Influence of Austempering Temperature and Time on the Microstructure and Mechanical Properties of Ductile Iron Weldment Using Developed Coated Electrode T. Sarkar, Ajit Kumar Pramanick, S.K. Sahoo, T.K. Pal, and Akshay Kumar Pramanick (Submitted November 4, 2017; in revised form October 30, 2018) Sound welded joints of ductile iron were first made by developing coated electrodes as well as weld procedure. The weldments were then austenitized at 900 °C for 2-h holding time followed by austempering at 300 and 350 °C for three different holding times (1.5, 2 and 2.5 h) at each austempering temperature. The influence of austempering temperature and time on the microstructural and mechanical properties was studied. Microstructural characterization, phase analysis, microhardness and mechanical properties were performed to understand the effect of austempering heat treatment on microstructure, austempering kinetics and mechanical properties of welded joints. In spite of significant variation in microstructures among all the three zones of DI weldment before austempering, the response of both weld metal and HAZ to austempering heat treatment similar to a base metal was noted. However, a significant variation in matrix structures (bainitic ferrite, retained austenite) as well as graphite nodules among the three zones was observed with changing the austempering temperature and holding time. Also, 100% joint efficiency of the welded joints was achieved at both 300 and 350 °C for 2-h holding time. Hardness and charpy impact toughness mostly depended on the volume fraction of retained austenite and carbon content. Based on the results of tensile and charpy impact testing, 350 °C and 2-h holding time could be considered as an optimum austempering condition for ADI weldment. Keywords austempering heat treatment, ductile iron (DI), me- chanical properties, microstructure, SMAW electrode development 1. Introduction Austempered ductile iron (ADI) with an ausferrite matrix is a new type of engineering material and has gained increasing interest in academic research and industrial application due to its attractive mechanical properties such as high ultimate tensile strength (850-1400 MPa), high fatigue strength (Ref 1), reasonable ductility (elongation 4-10%) (Ref 1-3), excellent wear resistance and fracture toughness (Ref 4, 5). The excellent combination of these several attractive properties of ADI is now finding more and more applications as structural components in various industries, such as automotive, agricultural, defense, rails and mining industries. Furthermore, ADI has economically replaced as-forged steels for many of its applications (Ref 6). ADI is produced from ductile iron (DI) casting with the help of the isothermal heat treatment process. The total heat treatment is conducted in two steps, i.e., austenitization and austempering. Austenitization is normally done at 815-927 °C for 1-2-h holding time and austempering at 260-400 °C for 1-4- h holding time (Ref 7, 8). The microstructure of heat-treated ADI consists of nodular graphite with bainitic ferrite and retained austenite base matrix. At lower austempering temper- ature the microstructure consists of needle-like (acicular) bainitic ferrite with high-carbon untransformed austenite, i.e., called retained austenite (Ref 9). With increasing austempering temperature, the shape of the bainitic ferrite (acicular) changes and it transforms to plate-like (feathery) shape. If the austem- pering time is too long, a second-stage reaction will start and high-carbon austenite transforms to carbide (e carbide) and ferrite which makes the material hard and brittle (Ref 9). The optimum microstructure and mechanical properties are achieved after completion of the first-stage reaction but before the second-stage reaction. The time period between the two stages of reactions is called process window. The mechanical properties and transformation kinetics of DI depend on various factors such as chemical composition, graphite nodularity, number of graphite nodules per unit area, size of the nodules, as well as the amount of ferrite present in addition to isothermal heat treatment and holding time (Ref 10). The graphite nodule count and its size distribution play a very important role in determining the final mechanical properties of ADI (Ref 11). A sufficient number of graphite nodules not only avoid the formation of carbides/cementite during solidification of the melt, but also influence the mechanical properties through changing the matrix structure of as-cast DI. The nodularity and number of nodules also influence the fatigue strength and become an important parameter in the characterization of the microstructure of DI (Ref 12). Nodularity in DI increases with T. Sarkar, T. K. Pal, and Akshay Kumar Pramanick, Welding Technology Centre, Jadavpur University, Kolkata 700032, India; Ajit Kumar Pramanick, Department of Forge Technology, NIFFT, Ranchi, Jharkhand 834003, India; S. K. Sahoo, Metallurgical and Materials Engineering Department, NIT Rourkela, Rourkela, Odisha 769008, India. Contact e-mail: tkpal.ju@gmail.com. JMEPEG ÓASM International https://doi.org/10.1007/s11665-019-03989-1 1059-9495/$19.00 Journal of Materials Engineering and Performance