The influence of cast structure on the austempering of ductile iron

Abstract

Microsegregation modelling shows that increasing the graphite nodule count can reduce the solute segregation to intercellular boundary significantly. Microanalysis on ductile irons of comparable composition but of different nodule counts confirms these predictions. Higher nodule count also increases the interfacial area between the graphite and matrix, decreases the volume of individual intercellular regions and decreases C diffusion length in the matrix. As a result, the stage I reaction in the austempering process is accelerated whilst there is little effect on the stage II process. Austempering studies at 375°C are used to detail the effect of increased nodule count on microstructure and mechanical properties. The results emphasise the importance of a sound casting and good quality cast structure if ADI specifications are to be satisfied. Increased nodule count is an effective, efficient and economical way of improving the cast structure and enhancing ADI properties.

Key wards:

• ductile iron
• cast structure
• nodule count
• austempering
• mechanical properties