Effect of Austempering Conditions on the Microstructure and Tensile Properties of Low Alloyed Sintered Steel
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SELECTION of a proper heat treatment is a key factor in research on iron-based alloys with the aim of a desired cost and performance. Austempering (also known as bainitic quenching) is a highly promising option because it ensures the formation of a 100 pct bainitic microstructure, which leads to a steel with balanced strength and toughness. In many applications, this treatment can replace the quenching and tempering steps, resulting in improved toughness and ductility while maintaining high values of hardness with less dimensional distortion, residual stresses, and the usual risk of cracking after quenching and tempering.[1] The process consists of austenitizing the sample (to completely dissolve the carbides, usually between 1063 and 1198 K (790 and 925 C)) followed by rapid cooling in a salt bath at a temperature that is above the martensite start (Ms) temperature of the processed steel. This is done to avoid the formation of proeutectoid ferrite and pearlite during the isothermal process. The steel is then maintained at this temperature for sufficient time to allow for full transformation of the austenite into bainite. The steel is then cooled to room temperature at a slow rate. Mechanical components of ductile M. CAMPOS, Assistant Professor, J. SICRE-ARTALEJO, Postdoctoral Researcher, and J.J. MUN˜OZ, Ph.D. Student, are with the Department of Materials Engineering, IAAB, Universidad Carlos III de Madrid, Avda. Universidad 30, E-28911 Legane´s, Madrid, Spain. Contact e-mail: [email protected] J.M. TORRALBA, Full Professor, is with Department of Materials Engineering, IAAB, Universidad Carlos III de Madrid, and Deputy Director, with Institute IMDEA Materials, C/ Profesor Aranguren s/n, 28040 Madrid, Spain. Manuscript submitted July 22, 2009. Article published online April 27, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
cast iron and TRIP steel with high manganese have been austempered for decades, but sintered parts exhibit unique complications associated with heat transfer and corrosion.[2–6] Both are a function of material density and, more precisely, effective extension of the exchange surface with consideration of porosity. In sintered steel, the presence of open porosity limits the application of this method when salts are used to perform the isothermal treatment. The salts can corrode both at the surface and inside the pore network. Currently, thanks to advanced uniaxial compaction methods that allow high green densities, parts with densities over 93 pct of the theoretical value can be obtained.[2,7–10] This level of density produces a close porosity at the surface, as was demonstrated in References 11 and 12. In those cases, bainitic quenching is highly recommended and attractive for industrial applications. The present study was undertaken to understand the influence of austempering on the nature of the resulting microstructures and its influence on the tensile properties of alloyed sintered steels. Thus, alloying systems that can promote the bainitic transformation with a proper cooling rate from a
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