High operating steam pressure and localized overheating of a primary superheater tube

Intergranular stress corrosion cracking (IGSCC) has often been reported in hard or semi-hard tubes of commercially pure copper in cooling systems. Among the simultaneous conditions required for SCC, i.e., tensile stress, susceptible materials and corrosive environments, the influence of materials factor is focused. For simple pure metals with limited materials or metallurgical variables, substructures arising from cold work were targeted as the critical factors assisting IGSCC, which otherwise rarely occurs in pure annealed metals. Microstructural analysis has been conducted in detail on the same type of semi-hard U-bend tubes, where IGSCC have often been reported, using scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD), and transmission electron microscopy (TEM). The local residual strain at the grain boundaries arising from inhomogeneous dislocation accumulation was revealed by the backscattering electron image and kernel average misorientation (KAM) of EBSD at the locations where IGSCC were observed. It is considered that it may enhance the sensitivity of the grain boundaries to corrosive media and assist in initiating IGSCC.