INFLUENCE OF AUSTEMPERING TEMPERATURE ON FRACTURE TOUGHNESS OF A LOW MANGANESE AUSTEMPERED DUCTILE IRON (ADI)

Abstract

The influence of microstructure on the plane strain fracture toughness of an unalloyed, austempered ductile cast iron (ADI) with low manganese content (<0.15 wt %) and with predominantly as-cast (solidified) ferritic structure was studied. Test specimens were austenitized at 927°C (1700°F) for 2 hr and then austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and correlated with the microstructure. The results of the present investigation indicate that the alloy (with an initially ferritic as-cast microstructure) has higher fracture toughness with an upper ausferritic structure, i.e., when austempered in the upper bainitic temperature range (above 316°C [600°F]). This behavior was markedly different from conventional ADI with a pearlitic as-cast microstructure because the pearlitic structure shows higher fracture toughness with a lower ausferritic structure, i.e., when austempered in the lower bainitic temperature range (<316°C [600°F]). The fracture toughness was found to increase with the increase in total austenitic carbon, i.e., XγCγ, where Xγ is the volume fraction of austenite and Cγ is the carbon content of austenite.

Acknowledgments