Influence of Austempering Temperature and Time on the Microstructure and Mechanical Properties of Ductile Iron Weldment

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JMEPEG https://doi.org/10.1007/s11665-019-03989-1

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), mechanical 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 T. Sarkar, T. K. Pal, and Akshay Kumar Pramanick, Welding Technology Centre, Jadavpur University, Kolkata 700032, India; Ajit Kumar Pramanick, Department of For

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