Effect of Aluminum Content on the Microstructure and Mechanical Properties of Hypereutectoid Steels
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HYPEREUTECTOID pearlitic steels are of importance in structural applications such as high strength wires and T-rails.[1,2] Typically, these steels form grain boundary cementite networks that are detrimental to mechanical properties. Alloying with silicon has long been known[3] to retard the formation of grain boundary cementite. Heckel and Paxton[3] showed that the precipitation behavior of cementite allotriomorph is a function of the silicon content. They suggested that the growth of grain boundary cementite was limited by silicon removal from austenite grain boundary because of the negligible solubility of silicon in cementite. Copper,[4] cobalt,[5] and aluminum[6] are the other alloying elements that have negligible solubility in cementite, similar to silicon, and have been investigated for their effect on grain boundary cementite precipitation. Kanetsuki et al.[5] showed that cobalt addition did suppress the formation of grain boundary cementite and the effective amount of cobalt was higher for steels with higher carbon content. Furthermore, cobalt addition was effective only when the steels were subjected to the patenting treatment. Cobalt was not effective in the YOON SOO JANG, Master’s Student, is with the Department of Materials Science and Engineering, Korea University, and the Materials Science and Technology Research Division, Korea Institute of Science and Technology. MOO-YOUNG HUH, Professor, is with the Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea. M.P. PHANIRAJ, Visiting Scientist, and DONG-IK KIM and JAE-HYEOK SHIM, Senior Research Scientists, are with the Materials Science and Technology Research Division, Korea Institute of Science and Technology, Seoul 136-791, Korea. Contact e-mail: [email protected] Manuscript submitted April 1, 2009. Article published online May 7, 2010 2078—VOLUME 41A, AUGUST 2010
air-cooled specimens. Studies[4] on the effect of copper content on ultra-high-carbon steels (UHCSs) showed that the addition of up to 4.9 wt pct copper is not effective in suppressing the formation of grain boundary cementite. Lesuer et al.[6] reported the effect of 1.6 wt pct Al on the microstructure of hot-extruded and air-cooled UHCSs. They found that aluminum inhibited the formation of grain boundary cementite at low extrusion temperature. The microstructural features of pearlite, viz. interlamellar spacing, nodule size, colony size, and cementite width, decide the strength and ductility of hypereutectoid steels.[7–11] The alloying additions used for grain boundary cementite control may affect these features. The interlamellar spacing is reduced by alloying with silicon[12] but remains unchanged with cobalt additions.[5] The microalloying element vanadium that has been used[13] in combination with silicon to facilitate the fragmentation of grain boundary cementite also refines the pearlite structure.[14,15] Leseur[6] compared the additions to UHCS of 1.2 wt pct Si with that of 1.6 wt pct Al and found that the interlamellar spacing was finer in the lat
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