Electromagnetic detection and monitoring of creep induced damage in high temperature resistant steels

Monitoring and remaining life assessment of ferritic-martensitic alloys exposed to creep was addressed using electromagnetic evaluation. In order to determine the correlation between the creep damage and the change in magnetic properties, two steels were exposed to different extent of creep and magnetic properties were evaluated for each sample. A close evaluation of the creep damage was performed in each sample using optical microscopy, as well as SEM and TEM techniques. It was found that the microstructural changes occurring during the creep progress have a correlation with variations in the magnetic response at the different levels of creep damage. Saturation decreases as creep damage progress due to the increases of demagnetized sites. Remanence shows the characteristic behavior of isotropic materials and coercivity changes as a function of the progress of the creep damage. Even though this established correlation may be used to directly monitoring the creep damage evolution, a magnetically determined damage factor was defined using the relationship of the hysteretic Jiles-Atherton factors with the extent pf creep damage. On the base of existing Continuous Damage Mechanics (CDM) models for creep, a model has been proposed for the monitoring and assessment of creep damage using the described magnetic damage factor.