Ageing Behavior of Ductile Iron with Dual Matrix Structure

Abstract

Although, extensive ageing and strain ageing (bake hardening, BH) studies have been carried out on dual phase steels, the ageing behavior of the dual matrix structure (DMS) ductile iron (DI), as a potential way to improve its mechanical properties, has not been addressed until now. This thesis was designed to study the ageing behavior of the DMS-DI with ferrite – martensite matrix structure. For produced the ferrite – martensite matrix, the intercritical region of the as-cast ductile iron with a chemical composition of 3.39 % C, 2.63 % Si, 0.33 % Mn, 0.03 % Mg, 0.01 % S, and 0.05 % P was calculated with the empirical equations and ThermoCalc software. To confirm the resulting calculations, the intercritical region verified practically using dilatometry and partial austenitizing heat treatment cycles. After defining the intercritical region, the DMS-DI with martensite volume fraction MVF of 30 % was produced by intercritical austenitizing at 780 °C followed by quenching in water to room temperature. Ageing treatments were carried out without pre-straining at ageing temperatures of 140, 170, and 220 °C for 2 – 10000 minutes. DMS-DI was investigated by light optical microscopy (LOM) and scanning electron microscopy (SEM) for unaged and selected samples after ageing treatments. The effect of ageing conditions (ageing temperatures and times) on the mechanical properties were investigated. Microhardness measurements for ferrite and martensite were also examined as a function of ageing conditions. The increase in yield strength YS due to ageing effect was determined.