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RECRYSTALLIZATION OF THORIUM

OAK RIDGE NATIONAL LABORATORY

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ORNL 1467

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RECRYSTALLIZATION OF THORIUM

F. H. Eckert and E. J. Boyle

Work carried out by: F. H. Eckert, D. S. Eaton

Metallography by: R. J. Gray

Fabrication by: J. H. Erwin

DATE ISSUED

FEB 1 8 19SS

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RECRYSTALL IZATION OF THORIUM

F. H. Eckert E. J. Boyle

ABSTRACT

The recrystallization characteristics of Ames thorium and iodide thorium were de

termined. The effects of annealing temperature, annealing time, and cold work were

followed by metallographic and hardness determinations. The results show that Ames

thorium and iodide thorium recrystal Iize in the temperature range from 500 to 700 C.

Iodide thorium recrystal Iizes at a slightly lower temperature than the Ames material.

Increasing the carbon content to 0.13% has no significant effect on the recrystalli

zation temperature of iodide thorium. The higher carbon-content material shows some

increase in hardness after completion of recrystallization. This behavior is believed to

be a result of precipitation of carbon at the higher annealing temperatures.

INTRODUCTION

This investigation was undertaken to study therecrystallization characteristics of pure thoriumthat had been plastically deformed specifiedamounts and then isothermally annealed, with time,temperature, per cent reduction in area, and carboncontent as variables. The effect of the carbon

content was studied because carbon is the principalcontaminant in thorium.

A previous study of the recrystallization of commercially pure Ames thorium was reported byJackson and Beaver ' at Battelle Memorial Insti

tute. The present study include both high-purityiodide thorium and high-purity Ames thorium.

CONCLUSIONS

On the basis of the results of the present experimental work, the following conclusions appear tobe justified:

1. Pure thorium, prepared from decomposition ofthe iodide, recrystallizes in the temperature rangefrom 500 to 700°C after 20 to 80% reduction bycold rolling.

2. Ames thorium that has been cold rolled 20 to

80% recrystallizes in the temperature range from520 to 700° C.

3. Carbon, which is the principal contaminant inAmes thorium, does not appear to have a significanteffect on the recrystallization temperature rangeof the thorium. However, the hardness levels,following recrystallization, of alloys containing

(1)L. R. Jackson and W. W. Beaver, Journal of Metallurgy and Ceramics, p. 74-88, TID-68 (Sept. 1949).

carbon in excess of the normal range for Amesthorium are considerably higher than those of thestandard material.

EXPERIMENTAL PROCEDURE

Material. The Ames thorium used in this investi

gation was produced bycalcium reduction of thoriumfluoride at the Iowa State College at Ames, Iowaand was received as a3-in.-dia cast billet (A-349B).The higher purity iodide thorium was produced atBattelle Memorial Institute by decomposition ofthorium iodide on a hot filament. The analyses ofthis material and a thorium-carbon alloy series areshown in Table 1. The high oxygen and nitrogencontents in the thorium-carbon alloys with increasing carbon content are believed to be due tocontamination by the master alloy used in preparingthese alloys. Iodide thorium was used in theirpreparation, and all fabrication procedures wereidentical except for greater additions of the masteralloy to the higher carbon alloys.

Fabrication. The thorium specimens were prepared in the following manner. The 3-in.-dia Amesthorium billet was extruded to 0.625-in.-dia rod,and four equal portions of this rod were cold rolledwith various specified reductions. The rolledthorium plate was heated to 750°C for 30 min in avacuum and furnace cooled to precipitate, initially,any impurities that might confuse the recrystallization studies by spurious hardening effects fromprecipitation. The annealed thorium plates werecold rolled to 20, 40, 60, and 80% reduction in areaand sectioned into 0.071- by 0.75- by 0.50-in.specimens.

The iodide crystal-bar thorium was melted in aninert-atmosphere arc furnace by using a non-consumable tungsten electrode. The thorium meltswere fabricated into 0.061- by 0.5- by 0.4-in.specimens with 20, 40, 60, and 80% cold work bythe same procedure as outlined above for the Amesmetal,.

Thorium-carbon alloys were prepared by masteralloy additions to the crystal-bar thorium in theinert-atmosphere arc furnace and were then coldreduced approximately 35%, homogenized 12 hr at1275°C, furnace cooled to room temperature, andcold rolled to 70% reduction in area. Specimens,0.07 by 0.6 by 0.5 in., were cut from the finishedmaterial.

Heat Treatment. A molten lead bath was found to

be most suitable for heating the specimens sincethis provided a rapid way to attain bath temperatures and prevented oxidation of the thorium duringthe annealing experiments without the use of aninert atmosphere or a jacketing material. The leadwas held in a resistance pot furnace with thetemperature controlled to ±2°C by means of achromel-alumel thermocouple.

Individual specimens of iodide and Ames thoriumwere isothermally annealed for varying times attemperatures ranging from 450 to 740°C for studyof the effect of time on recrystallization. Thespecimens that had been cold worked 20, 40, and60% and the thorium-carbon alloys were isothermally annealed for 1 hr at temperatures varyingbetween 450 and 740°C for study of the effects ofcarbon and cold work on the recrystallization ofthorium.

After heat treatment in the lead bath, all sampleswere quenched in water, and the progress of recrystallization was traced by Vickers hardnessmeasurements and metallographic examination.

RESULTS AND DISCUSSION

The effect of annealing time on the recrystallization of Ames thorium and iodide thorium is shown

in hardness vs. annealing time plots in Figs. 1and 2. No apparent changes in microstructure wereseen for the initial drop in hardness to 100 VHN inthe Ames thorium, whereas the iodide specimensshowed new strain-free grains forming at the firstdrop in hardness.

The effect of cold work on the recrystallizationof Ames thorium and iodide thorium is shown in

Figs. 3 and 4. Ames thorium exhibits a possible"recovery" zone, as shown by the break in therecrystallization curves, above which the micro-structures revealed no evidence of recrystalli

zation. This effect was not evident in the iodidethorium. The "recovery" zone in the Ames thoriummay be due to the precipitation of another phaseassociated with the higher level of impurities inthe Ames thorium. This hypothesis is supportedby the fact that iodide thorium does not show thischange under identical conditions. However,investigations have not been made to find outwhether the iodide thorium shows this effect at

lower temperatures.Table 2 shows the annealing temperature for the

start and completion of recrystallization with varying amounts of cold work. These temperatures weredetermined by metallographic examination. Examination of Table 2 and Figs. 3 and 4 shows thatincreasing cold work decreases the annealingtemperature necessary for recrystallization to takeplace at any given time. Increased annealing timeis necessary for recrystallization at lower temperatures.

It was generally observed that hardness aftercomplete recrystallization was unaffected by previous cold work but was slightly higher, the lowerthe annealing temperature used. Iodide thoriumrecrystallizes at a slightly lower temperature thanAmes thorium, all conditions being equal.

Figure 5 shows the effect of carbon content onthe recrystallization of iodide thorium. Table 3lists the temperatures at which recrystallizationstarts and finishes for different carbon alloys asdetermined by metallographic examination. It canbe seen that the addition of carbon up to 0.13%apparently has no significant effect on the recrystallization temperature range of the iodidethorium. The increases in hardness in the temperature range from 675 to 740°C are believed to resultfrom precipitation of carbides atthese temperatures.

Photomicrographs showing the progress of recrystallization under various conditions are shownin Figs. 6 through 12. Figures 6 through 10 showthe progress of recrystallization and grain growthin iodide thorium that was cold rolled 80% and

annealed for various times at 600°C. Figures 11and 12 show the structures obtained in Ames

thorium treated according to some of the sameconditions. It will be noted, in the case of theiodide thorium, that recrystallization started atthe shortest annealing time. Ames thorium alsoshows the start of recrystallization at the shortestannealing time. However, the grain size of theAmes thorium after 1 hr at temperature is very muchsmaller than that of the iodide material, whichindicates the grain growth restraining effect of theimpurities in the former material.

TABLE 1. CARBON, VACUUM-FUSION, AND SPECTROGRAPHIC ANALYSES OF THORIUM

MATERIALCOMPOSITION (ppm)

C °2 N2 H2 Al Si Fe Be Other Elements

Ames thorium billet*

(A-349B) 370 120 150 <100 90 90 140 30 Faint traces

Iodide thorium 200 no 10 <100 Trace Trace Faint traces

Thorium-carbon

alloy 1340 500 600 <100 Trace Trace Faint traces

1010 400 70 <100 Trace Trace Faint traces

850 200 40 <100 Trace Trace Faint traces

490 <100 <10 <100 Trace Trace Faint traces

*Ames analysis, except carbon and oxygen*

TABLE 2. EFFECT OF COLD WORK ON RECRYSTALLIZATION OF THORIUM

REDUCTION START OF RECRYSTALLIZATION (^C) RECRYSTALLIZATION COMPLETE (°C)

IN AREA (%) Ames Thorium Iodide Thorium Ames Thorium Iodide Thorium

80 520 500 625 575

60 520 510 to 520 625 600

40 540 to 550 520 650 600

20 550 to 575 550 to 575 650 700

TABLE 3. EFFECT OF CARBON ON RECRYSTALLIZATION OF IODIDE THORIUM*

CARBON CONTENT START OF RECRYSTALLIZATION (°C) FINISH OF RECRYSTALLIZATION (°C)

0.134 510 to 520 650

0.101 510 to 520 650

0.085 500 to 510 650

0.049 500 to 510 650

*70% reduction in area for all alloys.

20 30 40

ANNEALING TIME (min)

Fig. 1. Isothermal Recrystallization Curves for Iodide Thorium.

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Fig. 6. Iodide Thorium Cold Rolled 80%. 250X

Fig. 7. Iodide Thorium Cold Rolled 80% and Annealed 1 min at 600 C. 250 X

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