feeding modulus value. Note, in the final estimation of the section size is important to include any gating or risers in this calculation to properly estimate the cooling rate.
These graphs are helpful in showing the theoretical predic- tions of the influence of the aluminum and nitrogen com- position on AlN phase precipitation. However, there are several important limitations on these “rules of thumb” for AlN embrittlement prediction. First, the fundamental driv- ing force for the precipitation is the cooling rate which may be influenced by other factors than section size. For instance, the difference in sand properties or the presence of a large chill/riser adjacent to a section can influence the cooling rate in three dimensional geometries. To include these additional variables the calculation must be made on a setup which closely resembles the casting conditions. Second, casting geometry section size is not the final section size. The rig- ging can play an important role in increasing the feeding modulus or section size of the casting.
AlN Embrittlement Indicator
Because of the inadequate information about the cooling rates, the “rules of thumb” provided above may be mislead- ing. Therefore, the TTT diagrams have also been coded into an API post-processing routine for the estimation of the em- brittled volume. This would allow the calculation of the em- brittled volumes from the actual cooling curves for the total geometry. The cooling curves in casting simulation include the details which act independently of the section size like sand properties and adjacent risers, chills, etc. The resulting indicator is a conservative predictor of AlN embrittlement.
To illustrate the change that rigging can have on changing the section thickness and feeding modulus, the following example is given. The first example casting geometry and feeding modulus calculated without the rigging is shown in Figure 5. Adding the rigging increases the maximum feeding modulus from 7.04 cm to 8.38 cm. The location of the maximum has also changed as shown in Figure 6. The increase in feeding modulus and change in location should be considered in the final design for avoiding AlN embrittlement. From Figure 4, this feeding modulus, 8.38 cm, gives us a maximum allowable aluminum content of
Figure 4. Using the simulated feeding modulus and cooling curve and adjusting the TTT curve, the maximum aluminum content is determined and plotted here for three nitrogen levels.
Figure 3. Simulated thermocouples in various slab castings show the cooling curves for each section half-thickness increasing from left to right; 0.25”, 0.5”, 1”, 2”, 4”, and 8”. The cooling curves are plotted with a corresponding Hannerz calculated TTT curve at 130 ppm nitrogen. The aluminum content for the TTT curve decreases from left to right: 0.111, 0.077, 0.053, 0.037, 0.0285, and 0.0231.
30
Figure 5. The color contour plot of feeding modulus is shown on the geometry.
International Journal of Metalcasting/Summer 10
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 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81