ORNL-TM-1353.txt

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3 4456 0549331 0 joNAL LABORATORY
operated by

UNION CARBIDE CORPORATION w
for the

U.S. ATOMIC ENERGY COMMISSION
ORNL- TM- 1353

 

 

 

 

 

 

  

   

STUDIES OF THE CARBON DISTRIBUTION IN HASTELLOY N

H. E. McCoy

CENTRAL RESEARCH LIBRARY
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NOTICE This document contains information of a preliminary nature
and was prepared primarily for internal use at the Oak Ridge National
Laboratery. It is subject to revision or correction and therefore does
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-

ORNL-TM~1353

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

STUDIES OF THE CARBON DISTRIBUTION IN HASTELLOY N

H. E. McCoy

FEBRUARY 1966

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U.5, ATCMIC ENERGY COMMISSION

OAK RIDGE NATIONAL LABORATORY LIBRARIES

3 44565 0549331 O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
STUDIES OF THE CARBON DISTRIBUTION IN HASTELLOY N

 

H. E. McCoy
ABSTRACT

A small heat of Hastelloy N was prepared in
which a portion of the carbon atoms were tagged as
carbon-14. The response of this alloy to heat treat-
ment was studied in an effort to determine whether the
changes in mechanical properties could be correlated with
the observed changes in the carbon distribution. Although
marked segregation resulted, the changes in mechanical
properties did not appear to be related.

A second objective of this study was to determine
whether the relatively large precipitates in this alloy
were carbides. These precipitates, in both their
stringer (low-temperature) and lamellar (high-temperature)
forms, were found to be as low or lower in carbon than
the matrix. It is hypothesized that the other alloying
elements reduce the solubility of molybdenum in nickel
so that the precipitates are basically nickel-
molybdenum intermetallic compounds.

INTRODUCTION

Previous studies® of Hastelloy N have shown that the fracture
ductility of this alloy at elevated temperatures is very sensitive to
the thermal history of the alloy. Although numerous metallographic
changes have been observed, it has not been possible to correlate these
changes with the changes in the properties. Most of the metallographic
changes involve the fairly coarse precipitate particles (0.1 to 1 mil in
diameter) which are normally present as stringers. As the alloy is
taken to temperatures above 1260°C, these precipitates transform to a

lamellar phase., Quite early in the development of this alloy, these

 

H. E. McCoy, Influence of Several Metallurgical Variables on
the Tensile Properties of Hastelloy N, ORNL-3661 (August 1964).
2 However, more recent

precipitates were reasoned to be carbides,
studies indicate that these particles may not be carbides.

In order to help clariflyy this problem, a small heat of Hastelloy
N was made in which part of the carbon atoms were carbon-14. Samples
of this alloy were given the varicus heat treatments, which were
observed to alter the mechanical properties and subsequent autoradio-
graphic studies were carried out. It was felt that this approach would
provide the answers to two important questions: (1) are the coarse
precipitates and the lamellar phase in Hastelloy N carbides, and (2) how
does the carbon distribution in the alloy change with heat treatment and
can the changes in mechanical properties be correlated with the carbon
distribution? The following report will present the details of this
study.

EXPERIMENTAL DETATLS

A small heat of Hastelloy N was prepared from a charge of
commercial air-melted Hastelloy N and a small amount of nickel that had
been carburized by heating in a mixture of graphite and BaCO; (carbon
atoms present as carbon-14). The charge was nonconsumably arc melted
and cast into a 3/8-in,-diam mold. The ingot was sheathed in stainless
steel, swaged to 1/4 in, in diameter at 1177°C, and swaged to 1/8 in.
in diameter at ambient temperature. The chemical analysis of the
resulting heat is given in Table 1. The composition is within the
specified limits® for Hastelloy N.

The resulting l/8-in.-diam rod was cut into small pieces that
were given various heat treatments in argon. The specimens were then
prepared for metallographic examination. Eastman NTB Liquid Emulsion

was used for the autoradiographic studies and was applied directly on

 

°T, K. Roche, The Influence of Composition Upon the 1500°F
Creep-Rupture Strength and Microstructure of Molybdenum-Iron-Nickel-
Base Alloys, ORNL-2524 (June 24, 1958).

3J. T. Venard, Tensile and Creep Properties of INCR-8 Ffor the
Molten-Salt Reactor Experiment, ORNL-TM-1017 (February 1965).

 

 

 

 
Table 1. Analysis of Experimental Material

 

 

Element Content
(vt %)
Nickel 73.4
Chromium 6.34%
Iron 3.11
Molybdenum 15.5
Silicon 0.71
Manganese 0.50
Carbon 0.058

 

the surface of the polished specimen. In order to obtain high resclution,
very thin layers of emulsion were used. The emulsion was left undisturbed
until the desired exposure was obtained, after which the emulsion was
developed in situ. The carbon distribution was determined by microscopic
examination of the specimen-emulsion composite.

One problem associated with the interpretation of autoradiographs
was the fact that when viewed at about 250X, the emulsion was always
exposed in "spots" rather than uniformly. The reported grain size for
the emulsion is of the order of 1 U and the type of exposure expected
would be a uniform darkening even when the developed emulsion is viewed
at 1000%X, This apparent inconsistency is as yet unresolved, but the
"uniform" exposure has never been observed in any material studied., It
is felt that the small spots represent a uniform carbon distribution
and that these spots arise because all the material in the emulsion is

not capable of being exposed.

FEXPERIMENTAL: OBSERVATIONS

Figure la shows the microstructure of the material in the as-
worked condition. Figure 1lb shows an autoradiograph of the material

in the as-worked condition, The carbon seems to be inhomogeneously
s Y.61447

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Fig, 1.

micrograph of specimen etched with aqua regia.
specimen in the as-polished condition, 16-hr exposure.

Hastelloy N in the As-Worked Condition.
(b) Autoradiograph of

100X,

(a) Photo-
o

r—

distributed on a microscale, Figure 2a shows the material after a l-hr
anneal at 1177°C, followed by rapid cooling. Figure 2b is an autoradio-
graph of the same specimen., This anneal distributes the carbon almost
uniformly in the material, although there does seem to be enough
grain-boundary segregation to delineate the grain structure. Figure 3a
shows the alloy after a l-hr anneal at 1232°C, This photomicrograph is
at high enough magnification to resolve the large precipitates that are
characteristic of this alloy. The autoradiograph of this specimen is
quite similar to that shown in Fig. 2b. However, Fig. 3b shows the
autoradiograph at a high magnification. The light spots are the
precipitate particles in the alloy, Note that the autoradiograph does
not show any darkening above these particles, indicating that they are
low in carbon,

Figure 4a shows the microstructure which results from a l-hr
anneal at 1260°C. Many of the precipitates become associated with grain
boundaries so that an almost continuous network results. Figure 4b shows
an autoradiograph of this specimen which has a 48-hr exposure. The
importance of the exposure time will be shown later. There is a definite
segregation of carbon to the grain boundaries.

Figure 5a shows the microstructure of a specimen annealed 1 hr
at 1316°C. The precipitate particles have transformed almost entirely
to the lamellar phase. Figure 5b illustrates the microstructure in
greater detail, Figure 6a shows that one constituent of the lamellar
product is quite low in carbon and the the other constituent contains
about the same amount of carbon as the matrix. Figure 6b shows the
grain-boundary phése in a somewhat different morphology, but again, one
constituent is low in carbon and the other contains about the same amount
of carbon as the matrix.

As shown in Fig. 7a, a l-hr anneal at 1371°C increases the con=-
centration of the intergranular phase. Figure 7b is an autoradiograph
with a 72-hr exposure which indicates that the grain boundaries are
enriched in carbon, However, a shorter exposure and the use of a higher

magnification help delineate the actual lccation of the carbon.
 
  

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g iy 0

Y-60189

 

Fig. 2. Hastelloy N After a l-hr Anneal at 1177°C. (a) Photo-
micrograph of specimen etched with aqua regia. (b) Autoradiograph of
specimen in the as-polished condition, l6-hr exposure. 100x.
 

Fig. 3. Hastelloy N After a l-hr Anneal at 1232°C. (a) Photo-
micrograph of specimen etched with aqua regia. 250x. (b) Autoradiograph
of specimen in the as-polished condition, 16-hr exposure. 750x.

 
 

Fig. 4.
micrograph of specimen etched with aqua regia.
specimen in the as-polished condition, 48-hr exposure.

i

Hastelloy N After a l-hr Anneal at 1260°C,

‘ f‘ > - L% Y-63024

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Y -59961

 

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Hastelloy N After a 1l-hr Anneal at 1316°C. Etched
with aqua regia. (a) 250x., (b) 1000x.

 
 

10

Y-60257

 

Fig, 6., Autoradiographs of Hastelloy N After a l-hr Anneal at
1316°C. Exposed in the as-polished condition, 16-hr exposure, 750x.

 
Y59962

 

Fig. 7. Hastelloy N after a l-hr Anneal at 1371°C. (a) Photomicro-
graph etched with aqua regia. (b) Autoradiograph made in the as-polished
condition, 72-hr exposure. 250X.
12

Figure 8 shows that the lamellar product in the grain boundary contains
a constituent that is low in carbon., The carbon enrichment seems to be
adjacent to the grain-boundary product rather than in the grain~boundary
phase.

Figure 9 shows the microstructure of the alloy after a 1-hr anneal
at 1177°C followed by 100 hr at 649°C. Figure 10a and b show that rather
gross segregation of carbon to the grain boundaries occurs as a result
of this treatment.

Annealing for 1 hr at 1260°C followed by 24 hr at 871°C produces
the microstructure shown in Fig, lla., The autoradiograph shown in
Fig. 11b illustrates the inhomogeneous distribution of carbon in the alloy
after this heat treatment.

Two pieces of the alloy were rolled into 1 x 1/2 x O.%40-in.
sheets. They were fused together by Heliarc welding without the addition
of any filler metal. A transverse section of the weld wag prepared for
metallographic examination. The base metal has a microstructure similar
to that shown in Fig. 1. Figure 12a shows the heat-affected zone and the
weld metal., TFigure 12b is an autoradiograph of the area shown in Fig, 1l2a.
There does not seem to be any segregation of carbon in the part of the base
metal that recrystallized during welding. Near the fusion line, the car-
bon segregated to the grain boundaries. Figure 13a is a high magnifica-
tion photograph which shows the carbon segregation near the fusion line.
Figure 13b shows that the carbon distribution in the weld metal is quite
uniform,

Two variations in experimental techniques with respect to metallo-
graphic study of the weld specimen should be mentioned. First, the weld
specimen was etched lightly before the liquid emulsion was applied. This
was necessary because the carbon segregation was not sufficient to deline-
ate the various regions of the weld. Secondly, the exposure time for the
autoradiographs was 300 hr as compared with times of 16 to 72 hr for the
other specimens. This resulted from differences in the properties of the
NTB Liquid Emulsion. One lot of emulsion was used for the wrought speci-
mens and a second lot was used for the weld. By appropriate cross checks
1t was found that both lots of emulsion revealed similar details if the

exposure times were varied by a factor of about ten.

 
 

Fig. 8. Autoradiograph of Hastelloy N after a l-hr Anneal at
1371°C. As-polished condition, l6-hr exposure. 750x.

 

. ¥ XN SR bt Y-63027
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f v’ o e e aF N

Fig. 9. Hastelloy N after a l-hr Anneal at 1177°C Followed by
100 hr at 871°C. Etchant: aqua regia.

 
14

 

Fig. 10. Autoradiographs of Hastelloy N After a 1l-hr Anneal at
1177°C Followed by 100 hr at 649°C. As-polished condition. (&) 16-hr
exposure. (b) 72-hr exposure. 250x,.

 
15

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w -, - B -
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NN 0530
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=

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R

 

Fig. 11. Hastelloy N After a l-hr Anneal at 1260°C Followed by
24 hr at 871°C. (a) Photomicrograph of specimen etched with aqua regia.
(b) Autoradiograph of specimen in as-polished condition, 72-hr exposure,
250x%.

 
 

     
  
 

Y-62550

   
    

o o 5
L . -
i - i A "'1, »
% < : 4= T e e '.-‘ t- ¥ : 3
P ‘ f l.," f.'."" "4‘.'?-‘;'.'-. -i-.""g-' ‘)n". wy " '|J : - .
£ Ve ij . 1 =X 1 A "-'{.'- =
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} o ] - T s ‘i
i e = Yt YT ) k
" -
(b) - . P ~N }j I
b »
- - SR .

Fig. 12. Weld Metal and Heat-Affected Zone of a Hastelloy N Weld.
() Photomicrograph of specimen etched with aqua regia. (b) Autoradio-
graph of specimen after a light etch, 300-hr exposure. 250x.
Y-62552

 

Fig. 13. Autoradiographs of Hastelloy N Welds After a Light Etch,
300-hr Exposure. (a) Heat-affected zone., 750x. (b) Weld metal. 250x.

 
18
DISCUSSION OF RESULTS

The results which have been presented may be summarized as follows:

1. DNeither the coarse precipitate particles which are present as
stringers nor the lamellar phase are enriched in carbon. In fact, the
coarse precipitate particles and one constituent of the lamellar phase
are depleted in carbon with respect to the matrix.

2. The carbon is distributed fairly uniformly at annealing tempera-
tures up to 1260°C,

3. At temperatures above 1260°C the carbon is segregated to the
grain boundaries, although the areas adjacent to the grain-boundary pre-
cipitate are more enriched than the precipitates themselves.

4. Annealing at 1177°C followed by 100 hr at 649°C and annealing
at 1260°C followed by 24 hr at £71°C both produced gross segregation of
carbon to the grain boundaries.

5. Welding produced some intergranular carbon segregation in a
small region of the heat-affected zone.

The first observation indicates very strongly that Hastelloy N is
not basically a solid-solution alloy and that intermetallic phases are
present. There are numerous possibilities of compounds which could be
formed from the various alloying elements present. However, the observa-
tion that the quantity of precipitate in Hastelloy N does not depend on
the concentration of minor alloying elements leads one to believe that
the second phase in actually a nickel-molybdenum intermetallic. The
binary nickel-molybdenum phase diagram4 shown in Fig. 14 indicates that
a binary alloy of 16 wt % Mo and nickel would be single phase up to about
1400°C. However, studies by Lundy on the nickel-rich corner of the nickel-
molybdenum-chromium system have shown that the addition of chromium to
nickel reduces the solubility of molybdenum in the alloy. This is equi-

valent to saying that the alpha filed in Fig. 14 is reduced in size and

 

“Metals Handbook, American Society for Metals, Cleveland, 1948,
p. 1230,

>T. S. Lundy, A Metallographic and X-Ray Study of Nickel-Base Alloys
of 20-25 Per Cent Molybdenum and 3—15 Per Cent Chromium, M.S. Thesis, The
University of Tennessee, 1957.

 

 
£G

19

Y.12802

°C Atomic Percentage Molybdenum °F

20 4 80
2000 0 eo 3600

1800 3200

1600
2800
1460
2400
1200

2000
1000

1600
800

600 1200
400 800

400

a
+
p

 

ONi 10 20 30 40 30 60 70 80 90 Mo

Weight Percentage Molybdenum

Fig, 14, DNickel-Molybdenum Equilibrium Diagram.

that the alpha and beta fields are moved to the left. Lundy also noted
that the addition of chromium suppressed the formation of beta. Recent

6

studies by Norman® on the nickel-molybdenum-iron system have shown that

iron has very little effect on the solubility of molybdenum in nickel,
but that iron also suppresses beta formation. Both ILundy and Norman
noted that the phase transformations in these alloys are quite sluggish.
In light of these facts, it is hypothesized that the coarse precipitates
that are present in Hastelloy N after fabrication are a modified delta
phase which is formed in the melt as it cools. Reheating for 1 hr at
1260°C or at higher temperatures causes the delta to decompose to form
alpha and gamma (the lamellar phase).

The second, third, and fourth observations which concern carbide

segregation indicate that the ductility of this material is not controlled

 

%W. E. Norman and E. E. Stansbury, An Investigation of Nickel-Rich
Alloys Containing Molybdenum and Iron, The University of Tennessee
(August 1963).
20

by the carbon distribution. The studies reported previously’ showed

that the minimum fracture elongation (measured in a tensile test at
871°C) is reduced by a factor of 3 as a result of annealing at 1204°C

as compared with that obtained by annealing at 1177°C. Annealing at
1232°C produced another twofold decrease in elongation. However, the
autoradiographs show no significant carbon segregation until annealing
temperatures of 1260°C were used. Both of the aging treatments that were
used (1 hr at 1177°C followed by 100 hr at 871°C, and 1 hr at 1260°C
followed by 16 hr at 871°C) produced gross carbide segregation. The
material receiving the first heat treatment was found to have about one-
half the ductility of the material given the second treatment. From
Figs. 9, 10, and 11 it is not obvious that any great difference exists in
the degree of carbon segregation.

In view of these results, it is felt that the loss in ductility in
this alloy does not correlate with the grain-boundary segregation of car-
bon. A close examination of the test results’ obtained on various heats
of Hastelloy N and the compositions of these various heats leads one to
suggest that the troublesome element may be silicon, although no direct
evidence exists for this conclusion.

The carbon segregation that occurred in the weld can be rationalized
in terms of the microstructures that were obtained during 1-hr anneals.
The carbon was segregated to the grain boundaries by l-hr anneals above
1260°C. For the short time that heat was applied to the weld, higher
temperatures would probably be required to cause carbon segregation. The
weld metal did not show any carbon segregation., Hence the observation
that the carbon was segregated only in a small region in the heat-affected

zone is as expected.

 

7H. E. McCoy, Influence of Several Metallurgical Variables on the
Tensile Properties of Hastelloy N, ORNL-3661 (August 1964).

 

 
F P

21

SUMMARY AND CONCLUSIONS

It has been observed by autoradiographic studies on a heat of
Hastelloy N containing carbon-14 that (1) the large precipitates in this
alloy are not enriched in carbon, and (2) the loss in ductility cannot
be associated with the segregation of carbon. It is hypothesized that
the major precipitates in this alloy are nickel-molybdenum intermetallic
compounds which form as a result of the solubility of molybdenum in
nickel being reduced by the presence of chromium in the alloy. The loss
in ductility in this alloy as a result of annealing at elevated tempera-
tures is probably associated with a minor alloying element other than

carbon. It is hypothesized that silicon is this troublesome element.

ACKNOWLEDGMENT

The author is grateful to B, J. Massey of the Isotopes Division for
assistance in preparing the mixture used to saturate nickel melting stock
with carbon-1l4, This study would not have been possible without the
Metal Forming and Casting Group who made this alloy. The metallography
and auvtoradiography were performed by M. D. Allen and his efforts and
perscnal Jjudgement were of great value. The author is also grateful to
J. R. Weir, R. 5. Crouse, and D. A, Douglas for reviewing this work and
making several helpful suggestions. The Reports Office is acknowledged

for their assistance in preparing this document.
23

ORNL-TM-1353
INTERNAL, DISTRIBUTION
1-3, Central Research Library 57. J. H Frye, Jr.
4—5, Reactor Division Library 58, C. H. Gabbard
6. ORNL - Y-12 Technical Library 59, W. R. Gall
Document Reference Section 60, R. B. Gallaher
7—16. Laboratory Records Department 6l. R. J. Gray
17. Laboratory Records, ORNL R.C. 62. W. R. Grimes
18. ORNL Patent Office 63. A. G. Grindell
19. G. M. Adamson 6. R. H. Guymon
20, L. G. Alexander 65. G. Hallerman
21, C, F, Baes 66, P. H. Harley
22. S. E. Beall 67. D. G. Harman
23. E. 8. Bettis 68, C, S, Harrill
24, D, S. Billington 69. P. N. Haubenreich
25, F, F, Blankenship 70. P. G. Herndon
26, E. P, Blizard 71-73, M. R. Hill
27. R. Blumberg 7. B, C. Hise
28. A. L. Boch 75. H. W. Hoffman
29, E. G. Bohlmann 76. V. D, Holt
30, C., J. Borkowski 77. P. P. Holz
31, G. E. Boyd 78. A. Houtzeel
32. E. J. Breeding 79. T. L. Hudson
33. R. B. Briggs 80. H. Inouye
34. F., R. Bruce 81, P, R, Kasten
35. G, H. Burger 82. R. J. Kedl
36. S. Cantor 83, T. M. Kegley
37. D. W. Cardwell 84, M. T. Kelley
38. 0. B. Cavin 85. M. J. Kelly
39, J. A. Conlin 86. C. R. Kennedy
40, W, H. Cook 87. A. I, Krakoviak
41. L. T. Corbin 88, J. W. Krewson
42. G, A. Cristy 89, C, E, Lamb
43. R. S. Crouse 90. C. F. Leitten, Jr.
44, J., L. Crowly 91. R. B. Lindauer
45, F, L, Culler 92. R. S. Livingston
46. J. E. Cunningham 93. M. I. Lundin
47. D. G. Davis 9., T. S. Lundy
48, W. W. Davis 95, W. R. Martin
49, J. H. DeVan 96—-105. H, E, McCoy
50, R. G. Donnelly 106, W. B. McDonald
5l. D. A. Douglas 107. C. K. McGlothlan
52. N. E. Dunwoody 108, C. J. McHargue
53. J. R. Engel 109, E. C, Miller
5., E. P. Epler 110. C. A, Mills
55, W, K. Ergen 111, W, R. Mixon
56. A. P. Fraas 112. R. L. Moore
113, J. C. Moyers 136, W. F. Spencer
114, T. E. Northup 137. I, Spiewak
115, W. R. Osborn 138, R. Steffy
116, L. F, Parsly 139, C. E. Stevenson
117, P. Patriarca 140, C. D. Susano
118, H. R. Payne 141. A, Taboada
119, W. B. Pike 142, J. R. Tallackson
120, H. B. Piper 143. E. H. Taylor
121, B, E. Prince 144, R. E. Thoma
122. J. L. Redford 145, G. M, Tolson
123. M, Richardson 146. D. B. Trauger
124, R. C. Robertson 147. R. W. Tucker
125, H. C, Roller 148, W. C. Ulrich
126. M. W, Rosenthal 149, D. C. Watkin
127, H. C. Savage 150. G. M. Watson
128, H. W. Savage 151. B. H. Webster
129, D, Scott 152, J. R. Weir
130, J. H. Shaffer 153, J. H. Westsik
131, E, D, Shipley 154, J. C. White
132, G. M, Slaughter 155, L. V. Wilson
133. A. N. Smith 156, C. H. Wodtke
134, P. G. Smith 157. G. J. Young
135. A. H. Snell
EXTERNAL DISTRIBUTION

158. C. M. Adams, Massachusetts Institute of Technology
159-160. D. F. Cope, AEC, ORO

161. C. B. Deering, AEC, ORO

162. R. W. Garrison, AEC, Washington

163. J. L. Gregg, Bard Hall, Cornell University

164. R. W. McNamee, Manager, Research Administration, UCC, New York

165, M. Shaw, AEC, Washington

166. J. M. Simmons, AEC, Washington

167. E. E. Sinclair, AEC, Washington

168. W. L. Smalley, AEC, ORO

169, E. E. Stansbury, University of Tennessee

170. M. J. Whitman, AEC, Washington

171, Division of Research and Development, AEC, ORO
172-186. Division of Technical Information Extension