![Sample our Physical Sciences journals, sign in here to start your FREE access for 14 days]({"type":"image" , "src" : "/sda/110819/TOC-Subject-Sample-2021-Physical-Sciences.jpg"})
Abstract
The production of lightweight ferrous castings with increased strength properties became unavoidable to face the serious challenge of lighter aluminium and magnesium castings. The relatively new ferrous casting alloy austempered ductile iron (ADI) offers promising strength prospects, whereas the thermomechanical treatment of ductile iron may suggest a new route for production of thin wall products. This work aims at studying the influence of thermomechanical treatment, either by ausforming just after quenching and before the onset of austempering reaction or either by cold rolling (CR) after austempering. In the first part of this work, ausforming of ADI up to 25% reduction in height during a rolling operation was found to add a mechanical processing component to the conventional ADI heat treatment, thus increasing the rate of ausferrite formation and leading to a much finer and more homogeneous ausferrite product. The kinetics of ausferrite formation was studied using both metallographic and X-ray diffraction (XRD) techniques. The effect of ausforming on the strength values was quite dramatic (up to 70 and 50% increase in the yield and ultimate strength respectively). A mechanism involving both a refined microstructural scale and an elevated dislocation density was suggested. Nickel is added to ADI to increase hardenability of thick section castings, and ausforming to higher degrees of deformation is necessary to alleviate the deleterious effect of alloy segregation on ductility. In the second part of this work, the influence of CR on the mechanical properties and structural characteristics of ADI was investigated. The variation in properties was related to the amount of retained austenite (γ r) and its mechanically induced transformation. In the course of tensile deformation of ADI, transformation induced plasticity (TRIP) takes place, indicated by the increase in the instantaneous value of strain hardening exponent with tensile strain. The amount of retained austenite was found to decrease due to partial transformation of γ r to martensite under the CR strain. Such strain induced transformation resulted in higher amounts of mechanically generated martensite. The strength and hardness properties were therefore increased, while ductility and impact toughness decreased with increasing CR reduction.