a condition also in agreement with the Fe-N phase diagram. Wada and Pehlke determined that chromium increased the solubility of nitrogen in ductile iron without forming a ni- tride.29
The calculations showed no Fe4
N formation in liquid iron,
Other work by Wada, Pehlke, and co-authors re- viewed and/or experimentally determined that chromium increased solubility of nitrogen in both liquid and solid cast irons.29-32
Age Strengthening Effects on Machinability of Cast Iron
Edington et al. published the first research showing that, with aging, machinability improves in GCI.13
Research showed
This increase likely does not have an effect on aging. In practice, nitrogen does not usually reach the solu- bility limit in cast iron; therefore, increasing the solubil- ity limit usually will not increase nitrogen concentration in the melt. However, with an increase in the solid solubility limit of nitrogen, decreases in aging could be observed by reducing nitrogen supersaturation of the ferrite near room temperature. No study on chromium interaction with aging has been conducted in cast irons.
One mechanism briefly considered to explain the age strengthening behavior was titanium-nitrogen substitution- al-interstitial clusters interacting with dislocations, similar to those observed in steel.33
Since research has shown that
titanium additions reduce aging in cast iron or eliminate it altogether, if it is present in sufficient amounts relative to nitrogen, this mechanism is not a candidate to explain aging in cast iron.
On the other hand, if observations of aging in cast iron were from strain aging caused by interstitial atoms, then it would not be possible for samples to display overaging35 as can be seen in GCI (Fig. 7).15
The observed aging is certainly not strain aging. Strain ag- ing from substitutional elements is improbable because of the lack of their mobility at room temperature. Furthermore, if strain aging were caused by substitutional solute atoms, then increasing nitrogen content would act to reduce strain aging.34
In the discussion section of their first paper, Nicola and Rich- ards indicated that, although copper was used to stabilize pearlite, the aging could not be from an ε-copper precipitate like those possible in 17-4PH steel because GCI aging can occur at room temperature.6
Copper-rich epsilon pre-
cipitates require several hundred degrees Celsius to grow since they rely on the mobility of substitutional atoms whose room temperature diffusivity is low.
Neutron scattering experiments have been per- formed on GCI samples previously determined to demonstrate age strengthening. The neutron scat- tering revealed spherical particle precipitates with diameters ranging from 2nm to 4nm or an ordered array of particles with a spacing of 3.7nm.13
The
neutron scattering results are open to some doubt because inhomogeneities may have been removed from thinner samples; permitting peaks to appear that were not visible in thicker samples. The neutron scattering result has not been conclusively linked to a species of precipitate, but may be in the future.
International Journal of Metalcasting/Spring 10
Figure 4. Tool life data for foundry D showed dramatic improvements in the time that the tool was considered to be usable for machining.37
that tool life increases, while tilt-rate (a possible measure of tool wear rate), surface finish deterioration rate and ma- chine power usage all decrease with aging. The results of the tool life study may be seen in Fig. 4. The best surface finish and lowest required amperage appeared on days three and six of aging. Days of testing before and after this showed improvements over the unaged condition, but were interme- diate between the day three and day six behavior and the unaged behavior. Kountanya and Boppana observed similar
It is noteworthy that the
improvements in tool life appear to follow the same be- havior as the UTS during aging.
Figure 5. Decrease in tool wear rate observed when machining GCI disk-type castings.36
47
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89