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 microstruc-


ture on cast iron machinability, cast- ings from the same heat were tested further after ferritizing/resolutionizing heat treatment. Tis treatment trans- formed pearlite to ferrite and produced a resolutionizing effect, which allowed repeating the natural aging. Te effect


observed was opposite to the previ- ously discussed test of cast iron with pearlite matrix and steadite phase, in that aging of ferritized/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 machin- ability in the aged condition contained some amount of free ferrite, while gray


Sept/Oct 2013 | METAL CASTING DESIGN & PURCHASING | 27


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