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C. Monroe and R. Huff Caterpillar Inc., Champaign, IL, USA Copyright © 2010 American Foundry Society Abstract

Aluminum Nitride (AlN) embrittlement is a problem in heavier section (>4”) steel castings. AlN precipitates at higher residual aluminum and nitrogen levels and slow cooling rates. In load critical components, the formation of AlN will embrittle the casting, reducing the impact strength and ductility of the steel. The precipitation diagram for AlN from Hannerz is reviewed and his more accurate equation plotted. In addition, this information is matched to simulated cooling curves in slab castings to plot maximum aluminum content against section size to avoid embrittlement. However, these rules of thumb can be misleading in analyzing geometries without final rigging or


Aluminum Nitride (AlN) embrittlement is the result of a solid state precipitant in heavy section steel castings. Em- brittlement can lead to failure of sections with little load- ing, even during assembly.1

phase which precipitates to primary grain boundaries.2

production information like the sand properties. The most important information in predicting AlN is the cooling rates in the production setting. Therefore, the equations are incorporated into casting simulation software to use the simulated cooling curves to locate embrittled volumes. Two example castings serve to show the use of the AlN embrittlement indicator. This prediction will help to avoid AlN embrittlement in the design of heavy section steel castings and rigging.

Keywords: aluminum nitride, deoxidation, embrittlement, steel casting, heavy section, rock-candy fracture, simulation


The AlN phase is a brittle The

resulting fracture surface is dull, intergranular, and de- scribed as “rock-candy” fracture. It is also known that the AlN phase will not be dissolved by ordinary commercial heat treatment.3

Theoretical treatment of the problem by Hannerz produced some equations that show the relation- ship between the cooling rate, aluminum content, nitrogen content, and precipitation of the AlN phase. For known cooling rate and composition, the embrittlement by AlN phase can be predicted.

First presented is the background on controlling section size and composition as well as a discussion on the in- fluence of macro-segregation. Second, the Hannerz theo- retical model is shown and the calculation is compared to historical data. The equations are used on slab cast- ings to present a relationship of feeding modulus to the maximum aluminum content. Finally, the calculation is applied to the simulation of a three dimensional casting geometry, including any section size increases due to rig- ging determining the embrittled volume for an average steel chemistry.

Special care should be taken to avoid the precipita- tion of any AlN phase in heavy section steel castings. As discussed in the introduction, the two factors that control AlN embrittlement is the cooling rate and the composition. The cooling rate is strongly determined from the casting section size and the shape of the cast- ing geometry. It is not sufficient to assume the cooling rate of a flat plate with equivalent section thickness as the shape of the casting can greatly reduce the cooling rate. Often the section size or shape in the casting ge- ometry cannot be modified to substantially change the cooling rate. However, the addition of a heavy rigging system can substantially increase the effective section thickness in the casting and decrease the cooling rate. Also, chills may increase the cooling rate locally. Simi- larly, the change in a sand system from green sand to chemically bonded sand can decrease the cooling rate. Thus a borderline, heavy section casting can also be at risk for embrittlement if the rigging/sand properties are not properly considered. Both the composition and cool- ing rate through proper rigging design should be used as tools for avoiding AlN embrittlement. Once the rigging has been determined, control of the aluminum and ni- trogen content in the steel is the best prevention of AlN embrittlement.1 in steel originate?

Where does the aluminum and nitrogen

International Journal of Metalcasting/Summer 10


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