Comparison between Alloy 600 and Alloy 690 ODSCC behavior

Alloy 690TT showed significant SCC susceptibility in combined caustic solution including PbO and chloride of pH(T) of 9.5. Unlike Alloy 690TT, SCC resistance of Alloy 600MA was stronger than Alloy 690TT in highly caustic solution. The difference in SCC behavior was expressed in terms of passivity, chemical stability and alloying element. Environment weighted FOI for Alloy 690 versus Alloy 600 need to be revised based on accurate and wide data base. Many efforts to reduce SCC degradation were tried and then SCC resistance was improved by applying a heat treatment process from LTMA (low temperature mill annealed) Alloy 600 to HTMA (high temperature mill annealed) Alloy 600, and then TT (thermally treated) Alloy 600. Intra-granular carbide widely spread in LTMA Alloy 600 dissolves, and inter-granular carbide is then formed during high-temperature mill annealing and cooling, which leads to a great SCC resistance enhancement. Inter-granular carbide is well developed, healing chromium depletion at a grain boundary, and residual stress is removed during additional thermal treatment following mill annealing, which improves the SCC resistance more