Einfluß von Korrosion und mechanischer Belastung auf das Rißwachstum niedriglegierter,ferritischer Stähle in sauerstoffhaltigem Hochtemperaturwasser

Influence of corrosion and mechanical loading on the crack growth of low-alloyed ferritic steels in oxygenated high temperature water The mathematical and with regards to the contents main features of the mostly developed analytical model for corrosion-assisted crack growth are presented and the crack growth velocities resulting for low-alloyed ferritic materials in high-temperature water are given. Experimentally determined crack growth velocities for the ferritic material 20 MnMoNi 5 5 with two different sulfur contents as well as for the similar material 22 NiMoCr 3 7 in deionized, oxygenated (0.4 and 8 ppm O₂) high temperature water at 240°C are compared with calculated ones. The constant load experiments at different level were performed on compact tension specimens with a thickness of 50 mm (2T-CT-specimens). The experimental results show, that up to a stress intensity factor K_I of 60 MPa $ sqrt m $ the corrosion-assisted crack advance is neither dependent on the oxygen content of the medium and the K₁-value, nor on the sulfur content of the steel. A deviation up to 3 magnitudes compared to the calculated values exists. Furthermore, an increasing crack growth velocity with decreasing test duration is observed. Between 60 and 75 MPa $ sqrt m $ the crack growth velocity increases by several magnitudes also independent of the above mentioned parameters. Above 75 MPa $ sqrt m $ fracture of the specimen occurs soon after loading. In this region the experimentally derived crack growth velocity fits well with the analytical model. A possible explanation for the deviation between experimental and analytical results could be seen in low-temperature creep processes at the crack tip. Results of preliminary investigations on low-temperature creep processes of the material 20 MnMoNi 5 5 in air at 240°C are presented.