Nickel-chromium alloy

Abstract: The invention is directed to a method of preparing a nickel-chromium alloy of excellent cracking resistance to stress corrosion and is characterized by carring out an annealing treatment in range (Y) in Figure 1.

Description: The present invention relates to a method of preparing a nickel-chromium alloy of excellent cracking resistance to stress corrosion, 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 vapour, 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. US 35 73 901 discloses a high nickel alloy resistant to stress-corrosion cracking in leaded high purity water. Said alloy consists of about 24 to 32 % Cr, 50 to 67 % Ni, up to 10 % Mo, up to 10 % V, up to 10 % W. 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. 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 gist of the present invention is directed to a method of preparing a nickel-chromium alloy excellent in a stress corrosion cracking resistance which is obtained by carrying out an annealing treatment under required conditions, said alloy having the following composition: in terms of % by weight, 0.04% or less of C; 1.0% or less of Si; 1.0% or less of Mn; 0.030% or less of P; 0.02% or less of S; 40 to 70% of Ni; 25 to 35% of Cr; 0.1 to 0.5 % of Al; 0.05 to 1.0% of Ti; 0.5 to 5.0% in all of one or more of Mo, W and V; and the residue comprising Fe plus impurities. The above-mentioned required conditions mean annealing conditions within a range (Y) surrounded by points A, B, C, D and E in Figure 1 attached hereto. The aforesaid range (Y) is determined by A (C = 0%, 910°C), B (C = 0%, 850°C), C (C = 0.02%, 850°C), D (C = 0.04%, 900°C) and E (C = 0.04%, 1000°C). If the operation is made under the annealing conditions in the range (Y), the stress corrosion cracking resistance of the Ni-Cr alloy, which is heretofore insufficient, can be remarkably improved. Such an unexpected effect would be considered to be due to a synergistic effect of (i) the requirement that the C content is limited to 0.04% or less and a final annealing is carried out at a relatively low temperature in compliance with the C content, and (ii) the requirement that at least one of Mo, W and V is added as an element for reinforcing the coating. Then, the present invention, referring to FIGS. 1 to 5, will be described below. The reason why a composition of the alloy according to the present invention is defined as mentioned above is as follows: C: Since C is an element harmful to the SCC resistance, its content is limited to 0.04% or less. Si, Mn and Al: These elements all are deoxidizers, and they are added in a suitable amount in accordance with melting conditions. However, when the contents of Si, Mn and Al exceed upper limits of 1.0%, 1.0% and 0.5%, respectively, the formed alloy will be deteriorated in cleanness. Further, when being less than 0.1%, Al is not effective. Ni: This element is effective to improve a corrosion resistance, particularly it serves to improve an acid resistance and the SCC resistance in a high-temperature water including Cl⁻ ions. For the achievement of these effects, the content of Ni is required to be 40% or more, and its upper limit is set to 70%, taking addition proportions of alloy elements of Cr, Mo, W, V and the like into consideration. Cr: The element Cr is essential for the improvement in the corrosion resistance, but its amount less than 25% is insufficient to enhance the SCC resistance. On the contrary, when it is more than 35%, a hot workability will remarkably deteriorate. Therefore, the content of Cr is limited to the range of 25 to 35% in the present invention. P: The element P is present in the alloy as an impurity. If its content is above 0.030%, it will exert a harmful influence upon the acid resistance and the hot workability. S: The element S is also one of the impurities. If being present in an amount more than 0.02%, it will be deleterious to the acid resistance and hot workability, as in the case of P. Ti: This element Ti is added as a stabilizing agent. That is to say, even if the contents of P and S are controlled below the above-mentioned levels, a remarkable effect cannot be obtained. Therefore, in the present invention, Ti is added in an amount of 0.05% or more to assure the desired hot workability. On the contrary, when the content of Ti is more than 1.0%, its effect will reach a ceiling level. Therefore, the upper limit of this element is to be set to 1.0%. Mo, W and V: These elements 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 less than 0.5% in all, the passive coating on the surface will not be heightened and a pitting corrosion will occur, thereby deteriorating the stress corrosion cracking resistance. On the contrary, when the content of at least one of them is more than 5.0% in all, the effect of the improvement in the pitting corrosion resistance will reach a ceiling level, and the hot workability will noticeably be deteriorated. Therefore, in the present invention, the amount of one or more of Mo, W and V to be added is limited to the range of 0.5 to 5.0% in all. Nb: In the nickel based alloy (which includes 40% or more of Ni), Nb is greater in the effect of a carbon fixation than Ti. In the present invention, the content of Nb is set to the range of 0.2 to 5.0%. In this range, the ratio of Nb/C will become 10 to 125. In the case of its amount being 0.2% or less, the effect of fixing carbon is small and a sensitization will thus occur, thereby generating the SCC (stress corrosion cracking). On the contrary, when the content of Nb is more than 5%, the effect (carbon fixation) will reach a ceiling level, and additionally the hot workability will noticeably be deteriorated. Therefore, its upper limit is set to 5.0%. I. Now, reference will be made to the annealing treatment under annealing conditions in the above-mentioned range (Y). Referring first to Figure 1, lines BC and CD represent recrystallization lines of the alloy according to the present invention. If the annealing treatment is carried out at a temperature below the levels of the lines BC and CD, no recrystallization will occur, so that the strength of the annealed alloy will be high and its corrosion resistance will be bad. Therefore, the annealing treatment is required to be carried out at a temperature above the levels of the lines BC and CD in accordance with a C content in the alloy. On the other hand, a line AE in the same drawing means an upper limit of temperatures at which the carbon in the alloy is not thoroughly solubilized. Accordingly, so long as the annealing treatment is carried out at a temperature below this upper limit, a carbide will be present in the grains. However, if the annealing operation is done at a temperature above a level of the line AE, all the carbide will be deposited on crystal boundaries in the case that a sensitization treatment is accomplished at at a temperature of 600°C for a period of 3 hours. This will lead to the deterioration in the crystal boundary etching resistance. Therefore, the final annealing is required to be carried out at a temperature below the level of the line AE. Now, the present invention will further be described in detail in accordance with examples below. Examples 1 to 29 Alloys (Alloy Nos. of the present invention 1 to 29, conventional alloys Nos. 30 to 37 and comparative alloys Nos. 38 to 41) of compositions comprising chemical components exhibited in Table 1 below were dissolvingly formed in a 17-kg vacuum furnace and subjected to a forging, hot rolling and thermal treatment under usual conditions, and they were then cold rolled as much as 30%, followed by annealing at a variety of temperatures. Further, a thermal treatment, i.e. a sensitization treatment on conditions, 600°C x 3 hours, which were set on the basis of a supposed life in practical use was carried out, and 3-mm-thick x 10-mm-wide x 40-mm-long speciments for crystal boundary etching tests and 2-mm-thick x 10-mm-wide x 75-mm-long specimens for stress corrosion cracking tests were then prepared. These speciments were polished by the use of emery paper No. 320 and were then employed for the tests below. First, the specimens for the stress corrosion cracking tests were, after polished, caused to overlap each other every 2 specimens and each pair of them was bent into a U-shape to prepare double U-bent speciments. The thus prepared specimens were immersed in a solution including 1000 ppm of Cl⁻ (as NaCl) at 325°C for 1500 hours by the use of an autoclave (a high-temperature and high-pressure container). After the completion of the tests, cracks on inside surfaces of the specimens were measured for their depth by a microscope. On the other hand, the specimens for the crystal boundary etching tests were immersed in a boiling solution including 60% of HNO₃ and 0.1% of HF for 4 hours, and a weight loss caused by the corrosion was measured. Obtained test results are shown by graphs in Figures 2 to 5. Reference numerals in the graphs represent the numbers of the specimen alloys in Table 4. A variety of amounts of Ni was added to each fundamental composition comprising 0.02 to 0.03% of C, 25% of Cr and 0.6% of Mo according to the present invention to prepare alloy specimens, and an annealing treatment was then carried out by heating the specimens at 1150°C for 30 minutes. After water cooling, a sensitization treatment was carried out by heating them at 600°C for 3 hours and they were then cooled. The aforesaid crystal boundary etching tests were accomplished on the specimens to prepare data. Figure 2 exhibits the thus obtained data. The aforesaid annealing temperature was higher than that of the present invention. Even in the case of the alloy having the same composition as the alloy according to the present invention, if the annealing temperature is high and when 3 hours' heating at 600°C (the sensitization treatment) and an air cooling operation are carried out, the carbide of Cr will all deposit on the crystal boundaries and Cr-free layers will be formed in the vicinity of the crystal boundaries, so that corrosion will occur. Therefore, it is necessary to lower the annealing temperature. The graphs in Figure 3 show the crystal boundary etching resistances of the alloys comprising the compositions regarding the present invention and conventional alloys. The alloys in both the groups which had the composition of 0.02 to 0.03% of C and 0.6% of Mo were heated at 900°C for 30 minutes to accomplish the annealing treatment. After water cooling, they were heated at 600°C for 3 hours to accomplish the sensitization treatment, followed by air cooling. In Figure 3, white and black circles represent the alloys including more than 30% of Cr and those including 25 to 30% of Cr, respectively. As understood from the graphs in this drawing, the alloys including an Ni amount below 40% are all great in a corrosion rate; the alloys including an Ni amount of 40% or more have an improved crystal boundary etching resistance. Therefore, the Ni content of 40% or more is necessary. One or more of Mo, V and W were added to each fundamental composition comprising 0.02% of C, 25% of Cr and 50% of Ni in order to prepare alloy specimens, and an annealing treatment was then carried out by heating the prepared specimens at 900°C for 30 minutes. After water cooling, the sensitization treatment was carried out by heating them at 600°C for 3 hours and they was then air cooled. Thus obtained results of the crystal boundary etching tests are exhibited in Figure 4. This drawing indicates that when the total amount of at least one of Mo, V and W is less than 0.5%, any improvement in corrosion resistance is not seen, but when its content is 0.5% or more, the crystal boundary etching resistance is built up. This would be considered to allow a Cr₂O₃ coating formed on the alloy surface to stably exist, because the added Mo, V and W strengthen the passive coating. Hence, the total amount of one or more of the added Mo, V and W is required to be 0.5% or more. The graphs in Figure 5 show influences of an Ni content (%) and Cr content (%) upon the SCC resistance. Used alloy specimens were prepared through the annealing treatment of 30 minutes' heating at 900°C, water cooling, sensitization treatment of 3 hours' heating at 600°C, and air cooling. In this drawing, white and black circles represent the alloys without stress corrosion cracks and those with some cracks of 20 µ or more. It is apparent that even if the Cr content is 20% or more as in the present invention, when the Ni content is less than 40%, crystal boundary type stress corrosion cracks will occur. Therefore, the Ni content is required to be 40% or more.

Claims: A method of preparing a nickel-chromium alloy of excellent cracking resistance to stress corrosion, characterized by carrying out an annealing treatment depending on the carbon content of said alloy in a range (Y) surrounded by points, A, B, C, D and E in Fig. 1, said alloy consisting of, in terms of % by weight, 0,04 % or less of C; 1,0 % or less of Si; 1,0 % or less of Mn; 0,030 % or less of P; 0,02 % or less of S; 40 to 70 % of Ni; 25 to 35 % of Cr.; 0,1 to 0,5 % of Al; 0,05 % 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.

Inventors: INOUE MAMORU, KAWAGUCHI KATSUJI TAKASAGO TEC, KUSAKABE TAKAYA KOBE SHIPYARD, MINAMI TAKAO, NAGANO HIROO, OKADA YASUTAKA, ONIMURA KICHIRO TAKASAGO TECHN, SASAGURI NOBUYA TAKASAGO TECHN, SUSUKIDA HIROSHI TAKASAGO TECH, YAMANAKA KAZUO, YONEZAWA TOSHIO TAKASAGO TECHN, INOUE, MAMORU, KAWAGUCHI, KATSUJI TAKASAGO TECHNICAL INSTITUTE, KUSAKABE, TAKAYA 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 Kobe Shipyard &amp; Engine Works
Applicants: MITSUBISHI HEAVY IND LTD, SUMITOMO METAL IND, MITSUBISHI JUKOGYO KABUSHIKI KAISHA, SUMITOMO METAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C22C19
IPC codes: C22F1, C22C19