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Citation: Da Cruz Junior, E.J.; Seloto,
B.B.; Ventrella, V.A.; Varasquim,
F.M.F.A.; Zambon, A.; Calliari, I.;
Gennari, C.; Settimi, A.G. Correction
of Phase Balance on Nd:YAG Pulsed
Laser Welded UNS S32750 Using
Cobalt Electroplating Technique.
Crystals 2023,13, 256. https://
doi.org/10.3390/cryst13020256
Academic Editors: Pavel Lukáˇc,
Shenghu Chen and Qingsong Pan
Received: 23 December 2022
Revised: 30 January 2023
Accepted: 31 January 2023
Published: 2 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
crystals
Article
Correction of Phase Balance on Nd:YAG Pulsed Laser Welded
UNS S32750 Using Cobalt Electroplating Technique
Eli J. Da Cruz Junior 1, * , Bruna B. Seloto 2, Vicente A. Ventrella 2, Francisco M. F. A. Varasquim 1,
Andrea Zambon 3, Irene Calliari 3, Claudio Gennari 3and Alessio G. Settimi 3
1São Paulo Federal Institute of Education, Science and Technology, Campus Itapetininga,
Itapetininga 18202-000, SP, Brazil
2Mechanical Engineering Department, Campus of Ilha Solteira, UNESP, São Paulo State University,
Ilha Solteira 15385-000, SP, Brazil
3Department of Industrial Engineering, University of Padua, Via Marzolo 9, 35131 Padova, Italy
*Correspondence: dacruz.eli@ifsp.edu.br
Abstract:
Super-duplex stainless steel (SDSS) shows high mechanical and corrosion resistance because
of the balanced structure of austenite and ferrite. However, maintaining this phase ratio after
welding is a challenge. The use of austenite stabilizing components is recommended to balance the
microstructure. The addition of alloying elements presents a challenge because of the characteristics
of Nd:YAG pulsed laser welding. An approach, which has proven to be effective, is to use metal
electroplating to prepare the surfaces of the mechanical SDSS components that will be welded,
therefore promoting the phase balance in the fusion zone. While the effects of metals such as
nickel as an austenite stabilizer are well recognized, cobalt’s effects require more research. The
present work investigated the influence of the use of cobalt addition in the joining process by
preliminary electroplating on UNS S32750 SDSS Nd: YAG pulsed laser welding, specifically regarding
microstructure and microhardness. Three conditions were investigated, changing the thickness of the
deposited cobalt layer. The addition of cobalt modified the morphology and increased the volume
fraction of austenite. An austenite volume fraction of around 48% was obtained using a 35
µ
m thick
cobalt coating. The microhardness was affected by austenite/ferrite proportions. The microhardness
dropped from about 375 HV to 345 HV as the cobalt layer’s thickness rose, being similar to that of the
base metal. The effect of cobalt as an austenite stabilizer was observed, and the cobalt electroplating
technique was effective to correct the phase balance on UNS S32750 laser welding.
Keywords:
super duplex stainless steel; Nd:YAG pulsed laser welding; cobalt electroplating; volume
fraction; microstructure; microhardness
1. Introduction
Stainless steel is known as the steel that resists corrosion. The chromium oxide film
that is produced on the surface of the metal materials, forming a passive layer that isolates
and protects the surface, is what gives the materials their corrosion resistance [
1
]. To reach
excellent mechanical and corrosion resistance in Duplex Stainless Steel (DSS) it is required
to attain a balanced proportion of face-centered cubic (FCC) austenite and body-centered
cubic (BCC) ferrite in its biphasic microstructure. Super Duplex Stainless Steel (SDDS) is a
special high alloyed grade of DSS and it has been used in environments containing chloride
due to its excellent resistance to pitting and stress corrosion cracking [2–4].
DSS is stronger than austenitic steels and tougher than ferritic stainless steels. These
alloys are about twice as strong as austenitic steels. These steels have a wide range of uses
in the chemical, food and beverage, nuclear, oil and gas, petrochemical, pulp and paper,
offshore, and marine industries because of their excellent characteristics. The difference
between DSS and SDSS beyond the concentration of alloying elements is their Pitting
Resistance Equivalent Number (PREN) [
5
]. PREN is a measure of the alloy’s resistance
Crystals 2023,13, 256. https://doi.org/10.3390/cryst13020256 https://www.mdpi.com/journal/crystals
Crystals 2023,13, 256 2 of 11
to pitting corrosion. PREN is calculated from a simple formula: PREN = %Cr + 3.3%Mo
+16%N. DSS has PREN between 30 and 40, while SDSS has PREN above 40. SDDS is
frequently used when mechanical and corrosion resistance are needed [6].
Despite the broad advantages of SDDS, after being subjected to welding process, SDSS
presents an unbalanced microstructure with a high content of ferrite. In DSS welding,
austenite is formed from solid-state ferrite. Low cooling rates during welding promote
ferritic grain growth in the heat-affected zone (HAZ) and the formation of nitrides and
carbides. Due to the lack of time needed for austenite growth, rapid cooling rates promote
a larger concentration of ferrite [6].
According to the literature, to have an industrial application, an austenite volume
fraction higher than 25% is mandatory [
7
–
9
]. Another problem related to SDSS welding is
that due to the large amount of alloying element, depending on the welding parameters,
carbides, nitrides, and others detrimental secondary phases can precipitate in the temper-
ature range between 700 and 950
◦
C. Even a small amount of sigma phase is enough to
cause a huge reduction in ductility and corrosion resistance of SDSS [10–12].
There are several studies concerning the addition of austenitising elements to obtain
a balanced microstructure [
13
]. The effects of nickel and nitrogen addition on the mi-
crostructure and mechanical properties of electron beam welded DSS were investigated by
Muthupandi et al. [
14
,
15
]. Zhang et al. [
16
] studied the effects of nitrogen in shielding gas
on microstructure evolution and localized corrosion behavior of a duplex stainless steel
welding joint. Migiakis and Papadimitriou [
17
] also studied the effects of nickel and nitro-
gen on the microstructure and mechanical properties, but applied to SDSS plasma welding.
Pilhagen and Sandström [
18
] have investigated the role of nickel on the toughness of lean
DSS weld metal prepared by submerged arc welding (SAW). Tahaei et al. [
19
] studied the
effect of nickel and post-weld heat treatment applied to gas tungsten arc welding (GTAW).
The influence of nickel and nitrogen was examined in all works, but the laser welding
procedure wasn’t employed in any of them.
Cobalt is an austenitising element and is even used in the calculation of the nickel
equivalent number (Schaeffler–Delong Diagram). Although cobalt is an austenitising el-
ement [
20
,
21
], its application has not been described in detail yet to balance the SDSS
microstructure during a welding process. Cobalt is used to improve wear resistance,
corrosion resistance, and heat resistance in cobalt-base alloys and nickel-base superal-
loys [
22
–
24
]. Cobalt reduces the grain boundary carbides precipitation, improving corrosion
resistance [22].
The development of new DSS grades, such as SDSS, demands the use of more ap-
propriate welding techniques. Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet)
pulsed laser welding has some benefits over traditional methods, e.g., narrow heat affected
zone (HAZ), higher reliability, minimal distortion, simplicity of automation, higher traverse
speed, and short cycle time, making it an advantageous process in high-scale produc-
tion [
25
,
26
]. The characteristics of Nd:YAG pulsed laser, e.g., high cooling rates and low
heat input, prevent the precipitation of secondary phases but also result in a predominantly
ferritic welded microstructure [
27
]. The amount of ferrite phase in the solidified structures
in some DSS grades following the laser welding process is very close to 100% [28].
It is quite challenging to add alloying elements in Nd:YAG pulsed laser welding
due to process characteristics, using either shielding gas or filler wires [
29
]. Da Cruz
Junior et al. reported the use of a nickel electroplating technique for preparing surfaces of
SDSS mechanical elements to be welded [
30
]. They submitted the surface to be welded
to the Watts bath, promoting the formation of a nickel layer. The technique was efficient
for correcting the phase balance on SDSS laser welding. The use of metal electroplating
techniques associated with the laser welding process have been shown to be very promising
solutions for the phase balance correction on SDSS welding. However, further studies are
needed using alloying elements other than nickel.
Given the significance of maintaining phase balance to preserve the characteristics of
SDSSs and, consequently, their applications, as well as the suitability of metal electroplating
Crystals 2023,13, 256 3 of 11
techniques for forming an electrodeposited cobalt layer, and due the paucity of studies
regarding cobalt’s effect as an austenite-promoting element in SDSS welding, this work
studied the use of cobalt electroplating to promote austenite formation on UNS S32750
pulsed laser welding for the purpose of correcting the resulting microstructure phase
balance in the fusion zone.
2. Materials and Methods
Sheets 1.5 mm thick of UNS S32750 SDSS were used as base metal. DSS contains ferrite
stabilizing elements, such as Cr, Mo, Si, and W, as well as austenite stabilizing elements such
as Ni, Mn, C, N, and Cu. The Ni-equivalent and Cr-equivalent are frequently calculated to
estimate the total effect of the chemical composition on the microstructure; they demonstrate
the ability of the alloying elements to stabilize the ferrite and austenite structures. The
Ni-equivalent and Cr-equivalent were calculated using the universe formulas according to
the WRC 1992 Constitution Diagram as proposed by Jiang et. al. [31]. The used equations
were: Cr equivalent = %Cr + %Mo + 7.5%Nb and Ni equivalent = %Ni + 35%C + 20%N +
0.25%Cu. The chemical composition (provided by the manufacturer) and other important
information is given in Table 1.
Table 1. Chemical composition of UNS S32750 SDSS (%wt.), Cr and Ni equivalent and PREN.
Cr Ni Mo Mn Si N Cu P C S Fe
25.61 6.97 3.84 0.63 0.29 0.27 0.15 0.02 0.018
0.001
balance
PREN = %Cr + 3.3%Mo +16%N = 42.6
Cr equivalent = 29.45
Ni equivalent = 13.01
Part of the sample’s surface was immersed in the bath’s solution (containing 31.2 g of
CoSO
4
.7H
2
O, 2.0 g of NaCl, and 7.6 g of H
3
BO
3
in 100 mL of H
2
O) for different immersion
times, resulting in different electroplated layer thickness. The current density adopted was
3.7 A/dm
2
. The pH and temperature of the solution were 2
◦
C and 30
◦
C, respectively [
32
].
A Nd:YAG pulsed laser facility model UW-150 A (United Winners), with a 150 W
maximum power and beam spot diameter 0.2 mm was employed. The weld that achieves a
regular surface, without porosity, and with a deep weld pool greater than 50% of the sheet
thickness was analyzed to determine the welding parameters. The samples were welded in
pairs, with just one of the parts being coated with the electrodeposited layer of cobalt as
presented in Figure 1.
Crystals 2023, 13, x FOR PEER REVIEW 4 of 11
Figure 1. Samples dimensions and schematic representation of the butt weld joint configuration.
The conditions were rated in terms of cobalt coating deposition time, obtaining three
layers of thickness. Table 2 lists the welding parameters and the resulting thickness of the
electrodeposited layer for each condition.
Table 2. Cobalt electrodeposited layers and welding parameters.
Condition Cobalt Layer Thickness [µm] Welding Parameters
Co12 12.1 ± 1.1 Peak power: 2.0 kW
Welding speed: 1 mm/s
Pulse duration: 5 ms
Frequency: 9 Hz
Shielding gas: Argon
Flow rate: 20 L/min
Co20 20.2 ± 1.3
Co35 35.1 ± 1.2
For the metallographic analysis, the samples were sliced transverse to the weld
direction after the welding operation. Using a silicone mold, the samples were set into
cold-curing transparent epoxy resin. The samples were hand sanded, with each sanding
change rotating the samples by 90 degrees. Alumina suspension was employed for
polishing. The microstructure was revealed using Beraha’s reagent and then the samples
were observed with scanning electron microscope (SEM) (Carl Zeiss EVO LS15). The
austenite and ferrite volume fractions were calculated using the free image software
Image J. For each condition, three samples were prepared. Ten SEM images (fusion zone)
of each sample were examined. The micrographs were binarized, with ferrite appearing
dark and austenite appearing light. The austenite/ferrite ratio was obtained by comparing
the areas in the micrograph related to austenite and ferrite.
Vickers microhardness was evaluated perpendicular to the welding direction across
the entire width of the weldments, with a test load of 10 gf for 10 s and intervals of 0.08
mm respecting the ASTM-E384 standard. Energy-dispersive X-ray spectroscopy (EDS)
analyses were performed to determinate the amount (wt. %) of cobalt on the
electrodeposited layers and in the fusion zone.
3. Results and Discussion
The optical micrograph and EDS diagram for the sample Co35 are presented in
Figure 2. The other two samples (Co12 and Co20) had the same behavior. The composition
of the electrodeposited layer was approximately 95% Co; the remaining 5% of Fe and Cr
were probably from the base metal. The layer showed constant thickness, adhered well to
Figure 1. Samples dimensions and schematic representation of the butt weld joint configuration.
Crystals 2023,13, 256 4 of 11
The conditions were rated in terms of cobalt coating deposition time, obtaining three
layers of thickness. Table 2lists the welding parameters and the resulting thickness of the
electrodeposited layer for each condition.
Table 2. Cobalt electrodeposited layers and welding parameters.
Condition Cobalt Layer Thickness [µm] Welding Parameters
Co12 12.1 ±1.1 Peak power: 2.0 kW
Welding speed: 1 mm/s
Pulse duration: 5 ms
Frequency: 9 Hz
Shielding gas: Argon
Flow rate: 20 L/min
Co20 20.2 ±1.3
Co35 35.1 ±1.2
For the metallographic analysis, the samples were sliced transverse to the weld di-
rection after the welding operation. Using a silicone mold, the samples were set into
cold-curing transparent epoxy resin. The samples were hand sanded, with each sanding
change rotating the samples by 90 degrees. Alumina suspension was employed for polish-
ing. The microstructure was revealed using Beraha’s reagent and then the samples were
observed with scanning electron microscope (SEM) (Carl Zeiss EVO LS15). The austenite
and ferrite volume fractions were calculated using the free image software Image J. For
each condition, three samples were prepared. Ten SEM images (fusion zone) of each sample
were examined. The micrographs were binarized, with ferrite appearing dark and austenite
appearing light. The austenite/ferrite ratio was obtained by comparing the areas in the
micrograph related to austenite and ferrite.
Vickers microhardness was evaluated perpendicular to the welding direction across
the entire width of the weldments, with a test load of 10 gf for 10 s and intervals of 0.08 mm
respecting the ASTM-E384 standard. Energy-dispersive X-ray spectroscopy (EDS) analyses
were performed to determinate the amount (wt. %) of cobalt on the electrodeposited layers
and in the fusion zone.
3. Results and Discussion
The optical micrograph and EDS diagram for the sample Co35 are presented in
Figure 2. The other two samples (Co12 and Co20) had the same behavior. The composition
of the electrodeposited layer was approximately 95% Co; the remaining 5% of Fe and Cr
were probably from the base metal. The layer showed constant thickness, adhered well
to the surface of the base metal, and was not affected by relevant present porosity. These
characteristics contribute to a weld bead without discontinuities and achieves a uniform
austenite/ferrite proportion distribution [29,33].
Crystals 2023, 13, x FOR PEER REVIEW 5 of 11
the surface of the base metal, and was not affected by relevant present porosity. These
characteristics contribute to a weld bead without discontinuities and achieves a uniform
austenite/ferrite proportion distribution [29,33].
Figure 2. Co35 (a) Optical micrograph and (b) EDS diagram.
The SEM micrographs of the entire weld bead, which includes the base metal and the
cross section of the weldment for the Co12 sample are shown in Figure 3 in order to
display the weld bead geometry and dimensions. Because they all (Co20 and Co35) had
the same geometry, only the weld bead for the Co12 condition is shown. It is the
parameters and the welding mode that determine the geometry of the bead, and for all
conditions, these variables were kept constant.
Figure 3. SEM micrograph of the entire weld bead.
The weld bead had typical keyhole mode welding geometry: it was narrow and deep.
The width and depth of the weld bead were 1.2 mm and 0.8 mm, respectively. As
mentioned in the methodology, the welding parameters were selected to give a regular
surface without porosity and a deep weld pool higher than 50% of the sheet thickness.
The depth reached (0.8 mm) was greater than half of the total thickness of the base metal
sheet (1.5 mm).
Figure 2. Co35 (a) Optical micrograph and (b) EDS diagram.
Crystals 2023,13, 256 5 of 11
The SEM micrographs of the entire weld bead, which includes the base metal and the
cross section of the weldment for the Co12 sample are shown in Figure 3in order to display
the weld bead geometry and dimensions. Because they all (Co20 and Co35) had the same
geometry, only the weld bead for the Co12 condition is shown. It is the parameters and
the welding mode that determine the geometry of the bead, and for all conditions, these
variables were kept constant.
Crystals 2023, 13, x FOR PEER REVIEW 5 of 11
the surface of the base metal, and was not affected by relevant present porosity. These
characteristics contribute to a weld bead without discontinuities and achieves a uniform
austenite/ferrite proportion distribution [29,33].
Figure 2. Co35 (a) Optical micrograph and (b) EDS diagram.
The SEM micrographs of the entire weld bead, which includes the base metal and the
cross section of the weldment for the Co12 sample are shown in Figure 3 in order to
display the weld bead geometry and dimensions. Because they all (Co20 and Co35) had
the same geometry, only the weld bead for the Co12 condition is shown. It is the
parameters and the welding mode that determine the geometry of the bead, and for all
conditions, these variables were kept constant.
Figure 3. SEM micrograph of the entire weld bead.
The weld bead had typical keyhole mode welding geometry: it was narrow and deep.
The width and depth of the weld bead were 1.2 mm and 0.8 mm, respectively. As
mentioned in the methodology, the welding parameters were selected to give a regular
surface without porosity and a deep weld pool higher than 50% of the sheet thickness.
The depth reached (0.8 mm) was greater than half of the total thickness of the base metal
sheet (1.5 mm).
Figure 3. SEM micrograph of the entire weld bead.
The weld bead had typical keyhole mode welding geometry: it was narrow and deep.
The width and depth of the weld bead were 1.2 mm and 0.8 mm, respectively. As mentioned
in the methodology, the welding parameters were selected to give a regular surface without
porosity and a deep weld pool higher than 50% of the sheet thickness. The depth reached
(0.8 mm) was greater than half of the total thickness of the base metal sheet (1.5 mm).
In general, the weld bead showed a very uniform microstructure along the fusion
zone, indicating that there was efficient mixing of cobalt in the welding pool. The HAZ
was relatively narrow, which was to be expected for laser welding [
29
]. It is possible to
observe the balanced microstructure of the base metal, with the ferrite and austenite grains
aligned in the rolling direction.
Figure 4displays the SEM micrographs of the fusion zone for the Co12, Co20, and Co35
samples (a, b, and c, respectively). Austenite appears as the lightest phase and ferrite as the
darkest. Thicker electrodeposited layers lead to an increase in cobalt concentration, which
altered the morphology of austenite and increased its volume fraction. The sample Co12
had an unbalanced microstructure with several polygonal ferrite grains and allotriomorphic
austenite at the boundaries. In SDSS welding, austenite is formed from a diffusional solid-
state transformation so that, under high cooling rates as is usual for pulsed laser welding,
the austenite formation cannot fully develop [
6
,
33
]. The effect of cobalt, as an austenite
former, was not sufficient to overlap the thermal cycle effects imposed by laser welding.
Crystals 2023,13, 256 6 of 11
Crystals 2023, 13, x FOR PEER REVIEW 6 of 11
In general, the weld bead showed a very uniform microstructure along the fusion
zone, indicating that there was efficient mixing of cobalt in the welding pool. The HAZ
was relatively narrow, which was to be expected for laser welding [29]. It is possible to
observe the balanced microstructure of the base metal, with the ferrite and austenite
grains aligned in the rolling direction.
Figure 4 displays the SEM micrographs of the fusion zone for the Co12, Co20, and
Co35 samples (a, b, and c, respectively). Austenite appears as the lightest phase and ferrite
as the darkest. Thicker electrodeposited layers lead to an increase in cobalt concentration,
which altered the morphology of austenite and increased its volume fraction. The sample
Co12 had an unbalanced microstructure with several polygonal ferrite grains and
allotriomorphic austenite at the boundaries. In SDSS welding, austenite is formed from a
diffusional solid-state transformation so that, under high cooling rates as is usual for
pulsed laser welding, the austenite formation cannot fully develop [6,33]. The effect of
cobalt, as an austenite former, was not sufficient to overlap the thermal cycle effects
imposed by laser welding.
Three different types of austenite can occur in SDSS weld metals: allotriomorphs,
formed at the ferrite grain boundaries; Widmanstätten side plates that develop into the
grain from the allotriomorph grain boundaries; and intragranular austenite [15]. The effect
of the cobalt, deriving from the coating, in the morphology of austenite in the samples
Co20 and Co35 can be observed, with the formation of Widmanstätten austenite nucleated
from allotriomorph grain boundary austenite and intragranular austenite. The cobalt not
only modified the morphology of austenite, but also increased its volume fraction,
resulting in an approximately balanced microstructure for the sample Co35.
Figure 4. Microstructure of fusion zone (SEM micrographs) for (a) Co12, (b) Co20 and (c) Co35.
Figure 4. Microstructure of fusion zone (SEM micrographs) for (a) Co12, (b) Co20 and (c) Co35.
Three different types of austenite can occur in SDSS weld metals: allotriomorphs,
formed at the ferrite grain boundaries; Widmanstätten side plates that develop into the
grain from the allotriomorph grain boundaries; and intragranular austenite [
15
]. The effect
of the cobalt, deriving from the coating, in the morphology of austenite in the samples Co20
and Co35 can be observed, with the formation of Widmanstätten austenite nucleated from
allotriomorph grain boundary austenite and intragranular austenite. The cobalt not only
modified the morphology of austenite, but also increased its volume fraction, resulting in
an approximately balanced microstructure for the sample Co35.
The cobalt enrichment, promoted by the electroplating technique, is estimated in
Figure 5, which shows the composition (wt.%) of the fusion zone obtained by EDS analysis
for the samples Co12, Co20, and Co35 (a, b, and c, respectively). The values shown are the
average amounts (wt.%) of Fe, Cr, Ni, Mo, and Co in the melting zone. The Cr, Ni, Fe, and
Mo concentrations remained the same of the base metal (Table 1). The cobalt amounts in
the fusion zones were from the cobalt coating since the base metal does not have cobalt in
its composition. Figure 5d presents the increase in cobalt amounts in the fusion zones for
all the samples. The increase in cobalt amount increased the volume fraction of austenite,
corroborating the results observed in the micrographs and proving the efficiency of the
cobalt as austenite former [20,21,23].
Crystals 2023,13, 256 7 of 11
Crystals 2023, 13, x FOR PEER REVIEW 7 of 11
The cobalt enrichment, promoted by the electroplating technique, is estimated in
Figure 5, which shows the composition (wt.%) of the fusion zone obtained by EDS analysis
for the samples Co12, Co20, and Co35 (a, b, and c, respectively). The values shown are the
average amounts (wt.%) of Fe, Cr, Ni, Mo, and Co in the melting zone. The Cr, Ni, Fe, and
Mo concentrations remained the same of the base metal (Table 1). The cobalt amounts in
the fusion zones were from the cobalt coating since the base metal does not have cobalt in
its composition. Figure 5d presents the increase in cobalt amounts in the fusion zones for
all the samples. The increase in cobalt amount increased the volume fraction of austenite,
corroborating the results observed in the micrographs and proving the efficiency of the
cobalt as austenite former [20,21,23].
Figure 5. Fusion zone composition for (a) Co12, (b) Co20, (c) Co35 and (d) Co wt.% increase for all
the conditions.
Figure 6 displays the SEM micrographs of the transition between the base metal and
fusion zone for each sample. Austenite appears as the lightest phase and ferrite as the
darkest.
Figure 5.
Fusion zone composition for (
a
) Co12, (
b
) Co20, (
c
) Co35 and (
d
) Co wt.% increase for all
the conditions.
Figure 6displays the SEM micrographs of the transition between the base metal
and fusion zone for each sample. Austenite appears as the lightest phase and ferrite as
the darkest.
All weldments in the heat-affected zone (HAZ) have a phase balance of austen-
ite/ferrite that is quite unbalanced, with ferrite taking up the highest volume fraction [
16
].
The austenite proportion at the HAZ is significantly shorter than at the weld center, even for
Co35 sample, which displayed a balanced microstructure in the fusion zone. This is because
the cooling rate has increased in this area without enough time for more austenite forma-
tion. The HAZ’s corrosion resistance is significantly impacted by the ferrite preponderance,
which also increases the risk of pitting corrosion [29,34].
Table 3presents the average volume fraction of ferrite and austenite in the fusion
zone for each sample. The addition of cobalt, using the electroplating technique, was
effective to balance the microstructure once austenite and ferrite were present in almost
equal volume fraction in the Co35 sample. When all other welding conditions were kept
constant throughout all the tests, this impact was a result of the cobalt coating. According
to the literature, the weld bead must have a minimum austenite ratio of 25 to 30 percent
for most industrial uses [
6
–
9
]. Considering these references, Co35 sample presents a good
austenite volume fraction result.
Table 4displays the average microhardness of the weld bead for each condition. The
base metal average microhardness was 307
±
4 HV. The Co12 sample presents the highest
hardness due the predominantly ferritic microstructure. Higher ferrite content leads to high
strength [
35
]. As envisaged, a drop in hardness is shown as the amount of austenite in the
weld bead increases. Despite that the austenite/ferrite ration for the Co35 sample is nearly
identical to that of the base material, the microhardness is slightly higher. The hardness
increase is a consequence of the intergranular austenite development associated with a
Crystals 2023,13, 256 8 of 11
rapid cooling that introduce a strain hardening effect in microstructure, and also an effect
from the higher overall extension of boundaries within single ferritic grains [19,29,36].
Crystals 2023, 13, x FOR PEER REVIEW 8 of 11
Figure 6. SEM micrographs of transitions for (a) Co12, (b) Co20 and (c) Co35.
All weldments in the heat-affected zone (HAZ) have a phase balance of
austenite/ferrite that is quite unbalanced, with ferrite taking up the highest volume
fraction [16]. The austenite proportion at the HAZ is significantly shorter than at the weld
center, even for Co35 sample, which displayed a balanced microstructure in the fusion
zone. This is because the cooling rate has increased in this area without enough time for
more austenite formation. The HAZ’s corrosion resistance is significantly impacted by the
ferrite preponderance, which also increases the risk of pitting corrosion [29,34].
Table 3 presents the average volume fraction of ferrite and austenite in the fusion
zone for each sample. The addition of cobalt, using the electroplating technique, was
effective to balance the microstructure once austenite and ferrite were present in almost
equal volume fraction in the Co35 sample. When all other welding conditions were kept
constant throughout all the tests, this impact was a result of the cobalt coating. According
to the literature, the weld bead must have a minimum austenite ratio of 25 to 30 percent
for most industrial uses [6–9]. Considering these references, Co35 sample presents a good
austenite volume fraction result.
Table 3. Volume fractions for all the conditions.
Condition Ferrite [%] Austenite [%] SD
Co12 93.5 6.5 0.9
Co20 79.5 20.5 1.1
Co35 52.1 47.9 1.2
Figure 6. SEM micrographs of transitions for (a) Co12, (b) Co20 and (c) Co35.
Table 3. Volume fractions for all the conditions.
Condition Ferrite [%] Austenite [%] SD
Co12 93.5 6.5 0.9
Co20 79.5 20.5 1.1
Co35 52.1 47.9 1.2
Table 4. Weld bead hardness.
Condition Fusion Zone Hardness [HV]
Co12 375 ±6
Co20 361 ±7
Co35 345 ±6
While a high ferrite content boosts mechanical strength at the expense of corrosion
resistance, a high austenite content boosts corrosion resistance while lowering mechanical
strength. Balanced austenite/ferrite proportions are essential for a desirable combination
of mechanical strength and corrosion resistance [33,37], like sample Co35.
Crystals 2023,13, 256 9 of 11
4. Conclusions
1.
It is feasible to use cobalt as an austenite-promoting element in SDSS welding. Once
the volume fraction and morphology of austenite were changed, cobalt addition
influenced its formation. An increase can be observed in the austenite proportion in
the weld bead.
2.
The use of the cobalt electroplating technique was effective to promote austenite for-
mation and, consequently, correct the resulting microstructure in SDSS laser welding.
Given the difficulty of using alloying elements in Nd:YAG pulsed laser welding, the
technique presented may expand the use of this welding in SDSS, ensuring proper
phase balance in the weld bead.
3.
A higher austenite volume fraction in the fusion zone was obtained by thicker elec-
trodeposited cobalt layers. The ideal scenario was using a cobalt coating that was 35
µ
m thick. The phase balance was confirmed by the austenite volume percentage of
Ni35, which was close to 48 percent.
4.
The increase in austenite proportion caused a decrease in the microhardness of the
weld bead. The microhardness dropped from about 375 HV (Co12) to 345 HV (Co35)
as the cobalt layer’s thickness rose, being similar to that of the base metal. Although
the volume fraction of austenite in the Co35 sample was close to that of the base metal,
the hardness was approximately 15% higher because intragranular austenite formed
as a result of the rapid cooling rates during the laser welding, and also showed an
effect from the higher overall extension of boundaries within single ferritic grains.
5.
The results from the method outlined improves the application of Nd:YAG pulsed
laser welding on DSS and might be used to broaden the design of the welding settings
to include other DSS and laser sources.
Author Contributions:
Conceptualization, E.J.D.C.J., B.B.S. and V.A.V.; methodology, E.J.D.C.J., B.B.S.
and A.G.S.; validation, A.Z. and V.A.V.; formal analysis, F.M.F.A.V. and C.G.; writing—original
draft preparation, E.J.D.C.J., B.B.S., F.M.F.A.V., C.G. and A.G.S.; writing—review and editing, I.C.;
supervision, I.C., A.Z. and V.A.V. All authors have read and agreed to the published version of the
manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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