Trace Element Sn Segregation in Cu-rich Precipitates during Thermal Ageing of Pressure Vessel Steels

Two SA533B type steel weld specimens with different Ni concentrations have been investigated during long-term thermal ageing up to 49800 hrs at three temperatures (330degC, 365degC and 405degC). Although Sn as trace element is only present in the steels at a level of about 0.007 at%, it has been observed to segregate to Cu clusters at significant levels (up to 2 at%) in the low Ni steel. In contrast, no clear Sn segregation to Cu-rich clusters was observed in high Ni steel. A combination of TEM (transmission electron microscopy) and 3DAP (three-dimensional atom probe) characterization confirmed that strong segregation of Sn not only occurs to Cu-rich precipitates on dislocations but also to Cu-rich precipitates in the matrix of the low Ni steel. The level of segregation was found to be higher at higher ageing temperatures, but showed no variation with different ageing time