The intergranular stress corrosion cracking (IGSCC) of alloy 600 (UNS N06600) in high temperature water depends upon the microstructure and particularly upon the grain boundary composition and the intergranular carbide distribution. However, has been found that the presence of dissolved hydrogen in pure water increases the susceptibility to IGSCC and accelerates the crack growth rate. In this work, the role of heat treatment and microstructure produced was investigated. In order to analyze the susceptibility of alloy 600 to IGSCC induced in steam generators. The samples were heat treated and quenched in range of 600-1100 oC and compared with the mill annealed treatment at 930oC and the microstructure was characterized using SEM-EDX and TEM. The resistance to the susceptibility to intergranular attack corrosion (IGA) in boiling acid solution in concern to the weight loss was investigated. The crack growth was measured using the potential drop technique in modified wedge opening displacement (M-WOL) samples immersed in an instrumented vessel at temperature between 200 to 350 oC, in pure water saturated with H2 at 200 kPa. It was found that the microstructure produced by heat treatment produced a strong increase in precipitation carbides in grain boundaries and transgranular precipitation compared with the as-received condition, due to the effect of temperature and time. The results of crack growth showed that the samples in the mill-annealed-condition and asreceived condition, show a profile of transgranular crack growth. The sample quenched at 800 oC shows a wide precipitation of M23C6 carbides, preferentially at grain boundaries with a few transgranular precipitates.
Over the past 20 years extensive studies has been conducted to determine the susceptibility of the alloy 600 (UNS N06600) to intergranular stress corrosion cracking (IGSCC) 1. A combination of IGSCC and IGA has been observed as typical failures in tubing of alloy 600 (UNS N06600) on steam generators and pressurized water reactor nuclear power plants. However, is not clear if IGSCC and IGA are two aspects of the same phenomenon or they are governed by independent processes. There is no definite explanation to account for the phenomena of IGSCC and IGA 2. Three kinds of mechanism are usually proposed: (1) Dissolution/oxidation mechanisms, in which the crack grows by dissolution/oxidation of the material; (2) Hydrogen embrittlement mechanisms, in which the fracture results from a decohesion due to hydrogen accumulation in zones of maximum triaxial stresses; and (3) Corrosion-enhanced plasticity mechanisms, which predict that a brittle fracture occurs as result of local interactions between corrosion and plasticity 3. It has been found that chromium depletion due to the precipitation of grain boundary carbides during heat treatment is the responsible for the degradation of its resistance to intergranular corrosion, especially in the oxidizing environments 4. In recent years it has been found that the presence of dissolved hydrogen in pure water increases the susceptibility to IGSCC of the alloy 600 (UNS N06600) and accelerates the crack growth rate. However, it does not exist a relationship between concentration or partial pressure of hydrogen and IGSCC to the date 5. The chemistry of grain boundaries has emerged as a factor of prime importance for understanding IGSCC and IGA in these alloys 6. It has been shown that the microstructure of the grain boundary is one of the most important variables that influence the alloy 600 crack propagation rate. Previous studies have shown the grain boundary microstructure can be altered by heat treatment 1, which can dissolve the carbon in the matrix or can precipitate as carbides.
