Impact toughness of high-chromium martensitic steel with low nitrogen and high boron content
DOI:
https://doi.org/10.54708/26587572_2026_83265Keywords:
creep-resistant steels, heat treatment, impact toughness, structure, secondary phase particlesAbstract
9% Cr martensitic steels are regarded as advanced structural candidates for thermal power equipment operating under ultra‑supercritical steam conditions. The long‑term creep resistance of these materials is largely governed by the M₂₃C₆ carbide phase, whose thermal stability can be improved via boron microalloying. This work examines the influence of boron on nucleation and Ostwald ripening of M23С6 carbides in four experimental steel grades differing in Co, W, B, and N contents. Following standard heat treatment (austenitization and tempering), the specimens were subjected to creep at 650 °C for up to 16 000 h. Employing TEM, atom‑probe tomography, and thermodynamic simulations, we found that raising boron to 0.012 wt% promotes the formation of the M23(С,В)6 phase, which exhibits a 20% finer average particle size and a 1.5‑fold higher linear density along martensite lath boundaries relative to M23С6 carbides in low‑boron steels. The interfacial energy of the M23(С,В)6/ferrite boundary decreases to 0.12 J/m², indicative of coherent interface bonding. Nevertheless, despite this pronounced reduction in interfacial energy, the coarsening kinetics of M23(С,В)6 particles remain essentially the same as those of conventional M23С6 carbides, which is attributed to nickel and manganese segregation at particle/matrix interfaces. To fully exploit the beneficial effect of boron, the combined nickel‑plus‑manganese content must be decreased by at least a factor of two.References
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