Effect of two-step austempering process on carbidic ductile iron

Abstract

The two-step austempering process when applied to ductile iron, results in significant improvement in mechanical properties due to the strengthening of the ausferitic matrix. The present work was undertaken to investigate the effect of the two-step austempering process on carbidic ductile iron. Carbidic ductile iron is a new member to the ductile iron family, it is produced by alloying the ductile iron composition with chromium. In the present work, the carbidic ductile iron thus produced, was austempered by both conventional and two-step austempering route. For both processes, all specimens were austenitized at 975°C and soaked for 180 min. In the conventional process, the specimanwas quenched in salt bath held at to 350°C, 400°C, and 450°C and soaked at respective temperatures for 180 min. Whereas, in the two-step austempering process, specimenwere first quenched in salt bath heldat 250°C. After stabilization at 250°C, the salt bath temperature was raised to 350°C, 400°C, and 450°C. After the respective temperature was attained, the specimens were soaked for 180 min. The austempered samples obtained by both processes were analyzed with the help of SEM, XRD, and TEM. Two-step process resulted in finer ausferitic matrix than the conventional process. The XRD analysis revealed an increase in carbon content of austenite and volume fraction of Carbon Stabilized Austenite (CSA) in sample austempered by a two-step process. The TEM analysis indicated that the lath width of austenite and ferrite was small in samples austempered by two-step process than the conventional process, due to a high degree of super-cooling.