Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States
Premature fracture of steam pipelines from 14MoV6 3 steel designed for 1000,000 h service life at 540°C has been studied. Test specimens are manufactured from steel in the initial state and after 117 h of operation. Application of the local approach to fracture and the metallographic analysis,...
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irk-123456789-470642013-07-09T18:52:29Z Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States Zrilic, M. Burzic, Z. Cvijovic, Z. Научно-технический раздел Premature fracture of steam pipelines from 14MoV6 3 steel designed for 1000,000 h service life at 540°C has been studied. Test specimens are manufactured from steel in the initial state and after 117 h of operation. Application of the local approach to fracture and the metallographic analysis, in addition to classical methods (tensile, crack resistance, and fatigue strength tests), provided a more precise evaluation of steel properties degradation due to elevated temperatures and stresses. Urgency of further development of the local approach to predicting material fracture after long-term operation at high temperatures has been substantiated. Исследуется проблема преждевременного разрушения паропроводов (сталь 14MoV6 3) с расчетным ресурсом 100 тыс. ч при температуре 540°С. При изготовлении экспериментальных образцов использовалась сталь в исходном состоянии и после 117 тыс. ч эксплуатации. Применение локального подхода механики разрушения и металлографического анализа наряду с классическими методами (испытания на растяжение, трещиностойкость, усталостную прочность) позволило более точно оценить деградацию свойств стали под действием высоких температур и напряжений. Обоснована необходимость дальнейшего развития локального подхода к прогнозированию условий разрушения материалов при длительной эксплуатации в условиях высоких температур. Досліджується проблема передчасного руйнування паропроводів (сталь 14MoV6 3) із розрахунковим ресурсом 100 тис. год за температури 540оС. При виготовленні експериментальних зразків використовувалася сталь у первинному стані та після 117 тис. год експлуатації. Використання локального підходу механіки руйнування і металографічного аналізу поряд із класичними методами (випробування на розтяг, тріщиностійкість, міцність від утомленості) дозволило більш точно оцінити деградацію властивостей сталі під дією високих температур і напружень. Обгрунтовано необхідність подальшого розвитку локального підходу до прогнозування умов руйнування матеріалів при тривалій експлуатації в умовах високих температур. 2004 Article Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States / M. Zrilic, Z. Burzic, Z. Cvijovic // Проблемы прочности. — 2004. — № 1. — С. 68-83. — Бібліогр.: 12 назв. — англ. 0556-171X http://dspace.nbuv.gov.ua/handle/123456789/47064 539.4 en Проблемы прочности Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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Научно-технический раздел Научно-технический раздел |
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Научно-технический раздел Научно-технический раздел Zrilic, M. Burzic, Z. Cvijovic, Z. Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States Проблемы прочности |
| description |
Premature fracture of steam pipelines from
14MoV6 3 steel designed for 1000,000 h service
life at 540°C has been studied. Test specimens are
manufactured from steel in the initial state and after
117 h of operation. Application of the local approach
to fracture and the metallographic analysis,
in addition to classical methods (tensile, crack resistance,
and fatigue strength tests), provided a
more precise evaluation of steel properties degradation
due to elevated temperatures and stresses.
Urgency of further development of the local approach
to predicting material fracture after
long-term operation at high temperatures has been
substantiated. |
| format |
Article |
| author |
Zrilic, M. Burzic, Z. Cvijovic, Z. |
| author_facet |
Zrilic, M. Burzic, Z. Cvijovic, Z. |
| author_sort |
Zrilic, M. |
| title |
Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States |
| title_short |
Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States |
| title_full |
Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States |
| title_fullStr |
Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States |
| title_full_unstemmed |
Fracture Toughness and Crack Resistance of Steam Pipeline Steel in Initial and Used States |
| title_sort |
fracture toughness and crack resistance of steam pipeline steel in initial and used states |
| publisher |
Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| publishDate |
2004 |
| topic_facet |
Научно-технический раздел |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/47064 |
| citation_txt |
Fracture Toughness and Crack Resistance of Steam Pipeline Steel
in Initial and Used States / M. Zrilic, Z. Burzic, Z. Cvijovic // Проблемы прочности. — 2004. — № 1. — С. 68-83. — Бібліогр.: 12 назв. — англ. |
| series |
Проблемы прочности |
| work_keys_str_mv |
AT zrilicm fracturetoughnessandcrackresistanceofsteampipelinesteelininitialandusedstates AT burzicz fracturetoughnessandcrackresistanceofsteampipelinesteelininitialandusedstates AT cvijovicz fracturetoughnessandcrackresistanceofsteampipelinesteelininitialandusedstates |
| first_indexed |
2025-07-04T06:42:11Z |
| last_indexed |
2025-07-04T06:42:11Z |
| _version_ |
1836697603838836736 |
| fulltext |
UDC 539.4
Fracture Toughness and Crack Resistance of Steam Pipeline Steel
in Initial and Used States
M. Zrilic,a Z. Burzic,b and Z. Cvijovica
a Belgrade University, Faculty of Technology and Metallurgy, Belgrade, Serbia and
Montenegro
b Military Technical Institute, Belgrade, Serbia and Montenegro
УДК 539.4
Вязкость разрушения и трещиностойкость стали паропровода в
исходном состоянии и после эксплуатации
М. Зрилича, 3. Бурзич6, 3. Ц вийовича
а Белградский университет, факультет технологии и металлургии, Белград, Сербия и
Черногория
б Военно-технический институт, Белград, Сербия и Черногория
Исследуется проблема преждевременного разрушения паропроводов (сталь 14MoV6 3) с
расчетным ресурсом 100 тыс. ч при температуре 540°С. При изготовлении эксперимен
тальных образцов использовалась сталь в исходном состоянии и после 117 тыс. ч эксплу
атации. Применение локального подхода механики разрушения и металлографического ана
лиза наряду с классическими методами (испытания на растяжение, трещиностойкость,
усталостную прочность) позволило более точно оценить деградацию свойств стали под
действием высоких температур и напряжений. Обоснована необходимость дальнейшего
развития локального подхода к прогнозированию условий разрушения материалов при дли
тельной эксплуатации в условиях высоких температур.
Ключевые слова : паропровод, разрушение, локальный подход механики
разрушения, металлографический анализ, испытания на растяжение, трещино
стойкость, усталостная прочность.
Introduction. In late seventies, application of 14MoV6 3 steel (DIN) for
highly loaded steam pipelines (temperature up to 540oC and pressure as high as 45
bar for service life of 100,000 hours) offered significant benefits compared to the
steels of previous generation [1, 2] and allowed reduction of pipe wall thickness.
However, frequent premature failures of steam lines produced of this steel,
sometimes after only 30,000 service hours, imposed the necessity to retrofit
damaged steam pipelines, e.g., steam lines of thermoelectrical power plants in
Greece [3]. This unexpected repair cost hampered further application of 14MoV6
3 steel and the designers preferred to replace it by other, highly alloyed steels
(e.g., alloyed steel X12 CrMoV 1, according to DIN, and low alloyed steel
10CrMo9). There is no clear explanation for failure occurrence, and steel
© M. ZRILIC, Z. BURZIC, Z. CVIJOVIC, 2004
68 ISSN 0556-171X. Проблемы прочности, 2004, № 1
Fracture Toughness and Crack Resistance
producers claim that steel 14MoV6 3 is a proper one for the intended application
[2]. Better understanding of in-service behavior of 14MoV6 3 steel can help in
reducing forced shutdowns and improve the reliability and service safety of
thermoelectric power plants.
The rate of deterioration of properties and their level after long-term service
at high temperature is of importance for the evaluation of residual life and the
decision about next service of damaged pipes [4]. For that, experimental analysis
is performed in order to get more insight in 14MoV6 3 steel properties’
deterioration.
The rate of deterioration of properties can be assessed by comparing the data
of new and used steel. In addition to microstructure, mechanical and creep
properties, resistance to fracture has to be analyzed, applying fracture mechanics
and fatigue tests, as well as a new developed method based on local approach to
fracture [5, 6]. The latter relates the local stresses and strains in the considered
component volume to the mechanical damage parameters, directly depending on
material microstructure and fracture micromechanisms. The fracture parameters,
so far developed and recommended by standards, such as stress intensity factor
K i or J -integral, cannot confidently define and predict the behavior of materials
under external load in all conditions. Hence, a local approach to fracture is
introduced as a promising one, developed in theoretical, experimental and
numerical consideration of different stress concentration levels [7]. The purpose
of applying the local approach is to define more accurate and less conservative
material characteristics. This can be achieved by accurately determined stresses,
strains and variables, which describe physical damage mechanisms in cracked
material.
The specimens, produced from samples taken of new (initial) and old (used)
14MoV6 3 steel have been tested. Mechanical properties are evaluated by tensile
and impact tests. Resistance to fracture has been determined by fracture
mechanics testing, fatigue crack growth rate determination and by local approach,
analyzed for ductile fracture properties, which is dominant in the considered case.
Determination of M aterial Properties. Material and Specimen Preparation.
Specified chemical composition of 14MoV6 3 steel according to standard DIN
17175 is given in Table 1. The mechanical properties, as prescribed by standard
DIN 17175, are given in Table 2.
T a b l e 1
Chemical Composition of 14MoV6 З Steel
DIN C Si Mn P (max) S (max) Cr Mo V
17175 0.10-0.18 0.15-0.35 0.30-0.60 0.035 0.035 0.30-0.60 0.50-0.65 0.25-0.35
T a b l e 2
Mechanical Properties of 14MoV6 З Steel at Room Temperature
Tensile
strength
Rm, MPa
Yield
stress
Re, MPa
Longitudinal
elongation
A, ,%
Transverse
elongation
At,%
Longitudinal
impact energy
E, , J
Transverse
impact energy
Et, J
490-690 365 20 18 62 41
ISSN Ü556-171X. Проблемыг прочности, 2ÜÜ4, N І 69
M. Zrilic, Z. Burzic, Z. Cvijovic
The samples of 14MoV6 3 steel were taken from new and used pipes
(D X t = 609 X 40 mm), with drawn from service after 117,000 service hours at
maximum 540°C under pressure of 42 bars in steam pipeline because of detected
serious damage in elbows and welded joints.
Following specimens (Fig. 1) have been taken from samples for:
- tensile testing - ST;
- impact toughness testing - FT;
- local-fracture-approach testing: with notch root radius 2 mm - NT2, 4 mm
- NT4, and 10 mm - NT10;
- /-integral fracture toughness testing - SE(B);
- da/dN - fatigue crack propagation assessment - FT.
Fig. 1. Samples and specimens made from pipes.
Chemical Composition o f Tested Material. Chemical composition of new and
used material is given in Table 3.
T a b l e 3
Chemical Composition of 14MoV6 3 Steel
Steel C Si Mn P (max) S (max) Cr Mo V
new 0.13 0.20 0.36 0.014 0.021 0.55 0.51 0.28
used 0.12 0.21 0.36 0.013 0.019 0.56 0.49 0.31
Tensile Properties o f Initial and Used Steels. Tensile properties of new and
used 14MoV6 3 steels are determined by testing smooth standard specimens at
150 and 250°C. Mechanical properties at 150°C are necessary as basic data for
/-integral determination. It is noteworthy that tensile properties of this class of
steels have similar values in the range from room temperature up to 150°C. The
temperature of 250°C is selected for plastic analysis within the local approach
framework. Typical plots obtained in tensile testing are presented in Fig. 2, and
corresponding values are listed in Table 4.
In general, tensile properties of used steel are still not critical regarding next
service. The effect of elevated temperature under operating stresses on tensile
properties of steel 14MoV6 3 is more pronounced in the yield stress than in
70 ISSN Ü556-171X. Проблемыг прочности, 2ÜÜ4, N 1
Fracture Toughness and Crack Resistance
tensile strength. The difference between yield stress and the ultim ate tensile
strength is more pronounced at higher testing temperature (250oC). The
elongation reduction value is of the same level at both testing temperatures.
T a b l e 4
Mechanical Properties of New and Used 14MoV6 3 Steels at 150 and 25Q0C
Steel Temperature,
OC
Tensile
strength
Rm > MPa
Yield
stress
Re, MPa
Longitudinal
elongation
el , %
Radial
elongation
ed, %
Young’s
modulus
E, GPa
new 150 490 33S 20.6 44.S 19S
used 4З5 264 17.S 40.9 197
new 250 4S0 31S 21.0 46.7 194
used 415 227 1S.5 42.7 192
Fig. 2. Typical stress-strain diagrams for new and used 14MoV6 3 steels, tensile test at 2500C,
stress vs longitudinal strain (a), stress vs diameter contraction strain (b).
Impact Notch Toughness. Again, as in the case of tensile properties, the
reduction in impact toughness is not critical [8]. There is no reduction in impact
energy of used steel when testing at 1500C, and impact toughness is almost the
same for new and used steel. Reduction in impact energy at room temperature is
expressed only in crack propagation energy, not in crack initiation energy. Nil
ductility transition temperature for this steel is above room temperature, and this
explains the significantly higher values of impact toughness at 1500C.
Local F ractu re A pproach. The local fracture approach defines critical
parameters, which describe brittle fracture (cleavage) and ductile fracture
(tearing) [9]. Determination of these parameters is based on theoretical,
experimental and numerical procedures proposed by ESIS [6]. In the considered
case, only ductile fracture is of importance because steel is exposed in service to
elevated temperature.
Theoretical Basis o f Local Fracture Approach. Process of ductile fracture
initiation develops through three independent stages:
(i) void formation due to presence of inclusions;
(ii) void growth;
(iii) void coalescence [10].
ISSN Ü556-171X. Проблемыг прочности, 2ÜÜ4, N І 71
M. Zrilic, Z. Burzic, Z. Cvijovic
Voids are formed during material loading under the critical normal stress
inside the inclusions or at the boundary inclusion surface. Voids’ formation is
caused by breaking of interfaces between matrix and particles (inclusions and
second phase particles). Next stage in ductile fracture development is growth of
voids, and it is considered to be the most important stage. The growth of formed
voids depends on the effect of the external load, which produces additional plastic
strains and affects the hydrostatic stress component. The latter factors affect
further growth of voids and eventual coalescence of voids.
Ductile fracture is described by numerous models. One of the most
frequently applied is the model of Rice and Tracey [11], in which the growth of a
void insulated within a volume is considered, as presented by the expression:
I R ln I —
I R 0 /
= / 0.283
/ 3a m d£ pq . (1)
Here, — R 0 is the void growth rate, R - stands for actual void size, R 0 - for
initial void size. Void growth rate can be determined by integrating the right side
from the initial value of equivalent deformation to its actual value £ , for each
point of calculated values of hydrostatic pressure a m and equivalent stress a eq.
This model takes into consideration the stress triaxiality represented by the
ratio of hydrostatic stress a m and von Mises equivalent stress a eq, given by
components of stress tensor a ij:
a = a rr + a ee + a zz = a 11 + a 22 + a 33 (2)
a m o o , (2)
a eq = ^ 2 [(a 11 a 22 ) 2 + (a 22 a 33 )2 + (a 33 a 11)2]. (3)
Experimental Analysis o f Notched Specimens fo r the Local Fracture
Approach. Design of tensile specimens for experimental analysis is given in [12].
Specimens are produced with notch root radius R = 10,4, and 2 mm. In this way,
the triaxiality effect is taken into account. The critical void growth, determined as
a local-approach parameter by the proposed procedure, does not depend on
geometry. Ductile fracture is dominant at elevated (here, 250°C) temperature
testing, indicating that calculated critical void growth does not depend on
temperature as well [8]. In further FEM calculations, we used the true stress - true
strain curve obtained at the temperature for which calculation is performed. Strain
parameter in this case is contraction of the diameter in specimen notch root.
Using the “load vs diameter contraction” records (Fig. 3), true stress vs true
strain relationships can be determined for the new and used steel tested at 250°C
(Fig. 4), which are to be compared with corresponding curves obtained by FEM
calculation. The reduction in strength and plasticity of used steel is visible. True
stress reduction (A af ) is the greatest for notch root radius 2 mm, and reduction
in plasticity (A£ f ) is most expressed for radius 10 mm, indicating that stress
72 ISSN 0556-171X. npo6n.eubi npounocmu, 2004, № 1
Fracture Toughness and Crack Resistance
concentration is involved in the local approach method. This can be explained by
fracture mechanics approach because the critical triaxiality is achieved for plane
strain conditions, as in the case of notch root radius 2 mm. Used steel properties
degradation produces faster void activation, followed by faster occurrence of
critical growth rate and material fracture. For plane stress case, dominated for
lower triaxiality level (notch root radius 10 mm), the influence of strain is
dominant. In order to obtain an average result, critical void growth should be
determined for all three specimen geometries (R = 2, 4, and 10 mm) [6].
A d [mm] £f
Fig. 3 Fig. 4
Fig. 3. Load (F) vs contraction of notch root diameter (Ad), for new (N) and used (O) steel, notch
radius 2 mm (NT2), 4 mm (NT4), and 10 mm (NT10).
Fig. 4. True stress (aF) vs true strain (eF) relationships for new (N) and used (O) steel specimens,
with notch radius 2 mm (NT-2), 4 mm (NT-4), and 10 mm (NT-10).
Calculation o f Local Approach Parameters. In order to determine void
growth rate R /R 0 according to Eq. (1), the equivalent uniaxial strain e f and
true stress o f values need to calculated by the following formulas:
= 2lnl
, d
(4)
- _ 4 F f
0 F = x d 2 ■ (5)F
The calculation of the above parameters of ductile fracture is carried out
using a post-processor program [6]. The cavity growth rate R /R 0 is calculated in
each mesh element using the equivalent uniaxial strain and true stress averaged
values of the strains and stresses, respectively, over the Gauss points. Critical
cavity growth rate (R /R 0 ) c can be obtained by replacing von Mises equivalent
stress, o eq, in Eq. (2) and triaxial stress, o m, which represents hydrostatic stress,
in Eq. (3). True stress-strain curves obtained by testing the specimens with three
different notch root radii are compared with corresponding curves obtained by
ISSN 0556-171X. npo6n.eubi npounocmu, 2004, № 1 73
M. Zrilic, Z. Burzic, Z. Cvijovic
FEM analysis. When sufficient accordance between two sets of curves [6] is
achieved, critical void growth rate can be calculated using Eqs. (1)-(3). The
hydrostatic stress o m value is determined from the maximum normal stress
component obtained in a finite element critical node, positioned in the minimum
cross section of specimen axes (marked by CN in Fig. 5). The von Mises
equivalent stress o eq is calculated in the same node.
CN
339.5
275.2
—
■239.1
Fig. 5. Notched tensile specimen calculation: (a) mesh, notch root radius R = 2 mm; (b) stress
distribution, R = 4 mm; (c) mesh, R = 4 mm; (d) mesh, R = 10 mm.
74 ISSN 0556-171X. npo6neMbi npouuocmu, 2004, № 1
Fracture Toughness and Crack Resistance
Elastic-plastic analysis in NASTRAN software is used for FEM calculation
with 3D elements. Upper specimen half is modeled as a wedge element, with
angle of 5°, corresponding to 1/72 part of upper specimen part (Fig. 5). This
affects the shape of finite elements mesh, producing wedge type elements around
axes, and the other elements are of brick type.
Calculation by the local criterion yields the material characteristic
representing the local stressed state, which corresponds to the critical void growth
and to the onset of unstable fracture. It has a convenient form, applicable in FEM
calculation. When high stress level is found in a structure and void growth rate
R/Ro is verified, failure of the structure can be expected at the critical (R /R 0) c
level. From the engineering point of view, this verification should be performed
for the maximum load, e.g., for the ultimate tensile strength, (R /R o)m, as the
indication of critical behavior of the structure.
Experiments have been performed with five specimens of new and used steel
of various geometries for determination of values for critical void growth rate (the
final fracture) (R /R 0 ) c and void growth rate (R /R 0) m, corresponding to the
ultimate tensile strength (the maximum load). Columns 4-5 in Table 5 present
average values of measurement of five specimens taken from new (N) and used
steel (O) for three geometries (notch root radius 10 mm, 4 mm, and 2 mm). Columns
8-9 present average final values of critical void growth rate (R /R 0) c and void
growth rate (R /R 0 ) m, for the maximum load. Solution of Eq. 1 produces
ln(R /R o), and thus values of void growth rate R /R o presented in this form start
from 1, insofar as ln(R /R o) = 0. For this reason, the values (R /R o — 1) should be
used for comparison of the results obtained.
T a b l e 5
Values of Void Growth Rate for the Final Fracture (R/R0)c
and for the Maximum Load (R/R0)m
Speci
men
Average values for 5 specimens Average final values
h'( R )c
ln( R ]
V R0 )m ( R )c ( R )m h' ( R i
ln( R ]
V R0!m ( R 1 ( R )m ( R )c - (R ] ->V R0 !m
1 2 3 4 5 6 7 8 9 10 11
10N 0.6136 0.0792 1.8470 1.0824 0.5131 0.0734 1.68 1.08 0.68 0.08
4N 0.5467 0.0751 1.7275 1.0780
2N 0.3790 0.0659 1.4609 1.0682
100 0.5612 0.0742 1.7670 1.0770 0.4526 0.0688 1.59 1.07 0.59 0.07
40 0.4855 0.0702 1.6250 1.0727
20 0.3112 0.0621 1.3650 1.041
The bearing capacity of steel is exhausted as soon as the maximum load is
reached, e.g., it corresponds to the ultimate tensile strength. For that, relevant
void growth rate should be taken as the value (R /R 0) m, and not as critical value
defined by the local approach (R /R o) c. This also explains the difference in
values (R /R o ) c - 1 and (R /R 0)m - 1 in Table 5.
ISSN 0556-171X. npoôëeubi npounocmu, 2004, № 1 75
M. Zrilic, Z. Burzic, Z. Cvijovic
C rack Resistance. J-Integral. Single-edge notched bending specimens -
SE(B) type - have been produced from pipe in direction L-R. For new steel,
specimes are sized B = 17.5 mm in thickness and W = 33.6 mm wide, with
fatigue precrack of final length a 0 = 19.86 mm. Specimens from used steel were
of B = 17.5 mm, W = 19.8 mm, with fatigue precrack o f final length
a o = 12.79 mm. Testing is performed according to ESIS Recommandations
P1-92 and standard ASTM E1152, based on elastic-plastic fracture mechanics.
Thus, crack growth could be monitored also in the plastic range, and the critical
J-integral value, J i c, obtained as a measure of fracture toughness, which can be
used to calculate the critical stress intensity factor, K ic. Tests are performed at
150oC. Single specimen compliance method with successive unloading is used
(ASTM E1152). In this way, J — R curve, e.g., relationship J — Aa (Fig. 6) is
obtained, from which the critical value for crack initiation can be determined.
0.637 A a, mma
0.432 b A a,m m
Fig. 6. J — R curves (J — Aa), for new (a) and used (b) 14MoV6 3 steel tested at 150oC.
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Fracture Toughness and Crack Resistance
By measuring slope of unloading line it is possible to calculate the crack
extension, Aa, in one step. In general, crack starts to grow when the maximum
load in record “load F vs load line displacement 6” is reached (Fig. 7). It is
possible to obtain two sets of data:
(i) energy spent on crack extension in individual steps (area under record
F - 6 ) ;
(ii) change of compliance (change of unloading line slope).
S, nun
b HHJ1
Fig. 7. Records “load F vs load line displacement 6” obtained at 150oC by single specimen
successive unloading method, for new (a) and used (b) 14MoV6 3 steel.
Crack extension A a t in one step is calculated from the ratio 6/ F taken as
difference in unloading line slopes, applying formula:
Aai = A ai_1 +
2
Ci-1 _ Ci
Ci (6)
where a J— 1 stands for the previous crack length, C i = tan a i - for the considered
unloading line slope, and C J—1 = tan a J—1 - for the previous unloading line slope.
a
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M. Zrilic, Z. Burzic, Z. Cvijovic
From directly recorded relationship F — d (Fig. 3), /-integral can also be
calculated by formula
J i = J ii—1
2 \ A t — A —
+ l b ) B
a — a i—1
bi
(7)
where A is the area below the curve, B is specimen thickness, and b is the
ligament length.
Using parameters calculated by Eqs. (6) and (7), the relationship J — Aa is
obtained (Fig. 6), and the critical J-integral value, J Ic, can be then obtained in the
cross section of fitted parabolic line and ordinate for 0.2 mm crack length. The
regression line for J ic evaluation according to ASTM E813 is also given in Fig. 6.
From the critical J-integral value J ic it is possible to calculate the critical
stress intensity factor (plane strain fracture toughness), K ic, from
K Ic =
J IcE
(8)
The obtained values of critical J Ic integral and critical stress intensity
factor, K ic, are given in Table 6.
T a b l e 6
Fracture Toughness Values of new and Used 14MoV6 3 Steel at 150°C
Steel Temperature
(oC)
Young’s
modulus E,
GPa
Poisson’s
ratio v
Critical J-integral
value J Ic,
kJ/m2
Critical stress
intensity factor Kic,
MPaVm
new 150 198 0.3 170 192
used 150 197 0.3 130 160
Fatigue Crack Growth Rate (da/dN). This testing is performed for
determination of coefficients C and n in the Paris equation for fatigue crack
growth, as well as of threshold value. Standard Charpy specimen with V-notch
had been tested by variable bending with sinusoidal cycle and stress ratio R = 0.1
using the resonant high frequency pulsator CRACKTRONIC.
The mean load and amplitude were controlled with an accuracy of ± 5 N • cm.
Testing was performed in compliance with standard ASTM E-647-88a on
determination of fatigue crack growth rate, da/dN , and stress intensity factor
range, AK. This standard imposes two limitations:
_8
(i) fatigue crack growth rate must be higher than 10 m/cycle in order to
avoid fatigue threshold region, AKth ;
(ii) the amplitude of load must be constant.
Foil crack gauges RUMUL RMF A-5 of measuring length 5 mm are bonded
on machined specimens, enabling monitoring of crack growth on fatigue test
device FRACTOMAT operating by the potential drop principle. Crack growth is
followed by foil separation, producing the change of electric resistance (Fig. 8).
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Fig. 8. Scheme of crack growth measurement by the potential drop in crack gauge foil.
During testing the number of cycles was automatically registered for each
0.05 mm of crack extension. Crack growth rate da/dN is calculated based on this
registration.
Dependence “crack growth rate da/dN vs stress intensity factor range A K ”
can be expressed using the Paris coefficients C and n. Stress intensity factor
range AK for the actual crack length a depends on the specimen geometry, crack
length and variable load range, AF = Fg - F d (Fg is the upper load in a cylce
and Fd is the lower load). It can be calculated from the following formulas:
^ L
M : = - ^ = f ( a/W ),
bVW 3 '
(9)
f ( a/W ) =
a 1/ a
2|1 + 2 — 1|1------
w Zl w
3/2
a / a
1 .99- — 11- —
W l W
/ a / a
2 .1 5 -3 .9 3 — + 2 .71 —
w Iw
(10)
Here, L is span, B is the specimen thickness, W is the specimen width, and a is
the crack length.
Calculated values for the Paris equation, presented in the form log da/dN vs
log AK, are shown in Fig. 9 and given in Table 7 for new and used steel.
T a b l e 7
Fatigue Crack Growth Parameters for New and Used 14MoV6 3 Steel at 20°C
3
Steel Temperature fC) Paris
coefficient, C
Paris
exponent, n
Threshold value
Kth, MPaVm
new 20 1.47 -10"15 6.114 7.454
used 20 2.06 -10“ 14 4.936 7.231
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M. Zrilic, Z. Burzic, Z. Cvijovic
1 10 100
Fig. 9. Crack growth rate dajdN vs stress intensity factor range AK for new and used steel.
M etalographic Exam ination. Light metallography had been performed
with samples, taken from new and used steel, in order to compare the
microstructural change after 117,000 service hours. Finer microstructure of new
steel samples is clearly visible for magnification X200, and more expressed for
magnification X500 (Fig. 10). Due to exposure to high temperature and high
stresses, used steel becomes deteriorated by grain coarsening. On the other hand,
it is possible to find voids between grains in different positions, uniformly
distributed in the considered area. The clear difference in ferrite-pearlite
microstructure vanishes slowly. This is finally confirmed by higher magnification
of X 1000. It is possible to conclude that deterioration level in the used steel is
close to the initial creep stage. For that, it is recommended to reduce temperature
and pressure in the steam pipeline in the next limited operation and to replace
pipes after that. Having in mind that examined samples are taken from straight
part of the pipeline, the situation in the knee zone can be considered as more
critical.
Ductile behavior of new steel is confirmed by scanning electron examination
of fractured new steel specimen NT10 with notch root raidus 10 mm tested at
250°C (Fig. 11). Inclusions, from which void formed, can be recognized in the
bottom of dimples. Formed voids coalesce during fracture process up to the final
breaking.
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Fig. 10. Microstructure of samples taken from pipe outer surface: (a, c, e) new steel and (b, d,
f) used steel; (a, b) X200; (c, d) X500; (e, f) X1000.
a b
Fig. 11. Ductile fracture of new 14MoV 63 steel: (a) indication 1 mm and (b) indication 10 ^m.
ISSN 0556-171X. Проблемы прочности, 2004, N2 1 81
M. Zrilic, Z. Burzic, Z. Cvijovic
Analysis of Results. Tensile properties of used steel are reduced as
compared to the corresponding values of new steel for both testing temperatures
(150 and 250oC), as given in Table 4. This reduction percentage is presented in
Table 8. Tensile properties’ reduction is more pronounced at higher testing
temperature (250OC). The reduction is the most pronounced for the yield stress.
T a b l e 8
Reduction of Tensile Properties of Used Steel as Compared to that of New Steel (%)
Temperature
t, OC
Ultimate tensile
strength Rm, %
Yield stress
Re,%
Elongation
El > %
Contraction
Ed > %
150 11.2 22 14 8.71
250 13.5 29 12 8.77
Reduction in the void growth rate R/Ro is presented in Table 9 for the final
fracture and for the maximum load values in percentage. As it is already said, for
the proper analysis of steel deterioration, the value corresponding to the
maximum load should be used, e.g., (R /R 0) m -1 .
T a b l e 9
Reduction of Void Growth Rate RjR0 for the Used Steel (%)
ln(RRo)c ln(RR0)m (R/R>)c (R/ R0 )m RR0)c — 1 (RR0 )m 1
11 8 6.3 5.5 0.5 13.7 6.5
Comparison of properties at 250OC shows that the reduction is more
expressed for tensile strength R m (13.5%, Table 6), than for corresponding void
growth rate (R /R 0)m given through (R /R 0)m — 1 (6.5%, Table 9). This is the
indication of conservatism of classical approach. Moreover, ductile fracture
criterion, applied in local approach, is directly applicable in FE method, defining
limit stress value in a structure.
Conclusions. The agreement of records obtained by tensile testing of
notched specimens with diagrams obtained by the FEM analysis is a prerequisite
for calculation of local criterion for ductile fracture. In this way, the material
characteristics, precisely describing material properties, are available for modern
computational methods, such as FEM.
Described properties of the local fracture approach enabled more precise
evaluation of deterioration of steel 14MoV6 3 properties after long-term exposure
to elevated temperatures. Obtained results have shown that the maximum load in
tensile test, and not the value corresponding to the final fracture, is a relevant one
for the comparison of new and used steel properties. The reduction in void growth
rate obtained in the conducted tests is only 6.5%, whereas, being expressed in
term of tensile strength, it is 13.5%, and finally, being expressed through yield
stress, it is as high as 39%.
It is possible to conclude that the steel under study can be used for future
service under condition of prescribed in-service inspections, notwithstanding that
the specified service life is spent. The economical benefit could be significant.
Anyhow, further refinement of the local fracture approach is necessary for its
practical application.
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Резюме
Досліджується проблема передчасного руйнування паропроводів (сталь
14MoV6 3) із розрахунковим ресурсом 100 тис. год за температури 540оС. При
виготовленні експериментальних зразків використовувалася сталь у
первинному стані та після 117 тис. год експлуатації. Використання локаль
ного підходу механіки руйнування і металографічного аналізу поряд із кла
сичними методами (випробування на розтяг, тріщиностійкість, міцність від
утомленості) дозволило більш точно оцінити деградацію властивостей ста
лі під дією високих температур і напружень. Обгрунтовано необхідність
подальшого розвитку локального підходу до прогнозування умов руйну
вання матеріалів при тривалій експлуатації в умовах високих температур.
1. H. Kaes, Erfahrungen mit dem Stahl 14MoV6 3, in: Bericht des VGB-
Ausschusses für Materialfragen, Essen (1966).
2. W. Arnswald und H. Kaes, Verwendung des Stahles 14MoV6 3 fü r
Rohrleitungen, VGB Konferenz (1985), SS. 356-358.
3. H. Kessler und W. Hoppe, Teilaustausch von HD- und HZU-Leitungen in
einem 2 X 300 M W in Griechenland, Lebensdauer von Rohrleitungen in
Kraftwerken, Mannesmann Anlagebau AD, Düsseldorf (1987), SS. 373-385.
4. H. Weber, H. Kessler, und H. Musch, “Restlebensdauer- und Schadens
analysen von Rohrleitungssystemen im Kraftwerksbau - das Know-How von
M an n esm an n ,” L eb en sd a u er von R o h rle itu n g en in K ra ftw erken ,
Mannesmann Anlagenbau, Düsseldorf, 5 (1987).
5. A. Pineau, “Review of fracture micromechanisms and a local approach to
predicting crack resistance in low strength steels,” in: Advances in Fracture
Research, Pergamon Press (1981), Vol 2, pp. 553-577.
6. ESIS P6-98. Procedure to Measure and Calculate Material Parameters fo r
the Local Approach to Fracture Using Notched Tensile Specimens, European
Structural Integrity Society (ESIS) (1998).
7. F. Mudry and M. Di Fant, A Round Robin on the Measurement o f Local
Criteria, IRSID-SG Report, RE 93-319 (1993).
8. M. Zrilic and R. Aleksic, “The effect of long term exposure to elevated
temperature on steam line steel properties,” Thermal Science, 7, No. 1, 33-46
(2003).
9. M. Bethmont, “The principles of the local approach of fracture,” in: 5th
International Fracture Mechanics Summer School, EMAS (1990), pp. 45-70.
10. V. Tvergaard, “Ductile fracture by cavity nucleation between larger voids,”
J. Mech. Phys. Solids, 30, No. 4, 265-286 (1982).
11. J. R. Rice and D. M. Tracey, “On the ductile enlargement of voids in triaxial
stress fields,” J. Mech. Phys. Solids, 17, 201-217 (1969).
12. M. Zrilic, M. Rakin, A. Sedmak, and S. Sedmak, “Assessment of in-service
degradation of steam pipeline steel by local approach method,” in: Life
Assessment and Management fo r Structural Components (Resource 2000),
Kiev (2000), Vol. 1, pp. 247-253.
Received 26. 05. 2003
ISSN 0556-171X. Проблеми прочности, 2004, № 1 83
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