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Evaluation of nitrogen influence on
microstructure of duplex stainless steel laser
welds
Conference Paper · July 2014
DOI: 10.13140/2.1.1886.5928
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65th Annual Assembly & International Conference of the International Institute of Welding
13th-18th July 2014, Seoul, Korea
Evaluation of nitrogen influence on microstructure of duplex stainless steel
laser welds
Jozef Barta1, Milan Maronek2, Miroslav Sahul1, Katarina Bartova1, Erika Hodulova1
Institute of Production Technologies ,Faculty of Materials Science and Technology, Slovak University of Technology, Slovakia1
Institute of Production Technologies ,Faculty of Materials Science and Technology, Slovak Universiity of Technology, Slovakia 2
Abstract
Welding of duplex stainless steel (DSS) by conventional
welding methods is currently relatively well explored. A
wide scale of filler materials as well as appropriate
procedures for achievement of suitable weld joint
microstructure with regard to its mechanical properties and
corrosion resistance are available on the market. Welding of
DSS by the laser and electron beam leads to faster cooling of
weld metal. Therefore, there is insufficient time for
transformation of delta ferrite to austenite, and thus the
excess amount of ferrite in comparison to the base material
is present in the final microstructure of weld metal. This
disequilibrium can negatively influence the weld joint
corrosion
resistance.
One of the possible ways to promote austenite formation in
weld metal is application of the austenite supporting
elements, either in filler materials or in protective gas. In
case of welding without filler material, the addition of
nitrogen to the protective gas seems to be eventual solution.
The paper evaluates the influence of nitrogen as a protective
atmosphere on promotion of austenite formation in the laser
beam weld joints. The microstructure was examined by
optical microscopy. The results revealed that welding in
nitrogen shielding gas did not meet the required austenite
and ferrite ratio, neither on the weld surface nor in the layers
beneath.
Keywords: duplex steel, laser beam welding, nitrogen,
phase ratio
1. Introduction
Duplex stainless steels (DSS), meaning those with a mixed
microstructure of about equal proportions of austenite and
ferrite, have existed for nearly 80 years. These materials are
a family of grades combining good corrosion resistance with
high strength and ease of fabrication. Their physical
properties are between those of the austenitic and ferritic
stainless steels but tend to be closer to those of the ferritic
and to carbon steel. All DSS provide significantly greater
strength than the austenitic grades while exhibiting good
ductility and toughness. The typical yield strengths of
several duplex stainless steels are compared with that of
316L austenitic stainless steel between room temperature
and 300°C (570°F) in Figure 1 [1, 2].
Figure 1. Yield strengths comparison of several stainless steels [1]
Yield strength of DSS at room temperature in the solutionannealed condition is more than double that of the standard
austenitic stainless steels not alloyed with nitrogen. This may
allow the design engineer to decrease the wall thickness in
some applications [3].
The iron-chromium-nickel ternary phase diagram is a
roadmap of the metallurgical behavior of the duplex stainless
steels. A section through the ternary at 68% iron (Figure 2)
illustrates that these alloys solidify as ferrite (α), some of
which then transforms to austenite (γ) as the temperature
falls to about 1000°C depending on alloy composition.
Small changes in composition can have a large effect on the
relative volume fraction of these two phases, as the phase
diagram indicates. Beneficial effect of nitrogen, evidenced in
Figure 2, is that it raises the temperature at which the
austenite begins to form from the ferrite [1].
shielding gas containing, besides argon or helium, also 1%
to 2 % of nitrogen [1, 2, 5]. Since there are no arc
instabilities during laser beam welding, the aim of the paper
was to use pure nitrogen as a shielding gas in order to
maximize the austenite content in weld metal.
2. Material properties
Sheets with 1 mm thickness made of duplex stainless steel
grade 2205 were used in the experiment. The typical
chemical composition of 2205 DSS is provided in Table 1
[4].
Table 1 Typical chemical composition of 2205 duplex stainless steel [4]
Grade
C
Cr
Ni
Mo
N
Mn
Cu
[%]
[%]
[%]
[%]
[%]
[%]
[%]
3.5–5.5
0.1–0.6
2205 0.03 22.0–24.0
Figure 2. Section through Fe-Cr-Ni ternary phase diagram
at 68% iron content [1]
0.05–0.20 2.00 1.0–3.0
Mechanical properties measured in the direction parallel to
the rolling direction are shown in Table 2.
Table 2 Mechanical properties of DSS 2205
Duplex stainless steels have very good hot cracking
resistance due to the high ferrite content; hot cracking is
rarely a consideration when welding these steels. The
problems of most concern in duplex stainless steels are
associated with the Heat Affected Zone (HAZ), not with the
weld metal. The HAZ problems are loss of corrosion
resistance, toughness, or post-weld cracking [3].
Grade
Proof strength
Rp0.2 [MPa]
Tensile strength
[MPa]
Elongation A5
[%]
2205
556
810
23
The microstructure of DSS 2205 documented in Figure 3
consisted of approximately equal amount of austenite and
ferrite with typical elongated grains after rolling process.
As a general rule, preheating is not recommended because it
may be detrimental. Postweld stress relief is not needed for
duplex stainless steels and is likely to be harmful because the
heat treatment may precipitate intermetallic phases or alpha
prime (475°C) embrittlement causing a loss of toughness and
corrosion resistance [2].
Duplex stainless steels can tolerate relatively high heat
inputs. Exceedingly low heat input may result in fusion
zones and HAZ which are excessively ferritic with a
corresponding loss of toughness and corrosion resistance.
Exceedingly high heat input increases the danger of forming
intermetallic phases. To avoid problems in the HAZ, the
weld procedure should allow rapid cooling of this region
after welding [1].
Laser beam welding produce very limited heat affected
zones and rapid cooling that prevents intermetallic phase
formation. However, the high cooling rate associated with
these techniques can result in the excessive ferrite formation
in the weld, so weld qualification of the procedure is critical
when using these methods [3].
The most frequent way to prevent nitrogen diffusion from
weld metal in standard welding methods is usage of
Figure 3 Microstructure of duplex stainless steel 2205
3. Experiment
The experiment was carried out with the solid state disc
laser. The laser head was mounted on the Fanuc M-710iC/50
robotic arm for precise guidance of laser beam. The
65th Annual Assembly & International Conference of the International Institute of Welding
13th-18th July 2014, Seoul, Korea
workplace is shown in Figure 4. Shielding gas was supplied
by four parallel nozzles.
The butt welds were made by laser beam welding of two
duplex stainless steel sheets with the dimensions of
1x50x100 mm. The materials were fixed during welding in
the fixture shown in Figure 5. Design of the welding fixture
provided the possibility of both weld surface and weld root
protection by shielding gas.
Figure 4 Laser beam welding workplace
Figure 5 Welding fixture design
The specifications of welding equipment are provided in
Table 3.
Table 3 Specifications of welding equipment
Welding source
Maximum output power
Welding optics
Focal distance
Laser beam spot diameter
Focus position
TRUMPF TruDisk 4002
2 kW
TRUMPF D70
200 mm
200 µm
On material surface
The 99,998% nitrogen shielding gas was used in order to
evaluate its influence on microstructural changes. The flow
rate was constantly set to 16 l/min. Other welding
parameters are provided in Table 4.
Table 4 Welding parameters
Specimen
No.
30
35
40
45
50
30N
35N
40N
45N
50N
Welding
speed
[mm/s]
30
35
40
45
50
30
35
40
45
50
Shielding gas
Heat input
[kJ/mm]
–
–
–
–
–
Nitrogen
Nitrogen
Nitrogen
Nitrogen
Nitrogen
0.033
0.029
0.025
0.022
0.02
0.033
0.029
0.025
0.022
0.02
Worthy of note is that the recommended heat input for DSS
welding by arc welding methods is between 0.5 and
2.5 kJ/mm.
Specimens for microstructural analysis were prepared by
standard metallographic procedure (grinding, polishing and
etching). Neophot 32 Optical microscope was used for
microstructural analysis.
4. Results
As anticipated, the microstructural analysis revealed mostly
ferritic structure of weld metal. The specimens produced
using nitrogen as a shielding gas showed a weld shape
typical for laser beam welding, unlike the specimens welded
without shielding gas (Figure 6). Weld joint did not show
any abnormalities or porosity. Weld metal contained mostly
columnar ferritic grains with little amount of austenite
segregated on the ferritic grain boundaries.
Figure 5 Macrostructure of weld joints
nitrogen (left), no shielding gas (right)
Examples of the weld metal microstructure with and without
nitrogen shielding gas for specimens No. 35 and 35N are
shown in Figures 5 and 6 respectively. Results revealed low
amount of the austenite present at the ferritic grain
Specimen
No.
45
45N
50
50N
boundaries. The specimens welded without shielding gas
showed ferritic grain coarsening in comparison to the
specimens welded with nitrogen shielding gas.
Shielding gas
–
Nitrogen
–
Nitrogen
Heat input
[kJ/mm]
0.029
0.025
0.022
0.02
Ferrite content
[%]
91,1
90,6
90,6
87,5
Published research papers indicated that nitrogen addition to
shielding gas may help obtain balanced phase ratio in
common arc welding processes. Based on the research
results, this information was not proved in case of laser
beam welding.
5. Conclusion
Figure 5 Microstructure of weld metal in specimen No. 35
The attained results showed that nitrogen shielding gas had
minimum influence on the austenite content in weld metal
of laser beam welds. Suitable solution may lie in the
application of filler metal enriched by the elements
promoting austenite formation, e.g. nickel. On the other
hand, surpisingly, lower heat input caused higher austenite
content. This discrepancy is therefore the subject of further
research. Further research will be also focused on the
modification of heat input for example by the application of
subsequent laser beam pass in order to prolong the weld
metal cooling time.
Acknowledgement
This research was supported by Slovak Research and
Development Agency using financial support no. APVV0248-12.
Figure 6 Microstructure of weld metal in specimen No. 35N
Quantitative image analysis was performed on the
micrographs taken from the weld bead in order to determine
the volume fraction of ferrite and austenite. Determination of
the phase volume fractions was done by the manual point
count method on the micrographs taken at 500x
magnifications minimum, in accordance to ASTM E-562
standard.
Weld metal of the specimens welded with different heat
input showed similar microstructure with the minimum
content of austenite as illustrated in Table 5.
References
[1]
[2]
[3]
[4]
Table 5 Results of the ferrite content measurement
Specimen
No.
30
30N
35
35N
40
40N
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Shielding gas
–
Nitrogen
–
Nitrogen
–
Nitrogen
Heat input
[kJ/mm]
0.033
0.029
0.025
0.022
0.02
0.033
Ferrite content
[%]
95,6
92,4
92,8
92
92,8
91,1
[5]
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