Effect of Ti Addition on Carbide Modification and the Microscopic Simulation of Impact Toughness in High-Carbon Cr-V Too
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tool steels are used as structural components under severe working conditions, the relationship between chemical and microstructural parameters and mechanical properties, such as hardness, wear/fatigue-resistance, and toughness, has been of great interest.[1–8] For the case of tool steels containing high levels of carbon and carbide-forming elements, such as Cr, V, Mo, W, Nb, and Ti, significant amounts of primary carbides guarantee very high wear resistance, but make controlling toughness more difficult.[9–15] However, only the effect of microstructural variation on the toughness of tool steels has been frequently reported in literature.[16–18] The optimization of the microstructural parameters of carbides distributed within the steel matrix is important to achieve the best combination of the two competing properties of KI SUB CHO, Research Professor, SANG IL KIM and WON SUK CHOI, Students (Master Course), SUNG SOO PARK, Student (Ph.D. Course), and HOON KWON, Professor, are with the Center for Advanced Materials Technology (CAMT), Kookmin University, Seoul 136-702, Korea. Contact e-mail: [email protected] HEE KWON MOON, Researcher, is with the Human Resources Development Service of Korea, Ulsan 44538, Korea. Manuscript submitted January 8, 2015. Article published online November 3, 2015 26—VOLUME 47A, JANUARY 2016
hardness vs toughness. Thus, researchers have recently focused on improving toughness in wear-resistant tool steels with high volume fractions of carbides. In this study, significant amounts of Ti, one order of magnitude greater than that used for grain refinement, were added to high-C-Cr-V tool steel in order to modify coarse primary carbides, that is, M7C3 and MC. For the microscopic simulation of impact toughness from the viewpoint of carbide modification, we quantitatively analyzed cracked carbides in the modified steel under Vickers and Rockwell hardness indentation testing. Based on D7 tool steel (2.3C-0.4Si-0.4Mn-12.5Cr-1. 1Mo-4V), the experimental alloys were created by adding 0.2 and 0.5 wt pct. Ti; these were designated KD7 and K2D7, respectively. The alloys were prepared using vacuum induction melting. All specimens were austenitized at 1323 K (1050 C) for 1.5 hours and then oil-quenched. The quenched samples were tempered at 423 K (150 C) for 4 hours, which is normally performed to improve wear resistance, and then at 783 K (510 C) for 4 hours, in order to increase toughness. All tempering treatments were carried out by multi-step tempering; 2 hours + 2 hours, for the purpose of minimizing retained austenite (RA). The primary carbides as the main microstructural factor were investigated using optical microscopy and scanning electron microscopy (SEM: JEOL 7401f), and the corresponding results were characterized by energy dispersive X-ray analysis (EDX). Microstructural analysis was performed using back-scattered electron SEM images (SEM-BSE) coupled with Image-Pro Plus software. Two important aspects of the carbide particles are their mean diameter and roundness, evaluated using the following expr
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