CORROSION ANALYSIS: LOW-CAPACITY PUMP. Large Sodium Pump Study, Phase II. Page: 20 of 30
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function of both time and temperature.
Figure 6 and 7 give the available data for Types 304 and 304L stainless
steels at 900 and 1200*F, respectively f8'9. Assuming the changes to be
additive, an estimate can be made of the changes in mechanical properties
over the design lifetime of the pump. Mean starting values from Figures 6
and 7 are given in Table 3 together with the estimated changes due to 20
years at 900 F, and 1000 hours at 1200*F. The estimated final value is
given in the last column.
Although measurable changes will occur they are not considered to be large
enough to be deleterious, therefore, no problems are expected to arise from
changes in mechanical properties.
Sigma Phase Formation
Sigma phase, an iron-chromium intermetallic compound, can precipitate in
austenitic stainless steels after extended exposure at elevated tempera-
tures. The formation of sigma is a function of composition, time,
temperature, and the amount of cold work. The generally quoted temperature
range for sigma formation is 1000 to 1600*F; but, it can form at lower
temperatures in cold-worked materials, at least. Annealed Types 316 and
304 stainless steels, however, showed no sign of sigma precipitation after
2000 hours at temperatures up to 1500*F1ll].
Welds in austenitic materials usually contain between 3 to 7% ferrite.
This high chromium material is more susceptible to sigma formation than
the parent metal. However, results on ferritic materials in the annealed
condition, showed no evidence of sigma formation after 2000 hours at
temperatures up to 1500*F110]. On the basis of this information, sigma
phase formation is not expected to present any problems under the proposed
Thermal shock damage at high temperatures is cumulative, especially if
the dwell time after the temperature transient is of sufficient duration
to allow relaxationlll]. The resistance to thermal shock is a function
of the material fatigue behavior, and allowable stresses are estimated
with high-temperature fatigue property data as given in Code Case 1331-1.
Some cyclic strain fatigue data (105 cycles) are available for Types 316
and 30 st i ess steels at 1200*F (650*C) in sodium of varying impurity
contents 12' . The fatigue limit in clean sodium is higher than in air;
high carbon causes little change; and a high-oxygen sodium causes a
decrease in the fatigue limit.
It is believed, therefore, that the established fatigue data given in
Code Case 1331-1 should be adequate for calculating allowable stresses
under pump operating conditions. It would be advisable, however, to
establish new fatigue curves for a sodium environment for future use.
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Thomas, K.C. & Shiels, S. CORROSION ANALYSIS: LOW-CAPACITY PUMP. Large Sodium Pump Study, Phase II., report, October 31, 1970; Madison, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc1034653/m1/20/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.