The Effect Of Prior Cold Work On Tensile Resfidual Stress Development In Nuclear Weldments

Austenitic alloy weldments in nuclear reactor systems
are susceptible to stress corrosion cracking (SCC) failures.
SCC has been observed for decades and continues to be a
primary maintenance concern for both pressurized water
and boiling water reactors. SCC can occur if the sum of
residual stress and applied stress exceeds a critical
threshold tensile stress. Residual stresses developed by
prior machining and welding can accelerate or retard SCC
depending on their sign and magnitude.
The residual stress, cold work and yield strength
distributions on the inside diameter of an Alloy 600 tube Jwelded
into a pressure vessel were determined by a
combination of x-ray diffraction (XRD) and mechanical
techniques. A new method was used to relate the XRD line
broadening to the percent cold work or true plastic strain in
the Alloy 600 tube. The accumulated cold work in the
layers deformed by prior machining, in combination with
the true stress-strain relationship for Alloy 600, was used to
determine the increase in yield strength as a result of
deformation due to machining and weld shrinkage. The
yield strength of the deformed layer was found to be well in
excess of the bulk yield for the alloy, and is therefore
capable of supporting residual stresses correspondingly
higher.
Tension as high as +700 MPa, exceeding the SCC
threshold stress, was observed in both the hoop and axial
directions on the inside diameter of the Alloy 600 tubing
adjacent to the weld heat affected zone (HAZ). The cold
worked high tensile zones correlated with the locations of
field SCC failures. The tensile residual stresses are shown
to result from a combination of the high cold working from
initial machining followed by weld shrinkage. The
development of surface tension during weld shrinkage has
been modeled using finite element methods, and the
benefits of minimizing or removing the cold worked layer
prior to welding are demonstrated. Further laboratory
studies showing the influence of prior cold working on the
formation of residual stresses following bulk plastic
deformation are presented.