J. GÓRKA, A. OZGOWICZ: STRUCTURE AND PROPERTIES OF LASER-BEAM-WELDED JOINTS ...
189–193
STRUCTURE AND PROPERTIES OF LASER-BEAM-WELDED
JOINTS OF LOW-ALLOY HIGH-STRENGTH STEEL DOCOL
1200M WITH A MARTENSITIC STRUCTURE
STRUKTURA IN LASTNOSTI LASERSKO VARJENIH SPOJEV
MALOLEGIRANEGA VISOKOTRDNOSTNEGA JEKLA DOCOL
1200M Z MARTENZITNO MIKROSTRUKTURO
Jacek Górka1, Andrzej Ozgowicz2
1Silesian University of Technology, The Chair of Welding, 18a Konarskiego Street, 44-100 Gliwice, Poland
2Styvo AS, Vipevegen 8, N-6783 Stryn, Norway
jacek.gorka@polsl.pl
Prejem rokopisa – received: 2017-06-20; sprejem za objavo – accepted for publication: 2017-08-03
doi:10.17222/mit.2017.077
This article presents studies on the effects of the linear energy of the laser-beam welding of the DOCOL 1200M high-strength
low-alloy steel with a martensitic structure, 1.8-mm thick, on the properties and structural changes of the joint area. The welding
process was carried out in the horizontal position, without a filler material, at a variable linear energy of the welding ranging
from 25 J/mm to 55 J/mm. The non-destructive testing allowed the joints to be classified as the quality level B in accordance
with ISO 13919-1. In order to determine the structural changes and changes in the properties, the joints were subjected to
destructive testing including macro- and microscopic metallographic examinations, a hardness measurement, a static bend test
with the face and root of the weld in tension and a static tensile test. The destructive testing showed that with an increase in the
linear energy of the welding, the plastic properties of the joints are enhanced while, at the same time, their strength properties
are reduced to the values below that of the base material. The results showed that with the use of the lowest linear energies of
laser-beam welding (around 25 J/mm), it is possible to produce welded joints of the DOCOL 1200M steel, whose strength is
equal to that of the base material, which is not achievable when arc welding the steels with such a high strength (1200 MPa).
The joints made with the liner energy of 25 J/mm were characterised by a tensile strength of about 1240 MPa, obtaining a
bending angle of 70°. The weld hardness was about 440 HV1 and it was similar to that of the base material while in HAZ, the
hardness decreased to 360 HV1.
Keywords: martensitic steel, DOCOL 1200M, laser welding, linear energy of welding, martensite
V pri~ujo~em ~lanku avtorji predstavljajo {tudijo vplivov linearne energije varjenja z laserskim snopom na malolegirano jeklo z
martenzitno mikrostrukturo vrste DOCOL 1200M. Raziskovali so lastnosti in strukturne spremembe po varjenju v podro~ju 1.8
mm debelega zvara. Postopek varjenja so izvedli v vodoravnem polo`aju, brez polnilnega materiala in pri spreminjanju linearne
energije varjenja med 25 J/mm in 55 J/mm. Neporu{ne preiskave zvarov so omogo~ile njihovo uvrstitev v kakovostni nivo B v
skladu z ISO 13919-1. Da bi dolo~ili strukturne spremembe in spremembe lastnosti v preiskovanem materialu so bili izdelani
zvari izpostavljeni tudi poru{nim preiskavam. Te so obsegale makro- in mikroskopske metalografske preiskave, meritve trdote,
stati~ni upogibni preizkus na ~elu in v korenu zvarov, ter stati~ni natezni preizkus. Poru{ne preiskave so pokazale, da se z
nara{~ajo~o linearno energijo varjenja izbolj{ajo plasti~ne lastnosti zvarov, medtem ko se isto~asno trdnostne lastnosti
zmanj{ajo pod vrednosti, ki jih ima osnovni material. Rezultati so pokazali, da je bilo mo`no z uporabo najni`jih linearnih
energij varjenja z laserskim snopom (okoli 25 J/mm), variti jeklo DOCOL 1200M s trdnostjo enako osnovnemu materialu, kar
ni dosegljivo pri oblo~nem varjenju s tak{no visoko trdnostjo (1200 MPa). Zvari, izdelani z linearno energijo 25 J/mm, so imeli
natezno trdnost okoli 1240 MPa in upogibni kot 70° po upogibnem preizkusu. Trdota zvarov je bila pribli`no 440 HV1 in enaka
tisti, ki jo je imel osnovni material, medtem ko je trdota v toplotno vplivani coni (HAZ; angl.: Heat Affected Zone) padla na 360
HV1.
Klju~ne besede: martenzitno jeklo, DOCOL 1200M, lasersko varjenje, linearna energija varjenja, martenzit
1 INTRODUCTION
Steel producers manufacture newer and newer ma-
terials with enhanced properties as a response to the need
of, but not limited to, the automotive industry. The
reasons are high requirements related to the safety of
users and a continuous trend towards reducing the
weight of manufactured structures. This development
allows a considerable reduction in the weight of vehicles,
their fuel consumption and emissions of harmful gases to
the environment. To meet this demand of the automotive
industry, advanced high-strength steels (AHSSs) were
developed. These materials have performed very well in
the production of vehicles due to three very important
features: a high tensile strength, up to 1700 MPa; a high
yield point, up to 1450 MPa; and a high elongation A80,
up to 30 %.1–5 It is also important that plastic working
and machining of these steels are also relatively easy.
AHSSs are commonly used in the automotive indus-
try because they make it possible to reduce the thickness
of car-body sheets and car-body bearing elements, while
enhancing their mechanical properties compared to
conventional steels. An additional advantage of AHSSs
is their relatively low price due to small amounts of
alloying additives in steel. Despite the fact that the steels
Materiali in tehnologije / Materials and technology 52 (2018) 2, 189–193 189
UDK 620.1:621.791.725:669.14.018.62 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(2)189(2018)
were designed for being joined with bonding procedures,
some of them still remain problematic for the deve-
lopment of the methods and optimum parameters of
welding.6–12 It is a great challenge to weld high-strength
DOCOL 1200M steel with a martensitic structure,
primarily intended for manufacturing car bumpers, side
beams and other user-safety elements of motor vehicles.
In this respect, laser-beam welding has become increas-
ingly important. Thanks to the technological progress
and an increasing power of lasers, laser welding, one of
the many techniques for joining materials, including
simple and small items, for which this technique has
proved to be successful due to its advantages, has now
expanded into the area, which was until recently reserved
only for conventional welding techniques.
The process of replacing the welding techniques used
for a long time is often initiated by the use of new high-
strength materials that require precise techniques and
relevant technologies for joining to maintain high-
strength properties of the base material. Due to high-
power densities obtained in the impact area of a laser
beam, laser welding allows single-pass welding of the
sheets put together, without using butt welds with a
square joint geometry and with relatively high welding
rates (high performance).13–17
2 EXPERIMENTAL PART
The purpose of the examinations was to determine
the effects of the linear energy of welding (from 25 J/mm
to 55 J/mm) on the properties and structure of butt joints
of 1.8-mm sheets of the high-strength low-alloy DOCOL
1200M steel with a martensitic structure welded with a
laser beam without a filler material. For the actual che-
mical composition and properties of the DOCOL 1200M
steel (Table 1), and for its structure (Figure 1).
2.1 Welding process
The welded joints were made at a robotised station
equipped with a KUKA KR 20/2HA industrial robot
with a TRUMPF TruDisk 12002 disk-laser head (Fig-
ure 2). The focal length of the collimator lens was fc =
200 mm and the focal length of the converging lens was
fog = 300 mm. Optical-fibre cables with two different
diameters, dsw = 0.2 mm and dsw = 0.4 mm, were used for
carrying the laser beam, providing for an additional
modification of the linear energy of welding. The
diameter of the laser-beam focus for both cases was
calculated from Equation (1) and it was dog = 0.3 mm
and dog = 0.6 mm, respectively.
d
d f
fog
sw og
c
=
⋅
(1)
The welding process was conducted in the horizontal
position using a butt weld with a square joint geometry.
It was a shielded arc welding process with argon as the
shielding gas supplied through a four-pipe nozzle at a
flow rate of 20 L/min. The diameter of the laser-beam
focus, beam power and welding rate were changed
during the welding tests, Table 2. To reduce welding
deformations, the sheets were tacked at three points (at
the beginning, in the middle and at the end of the joint)
before welding.
Table 2: Laser welding parameters for DOCOL 1200M steel with a
thickness of 1.8 mm
Joint
designa-
tion
Power
(W)
Melting
rate
(mm/min)
Linear
energy
(J/mm)
Diameter of the
laser-beam focus
(mm)
1.x 2500 6000 25 0.3
2.x 3500 6000 35 0.6
3.x 1500 2000 45 0.3
4.x 1800 2000 55 0.6
2.2 Examinations of welded joints
Following the visual inspection in accordance with
PN-EN ISO 13919-1:2002, the welded joints were
subjected to destructive testing to the following extent:
• tensile strength tests in accordance with PN-EN ISO
6892-1:2010 using a ZWICK/ROELL Z 330RED
tensile-testing machine on samples taken in
accordance with PN-EN ISO 4136:2011;
J. GÓRKA, A. OZGOWICZ: STRUCTURE AND PROPERTIES OF LASER-BEAM-WELDED JOINTS ...
190 Materiali in tehnologije / Materials and technology 52 (2018) 2, 189–193
Figure 1: Microstructure of DOCOL 1200M steel
Table 1: Chemical composition and mechanical properties of martensitic DOCOL 1200M steel
Concentration of elements, % Mechanical properties
C Si Mn P S Al Nb V Ni Cr N Ce* Tensile strengthRm, MPa
Yield point Re,
MPa
Elongation A80,
%
0.113 0.22 1.58 0.01 0.002 0.035 0.016 0.01 0.04 0.04 0.006 0.39 1260 1060 5
* Ce – carbon equivalent
• a transverse bend test of a butt joint with the tension
from the face of the weld (FBB) and a bend test of a
butt joint with the tension from the root of the weld
(RBB), in accordance with PN-EN ISO 5173:2010.
The bend tests were carried out using the ZWICK/
ROELL Z 330RED machine with an additional
bend-testing module;
• macroscopic metallographic examinations using an
Olympus SZX9 stereoscopic light microscope; the
test samples were etched with Adler’s reagent;
• microscopic metallographic examinations using a
NIKON ECLIPSE MA100 light microscope; the test
samples were etched with nital;
• a hardness measurement using a Wilson Wolpert
401MVD Vickers device, at a load of 1 kg, along one
measuring line.
3 RESULTS AND DISCUSSION
The visual inspections of the welded joints revealed
no welding defects coming out to the surface, such as
cracks, porosity, incomplete fusions or a lack of joint
penetration. The welded joints complied with the
requirements for quality level B in accordance with ISO
13919-1. Only for the joints made with the linear energy
of 25 J/mm, a linear offset, which reduces the quality
level, can be observed. Neither did the macroscopic exa-
minations reveal any welding irregularities in the weld
area and HAZ (Figure 3). It can be observed that with an
increase in the linear energy of the welding, the width of
the weld and HAZ increased.
In each case, the microscopic examinations of the
weld area revealed a martensitic structure, and with the
increase in the linear energy of the welding, the size of
the martensite needles also increased, especially in rela-
tion to the base material. In HAZ, the martensitic struc-
ture was tempered. The degree of tempering grew with
the increase in the linear energy of the welding, Fig-
ure 4.
The analysis of the results of destructive testing of
the laser-beam-welded butt joints of the DOMCOL
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Figure 3: Mechanical and plastic properties of laser-welded joints of
steel DOCOL 1200M
Figure 2: Position of the robotic laser welding used for the studies
1200MC steel revealed that the test joints complied with
the requirements of ISO 15614-11, Table 3. As the linear
energy of the welding increased from 25 J/mm to
55 J/mm, the tensile strength decreased from 1240 MPa
to 1100 MPa, with the tensile strength of the base
material being at 1260 MPa. None of the tested welded
joints showed any noticeable elongation. At low linear
energies of the welding, a rupture took place in the
base-material area. The increase in the welding energy
resulted in the grain growth in HAZ where the rupture of
joints was initiated. The static-bend test revealed that the
highest plastic properties were shown by the joints made
at the highest linear energy (55 J/mm), for which a
bending angle of about 140° was obtained, Table 3. With
the reduction in the linear energy of the welding to 25
J/mm, plastic properties of the joints decreased and the
obtained bending angles were about 70°.
The measurements of hardness HV1 confirmed the
results of the microscopic metallographic examinations.
In the area of the weld made at the lowest linear energy
(25 J/mm), the obtained hardness results were similar to
the hardness of the base material, i.e., about 445 HV1.
With the increase in the linear energy of the welding, the
weld hardness decreased to 430 HV1 at 35 J/mm, 420
HV1 at 45 J/mm and 410 HV1 at 55 J/mm. In HAZ,
which was tempered, the hardness also decreased with
the increase in the linear energy of the welding, from 360
HV1 (25 J/mm) to 340 HV1 (35 J/mm), 310 HV1 (45
J/mm) and 280 HV1 (55 J/mm), Figure 5.
Table 3: Mechanical and plastic properties of laser-welded steel
DOCOL 1200M
Tension* Bending*, bending angle, °
Rm, MPa Place ofrupture Weld face Weld root
Joint 1; E = 25 J/mm
1240 Base material 68 69
Joint 2; E = 35 J/mm
1200 Fusion line 66 72
Joint 3; E = 45 J/mm
1150 HAZ 84 83
Joint 4; E = 55 J/mm
1120 HAZ 140 138
Comments: none of the tested joints showed any noticeable elongation
* the mean of two measurements
4 CONCLUSIONS
The examinations of laser-beam welding of the
DOCOL 1200M steel with a thickness of 1.8 mm
revealed that it is possible to make welds in compliance
with the criteria specified by ISO 15614-11. Despite the
high carbon equivalent of the DOCOL 1200M steel, it is
possible to make laser-beam welded joints with high
strength properties. In the weld area, all the joints are
characterised by a martensite structure with a variable
needle size, depending on the amount of the energy
supplied. With an increase in the linear energy of the
welding, the size of martensite laths increased. In HAZ,
the base material was tempered, which resulted in the
formation of a softened zone. The size of the weld and
the softened zone in the examined joints increased with
the increase in the linear energy of the welding. To
reduce the area of the softened zone, the linear energies
used should be as low as possible. The bend test revealed
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192 Materiali in tehnologije / Materials and technology 52 (2018) 2, 189–193
Figure 5: Distribution of hardness for laser-welded joints of DOCOL
1200M steel
Figure 4: Microstructure of laser-welded joints of DOCOL 1200M
steel
that the laser-beam welded joints of the DOCOL 1200M
steel with the applied parameters were characterised by a
low ductility in the joint area. However, the main
operating property of this steel is its tensile strength. By
using low linear energies of laser-beam welding, it is
possible to produce welded joints with a strength equal
to that of the base material, which is not achievable when
arc welding the steels with such a high strength.
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