Stability and transformations of retained austenite in ductile and Vermicular Austempered Cast Irons

Ductile and Vermicular Austempered Cast Irons (ADI and ACI, respectively), contain a large amount of retained austenite. This austenite, could transform into martensite when austerapered parts are exposed to service conditions that involve low temperatures, stresses or strains. The present article studies the stability of the retained austenite at sub-zero temperatures, and under plastic deformation. Austempered samples of both types of irons were cooled down to temperatures up to -196 degreesC (temperature of liquid Nitrogen)Brinell hardness indentations were used in order to produce plastic deformation. X-Ray diffraction techniques and Mossbauer spectroscopy have been used to measure changes in the volume fraction of retained austenite on ADI and ACI samples. The microstructure of the samples was also examined optically. The carbon content in the austenite was determined from the X-RD and Mossbauer spectra, in order to estimate the martensitic transformation starting temperature (Ms). Hardness tests were also performed to detect possible martensitic transformation and strain hardening. X-RD and Mossbauer spectra show that the volume fraction of retained austenite remains unchanged after cooling down to -196 degreesC. Nevertheless, metallographic observations using oblique illumination, performed on polished surfaces of samples cooled at sub- zero temperatures, reveals the presence of some martensite needles on the unreacted austenite, located at the last to freeze areas. The increase of hardness in areas next to Brinell indentations can be related to plastic deformation, rather than to the strain-induced transformation of austenite into martensite.