Gray (flake graphite) iron is well known to age at room temperature after casting, as evidenced by increasing strength and hardness over time, but there are conflicting theories/beliefs regarding the cause(s) of such aging response(s). Some researchers have concluded that those aging responses result from slow precipitation of submicroscopic nitrogen precipitates that are modified by interaction with titanium, but both of those elements are also known to have significant effects on graphite and matrix morphology. The results of this study correlate the logarithmic room temperature aging responses as measured by pitch-and-catch ultrasonic velocity to quantified metallography metrics and other properties in providing an alternate explanation for aging responses of gray iron. Since the positive logarithmic aging curves are characteristic of natural decay phenomena, of which stress relief is an example, stress relief is a viable alternate explanation for gray iron aging effects.

A review of the literature on room temperature strengthening of gray iron over time undisturbed by mechanical straining (natural aging) indicates that the increasing strength response function with aging time will be either “logarithmic” or “sigmoidal/Avrami.” A fundamental difference between those two disparate curve functions is that “logarithmic” entails no incubation period while ‘sigmoidal/Avrami’ does.

The type of response function for the natural aging of gray iron might be crucial in determination of the ultimate cause of such aging. Any validity of an incubation period might be more likely for elemental effects at the submicroscopic level, whereas a logarithmic function involving no incubation period would be most likely for a decay phenomenon such as stress relief.

Past research has shown that Type A graphite predominates in gray iron of greater free nitrogen content, whereas undercooled Types B, D and E graphite predominate in Ti-rich iron in which the Ti combines with much of the nitrogen. The undercooled graphite types are also associated with the formation of more ferrite abutting the graphite flakes relative to pearlite during eutectoid cooling than what occurs for iron containing Type A graphite.

Pitch-and-catch ultrasonic velocity (UV) testing is a nondestructive testing (NDT) method that has been used extensively for correlation with and quantification of ductile iron nodularity. Although this NDT method has also previously been used for evaluating gray iron properties, it has not been correlated to flake graphite morphology metrics nor used to quantify the effects of room temperature aging on gray iron.

The element nitrogen is a major reason for considering those two cases separately; nitrogen has been purported to be the driving force for the strengthening effect on gray iron by room temperature aging, but ductile iron typically contains lower nitrogen residuals as a result of the magnesium treatment, which purges much of the dissolved nitrogen contained in ductile base iron. Therefore, although past UV testing has revealed no quantifiable effect of room temperature aging on mold-cooled ferritic/pearlitic ductile iron by that test method, it cannot be concluded that the same would necessarily be true for mold-cooled gray iron of higher nitrogen content.

The differing morphologies, coupled with the unresponsiveness of mold-cooled ductile iron to room temperature aging as measured by ultrasonic velocity, uncloak the possibility of alternate causes besides submicroscopic chemistry precipitates for room temperature aging of gray iron.
For example, there is no reason at this point why stress relief should be dismissed as a possible cause of gray iron age strengthening. In that vein, it is possible that the relief of stress might effectively “tare/reset” the iron’s internal “tensometer” to zero such that the UTS measured by the external tensometer will be greater after stress relief than before.

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More information on this research can be found in the paper, "Gray iron aging effects as measured using NDT and microstructural analysis," by J. W. Cree and published in the International Journal of Metalcasting in 2017. https://doi.org/10.1007/s40962-016-0105-8