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The effect of sub-zero cooling on microstructure and mechanical properties of a low alloyed austempered ductile iron has been investigated. Austempering of samples was performed at 325 ℃ and 400 ℃after austenitizing at 875 ℃ and 950 ℃. The sub-zero treatments were carried out by cooling down the samples to -30 ℃, -70 ℃ and -196 ℃. The changes in volume fraction of austenite and mechanical properties were determined after cooling to each temperature. The austenite volume fraction of samples which were austenitized at 875 ℃ and austempered at 325 ℃ remained unchanged, whilst it reduced in samples austenitized at 950 ℃ and 875 ℃ for austempering temperature of 400 ℃. In these specimens, some austenite transformed to martensite after subzero cooling. Mechanical property measurements showed a slight increase in strength and hardness and decrease in elongation and toughness due to this transformation behavior.