Effect of Heat Treatments on Austempered Ductile Iron

Abstract

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, with the two-step process there was gain (4.7%) in the average hardness and loss (3.5%) in the impact resistance. The microhardness of the ausferrite was 6.2% higher in the one-step austempering when compared to the two-step process.

Keywords:

• Austempering
• Casting
• Ductile
• Iron
• Microstructure
• Properties
• Solidification
• Treatment