Dimensional instability in austempered ductile iron rotor shafts: the role of retained austenite and the influence of heat treatments

Dimensional stability in austempering ductile iron (ADI) rotor shafts is critical in compressor manufacturing. This study evaluated the influence of two austempering heat treatments, standard (N) and modified (A), on the dimensional variation of the shafts. The fraction of retained austenite was quantified by image analysis. Treatment (A) resulted in a fraction of retained austenite greater than 10%, versus 5% of treatment (N). Thermal tests at 170 °C showed no dimensional variation, but at 210 °C, the samples (A) expanded markedly. This behavior is due to the transformation of metastable austenite above the martensitic onset temperature (Ms), determined between 170-210 °C. It is concluded that strict control of austempering is essential to ensure dimensional stability, preventing the formation of thermally unstable austenite.