A new eddy current method for nondestructive testing of creep damage in austenitic boiler tubing

A new nondestructive eddy current technique is described for testing for incipient creep damage in austenitic steel boiler tubing in power plants. It is used because as incipient creep damage is formed, a magnetic oxide scale forms on the outside of the boiler tube in concentration proportional to the incipient creep damage; simultaneously in the base metal under the scale, a magnetic ferrite phase also forms in the grains and grain boundaries, which is in smaller concentration, but which is also in concentration proportional to the incipient creep damage. The eddy current signal can be processed in such a way that it varies monotonically and nearly linearly with the magnetic phase concentration and monotonically and nearly linearly with the incipient creep damage. Various aspects of the measurement are analysed and discussedfor example, liftoff, wall thickness, and diameter dependence. Using a zero point value to assess oxide layer permeability and dependence on conductivity is also discussed. Comparison is made between measurements and finite element modelling results.