Fretting

At my work,
During the disasembly of the turbine casing bolts, with a hydraulic bolt loosening tool, it can occure that the bolt and nut start fretting. Sometimes the hydraulic tool is not capable to loose them with a pressure of 700 B (10.000 psi) Then we have to make a cut-out on the nut with a grinding machine to release the tension.
On the picture the result how frettting looks like on a 1 1/2" bolt

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