Fatigue Characterization of AISI 321 Austenitic Steel by Means of HTC-SQUID

For AISI 321 austenicti steel the starting point of microcracks leading to macrocracks and finally to failure has to be expected at the locations of highest martensite concentrations. Information about the ferromagnetic martensitic phase fraction gives access to a possible way to determine the state of early fatigue damage of the material and reveaqls the risk for local failure by cracking. For realistic service temperatures of about 300 deg C, the fraction of martensite even up to failure is rathe r low, making highly sensitive maesuring techniques such as SQUIDs compulsory. Having shown the general potential of SQUIDs to characterize fatigue damage in austenitic steels future work will focus on the development of a testing system for measurements of in service components. This testing system eill not only focus on the measurement of the remanent magnetic field but also on dynamic measuring paramters such as the incremental permeability. This development will be based on the results o n the use of HTSL-SQUID-systems in eddy current testing.