The effects of heat treatment and composition on the stress corrosion cracking resistance of inconel alloy X-750
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INTRODUCTION
INCONEL alloy X-750 is an age hardened nickel-base superalloy. The alloy was developed in the late 1940's for high temperature applications where creep strength and resistance to oxidation were the primary requirements. Since that time the alloy has found many other applications. In particular the alloy has been widely used in light water nuclear reactors as bolts, springs, guide pins, and other structural hardware where high strengths are required. A typical operating environment in these applications is water at temperatures on the order of 300 ~ In recent years some stress corrosion failures have been observed in these reactor components that have prompted concern about the use of this alloy. The present program was conducted to see if the stress corrosion cracking resistance could be improved by modifications of the heat treatment and/or the minor element chemistry. Since the composition and heat treatment of the alloy had been designed for service at much higher temperatures, it was reasoned that these parameters could be modified to provide better service under the much different operating conditions found in light water reactors. The interest was to stay within the composition ranges specified for the alloy so that a new material would not have to be qualified for service. The study was divided into four series of tests. In the first series, the effects of different age-hardening heat treatments were evaluated on a commercial heat of alloy X-750. In the second series, 16 laboratory-made heats were studied to evaluate high and low levels of four minor elements using a standard aging treatment. Several other minor element variations were explored in the third series. Based on these stress corrosion results, the effects of combining the best heat treatment with the best minor element chemistry were evaluated in the final series. Additional examinations were carded out to explore the reasons for the observed changes in stress corrosion cracking resistance.
*Trademark of the Inco family of companies.
S. FLOREEN, Research Fellow, and J. L. NELSON, Section Manager, are both with Inco Alloy Products Company Research Center, Inco-Sterling Forest, Box 200, Suffern, NY 10901. Manuscript submitted June 22, 1982.
METALLURGICALTRANSACTIONS A
EXPERIMENTAL PROCEDURES
Twenty-four heats of alloy X-750 were evaluated in this investigation. The compositions of these alloys are given in Table I. In the first series of tests where different heat treatments were evaluated the samples were taken from heat A, which was commercially produced plate. The remaining heats were laboratory melted 14 kg vacuum induction melts. The heats were made from high purity charge materials with minor elements back-added either as pure metals or from master alloys. The ingots were initially hot worked at 1250 ~ and then hot rolled to approximately 15 mm thick plate stock starting at 1180 ~ In series II, a 16 heat statistically designed matrix was used to evaluate the following levels of C, S, Mg, and Zr: Nominal Low Nominal
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