12 A. I. Aydeniz, A. Göksenli, G. Dil, F. Muhaffel, C. Calli, B. Yüksel,
Electroless Ni-B-W Coatings for Improving Hardness, Wear And
Corrosion Resistance, MTAEC9, 47 (2013) 6, 803–806
13 T. S. N. Sankara Narayanan, K. Krishnaveni, S. K. Seshadri, Elec-
troless Ni–P/Ni–B Dublex Coatings: Preparation and Evaluation of
Microhardness, Wear and Corrosion Resistance, Mater. Chem. Phys.,
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14 W. X. Zhang, Z. H. Jiang, G. Y. Li, Q. Jiang, J. S. Lian, Electroless
NiP/NiB Dublex Coatings For Improving The Hardness and The
Corrosion Resistance of AZ91 D Magnesium Alloy, Appl. Surf. Sci.,
254 (2008), 4949–4955, doi:10.1016/j.apsusc.2008.01.144
15 A. B. Drovosekov, M.V. Ivanov, V. M. Krutskikh, E. N. Lubnin, Y.
M. Polukarov, Chemically Deposited Ni-W-B Coatings: Composi-
tion, Structure and Properties, Prot. Met., 41(2005) 1, 55–62,
doi:10.1007/s11124-005-0008-1
16 F. Z. Yang, Z. H. Ma, L. Huang, S. K. Xu, S. M. Zhou, Electro-
deposition and Properties of an Amorphous Ni-W-B Alloy before
and after Heat Treatment, Chin. J. Chem., 24 (2006), 114–118,
doi:10.1002/cjoc.200690004
17 S. Ruan, C. A. Schu, Mesoscale Structure and Segregation in
Electrodeposited Nanocrystalline Alloys, Scripta Mater., 59 (2008)
11, 1218–1221, doi:10.1016/j.scriptamat.2008.08.010
18 R. A.C. Santana, S. Prasad, A. R. N. Campos, F. O. Arau´ jo, G.P.
Sýlva, P. Lima-Neto, Electrodeposition and Corrosion Behaviour of a
Ni–W–B Amorphous Alloy, J. Appl. Electrochem., 36 (2006),
105–113, doi:10.1007/s10800-005-9046-2
19 R. A. Yildiz, A. Göksenli, B. Yüksel, F. Muhaffel, A. Aydeniz, Effect
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20 Z. Z. Hamid, H. B. Hassan, A. M. Attyia, Influence of Deposition
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B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
842 Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
843–847
SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW
STRESSES AND AN INSTANTANEOUS-STRESS-CHANGE
TESTING
KRATKOTRAJNO LEZENJE TOPLOTNO ODPORNEGA JEKLA P91
PRI NIZKIH NAPETOSTIH IN NENADNI MENJAVI NAPETOSTI
OBREMENJEVANJA
Jiang Zhe, Shen Junjie, Zhao Pengshuo
Tianjin University of Technology, Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent Control, National
Demonstration Center for Experimental Mechanical and Electrical Engineering Education, 391 Binshui Road, Xiqing District, Tianjin, China
sjj1982428@sina.com, j211209977@live.com
Prejem rokopisa – received: 2016-1'-20; sprejem za objavo – accepted for publication: 2017-03-21
doi:10.17222/mit.2016.305
For the short-term creep behavior to be evaluated and the creep mechanism of P91 heat-resistant steels at low stresses and high
temperatures to be clarified, stress-change testing was conducted with a “helicoidal-spring creep test” demonstrating a high
strain resolution. The creep deformation consists of the primary creep stage, whereas no secondary creep stage was observed.
Blackburn’s law was suggested to be the best choice for a short-term-creep-behavior description because it provides a good
representation of an experimental creep curve. An anelastic backflow at a low stress was confirmed, following a high reduction
in the stress. The absolute value of the instantaneous strain for a load increase was equal to the value for a load decrease and the
creep of the P91 steels at low stresses might have been controlled by the viscous glide of dislocations.
Keywords: P91 heat-resistant steel, creep, anelastic, stress change
Da bi bilo mo~ oceniti kratkotrajno lezenje in njegov mehanizem toplotno obstojnega jekla P91 pri nizkih napetostih in visokih
temperaturah, so bili izvedeni preizkusi spremembe napetosti z uporabo t.i. testa spiralne vzmeti, ki omogo~a veliko lo~ljivost
deformacije. Deformacija lezenja sestoji iz primarne in sekundarne faze lezenja, vendar sekundarne faze niso analizirali oz.
opazovali. Blackburnov zakon je najbolj uporaben zakon za opis obna{anja jekla med kratkotrajnim lezenjem, ker se z njim
najbolj pribli`amo rezultatom, dobljenim z eksperimenti. Neelasti~ni povratni tok pri nizki napetosti je bil potrjen z ve~jim
zmanj{anjem napetosti. Zaradi absolutne vrednosti trenutne deformacije za dano obremenitev je bil narastek deformacije enak
zmanj{anju obremenitve, medtem ko je lezenje pri nizkih napetostih jekla P91 kontrolirano z viskoznim drsenjem dislokacij.
Klju~ne besede: P91 jeklo za delo v vro~em, lezenje, neelasti~nost, sprememba napetosti
1 INTRODUCTION
The P91 ferritic heat-resistant steel is utilized for the
material production of thermal-power-plant components,
which are exposed to elevated temperatures for extensive
periods. This requires that the P91 structural materials
resist creep deformation at both high temperatures and
low stresses. The creep strength obtained with the extra-
polation method based on the data of the creep deforma-
tion at a high stress is higher than the true creep strength
under these conditions (at low stresses).1,2 The creep-
strength degradation is not only based on the microstruc-
tural degradation3–6, but it is also related to the change in
the creep mechanism.7–9 The creep-mechanism clarifica-
tion has to depend on the instantaneous creep behavior
because the long-term creep is associated with the micro-
structural degradation. The stress-change test constitutes
an effective method for a creep-deformation-mechanism
clarification based on the instantaneous-strain and
creep-rate-variation evaluation at the stress changes
during the creep testing.10–12
At a high strain rate (a high stress) and a high tem-
perature, a sudden-stress-change experiment has been
widely utilized for pure metals and solution-strengthened
alloys.10–12 In contrast, at a low strain rate 0, especially
the ultra-low 0, lower than 10–10 s–1, it is usually impossi-
ble for a conventional tension-creep technique to dist-
inguish the instantaneous plastic strain from the total
strain under a sudden stress change, due to the unsatis-
factory strain resolution, existing between 10–6 and 10–5.
As an additional creep technique, the helicoidal-spring
creep test based on torsion deformation provides a
significantly high strain resolution, even up to 10–9.13
Due to this high strain resolution, it is possible for very
low instantaneous strains to be measured during a
sudden stress change.14
In the present study, the instantaneous creep of the
P91 ferritic heat-resistant steels was studied during
sudden-stress-change experiments using the helicoidal-
spring-specimen technique for the creep mechanism
under both a low stress and a high temperature.
Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847 843
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.9.011:620.172.24:691.714 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)843(2017)
2 EXPERIMENTAL PART
2.1 Materials
The P91 heat-resistant steel was utilized in this expe-
riment. The chemical components are presented in Table 1.
2.2 Specimen processing
The steel was processed into helicoidal-spring speci-
mens using the method of high-speed wire-electrode
cutting machining; the outer and inner diameters of the
specimens were 12 mm and 8 mm, respectively. The
cross-section of the helicoidal-spring specimens was a
rectangle. The side length of the rectangle was 2 mm, as
presented in Figure 1.
The helicoidal-spring specimens were annealed at
1313 K for 60 min and consequently tempered at 1033 K
for 60 min.
2.3 Test methods
Figure 2 presents the experimental apparatus, which
includes an electric furnace with three adjacent heaters, a
high-precision optical micrometer, a load, a weight and a
system for receiving and processing information. By
measuring the helicoidal-spring-specimen pitch change,
the creep curves could be obtained. The test-apparatus
details were previously reported.14
During the testing, the number of weights was in-
creased or decreased for the instantaneous stress change
to be observed. The weight was suspended under a load
with a flammable thread. In order for a load to be
increased, the load was hooked quickly. For a load to be
decreased, the flammable thread was burned.
The test was performed at a temperature of 923 K
and a slight oscillation. The specimen was loaded sub-
sequently for five days when the strain rate was down to
10–9–10–8 s–1. Also, the stress consequently increased or
decreased at various applied levels and the instantaneous
strain was measured. The following equations15,16 were
utilized for calculating the mean surface shear stress, ,
and the surface shear strain, , with the assumption of the
pure torsion of the helicoidal-spring specimen:
 =
PD
a b2 2
(1)
 =
2
2
a
Dπ
Δ (2)
where P is the average load, D is the coil diameter
(12 mm) and  is the displacement of the mean
coil-pitch spacing. In this study, the torsion was the
dominant component of deformation, because D was
quite higher than d (D/d > 12)17 and the value of  was
between 2 mm and 4 mm.18 Since the stress and strain in
the helicoidal spring had essentially shear components,
the former could be transformed into the equivalent
tensile quantities with the von Mises equations for the
tensile stress  = 3 and the tensile strain  = / 3.
3 RESULTS AND DISCUSSION
Figure 3 demonstrates the creep curves obtained at
923 K and at various stresses: (34.85, 27.75 and 19.35) MPa.
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
844 Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 1: Chemical components of P91
Element Cr Mo Si V C Nb Cu N P Ni Ti Al S
Content (%) 9.50 0.91 0.60 0.20 0.10 0.10 0.07 0.04 0.02 0.02 0.01 0.01 0.01
Figure 2: Experimental apparatus including electric furnace, high-
precision optical micrometer, load, weight and information-receiving
and processing systemFigure 1: Helicoidal-spring specimen for the stress-change testing
The creep rate which corresponded to the slope of the
curves decreased as the duration increased.
Figure 4 presents the strain rate on a logarithmic plot
versus the strain at 923 K and various stresses. The creep
deformation of the P91 steels consisted of the primary
creep stages where the creep rate decreased along with
the strain and no apparent secondary (steady-state) creep
stage was observed where the creep rate would not
change along with the strain.
The constitutive creep equations expressing the pri-
mary stages should be utilized in the experimental
creep-curve analysis. The following constitutive creep
equations were utilized, being widely accepted as the
basic creep equations.19,20
Power law:
 = 0 + at
b (3)
Exponential law:
 = 0 + a[1-exp(-bt)] (4)
Logarithmic law:
 = 0 + a ln(1+bt) (5)
Blackburn’s law:
 = 0 + ac[1 - exp(-bt)]+c[1 - exp(-dt)] (6)
where  is the strain, 0 is the instantaneous strain on
dead weight, t is the elapsed time and a, b, c and d are
the parameters characterizing the primary creep region.
The a, b, c and d values were called the "scaling fac-
tors".21
Figure 5 presents representative results of a re-
gression analysis, for the creep at 923 K and 27.75 MPa.
The exponential-law and power-law equations did not
reproduce the experimental data. Compared to the loga-
rithmic-law equation, the Blackburn equation provided a
better representation of the experimental creep curve.
Therefore, the short-term creep for the P91 heat-resistant
steels at low stresses could be described with the
Blackburn equation.
In order for the creep mechanism of the P91 steels at
low stresses to be studied, the instantaneous creep
behavior was investigated. The specimens were loaded
for 4.3 × 105 s until the strain reached 6 × 10–3. In this
case, the dislocations could be expected to move. The
stress increased or decreased at various levels that were
consequently applied and the instantaneous strain was
measured.
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847 845
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Strain/time creep curves for P91 at 923 K and various
stresses
Figure 4: Strain rate/strain creep curves at 923 K and various stresses
Figure 6: Example of instantaneous elongation and contraction upon
low stress changes at a high temperature
Figure 5: Comparison of experimental data and predicted curves of
strain versus time at 923 K and 27.75 MPa
2 EXPERIMENTAL PART
2.1 Materials
The P91 heat-resistant steel was utilized in this expe-
riment. The chemical components are presented in Table 1.
2.2 Specimen processing
The steel was processed into helicoidal-spring speci-
mens using the method of high-speed wire-electrode
cutting machining; the outer and inner diameters of the
specimens were 12 mm and 8 mm, respectively. The
cross-section of the helicoidal-spring specimens was a
rectangle. The side length of the rectangle was 2 mm, as
presented in Figure 1.
The helicoidal-spring specimens were annealed at
1313 K for 60 min and consequently tempered at 1033 K
for 60 min.
2.3 Test methods
Figure 2 presents the experimental apparatus, which
includes an electric furnace with three adjacent heaters, a
high-precision optical micrometer, a load, a weight and a
system for receiving and processing information. By
measuring the helicoidal-spring-specimen pitch change,
the creep curves could be obtained. The test-apparatus
details were previously reported.14
During the testing, the number of weights was in-
creased or decreased for the instantaneous stress change
to be observed. The weight was suspended under a load
with a flammable thread. In order for a load to be
increased, the load was hooked quickly. For a load to be
decreased, the flammable thread was burned.
The test was performed at a temperature of 923 K
and a slight oscillation. The specimen was loaded sub-
sequently for five days when the strain rate was down to
10–9–10–8 s–1. Also, the stress consequently increased or
decreased at various applied levels and the instantaneous
strain was measured. The following equations15,16 were
utilized for calculating the mean surface shear stress, ,
and the surface shear strain, , with the assumption of the
pure torsion of the helicoidal-spring specimen:
 =
PD
a b2 2
(1)
 =
2
2
a
Dπ
Δ (2)
where P is the average load, D is the coil diameter
(12 mm) and  is the displacement of the mean
coil-pitch spacing. In this study, the torsion was the
dominant component of deformation, because D was
quite higher than d (D/d > 12)17 and the value of  was
between 2 mm and 4 mm.18 Since the stress and strain in
the helicoidal spring had essentially shear components,
the former could be transformed into the equivalent
tensile quantities with the von Mises equations for the
tensile stress  = 3 and the tensile strain  = / 3.
3 RESULTS AND DISCUSSION
Figure 3 demonstrates the creep curves obtained at
923 K and at various stresses: (34.85, 27.75 and 19.35) MPa.
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
844 Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 1: Chemical components of P91
Element Cr Mo Si V C Nb Cu N P Ni Ti Al S
Content (%) 9.50 0.91 0.60 0.20 0.10 0.10 0.07 0.04 0.02 0.02 0.01 0.01 0.01
Figure 2: Experimental apparatus including electric furnace, high-
precision optical micrometer, load, weight and information-receiving
and processing systemFigure 1: Helicoidal-spring specimen for the stress-change testing
The creep rate which corresponded to the slope of the
curves decreased as the duration increased.
Figure 4 presents the strain rate on a logarithmic plot
versus the strain at 923 K and various stresses. The creep
deformation of the P91 steels consisted of the primary
creep stages where the creep rate decreased along with
the strain and no apparent secondary (steady-state) creep
stage was observed where the creep rate would not
change along with the strain.
The constitutive creep equations expressing the pri-
mary stages should be utilized in the experimental
creep-curve analysis. The following constitutive creep
equations were utilized, being widely accepted as the
basic creep equations.19,20
Power law:
 = 0 + at
b (3)
Exponential law:
 = 0 + a[1-exp(-bt)] (4)
Logarithmic law:
 = 0 + a ln(1+bt) (5)
Blackburn’s law:
 = 0 + ac[1 - exp(-bt)]+c[1 - exp(-dt)] (6)
where  is the strain, 0 is the instantaneous strain on
dead weight, t is the elapsed time and a, b, c and d are
the parameters characterizing the primary creep region.
The a, b, c and d values were called the "scaling fac-
tors".21
Figure 5 presents representative results of a re-
gression analysis, for the creep at 923 K and 27.75 MPa.
The exponential-law and power-law equations did not
reproduce the experimental data. Compared to the loga-
rithmic-law equation, the Blackburn equation provided a
better representation of the experimental creep curve.
Therefore, the short-term creep for the P91 heat-resistant
steels at low stresses could be described with the
Blackburn equation.
In order for the creep mechanism of the P91 steels at
low stresses to be studied, the instantaneous creep
behavior was investigated. The specimens were loaded
for 4.3 × 105 s until the strain reached 6 × 10–3. In this
case, the dislocations could be expected to move. The
stress increased or decreased at various levels that were
consequently applied and the instantaneous strain was
measured.
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847 845
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Strain/time creep curves for P91 at 923 K and various
stresses
Figure 4: Strain rate/strain creep curves at 923 K and various stresses
Figure 6: Example of instantaneous elongation and contraction upon
low stress changes at a high temperature
Figure 5: Comparison of experimental data and predicted curves of
strain versus time at 923 K and 27.75 MPa
Figure 7 presents instantaneous-elongation and con-
traction examples upon low changes in the stress of the
P91 steels. In the figure, the – was the instantaneous
strain under a stress decrease and the + was the in-
stantaneous strain under a stress increase.
The relationship between the stress increment ||
and the instantaneous strain |E| is presented in Fig-
ure 7. The creep demonstrates a viscous behavior be-
cause the absolute values of the instantaneous strain for
the load increase were equal to the values for the load
decrease.
Two types of creep at low stress exist. One is creep
controlled by the diffusion including the lattice diffusion
creep (Nabarro-Herring type)22 at a high-T and grain
boundary diffusion creep (Coble type)22 at an inter-
mediate-T. The other is creep associated with dislocation
movement including free flight motion (climb con-
trolled) and viscous motion (glide controlled)23. When
creep is controlled by the diffusion, the creep should be a
non-viscous behavior. In this case, the absolute values of
the instantaneous strain for a small-load increase should
be apparently larger than the values for a small-load
decrease. If creep is controlled by free flight motion of
dislocation, plastic strain can occur instantaneously
when the stress is increased by a small amount. There-
fore, creep shows non-viscous behavior. When creep is
controlled by the viscous glide of dislocations, instant-
aneous plastic strain does not occur even if the applied
stress is increase suddenly. Thus, creep shows viscous
behavior. In this study, the creep demonstrates a viscous
behavior, because non instantaneous strain is observed
during the stress increase. It means creep may be con-
trolled by the viscous glide of dislocations.
Figure 8 gives an example of the anelastic backflow,
observed for a high reduction in the stress during the
transient-creep stage. Specifically, the helicoidal spring
specimen was loaded at 27.75 MPa and 923 K for 120 h
and consequently unloaded all the stress for 120 h. Two
different parts of the creep strain existed: the anelastic
strain was reversible, which is presented with solid
double arrows, whereas the plastic strain was irreversible
and it is presented with dashed double arrows. Therefore,
the instantaneous strain that occurred under a sudden
stress change included both elastic and anelastic compo-
nents.
4 CONCLUSIONS
The short-term creep behavior in the P91 heat-resis-
tant steels was investigated using instantaneous-stress-
change tests. The results were as follows:
1) The creep deformation consisted of the primary creep
stages, but no secondary creep stage was observed.
2) The Blackburn equation could be utilized for the
creep-curve description because it provided an im-
proved representation of the experimental creep
curve.
3) The viscous glide of dislocations might have been the
dominant creep mechanism because the creep dis-
played a viscous behavior.
4) The short-term creep at a low stress displayed an
anelastic behavior.
Acknowledgment
This project was supported by the National Natural
Science Foundation of China (Grant No. 51605330).
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ductility data, Materials at High Temperatures, 21 (2004) 1, 25–32,
doi:10.3184/096034004782750041
21 R. P. Reed, N. J. Simon , R. P. Walsh, Creep of copper: 4–300 K,
Materials Science and Engineering A, 147 (1991) 1, 23–32,
doi:10.1016/0921-5093(91)90801-s
22 W. D. Nix, Effects of Grain Shape on Nabarro-Herring and Coble
Creep Processes, Metals Forum, 4 (1981) 1, 38–43
23 H. Hiroyuki, N. Satoshi, K. Junichi, K. Akihiro, Creep deformation
characterization of heat resistant steel by stress change test,
International Journal of Pressure Vessels and Piping, 24 (2009) 9,
556–562, doi:10.1016/j.ijpvp.2008.06.003
24 K. Maruyama, S. Karashima, Theorethical Consideration of
Measurement of Wor-Hardening and Recovery Rates during high
temperature Creep, Trans. Japan Inst. Metals, 16 (1975) 11,
671–678, doi:10.2320/matertrans1960.16.671
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847 847
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 7 presents instantaneous-elongation and con-
traction examples upon low changes in the stress of the
P91 steels. In the figure, the – was the instantaneous
strain under a stress decrease and the + was the in-
stantaneous strain under a stress increase.
The relationship between the stress increment ||
and the instantaneous strain |E| is presented in Fig-
ure 7. The creep demonstrates a viscous behavior be-
cause the absolute values of the instantaneous strain for
the load increase were equal to the values for the load
decrease.
Two types of creep at low stress exist. One is creep
controlled by the diffusion including the lattice diffusion
creep (Nabarro-Herring type)22 at a high-T and grain
boundary diffusion creep (Coble type)22 at an inter-
mediate-T. The other is creep associated with dislocation
movement including free flight motion (climb con-
trolled) and viscous motion (glide controlled)23. When
creep is controlled by the diffusion, the creep should be a
non-viscous behavior. In this case, the absolute values of
the instantaneous strain for a small-load increase should
be apparently larger than the values for a small-load
decrease. If creep is controlled by free flight motion of
dislocation, plastic strain can occur instantaneously
when the stress is increased by a small amount. There-
fore, creep shows non-viscous behavior. When creep is
controlled by the viscous glide of dislocations, instant-
aneous plastic strain does not occur even if the applied
stress is increase suddenly. Thus, creep shows viscous
behavior. In this study, the creep demonstrates a viscous
behavior, because non instantaneous strain is observed
during the stress increase. It means creep may be con-
trolled by the viscous glide of dislocations.
Figure 8 gives an example of the anelastic backflow,
observed for a high reduction in the stress during the
transient-creep stage. Specifically, the helicoidal spring
specimen was loaded at 27.75 MPa and 923 K for 120 h
and consequently unloaded all the stress for 120 h. Two
different parts of the creep strain existed: the anelastic
strain was reversible, which is presented with solid
double arrows, whereas the plastic strain was irreversible
and it is presented with dashed double arrows. Therefore,
the instantaneous strain that occurred under a sudden
stress change included both elastic and anelastic compo-
nents.
4 CONCLUSIONS
The short-term creep behavior in the P91 heat-resis-
tant steels was investigated using instantaneous-stress-
change tests. The results were as follows:
1) The creep deformation consisted of the primary creep
stages, but no secondary creep stage was observed.
2) The Blackburn equation could be utilized for the
creep-curve description because it provided an im-
proved representation of the experimental creep
curve.
3) The viscous glide of dislocations might have been the
dominant creep mechanism because the creep dis-
played a viscous behavior.
4) The short-term creep at a low stress displayed an
anelastic behavior.
Acknowledgment
This project was supported by the National Natural
Science Foundation of China (Grant No. 51605330).
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22 W. D. Nix, Effects of Grain Shape on Nabarro-Herring and Coble
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International Journal of Pressure Vessels and Piping, 24 (2009) 9,
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24 K. Maruyama, S. Karashima, Theorethical Consideration of
Measurement of Wor-Hardening and Recovery Rates during high
temperature Creep, Trans. Japan Inst. Metals, 16 (1975) 11,
671–678, doi:10.2320/matertrans1960.16.671
J. ZHE et al.: SHORT-TERM CREEP OF P91 HEAT-RESISTANT STEELS AT LOW STRESSES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 843–847 847
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS