The effect of grain boundary chemistry on Intergranular stress corrosion cracking of Ni-Cr-Fe alloys in 50 Pct NaOH at 1
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INTRODUCTION
NICKEL-base alloys are used extensively in the primary and secondary systems of nuclear power reactors. Single-phase, solid solution Ni-Cr-Fe alloys exhibit excellent corrosion resistance in high-temperature water and steam environments. However, alloy 600 has proven to be susceptible to intergranular stress corrosion cracking (IGSCC) in high-temperature caustic solutions. Although this susceptibility has been known since the early 1960s, the basic mechanisms are not very well understood, u'2'31 Past and ongoing research has demonstrated that the susceptibility of alloy 600 to IG cracking in caustic solutions is a strong function of heat treatment, t4-8] Recently, Lee e t a / . [6] performed systematic IGSCC tests with samples heat-treated at 700 ~ for various times, with mill-annealed and mill-annealed plus solutionannealed samples in 50 pct NaOH at 140 ~ Mill-annealed samples experienced the maximum attack, with the IGSCC susceptibility significantly reduced by heat-treating at 700 ~ for 10 hours or longer. However, for solutionannealed samples, heat treatment at 700 ~ for 20 minutes increased the attack, while longer heat treatments decreased the attack. Among all the samples tested, the mill-annealed material was the most susceptible. Although the standard thermal treatment (mill anneal + 700 ~ h) has generally yielded a beneficial effect,/6'9J~ in high-purity water, such an effect on caustic IGSCC resistance is not always apparent. McIlree J.K. SUNG, T. ANGELIU, Graduate Research Assistants, Department of Materials Science and Engineering, J. KOCH, Undergraduate Research Assistant, Department of Nuclear Engineering, and G.S. WAS, Professor, Department of Nuclear Engineering and Department of Materials Science and Engineering, are with The University of Michigan, Ann Arbor, MI 48109. Manuscript submitted November 4, 1991. METALLURGICAL TRANSACTIONS A
et a l . U1] demonstrated that thermally treated alloy 600
(1130 ~ h + 675 ~ h) exhibited high IGSCC susceptibility in 50 pct NaOH solution at 316 ~ He later noted that thermal treatment [solution anneal (SA) + 649 ~ to 677 ~ h] was ineffective in 50 pct NaOH at various temperatures, u21 Pathania t~31 observed that a mill anneal + 650 ~ h improved IGSCC resistance of C-rings exposed to 40 to 150 g NaOH/kg H20 at 290 ~ but had no effect on cracking of pressurized capsules following a mill anneal + 600 ~ h tested in the same environment. This discrepancy in the beneficial effect of the thermal treatment may stem from the poor definition of the thermal treatment (time and temperature) and the thermomechanical history of materials before heat treatments (mill annealed or solution annealed). Heat-treatment temperature can also affect IGSCC in caustic solution. Pathania f131showed that a mill-annealed sample heat-treated at 870 ~ for 1 hour did not show improved IGSCC resistance in 40 to 200 g NaOH/kg H20 at 300 ~ Moreover, Aireyt91 observed that millannealed samples heat-treated at 870 ~ degraded IGSCC resistance in comparison with the mill-annealed
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