Heat treatment can be used to enhance the strength and elongation in ductile iron with coarse graphite nodules. Trough refinement of the grain struc- ture the mechanical properties can be improved. A recent study on ductile iron samples from 3-inch (76-mm) Y-blocks showed that normalizing just below the upper critical temperature can produce a fine-grained ferritic–pearlitic microstruc- ture. Te refinement of the microstruc- ture resulted in a significant increase in strength and elongation. Another form of degradation in

Fig. 3. Stress strain curves for two aluminum B319 test bars are plotted.

strength and tensile elongation to be re- duced. Such embrittlement was displayed in a series of aluminum alloy 356 test bars with various sized oxide inclusions. A recent investigation of gray cast iron

has shown that gray iron can also display a form of embrittlement. In that study, the manganese and sulfur concentrations were varied in order to determine the optimum combination of manganese and sulfur. When optimized, the tensile strength, response to inoculation (in terms of cell count), and chill tendency coincided with a specific level of sulfur for each of three levels of manganese. Te strength diminished significantly at higher levels of sulfur, even though the sulfur levels were in a commercially accept- able range. Precision stress–strain curves were

generated for the optimally alloyed and the high-sulfur versions from the study, and the results showed that the metal was embrittled.

Microstructure and Mechanical Properties

Several years ago, researchers

investigated the tensile properties at numerous locations in a large ductile iron casting. Te tensile elongation and strength varied significantly. When the fracture faces were examined, multiple types of discontinuities were observed, including oxide film inclusions and graphite nodule clustering. With ductile iron fractures, cracking begins with void formation around the

graphite nodules. Voids form more easily in castings with coarse graphite nodules. Te stress intensity around the nodules increases with increased nodule size, and the fracture strength of the pearlite ligaments between the nodules is reached at a lower bulk stress, resulting in lower tensile strength and tensile elongation. Te tensile properties in 1-in. (2.5 cm) Y-blocks and 3-in. (7.6 cm) Y-blocks of a recent study illustrate the reduction in strength and elongation with section size. As nodule count is increased and nodule size is reduced, tensile strength and elongation both increase.

properties is regularly displayed in ductile iron castings. In pearlitic–ferritic irons, failure occurs by uniform deformation during tensile elongation. As long as the pearlite is the continuous microconstitu- ent and ferrite is discontinuous, uniform deformation continues. When ferrite is confined to individual

graphite nodules, that is, in the form of bullseye ferrite, voids form at the poles of the nodules. Voids between neigh- boring nodules increase the local stress in the surrounding pearlite matrix and eventually, the voids link up when cracks propagate through the pearlite. When ferrite is continuous, form- ing a path linking nodule to nodule, local deformation in the ferrite regions occurs. As a result, premature fracture through the ferrite regions causes a somewhat reduced ductility.

Fig. 4. Stress–strain curves of two cast iron test bars show the embrittling effect of sulfur on strength and elongation.


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