Effects of processing on microstructure and properties of α-uranium formed parts

doi:10.1016/S1359-6462(02)00137-9

Scripta Materialia

Volume 47, Issue 4, 20 August 2002, Pages 261-266

https://doi.org/10.1016/S1359-6462(02)00137-9 Get rights and content

Abstract

The interrelated effects of thermomechanical processing and chemistry on the microstructure and tensile properties of α-uranium formed parts were characterized and presented.

Introduction

Thermomechanical processing of unalloyed uranium (U) is tailored to produce parts that possess a very fine, uniform grain size [1]. An initial hot round rolling of the billets at high α (phase field) temperatures is performed to promote continuous dynamic recovery [2], as a way of grain refinement. Subtle changes in the rolling temperature and chemistry may result in an inhomogeneity in the final microstructure [3].

A microstructural inhomogeneity observed in uranium hydroformed hemispherical shells is a nonuniform distribution of through-thickness grain-sizes. The inhomogeneity arises due to the decreased rolling temperature from 600 °C to as low as 210 °C (<0.35T_m of uranium), which is caused by the cold rollers. The temperature difference between the cold rollers and preheated ingot creates a quenching effect at the contact surfaces. As a result, higher strains are accumulated at the surface than at the center of the billet. The subsequent recrystallization heat treatment therefore establishes a nonuniform distribution of grain sizes through thickness. This inhomogeneity is enhanced during hydroforming of the shells, where nonuniform strain is applied to the formed blank, and during the final recrystallization heat treatment.

Given process limitations, an effective way to control grain size in the unalloyed uranium is through using grain growth inhibiting precipitates, such as U₃Si₅. Although submicron sized silicide precipitates are preferable, and thermodynamically favorable [8], uranium carbide precipitates may also form [4]. However, we must keep in mind that carbon and silicon are commonly present in unalloyed uranium as impurities, and molten uranium can also pick up carbon, as much as an additional 100 ppm, during casting. Recognizing that specified maximum allowable levels for Si and C are established at 625 and 600 wppm, respectively [1], controlling impurity concentrations is very difficult in a production plant environment and as a result these levels vary amongst individual castings.

The beneficial effects of precipitates in controlling the grain size were clearly seen [5], [6] when the microstructure of recycle grade uranium (225 ppm Si and 300 ppm C) contained much finer grains than that of high-purity uranium (54 ppm Si and 74 ppm C). Foote [4] has found that ideal impurity limits of silicon and carbon are between 200 and 500 ppm total. However, problems may arise if there are silicon-lean and carbon-rich regions.

The effects of processing history and Si/C chemistry differences on the microstructure of the α-uranium parts are presented. The differences in silicon and carbon content within the material are shown to have a marked effect on the subsequent microstructure, viz. grain size distribution. Tensile properties of the α-uranium parts are discussed in terms of the role of the final heat treatment, i.e., salt bath annealing versus vacuum annealing. Hydrogen absorption, as a result of a salt-bath pre-heat and anneal, is believed to have detrimentally affected tensile ductility [7], albeit with little effect on initial yielding. Subsequent vacuum annealing from the salt bath preheat reduces the internal hydrogen concentration from 5–12 ppm to <1 ppm, and improves tensile ductility [3].

Section snippets

Experimental procedure

The α-uranium formed parts were provided by the production plant in the wrought and fully recrystallized condition. To achieve this condition, a 178-mm diameter by 36-mm thick α-uranium ingot is preheated in a salt bath to 630 °C for 30 min and subsequently “round-rolled” by rotating either 45° or 90° from the initial rolled direction. Note that the purpose of round rolling is to avoid the in-plane anisotropy in the rolled blanks. After ten passes, the final blank dimensions are a 430-mm

Microstructure

The effect of silicon, i.e., silicides, in controlling the microstructure is shown in Fig. 2. The samples were taken from two different parts at the inner (contour) surface of the pole region, which had different silicon concentrations but comparable carbon concentrations. The differences seen in the microstructure of the pole region are therefore primarily due to silicides. The grains of α-uranium containing 200 ppm Si and 320 ppm C are much finer and uniform than those of α-uranium containing

Conclusions

The characterization of the α-uranium formed parts revealed that a subtle change in the initial rolling temperature and chemistry had a significant influence on the final microstructure. The microstructural inhomogeneity present in the billets is exacerbated with nonuniform strain induced during hydroforming. The bimodal microstructure is seen at the pole and in the 45° region of the formed parts. One of the recommendations was to increase the silicon content in α-uranium to prevent grain

Acknowledgements

This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48.

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