The two papers suggesting that aging is a residual stress effect can each be addressed. Novichkov noted that free nitrogen was a necessary component for aging to occur in GCI but never con- sidered a precipitation process.5
Although the author proposes a
residual stress relief process, he does not rule out a precipitation process. Perhaps he never considered a precipitation process and, had he thought more on the matter, he may have proposed nitride precipitation instead of residual stress relief. Kuehl pres- ents RF data collected for tensile bars and brake rotors.58
The
data showed a logarithmic increase with storage time of the ten- sile bars and brake rotors. After this apparent aging, tensile bars were shot-blasted, and the logarithmic increase in RF appeared again. The author concluded that aging could be removed and restarted by shot blasting. The study does not report the chemis- try of the iron; therefore, whether it possessed sufficient free ni- trogen to age strengthen is uncertain. Changes in RF were never directly correlated to a change in tensile strength. The changes of tensile strength, which were only indirectly measured, could have been as significant as 10 MPa or as insignificant as 0.1 MPa. This range leaves open the possibility that the chemistry of the iron did not promote aging and that the author did in fact observe some residual stress relief. This relief would have been minimal at room temperature, perhaps only about 6% in GCI, as determined by Hallett and Wing.64
Although Hallett
and Wing did detect the small relief of residual stresses at room temperature, they noted that “…treatments extending to three months do not appear to make a really useful contribution.” Based on their results and on the mounting body of evidence suggesting a precipitation process, residual stress relief is not an explanation for age strengthening. This conclusion does not imply; however, that residual stresses are unimportant and can- not affect casting performance.
Applications
Application of aging through nitride precipitation in cast irons has been successfully used to improve tool life in machine shops. Aging castings does increase the time between pro- duction and shipping and can require floor space. For these reasons, a balance should be found between improved ma- chinability from aging and cost of casting storage time. Many foundries producing automotive castings have determined that five days of room temperature aging provides a good bal- ance. Only Mereau has attempted use of accelerated aging to improve tool life and reduce required storage time.65
In his
work, a set of castings was artificially aged for 75 minutes at 271°C (520°F). Although the artificially aged castings ap- peared to show improved machinability, the wear area data for tools used to machine unaged castings showed significant scatter, thus results were statistically inconclusive. Wert ex- plored another issue relevant to artificial aging in high purity Fe-N materials. He showed that by precipitating 6% of the nitrogen from solid solution at 27°C (80°F) and then proceed- ing with precipitation at 50°C (122°F), the time to complete precipitation was shortened by about 80 hours compared to precipitation at only 50°C (122°F).51
This process, called “seeding,” increases available nucleation sites for precipitates 54
to grow on at the higher temperature. It may be applicable in cast irons to increase artificial aging rates. Foundries applying artificial aging might wish to know how overaging affects ma- chinability, but this has not yet been determined. If a foundry does not perform in-house machining, an engineer could sug- gest aging of castings to their customers.
Age strengthening could be applied to cast an iron at a higher CE, providing more fluidity, but have the same strength of a lower CE iron that did not age. This practice could be tried based on the current understanding of aging, but it would be more implementable once there is additional understanding of how the composition of cast irons affects the magnitude and rate of aging. For the time being, foundries can consider an equation presented by Anish for class 30 and 35 gray irons to estimate the increase in strength after 20 days of room temper- ature aging.28
This equation, presented as equation 3, applies to irons in which nitrogen and titanium weight percents are variables, but it assumes that nitride-forming elements other than titanium are not present in significant amounts.
∆UTS20 = 0.267 + 704.4(%N) - 99.1(%Ti) Equation 3
In Equation 3 a multivariable regression correlation determined that for the constants preceding the weight percents of nitrogen and titanium, the statistical confidence for both was greater than 95%. The stand-alone constant, however, only had a confidence of 18%, which suggests it is not significant. Furthermore, the 0.267 MPa should be excluded from consideration because it is such a small value and because with the assumption that nitrogen and titanium contents are both zero, there should be no change in strength when the iron is held at room tempera- ture. Further, the equation is only valid for age strengthening gray irons; that is, the equation is not meant to describe any loss of strength as would be calculated if nitrogen were essen- tially zero but titanium were a positive value. In light of these considerations, Equation 3 can be restated as Equation 4. Both equations represent empirical observations from laboratory tri- als and should not be treated as absolutes; they come with no guarantees, and should not be used for design.
∆UTS20 = 704.4(%N) - 99.1(%Ti); for ∆UTS20 ≥ 0 MPa Equation 4
According to an article published in ASM’s Heat Treating Progress, another benefit of aging cast iron is that it is less likely to crack during heating and quenching experienced from induction hardening.66
Controlled vanadium carbide precipitation in ductile iron can be used to increase yield strength without the propor- tional loss of elongation that would normally be expected. At this time, the effect of the process on machinability is un- known compared to an iron of equivalent strength that does not possess the precipitates. As with most aging steel alloys, the precipitation process requires a heat treatment; therefore, the strength gain must be weighed against the cost of alloy addition and heat treatment.
International Journal of Metalcasting/Spring 10
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