Effect of Ti as a trace element on mechanical properties and microstructural evolution in direct austempered ductile cast iron

Abstract

Abstract In this study, the effect of Ti as a trace element on acicular ferrite formation in direct austempered ductile cast irons was investigated. A step-block shape sample was used and the melt was poured into a sand mold at about 1450 °C. After cooling in the mold, the melt was hot knocked out at approx. 900 °C and then medium cooling to 350 °C was applied to the step-block shape sample. Afterwards, isothermal treatment was conducted in a commercial furnace at this temperature for 1 h, and then the sample was air-cooled down to room temperature. It was found that Ti as a trace element has an important effect on the nucleation of beneficial phases such as acicular ferrite in ductile irons which can result in increasing of mechanical properties. The microstructural examination confirmed that Ti actually acts as nucleation site for acicular ferrite. Moreover, the results show that in direct austempered ductile cast iron samples contain 0.25 wt.-% Ti, an increase from 820 to 950 MPa in tensile strength properties can be seen. Furthermore, the impact test results show that with an increase of tensile strength, the impact toughness does not decrease, but rather remains constant.