Structural and Magnetic Studies of an Ex-service Cast Austenitic Steel Tube Used in Hydrogen Reformers
Quintana, Oscar A.
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In the present work, a retired centrifugally cast austenitic tube that underwent different levels of creep degradation along its length, was studied. The tube life was shortened to about 5 years due to overheating. The exposure temperatures were in the range of 750 – 1150 °C; from the top to the center of the tube; where a bulge developed as a result of excessive heat. Selected samples were extracted taking in consideration the location and exposure temperature. Rough magnetic studies were initially done on the bulk material of each selected sample using a scale-magnet fixture; revealing a progressive increase in the magnetic response with the degree of degradation. Consequently, these samples were studied metallographically using several analytical techniques in order to identify changes in the matrix composition, new phase formation or carbide transformation that can impart any magnetic response. Furthermore, magnetic and micro-magnetic studies were carried out to correlate quantitatively and qualitatively these microstructural changes with magnetic behavior. Moreover, microhardness measurements were performed on the austenitic matrix of each selected sample to evaluate any change in mechanical properties caused by creep deterioration. Our goal is to find a relationship between magnetic behavior and changes in microstructural morphology, mechanical properties, and creep degradation at different exposure temperatures along the tube length. Ultimately develop a non-destructive method based on changes in the magnetic response of samples with different degree of creep deterioration; that allow us to anticipate or signal the onset of critical damage, and calculate the remaining life of service-exposed samples; so that proper maintenance schedules can be arranged
Magnetic force microscopy
magnetic etching with ferrofluids
remaining life calculation
Non destructive evaluation