No one, to this reviewer's knowledge, has attempted to control austenite content, i.e. reacted austenite content. It seems clear that much of the variations in yield strength and tensile strength of ADI among various alloys investigated are related to the volume fraction austenite in the ADI structure. The attached table from the literature illustrates the range of properties observed in ductile iron alloys austempered under the same heat treatment conditions. Note the wide range in strength, particularly yield strength. It is our contention that the variations in yield strength are due to significant variations in austenite content.
Less austenite leads to a higher flow stress.
Preliminary examinations of data from the literature indicate that certain elements increase yield strength for a given heat treatment, while others decrease yield strength. Irons that contain Ni, which stabilizes austenite, have always displayed lower yield strengths. A review of the literature indicated that five different investigators were able to achieve these exceptional properties. Our experiments in the laboratory with existing castings of ADI grade 1.5 proved we could raise yield strength by 15,000 psi with no apparent loss in ductility. The results were quite encouraging.