Article
citation information:
Szczucka-Lasota, B., Węgrzyn,
T., Łazarz, B., Lukaszkowicz, K., Jurek, A., Wilczyński, K.I. Welding of mobile
platform elements made from mixed grades of high-strength steels. Scientific Journal of Silesian University of
Technology. Series Transport. 2020, 107, 197-203. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2020.107.15.
Bożena
SZCZUCKA-LASOTA[1], Tomasz WĘGRZYN[2], Bogusław ŁAZARZ[3],
Krzysztof LUKASZKOWICZ[4], Adam JUREK[5], Krzysztof Ireneusz
WILCZYŃSKI[6]
WELDING
OF MOBILE PLATFORM ELEMENTS MADE FROM MIXED GRADES OF HIGH-STRENGTH STEELS
Summary. In the mobile platform structure, there is an
increasing necessity to weld steels from the various groups. For instance, it
has become necessary to connect AHSS (Advanced High-Strength Steel) with
fine-grained steels. This article verifies the possibility of obtaining
accurate mixed welded joints from different grades of DOCOL 1400 steel (from
the AHSS group) with S700 MC steel (from the fine-grained steel group). The structure,
quality and mechanical properties of the obtained welded joints were thoroughly
analysed. The joints were made at various welding speed with preheating to
80°C and without preheating.
Keywords: civil engineering, transport, mobile platforms,
welding
1. INTRODUCTION
In the automotive industry, mixed
joints made of AHSS steel and fine-grained steel play an important role [1, 2]. High-strength AHSS steels find increasing
use in civil engineering and automotive industry due to their high tensile
strength at the level of around 1300 MPa [3-5]. This article presents the
results of tests designed for proper selection of welding parameters of mobile
platforms thin-walled structure elements made of tested steel grades (DOCOL
1400M and S700 MC). It was decided to create mixed AHSS/fine-grained steel
joints using the MAG process with various welding parameters [6]. Welding of
AHSS steel with fine-grained steel is not widely presented in technical
literature. The possibility to weld correct and repeatable AHSS/fine-grained
steel mixed joints could affect new design constructions and usage of mobile
platforms with increasing lifting capacity and working range. The joints were made at a various
welding speed from 300 to 400 mm/min. Mixed welds with preheating to 80°C and
without preheating, were made.
2. RESEARCH MATERIALS AND WELDING PARAMETERS
From the available AHSS materials,
two grades were selected (Tab. 1) and combined with a non-alloy steel S355J2.
The welded steel grades differ significantly due to their chemical composition
(Tab. 1). Steels from the AHSS group are typically considered as difficult to
weld because of cracks that appear in the weld and in the heat-affected zone
after the welding [7-10]. Generally, high strength steels are not considered as
a well-weldable.
Tab.
1
Chemical composition of selected
steel grades [10]
|
Steel grade |
C% |
Si% |
Mn% |
P% |
S% |
Al% |
Ni% |
Ti% |
|
Docol 1400M |
0.11 |
0.20 |
1.70 |
0.01 |
0.002 |
0.04 |
- |
0.025 |
|
S700MC |
0.12 |
0.1 |
2.1 |
0.025 |
0.01 |
0.015 |
0,09 |
0.15 |
For metallurgical reasons, it should
be noted that both steel grades differ in the content of Al, Ti, Nb, which
affects the strength of the structure. The introduction of these elements into
the weld deteriorates their plastic properties, which can be measured by
relative elongation.
In welding AHSS and fine-grained steel, it is recommended to limit the
linear energy during welding to 5 kJ/cm level [4, 5, 7]. The weldability of
mixed joints has not been sufficiently investigated yet and there is scarce
literature information on it, especially regarding its use in the structure of
the transport and civil engineering means. It can be assumed that these steels
should be welded with low oxygen and low nitrogen methods [8, 9]. New
technologies such as micro-jet cooling have not been tested so far for
high-strength steels (neither AHSS nor fine-grained) [9]. During the welding of
low-alloy steels, micro-jet cooling is employed with an increasing frequency to
control the structure and enhance the plastic properties of the joint. It can
be assumed that micro-jet cooling would be used in welding high-strength steels
[10].
3. PREPARATION OF SAMPLES FOR
TESTING
MAG mixed welded joints were made
using the following steel grades: DOCOL 1400M with S700 MC. Significant
differences in the chemical composition of the selected steel grades affect the
structure, weldability and mechanical properties of the joint. Main mechanical
properties of the tested steels are presented in Tab. 2.
Tab. 2
AHSS steels used to build mobile
platforms - mechanical properties [10]
|
Steel grade |
The yield point YS [MPa] |
Tensile strength UTS [MPa] |
Elongation A5 [%] |
|
Docol 1400M |
1150 |
1380 |
6 |
|
S700MC |
700 |
1050 |
8 |
It is worth highlighting the vast
difference in strength and yield strength for both material groups. It was
decided to make mixed joints using electrode wires: Union X96. The chemical
composition of the wire is presented in Tab. 3.
Tab.
3
Electrode wire UNION X96 - chemical composition [10]
|
C% |
Si% |
Mn% |
P% |
Cr% |
Mo% |
Ni% |
Ti% |
|
0.12 |
0.87 |
1.89 |
0.010 |
0.29 |
0.46 |
3.3 |
0.005 |
The chemical composition of the wire is not quite similar to welded steels. Noteworthy, it has at least four times higher silicon content in wire than in steel, which increases strength. In addition, chromium was introduced with a relatively high content of 0.45%, which greatly increases strength, but also reduces the impact toughness of the joint. To improve the plastic properties of the joints, nickel (2.45% Ni) and molybdenum (0.65% Mo) were introduced into the weld. Nickel delivered to the weld metal with 2% content and molybdenum introduced to the weld metal with 0.5% content, significantly increases the impact toughness of the joint at negative temperatures. For joints made of the C-Mn low alloy steel, these two elements introduced into the weld metal guarantee an impact strength of 50 J at -40°C, which meets the 4th impact class. Similarly, in AHSS and fine-grained steel mixed joints of two elements significantly should improve the plastic properties.
The steel welding parameters were as
follows: the diameter of the electrode wire was 1 mm, the arc voltage was 20 V
and the welding current was 118 A. The welded sheets had dimensions of
400×50×3 mm and the weld was of single stitch type. In the MAG
process, a gas mixture of 82% Ar-18% CO2 was selected to act as shielding
gases. The shielding gas flow rate was at a level of 14 l/min. A 3 mm thick
(single stitch) welded butt joint (BW) was made (KT sample). The welding
process was applied in the down position (PA) and according to the requirements
of the EN 15614-1 norm.
The joints were made at various
speeds of 300, 350 and 400 mm/min, respectively. Joints with preheating to
80°C and without preheating were made for each tested welding speed.
4. METHODS (SCOPE OF RESEARCH)
The scope of this
research included non-destructive testing (NDT):
·
visual tests (VT) of welded joints were
made with the eye fitted with a magnifying glass at 3x magnification - the test
was performed according to the PN-EN ISO 17638 standard and the assessment
criteria according to the EN ISO 5817 norm,
·
magnetic-powder tests (MT) - the tests
were carried out in accordance with the PN-EN ISO 17638 standard and the
assessment criteria according to the EN ISO 5817 norm using a magnetic flaw
detector device type REM - 230.
The destructive
tests included:
·
the tensile tests were carried out in
accordance with the PN-EN ISO 4136:2013-5 standard,
using a ZD-40 strength testing machine,
·
the bending tests were carried out in
accordance with the PN-EN ISO 5173 standard, using a ZD-40 strength testing
machine,
·
examinations of the microstructure of the
samples were investigated on a light microscope (LM).
5. RESULTS AND DISCUSSION
The
results of the non-destructive testing are presented below.
Visual inspection
of sheets with a thickness of 3 mm was carried out using standard auxiliary
equipment: 3x magnifiers and a lux meter with a white light of 520 Lx. It was
found that the welds were made correctly, met the quality requirements and were
characterised by the acceptability limit "B" according to the PN EN ISO
5817 norm. The magnetic-powder test for sheets with a thickness of 3 mm was
made using the wet method at the following conditions: field strength 3 kA/m,
white light 515 Lx, temperature 20°C, detection means MR -76, contrast MR
-72. The results of the magnetic-powder tests are presented in Tab. 4.
Tab. 4
Results of the
magnetic-powder tests
|
Speed [mm/min] |
Preheating [°C] |
Detected implications |
Test results |
|
300 |
without |
small cracks were observed |
negative |
|
300 |
80 |
small cracks were observed |
negative |
|
350 |
without |
small cracks were observed |
negative |
|
350 |
80 |
improper surface
indications were not detected |
positive |
|
400 |
without |
small cracks were observed |
negative |
|
400 |
80 |
improper surface indications were not detected |
positive |
Table data shows that pre-heating is
strongly required for the proper welding of DOCOL 1400M with S700 MC steel.
After assessing the joints with non-destructive tests, it was decided to the
tensile strength of the welded platform elements. Welding speed of 300, 350 and
400 mm/min was also tested (process with preheating and without
preheating). A speed of 300 mm/min cannot be considered as appropriate for
welding the tested joint due to welding defects. The welding speed should be in
the range of 350-400 mm/min. A preheat temperature of 80°C was accepted as
sufficient.
For further (destructive) tests, only positive joints (made with preheating and welding speed in the range of 350-400 mm/min) were considered. In addition, the destructive strength test was carried out on the ZWICK 100N5A strength testing machine. The results of the tensile test of the welds are presented in Tab. 5.
Tab.
5
Mechanical properties of the mobile
platform joint (mixed joint
S700 MC/Docol 1400M)
|
Welding speed [mm/min] |
YS [MPa] |
UTS [MPa] |
A5 [%] |
|
350 mm/min |
684 |
992 |
5.9 |
|
400 mm/min |
679 |
984 |
5.7 |
The analysis of the array data shows that the connectors were made correctly. The strength of all joints is at a similar level. The highest relative elongation value of the mixed joint was obtained when welding with the use of MAG method, preheating 80°C, welding speed 350 mm/min. Thereafter, the bending test of the created joints was performed. For the test, a sample with thickness of a = 3 mm, width of b = 20 mm, mandrel of d = 22 mm and support spacing of d + 3a = 31 mm was used, the required bending angle was at the level of 180°. Five bending test measurements were carried out both on the face side and on the root side of the weld. The same joints were subjected to the bending test. The tests results are summarised in Tab. 6.
Tab.
6
Mixed joints bending test results
|
Welding speed [mm/min] |
Side deformation |
Size [mm] |
Bending angle [°] |
Comments |
|
350 |
root of the weld |
3 × 20 |
180 |
no cracks, no incompatibilities |
|
350 |
face of the weld |
3 × 20 |
180 |
no cracks, no incompatibilities |
|
400 |
root of the weld |
3 × 20 |
180 |
no cracks, no incompatibilities |
|
400 |
face of the weld |
3 × 20 |
180 |
no cracks, no incompatibilities |
The analysis of Tab. 7 shows that the joints were made
correctly. No cracks or other incompatibilities were found in the tested
samples. Afterwards, microstructure analysis was performed. Similarly, the
structure of those mixed joints that ensured the best relative elongation was
analysed. The microstructure of the cross-section of the S700 MC/Docol 1400M
weld is presented in Fig. 1.
Fig. 1. Structure of the S700 MC/Docol 1400M weld (after preheating,
welding speed 350 mm/min)
The
dominant area with martensitic structure, bainite and ferrite is shown (Fig.
1). All analysed welds were made correctly, the metallography observation
result was positive and confirm good properties of obtained joints by the
selected parameters.
5. CONCLUSION
1.
It
is possible to create correct and repeatable joints made of two different
grades of AHSS steel and fine-grained steel.
2.
The
strength of mobile platforms elements’ joints made of AHSS and
fine-grained steel grades is at the required level of 900 MPa.
3.
The
electrode wire UNION X96, shielding gas mixture of 80% Ar-18%, preheating
before welding up to 80°C, welding speed in the range of 350-400 mm/min
provides the most preferred plastic properties of a joint during the MAG
welding.
4.
Bending
test, structure testing, as well as non-destructive tests, confirmed the
possibility of correct mixed welding elements of the mobile platform elements.
Acknowledgements
This
paper is part of the COST project, CA 18223.
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Received 21.02.2020; accepted in revised form 21.05.2020
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