Optimisation of Structure-Property of Thin-Wall Austempered Ductile Iron

摘要

Many cast parts are currently produced in Aluminium rather than cast iron, because of the drive to get high strength/weight ratio for application in automobile and aerospace. Ductile Iron casting of approximately 2-3mm section thickness better known as Thin-Wall Ductile Iron (TWDI) having high strength/weight ratio and better mechanical properties than aluminium alloys. The austempering treatment of these TWDI components may further lead to improvement in the strength/weight ratio and will be a suitable and economical alternative to use in automobile parts. The effect of various parameters like, gating and feeding, pouring temperature, post inoculation on the structure and properties of TWDI were studied and it was observed that for thin plates decreasing in pouring temperature leads to finer pearlitic structure and increase in hardness. The increase in inoculation percentage leads to increase in the nodule count, decrease in nodule size and increase in volume fraction of ferrite. Gating and feeding design effect was analysed by using simulation software "ProCAST" and actual casting. It was found that the casting with suitable design and location of riser eliminates the formation of chills and reduces the difference in nodule count at various sections. As expected, the hardness increased with decrease in section thickness. Short-term austempering treatment was used to obtain thin-wall ADI castings. The samples of 2 and 3 mm thickness were austenitised at 900℃ for 30 minutes followed by holding at 350℃, 400℃ and 450℃ for 5,15,30 and 60 minues for each temperature. The microstructures and mechanical properties were analysed in respect of various process parameters like austempering time, austempering temperature and section thickness.