NI-BASED ALLOY AND METHOD FOR PREPARING SAME

Abstract: The presnt invention is concerned with a high-strength Ni-based alloy excellent in resistance to stress corrosion cracking in high-temperature high-pressure water, charac­ terized by consisting essentially of, in terms of weight ratio, 0.08% or less of C, 0.15% or less of Si, 0.1 to 1% of Mn, 15% or less of Fe, 20 to 30% of Cr, 3.5% or less of Ti, the alloy at 980 to 1,200°C, cooling it, and subjecting it once or more to an aging treatment of additionally heating and maintaining it at 550 to 850°C.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to an Ni-based alloy which has an excellent resistance to stress corrosion cracking as well as a high strength and which is thus suitable for structural members in light-water reactors or new type nuclear converters, fastening members such as pins, bolts and screws used for fuel elements, spring members such as leaf springs and coiled springs, bolts for turbines, supporting structural members for heat exchangers, and it also relates to a method for preparing such an Ni-based alloy. Heretofore, as the above mentioned material applicable to the light-water reactor and the like, a precipitated and reinforced Ni-based alloy has often been used which is called Inconel X-750 (trade name) and which is composed of 72% or more of Ni, 14 to 17% of Cr, 6 to 9% of Fe, 1 to 2% of each of Al, Ti and Nb. However, Inconel X-750 is liable to undergo stress corrosion cracking under given circumstances of the above mentioned applications, depending on conditions for a used heat treatment, and the stress corrosion cracking would occur at times in the above mentioned fastening members and the like made from such a material. In general, the high-strength materials having a great 2% proof strength and tensile strength are considered to be poor in the resistance to stress corrosion cracking. Therefore, no materials have been present anywhere which are desirable as the above mentioned pins, bolts and springs requiring the high strength and the excellent resistance to stress corrosion cracking in high-temperature and high-pressure water. OBJECT AND SUMMARY OF THE INVENTION The present invention has been intended in view of the aforesaid disadvantage of the conventional alloy, and its object is to provide an Ni-based alloy which has a high strength and which is additionally excellent in resistance to stress corrosion cracking in high-temperature high-pressure water. The inventors of the present application have conducted researches intensively, and as a result, it has been found that with regard to the fastening members of the conventional Inconel X-750, a metallic construction varies with chemical composition, conditions for heat treatment, working conditions and the like, with the result that the sensitivity of the material to stress corrosion cracking is disadvantageously heightened. And on the basis of such a knowledge, a novel Ni-based alloy, which has a chemical composition and a metallic construction free from the above mentioned problem, and its manufacturing method have now been developed. That is, the present invention is directed to a high-strength Ni-based alloy excellent in resistance to stress corrosion cracking in high-temperature high-pressure pure water which is characterized by consisting of, in terms of weight ratio, 0.08% or less of C, 0.15% or less of Si, 0.1 to 1% of Mn, 15% or less of Fe, 20 to 30% of Cr, 0,76 to 3,5% of Ti, 2% or less of Al, 7% or less of Nb optionally up to 10% Mo and/or up to 0,1% of at least one of rare earth element, Mg and Ca, and the balance, apart from impurities, of Ni; having at least one of a γ' phase and a γ" phase in a γ base; and semicontinuously predominantly precipitating M₂₃C₆ in grain boundaries, and the present invention is also directed to a method for preparing this high-strength Ni-based alloy according to the features of claim 2. EP-A-56 480 discloses an alloy which is used in the same environment as said of the present invention and the composition is partially similar. However, nothing at all is described about the half-continous precipitation of M₂₃C₆ in the grain boundaries constituting the essential requirements of the present invention. The aforesaid and other objects, features and benefits of the present invention will be more apparent from the following explanation in reference to accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 (a), 1 (b) and 1 (c) all are explanatory views of test pieces used for tests in examples regarding the present invention; Figs. 2 (a) and 2 (b) both are graphs showing relations of heat treatment conditions to each amount of C and Cr; Fig. 3 is a similar graph showing a relation of heat treatment conditions to each amount of Si and Mn; Fig. 4 is a similar graph showing a relation between heat treatment conditions and an amount of Mo; Figs. 5 (a) and S (b) are graphs showing relations between heat treatment conditions and a cold working ratio; Fig. 6 is a graph showing a relation between amounts of Ti and Nb; Fig. 7 is a graph showing a relation between amounts of Al and Nb; Fig. 8 is a graph showing relations of a cold working ratio to a tensile strength and a 0.2% proof strength; and Figs. 9 and 10 show relations of a tensile strength and a 0.2% proof strength to amounts of C and Cr, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Now, reference will be made to reasons for restriction on amount values of the respective components in the aforesaid inventions. C: C is bound to Cr in order to form the Cr carbide of M₂₃C₆ in grain boundaries and to thereby heighten a binding power of crystal grains therein. However, when an amount of C is in excess of 0.08%, C will be bound to Nb and Ti in order to form NbC and TiC, and γ' and γ" phases which will be formed by binding Nb and Ti to Ni will be decreased, with the result that the strength of a produced alloy will decline. In consequence, the content of C therein is set to 0.08% or less. Si: Si has the function of removing oxygen, which is an impurity, from the alloy, but when its content is more than 0.15%, the semicontinuous precipitation of M₂₃C₆ will be prevented in grain boundaries, and in consequence, the stress corrosion cracking resistance of the produced alloy will decline. Accordingly, the content of Si is set to 0.15% or less. Mn: Mn is an element for accelerating the semicontinuous precipitation of M₂₃C₆ in grain boundaries, and it is necessary that its content is 0.1% or more. However, when it is in excess of 1%, a brittle phase for impairing the ductility of the produced alloy will be precipitated superiorly. Therefore, the content of Mn is set to the range of 0.1 to 1%. Fe: Fe is an element of heightening the stability of an alloy construction at the time of casting or plastic working, but when its content exceeds a level of 15%, the ductility of the produced alloy will be hurt. For this reason, the content of Fe is set to 15% or less. Cr: Cr is the most important element to retain the resistance to stress corrosion cracking, and its content is required to be 20% or more. However, when the content of Cr is more than 30%, solidification and segregation will occur remarkably and thus forging will be difficult to do. In addition, a uniform ingot will be hard to produce. Therefore, the content of Cr is set to the range of 20 to 30%. Mo: Mo improves the resistance to pitting corrosion and the resistance to gap corrosion, but when its amount is in excess of 10%, the precipitation of M₂₃C₆ will be inhibited in grain boundaries and the resistance to stress corrosion cracking will decline. Accordingly, the content of Mo is set to 10% or less. Ti: Ti is bound to Ni in order to precipitate γ′ of Ni₃Ti and to thereby build up the strength of the product. When a content of Ti is more than 3.5%, its ductility will be poor, and a η phase will precipitate, which fact will lead to the deterioration in the resistance to stress corrosion cracking. For this reason, the content of Ti is set to 3.5% or less. Al: Al is bound to Ni in order to precipitate γ′ of Ni₃Al and to thereby heighten the strength of the product, but when its content exceeds a level of 2%, the resistance to stress corrosion cracking will deteriorate. Therefore, the content of Al is set to 2% or less. Nb: Nb is bound to Ni in order to precipitate a γ″ phase of Ni₃Nb or a δ phase and to thereby heighten the strength of the alloy product, but when its content is in excess of 7%, the resistance to stress corrosion cracking will decline. In consequence, the content of Nb is set to 7% or less. Rare earth element, Mg and Ca: A rare earth element such as Hf or Y, Mg and Ca not only remove oxygen, which is an impurity, from the alloy but also enhance the binding power of grain boundaries. However, when each content thereof is in excess of 0.1%, the resistance to stress corrosion cracking will be poor. Therefore, the content of at least one of the rare earth element, Mg and Ca is set to 0.1% or less. As conditions for the heat treatment, there are required the solid solution treatment and the subsequent aging treatment so as to keep up the high strength and the high resistance to stress corrosion cracking of the alloy, the aforesaid solid solution treatment comprising the steps of heating and maintaining the alloy at 980 to 1,200°C, and then cooling the alloy at a cooling rate of an air cooling or more, the aforesaid aging treatment comprising the step of additionally heating and maintaining the alloy at 550 to 850°C, and being necessarily carried out once or more. In this connection, the heat treatment is preferably carried out for a period of 5 minutes to 5 hours in the solid solution treatment and further for 1 to 150 hours in the aging treatment. Generally, in the case of a material for casting, the above mentioned solid solution treatment and aging treatment alone are enough, but when a cold working and a hot working are additionally performed, the following conditions may be employed for the working operations. That is, the cold working, after the solid solution treatment, may be carried out uniformly at a high working ratio of 10% or more reduction of area in order to procure the excellent resistance to stress corrosion cracking. According to the aforesaid cold working conditions, there can be prepared the high-strength material having not only the excellent resistance to stress corrosion cracking but also a 0.2% proof strength of 90 kg/mm² or more and a tensile strength of 100 kg/mm². Further, the above mentioned hot working may be carried out uniformly at a working temperature of 850 to 1,250°C so as to prevent the cracking and an excessive grain growth, and at a draft percentage of 20% or more so as to retain the excellent resistance to stress corrosion cracking. According to the above hot working conditions, there can be prepared the high-strength material having not only the excellent resistance to stress corrosion cracking but also a 0.2% proof strength of 70 kg/mm² or more at room temperature and a tensile strength of 90 kg/mm². Next, reference will be made to tests of stress corrosion cracking. These tests were carried out by the following procedure. (1) Tests of stress corrosion cracking: For the purpose of evaluating, under circumstances in a light-water reactor, the stress corrosion cracking resistance of fastening members, bellows and the like in which the Ni-based alloy of the present invention was employed, tests of stress corrosion cracking were carried out by immersing U-bent test pieces shown in Fig. 1 into water having conditions in Table 1 which simulated a primary system water in a pressurized water type light-water reactor; then applying a high stress thereto for 4,000 hours; and afterward checking cracks in the test pieces. (2) Test pieces: Chemical composition of the test pieces used in the tests is set forth in Table 2, and conditions for the heat treatments and the workings of the test pieces are exemplarily set forth in Tables 3-1 and 3-2. In the test pieces, elements of P and S were each contained in an amount of at most 0.01% or so, Cu in an amount of at most 0.07% or so, and N in an amount of at most 0.01% or so, as impurities. (3) Results of tests: The results of the tests are set forth in Tables 3-1 and 3-2 and Figs. 2 to 10. As elucidated in Table 4, white and black symbols in the accompanying drawings indicate "not cracked" and "cracked", respectively, in the test pieces. With regard to each test piece in which no cracks occurred, its metallic construction was observed. The results made it apparent that a γ′ phase or a γ″ phase was dispersed in a γ base and that M₂₃C₆ was semicontinuously and predominantly precipitated in grain boundaries. Typical examples of such test pieces are set forth in Table 5. The crack occurrences due to the influence of the respective components and heat treatment conditions are exhibited in Figs. 2 (a) and 2 (b) as well as Figs. 3, 4 and Table 6, and it can be grasped that the test pieces in the range of the compositions and the heat treatment conditions of the present invention were more excellent in resistance to stress corrosion cracking than the other test pieces. Further, Figs. 5 (a) and 5 (b) show relations of the crack occurrences to ratios of the cold working and temperatures of the solid solution treatment, and it is indicated thereby that all the test pieces in the range of the conditions regarding the present invention were more excellent in resistance to stress corrosion cracking than the other ones. Figs. 6 and 7 show the influences of amounts of Ti and Al on the stress corrosion cracking resistance, and it is definite that all the test pieces in the range of the conditions regarding the present invention were more excellent in resistance to stress corrosion cracking than the other ones. In Fig. 8, there are shown relations between mechanical properties and ratios of the cold working, and all the test pieces in the range of the present invention were excellent in resistance to stress corrosion cracking and additionally in a 0.2% proof strength and a tensile strength, as shown in Figs. 5 (a) and 5 (b). Figs. 9 and 10 exhibit relations between chemical components and mechanical properties of the alloys which were subjected to the hot working at a 30% draft, and the test pieces in the range of the present invention were excellent in stress corrosion cracking resistance and additionally in mechanical properties. With regard to the respective drawings (except Fig. 1) having the subsections (a) and (b), as in Figs. 2 (a) and 2 (b), data are divided into the drawings of (a) and (b) by presence or absence of the Mo element. The drawing to which "(a)" is attached is concerned with the test pieces containing no Mo, and the drawing with "(b)" is about the test pieces containing Mo. As described above, the present invention permits obtaining the Ni-based alloy which has the satisfactory mechanical strength and stress corrosion cracking resistance simultaneosuly, and therefore the Ni-based alloy according to the present invention can be utilized extremely safely for a period of its prolonged life as fastening members, spring parts and the like, in addition to structural parts in the light-water reactor.

Claims: A high-strength Ni-based alloy excellent in resistance to stress corrosion cracking in high-temperature high-pressure pure water, characterized by consisting of, in terms of weight ratio, 0.08 % or less of C 0.15 % or less of Si, 0.1 to 1 % of Mn, 15 % or less of Fe, 20 to 30 % of Cr, 0.76 to 3.5 % of Ti, 2 % or less of Al, 7 % or less of Nb, optionally up to 10 % Mo and/or up to 0.1 % of at least one of rare earth element, Mg and Ca, and the balance apart from impurities of Ni; having at least one of a γ' phase and a γ" phase in a γ base; and semicontinuously predominantly precipitating M₂₃C₆ in grain boundaries. A method for preparing a high-strength Ni-based alloy excellent in resistance to stress corrosion cracking in high-temperature high-pressure pure water, characterized by heating and maintaining, at 980 to 1,200 °C, said alloy consisting of, in terms of weight ratio, 0.08 % or less of C, 0.15 % or less of Si, 0.1 to 1 % of Mn, 15 % or less of Fe, 20 to 30 % of Cr, 0.76 to 3.5 % of Ti, 2 % or less of Al, 7 % or less of Nb, optionally up to 10 % Mo and/or up to 0.1 % of at least one of rare earth element, Mg and Ca, and the balance, apart from impurities, of Ni; cooling said alloy; and subjecting said alloy once or more to an aging treatment of additionally heating and maintaining it at 550 to 850 °C. A method according to claim 2, wherein in the heat treated and cooled alloy is subjected to a cold working at a 10 % or more reduction of area before the aging treatment. A method according to claim 2, wherein the alloy, prior to being heated and maintained at 980 to 1.200 °C, hot worked at 850 to 1.250 °C at a draft percentage of 20% or more. A method according to any of claims 2 to 4, wherein a duration of said heating and maintaining step at 980 to 1.200 °C is within the range of 5 minutes to 5 hours, a cooling rate in said cooling step is an air cooling rate or more, and a duration of said aging treatment is 1 to 150 hours.

Inventors: OKADA, YASUTAKA, YAMAGUCHI, NORITAKE KOBE SHIPYARD & ENGINE WORKS, YONEZAWA, TOSHIO C/O TAKASAGO TECHN. INSTITUTE, Yamaguchi, Noritake Kobe Shipyard &amp; Engine Works
Applicants: MITSUBISHI JUKOGYO KABUSHIKI KAISHA, SUMITOMO METAL INDUSTRIES, LTD.
Application year: 1987
Main classification: C22C19
IPC codes: C22F1, C22C19