NICKEL-CHROMIUM ALLOY

Abstract: The present invention provides a nickel-chromium alloy in which it has an excellent mechanical character and improves a pitting corrosion resistance, a stress corrosion cracking resist­ ance and a crystal boundary etching resistance, and the Ni-Cr alloy is obtained by carrying out an annealing treatment under required conditions, said alloy having the following composi­ tion: in therms of by weight, 40 - 70% of Ni; 0.015% or less of C; 1.0% or less of Si; 25 - 35% of Cr; 1.0% or less of Mn; 0.030% or less of P; 0.020% or less of S and the residue comprising Fe and impurities.

Description: The present invention relates to a non-deposition hardening type nickel based alloy which will be subjected to a high-temperature and high-pressure water or vapor and which has a remarkably improved crystal boundary etching resistance, mechanical properties and pitting corrosion resistance, in addition to the maintenance of a stress corrosion cracking resistance, and further has a bettered stress corrosion resistance in an aqueous NaOH solution. The present invention relates to a nickel-chromium alloy excellent in a stress corrosion cracking resistance, more specifically, to a nickel-chromium alloy in which the stress corrosion cracking resistance is noticeably improved by depositing an insolubilized carbide in grains thereof and by strengthening a coating on the surface thereof. The present invention also relates to an alloy for a heat transfer pipe, particularly to an alloy for a heat transfer pipe on the secondary side of a nuclear reactor. Heretofore, as materials, for a container for giving off vapor in a nuclear reactor, which will be exposed to the high-temperature and high-pressure water or vapor, for example, at 200 to 400°C and at 50 to 200 atm, and as materials used under a cooling system environment in a nuclear reactor, there are nickel based alloys such as INCOROI 800 (trade name), and INCONEL 600 (trade name) and INCONEL 690 (trade name) set forth in Table 1 below. In recent years, these alloys have further been treated by heating them at a rather lower temperature than a level (hereinafter referred to as T°C), at which a carbide is thoroughly solubilized, alternatively by further additionally specifically heating and retaining them at a temperature of 650 to 750°C, in order to improve the crystal boundary etching resistance and stress corrosion cracking resistance. However, the nickel based alloys which have undergone such a conventional thermal treatment are still poor in the pitting corrosion resistance and stress corrosion cracking resistance. In view of the above-mentioned conventional techniques, an object of the present invention is to provide a method for a thermal treatment of a nickel based alloy without such drawbacks above, i.e. a method for a thermal treatment of a nickel based alloy by which its mechanical properties, pitting corrosion resistance, stress corrosion cracking resistance and crystal boundary etching resistance can be improved. For the aforesaid object, the summary of the present invention is directed to a method of preparing a nickel based alloy, in a first thermal treatment process, said nickel based alloy is heated and retained at a temperature of T °C (at which chromium carbides are thoroughly solubilized) to (T + 100) °C which corresponds to the hatched zone in Fig. 4 and is cooled at a cooling rate of a furnace cooling rate or more, and that in a second thermal treatment process, said nickel based alloy is then retained at a temperature of 600 to 750 °C and within a sensitization recovery range which corresponds to the hatched zone in Fig. 2 for a period of 0,1 to 100 hours and is cooled at a cooling rate of said furnace cooling rate or more, said alloy consisting of, in term of % by weight, 58 % or more of Ni, 25 to 35 % of Cr, 0,003 % or less of B, 0,012 to 0,035 % of C, 1 % or less of Mn, 0,5 % or less of Si, 0,015 % or less of P, 0,015 % or less of S, optionally 0,5 % orless of Al; 0,01 to 1,0 % of Ti; 0,5 to 5,0 % in all, of one or more of Mo, W and V, and the balance being Fe plus impurities. Heretofore, for tubes, containers and their fittings used in stress corrosion cracking environments including Cl⁻ ions in nuclear reactors, chemical plants and the like many nickel based alloys which are considered to be excellent in the stress corrosion cracking resistance have been used. However, it has been reported that even in the case of a 30 % Cr-60 % Ni system alloy which has generally been used, the occurrence of the stress corrosion cracking cannot be avoided in certain environments. Thus, an object of the present invention is to provide an alloy which can overcome such a drawback inherent in the 30% Cr-60% Ni system alloy and which is excellent in a corrosion resistance, especially the stress corrosion cracking resistance so that it may be used for the tubes, the containers and their fittings in the nuclear reactors, the chemical plants and the like in the form of thick-walled plates, round rods or pipes. The inventors of the present case have paid much attention to the fact that the aforesaid 30% Cr-60% Ni based alloy is finally annealed at a relatively high temperature of 980 to 1150°C in compliance with a carbon content and is used in a state of including no insolubilized carbide, and they have researched into a relation between a morphology of the carbide in the alloy system and its corrosiveness. As a result, it has been found that an active deposition of the carbide, if in the grains thereof, is rather effective for the improvement in the stress corrosion cracking resistance. Further, in view of the report that in environments of a high-temperature water including Cl⁻ ions, stress corrosion cracks would occur starting from pitting corrosions, the addition of Mo, W and V, which are known as elements effective for the improvement in the pitting corrosion resistance, has been attempted with the intention of strengthening the coating. In consequence, it has been found that the corrosion resistance, i.e. the stress corrosion cracking resistance of the obtained alloy is noticeably improved in cooperation with the aforementioned deposition effect of the carbide, and the present invention has now been achieved. Presently, a heat transfer pipe of a steam generator in a nuclear reactor such as a pressurized water reactor is made of an only annealed alloy or Alloy 600 (trade name, 75%Ni-15%Cr-8%Fe) which has further been subjected to a specific thermal treatment (700°C x 15 hr). However, recent researches have revealed that Alloy 600 which is the alloy for the heat transfer pipe has the following problems. That is to say, a stress corrosion cracking sometimes occurs owing to an alkaline concentrate in the gap between the heat transfer pipe and a pipe-supporting plate in envrironments (alkaline environments including ammonia and hydrazine and having a pH of 9.2 to 9.5 and a temperature of 280°C) on the secondary side of the nuclear reactor, and a pitting corrosion tends to take place owing to Cl⁻ ions in leaked seawater in the same environments on the secondary side of the nuclear reactor. Further, this pitting corrosion will deeply penetrate and the number of the pitting corrosions will augment with the increase in a concentration of the Cl⁻ ions. Heretofore, it is known that the spefific thermal treatment is given to a nickel based alloy for the sake of the improvement in a stress corrosion cracking resistance (hereinafter referred to as the SCC resistance). For example, Japanese Patent Disclosure No. 25216/1979 discloses a method in which after a final annealing treatment, the alloy is successively heated and retained at a temperature of 550 to 850°C for a period of 1 to 100 hours in order to deposit a carbide on crystal boundaries, thereby heightening the SCC resistance. Even by such a conventional technique, however, a resistance to the stress corrosion cracking caused by the alkaline concentrate, i.e. an alkali stress corrosion cracking resistance and the pitting corrosion resistance cannot be improved. Further, in fact, the nickel based alloy obtained by such a conventional method cannot always provide a satisfactory combination of the alkali stress corrosion cracking resistance and the pitting corrosion resistance. Thus, an object of the present invention is to provide a method of preparing an alloy for a heat transfer pipe excellent in the corrosion resistance, especially an alloy for a heat transfer pipe excellent in the alkali stress corrosion cracking resistance and the pitting corrosion resistance. Another object of the present invention is to provide a method of preparing an alloy for a heat transfer pipe which can be used particularly advantageously under alkali environments in a steam generator of a pressurized water reactor. The inventors of the present case have intensively carried on researches for the achievement of such objects, and they have acquired the following knowledges: (1) The addition of one or more of elements Mo, W and V which are effective for the pitting corrosion resistance permits heightening a passive coating on the alloy in order to improve the pitting corrosion resistance. (2) The alkali stress corrosion cracking resistance can be remarkably improved by first heating and retaining the alloy for 1 minute or more at a temperature not less than a level at which a carbide in the alloy is thoroughly solubilized, in order to carry out an annealing treatment; cooling it to a temperature of 200°C or less; and accomplishing the specific thermal treatment for it at a temperature of 600 to 750°C for a period of 0.1 to 100 hours. (3) Such an alkali stress corrosion cracking resistance can be improved, together with the aforesaid betterment in the pitting corrosion resistance, by virtue of the addition of one or more of Mo, W and V. Since the nickel based alloy, inter alia the high Cr-Ni based alloy including 25 to 35% of Cr is small in the solubility of C therein, Cr carbide deposits on crystal boundaries during the cooling process or practical use after the annealing step in order to form Cr-poor layers thereon, so that the stress corrosion cracking will take place thereon. Therefore, when carbon is enough solubilized during the annealing step and the alloy is once cooled to a temperature of 200°C or less and the thermal treatment is then carried out by heating it again at a temperature within the range of 600 to 750°C for at most 100 hours, the deposition of Cr carbide (Cr₂₃C₆) will be accelerated, but the formation of the Cr-poor layers will positively be inhibited owing to the facilitation of a Cr diffusion from its interior which is brought about by doing the treatment at a heating temperature for a retention time in Figure 2. Such an effect will lead to the improvement in the stress corrosion cracking resistance, particularly the alkali stress corrosion cracking resistance and the pitting corrosion resistance in combination with the aforesaid effect based on the addition of one or more of Mo, W and V. In this connection, the inventors have known that after the final annealing, by once cooling the alloy to a temperature of 200°C or less at which no diffusion of Cr substantially occurs in the alloy, the deposition rate of the carbide at the time of the subsequent thermal treatment is unexpectedly remarkably accelerated, as compared with the case where the thermal treatment is successively carried out after the final annealing. Accordingly, the present invention is characterized by an alloy for heat transfer pipes excellent in an alkali stress corrosion cracking resistance which is obtained by heating and retaining said alloy at a temperature within the range of a temperature (T°C), at which a carbide in said alloy is thoroughly solubilized, to T + 100°C for 1 minute or more; cooling it once to a level of 200°C or less; and carrying out a thermal treatment under conditions within a hatched range Z in Figure 5. As definite from the foregoing, the present invention is directed to the alloy for a heat transfer pipe which is excellent in the alkali stress corrosion cracking resistance and the pitting corrosion resistance in the alkaline environments, but in a preferred embodiment, it is directed to the alloy for a heat transfer pipe on the secondary side of a nuclear reactor, for example a heat transfer pipe of a steam generator in a pressurized water reactor. Brief Description of the Drawings : Figure 1 is a schematic diagram illustrating a solution temperature of a carbide in a nickel based alloy and a temperature range in a first thermal treatment process; Figure 2 is a diagram illustrating an influence of conditions of a second thermal treatment process regarding the present invention upon a crystal boundary ectching resistance; and Figure 3 is a diagram illustrating influences of a cooling rate of the first thermal treatment process and a temperature retaining time of the second thermal treatment process regarding the present invention upon the crystal boundary etching resistance. Figure 4 is a graph showing a temperature range of an annealing treatment regarding the present invention with respect to a content of carbon; Figure 5 is a graph drawn by plotting alkali stress corrosion cracking resistances with respect to heating temperatures and retention times of thermal treatment conditions; and Figure 6 is a graph showing relations between contents of Mo, V and W and corrosion amounts. Detailed Description of the Invention : Now, referring to FIGS. 1 to 3, the detailed description will be made to an alloy to be treated. Alloy to be treated: The content of Ni is 58% or more, since when it is below 58%, the alloy will be poor in an alkali stress corrosion cracking resistance. When the content of Cr is less than 25%, the alloy will have a less crystal boundary etching resistance and stress corrosion cracking resistance; when it is more than 35%, abnormal substances will deposit in the second thermal treatment process, which fact will lead to the deterioration in ductility. Therefore, the content of Cr is within the range of 25 to 35%. With regard to the element B, when its content is above 0.003%, the alloy will be poor in the crystal boundary etching resistance. Therefore, the content of B is 0.003% or less. When the content of C is less than 0.012%, the alloy will have an insufficient strength; when it is in excess of 0.035%, it will be poor in the stress corrosion cracking resistance. Therefore, the content of C is within the range of 0.012 to 0.035%. Elements P, S and the like are incorporated into the product as impurities during a process of a usual iron manufacture or steel manufacture, but too much impurities have bad influence upon the corrosion resistance. Therefore, the content of P is 0.015% or less and that of S is also 0.015% or less. Further, Mn and Si are added for the sake of a deoxidation, a reinforcement of a matrix and a reinforcement of grain boundaries, but when the content of Mn is more than 1%, the alloy will be hard to melt, and when the content of Si is more than 0.5%, the alloy will be poor in welding properties. Therefore, the content of Mn is 1% or less, and that of Si is limited to 0.5% or less. First thermal treatment process: A temperature T°C at which the carbide of the nickel based alloy is thoroughly solubilized varies with the content of C,as elucidated by the schematic view in Figure 1. If this thermal treatment process is carried out at a temperature less than T°C, the carbide will deposit, thereby unreasonably increasing a tensile strength, 0.2% yield point and hardness, and thus deteriorating the stress corrosion cracking resistant. On the contrary, if at a temperature more than (T + 100)°C, a grain size of crystals will become remarkably coarse, thereby deteriorating the crystal boundary etching resistance, and merely providing the insufficient tensile strenght, 0.2% yeld point and hardness. Further, it is natural that the retention time is prolonged with the increase in the wall thickness of the material, hence it is impossible to uniformly define the retention time. However, generally speaking, the retention time takes 30 minutes or so per 2.54 cm (1 inch) of the material thickness, and in the case that the material thickness is 2.54 cm or less, 1 to 30 minutes will be usually taken. Furhter, since an abnormally prolonged time will produce coarse crystals on the surface of the material and its strength will thus be lowered, it is preferred that the retention time is within the range of 1 minute to 2 hours. Then, the alloy is cooled, for example, from a level of 200°C to room temperature. With regard to a cooling rate of the alloy, the cooling rate less than a furnace cooling rate is not advantageous, but any rate of the furnace cooling rate or more is in fact satisfactory. The cooling rate of the furnace cooling rate or more can be obtained by, for example, the furnace cooling, an air cooling, gas cooling, oil cooling, water cooling and the like. Second thermal treatment process: After retained at a temperature of T°C to (T + 100)°C for a period of 30 minutes and water cooled in the first thermal treatment process, specimens of Table 2 below were retained at various heating temperatures for various periods of time and were cooled in the same manner as in the aforesaid first process. Then, they were immersed in a boiling solution of 65% HNO₃ and 0.1-N HF for a period of 4 hours. Obtained test results are shown in Figure 2 below. In a sensitization range in Figure 2, Cr-free layers are formed on crystal boundaries, and a crystal boundary etching and pitting corrosion thus tend to occur. Further, in the case of the alloys in an unsensitization range therein, there is a probability of their being sensitized during their use as the real materials at a high temperature, therefore they are also liable to bring about the crystal boundary etching. In consequence, the retention temperature in the second thermal treatment process must be in a sensitization recovery range in which the Cr-free layers recover. Furthermore, when the retention temperature is more than 750°C, a solubility of C will be great and a solubility difference will result from a temperature difference between such a temperature and a temperature at the time of a cooling or a practical use. As a result, a carbide tends to deposit on the crystal boundaries. When the retention temperature is less than 600°C, the retention time more than 100 hours will be required, which fact is not economical. Therefore, the retention temperature is limited to the range of 600 to 750°C. Moreover, when the retention time is less than 10⁻¹ hour, the sensitization recovery range cannot be prepared at the aforesaid temperature; the retention time more than 100 hours is not economical. Therefore, it should be within the range of 10⁻¹ to 100 hours. With regard to the cooling rate in this case, any rate of the furnace cooling rate or more is satisfactory, as in the first thermal treatment process. After retained at a temperature of T°C to (T + 100)°C for a period of 30 minutes in the first thermal treatment process, specimens were heated and retained at a temperature of 700°C and were air cooled, and they were then immersed in a boiling solution of 65% HNO₃ and 0.2 g of Cr⁶⁺/liter for a period of 24 hours. Obtained results of crystal boundary etching resistance tests are exhibited in Figure 3 below. As the same drawing indicates, the sensitization range in the second thermal treatment process varies with the cooling rate in the first thermal treatment process, and it has been found that any case gets into the sensitization recovery range within 100 hours. In Table 3, results of a variety of tests for the specimens in Table 2 are summarized. According to the results, it can be understood that present invention permits providing the nickel based alloy having the remarkably improved crystal boundary etching resistance, pitting corrosion resistance, mechanical properties and alkali stress corrosion cracking resistance, in contrast with the conventional. As described in detail in the foregoing, the nickel based alloy according to the present invention can noticeably improve the crystal boundary etching resistance, pitting corrosion resistance, mechanical properties and stress corrosion cracking resistance, therefore this invention is most suitable for the thermal treatment for materials which will be subjected to a high-temperature and high-pressure water of 200 to 400°C, for example, materials for a container for giving off vapor in a nuclear reactor and materials for a cooling system in the nuclear reactor. Furthermore, referring to Figures 4 to 6, the present invention will be described below. The reason why the composition of the alloy and the conditions of the thermal treatment are restricted as mentioned above in the present invention is as follows: Carbon (C): The element C is harmful to the SCC resistance, therefore its content in the present invention is 0,012 - 0.035%. Silicon (Si) and manganese (Mn): Si and Mn both are deoxidizers. The amount of Mn is required to be 1.0% or less, the one of Si to 0,5% or less. However, when the amount of the elements Mn and Si is above 1.0% or 0,5% respectively, the alloy will have deteriorated welding properties and cleanness. Chromium (Cr): The element Cr is an essential component for the maintenance of the corrosion resistance of the alloy according to the present invention. When the content of Cr is less than 25%, it will be impossible to obtain such a corrosion resistance as the present invention requires. On the contrary, when it is above 35%, a hot workability of the alloy will remarkably deteriorate. Therefore, the content of Cr is limited to the range of 25 to 35% in the present invention. Aluminum (Al): Al is also necessary as a deoxidizer, but when it is above 0.5%, the cleanness of the alloy will be poor. Therefore, its content is limited to 0.5% or less. Titanium (Ti): When 0.01% or more of Ti is added to the alloy, its hot workability will be enhanced; when it is added thereto in an amount above 1.0%, its effect will reach a ceiling level. Therefore, its upper limit is 1.0%. Phosphorus (P): The element P is included as an impurity in the alloy. If its content is in excess of 0.015%, it will be harmful to the SCC resistance and the hot workability. Sulfur (S): This element is also included as an impurity in the alloy. If its content is above 0.015%, it will be harmful to the crystal boundary etching resistance and the hot workability. Molybdnum (Mo), tungsten (W) and vanadium (V): These elements all are effective to heighten the pitting corrosion resistance especially in a high-temperature water including Cl⁻ ions. When the content of at least one of these elements is 0.5% or less in all, a passive coating on the alloy surface will not be heightened and the pitting corrosion will thus occur. On the contrary, when the total content thereof is more than 5.0%, its effect will reach a ceiling level, and additionally the hot workability will noticeably be deteriorated. Therefore, it is preferred that these elements are added to the alloy in an amount of 1.0% or more in all. Annealing treatment: When the annealing operation is carried out below a temperature (hereinafter referred to as T°C) at which the carbide in the alloy is thoroughly solubilized, a tensile strength, 0.2% yield point and hardness of the alloy will become unreasonably great. On the contrary, when it is done at a temperature above T + 100°C, the alloy will have remarkably coarse crystal grains, which fact will lead to the deterioration in the corrosion resistance, i.e. the crystal boundary etching resistance and the crystal boundary stress etching resistance, and the tensile strength, 0.2% yield point and hardness cannot be obtained at predetermined levels. Therefore, the annealing temperature in the present invention are from T°C to T + 100°C. For example, in an embodiment of the alloy including 0.02% of C, an annealing temperature of 1050 to 1150°C is preferable. Further, with regard to a retention time, for example, a period of 1 to 120 minutes or 1 to 30 minutes is necessary, though it varies with a wall thickness of the pipe to be formed. With regard to a cooling rate, a high cooling rate as in the case of a water cooling is suitable, but other rates in cases of air cooling and oil cooling as well as a low rate in the case of furnace cooling are also acceptable. Special restriction is not imposed on this point. By such a cooling means, the alloy is cooled to, e.g. 200°C to room temperature. The above-mentioned temperature at which the carbide in the alloy is thoroughly solubilized varies with a carbon content as exhibited in Figure 4, but it is, e.g. 950°C at 0.01% carbon content, 1050°C at 0.02% content and 1100°C at 0.03% content. Thermal treatment After the aforesaid annealing treatment, the specific thermal treatment is carried out by retaining a temperature of 600 to 750°C for 0.1 to 100 hours as shown in Figure 5, whereby the carbide will semicontinuously deposit on the crystal boundaries and the Cr-poor layers in the vicinity of positions where the carbide exists will recover, thereby increasing the crystal boundary stress corrosion cracking resistance. The reason why such specific thermal treatment conditions are restricted to the hatched range (Z) in Figure 5 is as follows: On the left side of the hatched range (Z) in Figure 5, the retention time is lacking. As a result, the Cr carbide will deposit on the crystal boundaries and the Cr-poor layers formed there-around will hot enough recover, so that the SCC resistance cannot be obtained to a satisfactory degree. On the right side in Figure 5, the hatched range (Z) terminates at a position corresponding to 100 hours. Such a restriction is for an economical reason, though the farther prolonged heating treatment is good for the SCC resistance. Moreover, with regard to temperature, when it is less than 600°C, diffusion rates of Cr and C will be low. Hence, in order to cause the Cr-poor layers to recover and to improve the SCC resistance at such a temperature, the very longtime heating operation will be required, which fact is not practical. Therefore, the lower limit of the temperature is set to 600°C. On the other hand, when the temperature is above 750°C, the recovery of the Cr-poor layers and the betterment in the SCC resistance will be achieved in an extremely short period of time. However, since a difference between this specific heating temperature and a practically used temperature (300°C or so) is great, the solubilized carbon will deposit in large quantities in the form of the carbide in accordance with a dimension of the difference at the time of a practical use, so that the crystal boundaries tend to be remarkably sensitized. However, if the specific heating temperature is 750°C or less, the sensitization will be lowered to a negligible degree at the practical use, because the absolute quantity of the solubilized carbon will be small. Therefore, the thermal treatment conditions in the present invention are restricted to the hatched range (Z) surrounded by points A (10¹ hours, 750°C), B (10² hours, 750°C) and C (10² hours, 600°C) in Figure 15. Now, the present invention will be described in accordance with examples, but they are merely exemplary and do not intend to limit the present invention at all. Examples 1 to 18 By a vacuum solubilization, 60%Ni-30%Cr alloys (Alloy Nos.1to 18 and comparative Alloys Nos. 19 to 26) chemical compositions of which were set forth in Table 6 were manufactured. The thus manufactured alloys were forged at a temperature of 950 to 800°C to form them into plates of 25 mm in thickness and were then hot rolled at 1100°C up to a thickness of 7 mm. Next, they were cold rolled up to a thickness of 4.9 mm and were retained at a final annealing temperature of 1100°C for 20 minutes. Subsequently, water cooling was carried out to cool them to room temperature and 3 hours' thermal treatment at 600°C followed (under conditions based on a supposed life in practical environments in use). From these materials, there were prepared 2-mm-thick x 10-mm-wide x 75-mm-long specimens for an alkali stress corrosion cracking test and 3-mm-thick x 10-mm-wide x 40-mm-long specimens for a corrosion test. The alkali stress corrosion cracking test was accomplished by polishing the specimens with emery paper No. 320; bending them into a U-shape and holding them with bolts and nuts; immersing them in a solution including 30% of NaOH in an autoclave container (a high-temperature and high-pressure container) at 325°C for 2000 hours; and, after the completion of the immersion process, measuring a depth of cracks by a microscope. On the other hand, the corrosion test was accomplished by polishing the specimens with emery paper No. 320; immersing them in a solution including 100 ppm of Cl⁻ ions and having a pH of 4.5 in an autoclave container at 288°C for 2000 hours; and measuring a corrosion amount. Obtained results are shown in Figures 5 and 6 in the form of summary graphs. Numerals in Figure 6 correspond to the alloy numbers in Table 6. Figure 5 presents the stress corrosion cracking test results of the specimens of alloy No. 1 under the above-mentioned alkaline conditions. In this drawing, white circles and black circles represent specimens having cracks less than 25 µ in depth and those having cracks more than 25 µ in depth, respectively. As be apparent from the drawing, the specimens in the hatched range (Z) surrounded by points A, B and C have good alkali stress corrosion cracking resistance. In this connection, it was confirmed that the alloys according to the present invention other than alloy No. 1 also had substantially similar results. The data regarding the corrosion resistance in Figure 16 indicate that when the total content of at least one of Mo, V and W is less than 0.5%, the effect of the corrosion resistance will not be seen, but if its content is 0.5% or more, the corrosion resistance will be improved. This reason would be that the elements of Mo, V and W permit forming the fine and stable passive coating comprising Cr₂O₃. Table 7 summarizes the results of the corrosion resistance under the same conditions as in Figure 6. In this table, circles, triangles and crosses represent specimens not having any pitting corrosion, those having the slight pitting corrosions and those having the pitting corrosions. It can be understood from these results that the alloys according to the present invention are more excellent in the pitting corrosion resistance, as compared with the conventional alloys. Particularly, when the total amount of Mo, V and W to be added is 1.0% or more, the alloy can have the extremely excellent pitting corrosion resistance. As be definite from the foregoing, the alloy according to the present invention is excellent in the pitting corrosion resistance, the stress corrosion cracking resistance and the alkali stress corrosion cracking resistance, and, in place of the conventional Alloy 600, the alloy according to the present invention can be thus used, for example, particularly for a heat transfer pipe of a steam generator in a pressurized water reactor.

Claims: A method of preparing a nickel-chromium alloy for a material which will be subjected to a high-temperature and high-pressure water or vapor, comprising that in a first thermal treatment process, said nickel based alloy is heated and retained at a temperature of T °C (at which chromium carbides are thoroughly solubilized) to (T + 100) °C which corresponds to the hatched zone in Fig. 4 and is cooled at a cooling rate of a furnace cooling rate or more, and that in a second thermal treatment process, said nickel based alloy is then retained at a temperature of 600 to 750 °C and within a sensitization recovery range which corresponds to the hatched zone in Fig. 2 for a period of 0,1 to 100 hours and is cooled at a cooling rate of said furnace cooling rate or more, said alloy consisting of, in terms of % by weight, 58 % or more of Ni, 25 to 35 % of Cr, 0,003 % or less of B, 0,012 to 0,035 % of C, 1 % or less of Mn, 0,5 % or less of Si, 0,015 % or less of P, 0,015 % or less of S, optionally 0,5 % or less of Al; 0,01 to 1,0 % of Ti; 0,5 to 5,0 % in all of one or more of Mo, W and V, and the balance being Fe plus impurities. A method according to claim 1, whereby that material is cooled after the said first treatment to a level of 200 °C or less and the second treatment is carried out under conditions within a hatched range (2) in Figur 5.

Inventors: INOUE, MAMORU, KAWAGUCHI, KATSUJI TAKASAGO TECHNICAL INSTITUTE, KUSAKABE, TAKAYA C/O KOBE SHIPYARD & ENGINE WORKS, MINAMI, TAKAO, NAGANO, HIROO, OKADA, YASUTAKA, ONIMURA, KICHIRO TAKASAGO TECHNICAL INSTITUTE, SASAGURI, NOBUYA TAKASAGO TECHNICAL INSTITUTE, SUSUKIDA, HIROSHI TAKASAGO TECHNICAL INSTITUTE, YAMANAKA, KAZUO, YONEZAWA, TOSHIO TAKASAGO TECHNICAL INSTITUTE, Kusakabe, Takaya c/o Kobe Shipyard &amp; Engine Works
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA, SUMITOMO METAL INDUSTRIES, LTD.
Application year: 1983
Main classification: C22C19
IPC codes: C22C19, C22F1