Inclusion-involved fatigue cracking in high temperature water

The nonmetallic inclusions in low-alloy pressure vessel steel A533B were carefully examined and the low cycle fatigue (LCF) behavior of the steel was investigated in 288 degrees C air and water. Much attention was paid to the roles of inclusions on fatigue crack initiation and propagation in high temperature water. Three types of inclusions were observed in the steel, consisting of isolated or clustered sulfide inclusions, duplex oxide-sulfide inclusions and isolated oxide inclusions. In high temperature air, fatigue cracks initiated predominantly from subsurface inclusions. In high temperature water, however, fatigue cracks initiated primarily at corrosion pits on the specimen surfaces resulted mainly from the dissolution of large or clustered sulfide inclusions. The subsurface and bulk inclusions also contributed to the fatigue cracking in high temperature water. Possible influence of the above three types of inclusions on environmentally assisted cracking (EAC) was evaluated. The fatigue fractographic features suggested a synergism between sulfide inclusions and hydrogen-induced cracking dominate the present EAC in high temperature water.