Recrystallization of nickel-270

Metallography

239

Recrystallization J. T. HOUSTON Metals

and Ceramics

Division,

of NickeL270a

AND

K. FARRELL

Oak Ridge National

Laboratory,

Oak Ridge,

Tennessee

Nickel-270, a commercially pure nickel, when cold-worked through reductions in area of 10 to 94 %, displays slight recovery hardening after annealing in the temperature range 200” to 300 “C. The 30-minute recrystallization temperatures are in the range 290 to 400 “C and are inversely proportional to the degree of prior cold work. The time dependence of recrystallization in specimens deformed 94 y0 yields an apparent activation energy of about 40 kcal/mole, which is in better agreement with that for zone-refined nickel than with that for substantially less pure nickel.

Introduction In 1937, Fe& reviewed recovery and recrystallization data on cold-worked nickel. He pointed out that some of the major factors influencing these phenomena were the amount,

method,

temperature,

annealing time, and, more important, recrystallization

temperatures

and speed of cold working, the

the purity of the nickel. At that time,

for most grades of nickel that had been cold-

worked more than 50% usually exceeded 600°C. Fetz discussed the effects of various impurities and alloying elements and then demonstrated that electrolytic nickel of 99.866 y. purity, cold-rolled 82 %, would recrystallize at approximately 450°C

during a 30-minute

anneal; lesser deformations

shifted recrystallization

to higher temperatures. Earlier, Ransley and Smithells had shown that refined and subsequently melted electrolytic nickel of only 99.62% purity, cold-drawn through a 70%

reduction in area, would recrystallize at 480°C in an unspecified

time. Since then, zone refining has brought about progressive reductions in recrystallization temperature. 3 The lowest value attained seems to be 235°C for zone-refined

nickel cold-rolled 80%

and annealed for 30 minutes.4 The purpose

of this paper is to report a low recrystallization temperature for commercially pure nickel and to show the effects of annealing time and degree of prestrain. 8.Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. Metallography, Copyright

0

1969 by American

2 (1969)

239-246

Elsevier Publishing Company,

Inc.

240

J. T. Houston and K. Farrell

Experimental Nickel-270

Procedure is prepared

from carbonyl

Nickel Company (INCO).

nickel powder by the International

Its nominal nickel content is 99.98%,

with specified

maxima of 200 wt ppm carbon, 50 iron, and 10 each of other impurities.5 Our material was purchased in the form of hot-rolled, 0.50-inch-diameter rod. Chemical analyses for interstitial impurities gave 20 wt ppm carbon, 6 hydrogen, 2 nitrogen, and 74 oxygen. A semiquantitative

spectrographic

analysis indicated

that most other impurities were each less than 10 wt ppm, with the exceptions of 15 wt ppm iron, t30 tungsten, <30 zirconium, and <200 zinc; these latter figures are the limits of measurement-the actual analyses may be considerably less. The

as-received

rod was cold-swaged

through

88 %, 75 %, and 34 y. ; we also cold-stretched

reductions

in area of 94’$$,

one piece of rod in tension through

a 10% reduction in area. Disks cut from the wrought rods were annealed in air for 30 minutes. This included a heatup period of approximately 4 minutes. The annealed specimens were mounted and polished for metallography measurements.

Diamond

pyramid hardness

impressions,

and hardness

using a 500-g

load,

were made randomly on the polished and etched cross sections of the rods, the average of at least three impressions being used for the hardness value. The. temperature”recrystallization temperature was read as the “half-hardness that is, that annealing temperature

that caused the room-temperature

hardness

of the specimen to fall to a value midway between that for the as-worked material and that for the fully annealed condition. This sort of indirect measurement

has

been commonly used on nickel in the past, and Bollmans has demonstrated its acceptability by using transmission electron microscopy to show that changes in the

room-temperature

recrystallization

hardness

transformation

of wrought

correspond

nickel

annealed

through

to the stages of recovery,

the

primary

recrystallization, and secondary recrystallization or grain growth. The sharpest change in hardness is associated with primary recrystallization, the hardness remaining

practically

unchanged

during secondary

recrystallization.

We con-

firmed these observations qualitatively by transmission electron microscopy of our annealed 94% deformed material. Our optical metallography, made at the same time as the hardness measurements, Hardness-annealing

temperature

also agreed with the hardness changes.

curves for the cold-worked

Nickel-270

are

shown in Figs. 1 and 3b. Noteworthy features of the curves are that the hardness transitions occur at low temperature and are very sharp, and that prior to recrystallization

slight hardening takes place at temperatures

at which recovery

processes are expected to be active. The fractional hardness increase, relative to the asdeformed hardness, is very much bigger for the 34 y. deformed mater ial than for greater or lesser degrees of deformation. hardening

during recovery

in nickel.

Bridges

Others1 have found slight

and Ball’ state that Stage IV

Recrystallization recovery,

of Nickel-270

centered

241

at 26O”C, is detectable

in material

50 wt ppm carbon. They postulate that precipitation might lead to an increase in hardness.

containing of carbon

less than or carbides

In Fig. 2 the half-hardness recrystallization temperatures are plotted against the degree of prior cold deformation. In agreement with the data of Fe& for 200

i50

200

0

ANNEALING FIG. 1.

Hardness-annealing

on the curves

denote

prestrain

temperature in terms

400 TEMPERATURE curves

of reduction

for cold-worked in area.

600 (“C) Nickel-270.

Figures

242

J. T. Houston and K. Farrell

cold-rolled

electrolytic

nickel,

curves are widely displaced the relative contributions differences

the relationship

is inversely

linear,

but the two

on the temperature scale and are of different slope; of purity and deformation procedure to these

are unknown.

The International Nickel Company has published some earlier measurements of the recrystallization temperature of Nickel-270.5 These were also determined from

hardness

changes

Fig. 2, where

during

30-minute

anneals.

The

it can be seen that they are significantly

700

600

-l

NICKEL

A

INCO

0

PRESENT

in

the present

ELEC TRO-

FETZ’COLD-ROLLED LYTIC

/

are included than

I

I

•I

data

higher

DATA

NICKE :L 270

WOR

500

400

I ---_I_-

300

-L

200 0

25

75

50

DEFORMATION

400

(%I

FIG. 2. Variation of half-hardness recrystallization temperature with work. The deformation procedure for the INCO specimens is not known.

prior

cold

Recrystallization of Nickel-270 values. The discrepancies

243

here may be due in part to differences in deformation

procedures and perhaps to improvements in quality control. We have seen that the amount and type of impurity strongly influence the recrystallization temperature of nicke1.l This is probably because of their effects on grain boundary migration rates, since such migration is likely to be the process controlling

recrystallization.

aluminum

containing

A good example of this retarding effect is shown in

small amounts

of dissolved

copper.8 In recent

years,

however, it has become evident that the form and disposition of the impurities must also be considered. Work on aluminum containing a small amount of gold impurity9 has shown that if the gold is present as easily visible precipitates the material recrystallizes

as though it were zone-refined

gold is present in some submicroscopic

aluminum;

form grain boundary

but when the migration

rates

can be reduced by a factor of 10,000. Since the nature and distribution of impurities are functions of thermomechanical history, it is obvious that their effects in a commercial material may be quite complex. The time dependence

of recrystallization

given in Fig. 3. Experiments

in the 94%

with a thermocouple

prestrained

spot-welded

material is

to a specimen

established that it took an average time of 4 minutes to reach the furnace temperature. Nominal annealing times of 6 minutes, 30 minutes, 1 hour, and 24 hours were used to show that the recrystallization

temperature was markedly

decreased with increasing time. There might be a suggestion, too, that recovery hardening

was pushed to lower temperatures

for annealing times of 1 hours or

more. From Fig. 3 we can determine an activation energy for recrystallization. The general equation relating recrystallization time and temperature in terms of a single activation energy, QT , is 1

t,== where

t, is the time

required

to achieve X

fraction

of recrystallization

at

temperature T; C and R are constants. Since changes in hardness appear to be an acceptable measure of the progress of recrystallization in nickel, we can replace t, with the time to reach a given hardness.

The Arrhenius

plot for the half-

hardness time is shown in Fig. 4. The apparent activation energy is approximately 40 kcal/mole. It is not clear how meaningful this is, since Qr may be sensitive to impurity level8 and to the extent of prior deformation.lOJ1 However, it may be more than coincidence

that a QT of 40 kcal/mole is much closer to Qr values

obtained for zone-refined nickel than it is to those for substantially less pure nickel. Detert and Dressier,* using quantitative metallography to study recrystallization

in cold-rolled,

zone-refined

nickel,

found Qr to be about 47 and

40 kcal/mole for the start and finish of recrystallization material,

and about 29 and 38 kcal/mole, respectively,

in 60%

deformed

for the case of 80%

244

J. T. Houston and K. Farrell

deformation.

Wensch

and Walker,ll

using 99.3 o/0 commercial-purity

nickel

cold-rolled 20%, 400/(, and 60%, found Qr for the finish of recrystallization 75.8, 73.3, and 69.5 kcal/mole, respectively.

to be

I

94%

R IN A

2oo F150 :

26 min

2 min

100

I-----

“E < -9

50

I--

(0)

m

w” z

g I

200

150 56 min 100

50

(cl 0

200 ANNEALING

400 TEMPERATURE

FIG. 3. Effect of annealing time on recrystallization cold-swaged through 94 o/0 reduction in area.

(“C) behavior

of Nickel-270

rod

Remystallization

of Nickel-270

245 TEMPERATURE

(“C)

400

250

200

1000

94%

R IN

A

100

1

0.1 1.4

1.6

1.0 “‘O/T

FIG. 4. Arrhenius annealing temperature

2.0

2.2

(OK)

plot of reciprocal of half-hardness time versus for 94 o/O reduction-in-area material.

reciprocal

of

Summary

In summary, Nickel-270, a commercially pure nickel, when cold-worked through reductions in area of 10 to 94%, displays slight recovery hardening after annealing in the temperature range 200” to 300°C. The 30-minutes recrystalli-

J. T. Houston and K. Farrell

246 zation temperatures

are in the range 290” to 400°C and are inversely proportional

to the degree of prior cold work. The time dependence of recrystallization in specimens deformed 94% yields an apparent activation energy of about 40 kcal/mole, which is in better agreement with that for zone-refined with that for substantially

nickel than

less pure nickel.

References 1. E. Fetz, Trans. Am. Sec. Metals, 2.5 (1937) 1030. 2. C. E. Ransley and C. J. Smithells, I. Inst. Metals, 49 (1932) 287. 3. B. Dubois, A. M. Wacht, F. Dabosi, and J. Talbot, Mm. Sci. Rev. Met., 60 (1963) 851. 4. K. Detert and G. Dressler, Acta Met., 13 (1965) 845. 5. Huntington Nickel Alloys, published by International Nickel Company, Inc., Huntington, West Virginia. 6. W. Bollman, I. Inst. Metals, 87 (1958-59) 439. 7. P. J. Bridges and C. J. Ball, Phil. Mug., 15 (1967) 1107. 8. P. Gordon and R. A. Vandermeer, Trans. AIME, 224 (1962) 917. 9. R. A. Vandermeer, p. 6 in Oak Ridge National Laboratory Report ORNL-3970(1966). 10. J. E. Bailey and P. B. Hirsch, Proc. Roy. SW., A267 (1962) 11. 11. G. W. Wensch and H. L. Walker, Trans. Am. Sot. Metals, 44 (1952) 1186.

Accepted

July 29, 1969