THE IGSCC, SENSITIZATION, AND MICROSTRUCTURE STUDY OF ALLOYS 600 AND 690

Both Alloys 600 and 690 were studied to understand the effect of heat treatment on the sensitization and SCC behavior of these alloys. The m icrostructural evolution and chromium depletion near the grain boundar ies were carefully studied using analytical electron microscopy. The m ajority of the precipitates formed in Alloy 600 was found to be M(7)C( 3) with a hexagonal structure (a(0) = 1.398 nm, c(0) = 0.45 nm); where as the carbides found in Alloy 690 were identified as M(23)C(6) with a n fcc structure (a(0) = 1.06 nm). Modified Huey test performed in boil ing 40% HNO3 was used to study the effect of heat treatment and degree of sensitization. Constant load tests and constant extension rate tes ts were performed in the solution containing sodium thiosulfate to stu dy the SCC resistance of these alloys. The results of the constant loa d tests for Alloy 600 indicated that the susceptibility to SCC is sens itive to the chromium depletion depth at grain boundary, and the minim um value to prevent SCC failure is approximately 8 wt%. No SCC was obs erved for Alloy 690 tested using constand load and CERT in the same en vironments. All tests showed that Alloy 690 has a far better resistanc e to intergranular attack and SCC than Alloy 600, which is believed du e to its high chromium content. It is therefore anticipated that Alloy 690 now a better substitute to Alloy 600 as a steam generator tubing material for pressurized water reactor will also offer a superior corr osion resistance when ''sensitized'' and in particular if exposed to s ulfur containing media such as thiosulfate solutions.