Effects of Copper and Malleablizing Time on Mechanical Properties of Austempered Malleable Iron
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UCTION
IT is well known that white iron is generally heat treated to produce malleable iron, which is an important engineering material with desirable properties of machinability, toughness, and ductility for certain applications such as universal joint yokes, railroad car hardware, gear cases, and housings.[1–4] The heat treatment called malleablization is performed, which is a long annealing process at elevated temperature for about 40 to 100 hours to obtain graphite and ferrite or pearlite constituents in the matrix.[4–6] The annealing treatment mainly involves two important graphitizing steps.[5] The first-step graphitization (FSG) causes the nucleation of graphite initiated during heating to a high holding temperature ranging from 900 °C to 970 °C. During FSG, massive eutectic carbides can theoretically be eliminated from the iron structure to form irregularly shaped graphite. After FSG, the iron is rapidly cooled to the temperature range of 725 °C to 740 °C to undergo the second-stage graphitization. After this step, a completely ferritic or pearlitic matrix is obtained when the cooling rate is suitably controlled. However, malleablization is an inefficient process, because it requires high-temperature treatment and very long processing. With the development of austempered ductile iron in recent years, many researchers[7–10] concluded that ductile iron after austempering heat treatment significantly improved its mechanical properties, including strength, hardness, toughness, wear, and fatigue resistance. For the other irons such as gray irons and CHENG-HSUN HSU and FAN-SHIONG CHEN, Professors, and JUNG-KAI LU, Graduate Student, are with the Department of Materials Engineering, Tatung University, Taipei 10451, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted February 5, 2007. Article published online August 28, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
compact graphite irons, austempering treatment has also been performed and yielded analogous results.[11,12] Furthermore, some studies[12–14] have suggested that an alloying addition such as copper, nickel, or molybdenum can promote the hardenability and effectiveness of the austempering process. In this study, white irons with and without copper alloying were subjected to a duplex heat-treating process of malleablizing and austempering to obtain austempered malleable iron (AMI). The effects of copper and processing variables on microstructural and mechanical properties of AMI were also investigated.
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EXPERIMENTAL PROCEDURES
A. Material and Heat Treatment White iron in Y-block castings of 30-mm thickness was produced by regular foundry practice. The irons were divided into two types, the unalloyed iron (iron A) and the 1 wt pct copper alloyed iron (iron B). The copper element in the form of copper wire (99.99 pct) was added to the pouring ladle and fully stirred; then, it was poured into green sand molds. A duplex-heating process was designed to treat these irons as AMI in this work, as shown in Figure 1. It mainly consiste
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