Micrograin structure evolution associated with grain boundary characteristics in grade 91 steel during long-term creep exposure

Abstract

Micrograin structure evolution in ASME Grade 91 steel after 233,823 h of creep exposure at 600 °C was characterized after grain boundary classification in terms of misorientation angle (MOA), boundary energy, and lath martensite substructure. Under long-term creep with an applied stress of 5.9 MPa, micrograin coarsening occurs by annihilation of lath boundaries, consisting of hexagonal dislocation networks (HDN), thanks to the “knitting-out” of dislocations from the boundary, and the absorption of interior mobile dislocations into the boundary. This annihilation is expectedly faster when the original lath boundary shows a small MOA, and is decorated by little precipitation (MX carbonitrides and boundary M23C6 carbides). Under long-term creep with an applied stress of 86.7 MPa, micrograin coarsening occurs not only by lath boundary annihilation, but also by the migration of all other boundary types found in the lath martensite microstructure. This migration is described as a strain-induced boundary migration (SIBM), and mainly occurs when boundaries are relatively precipitation-free, and a gradient of stored internal energy is present between adjacent grains. Even within the same class of boundary, the progress of SIBM differs due to the local evolution of precipitates and the actual boundary energy.