Primary water stress corrosion cracking of Alloy 600 - effects of processing parameters

Correlations of stress corrosion cracking (SCC) data in deaerated water with temperature, stress, metallographic characteristics, and processing for laboratory test specimens are presented. The initiation time data show that a low temperature anneal and the resulting absence of grain boundary carbides results in a material having increased susceptibility to SCC, consistent with numerous other studies. The data also show that hot worked and annealed Alloy 600 is more resistant than cold worked and annealed material, both having carbide decorated grain boundaries. In the absence of grain boundary carbides, both materials are equally susceptible. Low temperature thermal treatment (-1100 degrees F) reduces the SCC susceptibility with or without grain boundary decoration. Weld metal data and data correlations developed from approximately 700 double U-bends are presented. The data demonstrate the effect of increased carbon content to improve the SCC resistance. In addition the data shows that the general relationship of time, temperature and strain for the wrought material is followed for the weld metal. The weld process used did not affect the SCC susceptibility of EN-82 which showed a greater resistance to SCC than EN-62. Stress relief of weld deposits showed an improvement as is the case for wrought material. Heat affected zone resistance was improved if the starting material received a high temperature anneal (1850 to 2000 degrees F). The range of SCC initiation times for weld metal is comparable to the range of initiation times for wrought material. 12 figs., 4 tabs