FIELD OF INVENTION
The present invention relates to a method for maintaining
microstructural balance of laser welded duplex stainless steel with autogenous
high power laser welding process using nitrogen shielding gas.
BACKGROUND OF THE INVENTION
Duplex stainless steel with equal composition of austenite-ferrite,
finds applications in chemical processing plants, pressure piping in urea plants,
cross country pipelines, etc. due to its advantages like good stress corrosion
cracking resistance, oxidation resistance, etc. Laser welding with associated
advantages like narrow fusion and heat affected zone, high cooling rates, etc. is
one of the most suited process for welding duplex stainless steel. However,
welding will affect the microstructural phase balance in this alloy. Conventionally,
the microstructural balance is maintained by using nickel fortified fillers which is
a difficult methodology especially with power beam welding process like laser
welding where the process is carried out in autogenous mode. Hence, it is
proposed to maintain the balance by using appropriate shielding gas and
defocused laser beam to weld this alloy and the details are explained. The
defocused laser beam is the laser beam positioned above from the focal. The
interaction time which stabilizes the austenite phase and maintains the
microstructural phase balance is increased by using 100% nitrogen shielding gas
with defocused beam. Also, defocused laser beam improves the smoothness of
the weld bead especially while welding with low welding speeds. The advantage
of the present invention is that it eliminates the use of filler wire through
external wire feeder.

The method of adding small quantities of nitrogen in the gas
mixture for laser welding has been reported. However, the use of fully nitrogen
shielding gas coupled with defocusing the laser beam for laser welding has not
been reported before. Conventionally, the welding of duplex stainless steels is
carried out by using welding aid which is usually nickel fortified filler.
US patent 2204/6793119B2 uses 1-3% nitrogen in Argon shielding
gas for laser welding of pipe/tube components. The process is claimed to have
improved the corrosion resistance of the weldments.
US patent 2004/6831248 B2 uses the shielding gas mixtures
containing 40-60% nitrogen and 60-40% He for laser welding various grades of
stainless steel plate formed pipes of thickness ranging from 0.5-4mm. The
process is found to be suitable for obtaining full penetration and good corrosion
resistance with 12kW CO2 laser welding of austenitic, ferritic and martensitic
grades of stainless steels.
Japanese patent JP-08155662 describes a welding condition to
obtain good toughness and corrosion resistance in the weld
All the prior arts mentioned above are using different techniques to
achieve optimum penetration, mechanical properties and corrosion resistance.
OB3ECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method for
maintaining microstructural balance of laser welded duplex stainless steel with
autogenous high power laser welding process using nitrogen shielding gas which
is capable of increasing the interaction time to stabilize the austenite phase to
maintain the microstructural phase balance during welding of duplex stainless
steel.

Another object of the invention is to propose a method for
maintaining microstructural balance of laser welded duplex stainless steel with
autogenous high power laser welding process using nitrogen shielding gas which
is able to avoid the supply of filler wire through external wire feeder.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - Shows the top surface of weld bead showing good bead appearance.
Fig. 2 - Shows an autogenous laser weld with nitrogen shield (good phase
balance).
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The invention is related to achieving the microstructural phase
balance and good bead appearance in autogenous laser welding of duplex
stainless steel. In the present method, autogenous laser welding has been
carried out on 6mm thick duplex stainless steel with 100% nitrogen shield gas.
The welding is carried out using a defocused laser beam to increase the
interaction time which will reduce the cooling rates. The interaction time is the
time for which the laser beam will be impinging on the workpiece. The decrease
in cooling rates also will increase the austenite stabilization and results in the
formation of intra granular austenite in addition to intergranular austenite. The
use of such technique will avoid the usage of welding aid and costly nickel
fortified filler and also increases the processing speed.
The use of 100% nitrogen shielding gas with defocused beam will
increase the interaction time which, will stabilize the austenite phase thereby
maintain the microstructural phase balance. The steel has two phase
microstructure in equal amount. The same is maintained by modifying the
process. Hence, the proposed method is to use 100% nitrogen as shield gas

instead of using it as a minor addition and welding with defocused beam to
improve the bead appearance and phase balance.
This method of laser welding can be used to weld duplex stainless
steel without using additional welding aid or supplying filler wire through external
wire feeder. Duplex stainless steel of specification UNS 31803 of 6mm thickness
was welded using CO2 laser of high beam quality. 100% nitrogen has been used
as the shielding gas. Nitrogen, a near inert shielding gas is a good austenite
former promoting the formation of austenite phase in the weld. Nitrogen being a
lighter gas required a minimum flow rate of 301pm to achieve plasma
suppression. The gas was supplied through a 4mm nozzle in off-axial position @
45° with a standoff distance of 6mm above the workpiece surface. Nitrogen
stabilizes the austenite phase in the weld. Welding was carried out at a welding
speed of 800mm/min. The focal plane has been maintained at 3mm above
surface with 300mm focal length mirror to achieve a spot size of 410|jm which is
the spot size of the laser beam at the point of interaction with the workpiece.
This will reduce both power density and energy density thereby increasing the
interaction time. The increase in interaction time will further reduce the cooling
rates and increases the formation of austenite. Also, defocusing of laser beam
will improve the smoothness of the weld bead by reducing the chances of
undercut at low welding speeds. The welds were characterized by microstructural
analysis to study the phase composition.
The as welded top surface of the weld is shown in Fig. 1. The
microstructure of the weld made with nitrogen shielding gas is shown in Fig. 2.
The use of 100% nitrogen as shielding gas in the present condition has improved
the phase balance remarkably. This technique has resulted in the formation of
both intergranular austenite and intra granular austenite.
This method of laser welding will improve the phase balance in
duplex stainless steel with good bead appearance.

ADVANTAGES OF THE METHOD OF THE PRESENT INVENTION
i) It provides an autogenous laser welding of duplex stainless steel with
defocused laser beam using 100% nitrogen shielding gas.
ii) The method successfully maintains the phase balance of Austenite-
Ferrite.
iii) It results defect free, smooth weld bead
iv) It avoids the supply of filler wire through external wire feeder
v) It eliminates the usage of expensive nickel fortified filler material

WE CLAIM
1. A method for maintaining microstructural balance of laser welded
duplex stainless steel with autogenous high power laser welding
process using nitrogen shielding gas comprising and characterized in
that;
arranging the nozzle for supplying nitrogen gas over the workpiece at
a predetermined distance in off-axial angular position;
supplying nitrogen gas at a predetermined flow rate to stabilize the
austenite phase in the weld and to achieve plasma suppression;
wherein welding is carried out at a predetermined welding speed
maintaining the focal plane at a predetermined height above surface
with 300mm focal length mirror to achieve a spot size of 410pm to
increase the interaction time to reduce cooling rates and to increase
the formation of austenite when laser beam is defocused to improve
the smoothness of the weld bead by reducing the chances of undercut
of low welding speeds.
2. A method as claimed in claim 1, wherein the predetermined distance
of the nitrogen supply nozzle over the workpiece is 6mm and the
diameter of the nozzle is 4mm.
3. A method as claimed in claim 1-2, wherein the nitrogen supply nozzle
is disposed at an angle of 45° with the plane of the workpiece.
4. A method as claimed in claim 1-3, wherein welding is carried out at a
welding speed of 800mm/min.

5. A method as claimed in claim 1-4, wherein the focal plane is
maintained at 3mm above surface.

ABSTRACT

The method comprises of arranging the nozzle for supplying
nitrogen gas over the workpiece at a predetermined distance in off-axial angular
position and then supplying nitrogen gas at a predetermined flow rate to stabilize
the austenite phase in the weld and to achieve plasma suppression when
welding is carried out at a predetermined welding speed maintaining the focal
plane at a predetermined height above surface with 300 mm focal length mirror
to achieve a spot size of 410µm to increase the interaction time to reduce cooling
rates and to increase the formation of austenite when laser beam is defocused to
improve the smoothness of the weld bead by reducing the chances of undercut
at low welding speeds.