Effect of Precipitation on Cryogenic Toughness in N-Containing Austenitic Stainless Steels

Three types of austenitic stainless steels JK2, JJ1 and JN1 were isothermally aged at temperatures from 600 to 900°C for 10 to 1000 minutes in order to study the microstructural evolution and its effect on the fracture toughness at cryogenic temperatures. The Charpy V-Notch fracture energy at 77 K showed a significant decrease with aging time in JJ1 and JN1 steels because of their higher contents of C and N. In contrast, the fracture energy corresponding to the aged JK2 steel decreased gradually with aging time. The abundant intergranular precipitation of carbides and nitrides seems to be the responsible for the fracture toughness deterioration in the aged JJ1 and JN1 steels. On the other hand, the intergranular precipitation of carbides was less abundant in the aged JK2 steel. The scanning electron microscope fractographs of the CVN test specimens corresponding to the aged JJ1 and JN1 steels showed mainly an intergranular brittle fracture and its fraction increased with aging time and temperature. In general, the presence of a more abundant intergranular precipitation resulted in a more rapid decrease in toughness with aging time.

Cryoground Rubber-Natural Rubber Blends

Addition of cryoground rubber (CGR) causes changes in curing characteristics (decrease in Mooney scorch time, optimum cure time, and reversion time) and shows a detrimental effect on most of the vulcanizate properties (tensile, resilience, flex, hysteresis, set, and abrasion). Tear strength, however, is not adversely affected by CGR. Higher doses of curatives (sulfur 3.5 phr and acelerator 1.4 phr) and addition of reinforcing carbon black make up the losses in physical properties. Scanning electron microscope fractographs show that there is little adhesion between natural rubber and CGR. But addition of carbon black overshadows the effect of CGR, and the fractographs show the characteristics of black-filled vulcanizates.

Fracture Properties of Human Enamel and Dentin

Work of fracture measurements and scanning electron microscope fractographs show that both enamel and dentin can best be considered as brittle materials with anisotropic fracture properties. Enamel is highly anisotropic, with the weakest path of fracture parallel to the enamel rods. Dentin is less anisotropic, with easiest fracture perpendicular to the dentinal tubules. A model is proposed to explain the fracture behavior of enamel.