The role of TiN inclusions in stress corrosion crack initiation for Alloy 690TT in high-temperature and high-pressure water

Microchemistry and microstructure of TiN in Alloy 690TT were analyzed using SEM and TEM. Constant loading test was conducted on Alloy 690TT in 325 degrees C water to study effect of TiN on SCC initiation. It was found that TiN-inclusion stringers existed along the longitudinal axis. Numerous micron-size TiN with Al2O3 and/or MgO nuclei and submicron-size TiN existed in this alloy. After holding a constant load equal to 180% of the yield strength for 40 days, cracks about 1-3 mu m initiated at micron-size TiN and TiN-inclusion stringers in the near surface. The mechanisms of Alloy 690TT crack initiation at these TiN are discussed. (C) 2009 Elsevier Ltd. All rights reserved.