Microscopic fracture processes were studied which are associated with hydrogen attack of a medium carbon steel in a well-controlled, high-temperature, high-purity hydrogen environment. Exposure to a hydrogen pressure and temperature of 3.5 MN/m2 and 575 C was found to degrade room temperature tensile properties with increasing exposure time. After 408 hr, yield and ultimate strengths were reduced by more than 40 percent and elongation was reduced to less than 2 percent. Initial fissure formation was found to be associated with manganese rich particles, most probably manganese oxide, aligned in the microstructure during the rolling operation. Fissure growth was found to be associated with a reduction in carbide content of the microstructure and was inhibited by the depletion of carbon. The interior surfaces of sectioned fissures or bubbles exhibit both primary and secondary cracking by intergranular separation. The grain surfaces were rough and rounded, suggesting a diffusion-associated separation process. Specimens that failed at room temperature after exposure to hydrogen were found to exhibit mixed mode fracture having varying amounts of intergranular separation, dimple formation, and cleavage, depending on exposure time

Melson, H. G.

Moorhead, R. D.

English

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NASA TECHNICAL
	
NASA TM X•62,435
MEMORANDUM
	
Ln	 (NASA—TM—X-62435) FRACTOGRAPHY OF THE HIGH
	 N75-24899
	
m	 TEMPERATURE HYDROGEN ATTACK OF A MEDIUM
	
N	 CARBON STEEL (NASA) 20 p HC $3.25 CSCL 11F
	
W	 Unclas
0
	
x	 G3/26 25321
z
rRACTOGRAPHY OF THE HIGH TEMPERATURE HYDROGEN ATTACK
OF A MEDIUM CARBON STEEL
Howard G. Nelson and R. Dale Moorhead
Ames Research Center
Moffett Field, Calif. 94035
May 1975
1, Repave No. 2. Government A=mlon No. 3, Reclp!ent's Catalog No.
TM X-62,435
4, Title and Subtitle 6, Report Date
Fractography of the High Temperature hydrogen
Attack of a Medium Carbon Steel 6. Performing Organisation Coda
7.	 At thorlal B. Performing Organisation Report No,
Howard G. Nelson and R. Dale Moorhead A-6079
10, Work Unit No,
505-01-21-02-00-21g. Performing Orgenivdlon Name and Adchm
NASA Ames Research Center
Moffett Field, Calif. 94035
11. Contract or Grant No.
13, Type of Report and Period Covered
Technical Memorandum12, Sponsoring Agency Name and Address
National Aeronautics and Space Administration 14, Sponsoring Agency Code
Washington, D. L.	 20546
16, Supplementary Note
Presented at Symposium on "Fractography-Microscopic Cracking Processes,"
78th Annual Meeting of ASTM, Montreal, Canada, June 23-27, 1975
16. Abstract ,
A systematic study was made of the microscopic fracture processes asso-
ciated with hydrogen attack of a medium carbon steel in a well-controlled,
high-temperature, high-purityhydrogen environment. 	 Exposure to a hydrogen
pressure and temperature of 3.5 MN/m 2 and 575' C was found to progressively
degrade room temperature tensile properties with increasing exposure time.
After 408 ltr, the maximum exposure time, of this study, yield and ultimate
strengths were reduced by more than 40 percent and elongation was reduced to
less than 2 percent.	 Initial fissure formation was found to be associated
with manganese rich particles, most probably manganese oxide, aligned in the
microstructure during the rolling operation. 	 Fissure growth was found to be
associated with a reduction in carbide content of the microstructure and was
inhibited by the depletion of carbon.	 The interior. surfaces of sectioned
fissures or bubbles exhibit both primary and seconds-:y cracking by inter-
granular separation.	 The grain surfaces were not smooth and flat but instead
were rough and rounded, suggesting a diffusion-associated separation process.
Finally , specimens that were failed at room temperature after exposure to
hydrogen were found to exhibit mixed mode fracture having varying amounts of
intergra'nular separation, dimple formation, and cleavage, depending on
exposure time.
17, Key Words (Suggested by Authoria)) 16. Distribution Statement
Carbon steel Unclassified - Unlimited
Elevated temperature
Hydrogen attack
Decarburization
Tensile propt-rtip..,;
STAR Category - 26
19. Security Gauif, lot this report) 20. Security Cleulf, (of this pagel 21. No, of Pages
1	 18
22. Rica'
1Unclassified Unclassified $3,25
"For sale by the National Technical Information Service, Springfield, Virginia 22151
Introduction
A number of energy-related systems, particularly advanced energy conver-
sion systems such as coal gasifiers, involve the containment of hydrogen at
high temperatures and pressures.0) The extended exposure of some structural
steels to such an environment can cause severe degradation of their mechanical
properties. Such degradation is termed "hydrogen attack" and is thought to b.
the combined result of internal decarburization and fissuring. Fortunately
for the power industry, the requirement for hydrogen containment at high tem-
peratures and pressures has existed for a number of years in the ammonia and
petroleum hydrorefining industries and literature on the subject is quite
voluminous. Like most industrial problems, however, the great majority of
available data are based on specific service experience and may not be directly
applicable to the different operating conditions associated with the power
industry. For example, industrial hydrogen environments are far from pure and
in most cases of failures associated with hydrogen attack, no attempt has been
made to characterize in detail the specific environments in which operating
data were obtained; as a result, many observations may reflect the influence
of gaseous species other than hydrogen.
A few systematic studies have been performed in controlled hydrogen
environments and the phenomenon of hydrogen attack has been at least phenom-
enologically characterized.( 2-7) Briefly, it has been established that hydro-
gen in atomic or protonic form is absorbed into the steel and diffuses to some
location where it can react with carbon to form methane. The result is
internal decarburization of the steel which causes a corresponding decrease in
strength. The methane molecule, being large, will not rea::ily diffuse away
but instead will accumulate at internal interfaces, such as grain boundaries,
1
with an eventual buildup of large internal pressures. The result is internal
fissuring of the steel giving rise to a decrease in strength and ductility.
The objective of the present study was to better understand the microscoriL
fracture processes associated with hydrogen attack of a plain carbon steel in
a well controlled, high-temperature, high-purity hydrogen environment. The
fractography of gas-filled fissures and failed tensile specimens was analyzed
a
in detail in an effort to identify; (1) any predominant microstructural defect
associated with fissure formation, (2) the prevalent modes of fracture, and
(3) the contribution of gas-filled fissures to the overall failure process.
Procedure
The material used in this investigation was commercially produced
SAE-AISI 1025 steel sheet, 8x10 -2 -cm thick which consisted of a microstructure
of fine pearlite and ferrite having an ASW grain size number between 9 and 10.
Tensile specimens having a gauge length of 7.62 cm were cut with their long
axes perpendicular to the direction of rolling. Prior to environmental
exposure, specimen surfaces were polished through 3/0 paper, cleaned in ace-
tone and ethyl alcohol, and air dried.
Environmental exposures were conducted in an all-metal-sealed, stainless
steel tubular chamber surrounded by a 40-cm long, resistance-heated, tube
furnace. Prior to exposure of the specimens to the test environment, the
following procedure was follo4,ed. The chamber containing thermocoupled
specimens was evacuated to ;.csa roan 6x10 -1 N/m2 (5x10-3 torr), charged at
room temperature with hydrogen to 3.5 MN/m2 (500 psig), evacuated, brought up .
to 300° C, charged with 69 kN/m 2 (10 psig) hydrogen for 16 hr, evacuated,
raised to 575° C, and given the final charge of hydrogen to 3.5 NW/m 2 (500 psig).
The hydrogen was of laboratory grade that initially contained less than 100 ppm
2
of active impurity gases (0 21 H2O, N2), as aetermined by mass spectrometric
methods; it was additionally purified by passing it slowly through liquid-
nitrogen-cooled coils. Specimen temperature was maintained, to within t5° C
during environmental exposure. Following exposure, the chamber containing t!`e
specimi:nb was evacuated for 30 min while remaining at 575° C and then furnace-
cooled. Tensile tests were always conducted at a fixed displacement rate of
1x10-2 cm/s within 48 hr after the environmental exposure.
Results and Discussion
A high-purity hydrogen environment at 575° C was found to significantly
degrade the tensile properties of SAE-AISI 1025 steel after as few as 136 hr
of exposure, the shortest exposure Lime of this study. These observations are
consistent with the operating limits for carbon steel suggested by Nelson(8)
based on experience in industrial environments and with published studies of
observations made in hydrogen environments.( 3 - 5) Figure 1 is a summary of the
tensile curves observed at room temperature after specimen exposure at 575° C
to a vacuum environment and a hydrogen environment for various times. Each
curve represents an average of two tests. As shown in this figure, yield
strength, ultimate strength, and elnngatioz: at fracture are all reduced pro-
gressively as exposure time is increased. After 408 hr, the maximum exposure
time of this study, yield and ultimate strengths are reduced more than 40 per-
cent and elongation is reduced to less than 2 percent.
Figure 2 is a photograph of a portion of a specimen given maximum exposure;
as can be seen, severe bubble formation occurred. The larger bubbles were
found only on specimens given the maximum hydrogen exposure. These larger
bubbles were lenticular, originated some distance below the specimen surfaces,
and grew most rapidly parallel to these surfaces. The smaller bubbles, found
3
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Y
!
	
	 on the surfaces of all specimens given hydrogen exposure, were aligned in
columns parallel to the rolling direction of the material. These observations
r	 suggest that at least at the temperature of this study, initial precipitation
occurs at some microstructural feature which has become aligned in the material
M1
'	
during the rolling operation. Additionally, the rapid initial formation of
i	 the smaller bubbles near the surface, and their lack of continued growth with
I
additional exposure, support the premise that lattice diffusion of the gaseous
I
reaction product is very difficult and that continued growth of near-surface
i
bubbles is inhibited because of depletion of necessary carbon due to surface
decarburization; away from the specimen surfaces, sufficient carbon is retained
for continued bubble growth.
The initial microstructure of this medium carbon steel was ferrite and
i
fine pearlite. After 136 hr of exposure at 575° C in vacuum, the structure
r
had transformed to spheroidized carbide in a ferrite matrix, and, in hydrogen,
i 4	
the amount of this spheroidized carbide had been significantly reduced. After
r,rt f
	
248 hr of hydrogen exposure, carbides could not be observed. Figure 3 is a
•	 photomicrograph of a specimen given this intermediate hydrogen exposure. As
I
i	 can be seen, the ferritic grain size has remained small, carbides cannot be
I
distinguished, and a number of fissured areas are present. One such fissured
area is shown in Fig. 4a at a higher magnification. In this figure, separa-
tion along grain boundaries can be seen to envelop what appear to be two
particles. Using energy dispersive X-ray analysis, it is seen in Figs. 4b
i
and 4c, which are the images for iron and manganese, respectively, that these
particles are not iron but that they contain manganese. Because no sulfur was
I.
evident, these inclusions are most probably manganese oxide. A great number
I
of such fissured areas were studied in this manner and in every case manganese
inclusions were observed to be present. This suggests that initial
4
precipitation of the gaseous reaction product occurs at the interface of the
iron/manganese particles which have been aligned during the rolling operation
(Fig. 2). Continued precipitation results in grain boundary separation in
the nearby material.
The interior surface of the largest bubble shown in Fig. 2 was examined
in an effort to establish the fracture mode associated with massive bubble
growth. Figure 5 consists of four SEM micrographs of the interior surface of
the sectioned bubble at various magnifications. At the higher magnifications
(Figs. Sc and 5d) it is seen that both primary and secondary cracking occurred
by intergranular separation. The grain surfaces, however, do not show the
flat, smooth facets normally observed with i.ntergranular fracture. Instead,
the grains are seen to be somewhat rough and rounded, more typical of the
fracture surface of material failed by a diffusion associated creep-rupture
mechanism at very high temperatures. (9) For carbon steel, a 575° C exposure
temperature is not high compared with true melting temperature of about
1500° C, and thus diffusion associated creep-rupture would not normally be
expected. This suggests that an additional influence may be important in
addition to the simple mechanical consideration of the formation of a high
internal pressure. Carbon, for example, has been shown to increase self-
diffusion in alpha-iron by as much as four orders of magnitude. 00) One pos-
sibility is that bubble growth may be enhanced at locations where carbon con-
centrations are high or, alternatively, similar enhancement may be realized
as the result of the presence of hydrogen in solution. Certainly, more work
is required to understand this observation.
Finally, Fig. 6 is a series of SEM fractographs typical of specimens
failed at room temperature after exposure to a vacuum environment (Fig. 6a)
and a hydrogen environment (Figs. 6b and 6c). Failure of specimens not
5
exposed to hydrogen occurred by the normally observed ductile mode of void
coalescence with the resulting dimpled surface morphology (Fig. 6a). After
hydrogen exposure, a mixed-mode fracture surface, consisting of varying amounts
of intergranular separation; and dimple formation (Fig. 6b), was observed
depending on exposured time. Because of the rounded nature of the grain
faces (Fig. 6b), similar to those previously observed (Figs. Sc and 5d), the
intergranular portion of the failure most probably occurred during hydrogen
exposure. Additionally, scattered areas of what appeared to be cleavage fail-
ure (Fig. 6c) were found on the fracture surfaces of the specimens given
maximum hydrogen exposure. This latter observation suggests that the strength
of the metal"Il ttice may have been affected by the presence of the reaction
product.
General Conclusions
Fractographic analysis has proved to be a useful tool in helping to
understand the microscopic fra'eture processes associated with hydrogen attack.
Although no previously unresolved\guestions were answered by this investiga-
tion, important points were raised wt\ ch require study in the future. For
example, what role do the manganese-ricla particles play in the initial process
of fissure formation? Or is diffusion-associated creep-rupture the primary
mechanism of fracture in hydrogen environments at elevated temperatures? If
so, how does this occur at temperatures which are so far below the melting
point of medium carbon steel? Work is being undertaken to address these
questions and other important points related to the hydrogen attack of steels.
6
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j References
i 1.	 Youngblut,	 K. C., Mat.	 Protect.	 and Preform., Vol. 12,	 1973,	 pp.	 33-36.
2.	 Allen,	 R.	 E.,	 Jansen,
	
11. J., Rosenthal,	 P. C., and Vitovec,	 F.	 H.,	 Proc.
f
l
Am.	 Petrol,	 Inst.,	 Vol.
—
41, 1961,	 pp.	 74-84.
3.	 Allen,	 R.	 E.,	 Jansen,	 R. J., Rosenthal,	 P. C.,	 and Vitovec,
	
F.	 H.,	 Proc.
i
Am.	 Petrol.	 Inst., Vol: 42, 1962,	 pp.	 452-462.
4. veiner, L. C., Acta Met., Vol. 8, 1960, pp. 52-53.
f
5. Weiner, L. C., Corrosion, Vol. 17, 1961, pp. 137t-143t.
a
F	 6. Vitovec, F. H., Proc. Amer, Petrol. Inst., Vol. 44, 1964, pp. 179-188.
ti	 7. Westphal, D. A. and Worzala, F. J., in Hydrogen in Metals, Thompson and
{	 Bernstein, Ed., ASM, Metals Park, Ohio, 1974.
8. Nelson, G. A., "Action of Hydrogen on Steel at High Temperature and High
Pressure," Welding Research Council Bulletin No. 145, October, 1969,
9. Metals Handbook, Vol. 9, Fractography and Atlas of Fractographs, ASM,
Metals Park, 1974, pp. 75-76.
10. Krishtal, M. A., Diffusion Processes in Iron Alloys, Metallurgizdat,
Moskva, 1963 (Translated from Russian, Israel Prog. Sci. Translations,
Jerusalem, 1970).
.^	 _I	 I	 I	 I	 I	 !_
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Fig. 2 - A portion of a tensile specimen exposed to 3.5 MN /m 2 hydrogen at
575° C for 408 hr.
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Fig. 3 - The microstructure of a specimen given intermediate hydrogen exposure.
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(a) SIEM ima
(b) Iron image using eneraY dispersive X-ray an,.lysis.
F— 20 um --^
(c) Afangcaiese image using ener;nf dispe;°live X-raD .znalysis.
Fig • 4 - A tRpica Z f 2 sourod r n tz c w,')? 1; F i 1. 3.
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Fig. 5 - SEM micrographs of the interior surface of a sectioned bubble
at various magnifications, (a) through (d).
(d)
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.'ig. 6


Fractography of the high temperature hydrogen attack of a medium carbon steel

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