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Zużycie korozyjno-mechaniczne połączeń śrubowych kolektora wytwornict pary

Tubielewicz K., Melechow R.

Problemy Eksploatacji

2000

nr 3

277-288

Przedstawiono wyniki badań wpływu smaru zawierającego dwusiarcze molibdenu na powstawanie pęknięć w otworach gwintowanych połączeń pokrywa--kołnierz kolektora wytwornicy pary elektrowni jądrowej.

The influence of molybdenum disulfide - a solid lubricant used to prevent seizure/galling in threaded connection of the steam generator collector of water cooled-water moderated nuclear reactor (WER - 440/213 NPP) -on cracking of 18Cr-10Ni type steel was investigated. It was found that transgranular cracking started from thread roots or from the hole bottom of steam generator flange madę of austenitic 08Khl8N10T steel. Autoclave testing of the unstressed specimes madę from of bottle filled alkaline solution with M0S2 at 300°C reveal that transgranular cracking even in the annealed metal after several days of exsposure. De-composition of molybdenum disulfide, that occurs in this process in high-temperature water with pHr > 7,3, leads to formation of such compounds (protons, HS~ and S2~ ions), which can promote hydrogen embrittlement of steel and adsorption of sulfur on bare metal surface of the advancing crack tip which increases anodic dissolution and inhibits formation of passive film. An extremely high concentration of residual hydrogen (50-100 ppm) in the metal closed to the crack walls was measured. The failure of a threaded steam generator collector of NPP was caused by: increasing of caustic concentration in thread haules due to negligible periodical leaking of primary water (treated with KOH) to the secon- dary circuit through gaskets between flagę and cover of the collector; realization of the active - passive transition in the environment with high pHT that cause dissolution of Fe and Ni, and hydrogen evolution due to these reactions; the compounds formed as a result of decomposition of the molybde num disulfide in water with high pHT at 300°C were caused by hydro gen embrittlement even in unstressed steel and by increased anodic dissolution metal in the tip of advancing craks due to adsorption of sulfur on bare metal surface of the advancing crack tip.