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change in location should be consid- ered in the fi nal design for avoiding AlN embrittlement. As shown in Fig. 2, the feeding modulus of 3.3 in. yields a maximum allowable alumi- num content of 0.0305 weight-% for nitrogen of 130 ppm. An AlN embrittlement indicator


was constructed using the simulated cooling curve at each point in the casting. The value of the indicator equaled time spent inside the AlN phase precipitation curve. This calcu- lation does not depend on assuming a constant cooling rate or tempera- ture fi eld. Therefore, the infl uence of the rigging should be considered and may not scale with the feeding modulus in previously shown fi gures. The resulting AlN embrittlement for a composition of 0.035 aluminum weight-% and 0.03 nitrogen weight-% is shown in Fig. 5. The embrittlement indicator for aluminum levels on the casting with rigging is shown in Fig. 6. Lowering the aluminum content from


0.035 to 0.03 weight-% reduced the em- brittled volume. According to the feeding modulus value alone, the lower alumi- num level should be free of predicted AlN embrittlement. However, based on the actual cooling curves, the volume still predicted some embrittlement. At the highest nitrogen and alumi-


num level, the entire end of the cast- ing was at risk of embrittlement. At an aluminum level of 0.028 weight-% or if the nitrogen level is decreased, all the embrittlement would be avoided. For castings that may have been over- looked or previously not considered as candidates for this type of embrittle- ment, this indicator serves to calculate the potential concern for reasonable maximum compositions.


Casting Example 2 In a second casting example, the feed-


ing modulus with the rigging reached a maximum of 2.53 in. (6.42 cm) (Fig. 7). The correlation of feeding modulus in Fig. 2 gives a maximum aluminum content of 0.0417 weight-% for 80 ppm nitrogen, 0.0337 for 130 ppm nitrogen and 0.028 for 180 ppm nitrogen. Plotting the cooling rate at the high-


est feeding modulus against three TTT curves for the high aluminum and nitro- gen levels showed the temperature vs. time curve intersected the transformation curve and just touched the curve in the calculated maximum cases (Fig. 8).


MODERN CASTING / November 2010 41


In this case, the model in Fig. 2


was able to predict aluminum levels to avoid embrittlement for typical nitrogen contents and section sizes. When complex geometries and/ or process limitations lead to non- conservative applications of the de- sign rule of thumb, a better strategy would be to test a few compositions in the 3-D geometry. A section view of the example casting showed AlN embrittlement would be completely


avoided at the lowest composition levels (Fig. 9).


MC About the Authors


Charles Monroe is senior research engineer, and Richard Huff is research supervisor at Caterpillar Inc., Champaign, Ill.


For More Information


This article is adapted from the paper “Prediction of Aluminum Nitride Embrittlement in Heavy Section Steel Castings,” AFS Proceedings 2010 (10-094), which can be downloaded in full at www.moderncasting.com


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