The microstructural response of mill-annealed and solution-annealed INCONEL 600 to heat treatment
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INTRODUCTION
INCONEL600 is an austenitic Ni-Cr-Fe-C alloy which is used extensively for tubing and other components in nuclear power plants. Consequently, there has been a great interest in the stress corrosion cracking (SCC) properties of this alloy. Studies have been performed evaluating the effect of factors such as electrochemical potential, environment chemistry, temperature, and alloy heat treatment, grain size, composition, and processing history on the SCC resistance of INCONEL 600 (for a review, see Serral). These studies have shown that an important metallurgical variable in the SCC process is the thermal treatment which the alloy has received. Many studies 2-~~have shown that if INCONEL 600 in the as-received condition (often referred to as "mill-annealed") is given a heat treatment in the temperature range of 650 to 700 ~ the SCC resistance in caustic solutions or pure high-temperature water is improved, but the material is more susceptible to attack in oxidizing solutions and sulfur-containing environments. ~1.i2 However, there is considerable confusion about the generality of these results since a number of counter-examples can be found. 1 The exact causes of the change in SCC response of INCONEL 600 as a result of heat treatment are not clear. Several microstructural changes are known to occur when *INCONEL is a trademark owned by the INCO family of companies. ERNEST L. HALL and CLYDE L. BRIANT are Members of the Technical Staff at the General Electric Corporate Research and Development Center, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted August 30, 1984.
METALLURGICALTRANSACTIONS A
INCONEL 600 is isothermally heated in the temperature range of 600 to 800 ~ One of these is the precipitation of Cr-rich carbides at grain boundaries in the material, 9'11'13'1a which causes chromium depletion (sensitization) at the grain boundaries and vulnerability to intergranular corrosion. If the alloy is heated for a sufficiently long time at the temperature which initially caused sensitization, the intergranular corrosion resistance will return (healing). ~5 In addition, heat treatment in the temperature range 600 to 800 ~ may cause segregation of impurities such as sulfur, phosphorus, and boron to grain boundaries. One major limitation in the attempt to describe microstructural changes accompanying the heat treatment of INCONEL 600 and their affect on SCC behavior has been that until recently changes in grain boundary chemistry could not be measured directly. However, in the past few years Auger electron spectroscopy (AES) has been used to measure impurity segregation to grain boundaries in INCONEL 600,16'17'18 and energy-dispersive X-ray spectroscopy in the analytical electron microscope (AEM) has been used to measure chromium depletion. 17,19 One major drawback of many of the previous corrosion studies has been the use of samples in either the mill-annealed or solution-annealed states without complete microstructural characterization of these conditions. In practice, tremendous variability is f
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