A second example casting was run to demonstrate the use of the above tools. The geometry and feeding modulus are shown in Figure 9.
The feeding modulus with the rigging reaches a maximum of 6.42 cm. The correlation of feeding modulus in Figure 4 gives a maximum aluminum content of 0.0417 for 80 ppm nitrogen, 0.0337 for 130 ppm nitrogen, and 0.029 for 180 ppm nitrogen. As an example, take the cooling rate at the highest feeding modulus plotted against three AlN TTT curves for the high Al and N levels. This is shown in Figure 10.
The temperature versus time curve does intersect the transfor- mation curve for the high aluminum or nitrogen contents and just touches the curve in the calculated maximum cases. This example demonstrates the use of Figure 4 to design aluminum levels to avoid embrittlement for typical nitrogen contents and section sizes. Sometimes, this will not be enough especially if complex geometries and/or process limitations lead to non- conservative application of the design rule of thumb. In this case, it is a better strategy to test a few compositions in the 3D geometry. Figure 11 shows a section view of the geometry at different composition levels. The AlN is shown to be com- pletely avoided at the lowest levels.
Discussion
The above analysis is important for heavier sections in steel and deoxidation practices which may lead to large residual aluminum and nitrogen contents. In this case, care should be taken to reduce the aluminum and nitrogen content to avoid embrittlement. How low depends on the cooling rate and thus the section thickness plus rigging and production information. The graphs provide a qualitative rule for avoid- ing embrittlement. In addition, a casting simulation tool is shown which can calculate the embrittled volumes for the production setup. In practice, the feeding modulus and the
simulation tool could be used to conservatively avoid em- brittlement in complex geometries where a simple section thickness is not sufficient.
Although it remains to be studied, the segregation of nitrogen may locally increase the nitrogen concentration. Segregation is particularly a concern in heavy sections. Future studies should include the non-uniform concentration of nitrogen.
Acknowledgments
Thanks for the support of Raymond Monroe at Steel Found- ers’ Society of America for discussion and assistance on the most useful presentation of data. Also, thanks to Barbara Al- lyn at Harrison Steel, who provided many helpful discussions to help a non-metallurgist understand AlN precipitation.
REFERENCES
1. Allyn, B., Carpenter, J., and Hanquist, B., “Deoxidation in Heavy Section Steel Castings” SFSA Technical and Operating Conference (2006).
2. Lorig, C.H. and Elsea, A.R., “Occurrence of Intergranular Fracture in Cast Steels”, AFS Transactions, vol. 55, pp 160-174 (1947).
3. Hannerz, N.E., “Influence of Cooling Rate and Composition on Intergranular Fracture of Cast Steel”, Metal Science Journal, pp 148-152 (1968).
4. Private Communication, Terry Myers, Caterpillar Inc. 5. Private Communication, Brent Hanquist, Chief Metallurgist at Harrison Steel
6. Banks, W., “Avoiding Aluminum Nitride Embrittlement in Steel Castings for Valve Components”, Proceedings of the 1985 Pressure Vessels and Piping Conference, vol. 98-2, pp 219-224 (1985).
7. Magma GmbH. Magmasoft v4.2, Aachen, Germany
International Journal of Metalcasting/Summer 10
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