Mechanical properties and microstructure analysis of 0.5% Niobium alloyed ductile iron under austempered process in salt bath treatment

The purpose of this work was to compare the mechanical and physical properties for 0.5% Niobium alloyed ductile iron (0.5% Nb-DI) with commercial ductile iron (DI) before and after austempering process. The materials for this investigation were produced by conventional CO₂ sand casting process and machined to TS EN 10001 standards for tensile test and ASTM E23 for Charpy impact test. Hardness test, density test and polarization test were also done to the samples. Microstructure observations were made after 2% Nital chemical etched and the fractured surfaces obtained through broken samples of tensile and impact test were evaluated through Scanning Electron Microscopy (SEM). The samples were austenitized at 900°C for 1 hour followed by austempered in a salt bath at 350°C for 3 different holding times which were 1 hour, 2 hours and 3 hours subsequently. The samples were then let cooled to room temperature. All testing were done before and after heat treatment process. The newly developed Nb alloyed DI possessed significant improvement in term of mechanical properties and corrosion behavior in comparison with pure ductile iron. Enhanced properties of the materials make them suitable for various technical and industrial applications.