Effect of inter-lamellar spacing and test temperature on the Charpy impact energy of extremely fine pearlite

Pearlitic steels with three different inter-lamellar spacing ranging from 79 to 120 nm have been developed in bulk by austenitization and subsequent isothermal transformation with varying degree of under-cooling. The colony size was found to decrease with reduction of transformation temperature. However, the pearlitic nodule size had a similar range for all the specimens. Charpy impact energy of the specimens has been determined for all the transformation conditions at different test temperatures. At 25 °C, the variation of impact energy was negligible with the variation of inter-lamellar spacing and the fracture was completely brittle with clear cleavage facets. The facet size had a strong correspondence with the pearlitic nodule size. However, as the test temperature was raised to 125 °C and 250 °C, the impact energy was enhanced and the pearlite with the finest inter-lamellar spacing fractured after absorbing the highest energy. At higher temperatures, a region of ductile fracture was observed in the fracture initiation region which expanded with an increase in test temperature and decreasing lamellar spacing. At lower testing temperatures, the crack seemed to be cutting through the colonies without any noticeable deformation of cementite lamellae. However, the cementite for all the specimens bent significantly before breaking at higher temperatures. The bent in the direction of crack propagation increased with refinement of inter-lamellar spacing. The study shows that although nodule size determines the impact toughness at lower testing temperatures, inter-lamellar spacing influences it at higher temperatures.