Eddy Current Techniques to Detect Incipient Creep Damage of Stainless Steel Boiler Tubes

As creep damage degradation proceeds in 304, 321, and 347 grade stainless steel boiler tubes, the tubes develop a ferro-magnetic component depending on length of service. Incipient creep damage occurs inside the steel, involving precipitation inside the grains, with precipitates getting larger and forming mostly at or near grain boundaries as degradation continues. This incipient creep damage eventually leads further to the development of cavities at grain boundaries, which in turn lead eventually to microcracking and cracking. The amount of ferromagnetic component has been correlated (albeit in a relatively small number of exploited specimens) to the amount of incipient creep damage. The ferromagnetic component appears to be primarily associated with the formation of ferromagnetic hard oxide scale on the outer surface of the boiler tube. The ferromagnetic part of the scale has been identified, using x-rays, as magnetite. This ferromagnetic oxide surface can be easily inspected in power plants using eddy current techniques. Because of the correlation with incipient creep damage, the suggestion is that measurement of the amount of ferromagnetic component can be used to nondestructively monitor the development of incipient creep damage in austenitic steel at stages of development well before cavitation and microcracking. We have found that a processed signal can be extracted from eddy current measurements, which is directly related to the amount of ferromagnetic component. We have shown mathematically why the signal behaves as it does. This same signal has been simulated using finite element modeling (FEM). Its linear dependence on the amount of ferromagnetic component is verified experimentally, mathematically, and by FEM. In addition, we also demonstrate a way in which frequency dependence of the eddy current signal can be used to separate out effects of conductivity from effects of change in permeability due to the ferromagnetic component, thereby reducing effects of experimental error in evaluating the amount of ferromagnetic component at low amounts of degradation. In the present year, the technique is now being tested via measurements at several power plant sites.