CA2149666A1 - Manufacture of materials and workpieces for components in nuclear plant applications - Google Patents

Abstract

Austenitic steel intended for use in radiation areas of nuclear reactors is largely resistant to irradiation-induced stress corrosion cracking if its silicon, phosphorus and sulfur contents are reduced in relation to standard commercial steel quantities and its grain structure has finely dispersed carbide precipitation, particularly of niobium carbide. The finely dispersed distribution can be induced in that larger niobium precipitation takes place at annealing temperatures between 1100 and 1150 ·C, and carbide is precipitated through the corresponding annealing at temperatures of approximately 750 ·C.

Description

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WO 94/14993 PCT/US92/11261 .
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MANUFACTURE O~ MATERIALS AND WORKPIECES
FOR COMPONENTS IN NUCLEAR PLANT APPLICATIONS.
TECHNICAL FIELD
This invention concerns the manufacturing of austenitic grade materials for radiation exposure applications.
E~ACKGROUND 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.qc, and a nickel content from 9 to 11.5 wt.%. lmpurities 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 impunty 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 are2 0 determined through the optimization of pertinent material properties Steel quahties 1.4301 and 14401, 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 rnat¢rials idepends 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 ' .~: - `.
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WO 94/14993 PCTrUS92/11261 grains can be reduced through recrystallization. Grain s~ructure affects material properties such as ductility and strength. I--Austenitic steels distinguish ~hemselves ~rom other steels because they have suitable mechanical propert:les while simultaneously possessing a high level of stability 5 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 s~ess, a matter which can be traced back to selectlve corrosion at grain boundanes 10 ("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 fall during inpile testing where radiation is present. The in-reactor 15 failure mechanism is therefore called "irradiation assisted stress corrosion cracking ("IASCC"~. It is suspected that phosphon~s and silicon are forced to the grain boundaries leading to a susceptible site for the onset of co~Tosion. Suppor~ed by . ~
outpile IGSC~ tests~, the articles "Behavior of Water Reactor Core ~alerials with Respect to Corrosion Attack" by Garzarolli and ~ubel and Steinberg's "Proceedings 2 0 of the International symposium on Environmental De~radation of Materials in Nuclear Power Systems - Water Reactors", Myrtle Beach, South CarGlina, August 22 ~
1983, Pages 1 through ~3, recommend that the silieon content be maintained under ~).1 wt.% and the phosphorus content be kept under 0.01 wt.%, while pointing out thatirradiation in a reac~or enhances theloccut~eince`o~ selective corr~sion.~ `
In "Deformability of Austenitic Stainless Steel and Ni-Base Alloys in the Core of a Boiling and a~ Pressunzed Water~ Reactor", Proceedings of the Znd International ~ i Symposium on Environmental Degradation of Materials in Nuclear Power Systems -Water Reactors, Monterey/California, September 9-12, 1985, Pages 131 to 138, ~. .
Garzarolli, Alter and Dewes repon results~ from inpiie tests that provide some insight 30:~ ~ i nto the Influence of phosphorus, silicon, and sulfur impurities on lASCC. Standard 21 l~fi~6 `~
wo 94/14993 PCT/US92/11261 steel qualities of stock numbers 1.4541, AISI 316 and 348, were subjected~ to annealing temperatures of 1050C and then cold worked approximately 10%. A
chemical analysis was performed to deteImine 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 addi~onal sarnples of "clean"
AISI 348, to 0.01% and 0.008% by a special cleaning procedure. The sulfur content was not analyzed but the rernainder of this "clean" steel was composed of 0.041% C, 11.1% Ni, 17.7% Cr, 1.6S% Mn and û.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 ASTI\I
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 diametrical-swelling of 0.7% and even 1.4% following irradiation.
Follow-on tests with newly manufactured tubes showed that these positive resultsoccurred at random and could not be reproduced. The factors and parameters obtained comcidentally 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, thatoxygen 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~c~htain frorh 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 (US-PS 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 L~SCC in individual tests. Systematically varying the N-content did not show any - 30 particular influence on the impurity content. All clean variants failed during inpile , ' 2 1 ~ 3 ~ G
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wo 94/14993 ` PCT/US92/11261 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 5 measures for its avoidance are largely unknown because of the rather extensive list of possible influences, longer reactor testing periods, and substanlial 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 10 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 is15 ~ impossible to "exactly" reproduce the known material pararneters 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 1040C and bearing the ASTM Number 9). It isalso 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 contributorltoward the pertinent beneficial property.
A controlled application is not required to obtain other parameters. They can begotten from the requirements of other mechanical processes or as coincident. Thematerial or corresponding workpiece manufactured according to the invention differentiates itself from the one-time or randomly manufaclured material by having a reproducible resistance to LASCC
The invention proceeds from the assumption that phosphorous, sulfur and silicon 21~G6 - i Wo 94/14993 PCT~US92/11261 impurities are particularly responsible for IASCC when they segregale 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 workpiece5 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 10 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 collectionpointsforthispropose. 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 15~ carbon conte~nt that~favors, wlth properly controlled thermal processing, the formation of finely~dispersed carbides of the alloying additions present in steelt as opposed to cosnntercial steel with their technically practical purities and mucrostructures.
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 -; 20 ~ ~ ran~ of nioblum 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~
Advantageous carbide precipitations woutd have a diameter betweèn 20 nm and 25~ 250 nm for sphencal 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 limlt on silicon is 0.1% by weight, while good test results are 30 obtainable with a maximum silicon content of 0.08%.
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wo 94t14ss3 PCT~S92/11261 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.()08%.
The invention provides that components or workpieces, that are to be made of steel 5 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 treatmer.t that will result in a finely dispersed carbide precipitate, with the alloyed carbide former, is desired. Annealing temperatures between 1000 and 1100C are sufficient with a standard annealing temperature of approximately 1050C
10 preferred to obtain a mean ~rain diameter (with an intercept length based on U.S.
Standard ASTM-E 112) under approximately 20 ~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 11 50C) can be used, particularly if coarser carbide 15 precipitations need to be dispersed, and if only one low temperature stabilizing process (under 800C) is anticipated to forrn the finely dispersed carbide distribution. These an~n~aiings can also be combined with mechanical processing steps at elevated tempera~ures (e.g., hot rolling) to get the desired structure.
, The fabrication process of the corresponding semi-finished steel customarily starts 20 with a blank which is already handled at temperatures of over 1100C. State of the art technology anticipates that blanks will be further processed at annealing temperatures of approximately 1050C ("standard annealing") so that any non-uniformides or other structural defects which could have formed during forging, extruding or other similar mechanical proces~ses, which could lead to a ripping or 25 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 content30 in thç base material can be realized though good melting practices or though refined : .

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wo 94/14993 PCT/US92/11261 7 1, cleaning procedures. Cleaning takes place through a one-time melting or multipleremelting 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 5 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 weighl content is advantageous as a carbide former when a niobium-carbon ratio is in the range of 10:1 to approximately 30:1.
Cornrnercial austenitie steels generally have a grain structure with grain diameters l 0 that can exceed 50 ~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, comrnercial austenitic steel can withstand relative length ~ expansion, dL, of up to 30% without incurring damage. This means that sealed pipes r~ l5 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 absorbingmaterial, 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 2 0 occur. The r~sulting values of dD fall in a large scatter band, with a typical value of only approximately 0.S%. The reasons for the scatter could be due to the ~; uncontrollabie 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 unkr~own. The reduced ductility is due to an 25 increase occurrence of IASCC, which means that austenitic steel has a lirnited use in - nuclearreactors. } `
- 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.
30 BRIEF DESCRIPI'ION OF THE DRAWINGS

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The invention is explained in more detail by means of an expanded material test series which is reproduced in Tables 1, 2 and 3, as well as in 16 figures, as follows:
Table 1 The chemical composition of different alloys of the test series Table 2 Temperature treatments and grain diameters of these ma~erials 5 Table 3 Chemical composition of additional successful test steels Figure 1 A pipe filled with material capable of expanding and specifically manufaceured for this test series Figure 2-4 The relationship of grain size to temperature treatment of the same composition materials 10 Figure S Relation between grain size and grain boundary surface Figure 6 Ductility levels attained at different temperature treatments - 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 15 inter-metallic inclusions for the structures of Figures 4, 7 and 8 Figure 12-14 The precipitates of niobium carbides which occur in the s~uctures 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 l.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 Al2OJB4C that acts as an expansion mandrel when subjected to a nèutroh flux. The ratio of this Al20JB4C mixture is 25 chosen depending on the amount of expansion desired. Samples are exposed to aneutron flux ranging between 133 and 2.5 X 10 21 n~cm2 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 30 on the maximum tolerated expansion at which no damage was observed~

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In order to manufacture these pipes, melts are produced from materials which aleclassified as highly pure materials or which only have a minimal amount of scrap.
lt 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 a~nosphere 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 4~0j 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 obt~ined under a low temperature process ("LTP"), with the test alloys arranged in the order of decreasing niobium content. The LTP material underwent three to five intermediate annealings at 850C
for a total of 240 minutes, and a final 60 rninute annealing~ at 850C.
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 interrnediate annealings) is also listed. The temperature for the final annealing (between 1075C and 1079C) and the duration (2 or 3 minutes) are also listed. All of these specimens lie within the standard annealing 3 0 process ("STP") whose temperatures are barely above the customar,v annealing 214~666 Wo 94/14993 PCTJUS92/1 1261 ..
temperature of 1050~ C.
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 1068C and 1100~C, lasting ? minutes, as well as a final 5 annealing period of 2 minutes at 1100C.
Specimen H is subjected to a high-temperature process, 2 minute interrnediate annealings at temperatures between 1138 and 1189C, and a final steady annealingwhich takes place at 748C for 100 hours.
In the following description of how temperature and niobium content effects the 10 structure and corrosion resistance of these test alloys, it is suspected that a coarser grained structure with its reduced grain ~oundary surface is formed as temperature and homogeneity increase. 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. Something sirnilar to this -15 is true for carbon which can lead to the formation of chromium carbide and a corresponding reduction in corrosion inhibiting chromium at grain boundaries.
~ ~ ~ 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 20 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-punty and unexpectedly small grains whose boundaries are less susceptible to local corrosion.
The mean grain diameter values which~were obtained through optical readings and 25 by counting the intercept lengths of a representative grain population, are listed in Table 2, next to the capital letters which are used to identify the specimens. Reliable data is rnissing for specimens D, C and E since the grain sizes were determined using methods which are customary for suppliers of senni-finished products, said methods, however, not being consistent with the reliable diameter readings which are obtained 30 by optically measuring the cross sectional photo. It is noted that the grain diarneter 21~666 ~`
wo 94/14993 PCT/US92/11261 increases from top left to bottom right, i.e., grain growth is less hindered by ~he decreasing niobium content and necessarily increases with annealing temperatures.
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 (Figure 2) were produced using a standard process and show a distribution around an average value of 7 ~m. Specimen G
(Figure 4), 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 llm range, produced using a high temperature process.
While enlarged grain size generally causes the grain surface of each individual grain to incre~se, the number of grains and the total grain surface of all grains actually ~1~5 decreased. Figure 5 shows the correlation between grain diameter in ,um and the grain bolmdary'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 L~SCC 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 symool o represents a samplè t~at withstood the applied expansion without damage.
while the symbol "(x)" points to light defects and the symbol "x" to considerable defects whlch renders the material useless. The combination of Figure 6 and Table 2 shows that specimens produced in accordance with this invention have a grain diameter of approximately 20 llm and can withstand relative expansions of up to 1.5%.
3 0 The influence that niobium content has on grain sizes (Table 2) is shown in Figure WO 94/1~1 PCT/-JS92/11261 4 (Specimen G~, Figure 7 (Specimen J) and Figure 8 (Specimen L). Cross sectio~alphotographs ~scale of 1000:l) taken of specimen treated using these low temperature processes are shown in Figure 9 (Specimen G), Figure lO (Specimen J) and Figure l l (Specimen L). In addition ~o occasionally occu Ting non-metallic inclusions which are 5 to be considered as production errors (e.g., oxide and sulfide), and islands of isolated iron arranged in the form of lines of delta femte, there is a distribution of niobium containing precipitates whose density decreases as the alloy ' s niobium contentdecreases. Figures l 2 (Specimen F), hgure l 3 (Specimen H) and Figure 14 (Specimen G), which are reproduced in a scale of 15,000: l, illustrate the relationship 10 between these precipitates and temperature treatments for alloys with a hi~h niobium content.
A non-uniform distribution of precipitates, caused by standard annealing temperatures, is indicated for Specimen F, whose maximum diameter lies between approximately 40 and 560 nm and are chemically ahke. Besides traces of iron, 15 chromium and nickel these precipitates have a niobium content of 90%. The niobium is actually in the form of niobium carbide. Almost no precipitates could be found that were~an ~intermetallic between niobium and iron, or chromium, or nickel. Finely ~, . . .
dispersed precipitates consisting primarily of niobium (and chromium poor) metalcarbldes, are typical for material with these chemical compositions.
Still higher intermediate annealings temperatures (high temperature process) -~ ~ partially promotes coarser carbide precipitates whereby the corresponding carbide precipitates take on a spherical shaped structure with particle diameters between 20 and S0 nm.
In Specimen H (Figure 13)!there are numerous needle-like precipitations with ~ `maximum diameters of 20 to 750 nm. Their composition consists of about 95%
-~ niobium, with residual amounts of iron, chromium and nickel, indicating niobium ~ ~ carbide.
- Specimen G (Figure 14) has a greater por~ion 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 ~ .
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- wo 94/14993 PCT/US92/11261 excess niobium while the matenal 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 Nb2Fe3 and Nb2Fe6, whereby there are also small traces of Cr and Ni instead of iron, which 5points ~o an interrnetallic phase. They are formed irregularly and have sizes between 0.25 and 1.5 llm (up to 3 lam), while the maximum diameter of fine dispersed carbide is only between 20 and 250 nm.
Different temperature treatments yield different results for expansion tests conducted under irradiation. Figure 15 repeats the results of Figure 6 with additional 10results for materials which are within the scope of temperature treatments contained in the present invention. These are plotte~ to the left of X line, while to the right of X line are listed the comparison statistics of other materials.
The chemical processes and conditions of the coolant in pressurized water reactors and- boiling water reactors differ from one another. While no differentiation was made .
15between these reactor types in Figures 6 and 15, Figure 16 does show a summary of results for a pressunzed water reactor. Expansion results for materials produced as per the invention are indicated with the symbol "o", while the symbol "x" is used to indicate relative diameter changes which resulted in damage to similar materials. The ; ~ ~ symbols "." and "+" represent undamaged and damaged diameter change, respectively, 2 0which occurred in commercial steel bearing German material No. 1.4981 which was also used in the comparison studies.
Other materials used which are listed in Table 3 were also prepared as per the invention and were subjected to practical reactor tests which yielded the same results.
To withstand irradiàtion assistedlstress' crack corrosion, the chemical composition 25of a material, particularly its high-purity with regards to Si, P and S (largely independent of other impunties such as, e.g., N) as well as its structure which is - formed dunng the tompenature treatment, is essendal.

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Claims (15)

We claim:

1. Austenitic steel for use in radiation zones of a nuclear reactor with a reduced silicon, phosphorous and sulfur content and a structure where carbon, contained in the steel, is almost completely precipitated as finely dispersed carbide, formed from an alloy additive.

2. Austenitic steel as per Claim 1, identified by a niobium weight content of approximately 0.9%, preferably at least 0.4%.

3. Austenitic steel as per Claim 2, identified by a niobium weight content of between approximately 0.6 and 0.9%, and a carbon content under approximately 0.06%. but preferably between 0.03 and 0.05%.

4. Austenitic steel as per Claim 2 or 3, identified by a niobium to carbon ratio from approximately 10 : 1 to 30 : 1.

5. Austenitic steel as per Claim 1, identified by carbide precipitations that largely have a diameter between 20 and 750 nm.

6. Austenitic steel as per Claims 1 through 5, identified by a silicon weight content of under approximately 0.1%, but preferably under approximately 0.08%.

7. Austenitic steel as per Claims 1 through 6, identified by a total phosphorous and sulfur content under approximately 0.03%, but preferably under approximately 0.02%.

8. Austenitic steel as per Claim 7, identified by a phosphorous and sulfur content each under 0.008%.

9. Any workpiece to be used in the radiation zones of a nuclear reactor made of austenitic steel as per one of the Claims 1 through 8.

10. Process for manufacturing austenitic steel to be used in a nuclear reactor'sirradiation zone, a solidified melt of base materials which contain reduced amounts of Si, P and S, a carbide forming alloy and, when subjected to a thermal treatment, precipitates carbides in a finely dispersed distribution.

11. Process as per Claim 10, identified by a cleaning procedure for the base materials or the melts where the silicon, phosphorous and sulfur contents are reduced.

12. Process as per Claim 11, identified in that melts are purified in a vacuum or control gas by a one-time melting or multiple remelting.

13. Process as per Claim 10, identified in that the silicon content is under 0.1% and the phosphorous and sulfur content is under approximately 0.03%.

14. Process as per Claim 13, identified in that niobium with an approximately weight content between 0.4 and 0.9% as the preferred alloy additive, and that the solidified melt is subjected to a heat treatments where there is at least a one-time annealing at temperatures between 1000°C and 1100°C, with a standard annealing at approximately 1050°C preferred.

15. Process as per Claim 13, identified in that niobium with a weight content between 0.4% and 0.9% (preferably with the lower bound at approximately 0.6%) and a carbon content of between 0.03 and 0.05% is added as an alloy and in that the solidified melt is subjected to several temperature treatments where it is annealed or heat treated at ah approximate temperature of 1150°C at least once, and where the temperature treatments following the annealing or heat process take place at temperatures at least under approximately 800°C, but preferably at 750°C