Patent Number: 059498382
Section: summary

TECHNICAL FIELD This invention concerns the manufacturing of austenitic grade materials for radiation exposure applications. BACKGROUND ART 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. The 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. The in-reactor failure mechanism is therefore called "irradiation assisted stress corrosion cracking" ("IASCC"). It is suspected that phosphorus and silicon are forced to the grain boundaries leading to a susceptible site for the onset of corrosion. Supported by outpile IGSCC tests, the articles "Behavior of Water Reactor Core Materials with Respect to Corrosion Attack" by Garzarolli and Rubel and Steinberg's "Proceedings of the International symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors", Myrtle Beach, S.C., Aug. 22-25, 1983, Pages 1 through 23, recommend that the silicon content be maintained under 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. In "Deformability of Austenitic Stainless Steel and Ni-Base Alloys in the Core of a Boiling and a Pressurized Water Reactor", Proceedings of the 2nd International Symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors, Monterey/Calif., Sep. 9-12, 1985, Pages 131 to 138, Garzarolli, Alter and Dewes report results from inpile tests that provide some insight into the influence of phosphorus, silicon, and sulfur impurities on IASCC. Standard steel qualities of stock numbers 1.4541, AISI 316 and 348, were subjected to annealing temperatures of 1050.degree. C. and then cold worked approximately 10%. A chemical analysis was performed to determine alloying constituents for each standard to be tested. 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.degree. 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 factors and parameters obtained coincidentally during the aforementioned successful tests, which could not be replicated or controlled, obviously have an influence on IASCC. The nuclear industry has learned from its experience with zirconium alloys, that oxygen causes embrittling and a higher incidence of corrosion. It is suspected that nitrogen has a similar influence on austenitic steel, and it was recommended that austenitic steels be used which contain from 0.025% to 0.065% carbon and 1.5 to 2% manganese, which then show a maximum content of 0.03% N, 0.005% P, 0.05% Si and 0.005% S (U.S. Pat. No. 4,836,976). Long term reactor tests show, however, that the use of these or similar materials, i.e., P, S, N and Si reduced, could not attain the ductility and resistance with regard to IASCC in individual tests. Systematically varying the N-content did not show any particular influence on the impurity content. All clean variants failed during inpile tests, which means that the previously found high resistance for the aforementioned one-time material must be considered coincidental, whose cause lies in the random, unavoidable variations of the composition and/or manufacturing processes. The exact mechanisms and contributing factors to IASCC as well as the suitable measures for its avoidance are largely unknown because of the rather extensive list of possible influences, longer reactor testing periods, and substantial cost associated with a comprehensive test series. The task of manufacturing tubes for absorber elements or other structural components for reactor irradiation zones out of a suitable austenitic steel, that are sufficiently resistant to IASCC and can be exposed to the stress of long term reactor operation, still remains unfulfilled. This invention is the key to finding the solution to this task. DISCLOSURE OF INVENTION 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.degree. 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. Finely dispersed carbides would be suitable collection points for this propose. 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. The grain size is sufficiently fine with an overall carbon content that favors, with properly controlled thermal processing, the formation of finely dispersed carbides of the alloying additions present in steel, as opposed to commercial steel with their technically practical purities and microstructures. The preferred alloying element for carbide formation is 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. Advantageous carbide precipitations would have a diameter between 20 nm and 250 nm for spherical shapes and/or up to 750 nm for needle shapes. The diameters are based on optical readings of the intercept lengths, which are similar to that used in US Standard ASTM E 112 for grain size, obtained from high magnification scanning electron micrographs. 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 O.008%. The invention provides that components or workpieces, that are to be made of steel and used in irradiation zones of a reactor, be manufactured from austenitic steel. This steel will require a base melt reduced in Si, P and S content after solidification. A thermal heat treatment that will result in a finely dispersed carbide precipitate, with the alloyed carbide former, is desired. Annealing temperatures between 1000 and 1100.degree. C. are sufficient with a standard annealing temperature of approximately 1050.degree. C. preferred to obtain a mean grain diameter (with an intercept length based on U.S. Standard ASTM-E 112) under approximately 20 .mu.m. This is the case when niobium in concentrations between 0.4 and 0.9% is used as the carbide former and only a small portion of the carbides present in a coarser distribution. Higher annealing temperature (e.g., at approximately 1150.degree. C.) can be used, particularly if coarser carbide precipitations need to be dispersed, and if only one low temperature stabilizing process (under 800.degree. C.) is anticipated to form the finely dispersed carbide distribution. These annealings can also be combined with mechanical processing steps at elevated temperatures (e.g., hot rolling) to get the desired structure. The fabrication process of the corresponding semi-finished steel customarily starts with a blank which is already handled at temperatures of over 1100.degree. C. State of the art technology anticipates that blanks will be further processed at annealing temperatures of approximately 1050.degree. 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. 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. The use of a cover gas (e.g. argon) is also possible and is advantageous for intermediate annealing process. 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. Commercial austenitic steels generally have a grain structure with grain diameters that can exceed 50 .mu.m, depending upon how much Si, P and S has been removed. This provides for a ductile material that is not only resistant to general corrosion but also resistant against stress corrosion cracking when in a non-irradiated condition. In the non-irradiated state, commercial austenitic steel can withstand relative length expansion, dL, of up to 30% without incurring damage. This means that sealed pipes can withstand large changes in diameter, dD, caused by an increase of internal pressure. This occurs, when the filling, such as nuclear fuel or other absorbing material, within a pipe swells and presses against the pipe from the inside. After this material has been subjected for an extended period to a high neutron flux, 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.