Effect of Titanium and Niobium on Modifying the Microstructure of Cast K100 Tool Steel
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CASTING is one of the major production processes and is widely used to manufacture near net shape as well as semi-finished products. In cast materials, microstructure controls the mechanical properties, thereby playing an important role in the service performance. The cast microstructure also affects the response of material to the complementary processing steps such as hot working.[1–4] The mechanical properties of a cast material originate from two sets of parameters: first, those dependent on the material specifications such as chemical composition and intrinsic mechanical properties and second, those dependent on the casting process. This is, therefore, clear that achieving good mechanical properties is possible through controlling the microstructure. In the past decades, improving the mechanical attributes of casting alloys by controlling the analysis, heat treatment, or even thermomechanical processing has been the topic of many studies.[5–10] Most of these researches have been focused on different types of tool steels. Due to the fact that tool steels are high-alloyed materials and naturally hard to work even at high MASOUD MIRZAEE, Academic Staff, is with the Department of materials, Mashhad Branch, Islamic Azad University, Mashhad, Iran. Contact e-mail: [email protected] AMIR MOMENI, Academic Staff, is with the Materials Science and Engineering Department, Hamedan University of Technology, Hamedan, Iran. HAMID KESHMIRI, Senior Engineer, is with the Esfarayen Steel Complex, Esfarayen, Iran. REZA RAZAVINEJAD, Senior Engineer, is with the Niroo Pajooh Shargh (nps.co), Mashhad, Iran. Manuscript submitted March 6, 2014. Article published online August 26, 2014. 2304—VOLUME 45B, DECEMBER 2014
temperatures, researchers have been interested in modifying the cast microstructures to produce cast-to-shape products.[11–13] Modification in the microstructure of cast tools opens up extensive possibilities for the effective improvement of the mechanical properties of tool steels. The most important aim of this modification is to break up the coarse network of interdendritic eutectic carbides or spheroidizing them as well as achieving a refined microstructure of austenite before quenching. Although the effect of heat treatments on modifying the microstructure of cast steels has been studied before,[7] the effect of chemical composition has not been well documented. Different carbides of types MC, M2C, M6C, M7C3, and M23C6 (M refers to all carbide formers such as Mo, Cr, Ti, Nb, Fe, Mn, and so on) may form in tool steels depending on the chemical composition[14,15] Great efforts have been made, in this way, to improve the morphology and distribution of the eutectic carbides[11] It is well known that coarse eutectic carbides are undesirable to the toughness of these materials because they may cause the nucleation of microcracks.[16] In addition, minimizing the microsegregation of alloying elements has been another important aim that necessitates the modification of casting products. The application of Nb, Ti, and Zr as alloying elements i
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