metals

Article
Influence of Heat Input on the Microstructure and Impact
Toughness in Weld Metal by High-Efficiency Submerged
Arc Welding
Jinjian Li 1,2 , Bing Hu 1,2 , Liyang Zhao 1,2 , Fangmin Li 1,3 , Jiangli He 1,2 , Qingfeng Wang 1,2,4, * and Riping Liu 1,2,4

1 State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Qinhuangdao 066004, China; ljj13315669823@163.com (J.L.); binghuysu19941631@163.com (B.H.);
zly13780385805@163.com (L.Z.); lfmcrsi@163.com (F.L.); hejiangli@stumail.ysu.edu.cn (J.H.);
riping@ysu.edu.cn (R.L.)
2 National Engineering Research Center for Equipment and Technology of Cold Strip Rolling,
Yanshan University, Qinhuangdao 066004, China
3 China Railway Science & Industry Group Co., Ltd., Wuhan 430066, China
4 Hebei Key Lab for Optimizing Metal Product Technology and Performance, Yanshan University,
Qinhuangdao 066004, China
* Correspondence: wqf67@ysu.edu.cn; Tel.: +86-335-2039067

Abstract: The development of high-efficiency multi-wire submerged arc welding technology in

bridge engineering has been limited due to the high mechanical performance standards required.
In this paper, weld metal was obtained by welding at three different high heat inputs with the
laboratory-developed high-efficiency submerged arc welding wire for bridges. The effect of changing
different high heat inputs on the microstructure and impact toughness of high efficiency submerged
arc weld metal was systematically investigated by cutting and Charpy V-notch impact tests at −40 ◦ C,
using optical microscopy, scanning electron microscopy, energy-dispersive electron spectroscopy,
electron backscatter diffraction, and transmission electron microscopy to characterize and analyze.
With the increase in heat input from 50 kJ/cm to 100 kJ/cm, the impact absorption energy decreased
significantly from 130 J to 38 J. The number of inclusions in the weld metal significantly decreased
Citation: Li, J.; Hu, B.; Zhao, L.; Li, F.; and the size increased, which led to a significant decrease in the number of inclusions that effectively
He, J.; Wang, Q.; Liu, R. Influence of
promote acicular ferrite nucleation, further leading to a decrease in the proportion of acicular ferrite
Heat Input on the Microstructure and
in the weld metal. At the same time, the microstructure of the weld metal was significantly coarsened,
Impact Toughness in Weld Metal by
the percentage of high-angle grain boundaries was decreased, and the size of martensite/austenite
High-Efficiency Submerged Arc
constituents was significantly increased monotonically. The crack initiation energy was reduced by
Welding. Metals 2023, 13, 1217.
https://doi.org/10.3390/
the coarsened martensite/austenite constituents and inclusions, which produced larger local stress
met13071217 concentrations, and the crack propagation was easier due to the coarsened microstructure and lower
critical stress for crack instability propagation. The martensite/austenite constituents and inclusions
Academic Editors: Andrea Di Schino
in large sizes worked together to cause premature cleavage fracture of the impact specimen, which
and Claudio Testani
significantly deteriorated the impact toughness. The heat input should not exceed 75 kJ/cm for
Received: 25 May 2023 high-efficiency submerged arc welding wires for bridges.
Revised: 26 June 2023
Accepted: 28 June 2023 Keywords: high heat input; weld metal; microstructure; M/A constituents; inclusions; impact toughness
Published: 30 June 2023

1. Introduction
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland. As the modern steel structure industry continues to develop in the direction of large-
This article is an open access article scale and high mechanical properties, it is increasingly important to use high-efficiency
distributed under the terms and welding methods for production. For high-efficiency welding methods, researchers all over
conditions of the Creative Commons the world have carried out in-depth research and exploration. Among them, Lincoln Electric
Attribution (CC BY) license (https:// Co. (Cleveland, OH, USA) of the United States has made very important contributions in
creativecommons.org/licenses/by/ the research and development and manufacturing of high-efficiency welding materials,
4.0/). high-efficiency welding power sources, and high-efficiency welding robots, especially

Metals 2023, 13, 1217. https://doi.org/10.3390/met13071217 https://www.mdpi.com/journal/metals

Metals 2023, 13, 1217 2 of 18

to promote the development of high-efficiency welding technology in the shipbuilding

industry. The FCB submerged arc welding line developed by Ogden Engineering Co.
for shipbuilding has promoted the development of shipbuilding. The company Canada
Weld Process developed the T.I.M.E. welding technology, which increases the wire feeding
speed with high current and is added to the special shielding gas to improve welding
efficiency. The laser-arc composite welding technology, which was first developed by
British scholars, has the advantages of large penetration, high efficiency, and small welding
deformation and has been widely used in many countries in Europe and the United States
in the manufacture of shipbuilding and marine engineering steel [1]. In the field of bridge
steel structures, submerged arc welding is one of the most important welding methods.
At the same time, multi-wire, high-heat input submerged arc welding is one of the most
common high-efficiency welding methods.
Compared to single-wire submerged arc welding (20 kJ/cm ≤ heat input ≤ 50 kJ/cm),
the heat input (Ej ) of double-wire submerged arc welding (heat input ≥ 50 kJ/cm) has a
large amount of deposited metal per unit time, which can significantly save production
time and cost and reduce the labor intensity of workers. However, with higher Ej , severe
coarsening of the microstructure occurs, which deteriorates the mechanical properties of
weld metal [2,3]. The weld metal composed of a large amount of acicular ferrite (AF) has
a higher impact toughness due to a higher proportion of high-angle grain boundaries
(HAGBs) and a precise interlocking structure [4–6]. However, as the Ej increases, the
proportion of HAGBs in the weld metal decreases, while the coarse weissite-ferrite appears,
leading to a deterioration of the impact toughness [7–9]. At the same time, the volume
fraction of coarse martensite/austenite (M/A) constituents increases significantly while
the HAGB spacing increases, thus deteriorating the impact toughness [10,11]. When the Ej
is too low, the impact toughness is reduced because of the formation of a large number of
bainite and martensite in weld metal [12].
The relationship between Ej and the size or amount of inclusions in the weld metal
has also been extensively investigated by researchers [7,8,13–15]. Among them, Kluke
et al. [13] proposed that the inclusions diameter was proportional to the cube root of the Ej ,
supported by the Ostwald ripening theory. Large inclusions tend to have a stronger ability
to promote ferrite nucleation than small inclusions; however, they also act as initiation
points for cleavage fracture [16]. According to these two opposite effects, the size of the
inclusions should neither be too large nor too small; otherwise, they will have a negative
impact on the impact toughness of the weld metal.
Due to the high mechanical performance standards required in bridge engineering,
single-wire submerged arc welding and gas shielded welding have been the main produc-
tion welding methods, and the development of high-efficiency submerged arc welding
technology has been slow in the bridge field. In this regard, to promote the application
of high-efficiency welding in the field of bridges, our team developed a submerged arc
welding wire by adding some micro-alloy elements based on the C-Si-Mn alloy and us-
ing a combination of mechanisms such as fine grain strengthening, phase transformation
strengthening, and solid solution strengthening to ensure the weld metal can meet the
mechanical properties of bridges under high-heat input. This work systematically investi-
gated the relationship between the microstructure and impact toughness of the weld metal
obtained by high-efficiency submerged arc welding wires under different high-heat inputs
from the perspectives of microstructure transformation, HAGB ratio, M/A constituents,
and inclusions. Thus, the applicable range of high-efficiency welding Ej was obtained
by this work to satisfy high-efficiency welding on bridge steel, and a reference basis for
high-efficiency welding in the field of bridges was provided by this work.

2. Experimental Procedure
The purpose of this work is to investigate the changes in the microstructure and me-
chanical properties of the weld metal with heat input changes and to derive the applicable
heat input for the high-efficiency submerged arc welding wire. Therefore, high efficiency
Metals 2023, 13, 1217 3 of 18

submerged arc welding wires were used for submerged arc welding with high heat input,
followed by sampling of the weld metal and testing the mechanical properties of the weld
metal at different high-heat inputs. Finally, the welded metal under different heat inputs
was observed and analyzed.

2.1. Welding Tests

Three bridge steel plates were used in the sizes of 300 × 150 × 32 mm and welded
using self-developed high-efficiency submerged arc welding wires for Ej = 50, 75, and
100 kJ/cm, respectively. The high-efficiency submerged arc welding steel was melted by
a vacuum induction furnace and then underwent forging of the welding wire ingot, hot
rolling of the wire rod, rough drawing, fine drawing to ϕ 5.0 mm, and copper plating. The
welding parameters are shown in Table 1, and the chemical compositions of the base metal,
weld wire, weld metal, and flux are shown in Table 2.

Table 1. Welding parameters.

Interpass
Welding Welding Welding Speed Heat Input
Sample Temperature
Current (I)/A Voltage (U)/V (υ)/mh−1 (Ej )/kJ cm−1
(T)/◦ C
680 (AW) 30 (AW)
WM-50 kJ/cm 24 150≤ 50
630 (PW) 32 (PW)
750 (AW) 32 (AW)
WM-75 kJ/cm 21.5 150≤ 75
700 (PW) 34 (PW)
800 (AW) 34 (AW)
WM-100 kJ/cm 21 150≤ 100
750 (PW) 36 (PW)
AW: Anterior wire; PW: Posterior wire.

Table 2. The chemical composition of welding flux, base metal, weld wire, and weld metal.

Sample Chemical Composition/wt%

SiO2 + TiO2 CaO + MgO AI2 O3 + MnO CaF2 S P
Weld flux
25–35 20–30 15–30 15–25 0.06 0.08
Element type C Si Mn P S Ni Cr Nb + V + Ti + Al + Mo + B
Base metal 0.07 0.22 1.52 0.013 0.002 0.027 0.04 0.106
Weld wire 0.06 0.20 1.55 0.014 0.003 0.39 0.03 0.851
WM-50
0.06 0.21 1.53 0.016 0.004 0.37 0.03 0.832
kJ/cm
WM-75
0.05 0.19 1.49 0.016 0.004 0.35 0.03 0.828
kJ/cm
WM-100
0.05 0.16 1.44 0.017 0.004 0.33 0.03 0.821
kJ/cm

2.2. Mechanical Tests

Three impact specimens were taken for each kind of heat input weld metal and
processed into 55 × 10 × 10 mm impact specimens in accordance with the ASTM standard
E2298, as shown in Figure 1. The weld metal impact test was performed by the JBN-300B
impact tester (Jinan Marxtest Technolggy Co., Ltd, Jinan, China) at a temperature of −40 ◦ C.
Three sets of impact tests were conducted on each Ej specimen and averaged to reduce
the error.
Metals 2023,13,
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18

Figure 1.
Figure 1. Schematic
Schematic diagram
diagram of
of weld
weld joints
joints with
with welding
welding thermal
thermal simulation
simulation and
and standard
standard impact
impact
samples (a,b), and microstructure observation area
samples (a,b), and microstructure observation area (c).(c).

2.3.
2.3. Fracture
Fracture Observation
Observation
The
The fracture surfaces
fracture surfaces of
of the
the impact
impact samples
samples were
were observed
observed via
via aa scanning
scanning electron
electron
microscopy (SEM SU5000, HITACHI, Tokyo, Japan). Subsequently,
microscopy (SEM SU5000, HITACHI, Tokyo, Japan). Subsequently, the samples the sampleswerewere
cut
cut
alongalong
the the cross-section
cross-section perpendicular
perpendicular to V-notch
to the the V-notch as shown
as shown in Figure
in Figure 1c, and
1c, and the
the dis-
distribution
tribution andand propagation
propagation path path of the
of the secondary
secondary cracks
cracks nearnear
the the main
main fracture
fracture werewere
ob-
observed under the
served under the SEM. SEM.

2.4. The Determination of Phase Transition Temperature

2.4. The Determination of Phase Transition Temperature
In order to measure the effect of Ej on the γ→α phase transition temperature Ar3, the
In order to measure the effect of Ej on the γ→α phase transition temperature Ar3, the
thermal expansion curve was recorded using a C-strain gauge, and the beginning and end
thermal expansion curve was recorded using a C-strain gauge, and the beginning and end
temperatures of the phase transition from austenite to ferrite at the corresponding cold
temperatures of the phase transition from austenite to ferrite at the corresponding cold
rate were determined using the tangent method. Figure 1a gives a schematic diagram of
rate were determined using the tangent method. Figure 1a gives a schematic diagram of
the sampling of the specimen for determining the phase change point of the weld metal.
the sampling of the specimen for determining the phase change point of the weld metal.
One round bar-type sample was taken for each heat input specimen. After the sampling
Onecompleted,
was round bar-type sample
the weld was taken for
microstructure eachspecimen
of the heat input specimen.
was After
etched with a 4%thenitric
sampling
acid
was completed, the weld microstructure of the specimen was etched with a 4% nitric
alcohol solution. Subsequently, the thermocouple wires were spot welded to the weld metal, acid
alcohol solution. Subsequently, the thermocouple wires were spot welded to
as shown in Figure 1a. The Gleeble-3500 thermal simulation tester (Dynamic Systems Inc., the weld
metal,York,
New as shown in Figure
NY, USA) 1a. The
was used Gleeble-3500
to conduct thermalthermal
the welding simulation
cycletester
from(Dynamic Sys-
Ej = 50 kJ/cm
tems Inc., New York, NY, USA) was used to conduct the welding thermal
to 100 kJ/cm on the specimens. Afterwards, the phase transition temperature Ar3 wascycle from Ej =
50 kJ/cm to 100 kJ/cm on
measured for each Ej sample.the specimens. Afterwards, the phase transition temperature Ar3
was measured for each Ej sample.
2.5. Microstructure Characterization
2.5. Microstructure Characterization
The remaining impact samples of three kinds of heat inputs were first cut in the cross-
The
section remaining impact
perpendicular samplessurface.
to the fracture of three Then
kindsthe
of heat inputswere
specimens weresanded
first cutand
in the cross-
polished,
section
then perpendicular
etched with a 4% to the acid
nitric fracture surface.
alcohol Then The
solution. the specimens werewas
microstructure sanded and pol-
observed by
ished,
an then etched
Olympus BX51M with a 4% nitric
(Olympus, acidJapan).
Aizu, alcoholThe
solution. The microstructure
percentage was observed
of the microstructures was
by an Olympus
calculated BX51M (Olympus,
using Image-Pro Aizu,(Image-Pro
Plus software Japan). The percentage
®Plus, Media of the microstructures
Cybernetics, Bethesda,
MD, USA). The specimens were repolished and etched with LePera solution for 120Be-
was calculated using Image-Pro Plus software (Image-Pro ® Plus, Media Cybernetics, s,
thesda,
then theMD,
M/A USA). The specimens
constituents were repolished
of the specimens and etched
were observed with the
under LePera solution for
metallographic
120 s, then theThe
microscope. M/A constituents
size and area of the specimens
fraction were observed
were statistically underby
calculated theImage-Pro
metallographic
Plus
microscope.
software. TEM Theslices
size and area to
parallel fraction were statistically
the metallographic calculated
specimens wereby obtained
Image-ProbyPlus soft-
cutting
ware.
and TEM slices
sanding parallel
to 40–50 to the metallographic
µm thickness. Small discs of specimens obtainedout
ϕ 3 mm were pressed by of
cutting and
the slices
sanding
and to 40–50 μm
subsequently thickness.
thinned using Small discselectrolytic
double-jet of φ3 mm were pressed
polishing. outthe
After of the slices was
thinning and
subsequently
completed, thethinned using double-jet
typical microstructure ofelectrolytic polishing.
the weld metal After theby
was observed thinning was high-
a JEM-2010 com-
resolution
pleted, thetransmission electron microscopy
typical microstructure of the weld (TEM,
metalJapan
was Electronics
observed by optics Corporation,
a JEM-2010 high-
Tokyo, Japan).
resolution The metallographic
transmission specimens
electron microscopy wereJapan
(TEM, polished again and
Electronics electrolytically
optics Corporation,
Metals 2023, 13, x FOR PEER REVIEW 5 of 18

Metals 2023, 13, 1217 5 of 18

Tokyo, Japan). The metallographic specimens were polished again and electrolytically
polished using perchloric acid (20%) and methanol (80%) solutions, followed by scanning
polished using perchloric acid (20%) and methanol (80%) solutions, followed by scanning
using the SEM with an electron backscatter diffraction detector in 0.25 μm steps to quan-
using the SEM with an electron backscatter diffraction detector in 0.25 µm steps to quantify
tifycrystal
the the crystal orientation
orientation of theof the metal.
weld weld metal. The reported
The reported mean equivalent
mean equivalent diameterdiameter
(MED)
(MED) statistics were the average values of at least 8 electron backscatter diffraction im-
statistics were the average values of at least 8 electron backscatter diffraction images used.
ages used. The specimens were sanded and polished again, and then the inclusions
The specimens were sanded and polished again, and then the inclusions in the weld metal in the
weld metal were observed
were observed via the SEM. via the SEM.

3. Results
3. Results
3.1.
3.1. Microstructure
Microstructure Observations
Observations ofof the
the Weld
Weld Metal
Metal
The
The microstructural metallographs andthe
microstructural metallographs and themicrostructure
microstructure percentage
percentageunder
underdifferent
differ-
high-heat are given in Figure 2. Under E
ent high-heat inputs are given in Figure 2. Underj Ej = 50 kJ/cm, the microstructure was
inputs = 50 kJ/cm, the microstructure was
mainly
mainlycomposed
composedofofAF AFand
and some
some grain
grainboundary
boundary ferrite (GBF),
ferrite granular
(GBF), bainite
granular (GB),(GB),
bainite and
polygonal ferrite ferrite
and polygonal (PF). With increasing
(PF). Ej , microstructures
With increasing underwentunderwent
Ej, microstructures significant coarsening,
significant
and the ferrite
coarsening, andsidetheplate (FSP)
ferrite appeared.
side plate (FSP) This kind of jagged
appeared. microstructure
This kind along the GBF
of jagged microstructure
growth
along thetoward the interior
GBF growth of the
toward theprior austenite
interior of thegrain
priorisaustenite
very detrimental to thedetrimental
grain is very toughness
of weld metal. At the same time, with increasing E , the percentages of
to the toughness of weld metal. At the same time, jwith increasing Ej, the percentages GBF, GB, and PF of
increased, and the percentage of AF decreased, as shown in Figure 2d.
GBF, GB, and PF increased, and the percentage of AF decreased, as shown in Figure 2d.

Figure 2. Microstructure
Figure 2. Microstructuremorphologies
morphologiesofof50
50kJ/cm
kJ/cm(a),
(a),75
75kJ/cm
kJ/cm (b),
(b), 100
100 kJ/cm
kJ/cm (c), and a schematic
schematic
diagram of
diagram of the
the percentage
percentageof
ofmicrostructures
microstructures(d).
(d).

The
The M/A constituentsofofthe
M/A constituents theweld
weldmetal
metalare
areshown
showninin Figure
Figure 3. 3. Separately,
Separately, under
under 10
10 fields
fields of of view,
view, thethe size
size andand area
area proportion
proportion of more
of more thanthan
300300 M/A
M/A constituents
constituents werewere
sta-
statistically calculated
tistically calculated byby Image-ProPlus
Image-Pro Plussoftware
softwaretotominimize
minimize local
local statistics.
statistics. The
The statistical
statistical
results
results are shown in Table 3. The M/A constituents were small, with an average size
are shown in Table 3. The M/A constituents were small, with an average size of
of
0.92 µm, the area proportion was 2.2% under E
0.92 μm, and the area proportion was 2.2% under Ej j = 50 kJ/cm. However, the average size
and = 50 kJ/cm. However, the average size
of
of inclusions
inclusions increased
increased significantly
significantly to
to 2.36 μm, and
2.36 µm, and the
the area
area proportion
proportion increased
increased toto 8.3%
8.3%
with the E increasing to 100 kJ/cm. At the same time, it can be found that with
with the Ej j increasing to 100 kJ/cm. At the same time, it can be found that with the increase the increase
in EEjj,, the
in the number
number of of stripy
stripy M/A constituents increased.
M/A constituents increased.
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Figure3.3.Four
Figure Fourpercent
percentLePera’s
LePera’sreagent-etched
reagent-etchedsamples
samplesofof5050kJ/cm
kJ/cm(a),
(a),7575kJ/cm
kJ/cm(b),
(b),and
and100
100kJ/cm
kJ/cm
Figure 3. Four percent LePera’s reagent-etched samples of 50 kJ/cm (a), 75 kJ/cm (b), and 100 kJ/cm
(c),where
(c), wherethe
thematrix
matrixisisgray
grayand
andthe
theM/A
M/A constituents
constituents are
are white.
white.
(c), where the matrix is gray and the M/A constituents are white.
Table 3.Quantification
Quantification resultsofof microstructuresinineach
each sample.
Table
Table3.3. Quantification results
results ofmicrostructures
microstructures in eachsample.
sample.
Heat input fM/A dM/A fMTA ≥ 15° MEDMTA ≥ 15°
Heat input
Heat Input Microstructures fM/A
fM/A dM/AdM/A fMTA f≥MTA
15◦ ≥ 15° MED
MED MTAMTA ≥ ◦15°
≥ 15
/kJ·cm −1 Microstructures
Microstructures /% /μm /% /μm
/kJ·cm−1 −1
/kJ · cm /% /% /µm/μm /% /% /µm
/μm
50 AF + PF + GBF + GB 2.2 0.92 46.3 3.1
50 50 AFAF+ +PF
PF++GBF
GBF++GBGB 2.2 2.2 0.920.92 46.346.3 3.13.1
75 AFAF
++PFPF+ +GBF ++GB + FSP 4.5 1.59 37.7 3.9
75 75 AF + PF + GBF + GB ++ FSP
GBF GB FSP 4.5 4.5 1.591.59 37.737.7 3.93.9
100100 AFAF
++PFPF+ +GBF
GBF + GB ++ FSP
+ GB FSP 8.3 8.3 2.362.36 24.1 24.1 5.35.3
100 AF + PF + GBF + GB + FSP 8.3 2.36 24.1 5.3
The TEM images of the weld metal are presented in Figure 4. As shown in Figure 4,
The TEM
The TEM images
images ofof the
the weld
weld metal
metal are presented
presented in Figure 4. As shown in Figure 4,
the main microstructure of the weld metal was AF with a little PF. As Ej increased, the size
the main microstructure
the microstructure of ofthe
theweld
weldmetal
metalwaswas AFAFwith
witha little PF.PF.
a little As As
Ej increased, the size
Ej increased, the
of AF increased significantly, and the number of inclusions decreased while its size in-
of AF
size of increased significantly,
AF increased andand
significantly, the the
numbernumberof inclusions
of inclusionsdecreased
decreased while its size
while in-
its size
creased. By comparison, it was found that not all inclusions produced effective promotion
creased. ByBycomparison,
increased. comparison,ititwaswasfound
foundthat
thatnotnotall
allinclusions
inclusionsproduced
produced effective promotion
promotion
of AF nucleation, and the small-sized inclusions shown in Figure 4a (indicated by the blue
of AF
of AF nucleation,
nucleation, and
and the
the small-sized
small-sized inclusions
inclusions shown
shown in in Figure
Figure 4a4a (indicated
(indicated byby the
the blue
blue
arrows) were engulfed by the growth of ferrite. The larger inclusions (indicated by the
arrows)
arrows) were
were engulfed
engulfed by by the
the growth
growth ofof ferrite.
ferrite. The
The larger
larger inclusions
inclusions (indicated
(indicated byby the
the
yellow arrows) had a stronger tendency to stimulate the nucleation of AF. Ferrite laths
yellow
yellow arrows)
arrows) had
had aa stronger
stronger tendency
tendency to to stimulate
stimulate the
the nucleation
nucleation of AF. AF. Ferrite
Ferrite laths
laths
nucleated on larger-sized inclusions and grew radiologically. Subsequently, neighboring
nucleated
nucleated on on larger-sized
larger-sized inclusions
inclusions and
and grew
grew radiologically.
radiologically. Subsequently,
Subsequently, neighboring
neighboring
ferrite laths nucleated on the previously existing laths by mutual induction, similar to the
ferrite
ferrite laths
laths nucleated
nucleated onon the
the previously
previously existing
existing laths
laths by
by mutual
mutual induction,
induction, similar
similar toto the
the
previous study[17].
previous [17].
previous study
study [17].

Figure 4. TEM micrographs of 50 kJ/cm (a), 75 kJ/cm (b), and 100 kJ/cm samples (c).
Figure 4.
Figure 4. TEM
TEM micrographs
micrographs of
of 50
50 kJ/cm
kJ/cm (a),
(a), 75 kJ/cm
kJ/cm(b),
(b),and
and100
100kJ/cm
kJ/cmsamples
samples(c).
(c).
The EDSmapping
The mapping imageofofa typical
a typical inclusion, as shown in Figure 5, revealed that
The EDS
EDS mappingimageimage of a typicalinclusion, as as
inclusion, shown
shownin Figure 5, revealed
in Figure thatthat
5, revealed the
the inclusions
inclusions werewere composite
composite oxideoxide
and and sulfide
sulfide composed
composed of of
Al, Al,
Ti, Ti,
Si, Si,
and and
Mn. Mn.
the inclusions were composite oxide and sulfide composed of Al, Ti, Si, and Mn.
The distribution of inclusions in the weld metal is shown in Figure 6, where the
inclusions that stimulate AF nucleation are circled in yellow. All inclusions were counted
under 25 fields of scanning electron microscopy using Image-Pro Plus software to minimize
errors. With Ej increasing from 50 kJ/cm to 100 kJ/cm, the total number of inclusions
decreased significantly. The average size of inclusions increased from 0.59 to 1.26 µm, and
the number of large-sized inclusions increased, with the maximum size reaching about
2.5 µm.
Metals 2023, 13,
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Figure 5.
Figure 5. Element
Element distribution
distribution by
by EDS
EDS of
of inclusions
inclusions in
in weld
weld metal.
metal.

The distribution
The distribution of of inclusions
inclusions in
in the
the weld
weld metal
metal is
is shown
shown inin Figure
Figure 6,
6, where
where the
the in-
in-
clusions that stimulate AF nucleation are circled in yellow. All inclusions
clusions that stimulate AF nucleation are circled in yellow. All inclusions were counted were counted
under 25
under 25 fields
fields of
of scanning
scanning electron
electron microscopy
microscopy using
using Image-Pro
Image-Pro Plus
Plus software
software to
to mini-
mini-
mize errors. With E increasing from 50 kJ/cm to 100 kJ/cm, the total number
mize errors. With Ej increasing from 50 kJ/cm to 100 kJ/cm, the total number of inclusions
j of inclusions
decreased significantly.
decreased significantly. The
The average
average size
size of
of inclusions
inclusions increased
increased from
from 0.59
0.59 to
to 1.26
1.26 μm,
μm, and
and
the number
the number of of large-sized
large-sized inclusions
inclusions increased,
increased, with
with the
the maximum
maximum sizesize reaching
reaching about
about
2.5 μm.
2.5 μm.
Figure
Figure 5. Element distribution by EDS of inclusions
5. inclusions in
in weld
weld metal.
metal.

The distribution of inclusions in the weld metal is shown in Figure 6, where the in-
clusions that stimulate AF nucleation are circled in yellow. All inclusions were counted
under 25 fields of scanning electron microscopy using Image-Pro Plus software to mini-
mize errors. With Ej increasing from 50 kJ/cm to 100 kJ/cm, the total number of inclusions
decreased significantly. The average size of inclusions increased from 0.59 to 1.26 μm, and
the number of large-sized inclusions increased, with the maximum size reaching about
2.5 μm.

Figure6.
Figure
Figure 6. Observations
6. Observations on
on the
the inclusion
inclusion of
of aa 50
50 kJ/cm
kJ/cm sample
kJ/cm sample (a)
sample (a) and
and aaa 100
(a) and 100 kJ/cm
100 kJ/cm sample
kJ/cmsample (b).
sample(b).
(b).

3.2.
3.2. Crystallographic
3.2. Crystallographic Characteristics
Crystallographic Characteristics of
Characteristics of the
of the Weld
the Weld Metal
Weld Metal
Metal
Inverse pole
Inverse pole
Inverse figures
pole figures (IPF)
figures (IPF) were
(IPF) were obtained
obtained using
were obtained using electron
using electron backscatter
electron backscatter diffraction,
backscatter diffraction, as
diffraction, as
as
shown
shown
shown in
in Figure
Figure 7a–c.
7a–c. Previous
Previous studies
studies [18–20]
[18–20] usually
usually defined
defined the
the
in Figure 7a–c. Previous studies [18–20] usually defined the threshold of HAGB atthreshold
threshold of
of HAGB
HAGB at
at
15 ◦ because grain boundaries with misorientation tolerance angles (MTA) greater than 15◦
15° because grain boundaries with misorientation tolerance angles
15° because grain boundaries with misorientation tolerance angles (MTA) greater than 15° (MTA) greater than 15°
have
have higher
higher energy
energy to to effectively force
effectively force
force thethe deflection
the deflection or
deflection or termination
or termination
termination of of crack
of crack propagation.
crack propagation.
propagation.
have higher energy to effectively ◦ between
Therefore,
Therefore, in in this
in this paper,
this paper, grain
paper, grain boundaries
grain boundaries
boundaries with with an
with an MTA
an MTA less
MTA less than
less than 15
than 15° between adjacent
15° between adjacent
Therefore, adjacent
grains
grains6.
Figure were
were defined
defined as
Observations as
as low-angle
low-angle
onlow-angle grain
grain
the inclusion boundaries
boundaries
of aboundaries (LAGBs),
(LAGBs),
50 kJ/cm sample while
while
(a) andwhile grain
kJ/cm boundaries
grain
a 100 grain (b). were
boundaries
sample were
grains were defined grain (LAGBs), boundaries ◦ .were
defined
defined as
as high-angle
high-angle grain
grain boundaries
boundaries (HAGBs)
(HAGBs) when
when the
the MTA
MTA was
was higher
higher than
than 15
15°. The
The
defined as high-angle grain boundaries (HAGBs) when the MTA was higher than 15°. The
HAGBs
3.2.
HAGBs were
were marked
Crystallographicmarked with
with black
Characteristics
black lines
of the
lines in Figure
Weld
in Metal
Figure 7.
7. As
As shown
shown in
in Figure
Figure 7,
7, HAGBs
HAGBs were
were
HAGBs were marked with black lines in Figure 7. As shown in Figure 7, HAGBs were
formed
formed between
between adjacent
adjacent AF
AF or PF,
or PF,
PF, which
which could effectively
couldusing
effectively hinder
hinder crack propagation
crack propagation
propagation [21].
[21].
formed between
Inverse poleadjacent
figures AF or
(IPF) were which could
obtained effectively hinder
electron crack
backscatter diffraction, [21].
as
shown in Figure 7a–c. Previous studies [18–20] usually defined the threshold of HAGB at
15° because grain boundaries with misorientation tolerance angles (MTA) greater than 15°
have higher energy to effectively force the deflection or termination of crack propagation.
Therefore, in this paper, grain boundaries with an MTA less than 15° between adjacent
grains were defined as low-angle grain boundaries (LAGBs), while grain boundaries were
defined as high-angle grain boundaries (HAGBs) when the MTA was higher than 15°. The
HAGBs were marked with black lines in Figure 7. As shown in Figure 7, HAGBs were
formed between adjacent AF or PF, which could effectively hinder crack propagation [21].

Figure7.
Figure
Figure 7. Inverse
7. Inverse pole
pole figures
figuresof
of50
50kJ/cm
kJ/cm (a),
kJ/cm (a), 75
(a), 75 kJ/cm
75 kJ/cm (b),
kJ/cm(b), and
(b),and 100
and100 kJ/cm
100kJ/cm sample
kJ/cmsample (c).
sample(c).
(c).

As shown in Figure 8a, the MED defined by different MTAs increased with the increase
in MTA. With the increase in Ej , the MED defined by MTA ≥ 15◦ (MEDMTA ≥ 15◦ ) increased
from 3.13 µm to 5.28 µm. Figure 8b gives the percentage of HAGBs. It can be found that
with the increase in Ej , the percentage of HAGBs decreased from 46.3% to 24.1%.

Figure 7. Inverse pole figures of 50 kJ/cm (a), 75 kJ/cm (b), and 100 kJ/cm sample (c).
 As shown in Figure 8a, the MED defined by different MTAs increased with the in-
As shown in Figure 8a, the MED defined by different MTAs increased with the in-
crease in MTA. With the increase in Ej, the MED defined by MTA ≥ 15° (MEDMTA ≥ 15°)
crease in MTA. With the increase in Ej, the MED defined by MTA ≥ 15° (MEDMTA ≥ 15°)
increased from 3.13 μm to 5.28 μm. Figure 8b gives the percentage of HAGBs. It can be
increased from 3.13 μm to 5.28 μm. Figure 8b gives the percentage of HAGBs. It can be
Metals 2023, 13, 1217
found that with the increase in Ej, the percentage of HAGBs decreased from 46.3% to
8 of 18
found that with the increase in Ej, the percentage of HAGBs decreased from 46.3% to
24.1%.
24.1%.

Figure 8. MED varied with MTA (a) and the distribution of boundary misorientation (b).
Figure 8. MED
Figure 8. MED varied
varied with
with MTA
MTA (a)
(a) and
and the
the distribution
distribution of
of boundary
boundary misorientation
misorientation(b).
(b).
3.3. Impact
3.3. Impact Toughness
Toughnessand andFracture
FractureBehavior
BehaviorofofthetheWeld
WeldMetal
Metal
3.3. Impact Toughness and Fracture Behavior of the Weld Metal
Table 44 gives
Table gives the the impact
impact energy
energy of of the
the weld
weld metal
metal samples,
samples, andand the
the average
average value
value
Table 4 gives the impact energy of the weld metal samples, and the average value
decreased from
decreased from 130 130 J to 38 J with Ejj increasing from 50 50 kJ/cm
kJ/cmtoto100 100kJ/cm.
kJ/cm. AsAsshown
shown in
decreased from 130 J to 38 J with Ej increasing from 50 kJ/cm to 100 kJ/cm. As shown in
Figure
in Figure9, the
9, thefibrous
fibrouszone waswas
zone more dominant
more dominant under Ej = E
under 50j =kJ/cm, and this
50 kJ/cm, andzonethis corre-
zone
Figure 9, the to
corresponds fibrous zone was
the ductile crack more dominant under Ejconsumption.
= 50 kJ/cm, andAccordingly,
this zone corre-
sponds to the ductile crack with awith
highera higher
energy energy
consumption. Accordingly, the cleavagethe
sponds tosurfaces
cleavage the ductilewere crack with a higher energy consumption. theAccordingly, the cleavage
surfaces were found to found
exist intotheexist in the
region region
away fromaway
the from
Charpy Charpy V-notch,
V-notch, i.e., the
i.e., the cleavage
surfaces were
cleavage fracture found to existlater
occurred in the regionEj away from the Charpy V-notch, i.e.,corresponds
the cleavage
fracture occurred later under Ej =under
50 kJ/cm. = The
50 kJ/cm.
cleavage The cleavage
fracture fracture
corresponds to the stage
fracture
to occurred later under E j = 50 kJ/cm. The cleavage fracture corresponds to the stage
of the stage
crack of crackpropagation,
instability instability propagation,
and the energy andconsumed
the energy consumed
is low. is low.
With the Withinthe
increase Ej,
of crack instability
increase in E , the propagation,
area of the and the
fibrous energy
zone was consumed
significantly is low. With and
reduced, the increase
the in Ej,
cleavage
the area of the j fibrous zone was significantly reduced, and the cleavage surfaces were
the areawere
surfaces of the fibrous closezone was significantly reduced, and thefracture
cleavage surfaces were
found close tofound the V-notch, to the
i.e.,V-notch,
the onseti.e.,
of the onset
cleavage of cleavage
fracture was
was significantly significantly
advanced.
found
advanced.close to the
Meanwhile, V-notch, i.e.,
thesurface the
fracturewas onset
surface of cleavage fracture was significantly advanced.
Meanwhile, the fracture very wasflat,very
whichflat,corresponded
which corresponded to theimpact
to the lower lower
Meanwhile,
impact the fracture surface was very flat, which corresponded to the lower impact
energy.energy.
energy.

Figure 9.
Figure 9. Fracture
Fracturesurfaces
surfacesofofthe
theweld
weldmetal
metalwith
withEjE=j =5050kJ/cm
kJ/cm(a),
(a),75
75kJ/cm
kJ/cm (b),
(b),100
100kJ/cm
kJ/cm (c),
(c), and
and
Figure
a high 9. Fracture surfaces
magnification image of
of the
the weld metal
fibrous with
zone Ej = 50and
(I/III/V) kJ/cm (a),zone
radial 75 kJ/cm (b), 100 kJ/cm (c), and
(II/IV/VI).
a high magnification image of the fibrous zone (I/III/V) and radial zone (II/IV/VI).
a high magnification image of the fibrous zone (I/III/V) and radial zone (II/IV/VI).
Metals 2023, 13, 1217 9 of 18

Table 4. CVN impact energy at −40 ◦ C of weld metal under different Ej .

Heat Input CVN Impact Energy at −40 ◦ C/J

kJ/cm Sample 1 Sample 2 Sample 3 Average Value
50 139 113 137 168
75 87 75 69 119
100 32 42 40 49

The fibrous and radial zones of the three specimens were further observed separately
by magnification, as shown in Figure 9I–VI. For the sample of Ej = 50 kJ/cm, the fiber zone
consisted of small, uniform, and deep dimples, while the radial zone had small cleavage
surfaces, and the cleavage surfaces extended from the cleavage crack to the periphery,
forming a cleavage step when encountering HAGBs. Tear ridges were found at the edges
of the cleavage surface, indicating a higher impediment to crack propagation. With the
increase in Ej , the dimples in the fiber zone became shallower and larger and developed
into a parabolic shape, which means that the degree of plastic deformation during the
ductile fracture stage was reduced. At the same time, the sizes of the cleavage surfaces in
the radial zone significantly increased, and the tear ridges were almost invisible, implying
that crack propagation became easier during the cleavage fracture [22]. It was worth noting
that when Ej increased to 100 kJ/cm, many large-size inclusions, or M/A constituents, were
formed and acted as crack initiation sources for the various cleavage surfaces. Accordingly,
it could be inferred that the generation of these coarse M/A constituents and inclusions
was inextricably linked to the reduction in impact toughness of the specimens, which will
be discussed later.
In order to further observe and analyze the initiation and propagation of cracks under
different Ej , specimens were cut from the impact fracture section to observe the morphology
and distribution of secondary cracks.
As shown in Figure 10a, a small number of microvoids were observed in the fracture
cross-section of the 50 kJ/cm sample in the fibrous region. The growth and coalescence of
the microvoids were limited by the HAGBs, such as the surrounding AF. Meanwhile, as the
large M/A constituents and inclusions that promoted the nucleation of microvoids were
less, the distance between microvoids was larger, and the energy required for microvoid
coalescence was correspondingly higher. As a result, the size of the microvoids was
relatively small under Ej = 50 kJ/cm, and the coalescence and growth of microvoids were
hardly seen. For the 100 kJ/cm sample, at the fiber zone, as shown in Figure 10b, it
was found that many microvoids were nucleated at the interface of M/A constituents or
inclusions and matrix, and the microvoids were close to each other and appeared to grow
and coalesce.
Under 50 kJ/cm, the radiation zone was relatively small, and the propagation paths of
secondary cracks were short, as shown in Figure 10c. When secondary cracks propagated on
the fine AF, there was blunting and deflection at the grain boundaries, indicating that large
amounts of AF have a strong arresting effect on crack propagation [23]. Under 100 kJ/cm,
the number and size of secondary cracks in the radial zone increased significantly. As
shown in Figure 10d, the long strip crack nucleated along the interface between the ferrite
matrix and M/A constituents. The secondary crack propagated straight through the ferrite
grain and the elongated M/A, eventually deflecting at the grain boundary and arresting at
the prior austenite grain boundary. The overall straightness of the cracks indicated that the
surrounding microstructures had a weak hindering effect on the cracks propagation.
Metals 2023, 13,
Metals 2023, 13, 1217
x FOR PEER REVIEW 10 of 18
10 18

Figure 10.
Figure 10. SEM
SEM observations
observationsofofmicrovoids
microvoidsand
andcrack
crackmorphologies
morphologiesatat
Ej E=j 50 kJ/cm
= 50 (a,c)(a,c)
kJ/cm andand
100
kJ/cm
100 (b,d).
kJ/cm (b,d).

4. Discussion
Under 50 kJ/cm, the radiation zone was relatively small, and the propagation paths
4.1. Effect of Ej cracks
of secondary on the Inclusions
were short, of the Weld Metal
as shown in Figure 10c. When secondary cracks propa-
gatedTheon the fine AF,
presence of there was blunting
convective flow ofand deflection
liquid in the at the pool
melt grainleads
boundaries, indicating
to collisions and
that large amounts
aggregation betweenofthe AFinclusions,
have a strongwhich arresting
are calledeffect on crack
gradient propagation
collisions, resulting [23]. Under
in coarse
100 kJ/cm, Therefore,
inclusions. the numberwith andthe size of secondary
increase cracks
in Ej , the in the flow
convective radialofzone
fluidincreased
in the weld signifi-
pool
cantly.
was As shown
enhanced. in Figure 10d,
Accordingly, the longgradient
the velocity strip crack nucleated
in the weld pool along
wasthe interface
increased, between
increasing
the
the possibility
ferrite matrixof collision
and M/Aand aggregation
constituents. Theofsecondary
inclusions. Thatpropagated
crack is the mainstraight
reason why the
through
number of inclusions in the weld metal decreased significantly,
the ferrite grain and the elongated M/A, eventually deflecting at the grain boundary and and the size coarsened
obviously
arresting at Ej increased
asthe from 50
prior austenite kJ/cm
grain to 100 kJ/cm.
boundary. The overall straightness of the cracks in-
Ostwald
dicated that theripening may occur
surrounding during thehad
microstructures finala stages of coolingeffect
weak hindering in the onweld pool,
the cracks
allowing further coarsening of the inclusions [16,24,25]. Kluken et al. [24] divided the melt
propagation.
pool into two regions, i.e., the “hot” and “cold” parts, as shown in Figure 11. The collision
and aggregation of inclusions and the floating processes mainly occurred in the hot part of
4. Discussion
the
4.1. Effectpool.
weld of Ej onInthe
contrast,
Inclusionsin the cold
of the part,
Weld collisions and aggregation of inclusions were
Metal
less likely to occur due to the low temperature gradient and the low convective intensity
of theThe
weldpresence of convective
pool. Therefore, flow
in the coldofpart,
liquid in the melt
Ostwald pool leads
ripening was the to collisions
main wayand ag-
for the
gregation between the inclusions, which are called gradient collisions,
growth of inclusions. The growth of large inclusions was at the expense of small inclusions resulting in coarse
inclusions.
during Therefore,
Ostwald with
ripening, and thethere
increase
was aindecrease
Ej, the convective flowofofinclusions.
in the density fluid in the Theweld pool
kinetics
was enhanced. Accordingly, the velocity gradient
of Oswald ripening can be calculated as shown in Equation (1). in the weld pool was increased, increas-
ing the possibility of collision and aggregation of inclusions. That is the main reason why
the number of inclusions in the weld metal
3 64DO CO Vsignificantly,
decreased M and the size coarsened
d3 = d + t (1)
obviously as Ej increased from 50 kJ/cminito 100 kJ/cm. 9RT
Ostwald ripening may occur during the final stages of cooling in the weld pool, al-
where dini is the average initial diameter; d is the average diameter of inclusions after a
lowing further coarsening of the inclusions [16,24,25]. Kluken et al. [24] divided the melt
time t; VM is the molar volume of oxide; DO is the diffusivity of oxygen; and CO is the
pool into two regions, i.e., the “hot” and “cold” parts, as shown in Figure 11. The collision
nominal oxygen concentration of the liquid.
and aggregation of inclusions and the floating processes mainly occurred in the hot part
At the same time, with the increase in Ej , the cooling rate of the weld pool became
of the weld pool. In contrast, in the cold part, collisions and aggregation of inclusions were
slower, and the nucleation rate of the first formed oxide inclusions was very low due to the
less likely to occur due to the low temperature gradient and the low convective intensity
low subcooling. Since the oxides formed first grew to larger sizes at high temperatures and
of theas
acted weld pool. Therefore,
heterogeneous sites forin subsequent
the cold part, Ostwald
oxide ripening
nucleation, theirwas the main
reduced way further
number for the
growth of inclusions. The growth of large inclusions was at the expense
led to a lower number of inclusions. At the same time, with the increase of Ej , the growth of small inclusions
during
of Ostwald
inclusions wasripening, and there
also promoted was atemperatures,
at high decrease in thesodensity
that theofnumber
inclusions. The kinet-
of inclusions
ics of Oswald
decreased withripening can be
the increase of calculated
E but the sizeas shown in Equation (1).
coarsened.
j
64DO CO VM
d3 =d3ini + t (1)
9RT
Metals 2023, 13, x FOR PEER REVIEW 11 of 18

where dini is the average initial diameter; d is the average diameter of inclusions after a
Metals 2023, 13, 1217 time t; VM is the molar volume of oxide; DO is the diffusivity of oxygen; and CO is 11 the
of 18

nominal oxygen concentration of the liquid.

Figure 11. Schematic diagram of two main reaction zones in the weld pool.

At the same time, with the increase in Ej, the cooling rate of the weld pool became
slower, and the nucleation rate of the first formed oxide inclusions was very low due to
the low subcooling. Since the oxides formed first grew to larger sizes at high temperatures
and acted as heterogeneous sites for subsequent oxide nucleation, their reduced number
further led to a lower number of inclusions. At the same time, with the increase of Ej, the
growth11.ofSchematic
inclusions was also
diagram promoted at high temperatures, so that the number of in-
Figure
Figure 11. Schematic diagram ofoftwo
two mainreaction
main reaction zones
zones inthe
in theweld
weldpool.
pool.
clusions decreased with the increase of Ej but the size coarsened.
In order to further analyze the relationship between inclusions and AF nucleation, nucleation,
At the same time, with the increase in Ej, the cooling rate of the weld pool became
the rates of ferrite nucleation promoted by inclusions at different sizes were calculated, as
slower, and the nucleation rate of the first formed oxide inclusions was very low due to
shown in Figure 12. (Inclusions smaller than 0.2 µm μm were neglected). The increase in the
the low subcooling. Since the oxides formed first grew to larger sizes at high temperatures
size of inclusions increased their ability to stimulate AF nucleation, which is consistent
and acted as heterogeneous sites for subsequent oxide nucleation, their reduced number
with
with the
the results ofofprevious studies [16,26,27]. However, an interesting phenomenon is that
further ledresults
to a lowerprevious numberstudies [16,26,27].
of inclusions. At theHowever,
same time, an with
interesting phenomenon
the increase of Ej, theis
the size of
that theofsize inclusions
of inclusions in the weld
in the metal of
weld metal a 50 kJ/cm sample
of a temperatures,
50 kJ/cm sample could stimulate AF nucleation
growth inclusions was also promoted at high so could
that the stimulate
numberAF nu-
of in-
at almostat100%
cleation almost when 100% reaching
when about 1about
reaching µm, while1 μm, the sizethe
while of size
inclusions
of neededneeded
inclusions to reach to
clusions decreased with the increase of Ej but the size coarsened.
about
reach 1.6 µm1.6
about to μmstimulate
to AF nucleation
stimulate AF at almost
nucleation at 100% with
almost 100% the
withEj the
up toE j 100
up kJ/cm.
to 100 That
kJ/cm.
In order to further analyze the relationship between inclusions and AF nucleation,
means
That the ability
means to stimulate
the ability AF nucleation
to stimulate AF of inclusions in the weld inmetal decreased
metalwith
the rates of ferrite nucleation promoted bynucleation
inclusions at of different
inclusions sizes the
wereweldcalculated, de-
as
the increase
creased with in E
the j . With
increase theinincrease
E in E , the increased transformation
j. With the increase in Ej, the increased transformation tem-
j temperature from
shown in Figure 12. (Inclusions smaller than 0.2 μm were neglected). The increase in the
austenite
perature to ferrite
from austenitemadetothe formation
ferrite made of theGBF and PF of
formation in GBF
the weld
and PF metal easier
in the than AF.
size
At of inclusions
the same time, increased
as the sizetheir
of ability
the to
inclusion stimulate
increases, AF there
nucleation,
will be which is weld
relaxation of
metal
consistent
stress
easier
with than
the AF. At
results of the same time,
previous studies as the size of the
[16,26,27]. inclusion
However, anincreases,
interesting there will be relax-
phenomenon is
around
ation ofthe interface
stress around between
the large-size
interface inclusions
between and austenite
large-size inclusions [28–31].
and These factors
austenite all
[28–31].
that
lead the size of inclusions in the weld metal of a 50 kJ/cm sample could stimulate AF nu-
Thesetofactors
cleation
a decrease
at almost all lead in the
100% aability
towhen
decrease of AF
reaching
nucleation
in the ability
about of
1 μm,
induced
AF by inclusions in by
nucleation
while the sizeinduced
weld
of inclusions
metal and
inclusions
needed to in
an increase
weld about
metal 1.6 in E
andμm j .
an toincrease in EAF j.
reach stimulate nucleation at almost 100% with the Ej up to 100 kJ/cm.
That means the ability to stimulate AF nucleation of inclusions in the weld metal de-
creased with the increase in Ej. With the increase in Ej, the increased transformation tem-
perature from austenite to ferrite made the formation of GBF and PF in the weld metal
easier than AF. At the same time, as the size of the inclusion increases, there will be relax-
ation of stress around the interface between large-size inclusions and austenite [28–31].
These factors all lead to a decrease in the ability of AF nucleation induced by inclusions in
weld metal and an increase in Ej.

12. Size
Figure 12. Sizeand
andnumber
numberdistribution
distributionof of
inclusions
inclusionsandand
statistics of nucleation
statistics rate of
of nucleation AFofpromoted
rate AF pro-
moted
by by inclusions
inclusions with different
with different sizesdifferent
sizes under under different Ej:kJ/cm
Ej : (a) 50 (a) 50 and
kJ/cm(b)and
100(b) 100 kJ/cm.
kJ/cm.

The inclusions formed in the weld metal cancan either

either contribute
contribute to
to the
the nucleation
nucleation of
ofAF,
AF,
increasing the percentage of HAGB, thus improving the impact toughness [23], or they
can act as crack sources for the cleavage fracture, which makes the cleavage fracture occur
prematurely and reduce the impact toughness [16,32,33], especially when the inclusions
are larger than 1 µm [34], which will be discussed further in the subsequent sections.
Figure 12. Size and number distribution of inclusions and statistics of nucleation rate of AF pro-
moted by inclusions
4.2. Effect with
of Ej on the different sizes under
Microstructures different
of the Weld Ej: (a) 50 kJ/cm and (b) 100 kJ/cm.
Metal
To better understand the influence of various Ej on final microstructures, the continu-
The inclusions formed in the weld metal can either contribute to the nucleation of AF,
ous cooling expansion curves of each specimen were measured. Subsequently, the phase
increasing the percentage of HAGB, thus improving the impact toughness [23], or they
transition temperatures (Ar3) from austenite to ferrite were measured using the tangent
method, as shown in Figure 13. As Ej increased from 50 kJ/cm to 100 kJ/cm, Ar3 decreased
from 701 ◦ C to 572 ◦ C.
 4.2. Effect of Ej on the Microstructures of the Weld Metal
To better understand the influence of various Ej on final microstructures, the contin-
uous cooling expansion curves of each specimen were measured. Subsequently, the phase
transition temperatures (Ar3) from austenite to ferrite were measured using the tangent
Metals 2023, 13, 1217 12 of 18
method, as shown in Figure 13. As Ej increased from 50 kJ/cm to 100 kJ/cm, Ar3 decreased
from 701 °C to 572 °C.

Figure 13.
Figure 13. Dilatometric
Dilatometric curves
curves of
of the
the weld
weld metal
metal obtained
obtained at
at different
different EEjj..

The phase
The phasetransition
transitionfrom from austenite
austenite to ferrite
to ferrite began began
with thewithformation
the formation
of grainofbound-
grain
boundary
ary ferrite along
ferrite (GBF) (GBF)the along the austenite
austenite grain boundaries
grain boundaries at atransition
at a higher higher transition tem-
temperature.
The increase
perature. Ej led toinaEcoarsening
Theinincrease of GBF, and
j led to a coarsening the volume
of GBF, and thefraction
volumeoffraction
GBF in of theGBF
weldin
metal
the weld increased [7]. Due [7].
metal increased to the
Duehigh formation
to the temperature
high formation of GBF,ofits
temperature C atoms
GBF, were
its C atoms
diffused more sufficiently
were diffused into the
more sufficiently surrounding
into environment,
the surrounding and theand
environment, GBFthewasGBFsoft.
wasUnder
soft.
the impact load, GBF deformed firstly resulting in stress concentration.
Under the impact load, GBF deformed firstly resulting in stress concentration. At the same At the same time,
because GBF was
time, because GBFcontinuously
was continuouslydistributed in a network
distributed along along
in a network the prior
the austenite grain
prior austenite
boundary, it oftenitpromoted
grain boundary, crack propagation
often promoted [35]. So,[35].
crack propagation the increase in its size
So, the increase in and propor-
its size and
tion deteriorates
proportion the impact
deteriorates thetoughness of the weld
impact toughness metal.
of the weldWith the increase
metal. Ej , the ferrite
With theinincrease in Ej,
sideplate
the ferrite(FSP) from(FSP)
sideplate the GBFfromside
the grew in a grew
GBF side pickaxe in ashape
pickaxe toward
shapethe austenite
toward interior.
the austenite
This kind of coarse microstructure cuts the grains apart and has a very
interior. This kind of coarse microstructure cuts the grains apart and has a very negative negative effect on
the toughness of the weld metal.
effect on the toughness of the weld metal.
With
With further
further cooling,
cooling, PF,
PF, AF,
AF,andandGBGBwere
wereformed
formed in in austenite.
austenite. The
The formation
formation of of PF
PF
is a diffusion phase transition controlled by the diffusion and migration
is a diffusion phase transition controlled by the diffusion and migration of solute atoms of solute atoms
near
near the
the phase
phase interface.
interface. When
When welded
welded at at 50
50 kJ/cm,
kJ/cm, thethe lower
lower transformation
transformation temperature
temperature
(572 ◦ C) hindered the diffusion of carbon atoms and inhibited the formation of PF. The
(572 °C) hindered the diffusion of carbon atoms and inhibited the formation of PF. The
higher
higher undercooling
undercoolingwas wasconducive
conducivetotothe thetransformation
transformation ofof
AFAFandandGB.
GB.TheTheexpression
expression of
the AF nucleation rate is shown in Equation
of the AF nucleation rate is shown in Equation (2) [29,36].(2) [29,36].
γ→α 
∆Gγ→α

CC44 C C44∆G max
max
IV = CI3Vexp = −
=C3 exp= {- −- } (2)
(2)
RTRT CC2 RT
2 RT

whereCC33 is related
where related toto the
the number
number of of AF
AF nucleation
nucleationsites,
sites,and
andCC22 and
and CC44 are constants basedbased
on experimental measurements. In this work, for the AF phase
on experimental measurements. this work, for the AF phase transition, C3 transition, C 3 is related to
to
γγ→α
→α
the number density of effective inclusions. T is the temperature in Kelvin, and ∆Gmax
the number density of effective inclusions. T is the temperature in Kelvin, and ∆G is
max is
the driving
the drivingforce
forceofofthe
thetransformation
transformationfrom from austenite
austenite to to ferrite.
ferrite. AtAt lower
lower Ej ,Ethe
j, the number
number of
of inclusions
inclusions waswas large
large andandthe the value
value of Cof C3 was
3 was high,
high, which
which favored
favored AF AF nucleation.
nucleation. There-
Therefore,
fore,AFthe
the AF content
content was at
was higher higher
a lowerat aEj lower Ej of 50AskJ/cm.
of 50 kJ/cm. the Ej As the Ej increased,
increased, phase
phase transition
temperatures (Ar3) increased, and the C atoms diffused more fully, leading to a significant
increase in the PF content. At the same time, the various microstructures also underwent
significant coarsening with sufficient diffusion of C atoms [37]. The microstructure of AF
is small, and AF has a precise interlocking structure, forming HAGBs between each other.
Therefore, the reduction of AF content and the coarsening of the microstructure reduced
the hindering effect of weld metal on crack propagation.
During the cooling process of the welding thermal cycle, with the transformation
from austenite to ferrite, C diffuses from AF, PF, and GBF to the residual austenite γ0 ,
thus causing the enrichment of C in γ0 and improving the stability of γ0 [38]. On further
cooling to near room temperature, γ0 transformed into M/A constituents. When Ej was
low, the diffusion distance of C was shorter because of the decreased diffusion coefficient
 structure of AF is small, and AF has a precise interlocking structure, forming HAGBs be-
tween each other. Therefore, the reduction of AF content and the coarsening of the micro-
structure reduced the hindering effect of weld metal on crack propagation.
During the cooling process of the welding thermal cycle, with the transformation
from austenite to ferrite, C diffuses from AF, PF, and GBF to the residual austenite γ′, thus
Metals 2023, 13, 1217 13 of 18
causing the enrichment of C in γ′ and improving the stability of γ′ [38]. On further cooling
to near room temperature, γ′ transformed into M/A constituents. When Ej was low, the
diffusion distance of C was shorter because of the decreased diffusion coefficient due to
due to the cooling
the faster faster cooling rateAccordingly,
rate [37]. [37]. Accordingly,
the sizetheofsize
theof the constituents
M/A M/A constituents
formed formed
at the
at the boundaries
grain grain boundaries wasand
was small small and point
mainly mainly point
M/A. M/A.
With With theofincrease
the increase of Ejhad
Ej, C atoms ,C
atoms hadtime
sufficient sufficient time andtodynamics
and dynamics to diffuse;
diffuse; thus, thus, the
the number andnumber
size of and
M/Asize of M/A
constituents
constituents increased,
increased, while while more
more elongated M/Aelongated M/Awere
constituents constituents were the
formed along formed
grainalong the
bounda-
grain boundaries between the ferrite [39,40]. The corresponding schematic representation
ries between the ferrite [39,40]. The corresponding schematic representation of the micro-
of the microstructural
structural transformation
transformation is shown in isFigure
shown 14.in Figure 14.

Figure 14.
Figure 14. Schematically
Schematically illustrating
illustrating microstructural
microstructural transformation
transformationunder
underdifferent
differentEEj .j.

4.3.
4.3. Effects
Effects of
of EEjj on
on the
the Impact
Impact Toughness
Toughness and
and Fracture
Fracture Behaviors
Behaviors of
of the
the Weld
Weld Metal
Metal
Due
Due to the obvious hardness difference between the M/A constituents, inclusions,
to the obvious hardness difference between the M/A constituents, inclusions,
and
and the ferrite
the ferrite matrix,
matrix, when
when thethe stress
stress was
was conducted,
conducted, the
the ferrite
ferrite matrix
matrix with
with less
less hardness
hardness
deformed
deformed plastically
plastically first,
first, and
and aa large
large number
number ofofdislocations
dislocationsaccumulated
accumulated at atthe
theM/A
M/A
constituents
constituents and inclusions or near their interfaces with the surrounding ferrite matrix,
and inclusions or near their interfaces with the surrounding ferrite matrix,
resulting
resultingin instress
stressconcentration
concentrationand andthus
thuscausing microcrack
causing microcrackinitiation. The
initiation. kernel
The average
kernel aver-
misorientation
age misorientation mapmapcould be used
could be usedto to
evaluate the
evaluate thedislocation
dislocationdensity
densityand
andestimate
estimate the
the
stress distribution state of the material [41]. In this paper, it was mainly used to analyze the
stress distribution state of the material [41]. In this paper, it was mainly used to analyze
magnitude of local stress. The band contrast maps and kernel average misorientation maps
the magnitude of local stress. The band contrast maps and kernel average misorientation
are shown in Figure 15, which means that the stress concentration mainly occurred at M/A
maps are shown in Figure 15, which means that the stress concentration mainly occurred
constituents or at the M/A–matrix interface in the 100 kJ/cm sample. The M/A constituents
at M/A constituents or at the M/A–matrix interface in the 100 kJ/cm sample. The M/A
of the 50 kJ/cm sample were small, and there was rarely a high stress concentration. The
constituents of the 50 kJ/cm sample were small, and there was rarely a high stress concen-
degree of stress concentration increased significantly with the size of the M/A constituents.
tration. The degree of stress concentration increased significantly with the size of the M/A
Long-strip M/A constituents produced high stress concentrations, as did large-sized blocky
constituents. Long-strip M/A constituents produced high stress concentrations, as did
M/A constituents.
large-sized blocky M/A constituents.
As shown in Figure 10, with the increase in Ej , the cleavage fracture occurred near the
V-notch, i.e., the occurrence of the cleavage fracture was advanced. At the same time, it
was observed that a large number of cleavage surfaces of the 100 kJ/cm sample nucleated
at the large size inclusions and M/A constituents as shown in Figure 16. The bulging of
the M/A constituent at the origin of the cleavage surface indicated that crack initiation
should be due to the debonding of the M/A constituent at the interface with the ferrite
matrix [11]. Therefore, with the increase in Ej , the coarsening of inclusions and M/A
constituents significantly reduced the cleavage crack initiation energy.
Metals 2023,
Metals 2023, 13,
13, 1217
x FOR PEER REVIEW 14
14 of 18
of 18

Figure 15. Band contrast map and kernel average misorientation map of the 50 kJ/cm (a,b) and 100
kJ/cm (c,d) samples.

As shown in Figure 10, with the increase in Ej, the cleavage fracture occurred near
the V-notch, i.e., the occurrence of the cleavage fracture was advanced. At the same time,
it was observed that a large number of cleavage surfaces of the 100 kJ/cm sample nucleated
at the large size inclusions and M/A constituents as shown in Figure 16. The bulging of
the M/A constituent at the origin of the cleavage surface indicated that crack initiation
should be due to the debonding of the M/A constituent at the interface with the ferrite
matrix
Figure [11].
Figure 15. Therefore,
Band
Band contrastwith
contrast map the
mapand increase
andkernel in Ej,misorientation
kernelaverage
average the coarsening of of
map
misorientation mapinclusions
thethe
of 50 50 and(a,b)
kJ/cm
kJ/cm M/A con-
and
(a,b) 100
and
stituents
kJ/cm significantly
(c,d) samples.
100 kJ/cm (c,d) samples. reduced the cleavage crack initiation energy.

As shown in Figure 10, with the increase in Ej, the cleavage fracture occurred near
the V-notch, i.e., the occurrence of the cleavage fracture was advanced. At the same time,
it was observed that a large number of cleavage surfaces of the 100 kJ/cm sample nucleated
at the large size inclusions and M/A constituents as shown in Figure 16. The bulging of
the M/A constituent at the origin of the cleavage surface indicated that crack initiation
should be due to the debonding of the M/A constituent at the interface with the ferrite
matrix [11]. Therefore, with the increase in Ej, the coarsening of inclusions and M/A con-
stituents significantly reduced the cleavage crack initiation energy.
Figure M/Aconstituents
Figure 16. M/A constituents(a)
(a)and
andinclusions
inclusions(b)
(b)acting
actingas
asthe
thenucleation
nucleation site
site for
for cleavage
cleavage fracture
fracture
on
on the 100 kJ/cm sample.
kJ/cm sample.

According to
According to Griffith’s
Griffith’s theory
theory [42],
[42], the
the critical
critical stress for cleavage
cleavage crack
crack instability
instability
propagation can be expressed by the following equation.
!1/2
πEγ0 ’
πEγ
1/2
0
σ ’
= ( ) (3)
σc = c (1-υ22 )d (3)
(1−υ )d00
where σ′c is the critical stress, E is the Young’s modulus, γ′ is the effective surface energy
where 0 is the critical stress, E is the Young’s modulus, γ0 is the effective surface energy of
σ16.
Figure
of c M/A constituents
the fracture, (a) and inclusions
υ is the Poisson’s ratio, and(b)d acting
is theas the nucleation
microcrack site
size, for cleavage
where d0 canfracture
be re-
the fracture,
garded as kJ/cm
on the 100 issample.
the Poisson’s
theυ maximum widthratio, and
of the M/A d isconstituents
the microcrack size, where[43].
or inclusions d0 can
Frombe Equation
regarded
as the
(3), maximum
it can be seen thatwidththeofcritical
the M/A constituents
stress or inclusions
is mainly related [43]. From
to the effective Equation
surface energy(3),of
it
can be seen
ferriteAccording that
and the size the critical
to of
Griffith’sstress is mainly
theory [42],
M/A constituents related
or the to the
critical The
inclusions. effective
stress surface
for cleavage
smaller energy
the sizecrack
of M/A of ferrite
instability
constit-
and the
propagationsize of
canM/A be constituents
expressed by or inclusions.
the following The smaller
equation. the size
uents or inclusions, the larger the critical stress for cleavage cracks and instability propa-of M/A constituents or
inclusions, the larger the critical stress for
gation. With the increase of Ej, the stress concentration cleavage cracks and instability propagation.
1/2 at the large-size M/A constituents
With
’
the increase of Eincreased
j , the stress concentration ’ atcrack
πEγ
the large-size M/A constituents and inclusions
and inclusions significantly, σand
c = ( )
initiation as well as propagation became(3)
increased significantly, and crack initiation (1-υ 2 )d
as well 0as propagation became easier.
easier.
At the same time, the microstructure coarsening caused by the increase in Ej also
where σ′c is the critical stress, E is the Young’s modulus, γ′ is the effective surface energy
played an important role in deteriorating the impact toughness. The research results of Cao
of the fracture, υ is the Poisson’s ratio, and d is the microcrack size, where d0 can be re-
and Martín-Meizoso A et al. [44–46] show that the cleavage fracture can be divided into the
garded as the maximum width of the M/A constituents or inclusions [43]. From Equation
following processes: First, microcracks nucleate at the hardening phase or at the interface
(3), it can be seen that the critical stress is mainly related to the effective surface energy of
between the hardening phase and the matrix and then pass through the hardening phase or
ferrite and the size of M/A constituents or inclusions. The smaller the size of M/A constit-
the interface between the hardening phase and the matrix. Second, microcracks propagate
uents or inclusions, the larger the critical stress for cleavage cracks and instability propa-
in the matrix and reach the HAGBs of the matrix. Finally, the microcracks pass through the
gation. With
HAGBs the increase
and coalesce, of Ejto
leading , the
thestress
final,concentration
complete fracture. at the large-size M/A constituents
and inclusions increased significantly, and crack initiation as well as propagation became
easier.
 At the same time, the microstructure coarsening caused by the increase in Ej also
played an important role in deteriorating the impact toughness. The research results of
Cao and Martín-Meizoso A et al. [44–46] show that the cleavage fracture can be divided
into the following processes: First, microcracks nucleate at the hardening phase or at the
interface between the hardening phase and the matrix and then pass through the harden-
Metals 2023, 13, 1217 15 of 18
ing phase or the interface between the hardening phase and the matrix. Second, mi-
crocracks propagate in the matrix and reach the HAGBs of the matrix. Finally, the mi-
crocracks pass through the HAGBs and coalesce, leading to the final, complete fracture.
Based on
Based on the
the previous
previous studies
studies and
and combined
combined with
with the
the experimental
experimental results
results of of this
this
work, the
work, the schematic
schematic diagram
diagram of
of cleavage
cleavage crack
crack initiation
initiation and
and propagation
propagation of
of the
the impact
impact
samples is
samples is shown
shown inin Figure
Figure 17.
17.

Figure 17.
Figure Schematicdiagram
17. Schematic diagramofof cleavage
cleavagemicrocrack
microcrackinitiation
initiationand
andpropagation
propagationin
inthe
the weld
weldmetal
metal
under50
under 50kJ/cm
kJ/cm(a)
(a)and
and100
100kJ/cm
kJ/cm (b).
(b).

When the
When the specimen
specimen was
wassubjected
subjectedto tothe
theimpact
impactload,
load,aalarge
largenumber
numberof ofdislocations
dislocations
were generated that were easier to slip in AF, which was conducive to stress
were generated that were easier to slip in AF, which was conducive to stress release release[47].
[47].
AF reduces stress concentration and crack initiation tendency due to its fine structure
AF reduces stress concentration and crack initiation tendency due to its fine structure and and
superior deformation ability. The size of M/A constituents and inclusions was small under
superior deformation ability. The size of M/A constituents and inclusions was small under
Ej = 50 kJ/cm, and the nucleation of microcracks was difficult. At the same time, the grain
Ej = 50 kJ/cm, and the nucleation of microcracks was difficult. At the same time, the grain
size of AF was small, and the HAGBs accounted for a high proportion. Therefore, in the
size of AF was small, and the HAGBs accounted for a high proportion. Therefore, in the
subsequent process of microcrack propagation, the number of times that cracks met with
subsequent process of microcrack propagation, the number of times that cracks met with
the HAGBs increased, which consumed more energy.
the HAGBs increased, which consumed more energy.
However, with the increase of E to 100 kJ/cm, the proportion of AF decreased and
However, with the increase of Ejj to 100 kJ/cm, the proportion of AF decreased and
the size of M/A constituents and inclusions increased, which makes the initiation and
the size of M/A constituents and inclusions increased, which makes the initiation and
propagation of cleavage cracks very easy. This was crucial because it directly determined
propagation of cleavage cracks very easy. This was crucial because it directly determined
that the ductile fracture zone was very small, as shown in Figure 10, which means that
that the ductile fracture zone was very small, as shown in Figure 10, which means that the
the cleavage fracture occurred much earlier. At the same time, a serious weakening of the
cleavage fracture occurred much earlier. At the same time, a serious weakening of the ar-
arresting effect on crack propagation occurred as the grain size of the weld metal coarsened
resting
and theeffect on crack
proportion propagation
of HAGBs occurred
decreased. as the
As the grain force
external size of theapplied,
was weld metal coarsened
a large number
and
of microcracks nucleated on many large hardening phases and propagated at thenum-
the proportion of HAGBs decreased. As the external force was applied, a large same
ber of microcracks
time. These cracksnucleated on many
interconnected large hardening
without phases
going through and propagated
many HAGBs, leadingat the to
samethe
time. These cracks
catastrophic failureinterconnected
of the sample. without going through many HAGBs, leading to the
catastrophic failure of the sample.
5. Conclusions
5. Conclusions
In general, the influence of E on the microstructure and impact toughness in the weld
j
Inof
metal general, the influence
high-efficiency of Ej onarc
submerged thewelding
microstructure
wires has and impact
been toughness
studied infollowing
with the the weld
metal of high-efficiency submerged arc welding wires has been studied with the following
conclusions:
conclusions:
1. The increase in Ej from 50 kJ/cm to 100 kJ/cm led to a significant reduction in the
1. The increase
number in Ej fromin50
of inclusions thekJ/cm
weldto 100 and
metal kJ/cm led to a significant
a significant reduction
increase in their sizeinfrom
the
number of inclusions
0.59 to 1.26 in the weld
µm. The ability metal and
to stimulate AF anucleation
significantofincrease in their
inclusions size from
was decreased
0.59 to 1.26 μm. The ability to stimulate AF nucleation of inclusions was
with the increase of Ej due to the increased transformation temperature from austenite decreased
with the and
to ferrite increase of Ej due
the relaxation to the
of stress increased
around transformation
the interface temperature
between large-size from
inclusions
and austenite;
2. The microstructure of the weld metal welded by the high-efficiency submerged arc
welding wires included GBF, FSP, PF, AF, GB, and M/A constituents. With the Ej
increasing from 50 kJ/cm to 100 kJ/cm, the MEDMTA ≥ 15◦ of the weld metal increased
from 3.1 to 5.3 µm. At the same time, AF content decreased from 84% to 65%, with an
increase in PF from 5% to 13%. The average size of M/A constituents increased from
0.92 to 2.36 µm.
Metals 2023, 13, 1217 16 of 18

3. The high-efficiency submerged arc welding wires studied in this work are suitable
for Ej ≤ 75 kJ/cm. With the increase in Ej from 50 kJ/cm to 100 kJ/cm, the impact
absorption energy decreased significantly from 130 J to 38 J. The fracture behavior of
weld metal changed from mainly ductile fracture to mainly brittle fracture. With the
increase in Ej , the local stress around the large size inclusions and M/A constituents
was greatly improved, while the fraction of HAGBs decreased from 46.3% to 24.1%.
These two factors led to the premature cleavage fracture of weld metal, and the impact
energy decreased significantly.

Author Contributions: Conceptualization, Q.W. and J.L.; software, B.H.; validation, L.Z.; formal
analysis, F.L.; investigation, J.H.; resources, Q.W.; data curation, J.L.; writing—original draft prepara-
tion, J.L.; writing—review and editing, J.L. and Q.W.; visualization, J.L.; supervision, Q.W.; project
administration, Q.W.; funding acquisition, Q.W. and R.L. All authors have read and agreed to the
published version of the manuscript.
Funding: This work was funded by the National Natural Science Foundation of China (52127808),
the Innovation Ability Promotion Program of Hebei (22567609H), and the National Key Research and
Development Program of China (Grant No. 2017YFB0304800 and Grant No. 2017YFB0304802 for the
second subproject).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.

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