TECHNICAL PAPER Mechanical Characterization of Austempered Ductile Iron Obtained by Two Step Austempering Process Vinayak Dakre 1 • D. R. Peshwe 1 • S. U. Pathak 1 • Ajay Likhite 1 Received: 11 November 2016 / Accepted: 8 March 2017 Ó The Indian Institute of Metals - IIM 2017 Abstract Austempered ductile iron (ADI) is known to have a good combination of mechanical properties due its unique ausferrite microstructure. The strength of ADI is mainly a function of the austempering temperature and the stability of ausferrite matrix. To increase the stability of the ausferritic matrix, two stage austempering processes was developed. During this investigation, in the Ist step, ductile iron specimens were austenitized at 900 °C for 60 min followed by quenching to 250 °C in salt bath. In the IInd step, after quenching at 250 °C, the salt bath was gradually heated to 350 °C, 400 °C and 450 °C respectively where specimen were soaked for 120 min. The tensile strength and impact strength were evaluated according to ASTM standards. The results were compared with that obtained by conventional austempering process by quenching directly into salt bath at 400 °C for 120 min. Both tensile and impact strength were found to have improved by two step austempering process. During Ist stage of austempering, martensite was observed while during IInd stage of austempering microstructures revealed acicular ferrite and carbon stabilized austenite. The fractographic examination revealed mixed type of fracture mode and intergranular fracture was seen under SEM. It was further observed that the tensile strength decreased whereas the impact strength increased with IInd stage of austempering temperature. Keywords ADI Á Two step austempering Á Tensile test Á Impact strength 1 Introduction Austempered ductile iron (ADI) is very well established material in many applications such as automobile parts (camshafts) because, the ADI offers a unique advantage of high strength at low cost [1]. The special properties of ADI are attributed to its characteristic microstructure which consists of acicular ferrite and carbon stabilized austenite (CSA) together called as ‘‘ausferrite’’ [2]. ADI is produced by austempering treatment of ductile iron. In conventional austempering treatment, ductile iron is austenitized at temperature between 850 °C and 950 °C and then quen- ched in salt bath, maintained at a specific austempering temperature, to get ausferrite matrix followed by cooling in air up to room temperature. The austempering temperature plays vital in final properties of ADI. It is very well established that, at a lower austempering temperature, ADI has high tensile strength due to fine ferrite, but low duc- tility. On the other hand, at higher temperature, ADI has low tensile strength and high ductility [3, 4]. Fracture toughness is a property which describes the ability of a material to resist fracture. Fracture toughness of ADI is known to be directly proportional to yield strength and the product of carbon content in austenite (C ! ) with volume fraction of CSA (X ! )[5]. At lower temperature, yield strength is more due to fine ferrite cell size. At higher temperature, C ! and X ! are more due to higher diffusion & Vinayak Dakre vinayakdakre2007@gmail.com D. R. Peshwe dilippeshwe@mme.vnit.ac.in S. U. Pathak supathak_vnit@rediffmail.com Ajay Likhite alikhite@gmail.com 1 Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology (V.N.I.T.), South Ambazari Road, Nagpur, Maharashtra 440010, India 123 Trans Indian Inst Met DOI 10.1007/s12666-017-1099-5