Long-term creep mechanisms and microstructural stability of boron- and nitrogen-modified 9Cr-2 W ferritic/martensitic steels

The long-term creep behavior and microstructural stability of 9Cr–2 W ferritic/martensitic steels FC92B and FC92N, modified with boron and nitrogen, were systematically compared. Thermal creep tests for up to 20,000 h at 923 K and 32 MPa, along with electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM) analyses, revealed distinct evolution mechanisms in each alloy. FC92B demonstrated superior creep resistance due to the suppression of M 23 C 6 carbide coarsening and the stabilization of prior austenite grain boundaries by boron, retaining approximately 82% of its initial geometrically necessary dislocation (GND) density after 20,000 h. In contrast, FC92N retained only approximately 48% of its GND density because of the accelerated coarsening of M 23 C 6 carbides and the formation of the Laves phase, along with limited dislocation pinning by MX carbonitrides, which promoted rapid recovery and lath coarsening. These findings illustrate that the varying creep resistance of the two alloys originated from the different precipitate coarsening kinetics that influence dislocation mobility. The results highlight the crucial role of trace boron in enhancing the long-term creep stability, offering valuable insights for the alloy design of advanced ferritic/martensitic steels.