WO1994014992A1 - Fabrication de materiaux et de pieces a usiner a grain fin pour composants utilises dans des centrales nucleaires - Google Patents

Fabrication de materiaux et de pieces a usiner a grain fin pour composants utilises dans des centrales nucleaires Download PDF

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Publication number
WO1994014992A1
WO1994014992A1 PCT/US1992/011260 US9211260W WO9414992A1 WO 1994014992 A1 WO1994014992 A1 WO 1994014992A1 US 9211260 W US9211260 W US 9211260W WO 9414992 A1 WO9414992 A1 WO 9414992A1
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Prior art keywords
content
approximately
identified
per
niobium
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PCT/US1992/011260
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English (en)
Inventor
Dietrich Alter
Peter Dewes
Friedrich Garzarolli
Roland Hahn
J. Lawrence Nelson
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Electric Power Research Institute Inc
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Electric Power Research Institute Inc
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Priority to PCT/US1992/011260 priority Critical patent/WO1994014992A1/fr
Publication of WO1994014992A1 publication Critical patent/WO1994014992A1/fr
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
    • G21C3/02—Fuel elements
    • G21C3/04—Constructional details
    • G21C3/06—Casings; Jackets
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C7/00—Control of nuclear reaction
    • G21C7/06—Control of nuclear reaction by application of neutron-absorbing material, i.e. material with absorption cross-section very much in excess of reflection cross-section
    • G21C7/08—Control of nuclear reaction by application of neutron-absorbing material, i.e. material with absorption cross-section very much in excess of reflection cross-section by displacement of solid control elements, e.g. control rods
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E30/00—Energy generation of nuclear origin
    • Y02E30/30—Nuclear fission reactors

Definitions

  • the starting point is an austenitic steel whose alloying constituent quantities are largely standardized, e.g., steel carrying the German Stock Number 1.4550 which require a carbon content under 0.1%, a niobium content higher than the eight fold of the carbon content, as well as a chromium content of 17 to 19 wt.%, and a nickel content from 9 to 11.5 wt.%.
  • Impurities level limits are set at 2.0 % Mn, 1.0 % Si, 0.045 % P and 0.03 % S by weight.
  • the properties of iron are modified by the prescribed amounts of the alloying components with the upper limits on impurities dictated by the specified application zone. Higher impurity limits are generally allowed to make it possible to manufacture alloys from standard, inexpensive source materials which conform to commercial impurity standards. The upper limits of many impurities are the result of optimized manufacturing processes. Concentration limits on other alloying constituents are determined through the optimization of pertinent material properties.
  • Steel qualities 1.4301 and 1.4401 for example, contain niobium as an impurity, but otherwise correspond to the usual impurities of 1.4550 steel. In the U.S., the corresponding steel qualities approximately correspond to markings AISI types 348, 304, and 316.
  • microstructure of these materials depends upon their composition, thermal treatment and other procedural steps during the manufacturing process. If for example, the material is subjected to high temperatures for extended periods, large grains will form. Impurities and/or the use of lower temperatures during manufacturing discourages grain growth. The formation of coarse grains can be promoted in some cases during forging, where extensive deformation of grains at elevated temperature causes larger grains to be formed when the forging cools. These grains can be reduced through recrystallization. Grain structure affects material properties such as ductility and strength.
  • Austenitic steels distinguish themselves from other steels because they have suitable mechanical properties while simultaneously possessing a high level of stability in the face of general corrosion, the even removal of material from the surface of a component, a fact which led to early use of austenitic steels as the material of choice for high stress nuclear reactor internal structural components.
  • Industry experience and laboratory testing has show that these materials fail when exposed to low stress, a matter which can be traced back to selective corrosion at grain boundaries ("intergranular stress corrosion cracking", IGSCC). This selective attack on the grain boundaries can be examined outside the reactor in laboratory tests (“outpile test”) by conducting corrosion tests under special aggressive conditions. The results of such tests, show that austenitic steel which is resistant to IGSCC when not exposed to radiation, does fail during inpile testing where radiation is present.
  • IASCC irradiation assisted stress corrosion cracking
  • 0.1 wt.% and the phosphorus content be kept under 0.01 wt.%, while pointing out that irradiation in a reactor enhances the occurrence of selective corrosion.
  • AISI 348 steel samples had a silicon and phosphorus content (0.59% and 0.017%, respectively). This was lowered, for use as additional samples of "clean” AISI 348, to 0.01% and 0.008% by a special cleaning procedure.
  • the sulfur content was not analyzed but the remainder of this "clean” steel was composed of 0.041% C, 11.1% Ni, 17.7% Cr, 1.65% Mn and 0.76% Nb+Ta by weight.
  • Temperatures used during the annealing processes that followed the cold work were not closely monitored, but did not in any case exceed 1040° C, yielding a grain size of ASTM No. 9. The sample with the lowest impurity content showed a considerably reduced corrosion rate during outpile tests.
  • Tubes made of the two types of AISI 348 steel were filled with a ceramic that expands when exposed to irradiation, for inpile tests. These tests showed that only the cleaner material remand relatively undamaged with a diametrical- swelling of 0.7% and even 1.4% following irradiation.
  • follow-on tests with newly manufactured tubes showed that these positive results occurred at random and could not be reproduced.
  • the intent is to reliably reproduce the one-time, randomly produced material condition which possesses the desired mechanical and corrosive properties. It is impossible to "exactly" reproduce the known material parameters at a justifiable expense: (austenitic steel composed as follows: 11.1% Ni, 17.7% Cr, 1.65% Mn, 0.76% Nb and Ta, 0.01% Si, 0.008% P, manufactured by thermal treatment of a large-grained blank at temperatures up to 1040°C and bearing the ASTM Number 9). It is also unknown whether other material parameters, not studied or controllable, could be responsible for the observed positive results. According to the findings, specific parameters can be selected, controlled, and applied to obtain the desired results. With the said parameters being sufficient to attain the positive results, others, which may encompass previously examined or as of yet unexamined parameters could play an accompanying role as a contributor toward the pertinent beneficial property.
  • a controlled application is not required to obtain other parameters. They can be gotten from the requirements of other mechanical processes or as coincident.
  • the material or corresponding workpiece manufactured according to the invention differentiates itself from the one-time or randomly manufactured material by having a reproducible resistance to IASCC.
  • the invention proceeds from the assumption that phosphorous, sulfur and silicon impurities are particularly responsible for IASCC when they segregate to grain boundaries.
  • the content of these impurities can be reduced with regard to customary steel qualities by using appropriate cleaning procedures, but it is not possible to completely remove all impurities.
  • the average grain diameter of such a workpiece tends to increases as the impurity concentrations decrease; the number of grains and total grain boundary surfaces decrease to the point where it is now possible to end up with an accumulation of an excessive number and concentration of impurities on the reduced boundary surfaces.
  • the invention also proceeds from the premise that higher disruptive segregation of impurities can be avoided if there are enough collection points in the material where impurities could be captured. Since this occurs at grain boundaries, a higher grain boundary density is prescribed and can be induced by controlled thermal treatment of the material.
  • the invention provides an austenitic steel tailored for used in irradiation zones of a reactor.
  • This steel has a reduced silicon, phosphorous and sulfur content.
  • a fine grain size structure measured per US Standard ASTM E 112 where enlarged photos from an electron microscope are used to determine the "intercept length", is desirable. Grain size under 20 ⁇ m is acceptable but a size under approximately 18 ⁇ m is preferred. This corresponds to a total grain surface size greater than approximately 500 cm 2 /cm 3 .
  • the upper limit on silicon is 0.1% by weight, while good test results are obtainable with a maximum silicon content of 0.08%.
  • the total content of phosphorous and sulfur should be under 0.03%, and preferably under 0.02%. Good results can be obtained when the phosphorous and sulfur contents are under 0.008%.
  • niobium which could range in concentration from as low as 0.4 wt. % to as much as 0.9 wt. %.
  • the preferable range of niobium concentration is between 0.7 and 0.85% by weight.
  • the carbon content can be as much as 0.06%, but is preferred to be around 0.04% by weight.
  • the preferred niobium/carbon ratio range is from approximately 10:1 to 30:1.
  • the invention provides that components or workpiece, that are to be made of steel and used in irradiation zones of a reactor, be manufactured from this austenitic steel. This steel will require a base melt reduced in Si, P and S content which after solidification can be subjected to a coarse formation requiring the application of high temperatures, but that following this coarse formation, the solidified melt may only be subjected to temperature treatments below 950°C.
  • the desired grain size is achieved by using temperature treatments of 950°C and preferably 850°C. Should higher temperatures be necessary for other reasons, such as annealing or heat treatment (e.g., standard annealing at 1050°C), the invention provides the steel with niobium content between 0.4% and 0.9%, preferably higher than 0.43%, as a carbide former since adding such an amount will enlarge grain size. These higher niobium austenitic steels can be exposed to temperatures up to approximately 1075°C.
  • the fabrication process of the corresponding semi-finished steel customarily starts with a blank which is already handled at temperatures of over 1100°C.
  • State of the art technology anticipates that blanks will be further processed at annealing temperatures of approximately 1050°C ("standard annealing") so that any non-uniformities or other structural defects which could have formed during forging, extruding or other similar mechanical processes, which could lead to a ripping or bursting of the metal, can be removed.
  • standard annealing annealing temperatures of approximately 1050°C
  • the desired structure of the metal limits the temperatures which are available during fabrication, but lowering temperatures during the intermediate processes can be equalized by extending the duration of the processes.
  • the attainment of advantageously reduced silicon, phosphorous and sulfur content in the base material can be realized though good melting practices or though refined cleaning procedures. Cleaning takes place through a one-time melting or multiple remelting under vacuum.
  • a cover gas e.g. argon
  • a silicon content of 0.1% and a common phosphorous and sulfur content of less than 0.03% is advantageous to maintain a purity level. Carbon content is permissible in the 0.03 to 0.05% range and should generally not exceed 0.06%.
  • a niobium content of 0.9% by weight content is advantageous as a carbide former when a niobium-carbon ratio is in the range of 10:1 to approximately 30:1.
  • the limit for relative length expansion, dL, or relative diameter change, dD can occur.
  • the resulting values of dD fall in a large scatter band, with a typical value of only approximately 0.5%.
  • the reasons for the scatter could be due to the uncontrollable impurities which are present in the indicated maximum values, or due to the deviations in grain structure and size, dependent on random occurrences during the manufacturing process that are unknown.
  • the reduced ductility is due to an increase occurrence of IASCC, which means that austenitic steel has a limited use in nuclear reactors.
  • the invention's workpiece in contrast, still shows sufficient ductility following a neutron exposure. It is possible for values of 1.5% or higher, in dD, to be reliably withstood without damaging the workpiece.
  • Table 1 The chemical composition of different alloys of the test series Table 2 Temperature treatments and grain diameters of these materials
  • Figure 1 A pipe filled with material capable of expanding and specifically manufactured for this test series Figure 2-4 The relationship of grain size to temperature treatment of the same composition materials
  • Figure 7-8 The change in grain size in relationship to Figure 4 using the same temperature treatments but with different niobium contents
  • Figure 9-11 The formation of non-metallic precipitates and the precipitates of inter- metallic inclusions for the structures of Figures 4, 7 and 8
  • Figure 12-14 The precipitates of niobium carbides which occur in the structures of Figures 9 through 11 Figure 15 Resulting ductility and grain size.
  • Figure 16 Relationship of ductility to grain size.
  • the standard specimen geometry used in this test series is depicted in Figure 1.
  • the pipe wall (10) consists of one of the materials described in Table 1.
  • Each pipe is filled with a pellet composed of a mixture of that acts as an expansion mandrel when subjected to a neutron flux. The ratio of this mixture is chosen depending on the amount of expansion desired.
  • Samples are exposed to a neutron flux ranging between 1.33 and 2.5 X 10 21 n cm "2 which also results in different diameter changes that relatively increase up to 1.7. If the pipes withstand these expansions without damage, particularly without any stress corrosion cracks, then they have passed the test. If, however, damage occurs they are classified based on the maximum tolerated expansion at which no damage was observed.
  • melts are produced from materials which are classified as highly pure materials or which only have a minimal amount of scrap. It is advantages if these metals are remelted under vacuum, particularly when they have a higher scrap content, so that they may obtain the lowest possible content of silicon, phosphorous or sulfur.
  • the cooled billet from the melt is shaped into unfinished pipes with a 19 cm inner diameter and a 22 cm outer diameter in a resistance oven. From this rough pipe form a refined pipe form is shaped as illustrated in Figure 1, after being annealed several times. Intermediate annealing takes place with induction heating in an argon atmosphere at controlled annealing temperatures. Sample cross sections of materials manufactured in this manner, were examined using customary optical and electron microscope methods, both before and after corrosion tests.
  • Each material was tested for chemical composition, range of grain size, and inclusions content.
  • the chemical compositions of different test materials are listed in Table 1 and are identified by alloy numbers. Alloys bearing the numbers 460, 463, 480, 964, 965 and 966 correspond to Steel 1.4550 or AISI type 348, while Alloy Number 491 corresponds to Steel 1.4306 or AISI type 304, Each of these test alloys has a different niobium content.
  • the samples formed from these alloys were shaped into hallow pipe. Different annealing times and processing temperature were used, identified by capital letters in Table 2.
  • the first line lists the resulting grain size obtained under a low temperature process ("LTP"), with the test alloys arranged in the order of decreasing niobium content.
  • LTP low temperature process
  • the LTP material underwent three to five intermediate annealings at 850°C for a total of 240 minutes, and a final 60 minute annealing at 850°C.
  • the next line in Table 2 lists several specimens that were exposed to intermediate annealings at varying temperatures which lie within the indicated temperature ranges.
  • the annealing duration (2 minutes for intermediate annealings) is also listed.
  • the temperature for the final annealing (between 1075°C and 1079°C) and the duration (2 or 3 minutes) are also listed. All of these specimens lie within the standard annealing process ("STP") whose temperatures are barely above the customary annealing temperature of 1050° C.
  • STP standard annealing process
  • Specimen Q which is listed as part of the next group, represents a transition to a high temperature process.
  • the process involves four intermediate annealings at temperatures between 1068°C and 1100°C, lasting 2 minutes, as well as a final annealing period of 2 minutes at 1 100°C.
  • Specimen H is subjected to a high-temperature process, 2 minute intermediate annealings at temperatures between 1138 and 1189°C, and a final steady annealing which takes place at 748°C for 100 hours.
  • temperature and niobium content effects the structure and corrosion resistance of these test alloys
  • Damaging impurities with regard to SCC, Si, P and S are concentrated at the reduced grain boundary surfaces and aid selective corrosion there, despite the low level of these impurities in the test alloys.
  • Niobium carbide particularly in a fine dispersed distribution, can act as collecting point for these impurities (i.e., the remaining base substance can largely be considered as highly pure and homogeneous) and hinder grain growth, i.e., the remainder of these damaging impurities are distributed over a larger surface and once dispersed have a difficult time to become concentrated.
  • This invention gives rise to a material of high-purity and unexpectedly small grains whose boundaries are less susceptible to local corrosion.
  • Alloy number 964 i.e., specimens F, G and H, are examined next.
  • the grain structure of these specimen is illustrated in Figures 2 and 4 which are also shown on a scale of 200:1, as Figures 7 and 8.
  • the grain diameters in specimen F were produced using a standard process and show a distribution around an average value of 7 ⁇ m.
  • Specimen G which was produced with a low temperature process, also shows approximately the same average values.
  • the grain sizes, particularly for longer annealing periods, have a relatively small scatter range.
  • Specimen H Figure 3) clearly shows enlarged grains, whose mean diameter lies in the 26 ⁇ m range, produced using a high temperature process.
  • Figure 5 shows the correlation between grain diameter in ⁇ m and the grain boundary's overall surface or the corresponding ASTM Number which is contained in one cubic centimeter of the specimen.
  • Figure 6 shows the influence of grain size that comes about because of the niobium content when produced under the same temperature processes, on the ability of the alloy to deform in the reactor expansion tests.
  • the dotted line R shows that customary steel qualities, which have not been purged of Si, P and S, show a susceptibility to IASCC for relatively low diameter changes, dD, of approximate 0.2%. This means that those materials cannot be used.
  • the specimens shown in Figure 6 are arranged by grain size diameter where the symbol “o” represents a sample that withstood the applied expansion without damage, while the symbol “(x)” points to light defects and the symbol "x" to considerable defects which renders the material useless.
  • Figure 6 shows that specimens produced in accordance with this invention have a grain diameter of approximately 20 ⁇ m and can withstand relative expansions of up to 1.5%.
  • Table 2 The influence that niobium content has on grain sizes (Table 2) is shown in Figure 4 (Specimen G), Figure 7 (Specimen J) and Figure 8 (Specimen L).
  • Cross sectional photographs (scale of 1000:1) taken of specimen treated using these low temperature processes are shown in Figure 9 (Specimen G), Figure 10 (Specimen J) and Figure 1 1 (Specimen L).
  • Specimen G has a greater portion of the niobium rich precipitates in area 1 in relationship to the finely dispersed niobium carbide precipitates in area 2, which can more than likely be traced to formations which bind themselves to the excess niobium while the material is being manufactured, and which were not able to be transferred into the finely dispersed carbide during the low temperature process.
  • These precipitates have a varying-type metal content which fluctuate between Nb 2 Fe 3 and Nb 2 Fe 6 , whereby there are also small traces of Cr and Ni instead of iron, which points to an intermetallic phase. They are formed irregularly and have sizes between 0.25 and 1.5 ⁇ m (up to 3 ⁇ m), while the maximum diameter of fine dispersed carbide is only between 20 and 250 nm.
  • Figure 15 repeats the results of Figure 6 with additional results for materials which are within the scope of temperature treatments contained in the present invention. These are plotted to the left of X line, while to the right of X line are listed the comparison statistics of other materials.

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Abstract

L'acier austénitique pour composants destinés à être utilisés dans les zones d'irradiation des réacteurs nucléaires est plus ou moins résistant à la corrosion fissurante sous tension due à l'irradiation si sa teneur en silicone, en phosphore et en soufre est réduite par rapport aux quantités d'acier commercial et si sa structure de grain présente un diamètre de grain moyen inférieur à environ 20 νm. On obtient la structure de grain optimum lorsqu'on évite le recuit à des températures supérieures à 950 °C. Lorsqu'on ajoute du niobium en tant que carbure permettant la formation d'une dispersion fine de carbures, on obtient toujours une résistance à la fissuration par corrosion due à l'irradiation lorsqu'on utilise des températures de récuit inférieures à environ 1075 °C.
PCT/US1992/011260 1992-12-18 1992-12-18 Fabrication de materiaux et de pieces a usiner a grain fin pour composants utilises dans des centrales nucleaires Ceased WO1994014992A1 (fr)

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PCT/US1992/011260 WO1994014992A1 (fr) 1992-12-18 1992-12-18 Fabrication de materiaux et de pieces a usiner a grain fin pour composants utilises dans des centrales nucleaires

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0747497A1 (fr) * 1995-06-09 1996-12-11 Hitachi, Ltd. Acier austénitique de grande résistance mécanique et résistant à la corrosion pour les composants de réacteur nucléaire et méthode de fabrication
CN105203438A (zh) * 2015-10-14 2015-12-30 武汉钢铁(集团)公司 珠光体类盘条奥氏体晶粒度的测定方法

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA563843A (fr) * 1958-09-23 William C. Clarke, Jr. Acier inoxydable a temperature elevee
US3284250A (en) * 1964-01-09 1966-11-08 Int Nickel Co Austenitic stainless steel and process therefor
US3303023A (en) * 1963-08-26 1967-02-07 Crucible Steel Co America Use of cold-formable austenitic stainless steel for valves for internal-combustion engines
US3401036A (en) * 1967-08-11 1968-09-10 Crucible Steel Co America Valve steel
US4892704A (en) * 1988-04-28 1990-01-09 Sumitomo Metal Industries, Ltd. Low Si high-temperature strength steel tube with improved ductility and toughness

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA563843A (fr) * 1958-09-23 William C. Clarke, Jr. Acier inoxydable a temperature elevee
US3303023A (en) * 1963-08-26 1967-02-07 Crucible Steel Co America Use of cold-formable austenitic stainless steel for valves for internal-combustion engines
US3284250A (en) * 1964-01-09 1966-11-08 Int Nickel Co Austenitic stainless steel and process therefor
US3401036A (en) * 1967-08-11 1968-09-10 Crucible Steel Co America Valve steel
US4892704A (en) * 1988-04-28 1990-01-09 Sumitomo Metal Industries, Ltd. Low Si high-temperature strength steel tube with improved ductility and toughness

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0747497A1 (fr) * 1995-06-09 1996-12-11 Hitachi, Ltd. Acier austénitique de grande résistance mécanique et résistant à la corrosion pour les composants de réacteur nucléaire et méthode de fabrication
CN105203438A (zh) * 2015-10-14 2015-12-30 武汉钢铁(集团)公司 珠光体类盘条奥氏体晶粒度的测定方法

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