they are the only GCI with statistically significant results so far. Delaying casting shakeout decreases base GCI strength but does not decrease the percent strength gain from aging.8 Samples of grade 65-45-12 ductile iron have also displayed aging with >99% confidence of a 4.6% increase in UTS and a 4.1% increase in 0.2% yield strength.9
Aging tensile bars in an unmachined condition, or aging after machining, has been determined not to affect the magnitude or rate of strength increase.6
This finding is sig-
nificant because it rules out a link between hydrogen and aging. Hydrogen diffusivity is so high at room temperature that if aging were caused by diffusion of hydrogen out of the bars, then aging bars in the machined condition would significantly increase the rate of aging. This has not been observed. A hydrogen effect is considered further in the discussion with paper reviewers included in research by Nicola and Richards.8
Despite some erroneous discussion in early work attributed to the measurement method, hardness does increase with ag- ing.10
The behavior of GCI’s strength and hardness during aging may be seen in Figs.1 and 2, respectively. The same figures also show that regardless of the presence of aging, ni- trogen increases the strength and hardness of the iron because nitrogen acts to reduce flake length and round the tips of the flakes, an effect linked to higher strength and hardness.11,12
Work Towards a Greater Understanding of Age Strengthening
In the first study by Nicola and Richards aging appeared to be related to the nitrogen content of the irons studied.6
Based
on this observation, the effect of nitrogen on GCI aging be- came an area of research interest. Nitrogen is now known to be required to produce aging behavior, and is believed to cause the precipitates that cause aging. Several facts support this belief:
• •
Researchers continue to observe that free nitrogen is required for aging.4-8,13-15
Nitrogen is known to have significant diffusivity in iron-based alloys at room temperature.16
allow diffusion controlled phase transformations
• Iron nitride species are known to precipitate in some ferrous alloys at room temperature.17-27
• The magnitude of increase of average UTS from age strengthening shows a good linear fit as a func- tion of predicted equilibrium weight percent Fe4
(Fig. 10).14 •
The DSC results agree with observations by Enrietto of the presence of various nitrides.21 Figure 3 shows the DSC results, with appropriate labeling.
Differential scanning calorimetry (DSC) results appear to show the metastable Fe16N2 nitride transforming to the stable Fe4
N nitride at ~250°C (482°F).10 46 N This can
Further work examined the effects of titanium, aluminum and boron additions on aging.28
These elements were se-
lected for their known tendency to form nitrides. A multi- variable regression correlation led to the conclusion that titanium decreased the mechanical property gains from ag- ing with a confidence level of 99%. Results suggested that aluminum and boron both reduced age strengthening, as would be indicated by the thermal stability of their nitrides, but coexistence of the nitride formers in the tested samples affected the correlation and confidence levels were not re- ported. A 2008 study by Anish, et al. further examined the effects of titanium and nitrogen on age strengthening with- out interactions due to other nitride forming elements.14 The study found that additions of titanium could prevent aging in both experimental results. Thermodynamic model- ing and scanning electron microscope (SEM) observations determined that titanium prevented age strengthening by tying up nitrogen as titanium nitrides. Calculations based on iron compositions with higher nitrogen content showed calculated formation of Fe4 showed more of the Fe4
N at higher temperatures and N phase at room temperature, both
of which are indicated by the Fe-N phase diagram (Fig. 9a).
The first attempt to remove aging by a precipitate-resolution- izing heat treatment at 499°C (930°F) failed.8
A subsequent
tested immediately after resolutionizing and again after ag- ing at room temperature for 30 days. The strength gain from the first aging returned after resolutionizing and re-aging.10
test succeeded in removing the strengthening effect of ag- ing with treatment at 577°C (1070.6°F) for ten minutes. This temperature is below the iron-carbon eutectoid but above the iron-nitrogen eutectoid for Fe4
N. The heat-treated iron was
The large exothermic peak is the austenite eutec- toid transformation.
ing.10
Figure 3. Differential scanning calorimetry showed two exothermic peaks, one which may represent the forma- tion of the precipitate, Fe4
N (γ’), presumed to cause ag-
International Journal of Metalcasting/Spring 10
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