“heat input effect in the microstructure on high strength...
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MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
Tema A2a Materiales: Soldadura híbrida, Hybrid Welding.
“Heat input effect in the microstructure on High Strength Low Alloy Steel welded by Laser-GMAW Hybrid Welding”
Ana Soriaa, Melvyn Álvareza, Miguel Carrizaleza
aCorporación Mexicana de Investigación en Materiales S.A. de C.V., Ciencia y Tecnología No.790, Saltillo Coah. C.P.25290, México
*Contacto: [email protected]
R E S U M E N
Los ciclos de temperatura utilizados durante los procesos de soldadura tienen efecto en la evolución de la microestructura
y propiedades mecánicas de la unión. El objetivo del estudio es evaluar la microestructura obtenida en el acero HSLA550
de 8 mm soldado por el proceso de soldadura híbrida láser-GMAW en función de la entrada de calor del proceso. Se
evaluaron la microdureza en las áreas superior, media e inferior de la soldadura transversal y la resistencia a la tensión. Se
usó microscopía electrónica de barrido para analizar los cambios microestructurales en la zona de fusión (ZF) y la zona
afectada por el calor (ZAC). La microestructura observada en la zona del láser fue mayormente martensita en listones,
mientras que en la ZAC se encontró una mezcla de martensita en placa y listones y en la ZF se encontraron algunas áreas
de ferrita acicular y ferrita widmastäten secundaria.
Palabras Clave: Soldadura híbrida de laser- arco, acero de alta resistencia y baja aleación, microestructura.
A B S T R A C T
The temperature cycles used during the welding processes have an effect in the evolution of the microstructure, which is
mainly responsible for the mechanical properties of the joint. The aim of this study is to evaluate the microstructure
obtained in the 8 mm HSLA 550 steel welded by the laser-GMAW hybrid welding process depending on the heat input of
the process. The microhardness was evaluated in the upper, middle and lower areas of the transversal welding and tension
strength. Scanning electron microscopy was used to analyze the microstructural changes in the fusion zone (FZ) and heat
affected zone (HAZ). The microstructure observed in the laser zone was mostly lath martensite, while in HAZ a mixture
of martensite in plate and lath was found and in FZ some areas of acicular ferrite and secondary widmastäten ferrite were
found.
Keywords: Hybrid Laser Arc Welding, High Strength Low Alloy Steel, Microstructure.
1. Introducción
High strength and low alloy steels (HSLA) are used in the
construction, transportation and energy industry; some
grades of API HSLA steels, contain small amounts of
hardener elements such as Ti, Nb and V and low carbon
content, which provides good weldability [1]. High strength
steels are manufactured by advanced technologies achieving
a microstructure of fine ferrite grains and perlite bands
aligned in the direction of the deformation during the
manufacturing process providing high strength and good
toughness [2,3].
Welding processes are necessary for the fabrication of
structures, fusion welding employs thermal cycles that
modify the microstructure of the components and can affect
the mechanical properties of the joint as well as promoting
defects such as solidification cracking, hydrogen
embrittlement or segregations [4]. However, the effect of
each type of welding provides different heat input, welding
speed and cooling speed.
The effect of laser welding (LW), gas tungsten arc
welding (GTAW) and gas metal arc welding (GMAW) on
the microstructure and mechanical properties of high-
strength steel joints has been investigated [5,6]. In LW and
GTAW processes, greater hardness was obtained in the FZ
than in the base metal; while in GMAW the FZ showed
lower hardness than in the base metal [7]. In addition, in
laser welding, the change in the microhardness in the joint
was consistent, with hardening increments and softening
zones [8]. The microstructure resulting from a ultra-narrow
gap laser process and GMAW in a high strength steel S960
is also compared; obtaining in the FZ martensite and
martensite tempered in LW, the fracture resistance of the
welded metal and the base metal are similar. While in the
GMAW process the microstructure of the FZ is formed of
ferrite with some martensite, the welded metal was reduced
by 100 MPa compared to the base metal; as well showing a
softening in the HAZ [9].
MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
The differences between arc and laser welding are due to
the particularities of each process; laser welding is
characterized by producing narrow and deep welds while arc
welding induces high heat input, that could generate
distortion [10]. A novel process for the industry is hybrid
laser-arc welding (HLAW), which is the combined process
where laser and arc welding are used simultaneously
interacting in the same welding pool [11]. Narrower welds
and small HAZ are obtained in the HLAW process. These
welds minimize the residual tension and distortion, due the
greater penetration depth of the laser allowed by the arc
interaction in the weld pool [12]. In a comparison between
HLAW and laser welding, the microstructure of HLAW FZ
was found acicular ferrite. Meanwhile in laser process the
FZ show the presence of bainite and martensite phases. In
addition, microhardness in laser welding was 40% higher
than in HLAW, due less temperature gradient within the
HLAW process [13].
In the present study the microstructure of the HAZ of 8
mm thick HSLA 550 steel plates joined by the HLAW
process was evaluated, the welds were made with a power
variation of 7 and 8 kW and wire feeding rate of 10 and 12
m/min. The microstructure was analyzed in optical
microscopy and scanning electron microscopy, in addition,
the microhardness of the weld and the tensile strength were
evaluated to determine the effect of the hybrid welding
process in the joint.
2. Experimental materials and procedures
HSLA 550 steel of 50 mm x 50 mm and 8 mm thickness
were used as the base metal. Table 1 lists the chemical
composition of HSLA 550.
Table 1 – Chemical composition of the HSLA 550. (wt.%).
Fe C V Nb Ti Si Mn Al
98.3 0.12 0.0034 0.0055 0.045 0.25 1.13 0.049
The butt joints were welded by the laser-GMAW hybrid
welding process by the TRUMPF equipment consisting of a
disk laser and a GMAW torch (Fig. 1). The welding
parameters used are shown in Table 2.
Samples were prepared metallographically with grinding
and polishing, were etched with 5% Nital for 5 seconds and
Picral 2% for 8 seconds. Penetration deep and dilution
measurements as well as HAZ, and FZ areas were observed
by the Nikon SMZ-745T stereoscope. The metallographic
characteristics of the base material, FZ and HAZ of the
samples were observed using the scanning electron
microscope TESCAN MIRA3 at 2000X.
Vickers microhardness test was carried out with Wilson
Hardness Tukon 2500, thirty indentations with distance of
0.3 mm between each indentation and 500 gf of preload were
obtained. In addition, tensile tests were performed on a
Tinius-Olsen H300KU-0049 equipment, based on the
ASTM E8 / E8M-16a standard.
Table 2 – Hybrid laser-GMAW welding parameters used.
Weld
Laser
Power
(kW)
Welding
speed
(mm/s)
Wire
feeding
rate
(m/min)
Voltage
(V)
Current
(A)
Heat
input
(J/mm)
a 8.0 30 12 28.8 194 452.90
b 7.0 30 12 29.2 186 414.37
c 7.0 30 10 36.0 286 576.53
d 8.0 30 10 36.0 287 611.06
Figure 1 – Hybrid laser-GMAW welding equipment.
MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
3. Results and discussions
3.1. Weld cross section
The cross sections of the joints are shown in the Fig. 2 for
the penetration and width of the weld beads and Fig. 3 for
the HAZ and FZ areas. The numerical data are summarized
in Tables 3 and 4.
Figure 2 – Measurement of the weld’s widths and penetration
depths.
Table 3 – Welds width and depth penetration of HLAW. (mm).
Welds Depth Length FZ width HAZ width
a 8.92 4.40 6.98
b 9.36 4.07 5.57
c 8.98 4.19 6.22
d 9.16 4.55 6.07
Table 4 – Welds areas of HAZ and FZ in HLAW. (mm2).
Welds HAZ-1 FZ HAZ-2
a 8.13 16.47 7.97
b 6.98 20.98 6.97
c 8.91 16.40 8.02
d 7.88 16.33 7.59
Figure 3 – Measurement of the weld’s areas of HAZ and FZ.
In Fig. 2 (a) - (d) the full penetration of the welds is
shown. In the welds (a) (b) and (c) there is an excessive filler
metal on the top of the weld, however in Fig. 2 (d)
incomplete filling is observed in the upper part of the weld.
An increase in the area of heat affected and FZ is
observed by applying greater heat input during welding [14].
However, in this study no difference was found in the
different measurements of the FZ and HAZ in Fig. 3.
3.2. Microstructural analysis
Fig. 4 shows the optical microscopy image of the HSLA 550
base metal microstructure containing ferrite and perlite.
In welding of low carbon steels (less than 0.2% C)
microstructures commonly form is the martensite in shape
of laths grouped into shaves or packets. As the steel carbon
content increase, the martensite tends to form into individual
plates [15].
The weld metal in bottom zone revealed mainly lath
martensite (LM) showed in Fig.5. Coarse grain in heat
affected zone (CGHAZ) is shown in Fig. 6, exposed a
mixture of plate martensite (PM) and LM. In the Fig. 7, the
fine grain heat affected zone (FGHAZ) also shown PM but
with a grain refinement.
MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
Figure 4 – HSLA 550 microstructure, F: ferrite, P: perlite.
Figure 5 – SEM microstructure of Lath Martensite (LM).
Figure 6 – SEM microstructure of Plate Martensite (PM) of CGHAZ.
CGHAZ occur due the high temperature reached for an
austenitic grain growth but insufficient for the fusion of the
grain. In the other hand, the grain refinement in FGHAZ, the
temperature reached is close to the isotherm of eutectoid
transformation, so it limits the growth of austenitic grain.
[16].
Figure 7 – SEM microstructure of Plate Martensite (PM) in FGHAZ.
In Fig. 8 shows the SEM an image of the FZ, it shows
localized zones of acicular ferrite (AF); the intergranular
inclusions generate the transformation of the AF phase. The
AF consists of microphases of primary ferrite (PF),
secondary widmanstäten ferrite (IIWF) and intergranular
bainite (IB) and its morphology depends on the chemical
composition and characteristics of the steel or inclusions
[15]. Fig. 8, also shows areas of (IIWF). IIWF is formed
from the austenitic grain boundary with parallel
arrangements, although it requires less transformation
energy than bainite or martensite, it is often confused with
them due to its morphology [15].
Figure 8 – SEM microstructure of Secondary Widmanstäten Ferrite
(IIWF) y Acicular Ferrite (AF).
3.3. Microhardness and tensile test
In Fig. 9, the variation of the microhardness test is shown.
The indentations were obtained from the cross section of
each weld at three different positions; the top section where
arc welding is observed; the middle area where a mixture of
laser and arc welding can be found; the lower area where it
is only affected by laser welding.
MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
Figure 9 – Transverse microhardness profile of the weld
The upper welding zones were wider than the
intermediate and lower zones. In Fig. 9 (a) 514 HV in
FGHAZ was obtained, while in the CGHAZ the average
hardness was 486 HV, in addition, a softening zone is
observed in the FZ. Fig. 9 (c) shows a behavior similar to
Fig. 9 (a) but the arc welding zone is narrower. In Fig. 9 (b)
the GMAW area obtained less hardness than in the
intermediate zone and the lower one; the welding of (b)
received less heat input. On the other hand, the weld (d) was
the one that received the largest heat input, uniformity was
observed in the hardness profiles in the three zones.
For the tension test, two specimens of each weld were
performed. the welding samples examined in the stress test
fractured in the base metal, the stress-strain curves for each
specimen are showed in the Fig.10. The average ultimate
tensile strength is between 543 and 559 MPa and the
Young´s modulus 143 and 188 GPa. According to the
analysis carried out by SEM, Fig.11 shows a fracture of the
ductile type where dimples was observed on the surfaces.
This morphology is representative of ductile fractures [17].
Figure 10 – Stress-strain curve from tensile test in HSLA 550 steel.
Figure 11 – SEM microstructure of ductile fracture in HSLA 550
steel.
4. Conclusion
Plates of HSLA 550 steels were hybrid laser-GMAW
welded. The effect of the heat input on the microstructure
and mechanical properties was investigated. The results are
summarized below:
• Weld d had the higher heat input (611.06 J/mm), this
specimen shows narrow FZ and HAZ.
MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
• The lower zones were most affected by laser welding,
the microstructure obtained was lath martensite. The
upper and intermediate zones present a mixture of
Widmastäten Ferrite, Acicular Ferrite and Martensite
due to the most heat input supplied by the arc in the upper
welding, obtaining less temperature gradient. The
microstrctures along of the joint formed by the weld
process produces a difference in microharness.
• The microhardness profiles vary according to the
properties and characteristics of the obtained
microstructures. The grain refinement of the FGHAZ
shows a higher hardness than CGHAZ due to the growth
of austenitic grain during welding.
• Resistance strength was not affected by the weld process
due to the fracture didn’t occur in the FZ or HAZ.
Acknowledgements
This work has been conducted under the financial support of
CONACYT during the Master´s Degree studies of the first
author Ana Soria at Corporación Mexicana de Investigación
en Materiales S.A. de C.V.
MEMORIAS DEL XXV CONGRESO INTERNACIONAL ANUAL DE LA SOMIM 18 al 20 DE SEPTIEMBRE DE 2019 MAZATLÁN, SINALOA, MÉXICO
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