Abstract
The present article investigates the stability of the retained austenite, present in austempered ductile iron (ADI) after cooling at sub-zero temperatures, considering that the austenite could transform into martensite when austempered parts are exposed to low temperatures or stresses or strains. Optical microscopy with oblique illumination, X-ray diffraction techniques and microhardness tests were used to analyse the transformation of the austenite on samples with different austempering thermal cycles. The results indicated that the martensitic transformation took place mainly at the unreacted austenite present at the last to freeze areas of samples austenitised and austempered at the highest temperatures. On the other hand, the reacted austenite, present in the bulk of all the investigated samples, remains unchanged after cooling. Tensile tests were performed in order to evaluate the influence of the martensitic transformation, promoted by the sub-zero cooling, on strength and ductility.