H. LI et al.: MICROSTRUCTURE EVOLUTION AND FRACTURE ANALYSIS OF HOT-ROLLED ...
297–303
MICROSTRUCTURE EVOLUTION AND FRACTURE ANALYSIS
OF HOT-ROLLED EXPLOSIVE-WELDED 06Cr13/Q345R
COMPOSITES
RAZVOJ MIKROSTRUKTURE IN LOMNA ANALIZA KOMPOZITA,
NASTALEGA IZ EKSPLOZIJSKO ZVARJENIH MATERIALOV
06Cr13 IN Q345R
Huaying Li, Guanghui Zhao*, Juan Li, Lifeng Ma, Qingxue Huang, Yugui Li
Coordinative Innovation Center of Taiyuan Heavy Machinery Equipment, Shanxi Provincial Key Laboratory of Metallurgical Device Design
Theory and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China
Prejem rokopisa – received: 2018-04-10; sprejem za objavo – accepted for publication: 2019-01-10
doi: 10.17222/mit.2018.073
In this paper, with electron backscatter diffraction (EBSD) and SEM, the microstructure evolution and fractography of bonded
interfaces were analyzed at various reduction rates (0, 15, 30, 40, 50 and 60) %. A 06Cr13/Q345R explosive-welded composite
interface of 50–80 μm was the impact area of explosive cladding where the grains of 06Cr13 and Q345R were both low-sized
and uniform. Subsequent to hot rolling, the explosive-welding effect on the 06Cr13-side interface was rapidly weakened. Com-
pared to the 06Cr13-steel side, the explosive-cladding effect on the Q345R-side interface was slowly weakened. A continuous
orientation distribution occurred, which indicated that a continuous, dynamic recrystallization mechanism existed in the
explosive-welded composite plate. In contrast, the dominant recrystallization-deformation mechanism was the discontinuous,
dynamic recrystallization, taking place under the rolling deformation. Two peaks existed between the 30-% reduction rate and
the 60-% reduction rate, and they were apparently at positions <15° and 50°–60°. As the rolling reduction rate increased, the
two materials were significantly bonded together and the interface diffusion distances of the peaks and valleys decreased and
tended to be homologous, whereas the composite interface fault was increasingly less apparent.
Keywords: 06Cr13/Q345R, rolling reduction, microstructure, fractography
Avtorji opisujejo raziskave razvoja mikrostrukture in fraktografijo na eksplozijsko zvarjenem spoju dveh materialov s pomo~jo
spektroskopije na osnovi difrakcije povratno sipanih elektronov (EBSD) in vrsti~nega elektronskega mikroskopa (SEM). Razvoj
mikrostrukture in fraktografija sta bila raziskana pri razli~nih stopnjah redukcije: (0, 15, 30, 40, 50 in 60) %. Na eksplozijsko
zvarjenem spoju dveh materialov (06Cr13 in Q345R) je nastala pribli`no 50 μm do 80 μm mejna plast, znotraj katere so bila
kristalna zrna obeh materialov - oboja majhna in enovita. Nadaljnje vro~e valjanje kompozita je povzro~ilo hitro slabenje vpliva
predhodnega medsebojnega eksplozijskega zvarjenja predvsem na strani 06Cr13. Primerjava strani, kjer se je nahajalo jeklo
06Cr13 s stranjo, kjer je bil Q345R, je pokazala, da je postopek vro~ega valjanja na to stran manj vplival. Nastopila je
neprekinjena orientacija, kar ka`e na to, da je pri{lo do kontinuirne dinami~ne rekristalizacije v eksplozijsko vezani plo{~i. V
nasprotju z dominantnim rekristalizacijskim deformacijskim mehanizmom je med vro~im valjanjem pri{lo do diskontinuirne
dinami~ne rekristalizacije. Med 30 % in 60 % stopnjo redukcije sta bila dva maksimuma, ki sta se pojavila pri <15° in 50° do
60°. Z ve~anjem stopnje deformacije med vro~im valjanjem se je znatno krepila vez med obema materialoma in medmejne
difuzijske razdalje obeh maksimumov in minimumov so se zmanj{evale s tendenco homologacije, medtem ko so postajale
napake na mejni ploskvi med obema materialoma vedno manj o~itne (vidne).
Klju~ne besede: 06Cr13/Q345R, valjanje, stopnja redukcije, mikrostruktura, fraktografija
1 INTRODUCTION
In the past, a pressure-vessel composite plate was
produced from a carbon-steel or low-alloy steel plate
used as the base layer, whereas stainless-steel or nickel-
based alloys were utilized as the cladding layer. Not only
could this procedure ensure the strength of the matrix
and the other mechanical properties, it also improved the
material corrosion resistance.
1,2
At present, mainly three types of manufacturing me-
thods for a pressure-vessel composite plate exist, in-
cluding explosive welding,
3,4
direct hot-rolling welding
5,6
and explosive welding along with rolling.
7
However,
with explosive welding, the shockwave of the explosion
results in partial melting at the region of the bonding
interface that quickly solidifies, which can easily cause
solidification defects such as pores, cracks and shrinkage
at the interface.
8,9
These defects severely affect the utili-
zation of explosive panels, especially for high-tempera-
ture and high-pressure vessels.
10
With the re-rolling technology, the interface defects
of an explosive composite panel can be eliminated. Jiang
Haitao et al.
11
studied the microstructure and mechanical
properties of Ti explosive composite panels subsequent
to asymmetric rolling. Mamalis et al.
12
fabricated an alu-
minum/copper bimetal composite plate with explosion
and rolling, and also produced a nickel/titanium shape-
memory alloy bimetal strip.
13
M. Asemabadi and M.
Sedighi studied the effect of cold rolling on an Al/Cu
bimetal explosive clad plate.
14
Materiali in tehnologije / Materials and technology 53 (2019) 3, 297–303 297
UDK 620.1:621.791:544.016.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(3)297(2019)
*Corresponding author e-mail:
zgh030024@163.com

So far, re-rolling an explosive composite plate has
been mainly focused on non-ferrous metal composite
panels. Research on steel composite plates is rare. The
effect of the rolling reduction on interface defects and
interface recrystallization is not very clear.
In this paper, the interface microstructure of an
explosion-welded and rolled 06Cr13/Q345R composite
plate was analyzed, and the effect of the reduction ratio
on the defects and recrystallization at the composite
bonding interface was investigated.
2 EXPERIMENTAL PART
2.1 Fabrication of a hot-rolled 06Cr13/Q345R explo-
sive-welded composite
In this study, a 06Cr13/Q345R explosive-welded
composite was hot rolled. The thickness of the ferritic
stainless steel (06Cr13) was 3 mm and the low-alloy
steel thickness was 14 mm. The chemical compositions
of both 06Cr13 and Q345R are presented in Table 1.
Table 1: Chemical compositions of 06Cr13 and Q345R (mass
fractions)
Alloy C Si Mn P S Cr
06Cr13 0.04 0.93 0.26 0.021 0.002 13.13
Q345R 0.15 0.39 1.43 0.018 0.005 –
Dimensions of the 06Cr13/Q345R explosive-welded
samples were 17 mm × 80 mm × 150 mm, being parallel
to the explosion direction (Figure 1). The 06Cr13/
Q345R explosive-welded composite was preheated at
1250 °C and retained for 10 min. Consequently, the hot-
rolling experiments were performed at 0.2 m/s, the
stacks were roll-bonded at reduction rates of (15, 30, 40,
50 and 60) %, and cooled down in air, subsequent to
rolling.
2.2 Microstructural analysis and mechanical-property
tests
The explosion-welded and rolled composites were
polished and electro-etched in a solution of 10 % of
nitric acid and 90 % of ethyl alcohol at an operating
voltage of 20 V. The TD-ND plane microstructure was
observed with a scanning electron microscope (SEM)
(ZIESS SIGMA FE-SEM) and detected with EBSD. The
EBSD scan data was processed with the HKL Channel 5
software. This software analyzed the capability of IPF,
misorientation, grain boundary angle and recrystalli-
zation. Through EDS, diffusion alloy elements at the
bonding interface were investigated.
In order to evaluate the rolling-reduction effects on
the tensile strength and tensile fracture morphology, the
composite plates were prepared according to the Chinese
Standard of GB/T6396-2008, along the rolling direction.
For the study of the tensile-fracture type, tensile-fracture
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298 Materiali in tehnologije / Materials and technology 53 (2019) 3, 297–303
Figure 2: IPF maps of the 06Cr13/Q345R composite interface at various reduction rates: a) original explosion-clad plate, b) 15 %, c) 30 %, d) 40
%, e) 50 %, f) 60 % reduction rate
Figure 1: Schematic view of hot rolling

surfaces were observed with a scanning electron micro-
scope (SEM). Moreover, tensile-shear testing was con-
ducted for the bonded interfaces of the samples.
3 RESULTS AND DISCUSSION
3.1 Microstructural analysis
The morphology of the TD-ND bonding interface
was observed with EBSD and the microstructure evolu-
tion was analyzed for the 06Cr13/Q345R explosive-
welded composites at various rolling reduction rates. The
characteristics of the IPF map, the grain boundary size
and the interface-area recrystallization of the hot-rolled,
explosive-welded 06Cr13/Q345R composites were stu-
died. Moreover, the formation mechanism of the
hot-rolled, explosion-welded 06Cr13/Q345R composites
was described.
Figure 2 presents the IPF maps of the 06Cr13/
Q345R composite interface at various reduction rates
that were selected from the TD-ND direction of the
plane. The left-hand layer was made of the 06Cr13 steel,
whereas the right-hand layer was made of the Q345R
carbon steel. Between the two layers, there was a bonded
interface. Figure 1a shows that the interface of the
original explosion-welded plate was wavy, indicating
that the two layers of the metal plates were completely
joined by explosive cladding. The wavy joint region was
caused by the explosive jet when 06Cr13 was ejected
towards Q345R. Under periodic wave and shear stress,
the thin metal layer on both sides of the interface had a
strong plastic deformation, resulting in refined and uni-
form grains of both 06Cr13 and Q345R near the com-
posite interface.
Near the interface, there was an explosive-welding
impact area of 50–80 μm where the grains of 06Cr13 and
Q345R were both low-sized and uniform. Subsequent to
hot rolling, the explosive-welding effect on the 06Cr13
side was rapidly weakened. At the 15-% reduction rate,
the grains on the 06Cr13 side increased in size and the
size was inhomogeneous, not demonstrating any effect of
the explosive welding. As the rolling reduction rate
increased, the stainless-steel grain size was uniform and
refined. When the rolling reduction rate reached up to 50
%, the 06Cr13 grains were uniform, fine and equiaxed,
with no apparent preferred orientation. Compared to the
06Cr13 side, the explosive-cladding effect on the Q345R
side interface was slowly weakened. At the 30-%
reduction rate, there were both low-sized and high-sized
grains with a heavily mixed grain structure on the
Q345R side. When the reduction rate reached up to
40 %, the fine equiaxed grain structure tended to be
uniform on the Q345R side. The grains on the side of the
Q345R steel were uniform, fine and equiaxed, whereas
most of the grains were oriented in the o plane at the
50-% rolling reduction rate.
3.2 Misorientation and misorientation angle distribu-
tions
Figure 3 presents the orientation of the 06Cr13/
Q345R composite interface at various reduction rates. It
was demonstrated that a continuous orientation distri-
bution occurred, which indicated that a continuous dyna-
mic recrystallization mechanism existed in the explo-
sive-welded composite plate. In contrast, the dominant
recrystallization deformation mechanism was disconti-
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Materiali in tehnologije / Materials and technology 53 (2019) 3, 297–303 299
Figure 3: 06Cr13/Q345R composite-interface grain-boundary misorientation-distribution plots at various reduction rates: a) original
explosive-clad plate, b) 15 %, c) 30 %, d) 40 %, e) 50 %, f) 60 %

nuous dynamic recrystallization under the rolling
deformation. There were two peaks between the 30-%
reduction rate and 60-%reduction rate, which were
apparently at positions <15° and 50°–60°.
The toughness of a material is closely related to the
ratio of large-angle grain boundaries in the micro-
structure.
15–17
The difference in the orientation angle of
adjacent grain boundaries can determine the extension
path of the microcracks in the material. When the
microcrack extension encounters a large-angle grain
boundary, it will deflect and hinder its rapid propagation,
thus improving the crack propagation energy of the
material. Generally, when a grain boundary with the
phase difference angle greater than 15° is the effective
grain boundary, and the orientation difference angle is
less than 15°, it can be considered that such a grain
boundary has little effect on the crack propagation, but a
small-angle grain boundary has substructure features that
act as dislocation strengthening or grain-boundary
strengthening due to dislocation entanglement. After
rolling, a large number of small-angle grain boundaries
generated at the interface of the composite plate help to
improve the interfacial bonding strength of the compo-
site plate through dislocation strengthening and grain-
boundary strengthening. This can be seen in the analysis
of the shear; as the rolling progresses, the interface
bonding strength increases.
Above all, the hot rolling contributed to the high-
angle misorientation and high-energy grain-boundary
increase, which could improve the comprehensive me-
chanical properties of the composite plate.
3.3 Recrystallization analysis
Regarding the EBSD test results, the blue zones
indicate the recrystallization and dislocation soft regions
where the dislocation density is low; the yellow zones
indicate the subgrain regions where the orientation diffe-
rence is low; the red zones indicate high-deformation
zones where the dislocation density is high. The three
distinctions are the misorientation angles (2–7.5 de-
grees). The recrystallization/grain boundary map of the
06Cr13/Q345R composite interface at various reduction
rates is presented in Figure 4. It is clear from Figure 4
that the degree of recrystallization in the original explo-
sive-clad plate is high and the grains are low-sized. This
is due to the fact that the plastic deformation zone was
under high-speed detonation and shock during the
explosive welding. Subsequently, low-sized recrystal-
lized grains were formed during the normalizing process.
When the rolling reduction rate was low, the recrystal-
lization fraction was small and the sub-structure
increased. The recrystallization fraction increased as the
rolling reduction rate increased.
Figure 5 presents the recrystallization fraction of the
06Cr13/Q345R composite interface at various reduction
rates. The recrystallization fraction of the original
explosive composite reached 49.1 %. As the rolling
reduction rate increased, the recrystallization fraction
decreased. The recrystallization fraction at the rolling
reduction rate of 30 % was the lowest, 9.65 %. Sub-
sequently, the recrystallization fraction increased. When
the rolling reduction rate reached up to 50 %, the recrys-
tallization fraction increased to 31.3 %. When the rolling
reduction rate reached up to 60 %, the deformation grain
H. LI et al.: MICROSTRUCTURE EVOLUTION AND FRACTURE ANALYSIS OF HOT-ROLLED ...
300 Materiali in tehnologije / Materials and technology 53 (2019) 3, 297–303
Figure 4: Recrystallization and grain-boundary diagram of the 06Cr13/Q345R composite interface at various reduction rates: a) original
explosive-clad plate, b) 15 %, c) 30 %, d) 40 %, e) 50 %, f) 60 %

fraction was 72.6 % due to the final rolling temperature
decrease. The recrystallization of the composite interface
facilitated the element diffusion within the interface and
formed a strong metallurgical bond.
3.4 Tensile/shear strength
In accordance with the Chinese standard GB/T
6396-2008, tensile/shear testing was conducted for all
the samples. Shear testing was conducted under the shear
direction corresponding to the hot-rolling direction. If a
fracture occurred within the bonded surface, it indicated
that the tensile/shear strength of the bonded interface
was lower than the strength of the explosive-welded
materials. According to the observations of all the
tensile/shear test samples (of both as fabricated and with
different hot-rolling reductions), the fractures due to
different hot-rolling reductions always occurred on the
ferritic stainless-steel layer only, without a fracture
occurrence at the joining interfaces, except for the
explosive-welded sample. This indicated that the
bonding strength was stronger compared to the 06Cr13
layer. As shown in Figure 6, the interface bonding
strength exceeded 220 MPa (Chinese standard GB/T
8165-2008). As the rolling reduction rate increased, the
bonding strength increased. When the rolling reduction
rate exceeded 50 %, the interfacial bonding strength
increased slowly and the maximum bonding strength was
268.4 MPa.
Since the yield stress and ultimate-tensile strength of
06Cr13 were quite lower compared to Q345R, the for-
mer was easily broken. The original explosive-clad plate
had a low bonding strength due to the interface having a
melting zone as well as the effects of voids.
10
The hot-
rolling treatment can homogenize and densify the micro-
structure, further improving the strength of the bonding
interface for the explosively welded low-alloy steel/
ferritic stainless-steel plates. In the previous researches,
no separation occurred at the joining interfaces. As an
example, Durgutlu et al.
18
demonstrated that no separa-
tion occurred at the interface of stainless steel and
copper multilayer plates. Also, M. Asemabadi and M.
Sedighi
14
reported that no separation occurred at the
interface of aluminum and copper multilayer plates.
3.5 Fractography
The scanning electron microscope (SEM) was uti-
lized to study the tensile-fracture morphology. The frac-
ture morphologies of the 06Cr13/Q345R composite at
various reduction rates are presented in Figure 7. The
left-hand layer was made of the 06Cr13 steel, whereas
the right-hand layer was made of the Q345R carbon
steel. Between the two layers, a bonded interface was
formed. According to Figure 7, no delamination
appeared within the interface. This demonstrates that the
bonded interface was firm.
Except for the 06Cr13 side of the original 06Cr13/
Q345R composite, the metals in the other composite
sustained ductile fracture. The fracture of the explosive-
welded 06Cr13 steel was brittle with a wave structure
(Figure 7a), caused by the explosive welding. As the
rolling reduction rate increased, the two materials were
significantly bonded together, while the composite inter-
face fault was increasingly less apparent. The fracture
surfaces of the composite interface were similar when
the rolling reduction rate reached up to 50 %. The frac-
ture surfaces exhibited a consistent low-sized and dense
dimple, which indicated that the grains were further re-
fined and that the composite interface was firmly com-
bined.
The aforementioned fracture-surface analysis demon-
strated that the hot-rolling treatment caused the
microstructure of the 06Cr13/Q345R explosive-welded
composite to become uniform and dense. Subsequent to
hot rolling, the fracture in the 06Cr13/Q345R composite
plate was ductile. In similar studies,
9,11,14
such a situation
was also reported. Hot rolling could cause a dense mi-
crostructure and reduce the defects, such as the melting
zone on the composite interface of the 06Cr13/Q345R
explosive-welded composite, further improving the
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Materiali in tehnologije / Materials and technology 53 (2019) 3, 297–303 301
Figure 6: Bonding strength of the 06Cr13/Q345R composite at
various reduction rates
Figure 5: Recrystallization percentages for the 06Cr13/Q345R com-
posite interface at various reduction rates

bonding strength of the 06Cr13/Q345R explosion-clad
plate interface.
4 CONCLUSIONS
In this paper, the microstructure and tensile fracture
of a 06Cr13/Q345R explosion-welded and rolled compo-
site plate were studied:
1) The 06Cr13/Q345R explosion-welded composite
interface of 50–80 μm was the impact area of explosive
cladding where the grains of both 06Cr13 and Q345R
were low-sized and uniform. Subsequent to hot rolling,
the explosive-welding effect on the 06Cr13 side of the
interface was rapidly weakened. Compared to the
06Cr13 steel side, the explosive-cladding effect on the
Q345R side of the interface was slowly weakened. When
the reduction rate reached up to 40 %, the fine, equiaxed
grain structure tended to be uniform on the Q345R side
where most of the grains were oriented in the o plane at
the 50-% rolling reduction rate.
2) A continuous-orientation distribution occurred,
indicating that a continuous dynamic recrystallization
mechanism existed in the explosive-welded composite
plate. In contrast, the dominant recrystallization-defor-
mation mechanism was discontinuous dynamic recrystal-
lization under the rolling deformation. There were two
peaks between the 30-% reduction rate and 60-%
reduction rate, which were apparently at positions <15°
and 50°–60°.
3) The recrystallization fraction of the original
explosive-welded composite amounted to 49.1 %. As the
rolling reduction rate increased, the recrystallization
fraction decreased. The recrystallization fraction at the
rolling reduction rate of 30 % was the lowest, 9.65 %.
Later, the recrystallization fraction increased. When the
rolling reduction rate reached up to 50 %, the recrystalli-
zation fraction increased to 31.3 %. When the rolling
reduction rate reached up to 60 %, the deformation-grain
fraction was 72.6 % due to the final rolling temperature
decrease.
4) The hot-rolling treatment can improve the strength
of the bonding interface of the explosive-welded plates.
When the rolling reduction rate exceeded 50 %, the
interfacial bonding strength increased slowly and the
maximum bonding strength was 268.4 MPa. As the
rolling reduction rate increased, the two materials were
significantly bonded together, while the composite-inter-
face fault was increasingly less apparent. The fracture
surfaces of the composite interface were similar when
the rolling reduction rate reached up to 50 %.
Acknowledgments
This project was supported by the Coal Based and
Low Carbon Joint Funds of Shanxi (U1510131), the Key
Research and Development Program of the Shanxi
Province (201603D111004 and 201703D111003), the
Science and Technology Major Project of the Shanxi
Province (MC2016-01) as well as the Coordinative Inno-
vation Center of Taiyuan Heavy Machinery Equipment.
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Figure 7: Fracture surface of the 06Cr13/Q345R composite at various reduction rates: a) original explosive-clad plate, b) 15 %, c) 30 %, d) 40 %,
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