tion 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 possibil- ity 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.


age strengthening curves obtained at different temperatures (Fig. 1). An Ar- rhenius plot was constructed using the rate constants versus the reciprocal of the absolute temperature (Fig. 2).


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 variations in manganese and sulfur on cast iron’s aging rate is important. In a study of cast iron with 0.8%- 0.83% manganese, aging was com- pleted 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 ar- ticle 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 strengthening. Nitrogen, available at solidification to form metastable solid solution in ferrite, affects the age strengthening of cast iron. Low soluble nitrogen left after titanium nitride for- mation does not allow for the produc-


ability test articles recommended by the American Foundry Society were used for facing cuts on a computer nu- meric 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 Fe4N pre- cipitation in ferrite, it increases the iron’s strength and hardness and allows for


26 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2013 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.


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