Abstract
This paper considers the important factors of the production of high-strength ADI (Austempered Ductile Iron); namely, the austenitization stage during heat treatment. The two series of ADI with different initial microstructures were taken into consideration in this work. Experiments were carried out for castings with a 25-mm-walled thickness. Variable techniques (OM, SEM, dilatometry, DSC, Variable Magnetic Field, hardness, and impact strength measurements) were used for investigations of the influence of austenitization time on austempering transformation kinetics and structure in austempered ductile iron. The outcome of this work indicates that the austenitizing temperature has a very significant impact on structure homogeneity and the resultant mechanical properties. It has been shown that the homogeneity of the metallic matrix of the ADI microstructure strongly depends on the austenitizing temperature and the initial microstructure of the spheroidal cast irons (mainly through the number of graphite nodules). In addition, this work shows the role of the austenitization temperature on the formation of Mg–Cu precipitations in ADI.
Highlights
Austempered ductile iron (ADI) belongs to the spheroidal graphite cast iron (SGI) family, which is subjected to heat treatment; i.e., austenitization and austempering
It can be stated that the coarser the initial microstructure of SGI, the more sensitive the castings are to the austenitizing temperature
The austenitizing temperature plays an important role in the formation of a homogeneous structure
Summary
Austempered ductile iron (ADI) belongs to the spheroidal graphite cast iron (SGI) family, which is subjected to heat treatment; i.e., austenitization and austempering. The structure of ADI cast iron consists of spheroidal graphite nodules imbedded in a metallic matrix of plates of ferrite and high-carbon austenite. The austenite decomposes into ferrite plates and high-carbon austenite, and the final ADI structure formation takes place [6]. The number of graphite nodules, their shapes and distribution, the number and morphology of the ferrite plates, and the high-carbon austenite determine the final properties of ADI cast iron. The present research aims to enrich the knowledge of the significant impact of the melt treatment to have different initial graphite structures and the first heat treatment period (i.e., austenitization) on the austempering process kinetics, structural homogeneity, and properties of ADI cast irons as well as on the formation of highly disperse precipitations from the Mg–Cu system
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