HIGH-NICKEL AUSTENITIC STAINLESS STEEL RESISTANT TO DEGRADATION CAUSED BY NEUTRON IRRADIATION

Abstract: The present invention aims at providing structural materials having resistance to degradation by neutron irradiation, causing no SCC in the environment of light-water reactors even after being subjected to neutron irradiation of approximately at least 1 x 10 22 n/cm 2 (E > 1 MeV), and having thermal expansion coefficients approximately similar to that of structural materials. High nickel austenitic stainless steels having resistance to degradation by neutron irradiation can be produced by subjecting stainless steels having compositions (by weight %) of 0.005 to 0.08 % of carbon, at most 0.3 % of Mn, at most 0.2 % of (Si + P + S), 25 to 40 % of Ni, 25 to 40 % of Cr, at most 3 % of Mo or at most 5 % of (Mo + W), at most 0.3 % of Nb + Ta, at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe to a solution-annealing treatment at a temperature of 1000 to 1l50°.

Description: Technical Field This invention relates to high nickel austenitic stainless steels having excellent resistance to degradation by neutron irradiation, which are used as structural materials for nuclear power plants of light-water reactors. Background Technique Up to the present time, it has been known that when austenitic stainless steels such as SUS 304, 316, etc., which are used as structural materials for nuclear power plants of light-water reactors, are used for a long time and subjected to neutron irradiation of at least 1 x 10 21 n/cm 2 (E > 1 MeV), Cr is depleted and Ni, Si, P, S, etc. are enriched, at crystal grain boundaries, resulting in a tendency to cause stress corrosion cracking (SCC) in the presence of high load stress in the environment of light-water reactors. This is called "irradiation assisted stress corrosion cracking" (IASCC). It has eagerly been desired to develop materials with low IASCC susceptibility, but such low IASCC susceptibility materials (excellent resistance to degradation by neutron irradiation) have not been developed yet. Austenitic stainless steels such as SUS 304, 316, etc., have been used as structural materials for nuclear power plants of light-water reactors, but when these materials are subjected to neutron irradiation of at least 1 x 10 21 n/cm 2 (E > 1 MeV) for a long time, changes of concentrations of their elements take place which do not or hardly occur before use. That is, it is known that when Cr is depleted and Ni, Si, P, S, etc. are enriched at crystal grain boundaries (which will hereinafter be referred to as "radiation induced segregation") and there is a high load stress or residual stress, stress corrosion cracking (irradiation assisted stress corrosion cracking, IASCC) tends to occur in high temperature and pressure water as a neutron irradiation environment in light-water. Furthermore, it is known that the presence of oxygen in large amount in high temperature and pressure water accelerates generation of IASCC. Thus, the inventors have made various studies on properties of stainless steels and as a result of comparison of the inventors' calculation results on the amounts of change in concentrations of Cr and Ni at crystal grain boundaries, based on S. Dumbill and W. Hanks' measured values of the crystal grain boundary segregation of neutron irradiated materials (Sixth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors, 1993, p. 521) with the inventors' accumulated SCC test results of neutron-irradiated SUS 304, 316, etc., it is found that the above described IASCC occurs when, at grain boundaries after neutron irradiation, the amount of Cr is at most 15 % and the amount of Ni is at least 20 %, as shown in Fig. 2, in which the slant line part shows the occurrence zone of SCC. The inventors have considered that such a phenomenon of the occurrence of IASCC is due to the concentrations of elements at crystal grain boundaries being similar to the composition of Alloy 600 (NCF 600 of JIS). Specifically, IASCC is considered to be probably due to compositions at crystal grain boundaries becoming low in Cr and high in Ni by neutron irradiation and approaching the composition of Alloy 600 (non-irradiated material), resulting in stress corrosion cracking (PWSCC: primary water stress corrosion cracking) in water at high temperature and pressure, often taking place in Alloy 600. At the present time, however, the mechanism of occurrence of PWSCC in Alloy 600 is not elucidated. The inventors have made studies based on the above described knowledge and reached the present invention by specifying a composition of a suitable material and simultaneously, combining it with a heat treatment and post working method for rendering optimum a crystal form in an alloy. That is to say, the present invention aims at providing structural materials having a resistance to degradation by neutron irradiation, causing no SCC in the environments of light-water reactors (in high temperature and pressure water or in high temperature and pressure water saturated with oxygen) even after subjecting to neutron irradiation of approximately at least 1 x 10 22 n/cm 2 (E > 1 MeV), corresponding to the quantity of maximum neutron irradiation received up to the end of the plant life of light-water reactors and having a thermal expansion coefficient approximately similar to that of SUS 304, 316, etc. For information JP-A-3068737 describes a high nickel austenitic steel containing inter alia 40 to 60% nickel which has improved stress corrosion cracking resistance. Disclosure of Invention This invention provides high nickel austenitic stainless steels having resistance to degradation by neutron irradiation, which comprise a stainless steel having a composition (by weight %) of 0.005 to 0.08 % of carbon, at most 0.3 % of Mn, at most 0.2 % of (Si + P + S), 25 to 35 % of Ni, 25 to 40 % of Cr, at most 3 % of Mo, at most 5% of (Mo + W), at most 0.3 % of (Nb + Ta), at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe, said stainless steels being subjected to a solution-annealing treatment at a temperature of 1000 to 1150°C, and wherein optionally a cold working up to 30 % is carried out after the above defined solution-annealing treatment, wherein a heat treatment for a period of up to 100 hours is carried out at 600 to 750°C after the above defined solution-annealing treatment or optional cold working. This invention also provides a process for the production of high nickel austenitic stainless steels having resistance to degradation by neutron irradiation, which comprises subjecting stainless steels having compositions (by weight %) of 0.005 to 0.08 % of carbon, at most 0.3 % of Mn, at most 0.2 % of (Si + P + S), 25 to 35 % of Ni, 25 to 40 % of Cr, at most 3% of Mo, at most 5% of (Mo + W), at most 0.3 % of (Nb + Ta), at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe to a solution-annealing treatment at a temperature of 1000 to 1150°C, and wherein optionally a cold working up to 30 % is carried out after the above defined solution-annealing treatment, wherein a heat treatment for a period of up to 100 hours is carried out at 600 to 750°C after the above described solution-annealing treatment or optional cold working. Brief Description of the Drawings Fig. 1 is a flow sheet showing a process for the production of a test piece used in the Example, Fig. 2 is a graph showing the relationship between Cr and Ni concentrations and SCC susceptibility at crystal grain boundaries of an alloy, assumed from measured values of crystal grain boundaries segregation of neutron-irradiated materials, Fig. 3 is a graph showing the relationship between the fluence of a neutron-irradiated stainless steel and the quantity of (Si + P + S) at crystal grain boundaries thereof and Fig. 4 is a schematic view of the shape and dimension of a test piece used in an SCC accelerating test. High nickel austenitic stainless steels having resistance to degradation by neutron irradiation according to the present invention are materials having excellent SCC resistance in an environment of light-water reactors, i.e. in high temperature and high pressure water approximately at 270 to 350°c/70 to 160 atm and in high temperature and pressure water saturated with oxygen, even after neutron irradiation of up to at least 1 x 10 22 n/cm 2 (E > 1 MeV), and having a thermal expansion coefficient in a range of 15 x 10 -6 ∼ 19 X 10 -6 /K, near 18 x 10 -6 ∼ 19 X 10 -6 /K corresponding to an average thermal expansion coefficient of SUS 304 or 316 having hitherto been used or from room temperature to 400°C, which can be produced favorably on a commercial scale by the foregoing production processes (6) to (7), for example, by the flow sheet as shown in Fig. 1. As high nickel austenitic stainless steels having resistance to degradation by neutron irradiation, provided with such properties, when the environment is of high temperature and pressure water, there are high nickel austenitic stainless steels having resistance to degradation by neutron irradiation, which comprise stainless steels having compositions (by weight %) of 0.005 to 0.08 %, preferably 0.01 to 0.05 % of carbon, at most 0.3 % of Mn, at most 0.2 % of Si + P + S, 25 to 35 % of Ni, 25 to 40 % of Cr, at most 3 % of Mo, at most 0.3 % of Nb + Ta, at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe, said stainless steels being subjected to a solution-annealing treatment at a temperature of 1000 to 1150 °C, whereby solute atoms in the alloy are completely dissolved in the matrix. When the environment is of high temperature and pressure water saturated with oxygen, moreover, there are high nickel austenitic stainless steels having resistance to degradation by neutron irradiation, which comprise stainless steels having compositions (by weight %) of 0.005 to 0.08 %, preferably 0.01 to 0.05 % of carbon, at most 0.3 % of Mn, at most 0.2 % of Si + P + S, 25 to 35 % of Ni, 25 to 40 % of Cr, at most 5 % of Mo + W, at most 0.3 % of Nb + Ta, at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe, the said stainless steels being subjected to solution-annealing treatment at a temperature of 1000 to 1150°C, whereby solute atoms in the alloy are completely dissolved in the matrix. In these stainless steels, there are precipitated M 23 C 6 (carbide in which M is predominantly Cr) coherent with matrix in crystal grain boundaries. Crystal grain boundaries are strengthened by coherent precipitation of M 23 C 6 in the crystal grain boundaries which improves the SCC resistance. Furthermore, if necessary, high nickel austenitic stainless steels having been subjected to the above described solution-annealing treatment can be subjected to a cold working of up to at most 30 % at a temperature in the range up to at most the recrystallization temperature and dislocations due to slip deformation in the crystal grains are increased so as to raise the strength as bolt materials without losing SCC resistance. After the above described cold working, a heat treatment (aging treatment) is carried out at 600 to 750 °C and thus M 23 C 6 coherent with matrix can be precipitated sufficiently in the crystal grain boundaries, thereby improving the SCC resistance. For the purpose of the present invention, the cold working can be effected lightly to an extent of at most 30 %. The heat treatment (aging treatment) of up to 600 to 750°C is effective for a period of about up to 100 hours. The reason for specifying the composition range as described above (percent is to be taken as that by weight in the following composition) is as follows: As a result of studying the relationship between the phenomenon that materials are degraded by neutron irradiation, that is, the quantity of Cr depletes and that of Ni enriches at grain boundaries with stress corrosion cracking and susceptibility in an environment of light-water reactors, it is found that SCC occurs when the quantities of Cr and Ni at the grain boundaries are within the range of slant lines as shown in Fig. 2. Since the quantity of neutron irradiation which a high stress-loaded part receives among core parts of light-water reactors until the end of the plant life, is approximately at most 1 X 10 22 n/cm 2 (E > 1 MeV), the inventors have tried to obtain a required initial value of the Cr quantity (before neutron irradiation) for such an alloy so that the quantities of Cr and Ni are not within the range of the slant lines in Fig. 2 even if subjected to neutron irradiation of 1 x 10 22 n/cm 2 , from the amounts of change of the Cr and Ni concentrations at crystal grain boundaries, based on the measured values of crystal grain boundaries segregation of neutron-irradiated materials, which have been reported. Consequently, it is found that the initial value must be at least 25 %. The quantity of Cr should preferably be increased, but if increased too much, ductility is reduced which deteriorates the casting property, so the upper value is preferably adjusted to 40 %. When preparing an alloy containing at least 25 % of Cr, it is required to adjust a content of Ni to 25 to 35 % so that the austenitic phase may be stable and the thermal expansion coefficient may approach that of SUS 304 (17 x 10 -6 /K). In Fig. 2, the area ABCD represents the concentrations of Cr and Ni before neutron irradiation, while the area A'B'C'D' represents the concentrations at crystal grain boundaries after receiving neutron irradiation of 1 x 10 22 n/cm 2 (E > 1 MeV). When a relationship between the phenomenon that materials are degraded by neutron irradiation namely, the quantities of Si, P and S are enriched at grain boundaries and the phenomenon that the SCC susceptibility in the environment of light-water reactors is increased has been investigated, for example, it is found that SCC tends to occur when the sum of the quantities of Si, P and S at grain boundaries of SUS 316 is at least 3 % as shown in Fig. 3. It will clearly be understood from Fig. 3 that the initial value of the quantities of Si, P and S amounts to at most 0.2 %, from a calculation result from the amounts of change of the Cr and Ni concentrations at crystal grain boundaries, based on the measured values of the crystal grain boundaries segregation of a neutron irradiated material, having been reported, through such an initial value (before neutron irradiation) that the sum of the quantities of Si, P and S is not more than 3 % even if subjected to neutron irradiation of about 1 x 10 22 n/cm 2 (E > 1 MeV) as the maximum value of a quantity of the neutron irradiation, a high stress-loaded part receives among core parts of a light-water reactors until the end of the plant life. The quantity of C should be 0.005 to 0.08 %, preferably 0.01 to 0.05 %, since if less than 0.005 %, precipitation of M 23 C 6 excellent in SCC resistance does not take place sufficiently, while if more than 0.08 %, on the other hand precipitation of carbides is increased and corrosion resistance is reduced with the concentration of Cr at crystal grain boundaries. Even if Mo as another component is not added, structural materials for reactors can be used, but in order to further improve the corrosion resistance, Mo is preferably added with an upper limit of 3 % corresponding to at most the content level of SUS 316. The addition of Mo even in micro amount is effective for repassivation of a surface coating film. A preferred addition range thereof is 1 to 2 %, whereby the toughness at low temperature can be improved, but the addition of Mo exceeding 3 % accelerates precipitation of intermetallic compounds and δ phase, resulting in embrittlement of the material and marked deterioration of the workability and welding thereof. This is not preferable. Furthermore, in order to improve the SCC resistance in high temperature and pressure water saturated with oxygen, Mo + W is specified in at most 5 % with a provision that Mo does not exceed 3 %. Particularly, Mo improves the corrosion resistance as described above, and when the addition amount thereof is further increased, a localized corrosion occurs in crevices formed when using stainless steels in high temperature and pressure water saturated with oxygen, that is, crevice corrosion is moderated. A preferred amount is 2 to 3 %. W has a similar effect to Mo and is capable of improving corrosion resistance in an amount of 0.1 to 1 %. Accordingly, the addition amount of Mo + W should be at most 5 %, and it is preferable to specify the upper limit thereof as 4 % for the purpose of obtaining production stability. Amounts of Nb + Ta and Ti are specified in at most 0.3 weight %, corresponding to at most an impurity level when using them as a deoxidizer, and amounts of Mn and B are specified in a possible minimum value in practice from the steel making technique at the present time. The amount of Mn is at most 0.3 %, preferably at most 0.1 % and that of B is at most 0.001 %. Nb + Ta, Ti, Mn and B are optional components and may respectively be 0. In the present invention, compositions of the material and metallic texture are previously controlled so that the material degrades to such an extent as hardly causing IASCC even if it is exposed to neutron irradiation, based on the knowledge that irradiation assisted stress corrosion cracking (IASCC) occurs superimposedly with degradation of the material by high load stress and neutron irradiation. It has been known that IASCC, as grain boundary cracking, takes place due to Cr depleting and Ni, Si, P, S, etc. enriching at grain boundaries. The feature of the present invention consists in that 1 ○ an amount of Cr is previously and adequately increased so that IASCC may not occur even if Cr is depleted in grain boundaries by neutron irradiation and 2 ○ amounts of impurities such as Si, P, S, etc. are previously and adequately reduced so that IASCC may not occur even if Si, P, S, etc. are enriched in grain boundaries by neutron irradiation. Moreover, it is found as a result of the inventors' studies from the knowledge that IASCC is related to precipitated carbides at grain boundaries that the feature consists in that 3 ○ precipitated carbides at grain boundaries are previously maintained so that IASCC hardly occurs and 4 ○ such an alloy composition as described above is specified and the thermal expansion coefficient is not so much changed from that of the prior conventional materials even if a heat treatment is effected. Example From the foregoing point of view, a test piece having the shape and dimension as shown in Fig. 4 (numerals in Fig. 4 are mm) was prepared using materials having the chemical compositions shown in Tables 1 to 4 according to steps shown in Fig. 1 and then subjected to neutron irradiation up to a fluence of 5 X 10 22 n/cm 2 (E > 1 MeV) at 320 °C using a nuclear reactor for the material test. Test pieces (Sample 1 ○) with compositions of Tables 1 and 2 were subjected to an SCC accelerating test under a simulated environment in lightwater reactors (in high temperature and pressure water, 360 °C, 160 kgf/cm 2 G, strain rate: 0.5 µ m/min) and Test pieces (Sample 2 ○) with compositions of Tables 3 and 4 were subjected to an SCC accelerating test under a simulated environment in light-water reactors (in high temperature and pressure water saturated with oxygen, oxygen concentration: 8 ppm, 290 °C, 70 kgf/cm 2 G, strain rate: 0.5 µ m/min), thus obtaining the results shown in Tables 5 and 6. Mean thermal expansion coefficients of from room temperature to 400 °C of the resulting test pieces were all within a range of from 15.8 x 10 -6 to 17.1 x 10 -6 /K. In Tables 5 and 6, "IGSCC" means intergranular stress corrosion cracking and "IGSCC Fracture Surface Ratio" is a value represented by [(Σ Fracture Surface in Crystal Grain Boundaries)/(Σ Cross Sectional Area of Test Piece)] x 100 %. "SSRT" means a slow strain tensile test. Tables 5 and 6 teach that the material is most suitable when the value of Fracture Surface Ratio (IGSCC Fracture Surface Ratio), which can be considered as having the largest effect from a point of view of IASCC resistance, unlimitedly approaches 0, preferably at most 2 % and it can be understood from this aspect that the amount of C should be 0.01 to 0.08 %, preferably 0.03 to 0.05 % and the amount of Cr is the larger, the better. In addition, it is desirable that Mo does not exceed 3 % in high temperature and pressure water of Table 5 and Mo + W is added in an amount of about 3 to 4 % in high temperature and pressure water saturated with oxygen of Table 6. P, S, Si, Nb, Ta, Ti and B are preferably added in less amounts. The heat treatment is carried out in such a manner that M 23 C 6 is coherently precipitated with matrix in the crystal grain boundaries. In this Example, samples were prepared by subjecting them to only solution-annealing treatment at 1050 °C for 1 hour as shown in Fig. 1 (Heat Treatment [α]), by subjecting, after the solution-annealing treatment, to an aging treatment at 700 °C for 100 hour (Heat Treatment [β]), by subjecting, after the solution annealing treatment, to a cold working of about 20 % (Heat Treatment [γ]), by further subjecting, after the Heat Treatment [γ], to an aging treatment at 700°C for 10 hours (Heat Treatment [ δ ]), or to an aging treatment at 700 °C for 100 hours (Heat Treatment [η]). As shown in Tables 5 and 6, all of these samples showed a small IGSCC Fracture Surface Ratio in SSRT Test, i.e. excellent SCC resistance. Utility and Possibility on Commercial Scale High nickel austenitic stainless steels resistant to degradation by neutron radiation according to the present invention are better in degradation resistance to neutron irradiation and hardly tend to cause stress corrosion cracking in an environment of a light-water reactor even after neutron irradiation of approximately 1 x 10 22 n/cm 2 (E > 1 MeV), as the maximum value of the quantity of the neutron irradiation until the end of the plant life of light-water reactors. When using this alloy for core materials in light-water reactors, operation is possible until the end of the plant life of reactors without fear of IASCC and reliability of nuclear reactors can further be improved. Thus, this invention greatly serves the development of the present technical field.

Claims: High nickel austenitic stainless steels having resistance to degradation by neutron irradiation, which comprise a stainless steel having a composition (by weight %) of 0.005 to 0.08 % of carbon, at most 0.3 % of Mn, at most 0.2 % of (Si + P + S), 25 to 35 % of Ni, 25 to 40 % of Cr, at most 3 % of Mo, at most 5% of (Mo + W), at most 0.3 % of (Nb + Ta), at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe, said stainless steels being subjected to a solution-annealing treatment at a temperature of 1000 to 1150°C, and wherein optionally a cold working up to 30 % is carried out after the above defined solution-annealing treatment, wherein a heat treatment for a period of up to 100 hours is carried out at 600 to 750°C after the above defined solution-annealing treatment or optional cold working, thereby M 23 C 6 carbides coherent with the matrix are precipitated in the crystal grain boundaries. A process for the production of high nickel austenitic stainless steels having resistance to degradation by neutron irradiation, which comprises subjecting stainless steels having compositions (by weight %) of 0.005 to 0.08 % of carbon, at most 0.3 % of Mn, at most 0.2 % of (Si + P + S), 25 to 35 % of Ni, 25 to 40 % of Cr, at most 3% of Mo, at most 5% of (Mo + W), at most 0.3 % of (Nb + Ta), at most 0.3 % of Ti, at most 0.001 % of B and the balance of Fe to a solution-annealing treatment at a temperature of 1000 to 1150°C, and wherein optionally a cold working up to 30 % is carried out after the above defined solution-annealing treatment, wherein a heat treatment for a period of up to 100 hours is carried out at 600 to 750°C after the above described solution-annealing treatment or optional cold working, thereby M 23 C 6 carbides coherent with the matrix are precipitated in the crystal grain boundaries.

Inventors: AJIKI KAZUHIDE, FUJIMOTO KOJI, IWAMURA TOSHIHIKO, KANASAKI HIROSHI, NAKADA SHIZUO, NAKAMURA MITSUHIRO, YONEZAWA TOSHIO, AJIKI, KAZUHIDE, FUJIMOTO, KOJI, IWAMURA, TOSHIHIKO, KANASAKI, HIROSHI, NAKADA, SHIZUO, NAKAMURA, MITSUHIRO, YONEZAWA, TOSHIO, AJIKI, Kazuhide Mitsubishi Jukogyo K.K., FUJIMOTO, Koji Mitsubishi Jukogyo K.K., IWAMURA, Toshihiko Mitsubishi Jukogyo K.K., KANASAKI, Hiroshi Mitsubishi Jukogyo K.K., NAKADA, Shizuo Mitsubishi Jukogyo K.K., NAKAMURA, Mitsuhiro Mitsubishi Jukogyo K.K., YONEZAWA, Toshio Mitsubishi Jukogyo K.K. Takasago
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1996
Main classification: C22C38
IPC codes: C22C30, C22C38, C21D8, C21D6, G21D1