Optimising parameters in a variable temperature austempering process

Abstract

With the advent of new technologies, the demand for new materials with ever improved properties has led the metallurgist to study materials such as-cast iron in more detail. One of the main advantages of cast iron is that it can have a wide range of properties depending upon the type of iron and the heat treatment it is subjected to. The absolute diversity of properties of this material will be indicated in the following paragraphs. Cast irons are near eutectic alloys of iron and carbon with melting points around 12000C. Eutectics have the advantage of high fluidity, thus the· molten metal will easily fill intricate moulds. The main disadvantage of Grey cast iron is its low ductility which cannot be improved by heat treatments as the weaker point of the structure are the graphite flakes, which tend to have sharp-edged rims. These introduce areas of stress concentration resulting in medium strengths and low ductility. It was Morrogh and Mills who simultaneously discovered Nodular cast iron (also known as Spheroidal-graphite or Ductile iron). In S.G., the graphite flakes associated with Grey cast iron are replaced by spherical nodules of graphite resulting in a more homogeneous structure with less stress concentrations [...].