On-line Fatigue Monitoring of Austenitic Stainless Steel Using a GMR-Sensor

Austenitic steel of the grade AISI 321 (German Grade 1.4541 - Ti-stabilized and 1.4550 - Nb-stabilized) is often used in power station and plant constructions. The evaluation of early fatigue damage and thus the remaining lifetime of austenitic steels is a task of enormous practical relevance. Metastable austenitic steel forms ferromagnetic martensite due to quasi-static and cyclic loading. This presupposes the exceeding of a threshold value of cumulated plastic strain. The amount of martensite as well as its magnetic properties should provide information about the fatigue damage. Fatigue experiments were carried out at different stress and strain levels at room temperature (RT) and at T = 300°C. The characterization methods included microscopic techniques such as light microscopy, REM, TEM and scanning acoustic microscopy (SAM) as well as magnetic methods, ultrasonic absorption, X-ray and neutron diffraction. Sufficient amounts of mechanical energy due to plastic deformation lead to phase transformation from fcc austenite without diffusion to tetragonal or bcc ferromagnetic a'-martensite. As the martensitic volume fractions are especially low for in-service-temperatures of about 300°C highly sensitive measuring systems are necessary. Besides systems on the basis of HT-SQUID (High Temperature Super Conducting Quantum Interference Devices) special emphasis was on the use of GMR-sensors (giant magnetoresistance) which have the strong advantage to be sensitive also for DC-magnetic fields without any need for cooling. In combination with an eddy-current transmitting coil and an universal eddy-current equipment as a receiver the GMR-sensors were used especially to on-line monitoring the fatigue experiments in the servo-hydraulic fatigue machine.