allow for the production of detectible age strengthening of cast iron. Te temperature range of super-saturation of ferrite lies from room temperature to 572F (300C), and beyond this range the possibility of aging is limited according to thermodynamics.
Machinability of Aged Cast Irons Cutting Tool Forces: Te machin-
Fig. 2. This graph shows the effect of manganese on aging time estimated from maximum increase in tensile strength and electrical resistivity.
Effect of Alloying Elements
While elevated temperature aging is less dependent on alloy composition, cast iron chemistry strongly affects room-temperature aging kinetics. From a practical perspective, the effect of vari- ations in manganese and sulfur on cast iron’s aging rate is important. In a study of cast iron with 0.8%-0.83% manga- nese, aging was completed at 25 days, while this process needed only 15 days for cast iron with 0.51% manganese at similar 0.04%-0.06% sulfur levels. To study the effect of alloying
elements, aging kinetics of cast irons from six heats with variations in manganese, nitrogen and sulphur were evaluated. Strength change curves typically had a prestrengthening peak and a “relaxation valley” before achiev- ing a full age strengthening. Alloying with manganese affected both the time to prestrengthening and the full strengthening peak. Cast iron from a heat with 0.53% manganese
had the highest reaction rate. Iron with lower manganese and especially higher manganese contents each had a longer aging reaction time.
Effect of Carbide/Nitride Forming Elements
Natural age strengthening of cast
iron occurs in Fe-BCC (ferrite) by iron nitride precipitation. Carbide forming elements such as chromium promote the decrease of free ferrite in cast iron and reduce the total possible strengthening effect. An as-cast machinability test arti- cle produced from cast iron with 0.2% chromium did not show an improve- ment in machinability after aging. Nitride forming elements such as
titanium, aluminum and boron can fully suppress iron nitride precipitate strength- ening. Nitrogen, available at solidifica- tion to form metastable solid solution in ferrite, affects the age strengthening of cast iron. Low soluble nitrogen left after titanium nitride formation does not
ability test articles recommended by the American Foundry Society were used for facing cuts on a computer numeric control (CNC) lathe. Tese test articles were produced in a laboratory using nobake molds and in industrial metalcasting facilities using green sand molds. Pearlite/ferrite cast irons with variations in carbon equivalent from 3.9% to 4.3% were tested in as-cast condition and after 25 days of natural aging. In as-cast or in unaged condi- tion, the cutting forces increased with increasing hardness in irons having less carbon equivalent, which is typical and expected. At the same time, a reverse type of dependency appeared in which the cutting force decreased when the increasing hardness was due only to natural aging in each iron. Tis unusual behavior could be explained by the energy requirement for chip formation. In unaged cast iron, soft ferrite absorbs energy for significant plastic deformation. Tis effect results in edge build-up on the tool tip, which also could promote increasing cutting force by enlarging the deformation region (similar to tool wear). In contrast, when iron aging occurs as a result of Fe4
N pre-
Fig. 3. These graphs show (a) the effects of cutting speed and aging on cutting force of ferritized/resolutionized gray iron and (b) the effect of cutting speed and aging on an average distance between cracks formed in chips.
36 | MODERN CASTING January 2014
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