Influence of Grain Boundary Microstructure on the Susceptibility of Alloy 600 to Intergranular Attack and Stress Corrosion Cracking

The influence of grain boundary microstructure on the susceptibility of thermally treated (TT) Inconel Alloy 600 to intergranular attack (IGA) and stress corrosion cracking (SCO) in 30%NaOH + 10%Na₂SO₄ at 350 C was investigated. Both mill-annealed (MA) and laboratory solution-annealed alloys were aged at 700 C to establish the effect of initial processing on the material’s response to thermal treatment. Treatments of 1 h or more at 700 C greatly improved the SCC resistance of solution-annealed material but did not improve the performance of MA material containing a high density of intragranular carbides. The poor behavior of this material after thermal treatment was associated with the formation of small M₂₃C₆ carbides growing at a high angle to the boundaries and the development of these into cellular colonies. This is attributed to the use of a MA temperature that was too low to take into solution sufficient carbon for the development of semicontinuous grain boundary carbides during the subsequent TT process.