Grain refinement of uranium-chromium alloys by continuous cooling

Reactor scienceand Technology(JOUUMOfNu~kar Energy,Parts A/B) 1962,Vol. 16,pp. 369to 373. PergamonPress Ltd. Printed in

GRAIN

REFINEMENT OF URANIUM-CHROMIUM CONTINUOUS COOLING

Northern

ALLOYS

Ireland

BY

D. C. MINTY and B. R. BUTCHER Atomic Energy Research Establishment, Harwell, Berks. (Received 23 January1962) Abstract-The transformation temperatures and grain sizes of dilute uranium-chromium alloys have been studied as a function of cooling rate. The results suggest that an alloy could be developed which would transform from the /?-phase to fine equiaxed cc-grains by a non-orientation dependent mechanism, when it is cooled at rates comparable to those observed on air-cooling large bars. 1. INTRODUCTION THE simplest manufacturing method for uranium fuel is to cast it as bars, but unfortunately to achieve the required grain size in the as-cast state requires the uranium to be alloyed with what may be an unacceptably large amount of solute. The next most simple approach is to use very dilute alloys in which a fine grain size can be achieved by B-treating and quenching the as-cast bars. It has now been realized that this treatment may sometimes lead to the introduction of a small amount of preferred orientation into the material (GRAINGER,1961). The third alternative is to transform isothermally a dilute alloy from the p-phase. The transformation temperatures would be such that a fine grain size would result, but the transformation mechanism would not involve orientation dependence of the matrix and product phases. It was thought, however, that a similar result under similar conditions might be achieved by a simpler process of p-treating and allowing the bars to cool out of the furnace in a nonoxidizing atmosphere. To test this, the effects of cooling rate on the transformation temperatures and grain sizes of dilute uranium-chromium alloys have been studied. Chromium was chosen as a solute for two reasons. Firstly it promotes very good refinement, and secondly, more is known about the kinetics of transformation of this alloy than those of any other alloy in the relevant composition range. WHITE’S (WHITE, 1955) TTT curves showed the familiar two ‘C’‘s, the upper one being attributed to nucleation and growth and the lower to a martensitic type of transformation. Recently, however, BEAUDIER, CABANEand MOUTURAT (1961) have demonstrated clearly in a 0.6 atm per cent Cr alloy the different morphology of transformation between the upper and lower halves of the upper ‘C’, and DIXON and BURKE (1962), from their kinetic

studies of a O-5atm per cent Cr alloy, said that there may be a discontinuity in the upper ‘C’ at approximately 525°C. There is a suspicion, therefore, that the transformation in the lower half of the upper ‘c’ may be orientation-dependent. Fortunately WHITE showed that regardless of composition the nose of the upper ‘c’ always occurred at 575°C and so to a first approximation, the temperature above which the transformation is not orientationdependent can be taken as 550°C. The aim of these experiments was to produce a finegrained material which transformed from the /3 at 550°C or above by being cooled at rates approximating to atmospheric cooling rates in large bars.* 2.

EXPERIMENTAL DETAILS (a) Material. The uranium stock was magnesium reduced billet. Three alloys containing 0.5, 0.3 and 0.15 atm per cent Cr were vacuum-melted and cast in graphite. They were swaged to &in. dia., annealed for 4 hr at 720°C and then isothermally transformed at 550°C. Specimens of a double cone shape with an axial thermocouple hole were machined from the rod (Fig. lb). (b) Apparatus and methods. The quenching apparatus is shown in Fig. l(a). A silica ‘T’junction tube had, surrounding the vertical limb, a small split furnace (A) which could be controlled to f 1°C. The top of this limb held a Wilson seal (B) and through this passed a stainless-steel tube of Q-in. dia. The lower end of this tube was cut to leave a thin strip of metal which was bent at right angles and drilled. The top end carried a smaller Wilson seal (C) through which passed a &-in. outside dia. Pyrotenax thermocouple

369

* Previous experiments showed that the initial cooling rate of 1.2-in. dia. bars taken out of a salt bath at 730°C into air was 300400”C/min.

D. C. MINTY and B. R. BUTCHER

I

FIG. lb.

Intermediate rates were obtained by variations of the two extreme methods. The signal from the thermocouple was fed via screened leads to a Cambridge high-speed recorder. A two-way switch in the circuit enabled spot checks of the thermocouple e.m.f. to be made on a potentiometer. From the records the temperature of transformation was noted, the initial cooling rate was measured as the tangent to the cooling curve at the start of cooling, and the time taken to cool from 660°C to the start of transformation, called ‘the time to start transformation’, was also measured. The specimens were then sectioned, prepared by the normal metallographic means, examined under polarized light, and the grain size measured by a linear intercept method. The number of counts and the nature of the structures were such that quoted grain sizes were probably not accurate to more than 10 per cent. 3. RESULTS

VACUUM

FIG.

l(a, b).-Schematic

ARGON

Most attention was concentrated on the more dilute alloys, since it became obvious from the initial results that to fulfil requirements the alloy must contain less than O-5 atm per cent chromium. Figure 2 shows transformation temperatures as a function of cooling rate of the three alloys. At the

diagram of apparatus. 600

(D) containing O-026in. dia. thermocouple wires. The point of the specimen rested in the hole in the bent strip and was secured at the top by pushing the thermocouple home. This arrangement ensured that the specimen stayed steady in the gas stream, while exposing% large surface area to it, and hence smooth cooling curves were obtained. Accuracy of measurement was improved by partially filling the thermocouple hole with tinman’s solder. The control thermocouple (E) was brazed through the side of the Wilson seal (C). The apparatus was evacuated to a pressure of 1(Y5 mm mercury. The furnace temperature was raised to 730°C in 8 min and held there for 15 min. High quenching rates were achieved by rapidly pushing the stainless-steel tube so that the specimen lay at the ‘T' junction, whereupon an automatically activated magnetic valve released argon under pressure into the system. Slow cooling rates were obtained by reducing the furnace current to allow radiation cooling in vacua,

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550

i 2

500

: ” a I

450

? z 0

400

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FIG. 2.-Plot

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I

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RATE,

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of transformatcon vs. cooliw rate.

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temperature

A

Grain refinement of uranium-chromium

alloys by continuous cooling

371

between the equiaxed and the ‘bainitic’ structures is gradual, which one might expect, since some specimens could cool at rates such that they could transform by both mechanisms. An example of a mixed structure is shown in Fig. 6(b) for a specimen which transformed at 530°C. It is interesting (Fig. 7) that the grain size of the ‘as received’ specimen which had been allowed to transform isothermally at 550°C was coarser by a factor of at least 18 than the grain size of the specimen of the same alloy that transformed at 560°C on continuous cooling. 4. DISCUSSION

The main weakness of assuming that 550°C would be a temperature at which the ,B+ M.transformation 300 0.5aPCr would not be orientation-dependent is that the mechanism of transformation applying at a certain 250 \ ! 1 I 10 100 1000 temperature on continuous cooling need not necesT,ME TO START TRANSFORMATION, SEC sarily be the same as the mechanism of isothermal FIG. 3.-Plot of transformation temperature vs. time to transformation at the same temperature. On Fig. 8 start transformation. are plotted the temperature of the kinks in the postulated ‘safe’transformation temperature, of 55O”C, continuous cooling TTT curves as a function of the cooling rates were approximately 60, 450 and chromium content. Also plotted are the temperatures 1500”C/min respectively. Of these the nearest to the of the gaps between the two ‘C’‘s which WHITEfound and 655°C air cooling rates for large bars was 450”C/min, but in for 0.3,0*45, and0.6 atmpercentchromium, this range the transformation temperature seemed has been plotted as the temperature of this gap for pure very sensitive to cooling rate and could vary by 50 uranium. According to various opinions this figure or 60°C for nominally the same cooling rate. Each 0 curve is shown in two sections, which may seem to be 300 wishful thinking for the 0.5 atm per cent alloy. These \ 280 \ sections are however, demonstrated in Fig. 3, where \ + 0.5 a10 Cr the results from the same cooling curves that were 260 * 0.3aP Cr used to plot Fig. 2 are shown in the form of TTT 240 -+j-0.15a10 Cr curves. Figure 4 shows grain size plotted against cooling 220 rates. At the relevant cooling rates for each alloy, the 200 grain sizes were approximately the same at 60 ,D for 180 the 0.5 and 0.3 atm per cent alloy and rather higher at approximately 100 ,u for the most dilute alloy. % 160 Grain sizes are plotted as a function of transformation w N u, 140 temperature in Fig. 5 and again a kink will be noticed. Also indicated in this figure are the grain shapes z 120 which were obtained. Examples for 0.3 atm per cent 5 alloy are given in Fig. 6. The specimen shown in l.3 100 Fig. 6(a) transformed at 560°C and this structure is 80 equiaxed. That of Fig. 6(c) transformed at 510°C and 60 is called ‘bainitic’ while Fig. 6(d) shows a specimen of martensitic structure transformed at 410°C. There 40 is, however, no sharp transition between these I 1 1 20’ I04 I03 I02 structures, especially between the ‘bainitic’ and the IO martensitic, and a distinction is roughly drawn only COOLING RATE, “C WIN by the temperature of transformation. The transition FIG. 4.-Plot of grain size vs. cooling rate.

D. C. MINTYand B. R. BUTCHER

312

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0.3 aID cr

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Cr

-

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--

MARTENSITIC

MARTENSITIC

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GRAIN SIZE,

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I 240

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I I20

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I I80

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FIG. 5.-Plot of transformation temperature vs. grain size for 0.15, @3 and 0.5 atm per cent.

of 655°C could be varied by +12 or -40°C. There is no definite evidence for any particular temperature, and in the authors opinion 655°C is a fair compromise. These points combine to form a smooth curve and thus it is suggested that although the kinetics of transformation are different on isothermal transformation and continuous cooling, the same mechanisms of transformation will apply at the same temperature in each case. Thus the assumption that 550°C will be a temperature at which anon-orientationdependent transformation will take place is probably true for the 0.5 and 0.3 atm per cent alloys, but. not for the 0.15 atm per cent, especially when the microstructural evidence is considered. 700 t

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,

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CURVES SIZE

CURVES

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:

0.4

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FIG. 8.-Plot of temperature of ‘C’junction vs. chromium content.

0.6

In Fig. 8 are also plotted the temperatures of the kinks in the grain size-transformation temperature curves. They are consistently lower than the line. A possible explanation will be given with the help of Fig. 9, which shows the grain size and TTT curves for the O-3atm per cent alloy on the left and in the centre respectively. The detailed TTT diagram for continuous cooling, following DIXON and BURKE, may be as shown on the right of Fig. 9. Here there are three overlapping curves. At slow cooling rates (i) the transformation will be by ‘classical nucleation and growth giving a large grain size which will decrease with increasing cooling rate. At slightly higher rates (ii) the initial transformation will be by nucleation and growth, but before this is 100 per cent complete, the ‘bainitic’ transformation will start, giving a mixed structure and a very fine grain size. Similarly just below the overlap of the top two ‘C’ ‘s (iii) a very fine mixed ‘bainitic’ and nucleation and growth structure will result. When the ‘bainitic’ structure occurs the grain size may continue to decrease. The sequence of structures with increasing cooling rate will then be ‘bainitic’ plus martensitic (iv), martensitic plus ‘bainitic’ (v) and martensitic alone (vi). It is possible that the largest grain size is associated with the pure martensitic transformation at the highest temperature at which this is possible (JEPSON et al., 1958). This temperature will always be below the intersection of the middle and bottom ‘c’ ‘s. The grain size vs. cooling rate curves are not consistent (Fig. 4). That for the 0.5 atm per cent alloy follows the pattern predicted by Harper and Weaver (BUTCHER, 1962) for the structures in an

Grain refinement of uranium-chromium

GRAINSIZE

TIME

TO START

373

alloys by continuous cooling

TRANSFORMATION

TIME

FIG. 9.-Plot of transformation temperature vs. grain size, vs. time to start transformation, vs. time.

end-quenched bar. The same pattern is not apparent for the other two alloys, although it is thought from the TTT curves that exactly the same sequence of transformations are involved, and the sequence of structures is shown in the grain size-transformation temperature curves. The reason could be that in certain ranges the transformation temperature is a very sensitive function of cooling rate, so that the statistical scatter of results inherent in small specimens masked the true curve. Just as likely a reason is that measuring the initial cooling rate was not a very accurate procedure. 5. CONCLUSION

this work it would seem possible to develop an alloy in which grain refinement of large bars could be achieved by natural or forced cool;lng from the p-phase in a non-oxidizing atmosphere. The transformation mechanism involved in the grain refinement From

would not be orientation dependent, and the grain size achieved by continuous cooling would be finer than that achieved by isothermal transformation. It is also possible, however, that a very close control would have to be kept over the process to ensure that the transformation took place at the correct temperature. Acknowledgment-The authors would like to thank DR. P. H. DIXON for useful discussion. REFERENCES BEAUJXERJ., CABANEG. and MOUTURATP. (1961) Mem. sci. Rev. Met. 58, 176. BUTCHERB. R. (1962) .Z. Inst. Met. To be published. DIXONP. H. and BURKEJ. (1962) J. Inst. Met. To be published. GRAINGERL. (1961) Nucl. Engng. 6, 102. JEPSONM. D., KEHOER. B., NICHOLSP. W. and SLAVERY G. F. Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, Paper 15/P/27. United Nations, N.Y. WHITED. W. (1955) J. Metals 7, 1221.