Plastic Flow Behavior of an Advanced 9Cr Martensitic Heat‐Resistant Steel at High Temperatures

Abstract

Advanced 9Cr–1.5Mo–1Co–VNbBN martensitic steel is used for thick castings under ultra‐supercritical conditions. The microstructure, including martensitic laths, dislocations, and precipitations, is observed before and after deformation by transmission electron microscope. Steady‐state flow behavior and deformation mechanisms are investigated by analyzing strain‐rate jump test data at 600–650 °C. Results show that the plastic flow behavior of 9Cr steel is well characterized by a stress power law whose apparent stress exponent n is between 20.5 and 27.0 for the three test temperatures. The apparent activation energy of 621 kJ mol is estimated for the plastic flow of this steel, and the stress exponent and apparent activation energy are particularly high. The analysis incorporates the effect of a threshold stress, which is associated with the pinning effect of particles on dislocations. The truly modified stress exponent n0 is 4.6 at all test temperatures after removing the threshold stress, which is close to that of dislocation‐climb deformation, indicating the deformation of this steel can be described as a lattice diffusion‐controlled dislocation‐climb mechanism. Transmission electron microscopy observation is conducted to characterize the interaction of dislocation–precipitate to provide structural evidence for specific deformation mechanisms.