In 1982, Dawson presented results showing that ductile iron al- loyed with up to 0.5% vanadium developed increased strength and hardness after annealing, compared to annealed irons not alloyed with vanadium.56
More recently, a 1997 study by Rez-
vani et al. demonstrated that additions of vanadium to ductile iron, combined with changes to the annealing procedure, in- creased yield strength by up to sixty percent.57
The increase in
yield strength was obtained with 0.15% vanadium content, an increase in the austenitizing temperature from 900°C (1652°F) to 1050°C (1922°F), and an increase in the cooling rate from 1 to 80 °C/min (3 to 176°F/min). For this case, multivariable analysis was used on collected data to infer an increase of 88 MPa in yield strength from ferrite grain size refinement and 104 MPa from the vanadium-related age strengthening. Measure- ments of lattice parameters by Rezvani et al. in a TEM sug- gested the precipitate responsible for vanadium-related age strengthening was vanadium carbide (VC0.75
). The Residual Stress Question
As far back as 1970 Novichkov offered the explanation for the GCI aging phenomena that nitrogen was required to start a residual stress relief process.5
A 2003 paper by Kuehl used
resonant frequency (RF) measurements in GCI to support the claim that all observations of aging in cast irons are an effect solely of residual stresses.58
ual stress relief is the cause of aging in cast irons is refuted in this section.
Ebner’s research determined that using variations in ma- chining procedure did not affect the magnitude of aging.2 This observation led to the conclusion that there was no significant effect of residual stresses on aging phenomena because machining should relieve some stresses and redis- tributed the rest. Although bars aged faster in the machined condition, this could have been caused by improper coolant flow, or by a complete lack of coolant. The heat generated by the machining process, if not controlled, could have ac- celerated precipitation. Further, in Nicola and Richards early work both machined and unmachined bars age-strengthened; however, removal of the bar skin during machining should have relieved some residual stress that was due to the dif- ferential cooling rate of surface versus center.6
Further considerations support conclusions of both Ebner and Richards that residual stresses, or the relief thereof, do not cause aging in cast irons: •
The argument that resid-
• A 10-minute heat treatment at 577°C (1070°F) was able to remove the strength increase from ag- ing in GCI; this strength returned with room tem- perature aging.10
Although 10 minutes should have
been plenty of time for nitrogen to diffuse in fer- rite, there is doubt that this heat treatment could cause sufficient changes to residual stresses based on information provided in Campbell’s text.59
Even
if the heat treatment induced residual stresses it is improbable that the treatment would restore the stresses to their exact magnitude of the original state after solidification, and thus create iron with the same unaged strength.
• Certainly residual stresses cannot explain Bur- gess’s observation in a TEM of precipitates that accompanied age strengthening in post-annealed malleable irons.4
In addition to the above, comparison of the activation energy for aging in GCI to energy for nitride precipitation in steel and en- ergy for ferrite stress relief supports a convincing argument for a nitride precipitation process. The activation energy for aging in gray iron over a range of 20°C (68°F) to 100°C (212°F) has been found to be 36 kJ/mol.28
Rasek determined the activation energy
for nitride precipitation in a binary Fe-N alloy was 44 kJ/mol over the temperature range of 50°C (122°F) to 180°C (356°F).60 Residual stresses in iron are known to relieve by creep. The acti- vation energy for creep in alpha-iron, even at high temperatures, is ~300 kJ/mol.61
Although a difference of 18% between 36 kJ/ mol (for GCI aging) and 44 kJ/mol (in Fe-N alloy) is not negli- gible, the two energies are certainly on the same order of mag- nitude. The same cannot be said for comparison of activation energies for aging and residual stress relief in gray iron. Clearly, the activation energy for aging in GCI correlates more closely with a nitride precipitation process than residual stress relief. For sake of argument, the possibility of residual stress relief by iron atom diffusion can also be considered. Even above room tem- perature, in the range of 770–884°C (1418-1623°F) the activa- tion energy for iron self-diffusion is ~240 kJ/mol.62
This energy
is also an order of magnitude more than the energy for GCI aging and would be even greater at room temperature. The activation energy near room temperature for diffusion of nitrogen in fer- rite has been found to range from 74 to 77 kJ/mol.63
Since the
activation energy for nitrogen diffusion is many times lower than for iron self-diffusion, the diffusion of nitrogen will occur before iron self-diffusion. In practice, iron self-diffusion near room tem- perature will be negligible.
At this time, there is no proof that residual stress relief creates overaging behavior, as observed in GCI.15
• Residual stresses should not be able to cause the microhardness increases in a mounted and polished sample of ductile iron, as has been detected.9
Fur-
ther work by this paper’s authors, so far unpub- lished but submitted for peer-review, has shown that the microhardness of ferrite in nitrogen-treated and annealed CG iron also increases with aging and follows a sigmoidal pattern.
International Journal of Metalcasting/Spring 10 •
If residual stresses are still to be considered the source of cast iron aging, some questions must be answered: •
Titanium additions are known to prevent the aging process. How might titanium in gray iron remove re- sidual stresses or stop their room temperature relief?
• Can nitrogen be associated to a mechanism that in- creases residual stress relief in cast irons?
How could residual stress relief create overaging behavior in strength of cast iron?
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