The Role of Deformation in Coarsening of M 23 C 6 Carbide Particles in 9% Cr Steel
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CTURE, PHASE TRANSFORMATIONS, AND DIFFUSION
The Role of Deformation in Coarsening of M23C6 Carbide Particles in 9% Cr Steel E. S. Tkacheva, *, A. N. Belyakova, and R. O. Kaibysheva aBelgorod
State National Research University, Belgorod, 308015 Russia *e-mail: [email protected]
Received January 22, 2020; revised February 12, 2020; accepted February 13, 2020
Abstract—The kinetics of the growth of M23C6 carbide particles in 9% Cr martensitic steel with a high boron content is studied under conditions of long-term aging and creep. It is shown that the grain-boundary diffusion is a mechanism that controls the coarsening of these carbides during the aging. Deformation accelerates the coarsening of particles during creep, which can be explained by additional diffusion along dislocation lines. Keywords: thermal engineering steel, particle growth mechanisms, creep behavior, structure DOI: 10.1134/S0031918X20060162
INTRODUCTION An increase in the boron content with a simultaneous decrease in the nitrogen content makes it possible to increase the operating temperatures of high-chromium martensitic steels [1–8]. Using this approach to alloying, new thermal engineering steels MARBN, P93, and G115 for use as new-generation thermal engineering materials have been developed in Japan and China [3, 7]. It is known that the degradation of the initial lath structure of tempered troostite, which includes particle coarsening, depletion of the ferrite matrix by elements providing solid-solution strengthening, and a decrease in the density of intercrystalline boundaries due to the growth of the lath structure and its transformation into the subgrain structure are the reasons for a decrease in the strength of high-chromium martensitic steels under creep conditions. The contribution of the second phase particles to the creep strength is of particular importance, since they prevent the migration of lath/subgrain boundaries in addition to ensuring the dispersion strengthening, [1, 2, 5, 6, 8–11]. Kostka et al. [9] showed that the strengthening by second phase particles in martensitic steel leads to a decrease in the creep rate by more than two orders of magnitude. At present, dispersion hardening in high-chromium martensitic steels is ensured by several types of particles (M23C6, Fe2W, MX, Z-phase, etc.) with different morphologies, distributions, and growth rates [2, 5, 6, 9–12]. The introduction of MX carbonitride nanoparticles into 9% Cr steel made it possible to raise the operating temperature of these steels (P911 and P92) to 580–600°C [1, 10]. During the heat treatment, M23C6 carbide particles are released along the
boundaries of the prior-austenite grains (PAGs), packs, blocks, and laths [2, 5, 6, 11, 12] and characterized by the largest volume fraction among all types of particles. In high-chromium steels with a high content of boron, M23C6 carbide particles make it possible to raise the operating temperature to about 630°C, since they have a significant effect on the creep resistance. These particles play the role of obstacles
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