A Study on the Production and Characterization of Carbidic Austempered Ductile Iron

Abstract

Austempered ductile iron (ADI) is the family of ductile iron with wide range of mechanical properties. This material is known for its higher toughness, wear resistance and the ductility. This ADI will replace the steel forgings with low production cost and aluminium components where higher strength-to-weight ratio is required. The properties of this material are improved by alloying and also by varying the austempering heat treatment process parameters. The microstructure of ADI contains ausferrite matrix and graphite nodules. The ausferrite matrix gives higher wear resistance due to its strain hardening effect during the wear applications. The wear resistance of this ADI is further improved by introducing carbides because carbides are wear resistance compounds. ADI with carbides are called as carbidic austempered ductile iron (CADI). The production process, characterization and optimization of mechanical properties of CADI are done in this research work. Chapter 1 deals with the introduction, production process, microstructure, variables governing the properties and applications of ADI and CADI. An extensive literature review has also been carried out on the constitutions of microstructure, mechanical properties and their relationship with the alloying elements and the production process parameters. iv In the first phase of the research, ductile iron is formed without alloying, with copper alloying and with copper and molybdenum alloying. Suitable specimens are prepared from the ductile iron castings and subjected to austempering heat treatment. The unalloyed ductile iron does not show the ausferrite matrix after austempering but small improvement in the mechanical properties. Alloy additions form ausferrite matrix during austempering. This alloyed ADI gives higher improvement in the mechanical properties compared to the unalloyed ADI. A clear upper ausferrite matrix at higher austempering temperatures and lower ausferrite matrix at lower austempering temperatures are noticed on the microstructure of copper, copper and molybdenum alloyed ADI. Maximum hardness of 444 BHN [HBW] and ultimate tensile strength as high as 1781N/mm2 is noticed at 250°C austempering of copper and molybdenum alloyed samples. This hardness is 200% higher than the as-cast ductile iron of same composition. Maximum impact toughness of 109 joules is noticed at 350°C austempering of copper alloyed ductile iron. This is 76% higher compared to the as-cast ductile iron with pearlite matrix. Wear resistance of copper, copper and molybdenum alloyed ductile iron austempered at 250°C for two hours is higher when compared to the other heat treatment conditions.