Precipitation Kinetics of M 2 C Carbide in Severely Ausformed 13Co-8Ni Secondary Hardening Steels
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SECONDARY hardening alloy steels for aircraft, aerospace, and advanced structural applications have been regarded as significant structural materials by many researchers, thanks to their exceptional mechanical properties.[1–7] The superior performance traits of these steels—for example, high fracture toughness at ultrahigh strength levels of about a 2 GPa grade—arise as a result of secondary hardening reactions, normally due to nanoscale M2C carbides precipitating during the aging process. Initial studies of the secondary hardening reaction revealed that Co and Ni additions into Fe-(M)-C systems, where M represents one or more carbide formers (such as Mo, W, V, Cr, etc.), resulted in a higher rate of M2C nucleation and a corresponding refinement of the particles. These studies demonstrated that optimized compositional modification can enhance an alloy’s mechanical properties.[2,8–10] In addition to the alloying effects, the strengthening mechanism and carbide precipitation behavior, with particular emphasis on the M2C composition and its crystal structure, have been studied by Olson et al.[1] This study was the basis for the development of high-performance structural materials such as AF1410, AerMet, and Ferium.[11,12] As another strengthening mechanism for the enhancement of alloy properties, an ausforming process through thermo-mechanically controlled processing (TMCP) in the austenite region has become well established as a means for achieving high strength without sacrificing KI SUB CHO, Research Professor, SUNG SOO PARK, Graduate Student, and HOON KWON, Professor, are with the Center for Advanced Materials and Technology (CAMT), Kookmin University, Seoul 136-702, Korea. Contact e-mail: [email protected] HONG KYU KIM and YOUNG BEUM SONG, Researchers, are with the Agency for Defense Development (ADD), Daejeon, Korea. Manuscript submitted October 12, 2014. Article published online 21 January 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
toughness.[13–15] Although many studies have already been performed on high Fe-Ni(-C) steels having subzero martensite transition temperatures—where deformed austenite can be investigated—there were few studies of the ausforming process in secondary hardening steels, despite the fact that some beneficial effects (fineness of martensite packets and block sizes) can be expected. Moreover, the fundamental knowledge of the precipitation reaction for M2C carbides is thus far insufficient compared with that of commercial steels. In the current study, a differential scanning calorimetry (DSC) experiment, coupled with the well-known analytical description of transformation kinetics according to the Johnson–Mehl–Avrami (JMA) equation,[16–20] was performed to analyze the effects of the ausforming process on M2C precipitation behavior. Considering the variation of kinetic parameters including activation energy (E), the pre-exponential factor (k0), and the JMA exponent (n) with respect to the fraction of transformed M2C, we simulated the precipitation kinetics of M2C carbides under isotherm
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