Fractographic and Microstructural Analysis of Fatigue Specimens of A302 Grade B Steel Tested in Air at Room Temperature

摘要

Microscopic studies were undertaken to characterize the failure processes and to establish correlations with fatigue crack propagation data of Type A302-B ferritic steel tested in air at room temperature. A cooperative program was established between the Naval Research Laboratory (NRL) and the Centro Informazioni Studi Esperienze (CISE) to investigate the fatigue properties of this ferritic steel for nuclear pressure vessel applications. Failed specimens have been examined by scanning electron microscopy (SEM), by metallographic techniques, and by energy dispersive x-ray microanalysis. SEM examination of the fracture surfaces of specimens cut in the T-L direction revealed the presence of inclusion bands above an observed delta K threshold value. Subsequent x-ray image scans and corresponding energy spectra showed that the bands were manganese-sulfide inclusions. The inclusions bands resulted in increased crack growth rate in these specimens compared to those cut in the L-T orientation specimens. The delta K threshold value necessary to activate the inclusion bands is thought to be a measure of the cohesive strength between the inclusions and the material matrix. The failure mode of all the specimens tested was transgranular; extensive microcracking was also present in the fracture surfaces. The microcracking did not appear to have affected the crack propagation behavior. (Author)