Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel
Effects of ausforming through upsetting at 890 °C and following tempering on microstructure and mechanical properties under uniaxial tension of 12CrWMoNbVB ferritic/martensitic steel are studied. Electron microscopic investigations along with elemental analysis of the steel samples in various states...
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| Zitieren: | Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel / I.F. Kislyak, H.Yu. Rostova, N.F. Andrievskaya, A.S. Kalchenko, V.S. Okovit, M.A. Tikhonovsky, R.L. Vasilenko, I.G. Tantsiura, V.A. Panov // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 76-84. — Бібліогр.: 22 назв. — англ. |
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Kislyak, I.F. Rostova, H.Yu. Andrievskaya, N.F. Kalchenko, A.S. Okovit, V.S. Tikhonovsky, M.A. Vasilenko, R.L. Tantsiura, I.G. Panov, V.A. 2023-12-07T10:54:10Z 2023-12-07T10:54:10Z 2022 Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel / I.F. Kislyak, H.Yu. Rostova, N.F. Andrievskaya, A.S. Kalchenko, V.S. Okovit, M.A. Tikhonovsky, R.L. Vasilenko, I.G. Tantsiura, V.A. Panov // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 76-84. — Бібліогр.: 22 назв. — англ. 1562-6016 https://nasplib.isofts.kiev.ua/handle/123456789/195831 621.789 DOI: https://doi.org/10.46813/2022-137-076 Effects of ausforming through upsetting at 890 °C and following tempering on microstructure and mechanical properties under uniaxial tension of 12CrWMoNbVB ferritic/martensitic steel are studied. Electron microscopic investigations along with elemental analysis of the steel samples in various states provided detailed information about subgrain structure and typical precipitates, the latter mainly М₂₃С₆ and NbC carbides. Strength and plasticity parameters (σ₀, σb, δ) are examined and shown to enhance under applied thermomechanical treatment as compared to standard characteristics of the steel. Preferred treatment of the steel is preliminarily evaluated. Досліджено вплив аусформінгу шляхом осаджування при 890 °C та подальшого відпускання на мікроструктуру і механічні властивості за одновісного розтягу феритомартенситної сталі 12CrWMoNbVB (18Х12ВМБФР – за ГОСТ 5632-72). Електронно-мікроскопічні дослідження, а також елементний аналіз зразків сталі в різних станах дозволили отримати детальну інформацію про субзеренну структуру та типові виділення. Останні переважно представлені карбідами М₂₃С₆ та NbC. Досліджено параметри міцності та пластичності (σ₀, σb, δ), котрі покращуються після застосованої обробки у порівнянні зі стандартними характеристиками сталі. Попередньо оцінено переважну обробку сталі. Исследовано влияние аусформинга путем осадки при 890 °C и последующего отпуска на микроструктуру и механические свойства при одноосном растяжении ферритомартенситной стали 12CrWMoNbVB (18Х12ВМБФР – по ГОСТ 5632-72). Электронно-микроскопические исследования, а также элементный анализ образцов стали в различных состояниях позволили получить подробную информацию о субзеренной структуре и типичных выделениях. Последние в основном представлены карбидами М₂₃С₆ и NbC. Исследованы параметры прочности и пластичности (σ₀, σb, δ), которые улучшаются при применяемой термомеханической обработке по сравнению со стандартными характеристиками стали. Предварительно оценивается предпочтительная обработка стали. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Physics and technology of structural materials Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel Вплив аусформінгу на механічні властивості 12%Cr феритомартенситної сталі Влияние аусформинга на механические свойства 12%Cr ферритомартенситной стали Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
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| title |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel |
| spellingShingle |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel Kislyak, I.F. Rostova, H.Yu. Andrievskaya, N.F. Kalchenko, A.S. Okovit, V.S. Tikhonovsky, M.A. Vasilenko, R.L. Tantsiura, I.G. Panov, V.A. Physics and technology of structural materials |
| title_short |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel |
| title_full |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel |
| title_fullStr |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel |
| title_full_unstemmed |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel |
| title_sort |
effect of ausforming on mechanical properties of 12%cr ferritic/martensitic steel |
| author |
Kislyak, I.F. Rostova, H.Yu. Andrievskaya, N.F. Kalchenko, A.S. Okovit, V.S. Tikhonovsky, M.A. Vasilenko, R.L. Tantsiura, I.G. Panov, V.A. |
| author_facet |
Kislyak, I.F. Rostova, H.Yu. Andrievskaya, N.F. Kalchenko, A.S. Okovit, V.S. Tikhonovsky, M.A. Vasilenko, R.L. Tantsiura, I.G. Panov, V.A. |
| topic |
Physics and technology of structural materials |
| topic_facet |
Physics and technology of structural materials |
| publishDate |
2022 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Вплив аусформінгу на механічні властивості 12%Cr феритомартенситної сталі Влияние аусформинга на механические свойства 12%Cr ферритомартенситной стали |
| description |
Effects of ausforming through upsetting at 890 °C and following tempering on microstructure and mechanical properties under uniaxial tension of 12CrWMoNbVB ferritic/martensitic steel are studied. Electron microscopic investigations along with elemental analysis of the steel samples in various states provided detailed information about subgrain structure and typical precipitates, the latter mainly М₂₃С₆ and NbC carbides. Strength and plasticity parameters (σ₀, σb, δ) are examined and shown to enhance under applied thermomechanical treatment as compared to standard characteristics of the steel. Preferred treatment of the steel is preliminarily evaluated.
Досліджено вплив аусформінгу шляхом осаджування при 890 °C та подальшого відпускання на мікроструктуру і механічні властивості за одновісного розтягу феритомартенситної сталі 12CrWMoNbVB (18Х12ВМБФР – за ГОСТ 5632-72). Електронно-мікроскопічні дослідження, а також елементний аналіз зразків сталі в різних станах дозволили отримати детальну інформацію про субзеренну структуру та типові виділення. Останні переважно представлені карбідами М₂₃С₆ та NbC. Досліджено параметри міцності та пластичності (σ₀, σb, δ), котрі покращуються після застосованої обробки у порівнянні зі стандартними характеристиками сталі. Попередньо оцінено переважну обробку сталі.
Исследовано влияние аусформинга путем осадки при 890 °C и последующего отпуска на микроструктуру и механические свойства при одноосном растяжении ферритомартенситной стали 12CrWMoNbVB (18Х12ВМБФР – по ГОСТ 5632-72). Электронно-микроскопические исследования, а также элементный анализ образцов стали в различных состояниях позволили получить подробную информацию о субзеренной структуре и типичных выделениях. Последние в основном представлены карбидами М₂₃С₆ и NbC. Исследованы параметры прочности и пластичности (σ₀, σb, δ), которые улучшаются при применяемой термомеханической обработке по сравнению со стандартными характеристиками стали. Предварительно оценивается предпочтительная обработка стали.
|
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/195831 |
| citation_txt |
Effect of ausforming on mechanical properties of 12%Cr ferritic/martensitic steel / I.F. Kislyak, H.Yu. Rostova, N.F. Andrievskaya, A.S. Kalchenko, V.S. Okovit, M.A. Tikhonovsky, R.L. Vasilenko, I.G. Tantsiura, V.A. Panov // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 76-84. — Бібліогр.: 22 назв. — англ. |
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76 ISSN 1562-6016. ВАНТ. 2022. №1(137)
https://doi.org/10.46813/2022-137-076
UDC 621.789
EFFECT OF AUSFORMING ON MECHANICAL PROPERTIES
OF 12%Cr FERRITIC/MARTENSITIC STEEL
I.F. Kislyak, H.Yu. Rostova, N.F. Andrievskaya, A.S. Kalchenko, V.S. Okovit,
M.A. Tikhonovsky, R.L. Vasilenko, I.G. Tantsiura, V.A. Panov
National Science Center “Kharkov Institute of Physics and Technology”, Kharkiv, Ukraine
E-mail: kislyak@kipt.kharkov.ua
Effects of ausforming through upsetting at 890 °C and following tempering on microstructure and mechanical
properties under uniaxial tension of 12CrWMoNbVB ferritic/martensitic steel are studied. Electron microscopic
investigations along with elemental analysis of the steel samples in various states provided detailed information
about subgrain structure and typical precipitates, the latter mainly М23С6 and NbC carbides. Strength and plasticity
parameters (σ0, σb, δ) are examined and shown to enhance under applied thermomechanical treatment as compared
to standard characteristics of the steel. Preferred treatment of the steel is preliminarily evaluated.
INTRODUCTION
Heat-resistant 9…12%Cr steels of the
ferritic/martensitic class are promising structural
materials for cores and vessel internals of new
generation nuclear and thermonuclear power reactors
[1–8]. However, the operating temperature range limits
the use of steels of this class. On the one hand, they
have a tendency to low-temperature embrittlement
under radiation exposure, and on the other hand, they
have a low value of heat resistance at a level of
~ 500 °C [1, 3, 8].
To increase the thermal stability of these steels with
the aim of their further use in the power industry, it is
necessary to increase their heat resistance and simul-
taneously reduce the temperatures of low-temperature
embrittlement (brittle-ductile transition) [4, 9, 10]. The
main way to solve the low thermal resistance is to select
the modes of thermomechanical treatments (TMT) in
order to create more stable structure/phase states and
improved properties in ferritic/martensitic steels. This
will provide favorable conditions for the operation of
the planned nuclear and thermonuclear power reactors.
One of the simple, from a technological point of
view, TMT methods is high-temperature thermome-
chanical treatment (HTMT), which in the English
literature is called ausforming. HTMT or ausforming is
the heating of steel to temperatures of stability of
austenite, deformation at these temperatures (or
temperatures of austenite metastability) and subsequent
quenching for martensite.
According to [11], ausforming in high-strength
steels leads to significant hardening of martensite. Thus,
the study of steel with 13%Cr and 0.3%C showed that
ausforming leads to changes in the tempering stage of
martensite and promotes the precipitation of fine
carbides [12].
It was shown in [13] that HTMT of reactor steel EK-
181 leads to a significant increase in the dislocation
density in the martensite structure, as well as to an
increase in the dispersity and volume fraction of
vanadium carbonitride nanoparticles, which nucleate
directly during deformation in the temperature range of
the existence of austenite and subsequent quenching.
These processes provide a significant (by
≈ 300 MPa) increase in the values of the yield strength
of steel at room temperature. Subsequent tempering at
T = 720 °C (for 1 h) after HTMT also leads to a
significant (compared to standard heat treatment (HT))
increase in the values of the yield strength of steel both
at room temperature (by Δσ0.1 ≤ 290 MPa), and at
elevated temperature (T = 650 °C, by Δσ0.1 ≤ 100 MPa).
In this case, the plasticity remains at a fairly high level
(δ ≈ 10…13%) [13].
It was shown in [14] for steel Grade 91 (T91) that at
room and at elevated test temperatures after HTMT, an
increase in short-term mechanical properties is
observed. So, after HTMT, the value of the yield
strength of steel at 20 °C is 931 MPa (in the original
steel it was 520 MPa), and at 550 °C it is 564 MPa (in
the original steel it was 390 MPa). In this case, the
width of the martensite laths after HTMT is signify-
cantly smaller than after standard HT, and the sizes of
M23C6 carbides after ausforming are comparable to their
sizes after standard HT, while the sizes of MX carbides
(carbonitrides) are much smaller.
In this regard, in a number of works [12, 1517], it
is proposed to use thermomechanical treatment in
relation to 9…12%Cr ferritic/martensitic steels to
increase their high-temperature strength.
In this work, we investigated a steel of this class,
12CrWMoNbVB (EI993), which is used for the manu-
facture of products operating at temperatures up to
620 °C [18, 19]. To increase its strength characteristics,
the ausforming method was used, and a comprehensive
study of the effect of HTMT on the microstructure and
mechanical properties of the 12CrWMoNbVB steel was
carried out.
1. MATERIALS AND METHODS
Industrial steel 12CrWMoNbVB was used as a
starting material. Steel compositions according to
GOST 5632-72 and according to our measurements are
given in Table 1. As one can see, in contrast to the
standard content of elements in this steel, there are
traces of aluminum, cobalt, arsenic and a noticeable
content of nickel in our sample.
mailto:kislyak@kipt.kharkov.ua
ISSN 1562-6016. ВАНТ. 2022. №1(137) 77
Table 1
Content of elements in the test steel
Symbol Element
Composition according
to GOST 5632-72, wt.%
Our measurements,
wt.%
B Boron Up to 0.003 0.002
C Carbon 0.15…0.22 0.205
N Nitrogen – –
Al Aluminum – 0.029
Si Silicon Up to 0.5 0.244
P Phosphorus Up to 0.03 0.033
S Sulfur Up to 0.025 0.0075
V Vanadium 0.15…0.30 0.18
Cr Chromium 11.0…13.0 11.69
Mn Manganese Up to 0.5 0.21
Fe Iron rest rest
Ni Nickel – 0.59
Co Cobalt – 0.043
Cu Copper Up to 0.3 0.077
As Arsenic – 0.0035
Nb Niobium 0.2…0.4 0.24
Mo Molybdenum 0.4…0.6 0.42
W Tungsten 0.4…0.7 0.38
A cylindrical steel specimen Ø 20 mm was heated in
a vacuum chamber to a temperature of 1250 °C and held
for 10 min, and then cooled with the camera. After that,
the sample was subjected to ausforming. This treatment
included heating in air to ~ 890 °C, holding at this
temperature for 15 min, and following upsetting to
Ø 29 mm. The degree of true deformation of the steel
was e = ln(S/S0) ≈ 0.7 (S0 is the initial and S is the final
cross section of the sample).
By the electroerosion method, parallel to the axis of
the cylindrical billets of the initial steel and steel after
ausforming, specimens in the form of dog bone were cut
for testing under uniaxial tension. After that, the
samples were mechanically ground and polished. The
dimensions of the working area of the samples ready for
testing were ≈ 0.5×1.0×5.7 mm. The samples from the
ausformed steel were studied both in the initial state and
after tempering. This treatment was carried out in an
argon atmosphere at three modes: 720 °C for 3 h;
665 °C for 3 h; 550 °С for 25 h, with further cooling of
the samples in the air.
Uniaxial tensile tests were carried out on a tensile
testing machine equipped with a vacuum camera and a
furnace for heating the objects under study. The samples
were stretched at an average strain rate of 1·10
-3
s
-1
at
four temperatures: -196 (in liquid nitrogen); 20; 550,
and 650 °C (the last two tests were carried out in
vacuum). The microhardness of the samples was studied
using a PMT-3 microhardness tester with a load on the
indenter of 2 N, the holding time was 15 s.
Microstructural studies were carried out on an
Olympus GX51 metallographic microscope and a JSM
7001F scanning electron microscope equipped with an
INCA ENERGY 350 X-ray spectrum analyzer. The fine
structure of the samples was studied using a JEM-2100
transmission electron microscope (TEM) (accelerating
voltage 200 kV) equipped with a STEM JEOL EM-
24511 scanning attachment and a JEOL EX-24063 JGP
energy-dispersive X-ray microanalyzer. Samples for
TEM studies were prepared by standard jet electro-
polishing on a Tenupol installation in an electrolyte
containing 800 ml of C2H5OH, 100 ml of HClO4,
100 ml of C3H8O3, and a voltage of 40 V at room
temperature. Parameters of subgrain structure and
carbide precipitates (average subgrain sizes; size,
volume fraction, and density of carbides) were
determined by computer processing of microstructure
images and subsequent data calculation in the ImageJ
program.
2. RESULTS AND DISCUSSION
2.1. STRUCTURE
In the course of research, it was found that the
strength characteristics of the initial steel, as delivered,
significantly differ from the reference ones [20, 21]: σ0
values are lower than the reference ones by 40% or
more, σb values, too, lower by ≥ 20%. On the contrary,
the δ values exceed the reference values by 8% or more,
up to ~ 70%. In this regard, it was decided to perform
heat treatment of the steel in order to bring it to a
standard state [19, 21], namely, to process it according
to the following scheme: quenching in oil from 1050 °C
(15 min), subsequent tempering at 720 °C, for 3 h, and
air cooling. This material and samples from it are
hereinafter referred to as EI993+TT, or standard.
Subsequent tests have confirmed the correctness of this
approach, which will be discussed later.
The structure of a standard sample is tempered
martensite with an average transverse lath size of
220 nm and is shown in Fig. 1.
78 ISSN 1562-6016. ВАНТ. 2022. №1(137)
Fig. 1. The 12CrWMoNbVB steel structure in the
standard state
After upsetting at temperature of 890 °C with the
true deformation of 0.7, a banding microstructure with a
pronounced direction of deformation and a transverse
size of subgrains of 260 nm is observed (Fig. 2,a). In
some areas, there is no clearly pronounced direction, the
grains are oriented chaotically (see Fig. 2,b).
a
b
Fig. 2. The 12CrWMoNbVB steel structure after
ausforming
The structure of steel samples after ausforming and
subsequent tempering at three temperatures is shown in
Fig. 3.
In all three modes of heat treatment, a subgrain
structure with an initial stage of recrystallization is
observed, when, along with the formed larger grains
with clear boundaries, practically without dislocations
in the grain field, there are also subgrains of the initial
state 5–7 times smaller in size. There are noticeably
more areas with a smaller subgrain after tempering at
550 °С for 25 h than after tempering at 720 °С for 3 h
(Fig. 4). This structure can be called bimodal.
In electron microscopic images with a fine subgrain
structure, dark particles are visible, located mainly
along the boundaries of subgrains and lamellas (see
Fig. 3). Some of them are hardly detected due to the
increased density of subgrain boundaries and
dislocations. These particles are more clearly visible
when examining the samples in an electron microscope
in the STEM mode (see Fig. 4).
a
b
c
Fig. 3. TEM-structure of the 12CrWMoNbVB steel after
ausforming and subsequent temperings at 720 °С
for 3 h (а); 665 °С for 3 h (b); 550 °С for 25 h (c)
a b
Fig. 4. STEM-images of the samples after ausforming
and subsequent temperings at 720 °С for 3 h (а)
and 550 °С for 25 h (b)
On the basis of computer processing of
microstructure images and data calculation in the
ImageJ program, the structural characteristics of steel
samples in various states are determined. The obtained
data are summarized in Table 2. An energy-dispersive
X-ray microanalyzer was used to precisely study the
composition of the particles. Fig. 5 shows an image of a
ISSN 1562-6016. ВАНТ. 2022. №1(137) 79
section of a standard sample with one large dark particle
and many smaller ones.
Analysis of a large dark precipitate (Table 3, No. 1)
revealed that it consists mainly of niobium and a small
along the boundaries of subgrains and lamellas (see
Fig. 3). Some of them are hardly detected due to the
increased density of subgrain boundaries and
dislocations. These particles are more clearly visible
when examining the samples in an electron microscope
in the STEM mode (see Fig. 4).
Table 2
Structural characteristics of steel samples in various states
State EI993+TT
Subsetting+
tempering at 720 °С
for 3 h
Subsetting+
tempering at
665 °С for 3 h
Subsetting+
tempering at
550 °С for 25 h
М23С6
ρ, m
-3
7.2∙10
19
4∙10
19
5.5∙10
19
8.8∙10
19
d, nm 100 128 141 86
V, % 3.6 3.8 6.0 3.5
NbC
ρ, m
-3
1.8∙10
18
3.5∙10
17
9∙10
17
–
d, nm 343 385 506 –
V, % 1.5 0.3 0.2 –
Average size of subgrains
and lamellae, nm
220 310 290 305
Fig. 5. Carbide precipitates in the standard sample
Fig. 6. STEM-image of a sample after
tempering at 720 °C for 3 h
Table 3
Elemental composition of precipitates in the standard sample (see Fig. 5)
No.
at.%
C Si V Cr Mn Fe Nb Mo W
1 13.03 0.24 0.42 14.67 0.36 13.82 51.16 6.27 0.03
2 17.45 0.00 0.00 53.58 1.43 23.77 0.00 2.34 1.42
No
wt.%
C Si V Cr Mn Fe Nb Mo W
1 2.20 0.09 0.30 10.75 0.28 10.87 66.95 8.47 0.08
2 4.29 0.00 0.00 57.00 1.61 27.16 0.00 4.59 5.35
Analysis of a large dark precipitate (see Table 3,
No. 1) revealed that it consists mainly of niobium and a
small amount of chromium and molybdenum, while
small ones (see Table 3, No. 2) are composed of
chromium, iron, traces of manganese, molybdenum, and
tungsten. Fig. 6 gives a view of a sample area after
tempering at 720 °C for 3 h, and Table 4 shows the
elemental composition of individual particles (No. 1–3)
and matrix (No. 4). The particles show an increased
content of chromium, iron, the presence of manganese,
molybdenum and tungsten, which is typical for M23C6
carbides. The presence of all alloy elements with
predominance of iron, low chromium content, but with
out manganese and tungsten is determined in the matrix.
Chromium, manganese, and tungsten actively interact
with carbon to form carbides.
In scanning mode, cartograms were taken from some
areas of the studied samples. Fig. 7 shows a cartogram
of a sample area after tempering at 665 °C for 3 h.
Chromium, iron, molybdenum are concentrated in small
particles, and a significant part of the large particle is
niobium with the presence of molybdenum, vanadium,
and tungsten. In this case, carbon is not determined
unambiguously due to its low atomic weight. Electron
diffraction patterns confirm that large particles are
carbides of the MC type, where, in our case, niobium is
the predominant M-element, and small particles are
carbides of the M23C6 type.
80 ISSN 1562-6016. ВАНТ. 2022. №1(137)
Table 4
Elemental composition of precipitates in a sample after tempering at 720 °С for 3 h (see Fig. 6)
No.
at.%
C Si V Cr Mn Fe Nb Mo W
1 40.53 0.83 0.00 36.39 1.80 16.64 0.69 2.27 0.84
2 32.90 0.00 0.00 41.07 1.18 22.48 0.00 1.82 0.55
3 22.91 0.00 0.48 30.21 0.00 45.29 0.00 0.71 0.40
4 28.62 0.60 0.86 11.05 0.00 56.38 0.98 1.48 0.04
No.
wt.%
C Si V Cr Mn Fe Nb Mo W
1 12.59 0.61 0.00 48.93 2.56 24.03 1.65 5.64 4.00
2 9.57 0.00 0.00 51.75 1.56 30.42 0.00 4.23 2.46
3 6.06 0.00 0.54 34.59 0.00 55.69 0.00 1.50 1.62
4 7.87 0.38 1.00 13.16 0.00 72.10 2.09 3.24 0.15
BF 1.0 µm C K 1.0 µm V K 1.0 µm
Cr K 1.0 µm Fe K 1.0 µm Nb L 1.0 µm
Mo L 1.0 µm W M 1.0 µm
Fig. 8 shows a sample area after tempering at 550 °C
for 25 h, and Table 5 shows the elemental composition
of precipitates and matrix. In all small dark precipitates
(No. 1–3, 5, 6), a large amount of chromium with the
presence of matrix iron, as well as small amount of
molybdenum, niobium and tungsten, is determined. In
the matrix (No. 4), iron predominates with the presence
of chromium and molybdenum. Carbide-forming
Fig. 7. Elements distribution
in precipitations after tempering
at 665 °C for3 h
ISSN 1562-6016. ВАНТ. 2022. №1(137) 81
elements niobium and tungsten are not found in the
matrix, because they diffuse to precipitates.
Let us refer to Table 2, which shows the average
dimensions (d), density (ρ), and volume content (V) of
carbides at different tempering regimes. In all samples,
the density of М23С6 particles is 1.52 orders of
magnitude higher than the density of NbС precipitates,
and the size is three times smaller. Comparison of
various tempering regimes with each other leads to the
conclusion that the size of М23С6 carbides increases
from 85 to 135 nm with an increase in the tempering
temperature from 550 to 720 °С, and their density
decreases from 8.8∙10
19
to 4∙10
19
m
-3
. Apparently, this
can be explained by the combined action of diffusion
processes of dissolution and fusion of carbides.
Fig. 8. Carbide precipitates in a sample after
tempering at 550 °C for 25 h
Table 5
Elements distribution in a sample after tempering at 550°C for 25 h
No
at.%
C Si V Cr Mn Fe Nb Mo W
1 59.56 0.74 0.00 13.13 0.00 25.36 0.06 1.15 0.00
2 50.50 0.45 0.00 26.07 0.18 20.66 0.16 1.43 0.56
3 50.27 0.89 0.00 20.08 0.00 27.14 0.29 1.12 0.22
4 0.00 1.10 0.13 12.77 0.00 85.42 0.00 0.58 0.00
5 56.83 1.18 0.00 25.59 0.10 14.61 0.31 1.15 0.23
6 51.05 1.23 0.00 28.34 0.44 17.68 0.13 0.69 0.44
No
wt.%
C Si V Cr Mn Fe Nb Mo W
1 24.24 0.70 0.00 23.13 0.00 47.99 0.18 3.75 0.00
2 17.88 0.37 0.00 39.94 0.28 34.01 0.45 4.05 3.02
3 17.95 0.74 0.00 31.04 0.00 45.06 0.80 3.18 1.22
4 0.56 0.12 12.01 0.00 86.30 0.00 1.01 0.00
5 22.38 1.09 0.00 43.63 0.17 26.76 0.94 3.61 1.41
6 18.63 1.05 0.00 44.78 0.74 30.00 0.37 2.00 2.44
2.2. MECHANICAL PROPERTIES
The obtained experimental loading curves made it
possible to calculate the characteristics of strength (σ0,
the proportionality limit, σb, the ultimate tensile
strength) and plasticity (δ, the rupture elongation) of the
specimens under study.
The final results of examining samples for uniaxial
tension are shown in Fig. 9, as well as in Table 6; the
latter also contains data on the microhardness of the
samples. These results indicate the following:
1) the strength characteristics of materials decrease
monotonically as the test temperature rises (see Fig.
9,a,b);
2) upsetting led to an increase in steel strength
characteristics relative to their values for EI993+TT
steel in the range from ~ 10 to ~ 60%, depending on the
test temperature;
3) tempering after ausforming led to a drop in the
values of σ0 and σb in the entire temperature range of
research, but nevertheless these characteristics remained
predominantly higher than in steel EI993+TT;
4) the relative elongation of the sample after rupture,
δ, in steel after ausforming decreases relative to the
values in standard steel (see Fig. 9,c);
5) tempering increases the δ values, and at high test
temperatures they exceed those in the standard steel.
For greater clarity and the possibility of comparison
with the GOST indicators, the experimental results on
the mechanical properties of steel are presented in the
temperature range above 0°C (see Fig. 10). Fig. 10
demonstrates a good compliance of the strength
parameters of standard steel with the GOST conditions,
as mentioned earlier. Somewhat worse agreement is
observed for plasticity.
Fig. 10 illustrates well the effect of upsetting
(ausforming) and subsequent tempering on the
mechanical characteristics of steel. It can be seen that
these characteristics exceed the requirements of GOST
in terms of strength indicators at all temperatures,
including the region of high operating temperatures. The
plasticity of the studied samples in the range of
operating temperatures remains either at the level of
GOST requirements, or slightly decreases from them.
82 ISSN 1562-6016. ВАНТ. 2022. №1(137)
Table 6
Mechanical characteristics of samples of the 12CrWMoNbVB steel in different states
and at different test temperatures
State EI993+ТT Upsetting
Upsetting+
tempering
720 °С
for 3 h
Upsetting+
tempering
665 °С
for 3 h
Upsetting+
tempering
550 °С
for 25 h
Нμ, МPа 262 370 280 305 317
σ0,
МPа
Т, °С
-196 1240 1420 1280 1270 1390
20 680 1040 800 830 840
550 420 680 540 520 420
650 280 340 260 345 400
σb,
МPа
Т, °С
-196 1370 1520 1390 1355 1480
20 910 1170 950 990 1020
550 540 840 580 575 650
650 400 370 380 390 440
δ,
%
Т, °С
-196 10.4 7.3 13.6 – 9.5
20 11.4 5 9.3 12.2 8.4
550 >10 10 18.4 13.5 17.1
650 18 22.3 19.5 26
Fig. 9. Temperature dependences of the proportionality
limit σ0 (a), the ultimate strength σb (b), and the rupture
elongation δ (c) of steel samples in different states
The obtained experimental results on the mechanical
properties (Figs. 9, 10, and Table 6) quite convincingly
demonstrate that, among the three modes of heat
treatment after ausforming, the best combination of
mechanical properties is provided by tempering at
550 °C for 25 h.
Let us analyze the obtained results using the expres-
sion for the yield stress of an alloy according to the rule
of mixtures:
σ0 = σf + σss + σd + σcp + σgb, (1)
Fig. 10. Dependences of the proportionality limit σ0 (a),
the ultimate strength σb (b) and the rupture elongation
δ (c) of steel samples in different states on the test
temperature T > 0 °C and the area of the corresponding
characteristics according to GOST (shaded zones)
where σf is the stress caused by the friction of
dislocations when moving in iron; σss is solid solution
hardening associated with interstitial and substitution
atoms; σd is dislocation hardening; σcp is hardening due
to precipitation of carbides, and σgb is grain boundary
hardening. Due to the lack of the entire volume of nece-
ssary data, it is impossible to give a quantitative esti-
mate of σ0, but a qualitative assessment of the change in
this characteristic can be made. Note that the values of
σf and σss are constant and are determined by the atomic
structure of iron and chemical composition of the steel.
T, °C
T, °C
ISSN 1562-6016. ВАНТ. 2022. №1(137) 83
The dislocation density in the samples was not deter-
mined. The average size of subgrains in the samples
after tempering is practically the same (see Table 2).
Considering these factors, the analysis of formula (1) is
reduced to the analysis of behavior of the “carbide”
term σcp. As noted earlier, in the steel under study, the
density of M23C6 precipitates is 1.5–2 orders of
magnitude higher than the density of NbC precipitates,
and the size is three times less (see Table 2).
Consequently, all other things being equal, it is the
M23C6 precipitates that should play a decisive role in the
evolution of σ0.
According to the model of dispersed barrier harde-
ning [22],
σcp~(ρd)
1/2
, (2)
where ρ is the density of precipitates (barriers), d is their
average diameter. If we refer to Table 2, we can see that
when tempering at 550 °C for 25 h, the highest density
of carbide precipitates М23С6 (ρ = 8.8∙10
19
m
-3
) with the
smallest average diameter (d = 86 nm) is observed.
Direct calculation by relation (2) gives the following
values (aside from the dimension): 27, 27, and 22 for
the tempering modes 550 °C, 25 h; 665 °C, 3 h, and
720 °C, 3 h, respectively. Taking into account the strong
simplification of the assessment, these results can be
considered to be quite strong evidence in favor of the
550 °C, 25 h tempering regime.
CONCLUSIONS
1. A complex of electron microscopic studies of
steel was carried out, which provided information on the
subgrain structure, type and parameters of precipitates
in steel samples in different states. The main preci-
pitates are carbides of the М23С6 and NbC types, and the
density of the former is 1.5–2 orders of magnitude
higher than that of the latter, depending on the state of
the steel.
2. Tests for uniaxial tension of samples of the steel
12CrWMoNbVB, which underwent preliminary ausfor-
ming by upsetting at temperature of about 890 °C and
subsequent tempering in three modes (720 °C for 3 h;
665 °C for 3 h; 550 °C for 25 h), were carried out. The
samples were investigated at four temperatures: -196 (in
liquid nitrogen); 20; 550, and 650°C (the last two tests
were carried out in vacuum). Measurements of the
microhardness of these samples at room temperature
were also performed.
These studies made it possible to determine the
characteristics of the strength and plasticity of steel
samples in different states: the proportionality limit σ0,
the ultimate tensile strength σb, and the rupture
elongation δ.
It has been established that ausforming and sub-
sequent tempering provide an increase in σ0 and σb of
the steel relative to their values for the standard steel
(according to GOST requirements) in the entire range of
research temperatures. In this case, the ductility δ of the
treated steel in the region of high operating temperatures
also increases.
3. The best combination of mechanical properties,
especially in the area of high operating temperatures, is
achieved after tempering at 550 °C for 25 h. In this case,
precipitates of carbides of the М23С6 type can play a
significant role.
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Article received 06.12.2021
ВЛИЯНИЕ АУСФОРМИНГА НА МЕХАНИЧЕСКИЕ СВОЙСТВА
12%Cr ФЕРРИТОМАРТЕНСИТНОЙ СТАЛИ
И.Ф. Кисляк, А.Ю. Ростова, Н.Ф. Андриевская, А.С. Кальченко, В.С. Оковит,
М.А. Тихоновский, Р.Л. Василенко, И.Г. Танцюра, В.А. Панов
Исследовано влияние аусформинга путем осадки при 890 °C и последующего отпуска на микроструктуру
и механические свойства при одноосном растяжении ферритомартенситной стали 12CrWMoNbVB
(18Х12ВМБФР – по ГОСТ 5632-72). Электронно-микроскопические исследования, а также элементный
анализ образцов стали в различных состояниях позволили получить подробную информацию о субзеренной
структуре и типичных выделениях. Последние в основном представлены карбидами М23С6 и NbC.
Исследованы параметры прочности и пластичности (σ0, σb, δ), которые улучшаются при применяемой
термомеханической обработке по сравнению со стандартными характеристиками стали. Предварительно
оценивается предпочтительная обработка стали.
ВПЛИВ АУСФОРМІНГУ НА МЕХАНІЧНІ ВЛАСТИВОСТІ
12%Cr ФЕРИТОМАРТЕНСИТНОЇ СТАЛІ
І.П. Кісляк, Г.Ю. Ростова, Н.Ф. Андрієвська, О.С. Кальченко, В.С. Оковiт,
М.А. Тихоновський, Р.Л. Василенко, І.Г. Танцюра, В.А. Панов
Досліджено вплив аусформінгу шляхом осаджування при 890 °C та подальшого відпускання на
мікроструктуру і механічні властивості за одновісного розтягу феритомартенситної сталі 12CrWMoNbVB
(18Х12ВМБФР – за ГОСТ 5632-72). Електронно-мікроскопічні дослідження, а також елементний аналіз
зразків сталі в різних станах дозволили отримати детальну інформацію про субзеренну структуру та типові
виділення. Останні переважно представлені карбідами М23С6 та NbC. Досліджено параметри міцності та
пластичності (σ0, σb, δ), котрі покращуються після застосованої обробки у порівнянні зі стандартними
характеристиками сталі. Попередньо оцінено переважну обробку сталі.
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