behavior for tool life improvement with aging.36
Their re-
sults, provided in Fig. 5, showed a decrease in tool wear rate with aging that followed a sigmoidal behavior.
The improvement in machinability accompanied by increases in hardness from aging may be different than one would ex- pect in cast iron.38
It should be noted that, when aging alumi-
num alloys however, increased hardness can improve machin- ability. As stated in the ASM Handbook (Volume 16) “Heat- treatable alloys are more machinable in the heat-treated tem- pers than in the softer as-fabricated or annealed solution.” 39
Laboratory studies published by Teague et al. showed that aging in GCI can reduce forces acting on a single point tool in Computer Numerically Controlled (CNC) turning.40
The
same paper presented another tool wear study with industrial conditions. Fifty passenger car brake disks were machined as soon as possible after casting, and 250 were machined af- ter 10 days of room temperature aging. Results showed that the polycrystalline cubic boron nitride (PCBN) tools used in the machining centers had less wear on the tool flanks (sides) and the tool faces (tops) when machining aged cast- ings. Additionally, less wear was generated after machining 200 aged castings than when machining 50 unaged castings. A portion of the tool wear results are shown in Fig. 6.
Additional work41 has been done on passenger car brake
disks from the same foundry, process, and production line as those used for the tool wear study presented in Fig. 6. Tool force data was collected for five brake disks in the un- aged condition, five disks aged at room temperature for five days, and five disks aged at room temperature for 50 days. All data was collected on a CNC lathe, and cutting was per- formed dry and on the face of the flanges. The data showed that after aging for five days, tool forces in the first 15 thou- sandths of an inch (0.381mm) significantly decreased. Af- ter 50 days of room temperature aging, tool forces decreased with statistical confidence for all 0.12 inches (3.048mm) of material re- moved. The first region to show decreased tool forces with aging was the casting skin is a significant finding because it suggests an effect of free ferrite, which was present as ~50% of the matrix to a depth of 0.004inch- es (0.102mm). In addition to tool forces, tool wear and machined surface roughness were measured. The tool wear data suggested that tool wear was less following machin- ing of aged disks, but the results were not statistically significant. Machined surface roughness results showed clear statistical significance, with roughness being lowest after five days of room temperature aging. These results agree with machined surface roughness values from Edington, et al. dur- ing industrial trials in which roughness after three and six days of natural aging was less
48
than after nine days of aging or no aging.13
Furthermore, the
machining performed by Edington et al. on castings required less amperage in the machining center on days three and six.
Determining how machinability changes with aging of cast iron is of practical importance to the iron casting industry so that attempts to optimize machinability, whether measured by tool life, surface finish, or another method can be made.
No machinability work has been conducted to determine how aging affects the machinability of any cast irons other than gray iron. This is a topic of interest for future work.
Kinetics, Thermodynamics, and Crystallography of Nitride-Induced Aging in GCI
Artificial aging studies have been conducted to study GCI aging kinetics.8,15
follows standard sigmoidal Avrami-Johnson-Mehl precipi- tation behavior observed in many metallurgical systems.10 Figure 1 shows an example of this behavior. Aging slows when done below room temperature and elevated tempera- tures accelerate aging. Artificial aging at 182ºC (360°F) is comparable to experiential times for iron-nitride precipita- tion observed in steel by Wert at 250°C (482°F).42
In the
iron aging studies, a small decrease in UTS and hardness occurred a few hours prior to the onset of aging at room temperature and at 100°C (212°F).15
In addition to the
strength decrease, researchers noted that the time required before aging could be observed increased as the manganese content of the iron increased. A possible explanation for this observation is the occurrence of “interaction solid solu- tion strengthening” such as that found in steel, as discussed by Leslie16
and Enrietto.21 These Mn-N interaction regions
are not a separate phase. One hypothesis based on internal friction measurements suggested that Mn and N form an
The studies have shown that aging in GCI
Figure 6. Comparison of wear area on the flank (side) of tools after ma- chining 50 unaged, 50 aged, and 200 aged brake disks.40
Operation num-
ber labels on the x-axis are proprietary to the foundry and do not indicate order of operations.
International Journal of Metalcasting/Spring 10
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