Characterization of Creep Degradation of Modified 9Cr-1Mo Steel Using Magnetic Measurement

Microstructure evolution during the creep of a modified 9Cr-1Mo heat-resistant steel was characterized by a nondestructive method of magnetic hysteresis loop (MHL) technique at 893 K (620 degrees C). Two parameters of coercivity and remanence were introduced to evaluate the magnetic properties of interrupted crept specimens. Experimental results suggested that both the parameters were significantly sensitive to the microstructures, such as dislocations, precipitates, and microcracks. During the primary and secondary creep stage, the coercivity and remanence increased with creep time, which was mainly attributed to the increase in pinned dislocations and number density of precipitates. However, because of the coarsening of M23C6 carbides and presence of a demagnetizing field induced by the nonmagnetic precipitates and microcracks, the coercivity and remanence decreased during the tertiary creep stage. It suggested that the MHL technique has great potential to evaluate the microstructure evolution during creep degradation of heat-resistant steels.