The mechanical properties of the various heats that were peak aged at 460°C (860°F) and 482°C (900°F) failed to meet the minimum ductility and notch toughness goals set forth by the (AMC) (Table 6). Upon over aging at 538°C (1000°F) for four hours, the base heat failed to meet ten- sile strength goals and heat 3 met all specified goals except notch toughness. A study of the fracture surfaces showed that inclusions and (Nb, V)-rich carbonitrides initiated mi- cro-void formation. Large complex (Nb, V) carbonitrides contributed to lower ductility and the low notch toughness. Future work will be dedicated to improvement of notch toughness of modified cast CB7Cu-1 by decreasing size and volume of complex carbonitrides. Also, the effect of cleanliness on steel toughness will be evaluated. The pos- sibilities for optimization of steel composition will be stud- ied in Phase 2 of this project.
Conclusions
The results of Phase 1 of the project for increasing the strength and toughness of modified CB7Cu-1 steel are presented in this article. The possibilities of improve- ments in mechanical properties by increasing Nb content and V+N modification of CB7Cu-1 steel were explored. The homogenization treatment of 1200°C (2190°F) for 2 hours, followed by air cooling was found to be optimal and resulted in lower amount of ferrite than other homog- enization times and temperatures. Peak aging to obtain maximum hardness caused embrittlement, especially in the modified steels. Over aging at 538°C (1000°F) for 4 hours produced higher tensile strengths (1398±30 MPa/203± 4.5 ksi) in the heat modified with Nb, V, and N as compared to the base heat (1212± 12 MPa/176±1.75 ksi). A small loss of ductility was observed for the modi- fied alloy, but an average percent elongation to failure greater than 8% was obtained. The Charpy impact energy of the modified heat was lower as compared to the base heat in peak-aged as well as over-aged conditions. The decrease in notch toughness was associated with the for- mation of complex cuboid-shaped (Nb, V) carbonitrides of 1 to 2 microns in size. The origin of these particles is being investigated since they contribute to void formation during fracture.
Acknowledgements
The authors wish to thank the Steel Foundry Society of America, Defense Logistics Agency and Advanced Tech- nology Institute under the Castings for Improved Defense Readiness program for their support. They also thank Pro- fessor Robert Voigt of PSU for collaboration. The authors wish to recognize the contributions of Eric Bohannan and Clarissa Wisner who helped in analyzing XRD data and SEM imaging respectively and the undergraduate students who helped in the project.
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