[en] For the purpose of improving the cyclic oxidation resistance of Ni-base superalloy, Inconel 600, aluminide coating methods are studied. The formation rate of aluminide coating layers is measured as a function of time and pack composition to find out the optimum coating condition. The evaluation of cyclic oxidation is established by the change in weight, the microphotography and EPMA of cross sectional area during 200 deg C - 950 deg C and 200 deg C - 1100 deg C, respectively. The thickness of coating layer and weight gains are parabolic behavior in propotion to time and Al contents. In pack of low aluminum contents, 2wt%, however, weight gain is decreased when activator, NH₄Cl is higher than 2 wt%. The cyclic oxidation resistance of the coatings carried out at 1100 deg C are superior to those of the coatings diffusion-treated after pack cementation at 800 deg C. Aluminide oxide, which is formed in external scale, is barrier to the cyclic oxidation. (Author)

[en] Slow strain rate stress corrosion tests have been performed on specimens cut from four separate heats of alloy 600 (Inconel 600) steam generator tubing. The material was tested in the mill-annealed and thermally-stabilised conditions and after various low temperature ageing treatments. Only limited cracking was observed, even for tests at 340⁰C, but the initiation of intergranular cracking was easier on the inner than on the outer surfaces on the tubing. Polarization data has been obtained in high-temperature water and in saturated boric acid and saturated lithium hydroxide at the atmospheric boiling points, and slow strain tests were performed at controlled potentials in these environments. Again, only very short cracks formed during the slow strain rate tests which were performed at a strain rate of about 10^-6 s^-1. The data is discussed in terms of the probable crack tip strain rates that would exist in these tests and at other strain rates. It is argued that if cracking occurs, the main role of very slow strain rate tests is to provide time for initiation and crack growth, so that cyclic loading or intermittent loading long term tests are likely to be more successful in sustaining crack growth in this alloy. (orig./HP)

[en] Carbide formation in nickel-base alloys (Alloy 600, and Alloy 690) is evaluated using a thermodynamic model. The Kohler method was adopted for evaluating chromium activity internary Ni-Cr-Fe system. For calculation of carbon activity, an empirical relationship was used, which shows a linear relationship between carbon activity and carbon solubility limit in the given alloy systems. The results show that Cr₂₃C₆ was thermodynamically and kinetically stable for Alloy 690. In case of Alloy 600, there is no significant difference in free energy changes between Cr₂₃C₆ and Cr₇C₃. At low temperature such as 650.deg.C or below, Cr₂₃C₆ needs less carbon contents for carbide formation. However, for heat treatment temperatures higher than 650.deg.C, Cr₇C₃ needs less carbon contents for carbide formation. The accuracy and limits of these calculations are discussed

[en] An evaluation has been made of the efficiency of thermal treatment (15 H, 700⁰C) in conferring resistance to carbide precipitation and intergranular attack, following subsequent heat treatment. The resulting carbide precipitation and resistance to intergranular corrosion has been determined after treatments carried out over the temperature range 300-800⁰C for up to 1 h. Marked differences between nominally equivalent commercial materials have been found, but these differences are evident only after extended corrosion testing. The ability of the thermal treatment to eliminate chromium depletion and provide good corrosion resistance is critically dependent on the grain size and carbon content of the alloy. Thermal treatments which fall short of the required period appear to induce a susceptibility to sensitisation to intergranular corrosion at temperatures met in steam generator service. A thermal treatment index is derived, which allows calculation of the period at 700⁰C necessary to complete the equilibrating reactions. (author)

[en] The stress corrosion cracking (SCC) of sensitized Alloy 600 was investigated in aerated solutions of sodium thiosulfate containing 1.3% boric acid. Results indicate that in the borated thiosulfate solution containing 7 ppM sulfur, 5 ppM lithium as lithium hydroxide is sufficient to inhibit SCC in U-bends. The occurrence of inhibition seems to correlate to the rapid increase of pH and conductivity of the solution as a result of the lithium hydroxide addition. In the slow strain rate tests in the borated solution containing 0.7 ppM lithium as lithium hydroxide, significant SCC is observed at a sulfur level of 30 ppB, i.e., a lithium to sulfur ratio of 23. In a parallel test in 30 ppB sulfur level but without any lithium hydroxide, the SCC is more severe than that in the lithiated environment. In the constant load test on a specimen held initially at a nominal stress near the yield strength of the material, cracks continue to grow until fracture during controlled, progressive dilution of the bulk solution, leading to final lithium concentration of 1.5 ppM and sulfur concentration (as thiosulfate) of 9.6 ppB i.e., a lithium to sulfur ratio of about 156, although lithium hydroxide retards the rate of crack propagation to some extent. The crack growth rate is strongly influenced by the electrochemical potential which is primarily governed by the local crack tip chemistry

[en] A chemical cracking test has been used to quickly obtain intergranular stress corrosion cracking (SCC) as it occurs in Alloy 600 wrought metal and EN82 weld metal in deaerated high temperature water environments. The test, referred to hereafter as the doped steam test, involves exposing the specimen surface of interest to 3000 psig (20.7 MPa), 750 deg. F (400 deg. C) superheated stagnant steam raised from water that contains 100 ppm each of chloride, fluoride sulfate, and nitrate sodium salts and to 10 psia (69 KPa) hydrogen partial pressure. Alloy 600 and EN82 bent beam specimens loaded to various known stress levels were exposed to this doped steam environment for periods of one to eight weeks. Threshold behaviours were determined from this test series. For specimens loaded above the threshold stress, SCC occurred in less than one week. Welded specimens with partial penetration EN82 welds were also subjected to the doped steam environment in the built-in crevice associated with partial penetration welds. During this test, cracking occurred in both the weld and wrought materials. The weld cracks initiated at the root and grew through the entire thickness of the weld throat in two weeks. Metallographic sections in the crack region and fractographs of the weld crack surface confirmed the presence of the multiple branched intergranular cracking expected in SCC. The results clearly indicate that the superheated stagnant steam with hydrogen and these four dopants provides a useful environment to assess the tensile stress condition of Alloy 600 wrought metal and EN82 weld metal specimens. (author). 10 refs, 6 figs, 1 tab

[en] Electrochemical machining (ECM) provides a rapid, easily controlled and damage free method of metal removal. Utilization of this technique to reduce the cross-sectional area of a tube offers an accurate procedure for obtaining data to calculate the residual stress distribution using the Sachs technique. Results of such a study conducted on cold-drawn Inconel 600 tubes in the as-received and annealed condition indicate that ECM avoids many problems, associated with methods based on mechanical removal of material, which may lead to alteration of the internal streess state during the measurment. Principal stresses in as-received material cause the tube to be in or near the plastic state throughout 85% of the tube wall thickness. Annealing for two hours at 1200 K followed by air cooling reduces the principal stresses substantially

[en] Cold drawn INCONEL 600 samples had been submitted to creep rupture tests with constant load at 650⁰C. The controlling creep mechanism had been characterized in that condition. Some aspects of the mechanical and structural behaviour of the material after the creep rupture tests had been compared with those observed in the same material after the simple submission its to isothermal treatment. (Author)

[en] Parametric studies of laboratory initiated intergranular attack (IGA) in mill annealed Alloy 600 were conducted in advance of remedial action testing. In isothermal capsule tests an increase in test temperature from 600 to 650⁰F and, to a lesser extent, an increase in caustic concentration from near 10 to 50 weight percent enhanced IGA. The possibility that mildly alkaline silicates could concentrate in a superheated crevice and cause IGA was investigated. Vapor pressure measurements did not identify a composition of silicates that could concentrate. Exposures of capsules to organic solutions (sodium-acetate, sodium-phenoxide, and sodium formate) at 610⁰F for 180 days did not produce noticeable IGA or stress corrosion cracking (SCC). However, stressed C-rings exhibited IGA and SCC in the last two, less stable environments but not in the sodium-acetate solution. In superheated crevice experiments near 610⁰F, chemical additions were made of 10 ppM sodium carbonate or of sodium hydroxide plus potassium hydroxide. At the top of the tubesheet crevice, denting was observed for both environments as a consequence of caustic corrosion of the tubesheet. The denting rate was much higher for carbon steel than for A508 low alloy steel tubesheets. IGA rates were the highest near the bottom of the tubesheet crevice. Increasing the tubesheet temperature to 650⁰F and packing the crevices with chromium oxide, effected similar amounts of denting, but enhanced SCC and IGA. IGA rates up to 47.4 mpy were obtained, which could be suitable for remedial action testing. Such IGA rates are comparable to those obtained in Alloy 600 capsules at 650⁰F using the most aggressive environment of caustic plus chromium oxide. Neither sulfur contamination nor hydrogen markedly influenced the IGA rates. An attempt to remove crevice deposits with the EPRI crevice cleaning solvent was partially effective. 29 refs., 41 figs., 36 tabs

[en] Published in summary form only