This page contains a Flash digital edition of a book.
lated to the cooling rate of the steel in the TA cups. Secondary dendrites arm spacing are known to be strongly dependent on the local solidification time.17-19


Local solidification time does


not simply depend on the number of nucleation events, but also depends on the cooling rate during solidification. Unlike pri- mary dendrite arms, secondary dendrite arms coarsen during solidification where larger secondary arms grow at the expense of smaller ones. This coarsening depends on the length of time the dendrite grows. El-Bealy and Thomas derived the following relationship between cooling rate and SDAS (λ2


) in steels: Equation 2 where A1 the steel, and CR


and n are constants dependent on the composition of is the cooling rate. If the cooling rate is high


enough, it is possible the cooling rate has a greater effect on SDAS than the number of dendrites nucleated by the nuclei present in the steel. SDAS usually decreases with a greater number of heterogeneous nuclei because the local solidifica- tion time declines due to earlier dendrite arm impingement. In these experiments, it is possible that the cooling rate was sufficiently high that, once solidification was initiated by the heterogeneous nuclei, growth occurred rapidly. This model


does help explain why the MgO samples had a larger SDAS despite decreasing the undercooling required for solidifica- tion. Since the lower density MgO powder had a tendency to float on top of the TA cup, the powder would have decreased the cooling rate of the TA cup. A decrease in the cooling rate would result in the increased SDAS observed in Figure 6.


hK results


not reduce the undercooling to initiate solidification. The lack of undercooling reduction by ZrO2


O3


Undercooling measurements for the HK samples found a reduction in the average undercooling for the samples with La2


, MgO, and NbO additions (See Figure 7). ZrO2 did was consistent


This calculation was contrary to the statements of other au- thors; however, the observed behavior of ZrO2


was consistent with them acting as heterogeneous nuclei.


tent with the authors’ calculations.13,14 dercooling by the La2


O3


with the lattice disregistry between austenite and this mate- rial. Crystallographic calculations done by the author had an average disregistry between ZrO2


, MgO, and NbO addition samples


and austenite around 15%. was consis-


The reduction in un-


Figure 7. Undercooling measurements for the HK samples.


Figure 8. Representative micrograph of the no addition HK sample.


Figure 9. Micrograph of the HK sample with MgO powder addition.


32


Figure 10. SDAS measurements for HK samples. International Journal of Metalcasting/Winter 2012


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73