Influence of Complex Alloying Modifications and Carbide/Boride Evolution on Creep Resistance in B‐Doped 12% Cr Steels

Abstract

This study examines the effect of complex alloying on the creep resistance of two high‐B, low‐N 12% Cr steels with similar alloying. The Ta‐modified 12CrNbTa steel exhibits a threefold increase in rupture time at 650 °C compared to the 12CrNb reference. Lath/subgrain growth follows a power law D ∼ t 1/ N with N = 2 for 12CrNb and N = 3 for 12CrNbTa, indicating a transition from curvature‐controlled to pinning‐controlled boundary migration. Retarded microstructural degradation in 12CrNbTa is attributed to slower coarsening of M 23 (C,B) 6 carbides: its coarsening rate constant is three times lower, and a high fraction of fine particles (<50 nm) is retained at lath boundaries even after ~30,000 h. Interfacial energy calculations show carbides in 12CrNbTa are effectively B‐enriched ( γ = 0.1 J m −2 ), whereas those in 12CrNb exhibit higher energy ( γ = 0.3 J m −2 ) due to limited boron availability, likely because of coarse W 2 B boride formation. The finer W 2 B dispersion in 12CrNbTa is attributed to an indirect effect of Ta on boride coarsening during preliminary heat treatment. These findings demonstrate that creep resistance in B‐doped 12% Cr steels critically depends on M 23 (C,B) 6 carbide stability, governed by complex interactions among Ta, Nb, Co, and minor impurities such as Si, Mn, and Ni.