Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel
Tensile and fracture properties of X80 pipeline steel were studied in a mimic petrochemical environment. X80 pipeline steel specimens were firstly exposed to air or H₂S corrosive medium. Then their tensile properties and δ-Δa resistance curves were obtained in experiments. The influence of H₂S corro...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
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| Cite this: | Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel / L. Gu, J. Wang, C.B. Luan, X.Y. Li // Проблеми міцності. — 2019. — № 1. — С. 163-174. — Бібліогр.: 22 назв. — англ. |
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nasplib_isofts_kiev_ua-123456789-1883142025-02-09T16:51:19Z Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel Влияние коррозионной среды, содержащей сероводород, на прочность при растяжении и вязкость разрушения трубопроводной стали Х80 Вплив корозійного середовища, що містить сірководень, на міцність при розтягуванні і в'язкість руйнування трубопровідної стали Х80 Q345B Gu, L. Wang, J. Luan, C.B. Научно-технический раздел Tensile and fracture properties of X80 pipeline steel were studied in a mimic petrochemical environment. X80 pipeline steel specimens were firstly exposed to air or H₂S corrosive medium. Then their tensile properties and δ-Δa resistance curves were obtained in experiments. The influence of H₂S corrosion on the X80 pipeline steel’s crack growth resistance curve, fracture toughness, and plastic work loss was analyzed. The comparison between the test results after two pretreatments indicates that there was no significant difference in the X80 pipeline steel’s ultimate tensile strength before and after H₂S corrosion. But fracture elongation, area reduction and fracture toughness varied greatly (a substantial decrease in elongation and reduction of area). The crack growth resistance curve of the specimen in air was obviously higher than the crack growth resistance curve of the corroded one. Under stable crack growth stage, the crack initiation toughness δ0.2BL of the specimen in air was 0.732 mm, 2.02 times that of the corroded one (0.364 mm). In the case of similar crack growth Δa, the plastic work required in the crack growing process (Uр ) of the specimen in air was 2.29 times that of the H₂S-corroded specimen (U'р). H₂S corrosion resulted in a significant reduction of the fracture toughness of X80 pipeline steel. Hence, H₂S corrosion should be avoided in the process of natural gas transportation by pipelines, so as to protect the pipeline steel from toughness degradation. Исследованы прочность при растяжении и вязкость разрушения трубопроводной стали Х80 в среде, характерной для нефтехимических процессов. Образцы стали подвергали длительному воздействию воздуха и коррозионной среды, содержащей сероводород, а затем испытывали на растяжение и трещиностойкость при трехточечном изгибе с построением кривых напряжение деформация и раскрытие трещины в ее вершине длина трещины. Выполнен анализ влияния сероводородной коррозии на диаграмму роста трещины, вязкость разрушения и работу пластической деформации. Анализ результатов, полученных на образцах в исходном состоянии и подвергнутых воздействию сероводородной среды, показал, что данный фактор незначительно снижает предел прочности стали при растяжении, однако приводит к существенному удлинению, а также к уменьшению сечения образцов при разрушении и вязкости разрушения. На стадии устойчивого роста трещины раскрытие ее вершины при страгивании на воздухе составляло 0,732 мм, что более чем вдвое выше, чем в коррозионной среде (0,364 мм). При этом работа пластической деформации, необходимая для продвижения трещины на одну и ту же длину Δa, при воздействии воздуха в 2,29 раза выше, чем коррозионной среды. При транспортировке природного газа по трубопроводам следует избегать коррозионного воздействия сероводорода, существенно снижающего вязкость разрушения трубопроводной стали. Досліджено міцність при розтягуванні і в'язкість руйнування трубопровідної стали Х80 в середовищі, характерної для нафтохімічних процесів. Зразки стали піддавали тривалому впливу повітря і корозійного середовища, що містить сірководень, а потім випробовували на розтягнення і тріщиностійкість при трьохточкову згинанні з побудовою кривих напруга-деформація і розкриття тріщини в її вершині-довжина тріщини. Виконано аналіз впливу сірководневої корозії на діаграму зростання тріщини, в'язкість руйнування і роботу пластичної деформації. Аналіз результатів, отриманих на зразках в початковому стані і підданих впливу сірководневої середовища, показав, що даний фактор тільки незначною мірою зменшує межу міцності стали при розтягуванні, проте призводить до істотного подовження, а також до зменшення перетину зразків при руйнуванні і в'язкості руйнування. На стадії стійкого росту тріщини розкриття її вершини при зсуві на повітрі становило 0,732 мм, що більш ніж удвічі вище, ніж в корозійної середовищі (0,364 мм). При цьому робота пластичної деформації, необхідна для просування тріщини на одну і ту ж довжину, при впливі повітря в 2,29 рази вище, ніж корозійного середовища. При транспортуванні природного газу по трубопроводах слід уникати корозійного впливу сірководню, що істотно знижує в'язкість руйнування трубопровідної сталі. This article is supported by National Key Research and Development Program of China (Project No. 2016YFC0801905-16). 2019 Article Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel / L. Gu, J. Wang, C.B. Luan, X.Y. Li // Проблеми міцності. — 2019. — № 1. — С. 163-174. — Бібліогр.: 22 назв. — англ. 0556-171X https://nasplib.isofts.kiev.ua/handle/123456789/188314 539.4 en Проблемы прочности application/pdf Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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Научно-технический раздел Научно-технический раздел |
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Научно-технический раздел Научно-технический раздел Gu, L. Wang, J. Luan, C.B. Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel Проблемы прочности |
| description |
Tensile and fracture properties of X80 pipeline steel were studied in a mimic petrochemical environment. X80 pipeline steel specimens were firstly exposed to air or H₂S corrosive medium. Then their tensile properties and δ-Δa resistance curves were obtained in experiments. The influence of H₂S corrosion on the X80 pipeline steel’s crack growth resistance curve, fracture toughness, and plastic work loss was analyzed. The comparison between the test results after two pretreatments indicates that there was no significant difference in the X80 pipeline steel’s ultimate tensile strength before and after H₂S corrosion. But fracture elongation, area reduction and fracture toughness varied greatly (a substantial decrease in elongation and reduction of area). The crack growth resistance curve of the specimen in air was obviously higher than the crack growth resistance curve of the corroded one. Under stable crack growth stage, the crack initiation toughness δ0.2BL of the specimen in air was 0.732 mm, 2.02 times that of the corroded one (0.364 mm). In the case of similar crack growth Δa, the plastic work required in the crack growing process (Uр ) of the specimen in air was 2.29 times that of the H₂S-corroded specimen (U'р). H₂S corrosion resulted in a significant reduction of the fracture toughness of X80 pipeline steel. Hence, H₂S corrosion should be avoided in the process of natural gas transportation by pipelines, so as to protect the pipeline steel from toughness degradation. |
| format |
Article |
| author |
Gu, L. Wang, J. Luan, C.B. |
| author_facet |
Gu, L. Wang, J. Luan, C.B. |
| author_sort |
Gu, L. |
| title |
Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel |
| title_short |
Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel |
| title_full |
Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel |
| title_fullStr |
Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel |
| title_full_unstemmed |
Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel |
| title_sort |
influence of h₂s corrosion on tensile properties and fracture toughness of x80 pipeline steel |
| publisher |
Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| publishDate |
2019 |
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Научно-технический раздел |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/188314 |
| citation_txt |
Influence of H₂S Corrosion on Tensile Properties and Fracture Toughness of X80 Pipeline Steel / L. Gu, J. Wang, C.B. Luan, X.Y. Li // Проблеми міцності. — 2019. — № 1. — С. 163-174. — Бібліогр.: 22 назв. — англ. |
| series |
Проблемы прочности |
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| fulltext |
UDC 539.4
Influence of H2S Corrosion on Tensile Properties and Fracture Toughness of
X80 Pipeline Steel
L. Gu, J. Wang,
1
C. B. Luan, and X. Y. Li
Department of Mechanical Engineering and Applied Electronics Technology, Beijing University of
Technology, Beijing, China
1 wjing@bjut.edu.cn
Tensile and fracture properties of X80 pipeline steel were studied in a mimic petrochemical
environment. X80 pipeline steel specimens were firstly exposed to air or H2S corrosive medium. Then
their tensile properties and ���a resistance curves were obtained in experiments. The influence of
H2S corrosion on the X80 pipeline steel’s crack growth resistance curve, fracture toughness, and
plastic work loss was analyzed. The comparison between the test results after two pretreatments
indicates that there was no significant difference in the X80 pipeline steel’s ultimate tensile strength
before and after H2S corrosion. But fracture elongation, area reduction and fracture toughness
varied greatly (a substantial decrease in elongation and reduction of area). The crack growth
resistance curve of the specimen in air was obviously higher than the crack growth resistance curve
of the corroded one. Under stable crack growth stage, the crack initiation toughness �0 2. BL of the
specimen in air was 0.732 mm, 2.02 times that of the corroded one (0.364 mm). In the case of similar
crack growth �a, the plastic work required in the crack growing process (U P ) of the specimen in air
was 2.29 times that of the H2S-corroded specimen ( �U P ). H2S corrosion resulted in a significant
reduction of the fracture toughness of X80 pipeline steel. Hence, H2S corrosion should be avoided in
the process of natural gas transportation by pipelines, so as to protect the pipeline steel from
toughness degradation.
Keywords: X80 pipeline steel, tensile properties, resistance curve, fracture toughness, H2S
corrosion.
Introduction. The X80 pipeline steel is widely adopted in oil-and-gas pipelines of the
Second West-East Pipeline Project in China, with a total length exceeding 7,000 km [1].
However, a safe operation of gas pipelines is jeopardized by H2S impurities in the natural
gas, which is prone to react with vapor to form a corrosive environment, thus producing
stress corrosion in pipelines. According to Sosa et al. [2, 3], five pipeline rupture accidents
caused by corrosion occur worldwide per year. Hence, the analysis of mechanical
properties and fracture toughness of X80 pipelines in operational conditions is crucial for
the reliability assessment of oil-and-gas equipment components and prediction of their
service life.
There are numerous studies on the stress corrosion cracking (SCC) resistance of X80
pipeline steel due to its wide application. Thus, Xi et al. [4] investigated H2S-induced SCC
behavior of X80 pipeline steel and its welded joints using the three-point bending tests.
Wang et al. [5] used the welded joints of X80 pipeline steel to prepare wedge-open loading
(WOL) specimens and then conducted constant-displacement SCC tests in the H2S
environment. The threshold stress intensity factor (SIF) for SCC (K SCCI, ) and crack growth
rates were determined for the base metal, weld seam, and heat-affected zone in the tests.
Chen et al. [6] categorized SCC types and a principle of buried X80 oil pipelines, as well as
examined the influence of material, stress and environmental factors (temperature, applied
potential, and cathodic protection) on the SCC behavior of X80 pipeline steel, and offered
the respective protection measures.
This study is devoted to the X80 pipeline steel employed in the above project, given
the particular operational conditions. After being exposed to air and H2S corrosion,
© L. GU, J. WANG, C. B. LUAN, X. Y. LI, 2019
ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1 163
specimens from X80 pipeline steel were tersted to determine their tensile and fracture
properties. The respective resistance curve “crack opening displacement–crack extension”
( )�� �a and the characteristic values �0 2. BL were obtained. The influence of H2S
corrosion on the crack growth resistance curve, fracture toughness and plastic work loss of
the X80 pipeline steel was determined, providing the data support for its engineering
application.
1. Experimental.
1.1. Material. The X80 pipeline steel used in this study was obtained from pipelines in
the West-East Gas Pipeline Project, China. The chemical compositions (wt.%) of the tested
steel are illustrated in Table 1. The measured elastic modulus E is 206 GPa, tensile
strength Rm is 767 MPa, yield strength R p0 2. is 661 MPa. elongation A is 19.1%, and
reduction of area Z is 77.7%.
1.2. Specimen Geometry. Specimens for tensile tests (Fig. 1a) and three-point bending
tests (Fig. 1b) were designed according to the national standards GB/T 228.1-2010 [7] and
GB/T 21143-2007 [8], respectively. The geometry of a standard specimen for three-point
bending tests was employed. Given the actual wall thickness of pipeline (t � 19 mm), the
specimen thickness was determined as 15 mm, while its width was W � 30 mm and length
was S � 120 mm.
In compliance with the above national standard [8], initial fatigue cracks of 2 mm-
length were prefabricated using a high-frequency fatigue test machine, with the maximum
load of 14.87 kN and the stress ratio R = 0.1.
L. Gu, J. Wang, C. B. Luan, and X. Y. Li
164 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1
T a b l e 1
Chemical Composition of the Tested Steel (wt.%)
C Si Mn P S Cr Ni Ti Nb V Mo
0.065 0.27 1.82 0.011 0.0006 0.03 0.03 0.016 0.061 0.059 0.21
a
b
Fig. 1. Specimens for tensile (a) and three-point bending (b) tests.
1.3. Experimental Environment. The X80 pipeline steel specimens were subjected to
tensile tests and fracture toughness tests before and after H2S corrosion at room temperature.
Therein, the H2S corrosion environment was the standard Class A H2S corrosive solution,
according to NACE International requirements [9–11], i.e., H2S gas was injected into the
aqueous solution containing 5% NaCl and 0.5% CH3COOH until saturated (concentration
2300 10 6� � (volume fraction)), the rate of H2S gas bubbling was 300 cm3/min, per liter of
test solution for the first 60 minutes. Thereafter, a positive pressure of H2S in the test
solution was measured by the iodometric titration. The corrosion process duration was 48
hours.
1.4. Experimental Steps. Tensile tests were performed using a Zwick Z150 test
machine, according to the national standard GB/T228.1-2010 [7]. The machine automatically
recorded and plotted tensile force–displacement curves during the test. The elastic modulus,
yield strength, tensile strength, elongation, reduction of area, and the total work of the
stretching process were obtained after the test.
The multiple-specimen method was adopted for the test. Specifically, several specimens
were loaded to varying displacement levels, which were preset. Then, they were unloaded,
frozen in a liquid nitrogen, and broken. The original crack growth length a0 and final
crack growth �a were measured, and the crack opening displacement � was calculated.
The blunting line equation of �� �a curves was �� 187 0 2. ( ).R R am p � [8]; a line parallel
to the blunting line was drawn at �a� 0.1 mm, as the left boundary of the effective crack
growth, and another parallel line was drawn at �a max as the right boundary. The effective
crack growth area was quartered, each sub-area containing at least one data point. If there
were data missing in a subarea, a data point would be added according to the test result of
the adjacent sub-area. The test was terminated when six or more effective data points were
obtained and they were evenly spaced. The loading process is shown in Fig. 2.
For specimens subjected to the three-point bending test, � is calculated as follows
[8]:
�
�
�
�
�
�
�
�
�
�
�
�
�
�
�
�S
W
F
BB W
g
a
W R
N p( )
( )
.
.
0 5 1
0
2 2
0 2
1
2 E
a W a V
a a W z
P
�
�
�
�
�
��
� �
� � �
[ . . ( )]
. ( ) .
,
06 04
06 04
0
0
�
�
(1)
where S and W are specimen length and width, respectively, F is the load at break point,
B is the specimen thickness, B N is the net specimen thickness between two side troughs
(B BN � in this paper), g1 is the SIF coefficient for three-point bending test specimens,
Influence of H2S Corrosion on Tensile Properties ...
ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1 165
Fig. 2. Loaded specimen by using universal electronic testing machine.
a0 is the original crack length, � is Poisson’s ratio, R p0 2. is the proof strength at 0.2%
non-proportional extension of the material along the direction vertical to the crack plane at
the test temperature, E is elastic modulus, �a is the final crack growth increment, Vp is
the plastic component of crack opening displacement (COD), and z is the thickness of an
extensometer knife edge used for the COD measurement.
A line parallel to the blunting line is drawn at �a� 0.2 mm, and its intersection with
the resistance curve is defined as �Q BL0 2. . The validity of �Q BL0 2. is controlled by the
following conditions: (i) there is at least one data point between 0.1 and 0.3 mm offset lines
parallel to the blunting line, and at least two data points between 0.1 and 0.5 mm offset
lines; (ii) � �Q BL W a0 2 0 20. max ( ) ;� � � (iii) the slope of resistance curve at its intersection
with the 0.2 mm offset line satisfies the inequality 187 2
0 2 0 2
. .
. .
R
R
d
da
m
p BL
�
�
�
�
��
�
�
�
�
��
�
��
�
If �Q BL0 2.
satisfies the above conditions, the obtained �Q BL0 2. is insensitive to the scale effect.
2. Experimental Results.
2.1. Tensile Properties of X80 Pipeline Steel after H2S Pre-Corrosion. Tensile
properties of X80 pipeline steel were tested after pre-corrosion. The engineering stress–
strain curve was recorded, as seen in Fig. 3, while Table 2 lists the material tensile
properties.
166 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1
L. Gu, J. Wang, C. B. Luan, and X. Y. Li
T a b l e 2
Tensile Properties of X80 Pipeline Steel before and after H2S Pre-Corrosion
Specimen No. E,
GPa
Rp0 2. ,
MPa
Rm ,
MPa
A,
%
Z ,
%
WB ,
J
Before corrosion 1# 206.0 634.0 773 20.0 78.7 522.49
Before corrosion 2# 199.0 647.0 775 18.8 76.7 478.50
Average 202.5 640.5 774 19.4 77.7 500.50
Saturated H S2 , 48h, 1# 212.0 658.0 782 12.9 44.1 379.45
Saturated H S2 , 48h, 2# 209.0 659.0 782 13.7 41.0 402.61
Average 210.5 658.5 782 13.3 42.55 391.03
Note. Here and in Table 6: WB is total work of the stretching process.
Fig. 3. Engineering stress–strain curve of X80 pipeline steel before and after H2S pre-corrosion.
2.2. Crack Initiation Toughness �0 2. BL at Stable Crack Growth and Its Validity
Judgment. The �� �a resistance curve and crack initiation toughness �0 2. BL were
obtained for the X80 pipeline steel specimens subjected to three-point bending test before
and after H2S corrosion. Tables 3 and 4 show the test results of each specimen before and
after H2S corrosion, respectively. Figure 4 shows the �� �a resistance curve. Figure 5
displays the fracture of specimens exposed in air and corroded by H2S.
ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1 167
Influence of H2S Corrosion on Tensile Properties ...
T a b l e 3
Test Results for Specimens Exposed in Air
Sample
number
B,
mm
W ,
mm
S ,
mm
F ,
kN
g1 � Rp0 2. ,
MPa
E,
GPa
a0 ,
mm
�a,
mm
Vp ,
mm
z,
mm
�,
mm
A1 15.02 30.10 120 31.56 2.58 0.3 661 206 14.73 0.63 2.81 2.0 0.81
A2 15.03 30.00 120 28.50 2.62 0.3 661 206 14.84 1.34 3.78 2.0 1.05
A3 15.00 30.20 120 24.09 2.62 0.3 661 206 14.81 3.01 5.03 2.0 1.36
A4 14.99 30.20 120 26.81 2.66 0.3 661 206 15.01 2.15 4.63 2.0 1.25
A5 15.10 30.02 120 27.86 2.66 0.3 661 206 15.04 0.83 2.74 2.0 0.76
A6 15.01 30.02 120 27.07 2.58 0.3 661 206 14.71 2.28 4.18 2.0 1.15
T a b l e 4
Test Results for the Specimens after H2S Pre-Corrosion
Sample
number
B,
mm
W ,
mm
S ,
mm
F ,
kN
g1 � Rp0 2. ,
MPa
E,
GPa
a0 ,
mm
�a,
mm
Vp ,
mm
z,
mm
�,
mm
3# 15.10 29.82 120 27.88 2.58 0.3 661 206 14.57 0.25 0.78 2.0 0.26
4# 14.90 29.80 120 27.93 2.54 0.3 661 206 14.54 0.51 1.42 2.0 0.43
5# 15.02 29.72 120 29.40 2.54 0.3 661 206 14.43 0.35 1.13 2.0 0.36
8# 15.11 30.10 120 16.46 2.66 0.3 661 206 15.08 1.18 1.90 2.0 0.52
11# 15.00 30.00 120 19.87 3.63 0.3 661 206 15.90 1.87 2.31 2.0 0.61
12# 15.09 29.92 120 16.10 4.43 0.3 661 206 16.21 2.98 3.03 2.0 0.76
a b
Fig. 4. ���a resistance curves (1) of original specimens (a) and saturated H2S pre-corroded
specimens (b).
In Fig. 4b, a parallel line passing through the passivation line at �a� 0.1 mm is
defined as the left boundary of the effective crack growth length, and the parallel line
passing through �a max as the passivation line is defined as the right boundary of the
effective crack growth length. The entire dataset falls between the left and right boundaries.
At least six data points are required to define the R-curve, and when fitting the R-curve,
there is at least one data point in the four equally spaced crack extension regions (1d, 2d, 3d,
and 4d).
The resistance curve was fitted by the three-parameter exponential equation recommended
by the national standard [8]. The resistance curve equation and crack initiation toughness
after exposure to air and H2S corrosion were finally obtained as follows:
Before H2S corrosion: �� �020041 07093 0 44416. . ..�a
After H2S corrosion: �� �024544 023993 0 69532. . ..�a
A line parallel to the blunting line was drawn at �a� 0.2 mm. According to its
intersection with the resistance curve, the �Q BL0 2. after exposure to air and H2S corrosion
was obtained separately. Figure 6 and Table 5 summarize the data distribution and their
validity judgment.
Table 5 shows that �Q BL0 2. values of the specimens exposed in air and corroded by
H2S are both valid. Thus, the crack initiation toughness of X80 pipeline steel for stable
crack growth in these two test condition attains the following values:
Before H2S corrosion: �0 2. BL � 0.732 mm; after H2S corrosion: �0 2. BL � 0.364 mm.
3. Discussion.
3.1. Comparison of Tensile Properties. The tensile test results of X80 pipeline steel
before and after H2S corrosion were averaged and compared (Table 6). It can be seen that
the elastic modulus, yield strength, and tensile strength raised slightly after H2S corrosion,
but the overall variations were small (all within 4%). However, H2S produced a significant
168 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1
L. Gu, J. Wang, C. B. Luan, and X. Y. Li
a b
Fig. 5. Fracture surfaces of original specimens (a) and saturated H2S pre-corroded specimens (b).
T a b l e 5
Values of �Q BL0 2. and Their Validity
Test condition Data
distribution
requirements
�Q BL0 2. ,
mm
�max ,
mm 187
0 2
.
.
R
R
m
p
�
�
�
�
� 2
0 2
d
d a
BL
�
( )
.
�
�
�
�
�
�
�
�
�
�
Validity
judgment
Original Meet
conditions
0.732 0.749 2.26 0.732 Valid
Pre-corroded Meet
conditions
0.364 0.686 2.22 0.364 Valid
influence on the plasticity characteristics. Specifically, the elongation, reduction of area, and
total mechanical energy of the stretching process decreased by 31, 45, and 21%, respectively,
after H2S corrosion. This finding accords with other researchers’ achievements [12]. The
reason for these variations of plasticity characteristics is as follows. Hydrogen ions are
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Influence of H2S Corrosion on Tensile Properties ...
T a b l e 6
Strength and Plasticity Indices of X80 Pipeline Steel before and after H2S Corrosion
State of specimen E,
GPa
Rp0 2. ,
MPa
Rm ,
MPa
A,
%
Z ,
%
WB ,
J
Before corrosion 202.5 640.5 774 19.4 77.70 500.50
Corroded
by saturated H S2 for 48 h
210.5 658.5 782 13.3 42.55 391.03
Difference �8.0 �18.0 �8.0 �6.1 �35.15 �109.47
Percentage difference 3.95% 2.81% 1.03% 31.44% 45.23% 21.87%
a
b
Fig. 6. The value of �Q BL0 2. and validity judgment of original specimens (a) and saturated H2S
pre-corroded specimens (b).
generated through electrolysis in the H2S corrosive solution and then infiltrate into the
metal material. Once meeting such hydrogen traps as inclusions, grain boundaries, and
cavities, hydrogen atoms will bind to these strong traps, leading to an increase of local
hydrogen concentration and a ductility loss to the metal material [13]. As a result, both
elongation and the reduction of area decline, while the total work done in the stretching
process also declines.
3.2. Comparison of �0 2. BL . �0 2. BL refers to the crack opening displacement at the
intersection of the 0.2 mm offset line of blunting line and the resistance curve. The value
characterizes the material’s resistance against crack initiation and growth [14]. Test results
indicate that the �0 2. BL value of X80 pipeline steel in air was 0.74 mm, and it decreased to
0.365 mm after the corrosion of saturated H2S, which implies more than a twofold
reduction. This shows that H2S corrosion significantly degraded the fracture toughness of
X80 pipeline steel.
Crack opening displacement � has a close relationship with the stress field of crack
tip, the intensity of which can be described by SIF K . The critical SIF K cI is the minimum
SIF, at with an unstable crack growth occurs. It is a crucial parameter for measuring the
fracture toughness of materials. In engineering, the crack initiation toughness �0 2. BL under
stable crack growth can be used to estimate the value of K cI .
According to [15, 16], � can be reduced to the following form:
��
J
kR p0 2.
, (2)
where J is the experimental equivalent of J-integral and k is the crack opening
displacement (COD) reduction factor, with the value range of 1.1–2.0.
The relationship between J and K is as follows:
J
K
E
c
c
I
I�
�
2
, (3)
where K cI is the fracture toughness and �� �E E ( )1 2� for plane strain.
It can be deduced from Eqs. (2) and (3) that
K
kR E
c
BL p
I �
�
�
�
0 2 0 2
21
. .
. (4)
Set k � 1.1, and substitute the mechanical parameters of X80 pipeline steel and
values of �0 2. BL measured after exposure to air and H2S corrosion into Eq. (6). This
yields K cI � 349 MPa mm� for the specimen in air and K corr � 245 MPa mm� for the H2S-
corroded specimen.
It was found that the fracture toughness K cI of the steel X80 is reduced by 30% due
to H2S-corrosion, which further suggests that H2S corrosive medium will make the steel
more prone to brittle fracture. The reason is summarized as follows. High-stress area is
produced at the crack tip during the specimen loading. Due to the stress-induced hydrogen
diffusion, the hydrogen atoms in the metal will converge to the crack tip and form discoid
atomic-hydrogen atmospheres (Cottrell atmospheres). The shearing component �H of the
internal pressure works together with the shearing component induced by the external stress
field, to promote the dislocation multiplication and movement in part of the crack tip. It will
further promote local plastic deformations and reduce the material’s fracture toughness [17,
18].
170 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1
L. Gu, J. Wang, C. B. Luan, and X. Y. Li
Wang et al. [19] also concluded that H2S corrosive medium would degrade the
fracture toughness of metal material according to the result that the critical SIF K cI of
high-pressure cylinder material 4130 was 1.94 times higher than that after exposure to H2S
environment. As for the difference in the reduction scal with the present work, one reason
is that different experimental materials show varying sensitivity to H2S corrosive medium.
Another reason is that specimens in this study were only subjected to the unstressed H2S
corrosion, i.e., static hydrogen charging. In such a case, hydrogen atoms enter the metal
only through diffusion. But at ambient temperature the diffusion coefficient of hydrogen
atoms is very low, so there are less hydrogen atoms reaching the bulk of specimens. In
study [19], specimens were immersed into the corrosive environment during their loading,
i.e., were subjected to a dynamic hydrogen-charging process. Besides the diffusion-induced
mechanism, hydrogen atoms can also be carried to the inner layers of specimens by
dislocations. The crack growth of specimens was caused by the hydrogen-stress coupling.
During the dynamic hydrogen charging, the plastic deformation of specimen will induce
massive dislocation slips. At this moment, the hydrogen atoms produced by the electrolysis
will form atomic-hydrogen atmospheres near the dislocations and will be carried into the
specimen bulk by these dislocations. The hydrogen atoms reaching the specimen surface
due to electrolytic hydrogen charging will hence pour into the inner layers. Thus, a large
number of hydrogen atoms will be enriched in the specimen quickly, finally resulting in the
hydrogen-induced brittle fracture to the specimen.
3.3. Comparison of Crack Growth Resistance Curves. Fracture resistance curves of
the specimen in air and corroded by H2S were compared, as seen in Fig. 7.
Crack growth resistance curve shows the material’s resistance against crack initiation,
and also reflects the variation of fracture toughness at crack tip with the crack length
increment �a. The crack growth resistance curves of X80 pipeline steel after exposure to
air and H2S corrosion were plotted. As shown in Fig. 7, the crack growth resistance of
specimens after two treatments increased with the growth of crack, presenting a convex
ascent curve. It means that X80 pipeline steel has a good toughness and can maintain a
certain bearing capacity after crack initiation. The crack will grow to some extent from its
initiation to the specimen’s fracture. Araújo et al. [20] obtained the R-curves of API X52
pipeline steel, according to the ASTM E1820-01 standard [21]. In their test, along with the
increase of crack length increment �a, the COD also increased, but the increasing rate
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Influence of H2S Corrosion on Tensile Properties ...
Fig. 7. Fracture resistance curves of X80 pipeline steel specimen before and after H2S corrosion.
declined gradually. The increasing trend of their resistance curve conforms to ours, further
affirming findings of this study. It is seen in Fig. 7 that, regardless of the crack length, the
resistance curve of the specimen in air is always located much higher than that of the
H2S-corroded specimen. It means that the X80 pipeline steel in air has a much better
toughness than that subjected to H2S corrosion. As the crack grows, the difference between
the resistance curves of the specimens after varying treatments became greater. At the rear
of the curve(�a� 3 mm), the difference of crack opening displacement was about 1.78
times. It suggests that the superior toughness of the material in air was enhanced by the
growth of crack.
3.4. Comparison of Plastic Work of the Crack Growing Process. For the plastic work
required in the crack growing process (UP ), its value is equal to the area of the applied
force–crack opening displacement curve. F V� curves of two specimens (A3: exposed to
air; 12#: corroded by H2S) with similar crack growth (�a � 3 mm) were compared, as
shown in Fig. 8.
As seen from the curves in Fig. 8, both the specimen in air and that corroded by H2S
presented a certain linear feature at the initial stage, during which the curve slope of the
former was slightly higher. The difference between these two curves notably increased with
the crack growth. Under condition of similar crack growth (�a � 3 mm), the UP value of
H2S-corroded specimen approached 52.5 kN mm� , and the �UP value of uncorroded
specimen approached 120 kN mm� , i.e., was 2.29 times higher that of the former one. This
indicates that for the same crack growth, there was a large loss in the plastic work occurring
in uncorroded specimens. It indirectly proves the superiority in fracture toughness of
specimens in air over those corroded by H2S. Chatzidouros et al. [22] studied the fracture
toughness of X70 pipeline steel under conditions of electrolytic hydrogen charging, and
plotted displacement– force curves for the specimens pretreated by air and corrosive media
of different hydrogen concentration. They drew the similar conclusion as this study, that is,
with an increase in hydrogen concentration, the plastic work required for the crack growth
process decreased gradually. Thus, hydrogen corrosion would deteriorate the pipeline
steel’s fracture toughness.
A comparison was made between stretching and crack growth processes. It was found
that, compared with uncorroded specimen, the total work carried out on the specimen in the
stretching and crack growth processes declined by 22 and 56%, respectively, after
corrosion. Obviously, the defective specimen had a greater energy loss than the specimen
with a smooth surface. It is because hydrogen atoms are mostly concentrated at the
172 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1
L. Gu, J. Wang, C. B. Luan, and X. Y. Li
Fig. 8. Displacement–force curves of X80 pipeline steel specimens before and after H2S corrosion.
specimen surface after H2S corrosion. After the test began, these surface hydrogen atoms
entered into the specimen by the stress-induced diffusion and dislocation mechanisms.
During the stretching, the internal stresses in the specimen were relatively uniform. The
same was true for the hydrogen atoms redistributed inside the specimen by the stress-
induced diffusion. However, for the pre-cracked specimens, there was a high-stress area at
the crack tip after loading. Abundant hydrogen atoms converged in this area, thus
increasing the ductility loss, as compared to the stretching process.
C o n c l u s i o n s
1. The ultimate tensile strength Rm and the proof strength R p0 2. of X80 pipeline
steel showed no large differences before and after H2S corrosion for unnotched samples in
standard tensile testing. However, such properties as fracture elongation and reduction of
area were significantly lower for the corroded specimens.
2. The �� �a resistance curve and crack initiation toughness �0 2. BL were
experimentally obtained for the X80 pipeline steel pretreated in air and H2S corrosive
medium under conditions of stable crack growth. The following results were obtained: for
the specimen in air, the resistance curve equation was �� �020041 07093 0 44416. . .�a , and
the crack initiation toughness was �0 2. BL � 0.732 mm; for the H2S-corroded specimen, the
resistance curve equation was �� �024544 023993 0 69532. . ,.�a and the crack initiation
toughness was �0 2. BL � 0.364 mm.
3. The resistance curve of the specimen exposed in air was located much higher than
that after H2S corrosion. The crack initiation toughness, fracture toughness, and plastic
work in the crack growing process of the specimen in air were 2.02, 1.42, and 2.29 times
higher than those of H2S-corroded specimens, respectively. Thus, H2S corrosion significantly
degraded the fracture toughness of X80 pipeline steel.
Acknowledgments. This article is supported by National Key Research and
Development Program of China (Project No. 2016YFC0801905-16).
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Received 15. 03. 2018
174 ISSN 0556-171X. Ïðîáëåìè ì³öíîñò³, 2019, ¹ 1
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/HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. Stvoreni PDF dokumenti mogu se otvoriti Acrobat i Adobe Reader 5.0 i kasnijim verzijama.)
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/NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken die zijn geoptimaliseerd voor prepress-afdrukken van hoge kwaliteit. De gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe Reader 5.0 en hoger.)
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/ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.)
>>
/Namespace [
(Adobe)
(Common)
(1.0)
]
/OtherNamespaces [
<<
/AsReaderSpreads false
/CropImagesToFrames true
/ErrorControl /WarnAndContinue
/FlattenerIgnoreSpreadOverrides false
/IncludeGuidesGrids false
/IncludeNonPrinting false
/IncludeSlug false
/Namespace [
(Adobe)
(InDesign)
(4.0)
]
/OmitPlacedBitmaps false
/OmitPlacedEPS false
/OmitPlacedPDF false
/SimulateOverprint /Legacy
>>
<<
/AddBleedMarks false
/AddColorBars false
/AddCropMarks false
/AddPageInfo false
/AddRegMarks false
/ConvertColors /ConvertToCMYK
/DestinationProfileName ()
/DestinationProfileSelector /DocumentCMYK
/Downsample16BitImages true
/FlattenerPreset <<
/PresetSelector /MediumResolution
>>
/FormElements false
/GenerateStructure false
/IncludeBookmarks false
/IncludeHyperlinks false
/IncludeInteractive false
/IncludeLayers false
/IncludeProfiles false
/MultimediaHandling /UseObjectSettings
/Namespace [
(Adobe)
(CreativeSuite)
(2.0)
]
/PDFXOutputIntentProfileSelector /DocumentCMYK
/PreserveEditing true
/UntaggedCMYKHandling /LeaveUntagged
/UntaggedRGBHandling /UseDocumentProfile
/UseDocumentBleed false
>>
]
>> setdistillerparams
<<
/HWResolution [2400 2400]
/PageSize [612.000 792.000]
>> setpagedevice
|