Age Strengthening in Malleable Cast Iron
Although at least three publications discuss the age strengthening of malleable iron, the latest and most com- prehensive is by Burgess.4
His study revealed a great deal
about aging in cast iron. The research used samples of annealed malleable iron from production foundries and a laboratory. Various tests were done with samples aged at room temperature, and others at elevated temperatures. The aging behavior observed followed Avrami-Johnson- Mehl behavior, as shown in Fig. 11.
The increase of tensile strength was found to be greater with higher uncombined nitrogen and be less when alu- minum content was higher, just as observed for GCI. Silicon has also been shown by Burgess to decrease the effect of aging on strength. The loss of free nitrogen by formation of MgSiN2
nitrides, which have been previ-
ously seen by Wada and Pehlke to form in malleable and ductile iron, may explain why silicon decreases the magnitude of aging.29
Silicon can also act as a solid so-
If the solubility of nitrogen in liquid cast iron were reduced to below the levels encountered in pro- duction, then nitrogen would be rejected from the melt and less nitrogen would be present to form nitrides. In this case, because of the approximate equivalence of the solubility limit and nitrogen content, one would expect a rejection of nitrogen from the iron at solidification and industry would long since have correlated high silicon contents with the presence nitrogen porosity. Since this is not the case, the other explanations are more logical. The silicon content as a variable affecting aging has not been seriously considered in GCI and is an item for fu- ture investigation. Carbon, sulfur, and manganese did not appear to affect aging in the malleable irons stud- ied by Burgess. Burgess noted that the manganese lev- els were all similar; therefore, any effect may have been overlooked.
A TEM analysis showed that the aged irons possessed precipitates of approximately 1,000Å in diameter that were near, but not collected on, ferrite grain boundar- ies. Precipitates were not found in irons that did not age strengthen. Precipitates appeared to interact with dis- locations, which could explain the increase in strength from aging. While the precipitates observed in the TEM images were not identified, Burgess believed them to be nitrides, like those found in steel. Since the ferrite in malleable iron is essentially a Fe-C-Si-Mn alloy, he reasoned that it was comparable to the steel results for nitride precipitation.
52
lution strengthener in ferrite. This strengthening may reduce the observable effects of nitride age strengthen- ing. Another explanation to consider is that silicon could reduce strength gain from aging by affecting the content of nitrogen in liquid iron. Silicon will reduce solubil- ity of nitrogen in both liquid and solid solutions of cast iron.32,55
Age Strengthening In Ductile Cast Iron
One study by Richards et al. sought to determine the nitro- gen-related aging characteristics of ductile cast iron.9
Results
for grade 65-45-12 showed statistically significant increases, based on t-tests, at a >99% confidence level for UTS, 0.2% yield strength, and microhardness of free ferrite. Grade 80- 55-06 displayed 89% confidence for a UTS increase, but it also showed a decrease in 0.2% yield strength with 99% confidence. Microhardness was not tested for 80-55-06. Strain data was collected during the tensile tests and used to calculate a true strain-hardening rate. Data for both 65-45- 12 and 80-55-06 indicated increased strain hardening rates for the aged samples, possibly due to precipitates acting as barriers to dislocation movement, which would support the precipitation hypothesis of cast iron aging. A quenched and tempered 100-70-03 was included in the study by Richards et al. and showed no signs of age strengthening. The temper- ing temperature was in the range known to accelerate age strengthening. Given the amount of time that the bars were tempered, aging was probably completed by the end of the heat treatment.
The statistically significant decrease in yield strength for 80-55-06 suggests the possibility that ductile cast iron will behave differently with aging depending on the amount of pearlite present in the matrix. The results also suggest that more bars should be tested at the same time to provide a bet- ter representation of the iron’s behavior.
Although it is reasonable to speculate that nitrogen-based age strengthening would exist in compacted graphite cast iron, no published experimental results support this. There is, however, work being performed by this paper’s authors to determine effects of aging on compacted graphite iron’s mechanical properties and machinability.
Figure 11. Aging behavior of tensile and yield strengths observed in malleable iron. Data is adapted from Bur- gess.4
Two sets of samples were tested, both annealed
in nitrogen, then solution treated either at 721°C (1330°F) or 743°C (1370°F).
International Journal of Metalcasting/Spring 10
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