Revealing the difference of precipitation kinetics between TiC and VC in low-carbon tempered martensitic steels
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Revealing the difference of precipitation kinetics between TiC and VC in low-carbon tempered martensitic steels Xinjun Sun1,*, Junyu Kang1, and Qilong Yong1 1
Department of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China
Received: 25 May 2020
ABSTRACT
Accepted: 16 August 2020
The precipitation kinetics of TiC and VC during high-temperature tempering of low-carbon martensitic steels was studied by means of hardness measurement and theoretical analysis. The precipitation–temperature–time diagram, which was determined through the hardness versus tempering time curves, shows that the precipitation rate of TiC is more rapid than that of VC at 600 °C or above, while they are almost the same at 550 °C. The number density of TiC precipitates is larger than that of VC precipitates, thus leading to a larger secondary hardening effect. An analytical model to describe the precipitation kinetics of microalloying carbides during tempering of martensitic steel was presented. Using this model, the difference of precipitation kinetics between TiC and VC can be well explained, in terms of the shape and relative position of kinetics curves and the nose temperature.
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Introduction Precipitation of microalloy carbides like NbC, VC and TiC is a process of fundamental importance in the production of microalloyed steels. Therefore, extensive research has been conducted during the last half century on the precipitation of microalloy carbides [1–8]. It has been well documented that this precipitation process can be roughly divided into two types [1]. One is the precipitation in austenite, including static precipitation and strain-induced
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https://doi.org/10.1007/s10853-020-05176-3
precipitation in deformed austenite; the other is the precipitation in body-centered cubic Fe (a-Fe), which includes the interface precipitation formed at austenite/ferrite boundaries during austenite-to-ferrite transformation as well as the random precipitation in supersaturated a-Fe after ferrite transformation or during tempering of bainite or martensite. The strain-induced precipitation can effectively retard the recovery and recrystallization of deformed austenite, thus favoring the refinement of final microstructure of steels [8–10], but its precipitation strengthening effect is relatively weak due to
J Mater Sci
the coarsening of particles at high temperatures [11]. In contrast, the precipitation in a-Fe can give rise to a much greater strengthening effect since the number density of precipitates is extremely high due to its lower precipitation temperature [12, 13]. Compared with the grain refinement strengthening, the precipitation strengthening has more potential to improve the strength of steels without deteriorating the plasticity significantly [14–20]. Besides, fine particles that precipitated in a-Fe can act as irreversible hydrogen traps, thus rem
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