Future Work
Further work might consider the following: 1. Interaction of microstructure and aging to influ- ence machinability… • The ratio of free ferrite to pearlite may influence how aging affects machinability. (Formation of nitrides is known to take place in the ferrite, as shown in the iron-nitrogen phase diagram.)
2. Aging effects on machinability in CG iron and duc- tile iron
3. Ternary interaction of alloying elements such as Mn, Si, and Cr with the rate and magnitude of ag- ing. • Manganese and nitrogen are believed to under- go interaction solid solution strengthening. This could lead to a delay in the onset of aging.
• Other elements have been determined by re- searchers to display interaction solid solu- tion strengthening with nitrogen and carbon in steels.43,67,68
in cast irons include Si and Cr.
• Results in malleable iron suggest that silicon re- duces the magnitude of age strengthening.4
This
is possibly due to formation of nitrides contain- ing silicon or, more likely, because silicon solid solution strengthening decreases the effect of aging on strength.
4. As discussed above in section 8, Wert successfully applied a “seeding” process to increase the ag- ing rate of an Fe-N alloy.51
Given the number of
similarities believed to exist between the steel iron- nitrides and the nitrides in cast iron, this process may also work in cast irons, but further research is required.
5. Age strengthening has the potential to change tool wear mechanism and characteristics. Optimization of machinability will be important for industry.
6. A TEM experiment has been planned by this pa- per’s authors to take observations of samples known to age strengthen and others known not to age strengthen. Even if a nitride precipitate cannot be found, measurements of lattice parameters on a hot stage can be used with Vegard’s Law to check for precipitation processes.
7. Although residual stresses and their relief have been ruled out as a cause for age strengthening, they may effect the nucleation and growth of pre- cipitates, as observed in some aluminum alloys.
Acknowledgements
The authors wish to thank metallurgy students Allison Mill- er and Valerie Jamerson for their assistance in locating and cataloging literature. The authors wish to thank Missouri S&T Technical Editor Jeanine Bruening for providing a de- tailed review of grammar, punctuation, and spelling.
International Journal of Metalcasting/Spring 10 The elements that might be expected
REFERENCES
1. Kempka G.E., “Embrittlement, Toughening, and Sub- critical Thermal Treatment of Malleable Iron,” AFS Transactions, vol. 63, pp 675-82 (1955).
2. Ebner, R. “Influence of Ageing, Sampling Site and Machining of Test Specimens Upon Tensile Strength and Hardness of Grey Cast Iron,” BCIRA, vol. 50, pp 689-91 (1963).
3. Beiser C.A., Evans E.B., Modern Casting, American Foundry Society (1956).
4. Burgess P.B., “Age Hardening Ferritic Malleable,” AFS Transactions, American Foundry Society, vol. 77, pp 172-179 (1969).
5. Novichkov P.V., “Thermocyclic Aging of Cast Iron at 200-280 Degrees C,” Liteinoe Proizvodstvo (in Rus- sian), pp 31-35 (1970).
6. Nicola W.M., Richards V.L., “Age Strengthening of Gray Cast Iron, Phase I: Statistical Verification,” AFS Transactions, American Foundry Society, pp 749-55 (1999).
7. Nicola W.M., Richards V.L., “Age Strengthening of Gray Cast Iron, Phase II: Nitrogen and Melting Meth- od Effects,” AFS Transactions, American Foundry Society, vol. 108, pp 233-237 (2000).
8. Nicola W.M., Richards V.L., “Age Strengthening of Gray Cast Iron, Phase III: Effect of Aging Tempera- ture,” AFS Transactions, American Foundry Society, vol. 109, pp 1085-1095 (2001).
9. Richards V.L., Van Aken D.C., Mereau O.P., Nicola W.M., “Effects of Room Temperature Aging on Duc- tile Iron,” AFS Transactions, American Foundry Soci- ety, vol. 112, pp 801-812 (2004).
10. Richards V.L., Van Aken D.C., Nicola W., “Age Strengthening of Gray Cast Iron: Kinetics, Mechanical Property Effects,” AFS Transactions, vol. 111, pp 733- 741 (2003).
11. McGrath M.C., Richards V., Anish T.V, “Effects of Nitrogen, Titanium, and Aluminum on Gray Cast Iron Microstructure,” AFS Transactions, American Foundry Society, vol. 117, pp 497-505 (2009).
12. Stets W., Lötschert A., Wolf G., Casting Plant and Technology, pp 4-25, (2009).
13. Edington J., Nicola W., Richards V.L., “Age Strength- ening of Gray Cast Iron: Nitrogen Effects and Machin- ability,” AFS Transactions, American Foundry Soci- ety, vol. 110, pp 983-993 (2002).
14. Anish T., Lekakh S.N., Richards V.L., “The Effect of Ti and N on Iron Age Strengthening,” AFS Transac- tions, American Foundry Society, vol. 116, pp 653-663 (2008).
15. Richards V.L., Anish T.V., Lekakh S., Van Aken D.C., Nicola W., “Age Strengthening of Gray Iron - Kinetics Study,” AFS Transactions, American Foundry Society, vol. 115, pp 579-589 (2007).
16. Leslie W.C., The Physical Metallurgy of Steels, pp 74- 79, CBLS Publisher & Book Distributor (1981).
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