SCC BEHAVIOR OF MODEL ALLOY 600 CONTAINING MINOR ELEMENT CERIUM IN A CAUSTIC SOLUTION

摘要

High purity model alloys with major compositions of Ni-15Cr-9Fe-0.03C (600CE0) and Ni-15Cr-9Fe-0.03C-0.04Ce (600CE4) were produced. Using these model alloys the effect of an alloying element, Ce, on the SCC behavior of Alloy 600 was evaluated. To precipitate the carbides on the grain boundaries, a thermal treatment (TT) was performed on both the solution annealed alloys. Microstructural examinations using SEM and TEM EDX techniques showed that the same structural carbides, Cr_7C_3, were precipitated in both alloys and no significant difference in the amount of Cr depletion along the grain boundaries was observed between the two model alloys. However, it was found that the coverage of the grain boundary by the carbides was higher in the Ce-bearing alloy (600CE4). The SCC susceptibility of the alloys was investigated in a 40%NaOH solution at 315℃. Being evaluated in terms of the maximum crack depth, the SCC susceptibility of the alloy turned out to be reduced by the addition of Ce. The increased resistance to SCC of alloy 600CE4 was attributed to the increased coverage of the grain boundaries by carbides.