PWSCC of thermally treated alloy 600 pulled from a Korean plant Original Research Article

Three tubes of alloy 600 were pulled out from a Korean nuclear power plant. The microstructure was analyzed using an optical microscope and TEM. Information on the crack length and depth was obtained by metallography, and crack detection and evolution were evaluated by analyzing the eddy current data obtained from each in-service-inspection (ISI). The carbon content in the pulled tubes was higher (around 0.03 wt.%) than that (around 0.015 wt.%) of Alloy 600 tubing used in other operating nuclear power plants. Most carbides in the pulled tubing were distributed in the grains rather than along the grain boundaries. The poor microstructure might come from high carbon contents, low temperature annealing, or high residual stresses during tube straightening. Mill annealing temperature should be high enough to dissolve all carbon in order to decorate the grain boundaries with semi-continuous carbide precipitation during 700 °C thermal treatment. Shot peening seemed to suppress the growth of the axial cracks, while it was analyzed to play a role in increasing crack growth in the wall thickness direction.