Role of core–shell boron distribution in the long-term stability of M23C6 precipitates in modified 9Cr–1Mo steel

Boron (B) improves the creep resistance of ferritic–martensitic steels by suppressing coarsening of M 23 C 6 precipitates, though the mechanism remains unresolved. Using 3D atom probe tomography on tempered and aged modified 9Cr–1Mo steel, we report, for the first time, a core–shell B distribution within M 23 C 6 , with a B-enriched core, arising from early B capture during nucleation followed by growth in a B-lean matrix. TC-PRISMA simulations using an interfacial energy of 0.25 J.m⁻ 2 capture early-stage growth but overpredict coarsening at longer aging times at 650 °C; a lower value (0.1 J.m⁻ 2 ) matches long-term data better, suggesting interfacial energy decreases as coarsening progresses. This is attributed to the approaching interface of a small dissolving precipitate nearing the B-rich core, known to lower interfacial energy, slowing dissolution and coarsening kinetics at long ageing times. This mechanism offers a possible explanation for the exceptional long-term stability of M 23 C 6 in B-containing ferritic steels.