L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
541–549
EFFECTS OF GRAIN SIZE ON THE DYNAMIC
RECRYSTALLIZATION OF 25Cr2Ni4MoV STEEL FOR A
SUPER-LARGE NUCLEAR-POWER ROTOR
VPLIV VELIKOSTI KRISTALNIH ZRN NA DINAMI^NO
REKRISTALIZACIJO JEKLENE ZLITINE 25Cr2Ni4MoV ZA
OGROMNI ROTOR JEDRSKE ELEKTRARNE
Li-yan Ye, Yue-wen Zhai*, Le-yu Zhou, Xiao-mao He, Peng Jiang
Beijing Research Institute of Mechanical and Electrical Technology, Science and Technology Development and Innovation Center, 18 Xueqing
Road, Haidian District, Beijing 100083, P. R. China
Prejem rokopisa – received: 2019-11-17; sprejem za objavo – accepted for publication: 2020-03-23
doi:10.17222/mit.2019.281
The effects of grain size on the dynamic recrystallization of 25Cr2Ni4MoV steel for a super-large nuclear-power rotor were in-
vestigated on a Gleeble-1500 thermal-mechanical simulation tester with the temperature range of 1373 K-1523 K and the strain
rate range of 0.005–0.5 s
–1
. It was found that the flow stresses with fine grains are greater than those with coarse grains under
certain deformation conditions. At high temperatures and strain rates, the DRX of the alloy with coarse grains is complete,
while that of the alloy with fine grains is incomplete. In particular, the smallest DRX grain size is about 22 μm. The reconsti-
tuted microstructure by DRX with an initial grain size of 22 μm shows no refinement. Finally, it can be deduced that the model
can precisely predict the rate of DRX for 25Cr2Ni4MoV steel by comparing experimental results and predicted results about the
flow stress.
Keywords: 25Cr2Ni4MoV steel, flow stress, grain size, dynamic recrystallization
Avtorji ~lanka so raziskovali vpliv velikosti kristalnih zrn na dinami~no rekristalizacijo jekla 25Cr2Ni4MoV za zelo velik rotor
v jedrski elektrarni. Raziskave so izvajali na termo-mehanskem preizku{evalniku Gleeble-1500 v temperaturnem obmo~ju med
1373 K in 1523 K ter hitrostih deformacije med 0,005 s
–1
in 0,5 s
–1
. Ugotovili so, da je meja te~enja pri zlitini z drobnej{imi
kristalnimi zrni vi{ja, kot pri grobo zrnati mikrostrukturi zlitine za dane pogoje deformacije. Pri vi{ji temperaturi in hitrosti
deformacije je dinami~na rekristalizacija grobozrnate zlitine potekla popolnoma medtem, ko je bila pri finozrnati zlitini
nepopolna. Najmanj{a velikost kristalnih zrn po dinami~ni rekristalizaciji je bila okoli 22 μm. Pri zlitini z za~etno velikostjo zrn
22 μm ni pri{lo do udrobitve, t.j. zmanj{anja velikosti kristalnih zrn, po njeni dinami~ni rekristalizaciji. Avtorji v zaklju~ku {e
ugotavljajo, da so z izdelanim modelom lahko natan~no predvideli obseg dinami~ne rekristalizacije zlitine 25Cr2Ni4MoV in
napovedali njeno mejo te~enja v primerjavi z dejanskimi eksperimentalnimi rezultati.
Klju~ne besede: jeklena zlitina 25Cr2Ni4MoV, meja te~enja, velikost kristalnih zrn, dinami~na rekristalizacija
1 INTRODUCTION
A super-large nuclear-power rotor with the largest di-
ameter of 3 metres is produced by open die forging at
high temperature from a large cast ingot of about 650
tons. Owing to the super-large size, during the open-die
forging, the majority of the rotor is always at a high tem-
perature of no less than 1373K and inhomogeneous de-
formation with strain rate of no more than 0.5 s
–1
. The
movements of tools during open-die forging as one kind
of discontinuous and local forming process are clarified
into three main categories. Taking the drawing process of
the round billet, for example, the top die strikes the
workpiece, the top die feeds along the axial direction
without touching the workpiece, and the manipulator ro-
tates the workpiece. Three movements are alternated ac-
cording to open-die forging schedule.
1,2
Correspondingly,
for a certain part of the workpiece, dynamic recovery
(DRV), metadynamic recrystallization (MDRX) or static
recrystallization (SRX) can happen before the dynamic
recrystallization (DRX), which results in different initial
grain sizes for the DRX and makes the microstructure
evolution of DRX complex.
3
For example, the initial
grain size of the area with DRV is about 2 mm, while the
initial grain size of the area with complete SRX is quite
small. Due to the severe service environment, a super-
large nuclear-power rotor has strict requirements on
microstructure and properties. 25Cr2Ni4MoV has great
potential for super-large nuclear-power rotors. According
to the Hall-Petch relationship, the yield stress increases
with the inverse square root of the grain size, which
leads to fine-grain strengthening.
4
DRX with a different
initial grain size might reconstitute the microstructure
with different levels of fine grain. Therefore, to obtain
the desired property, it is important to quantitatively in-
vestigate the influence of the initial grain size on the evo-
lution of the DRX microstructure.
In recent years, many studies have been conducted to
investigate the hot-deformation behaviour of metals and
alloys.
5–10
For example, W. Wang and J. Zhao
11
deduced
Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549 541
UDK 620.1:620.192.4:669.14:621.311.25 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(4)541(2020)
*Corresponding author's e-mail:
yeliyan2006@sina.com (Zhai Yue-wen)

kinematic models of DRX to describe the dynamic
recrystallization behaviour of 20Cr2Ni4A steel, and a
dynamic recrystallization grain size model of
20Cr2Ni4A steel was also constructed. D. L. Sang and
R. D. Fu
12
investigated the dynamic microstructure evo-
lution and mechanism of the Fe-38Mn alloy during hot
deformation. It was found that the degree of the recrys-
tallization increased with the deformation temperature
and strain rate, and the occurrence temperature of full
DRX decreased with increasing strain rate. W. Wang and
R. Ma
13
established a kinetic model and a kinematic
model of DRX for 40CrNiMo. Y. K. Xu and P. Birn-
baum
14
proposed a JMAK-type DRX kinetic and dy-
namic recrystallization grain size model for 22MnB5
steel. A larger deformation degree is needed for a com-
pleted DRX of 22MnB5 steel when the deformation tem-
perature is decreased or the strain rate is increased. P.
Zhou and Q. X. Ma
15
studied the dynamic recrystal-
lization behaviour of as-cast 30Cr2Ni4MoV steel and de-
termined the kinetics model of the DRX. As one kind of
potential material for super-large nuclear-power rotors,
the DRX of 25Cr2Ni4MoV steel needs to be further
studied to guide the design of the open-die forging pro-
cess. In this paper, the rate and the progress of DRX with
initial grain sizes of 22 μm and 1970 μm was discussed
respectively during hot-compression tests.
2 EXPERIMENTAL MATERIALS AND
PROCEDURES
The chemical composition of the 25Cr2Ni4MoV
steel studied in this work is shown in Table 1. Based on
the possible manufacturing practice of the alloy, two
kinds of specimens are obtained. One is annealed sam-
ples with a grain size of 22 μm. The other is held for 2 h
at 1523 K with a grain size of 1970 μm. Compression
specimens with a diameter of 10 mm and a length of
12 mm were machined from each heat-treatment alloy
billet for hot-compression tests on Gleeble-1500 thermal
mechanical simulation tester. As shown in Figure 1, the
specimens were heated to 1523 K at a heating rate of
10 K/s and held for 5 minutes to achieve a homogeneous
austenitic microstructure, then cooled to the deformation
temperature at a rate of 10 K/s and held for 1 minute. In
order to minimize the effect of friction, graphite sheets
as the lubricant material were applied to both ends of the
specimen.
Four different forming temperatures (1373, 1423,
1473, and 1523) K and three strain rates (0.005, 0.05,
0.5) s
–1
were used in the hot compression tests. All the
specimens were compressed to a true strain of 90 %, and
then water quenched to room temperature to keep the
austenite structure after DRX. Finally, the quenched
specimens were sliced parallel to the axial direction, pol-
ished and etched using saturation picric for optical mi-
croscopic observation. As shown in Figure 2, the aver-
age grain sizes for six positions at the sliced section were
measured according to ASTM standards.
3 RESULTS AND DISCUSSION
3.1 Hot-deformation flow curves
Figure 3 shows a number of true stress-strain curves
with initial grain size of 22 μm and 1970 μm. The signs c
and f represent coarse and fine grains, respectively. It is
obvious that the initial grain size strongly influences the
flow curves. At high temperature (T 1473 K) and high
strain rate (
  0.5 s
–1
), specimens with coarse grains
show typical DRX flow stress, including a single peak
followed by a steady state, while specimens with fine
grains show no single peak, which indicates DRX is in-
complete. It is because dislocation movements including
gliding and sliding are inhibited more strongly in fine
grains than in coarse grains. When the temperature and
strain rate are constant, stress increases with decreasing
grain size, resulting in a behaviour similar to that de-
scribed by the Hall-Petch equation. The largest peak
stress difference between two kinds of specimen is
12 MPa, while steady stress difference is 7 MPa.
L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
542 Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549
Table 1: Chemical compositions of steel 25Cr2Ni4MoV (w/%)
C Mn P S Si Ni Cr Mo V Cu Fe
F 0.3 0.1 0.3  0.015  0.015  0.012 2.0 2.5 2.15 2.45 0.6 0.85  0.12  0.17 Bal.
Figure 2: Six positions at the sliced section for the optical microstruc-
ture observation
Figure 1: Experiment scheme of hot compression

3.2 DRX microstructure
Figures 4 and 5 shows the optical microstructures at
the central area of the section (around the position 5 in
Figure 2) with a strain rate of 0.05 s
–1
and temperatures
of (1373, 1423, 1473, and 1523) K, respectively. DRX
happens completely for all the specimens at a strain rate
of 0.05 s
–1
as indicated by the flow curves. For an initial
grain size of 22 μm, the average grain sizes (obtained
from the positions 1 to 6 in Figure 2) are 38.0 μm, 49.2
μm, 63.8 μm and 71.4 μm for the deformation tempera-
tures of (1373, 1423, 1473, and 1523), respectively. For
L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549 543
Figure 3: True stress-strain curves obtained at different temperatures: a) 1373 K, b) 1423 K, c) 1473 K, d) 1523 K
Figure 4: Optical microstructures of 25Cr2Ni4MoV steel after dynamic recrystallization at a true strain of 0.9 and a strain rate of 0.05 s
–1
for an
initial grain size of 22 μm with different temperatures: a) 1373 K, b) 1423 K, c) 1473 K, d) 1523 K

an initial grain size of 1970 μm, the average grain sizes
(obtained from the positions 1 to 6 in Figure 2) are about
45.2 μm, 50.2 μm, 97.1 μm, and 100.5 μm, respectively.
It can be concluded that the dynamic recrystallization
grain size decreases with decreasing temperature when
the initial grain size is constant. It can be explained that
grain growth instead of grain nucleation dominates the
microstructure evolution of DRX at high temperature.
The dynamic recrystallization grain size increases with
increasing initial grain size for a certain temperature. It is
mentioned above that a fine initial grain has more grain
boundaries for nucleation.
Figures 6 and 7 show the optical microstructures in
the central area of the section (around the position 5 in
Figure 2) with a temperature of 1523 K and strain rates
of (0.005, 0.05, and 0.5) s
–1
, respectively. For an initial
L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
544 Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549
Figure 5: Optical microstructures of 25Cr2Ni4MoV steel after dynamic recrystallization at a true strain of 0.9 and a strain rate of 0.05 s
–1
for an
initial grain size of 1970 μm at different temperatures: a) 1373 K, b) 1423 K, c) 1473 K, d) 1523 K
Figure 6: Optical microstructures of 25Cr2Ni4MoV steel after dynamic recrystallization at 1523 K and a true strain of 0.9 for an initial grain size
of 22 μm with different strain rates: a) 0.005 s
–1
, b) 0.05 s
–1
,c)0.5s
–1

grain size of 22 μm, the average grain sizes (obtained
from the positions 1 to 6 in Figure 2) are 155.5 μm, 71.4
μm and 40.6 μm for the strain rate of (0.005, 0.05, and
0.5) s
–1
, respectively. For an initial grain size of 1970 μm,
the average grain sizes (obtained from the positions 1 to
6inFigure 2) are 157.6 μm, 100.5 μm, and 60.3 μm, re-
spectively. Therefore, the average grain size decreases
with increasing strain rate for a certain temperature and
initial grain size. It can be clarified from two aspects. On
the one hand, dynamic recovery during the work-harden-
ing period consumes less distortion energy, then more
energy was dissipated to DRX nucleation. On the other
hand, higher strain rates mean less time for recrystallized
grains to grow.
The average dynamic recrystallization grain sizes are
summarized in Table 2. When the dynamic recrystal-
lization grain is larger, the difference in the dynamic
recrystallization grain size resulting from different initial
grain sizes is also larger and the maximum difference is
47.3 μm with the domain of 1473 K/0.005 s
–1
. The
smaller dynamic recrystallization grain size gap is
achieved when the dynamic recrystallization grain is
finer. Though at the deformation condition of 1373 K/
0.5 s
–1
with an initial grain size of 22 μm, DRX is incom-
plete, as indicted by flow curves. It is interesting to note
that the average dynamic recrystallization grain size is
about 22 μm, which is the finest dynamic recrystal-
lization grain among all the deformation conditions and
nearly the same as the initial grain size. Therefore, when
the initial grain size is very small, the microstructure re-
constituted by DRX will not be refined. It appears that
the smallest dynamic recrystallization grain size exists
for 25Cr2Ni4MoV steel, which is about 22 μm.
Table 2: Average dynamic recrystallization grain sizes (D
drx
) for all
deformation conditions
Temperature
(K)
Strain rate
(s
–1
)
D
drx
(μm) for
22/1970 μm
1373
0.005 47.9/61.9
0.05 38.0/45.2
0.5 22.0/37.1
1423
0.005 88.0/91.4
0.05 49.2/50.2
0.5 27.5/29.7
1473
0.005 103.8/151.1
0.05 63.8/97.1
0.5 33.2/58.0
1523
0.005 155.5/157.6
0.05 71.4/100.5
0.5 40.6/60.3
3.3 DRX kinetics
Two methods were used to calculate the DRX frac-
tion. One is measuring the DRX fraction from the
microstructure observation, which requires a large num-
ber of specimens and accurate identification of the new
grains. The second one is computing from the
stress-strain curves, which is simpler and quicker than a
quantitative metallographic measurement. Therefore, in
this study we used the second method to obtain the DRX
fraction. The onset of DRX depends on the dislocation
density and distribution during hot deformation. DRX
occurs when the dislocation density or strain reaches a
L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549 545
Figure 7: Optical microstructures of 25Cr2Ni4MoV steel after dynamic recrystallization at 1523 K and a true strain of 0.9 for an initial grain size
of 1970 μm with different strain rates: a) 0.005 s
–1
, b) 0.05 s
–1
,c)0.5s
–1

critical value.
16
To simplify the Avrami analysis with an
acceptable level of accuracy, the initiation of DRX was
intentionally considered at the peak point and the effect
of dynamic recovery on flow softening was not consid-
ered. Therefore, Equations (1) and (2) can be used to ex-
press the kinetics of DRX.
17–19
Xk
d
p
p
=− −
− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 1e x p()

 (1)
X
d
p
ps
=
−
−


(2)
where X
d
is the DRX fraction, k and n are material con-
stants, p
is the peak strain, is the instantaneous strain,
 p
is the peak stress,  s
is the steady stress and  is the
instantaneous stress at different strains. Figure 8 shows
the method used for determining the characteristic
points of flow curves by using the work-hardening rate
( =d  /dε). The peak and steady stress, and the peak
and steady strain, were detected from the  –  curves
and – curves.
20
According to the flow curves, except
for the incomplete deformation condition, all the flow
curves were used to calculate the DRX fraction, which
guarantees the accuracy of the DRX fraction model.
Figure 9 shows the linear relationship between
ln(–ln(1 – X
d
)) and ln( –  p
)/ p
) under different defor-
mation conditions. The values of k and n can be com-
puted from the intercept and slope of the line. There-
fore, for different initial grain sizes, the kinetics of DRX
is given by:
X
d
=0  <  p
(3)
X
d
p
p
=− −
− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 1 00122
4 295
exp .
.

 
p
and d
0
=22μm (4)
X
d
p
p
=− −
− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 1 0363
3 394
exp .
.

  <  p
and d
0
= 1970 μm (5)
Based on the calculated results, Figure 10 shows the
DRX fraction variation with different initial grain sizes.
The DRX fraction progresses in a sigmoidal manner with
L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
546 Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549
Figure 8: Determination of characteristic points of flow curves: a)
characteristic stresses, b) characteristic strains
Figure 9: The linear relationship between ln(–ln(1 – X
d
)) and
ln( – p
)/ p
) with different initial grain sizes: a) 22 μm, b) 1970 μm

L. YE et al.: EFFECTS OF GRAIN SIZE ON THE DYNAMIC RECRYSTALLIZATION OF 25Cr2Ni4MoV ...
Materiali in tehnologije / Materials and technology 54 (2020) 4, 541–549 547
Figure 10: Volume fraction of DRX with different initial grain sizes at different deformation temperatures: a) 1373 K, b) 1423 K, c) 1473 K,
d) 1523 K
Figure 11: Comparison of fitting results and experimental results of flow curves after peak stress with different initial grain sizes at different de-
formation temperatures: a) 1373 K, b) 1423 K, c) 1473 K, d) 1523 K

respect to strain. The volume fraction of DRX increases
with increasing strain. It can also be shown that the DRX
fraction is higher at a lower strain rate or at a higher tem-
perature for a certain strain. For all the deformation con-
ditions, except the domain of 1423 K/0.05 s
–1
,
1473 K/0.5 s
–1
and 1523 K/0.5 s
–1
with initial grain size
of 22 μm, the volume fraction of DRX is 100% at low
strain rates, which substantiates the stress-strain curves
with a single peak in Figure 3. The DRX with an initial
grain size of 1970 μm occurs at a lower strain than that
of 22 μm at a strain rate of 0.005 s
–1
, which makes the
former one have a higher DRX fraction at a certain
strain. It is because the peak strain of the former is lower
than that of latter. Therefore, combining the flow curves
in Figure 3, the flow curves can well predict the volume
fraction of DRX.
According to Equation (2) and (1), the flow stress be-
yond the peak stress could be derived:


 =−− −−
− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ pps
p
p
()e x p ( ) 1 k
n
(6)
Figure 11 shows the fitting results and experimental
results of the flow curves after peak stress for different
initial grain sizes. The root mean square error (RMSE) as
shown in Equation (7) was used to evaluate the ability of
Equation (6) to predict flow stress. P
i
is the predicted
value and E
i
is the experimental value. The RMSE for all
flow curves is 2.30 MPa, which shows that the volume
fraction of DRX is capable of predicting the flow curves
after the peak stress.
RMSE
N
PE
i
ii
i
N
=−
=
∑
1
2
1
() (7)
4 CONCLUSIONS
By investigating the effects of grain size on the dy-
namic recrystallization of 25Cr2Ni4MoV steel, the
microstructural evolution of 25Cr2Ni4MoV steel during
DRX is clearer, which is very important for the design of
an open-die forging process for a super-large nuclear-
power rotor. The following conclusions are drawn.
The initial grain size strongly influences the micro-
structural evolution of 25Cr2Ni4MoV steel. At high tem-
perature and strain rate, DRX of 25Cr2Ni4MoV steel
with coarse grains is complete, while that of the alloy
with fine grains is incomplete.
When the initial grain size is small, DRX will not re-
fine the microstructure. The smallest dynamic recrys-
tallization grain size of 25Cr2Ni4MoV steel is about
22 μm with the deformation condition of 1373 K/0.5 s
–1
and initial grain size of about 22 μm.
The dynamic recrystallization fraction (X
d
) can be
given as
X
d
=0  <  p
X
d
p
p
=− −
− ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 1 00122
4 295
exp .
.

 
p
and d
0
=22μm
The comparison of predicted results and experimental
results shows that the volume fraction model of DRX for
25Cr2Ni4MoV steel can be used to predict the flow
stress after the peak stress with an acceptable level of ac-
curacy.
Acknowledgement
The authors appreciate the financial support from Na-
tional Science and Technology Major Project of China
(Grant No. 2018ZX04044001).
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