Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 36 of 47
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One inherent weakness in the hot hardness test is, of course, the rapid increase
in resistance to flow as the indentor penetrates the specimen surface. It is,
therefore, a short-time test whereas high temperature creep is a long-time test
usually requiring many hours to reach a stable rate of deformation. The pitfall,
therefore, arises that while hot hardness may be a necessary characteristic of
an alloy possessing high creep strength, short-term hot hardness is no
guarantee of long-term resistance to flow. This difficulty was experienced
during the early period of World War II when a major program in hot hardness
testing was undertaken as a screening procedure to select superior candidates
for gas turbine alloys which would then be chosen for actual creep testing. It
was demonstrated at that time that high hot hardness was not a sufficient
condition for good creep strength properties, and the program was, therefore,
abandoned as a screening technique. Actually, the most fruitful screening
process is a so-called stress-to-rupture test which, in effect, is an
accelerated creep tes-t lasting only perhaps 50 to 100 hours. The rate of flow
under the constant stress of this test (in contrast to the steadily changing
applied stress of the hot hardness procedure) may be extrapolated to long-term
test conditions with remarkable fidelity.
These same uncertainties appeared to apply to an important degree in the
appraisal of resistance to in-reactor swelling, and in due course the experi-
mental program was discontinued.
Tensile and Impact Testing
A number of unusual fuel core ruptures* in the Hanford reactors in 1961 led to
an examination of the mechanical properties of alloyed dingot as compared with
ingot uranium. Impact tests were performed on an assortment of dingot alloys,
and on ingot samples were included as a control, over the range from -100 to
+350"F. Little difference between ingot-or dingot or between different alloys
was observed. All showed low impact strength at low temperatures, and all
showed toughness and ductility~at the upper temperature levels. All fractures
below +200*F. were brittle in nature whereas above this temperature most of the
samples did not actually break but were bent and dragged through the impact
machine. Samples oil.quenched from. the beta phase prior to testing revealed
*These ruptures were transverse splits which had not been observed before nor
since and which were never thoroughly understood. There appeared to be some
basis for the suspicion that- the possible combination of nonbonding of the
uranium to the aluminum spire at the core I.D., combined with an inadvertent
defect in the end weld closure, had permitted water entry and a rapid and
uniform generation of a U02 layer over the core I.D. It was postulated that
localized penetration of the oxidation toward the O.D. may have created wedges
that split the core by the pressure from the change in volume. Attempts to
duplicate this experimentally confirmed the tendency to oxidize but were never
really satisfactory in reproducing this type of transverse fracture.
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Fellows, J. A. Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966., report, January 1, 1966; Weldon Spring, Missouri. (https://digital.library.unt.edu/ark:/67531/metadc1033773/m1/36/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.