Deformation behavior and strength properties of spheroidal graphite cast iron which had been austempered and had various microstructures were studied. The matrix of the cast iron consisted of four kinds : pure ferrite, a mixture of ferrite and pearlite, fully pearlite or a mixture of banitic ferrite and retained austenite. Specimens used were cylindrically hollow tubes which were strained under torsional stresses combined either with tensile or compressive stresses. It was found that the deformation of austempered spheroidal graphite cast iron did not depend much on stress rate in an elastic region, but it depended to an extensive degree on the amount of retained austenite in a plastic region. Fracture strengths of spheroidal graphite cast iron can be predicted by Hoffman's criterion for the specimens having any matrix structure. Fracture morphologies differ quite drastically depending upon their microstructure and upon stress loading conditions.
