Development of Martempered Ductile Iron by Step-Quenching Method in Warm Water

Abstract

The evaluation of the suitability of warm water for interrupted quenching of ductile cast irons in the production of Martempered Ductile Iron (MDI) was carried out. Ductile cast irons used were normalized at a temperature of 850oC for 60 minutes. The normalized specimens were subsequently heat-treated in muffle furnace to a pre-determined temperature of 850oC for 30 minutes, then step-quenched in warm water maintained at temperature of 80oC for 40 seconds followed by tempering at different tempering temperatures (175oC-425oC) and times (30-180 minutes). Hardness test was used to evaluate the mechanical property while the metallographic examinations that produced different microstructures were used to evaluate the transformation behavior. The results showed that the developed MDI has a high average hardness value of 53Rc at the lowest transformation temperature, and 19.6Rc at the highest transformation temperature. Metallographic analysis showed that untempered martensite was obtained at holding temperature below 250oC, tempered martensite at 250oC to 325oC, tempered martensite at holding temperature of 350oC for short holding times, above which the specimen is over-tempered (depending on the holding time). This showed that warm water is a suitable quenchant for the production of MDI.