ELECTRON MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF SILICON AND ALUMINUM DUCTILE IRONS

Samples of unalloyed silicon and aluminium spheroidal graphite cast ir on have been studied in the austempered condition. Austempering times of up to 3 h at 400-degrees-C for Al SG and 1 h at 350-degrees-C for S i SG gives a typical ADI microstructure consisting of carbide-free bai nitic ferrite and stable, high carbon enriched, retained austenite. Th is has an attractive combination of elongation and strength. For longe r austempering times transition carbides are precipitated in the baini tic ferrite, eta-carbide in the upper bainitic range, i.e. 400-degrees -C for Al SG and 350-degrees-C for Si SG, and epsilon-carbide in the l ower bainite range. Increasing amounts of transition carbide reduce th e ductility and produce a mixed mode of fracture. For longer austemper ing times chi-carbide is precipitated at the ferrite/austenite boundar ies leading to a more brittle fracture mode.