Patent Number: 059498382
Section: description

MODES FOR CARRYING OUT THE INVENTION The standard specimen geometry used in this test series is depicted in FIG. 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 Al.sub.2 O.sub.3 /B.sub.4 C that acts as an expansion mandrel when subjected to a neutron flux. The ratio of this Al.sub.2 O.sub.3 /B.sub.4 C mixture is chosen depending on the amount of expansion desired. Samples are exposed to a neutron flux ranging between 1.33 and 2.5.times.10.sup.21 n.multidot.cm.sup.-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. In order to manufacture these pipes, 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 FIG. 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. The LTP material underwent three to five intermediate annealings at 850.degree. C. for a total of 240 minutes, and a final 60 minute annealing at 850.degree. 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.degree. C. and 1079.degree. 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.degree. 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 1068.degree. C. and 1100 .degree. C., lasting 2 minutes, as well as a final annealing period of 2 minutes at 1100.degree. C. Specimen H is subjected to a high-temperature process, 2 minute intermediate annealings at temperatures between 1138 and 1189.degree. C., and a final steady annealing which takes place at 748.degree. C. for 100 hours. In the following description of how temperature and niobium content effects the structure and corrosion resistance of these test alloys, it is suspected that a coarser grained structure with its reduced grain boundary 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 similar to this 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 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. The mean grain diameter values which were obtained through optical readings and 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 missing for specimens D, C and E since the grain sizes were determined using methods which are customary for suppliers of semi-finished products, said methods, however, not being consistent with the reliable diameter readings which are obtained increases from top left to bottom right, i.e., grain growth is less hindered by the decreasing niobium content and necessarily increases with annealing temnperatures. Alloy number 964, i.e., specimens F, G and H, are examined next. The grain structure of these specimen is illustrated in FIGS. 2 and 4 which are also shown on a scale of 200:1, as FIGS. 7 and 8. The grain diameters in specimen F (FIG. 2) were produced using a standard process and show a distribution around an average value of 7 .mu.m. Specimen G (FIG. 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 (FIG. 3) clearly shows enlarged grains, whose mean diameter lies in the 26 .mu.m range, produced using a high temperature process. While enlarged grain size generally causes the grain surface of each individual grain to increase, the number of grains and the total grain surface of all grains actually decreased. FIG. 5 shows the correlation between grain diameter in .mu.m and the grain boundary's overall surface or the corresponding ASTM Number which is contained in one cubic centimeter of the specimen. FIG. 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 FIG. 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. The combination of FIG. 6 and Table 2 shows that specimens produced in accordance with this invention have a grain diameter of approximately 20 .mu.m and can withstand relative expansions of up to 1.5%. The influence that niobium content has on grain sizes (Table 2) is shown in FIG. 4 (Specimen G), FIG. 7 (Specimen J) and FIG. 8 (Specimen L). Cross sectional photographs (scale of 1000:1) taken of specimen treated using these low temperature processes are shown in FIG. 9 (Specimen G), FIG. 10 (Specimen J) and FIG. 11 (Specimen L). In addition to occasionally occurring non-metallic inclusions which are to be considered as production errors (e.g., oxide and sulfide), and islands of isolated iron arranged in the form of lines of delta ferrite, there is a distribution of niobium containing precipitates whose density decreases as the alloy's niobium content decreases. FIG. 12 (Specimen F), FIG. 13 (Specimen H) and FIG. 14 (Specimen G), which are reproduced in a scale of 15,000:1, illustrate the relationship between these precipitates and temperature treatments for alloys with a high 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 alike. Besides traces of iron, 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) metal carbides, 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 50 nm. In Specimen H (FIG. 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 (FIG. 14) 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.sub.2 Fe.sub.3 and Nb.sub.2 Fe.sub.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 .mu.m (up to 3 .mu.m), 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. FIG. 15 repeats the results of FIG. 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. 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 between these reactor types in FIGS. 6 and 15, FIG. 16 does show a summary of results for a pressurized 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, which 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 irradiation assisted stress crack corrosion, the chemical composition of a material, particularly its high-purity with regards to Si, P and S (largely independent of other impurities such as, e.g., N) as well as its structure which is formed during the temperature treatment, is essential. TABLE 1 ______________________________________ Content (10.sup.-3 Wt. %) Content (Wt. %) Alloy Number Si P S C N Cr Ni Mn Nb ______________________________________ 460 20 &lt;5 4 7 12 17 10 1.7 0.17 463 20 &lt;5 5 11 31 18 10 1.7 0.19 480 10 &lt;5 4 15 70 18 10 1.5 0.1 491 &lt;10 &lt;5 4 5 10 19 11 1.7 &lt;0.01 964 60 &lt;5 44 44 16 18 10 1.6 0.81 965 50 &lt;5 30 30 9 18 11 1.8 0.43 966 40 &lt;5 30 30 15 18 10 1.6 0.26 ______________________________________ TABLE 2 __________________________________________________________________________ Alloy Number (Nb Content) Heat 964 965 966 463 460 480 491 Treatment (0.81%) (0.43%) (0.26%) (0.19%) (0.17%) (0.16%) (--) __________________________________________________________________________ "LTP" G(7 .mu.m)* J(15 .mu.m) L(6 .mu.m) O(7 .mu.m) N(9 .mu.m) P(14 .mu.m) T(18 .mu.m) "STP" F(7 .mu.m) I(15 .mu.m) K(21 .mu.m) D C E S(29 .mu.m) 1077-1113.degree. C. 1028-1140.degree. C. 1057-1113.degree. C. M(23 .mu.m) (2 min.) (2 min.) (2 min.) 1080-1126.degree. C. 1077.degree. C.(3 min.) 1075.degree. C.(3 min.) 1077.degree. C.(3 min.) (2 min.) 1079.degree. C.(3 min.) "HTP" H(26 .mu.m) Q(37 .mu.m) __________________________________________________________________________ *Alloy letter identification (grain size). TABLE 3 ______________________________________ Content (Wt. - %) Si P S C N Cr Ni Mn Nb ______________________________________ 0.02 0.002 0.007 0.410 0.008 17.7 11.0 1.56 0.70 0.08 0.003 0.006 0.42 0.016 18.2 10.6 1.75 0.81 ______________________________________