This page contains a Flash digital edition of a book.
PREDICTION OF ALUMINUM NITRIDE EMBRITTLEMENT IN HEAVY SECTION STEEL CASTINGS


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


Introduction


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


Background


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


27


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