Effect of Austempering Conditions on the Corrosion Behavior of HSADI Alloyed with Niobium

The present work studied the effect of the austempering heat treatment parameters on high silicon austempered ductile iron (HSADI) alloyed with 0.6 and 0.9 wt% Nb in the resulting microstructure and corrosion resistance. Samples were austenitizing at 1050 °C for 2 h followed by austempering treatment at 290, 320, and 350 °C for 60, 90, and 120 min and finally air-cooled. Scanning electron microscopy and X-ray diffraction were used to evaluate the microstructural evolution. These techniques revealed that the microstructures consisted of a mixture of graphite nodules, carbon-rich austenite, acicular ferrite, and niobium carbides in the matrix. To assess the influence of these carbides and heat treatment parameters on the corrosion resistance, potentiodynamic tests were conducted in 1N H₂SO₄ solution as an electrolyte. The results showed that with the Nb content in high silicon ductile iron, both carbon-rich austenite and the C content in the austenite also increased, and significantly reduced the corrosion rate on HSADI.