cipitation in ferrite, it increases the iron’s strength and hardness and allows for chip formation with a smaller amount of plastic deformation, which could decrease the cutting force. Similar results were achieved in
other cast irons having ferrite in metal matrix and different graphite shapes. For example, aging decreased cutting forces after aging ductile iron with spherical graphite and significant free ferrite. However, aging does not always
improve cast iron machinability. For example, aging cast iron containing carbide forming elements produced a completely opposite effect on cast- ing machinability. Tere was a visible and statistically significant increase of the average normal cutting forces for aged samples versus unaged samples. Te ratio of passive to normal cutting forces is used as an indicator of tool wear, because as a tool loses sharpness, it has an increasing passive reac- tion force. Tis ratio increased more significantly when cutting aged gray iron with carbide-promoting element
Fig. 4. This graph shows the comparison of tilt data from machining: tool wear (flank area).
content. Te microstructure in this case was pearlitic with some steadite and free carbide but no free ferrite. To verify the effect of microstructure
on cast iron machinability, castings from the same heat were tested further after ferritizing/resolutionizing heat treatment. Tis treatment transformed pearlite to ferrite and produced a resolution- izing effect, which allowed repeating the natural aging. Te effect observed was opposite to the previously discussed test of cast iron with pearlite matrix and
steadite phase, in that aging of ferri- tized/resolutionized gray iron improved machinability. Te cutting forces were decreased at all cutting speeds studied (Fig. 3). It can be concluded from these tests that all gray iron showing improved machinability in the aged condition contained some amount of free ferrite, while gray iron showing increased cut- ting forces after aging had no free ferrite but was entirely pearlitic with cementite/ steadite phases. Tis differing behavior of aged cast
January 2014 MODERN CASTING | 37
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