The Effect of alloying elements on the austenite-marstensite transformation in austempered ductile iron

ABSTRACTDevelopment of cast irons that are in use for more than one hundred year, still continues inorder to achieve higher stregth and lower cost.The birth of a new engineering material, Ductile Iron (DI), was announced at the 1948 ASMAnnual Foundry Conference following the announcement of Austempered Ductile Iron (ADI)as a new star to DI family. Combining the strength, ductility, fracture toughness and wearresistence of a steel with a castability and production economies of a conventional DI, ADIoffers the designer an exceptional opportunity to create superior components at reduced costs.Beside the common mechanical advantages, ADI found itself new advantageous fields due toits phase transformation mechanism which has been widely known in steels for decades. Theretained austenite in ADI transforms to martensite if subjected to plastic deformation. Thistransformation mechanism provides an extra strength to the material and enhances the life ofthe product.This study is supported by State Planning Organization (DPT). The aim of the main projectwas to develop casting technology for the production of wagon wheels which are currentlyimported. Austenite-martensite transformation at the surface of the wagon wheels duringoperation was the most important factor affecting the fatigue life and in turn success of thenewly developed centrifugal casting technology. In this doctorate study, effect of the mainalloying elements of ADI namely, Cu, Ni and Mo on the austenite-mertensite transformationhas been investigated. Miniature wagon wheel-shaped specimens were tested on a machinedesigned to simulate the wheels operating on a wagon.Hardness, martensitic transformation rate, fatigue strength and mass loss of specimens withdifferent amount of alloying elements have been investigated and effects of above alloyingelements on those properties are determined. Properties of the cast products are comparedwith the steel wheels which are currently used by the railway company.Study has also thrown a new light on the martensitic transformation. A mathematicalcorrelation as y = x + cFe between atomic radius of alloying elements and the martensitictransformation rates has been discovered.Since the same material was suitable for bucket teeth production, tooth are also cast for thispurpose. Fatigue life is very important for wagon wheels. But mass loss was also importantfor the bucket tips due to the field of use.At the end of the study, altougth our recommendations are mainly based on the mechanicalproperties, economical aspects are also taken into account. Composition which has high Cuand Ni (4) have been found suitable for the wagon wheel production, unalloyed ADI (3) forscraper bucket teeth and materials which has high Cu, Ni and Mo (8) are recommended forthe bulldozer bucket tips.Keywords: Casting, ductile iron, austempering, retained austenite, martensite, plasticdeformation, phase transformation, wagon wheel, grader bucket tip, fatigue strength, wearresistance, solidification simulation, stress analysis.3