Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels

Abstract

Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels has been studied using model steels with the initial microstructures consisting of the various combination of α″ MX and M23C6 to clarify the role of each precipitate on creep deformation and the resultant creep strength of the steels. A carbon free steel strengthened by the α″ phase has exhibited high creep resistance at the initial stage of transient creep region but a quick transition to the acceleration creep at a small strain, due to less precipitation along boundaries such as lath, block, packet and the prior austenite grain boundary. Precipitation of M23C6 along these boundaries has been found to delay the transition to the acceleration creep, which gives a larger offset creep strain. Fine dispersion of MX has reduced much the creep rate in the transient creep region but enhances the heterogeneous creep deformation in the acceleration creep region. It is thus concluded that an optimum microstructure consists of fine dispersion of the α″ phase and MX carbonitride inside lath grain decorated with M23C6 along lath, block, packet and the prior austenite grain boundary.