Evolution of Microstructures During Austempering of Ductile Irons Alloyed with Manganese and Copper
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TRODUCTION
AUSTEMPERED ductile iron (ADI) has gained popularity because of its excellent combination of strength, ductility, and toughness,[1] commonly achieved by implementing proper austempering heat treatments to ductile irons. During the heat-treatment cycle, ADI undergoes a two-stage reaction process. In stage I, the austenite (c) decomposes into ferrite (a) and highcarbon-retained austenite (cR). The (a + cR) mixture is known as ausferrite, a micro-constituent responsible for a good balance in mechanical properties.[2,3] Yet, the high-carbon austenite (cR) being metastable transforms to ferrite and carbide in the stage II reaction at longer austempering times, and this may result in poor mechanical properties. The influence of this two-stage austempering process on mechanical properties has been extensively investigated.[1–6] Production of ADI requires alloying for large sections to improve austemperability or hardenability. The usual alloys are nickel, copper, or nickel plus copper in combination with molybdenum.[4] Manganese content is generally restricted to 0.25 wt pct max because of its tendency to RAJAN KUMAR DASGUPTA, Scientist (Retired), is with the Central Mechanical Engineering Research Institute, Durgapur 713209, India. DIPAK KUMAR MONDAL, Professor, Dean-Academics, and AJIT KUMAR CHAKRABARTI, Ministry of Steel Chair Professor (Retired), are with the Department of Metallurgical and Materials Engineering, National Institute of Technology, Durgapur 713209, India Contact e-mail: [email protected] Manuscript submitted July 13, 2012. Article published online November 9, 2012 1376—VOLUME 44A, MARCH 2013
segregate in the intercellular regions. However, the potent alloying interaction effects suggest that smaller additions of multiple alloying elements are more effective in improving austemperability than large additions of single alloying elements. The effect of alloying and heat-treatment parameters on the characteristics of ADI has been summarized in published articles[7–9] and books.[10,11] Investigations on Cu-Ni-Mo-alloyed ADIs are still continuing, and the results on austenitizing at 1133 K (860 C) followed by austempering at 593 K (320 C) are reported to have produced the best combination of structure and mechanical properties.[12] Investigators[9,13,14] have also studied the influence of austenitizing temperature and austempering time and temperature on the structure and properties of ADI alloyed singly with copper alone or combined with copper and nickel. In most of these studies, alloy designs are based on the traditional low-manganese concept because of its strong segregation to intercellular areas and its effect of delaying the Stage I reaction leading to closure of processing window meant for production of ausferrite structure and corresponding optimum mechanical properties. Later, some results on austemperability of high-manganese, copper plus manganese-alloyed ductile irons are published in the 1980s and 1990s,[15–17] but no further progress has taken place in this area. In a recent art
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