Influence of thermal aging on the intergranular corrosion resistance of types 304LN and 316LN stainless steels
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I.
INTRODUCTION
A number of failure incidentsf~l which have occurred in the austenitic stainless steel (SS) components of nuclear power plants indicated that such failures are due to intergranular corrosion and/or intergranular stress corrosion cracking (IGSCC). Sensitization of austenitic SS, leading to the formation of Cr-depleted zones adjacent to Cr-rich M23C6 carbides developed along the grain boundaries, was attributed as a major reason for the failure of materials by intergranular corrosion (IGC) or IGSCC.E2-61Though a number of methods are available for eliminating IGC, E712] in recent days, the addition of nitrogen to austenitic SS was considered as a method to delay the onset of sensitization and improve the IGC resistance, t13-~9] The mechanism by which nitrogen addition improved the IGC resistance is still not clear, though attempts were made in the past by many inve stigators.t~5,t6,17] In the present work, nitrogen-bearing types 304 and 316 SS alloys chosen for the construction of the Prototype Fast Breeder Reactor (PFBR) to be built by the Department of Atomic Energy, Kalpakkam, Indiat2~ were assessed for their IGC resistances. This is necessary since the components made of austenitic SS may undergo IGC during many stages of fabrication, including welding and heat treatment, and also during commissioning and operation. The type of precipitate leading to chromium depletion in these alloys could be different from the M23C 6 carbides since nitrogen addition could alter the type of precipitates. To assess the degree of sensitization, ASTM A262 practice A tests and EPR tests were conducted on these alloys after thermally aging them at 823, 873, and 923 K for various durations. The precipitates in the aged specimens were extracted by an electrochemical method and were examined by X-ray diffraction (XRD) method. Hence, the purpose of the presU. KAMACHI MUDALI, Scientific Officer SE, R.K. DAYAL, Head, ACSS, and J.B. GNANAMOORTHY, Head, are with the Metallurgy Division, Indira Gandhi Centre for Atomic Research. P. RODRIGUEZ, Director, is with the Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Manuscript submitted January 21, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
ent investigation was (1) to determine the time-temperaturesensitization (TTS) diagram for these alloys based on the results of practice A and electrochemical potentiodynamic reactivation (EPR) tests and (2) to understand the type of precipitates developed during thermal aging. The appropriate sensitizing conditions for these alloys could be identified from the TTS diagrams for choosing the right combination of time and temperature conditions to avoid the incidence of IGC. II.
EXPERIMENTAL PROCEDURE
A. Materials and Heat Treatment
Nuclear-grade AISI types 304LN and 316LN SS were used for the present investigation. The chemical compositions of these alloys are given in Table I. Initially, the alloys were given a solution annealing heat treatment at 1323 K for 1 hour in a high-temperature, high-vacuum furnace, and que
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