Characterization and correlation of mechanical, microstructural and ultrasonic properties of power plant steel

In the present work 912% Cr tempered martensitic steel which is normally used in gas turbine-shrouds in electricity generation power plants was characterized through mechanical, microstructural and ultrasonic testing. The steel tested was in the as-received (virgin), artificially aged at 700 °C for three different periods of time namely 336, 840 and 1344 h, and retired from real operating conditions. The mechanical and microstructural test results were assessed and correlated with ultrasonic testing parameters. Artificial aging caused carbides to dissolute within the martensite-ferrite matrix and to coarsen and disintegrate at the prior austenite grain boundaries. Aging reduced the percentage of low angle grain boundaries and increased the average misorientation angle, both of which indicate the occurrence of static recrystallization. Energy dispersive spectroscopy elemental analysis identified that the dominant carbide types are Cr23C6 and VC, in which the former was mainly located at the prior austenite grain boundaries whereas, the latter was spread within the entire microstructure. Softening and the consequent loss in mechanical properties were detected as a function of prolonged aging times due to the accompanying microstructural phases and carbide dissolution which in turn were correlated with ultrasonic velocity and attenuation. The retired condition had the lowest strength, highest sound velocity and almost similar attenuation as that of the virgin which possessed highest strength and lowest sound velocity.