbon steels that the solidification time for a green sand casting appears to be long enough that TiN ad- ditions would dissolve. (The Role of Titanium on the Grain Refining of 1030 Steel Castings, AFS Transactions, 2011)


Reviewer: In describing procedure, the author states that 10 “TA cups” were cast in each heat and each in- oculant was tested three times (five including the base of no inoculant). This would add up to 15 tests. How- ever, in Figure 3 there are only 12 points. Assuming each point corresponds to one test, what happened to the other three? How many heats were poured? If two, shouldn’t there be 20 points? This confusion should be cleared up.


Author: The missing data points the reviewer is referring to are in Figure 3. For a couple NbO and NiAl cups, the mea-


19. El-Bealy, M., Thomas, B. G., “Prediction of Dendrite Arm Spacing for Low Alloy Steel Casting Processes,” Metallurgical and Materials Transactions B, vol. 27B, pp. 689-693 (1996).


sured undercooling was identical. The result is that these data points are on top of each other. This also happened in the HK data.


Reviewer: I am not surprised the author finds little effect of inoculation on dendrite arm spacing, but am surprised at his thinking there should be an effect. I am not famil- iar with the recent studies he quotes, but the preponder- ance of earlier work concludes dendrite arm spacing is independent of grain size. This is to be expected since dendrite arm spacing depends on “ripening,” where- as grain size is determined by nucleation or “dendrite breakup.” Of course, when the grain size is small enough that only a few dendrite arms are present in it, rate of ripening may be affected, and of course it is more dif- ficult to measure dendrite arm spacing. The author should clarify his position, and the literature, on this point.


Author: As cited in the paper, several other researchers have found that the SDAS can decrease with the addi- tion of a grain refiner. The work cited by the author is in aluminum, magnesium, and steels so it is a relatively common experimental observation in different alloy sys- tems. This is why the author thought there would be a difference. The reviewer is correct that the SDAS does not always indicate refinement. The spacing between pri- mary dendrite arms is directly related to the number of nucleation events. However, primary dendrite arm spac- ing measurements can be a problem if there are only a few within a field of view or it is difficult to distinguish between them. SDAS is frequently easier to measure, but can be misleading. As the dendrite grows the second- ary arms undergo coarsening with larger arms growing at the expense of smaller arms. The result is that the final SDAS is not the initial SDAS. However, this mechanism requires time which is limited by the local solidification time. Increasing the number of nuclei can decrease the local solidification time and result in a decrease in SDAS; however, it is also possible that the gradient and growth velocities are such that the SDAS is not affected by the presence of heterogeneous nuclei. The revised manu- script has been modified to clarify this.


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International Journal of Metalcasting/Winter 2012


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