Screen Bars Destroyed by Intergranular Corrosion
Fragments of screen bars which as structural elements of a condenser had come into contact with cooling water from the mouth of a river were received. The screen bars were made of stainless austenitic Cr-Ni-Mo steel X 5 Cr-Ni-Mo18 10 (Material No. 1.4401). The bars were fractured repeatedly. The ruptures did not occur exclusively or even preferentially at the loops, but just as frequently at locations between them. The mistake made in this case was annealing the steel at a temperature in the critical region. This was probably done to relieve stresses that originated during cold-forming and led to damage by stress corrosion. This would have been the correct method for a ferritic steel, but not austenitic steel, which requires the special heat treatment indicated. When an anneal in the critical region is unavoidable and the indicated additional treatment is impossible or difficult, a type of steel has to be chosen which is resistant to intergranular corrosion.