Abstract
The stainless steel connecting shafts of digester discharge screws of a paper production plant failed within a relatively short period after the production start of the plant. To investigate the reasons for these failures, stress analysis and material characterization of the broken shafts were performed. The test program included tensile, impact and hardness tests and metallographical and fractographical investigations. Fractographical examination verified that crack growth due to fatigue would have not been a contributing damage mechanism. Existence of chromium carbides together with a martensitic structure in the welded regions between the shaft body and its blades had provided potential zones for crack initiation. Stress corrosion cracking (SCC) was judged as the main damage mechanism in the failure of the shafts. The high temperature steam containing aggressive chemical species caused SCC. Thus, the material selected for the DDS shaft is not proper with respect to resistance to the corrosive atmosphere in the digester housing.
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