Influence of Intercritical Austenitizing Temperature, Quenching Media and Tempering Temperature on Mechanical Properties and Wear Behavior of Ductile Iron with Dual Matrix Structure

Abstract

Mechanical properties of ductile iron are controlled primarily by the distribution and amount of matrix phases and micro-constituents. In the moderately newly developed ductile iron with dual matrix, the structure consists of ferrite (soft phase) and martensite (hard phase) which is called dual matrix structure. This structure is obtained by a special heat treatment process in which the
ductile iron is subjected to an incomplete austenitization stage within the intercritical intervals
followed by a fast cooling step to transform austenite into martensite. Ductile iron with dual matrix structure holds promises for the production of suspension parts for automotive industries and safety components for military industries where high strength and ductility is required. The further development and applications of intercritical heat treatments in ductile iron requires an increasing understanding of different aspects of this kind of heat treatment cycle. The present dissertation was focused on studying the influence of intercritical austenitizing temperature, quenching media and tempering temperatures on mechanical properties and wear behavior of dual matrix structured ductile iron. Ductile iron (3.56 C wt. %, 2.07 Si wt. %, 0.17 Mn wt. %) was intercritically austenitized in two phase region (α +γ) at temperatures of 785°C and 815°C for 5 minutes and then quenched in two different quenching media paraffin liquid light oil and water to obtain dual matrix structured ductile iron with different ferrite and martensite volume fractions. Some specimens
were quenched from same intercritical austenitizing temperatures and tempered for 1 hour at three different tempering temperatures of 400°C 450°C and 500°C to produced quenched and tempered dual matrix structured ductile iron. The microstructures of all the treated ductile iron samples were analyzed using optical microscopy and metal pro image analyzer. X-Ray diffraction of dual matrix structured ductile iron was done for the phase detection. To study the mechanical properties tensile test were performed by using Instron 1195 and hardness measurement was determined using Vickers hardness tester. Fractographic analysis of tensile fractured surface was done using field emission scanning electron microscopy. Wear properties
of all the treated ductile iron samples was analyzed by using a ball-on-plate tribometer. The worn surface was studied under scanning electron microscopy in order to study the wear mechanism involved. The result shows that volume fraction of martensite and ferrite can be controlled to obtain the strength and ductility of ductile iron with dual matrix structure. The weight loss resistance and strength increased and ductility decreased with increased martensite volume fraction. The
ultimate tensile strength and yield strength initially drops and remains almost constant by increasing the tempering temperature from 400°C to 500°C. Moreover, weight loss of all the tested samples increased almost linearly as the applied load increased since an increase in the load might increase the contact stress leading to further damage.