The-validity of a recently proposed model for phenomena contributing to environmentally assisted cracking of:Alloy 600 is discussed on the basis of theoretical calculations and experimental as well as field data. The model is based-on an assumption that the crack tip environment consists of steam hydrogen bubbles, i.e., similar to the environment in hydrogen doped superheated steam tests. Hydrogen is produced by electrochemical and chemical reactions inside the crack.
The crack growth rate of Alloy 600 is presumed to be dependent on hydrogen diffusion into the material ahead of the crack tip along grain boundaries. This is influenced by steam hydrogen bubble formation along the crack tip and by the partial pressure of gaseous hydrogen at the crack tip.
Crack growth rate tests were performed in simulated pressurized water reactor environment on mill annealed Alloy 600. The applied stress intensity K-max was 40 MPa root m the load ratios R were 0.5, 0.75 and 0.9 and:the frequency was 0.05 Hz. The load ratio of 0.75 and 0.9 with:the frequency of 0.05 Hz produced mainly intergranular cracking. With the load ratio 0.5 and in one case with the load ratio 0.9, transgranular cracking was observed. The different cracking mode at load ratio 0.9 may be explained by the change of the thermodynamic conditions in-the crack tip, i.e., intergranular: cracking is enhanced when the crack tip environment contains steam hydrogen bubbles.
A visual observation of the fracture surface showed a transition line at a distance from the crack mouth which indicated that there was an environmentally dependent gradient inside the crack during the tests.
Experience of intergranular environmentally assisted cracking cracks in Ringhals nuclear power plants has shown that cracking occurs preferably in positions, where thermal loads resulting in alternating stresses are present or in positions with cold worked material or material which has fielded locally. These cracks have occurred in both water and steam hydrogen bulk environments. Steam hydrogen gas exists, e.g., at the top of the pressurizers in PWR plants. This cracking is in agreement with the low frequency fatigue test results where intergranular cracking also occurred.
|Title of host publication||PROCEEDINGS OF THE EIGHTH INTERNATIONAL SYMPOSIUM ON ENVIRONMENTAL DEGRADATION OF MATERIALS IN NUCLEAR POWER SYSTEMS - WATER REACTORS, VOLS 1 AND 2|
|Publisher||AMERICAN NUCLEAR SOCIETY|
|Number of pages||4|
|Publication status||Published - 1997|
|MoE publication type||A4 Article in a conference publication|
|Event||International Conference on Environmental Degradation of Materials in Nuclear Power Systems and Water Reactors - Amelia Island, United States|
Duration: 10 Aug 1997 → 14 Aug 1997
Conference number: 8
|Conference||International Conference on Environmental Degradation of Materials in Nuclear Power Systems and Water Reactors|
|Period||10/08/1997 → 14/08/1997|
- STRESS-CORROSION CRACKING