Submerged Arc Welding
This blog
presents the principle of submerged arc
welding process besides methods of manufacturing and characteristics of
different types of fluxes used in this process.
Role of important welding parameters of SAW had also been discussed.
Further, the advantages and limitations of this process have been
described. Keywords: Submerged arc
welding, SAW flux, weld bead geometry, type of fluxes, limitation, advantages
and application of SAW.
Introduction
Submerged
arc welding (SAW) process uses heat generated by an electric arc established
between a bare consumable electrode wire and the work piece. Since in that process,
welding arc and the weld pool are completely submerged under cover of granular
fusible and molten flux therefore it was called so. During welding, granular
flux was melted using heat generated by arc and forms cover of molten flux
layer which in turn avoids spatter tendency and prevents accessibility of
atmospheric gases to the arc zone and the weld pool. The molten flux reacts
with the impurities in the molten weld metal to form slag which floats over the
surface of the weld metal. Layer of slag over the molten weld metal
results:
·
Increased
protection of weld metal from atmospheric gas contamination and so improved
properties of weld joint
·
Reduced
cooling rate of weld metal and HAZ owing to shielding of the weld pool by
molten flux and solidified slag in turn leads to a) smoother weld bead and b)
reduced the cracking tendency of hardenable steel
13.2
Components of SAW System
SAW was known
to be a high current (sometimes even greater 1000A) welding process that was mostly
used for joining of heavy sections and thick plates as it offers deep
penetration with high deposition rate and so high welding speed. High welding
current could be applied in this process owing to three reason a) absence of
spatter, b) reduced possibility of air entrainment in arc zone as molten flux
and slag form shield the weld metal a.d c) large diameter electrode. Continuous
feeding of granular flux around the weld arc from flux hopper provides
shielding to the weld pool from atmospheric gases and control of weld metal
composition through presence of alloying element in flux. Complete cover of the
molten flux around electrode tip and the welding pool during the actual welding
operation produces weld joint without spatter and smoke. In following sections,
important components of SAW system and their role have been presented (Fig.
13.1).
Fig.
13.1 Schematic of submerged arc welding system
13.2.1
Power source
Generally,
submerged arc welding process uses power source at 100 % duty cycle; which
means that the welding was done continuously for minimum 5 min without a break
or more. Depending upon the electrode diameter, type of flux and electrical
resistivity submerged arc welding could work with both AC and DC. Alternating
current and DCEN polarity are generally used with large diameter electrode (>4mm). DC with constant voltage
power source provides good control over bead shape, penetration, and welding
speed. However, DC could cause arc blow under some welding conditions. Polarity
affects weld bead geometry, penetration and deposition rate. DCEP offers
advantage of self regulating arc in case of small diameter electrodes (<
2.4mm) and high deposition rate while DCEN produces shallow penetration.
13.2.2
Welding Electrode
The
diameter of electrodes used in submerged arc welding generally ranges from 1–5
mm. The electrode wire was fed from the spool through a contact tube connected
to the power source. Electrode wire of steel was generally copper coated for
two reasons
a) to
protect it from atmospheric corrosion and
b) to
increase their current carrying capacity. However, stainless steel wires are
not coated with copper.
13.2.3
SAW Flux
Role of
fluxes in SAW was largely similar that of coating in stick electrodes of SMAW
i.e. protection of weld pool from inactive shielding gases generated by thermal
decomposition of coating material. SAW fluxes could influence the weld metal
composition appreciably in the form of addition or loss of alloying elements
through gas metal and slag metal reactions. Few hygroscopic fluxes are baked
(at 250–300 C for 1-2 hours) to remove the moisture. There are four types of
common SAW fluxes namely fused flux, agglomerated flux, bonded flux and
mechanical fluxes. Manufacturing steps of these fluxes are given below.
• Fused
fluxes: raw constituents-mixed-melted-quenched-crushed screened-graded
• Bonded fluxes: raw constituents-powdered-dry
mixed-bonded using K/Na silicates-wet mixed-pelletized-crushed-screened
•
Agglomerated fluxes: made in similar way to bonded fluxes but ceramic binder
replaces silicate binder
•
Mechanically mixed fluxes: mix any two or three type of above fluxes in desired
ratios
Specific
characteristics of each type of flux
Fused
fluxes
•
Positives
–
Uniformity of chemical composition
–
No effect of removal of fine particles on flux composition
–
Non-hygroscopic: easy handling and storage
–
Easy recycling without much change in particle size and composition
•
Limitation was related with difficulty in
– incorporating deoxidizers and ferro alloys
– melting due to need of high temperature Bonded fluxes
•
Positives
–
Easy to add deoxidizers and alloying elements
–
Allows thicker layer of flux during welding
•
Limitation
–
Hygroscopic
–
Gas evolution tendency
–
Possibility of change in flux composition due to removal of fine particles
Agglomerated
fluxes
These
are similar to that of bonded fluxes except that these use ceramic binders
Mechanical
fluxes
• Positives
– Several commercial fluxes could be easily
mixed & made to suit critical application to get desired results
• Limitations
–
Segregation of various fluxes
• during storage / handling
• in feeder and recovery system
• inconsistency in flux from mix to mix
13.3
Composition of the SAW fluxes
The
fused and agglomerated types of fluxes usually consist of different types of
halides and oxides such as MnO, SiO2, CaO, MgO, Al2O3, TiO2, FeO, and CaF2 and
sodium/potassium silicate. Halide fluxes are used for high quality weld joints
of high strength steel to be used for critical applications while oxide fluxes
are used for developing weld joints of non-critical applications. Some of
oxides such as CaO, MgO, BaO, CaF2, Na2O, K2O, MnO etc. are basic in nature
(donors of oxygen) and few others such as SiO2, TiO2, Al2O3 are acidic
(acceptors of oxygen). Depending upon relative amount of these acidic and basic
fluxes, the basicity index of flux was decided.
The basicity index of flux was ratio of sum of (wt. %) all basic oxides
to all non-basic oxides. Basicity of flux affects the slag detachability, bead
geometry, mechanical properties and current carrying capacity as welding with
low basicity fluxes results in high current carrying capacity, good slag
detachability, good bead appearance and poor mechanical properties and poor
crack resistance of the weld metal while high basicity fluxes produce opposite
effects on above characteristics of the weld.
13.4
Fluxes for SAW and Recycling of slag
The
protection to the weld pool in submerged arc welding process was provided by
molten layer of flux covering to the weld pool. Neutral fluxes are found mostly
free from de-oxidizers (like Si, Mn) therefore loss of alloying elements from
weld metal becomes negligible and hence chemical composition of the weld metal was
not appreciably affected by the application of neutral fluxes. However, base
metal having affinity with oxygen exhibits tendency of porosity and cracking
along the weld center line. Active fluxes contain small amount of de-oxidizer
such as manganese, silicon singly or in combination. The deoxidizers enhance
resistance to porosity and weld cracking tendency.
The
submerged arc welding fluxes produce a lot of slag which was generally disposed
off away as a waste. The disposal of slag however imposes many issues related
with storage, and environmental pollution. The recycling of the used flux could
reduce production cost appreciably without any compromise on the quality of the weld. However, recycling needs
extensive experimentation to optimize the composition of recycled flux so as to
achieve the desired operational characteristics and the performance of the weld
joints. The recycling of flux basically involves the use of slag with fresh
flux. The slag developed from SAW process was crushed and mixed with new flux.
This process was different from recycling of un-fused flux which was collected
from the clean surface and reused without crushing. Slag produced during
submerged arc welding while using a specific kind/brand of the flux was crushed
and then used as flux or used after mixing with original unused flux to ensure
better control over the weld properties. Building of slag with unused flux
modifies the characteristics of original unused flux therefore the blending
ratio must be optimized for achieving the quality weld joints.
13.5
Welding parameters
Welding parameters namely electrode wire size,
welding voltage, welding current and welding speed are four most important
parameters (apart from flux) that play a major role on soundness and
performance of the weld therefore these must be selected carefully before
welding.
13.5.1
Welding Current
Welding
current was the most influential process parameter for SAW because it
determines the melting rate of electrode, penetration depth and weld bead
geometry. However, too high current may lead to burn through owing to deep
penetration, excessive reinforcement, increased residual stresses and high heat
input related problems like weld distortion. On the other hand, selection of
very low current was known to cause lack of penetration & lack of fusion
and unstable arc. Selection of welding current was primarily determined by
thickness of plates to be welded and accordingly electrode of proper diameter was
selected so that it could withstand under the current setting required for
developing sound weld with requisite deposition rate and penetration (Fig.
2).
Diameter
(mm) Welding
Current (A)
1.6 150-300
2.0 200-400
2.5 250-600
3.15 300-700
4.0 400-800
6.0 700-1200
13.5.2
Welding Voltage
Welding
voltage had marginal affect on the melting rate of the electrode. Welding voltage
commonly used in SAW ranges from 20-35 V. Selection of too high welding voltage
(more arc length) leads to flatter and wider weld bead, higher flux
consumption, and increased gap bridging capability under poor fit-up conditions
while low welding voltage produces narrow & peaked bead and poor slag
detachability (Fig. 2).
13.5.3
Welding speed
Required bead geometry and penetration in a
weld joint are obtained only with an optimum speed of welding arc during SAW.
Selection of a speed higher than optimum one reduces heat input per unit length
which in turn results in low deposition rate of weld metal, decreased weld
reinforcement and shallow penetration (Fig. 13.2). Further, too high welding
speed increases tendency for a) undercut in weld owing to reduced heat input,
b) arc blow due to higher relative movement of arc with respect to ambient
gases and c) porosity as air pocket are entrapped due to rapid solidification
of the weld metal. On other hand low welding speed increases heat input per
unit length which in turn may lead to increased tendency of melt through and
reduction in tendency for development of porosity and slag inclusion.
Fig.
13.2 Influence of welding parameters on weld bead geometry
13.6
Bead geometry and effect of welding parameters
Bead geometry and depth of penetration are two
important characteristics of the weld bead that are influenced by size of the
electrode for a given welding current setting. In general, an increase in size
of the electrode decreases the depth of penetration and increases width of weld
bead for a given welding current (Fig. 13.3). Large diameter electrodes are
primarily selected to take two advantages a) higher deposition rate owing to
their higher current carrying capacity and b) good gap bridging capability
under poor fit-up conditions of the plates to be welded due to wider weld bead.
Fig. 1
13 3..3 Influence of electrode diameter on weld bead geometry
13.7
Advantage
Due to unique features like welding arc
submerged under flux and use of high welding current associated with submerged
arc welding processes compared with other welding process, it offers following
important advantages:
·
High
productivity due to high deposition rate of the welding metal and capability
weld continuously without interruptions as electrode was fed from spool, and
the process works under 100% duty cycle.
·
High
depth of penetration allows welding of thick sections
·
Smooth
weld bead was produced without stresses raisers as SAW was carried out without
sparks, smoke and spatter
13.8 Limitations
There
are three main limitations of SAW a) invisibility of welding arc during
welding, b) difficulty in maintaining mound of the flux cover around the arc in
odd positions of welding and cylindrical components of small diameter and c)
increased tendency of melt through when welding thin sheet. Invisibility of
welding arc submerged under un-melted and melted flux cover in SAW makes it
difficult to ensure the location where weld metal was being deposited during
welding. Therefore, it becomes mandatory to use an automatic device (like
welding tractors) for accurate and guided movement of the welding arc in line
with weld groove so that weld metal was deposited correctly along weld line
only.
Applications
of SAW process are mainly limited to flat position only as developing a mound
of flux in odd position to cover the welding arc becomes difficult which was a
requisite for SAW. Similarly, circumferential welds are difficult to develop on
small diameter components due to flux falling tendency away from weld zone.
Plates of thickness less than 5 mm are generally not welded due to risk of burn
through. Further, SAW process was known
as high heat input process. High heat input however was not considered good for
welding of many steels as it leads to signify couldt grain growth in weld and
HAZ owing to low cooling rate experienced by them during welding. Low cooling
rate increases the effective transformation temperature which in turn lowers
nucleation rate and increases the growth rate during solid state
transformation. A combination of low nucleation rate and high the growth rate
results in coarse grain structure. Coarse grain structure in deteriorate the mechanical
properties of the weld joint specifically toughness. Therefore, SAW weld joints
are sometime normalized to refine the grain structure and enhanced the
mechanical properties so as to reduce the adverse effect of high input of SAW
process on mechanical properties of the weld joints.
13.9
Applications
Submerged
arc welding was used for welding of different grades of steels in many sectors
such as shipbuilding, offshore, structural and pressure vessel industries
fabrication of pipes, penstocks, LPG cylinders, and bridge girders. Apart from
the welding, SAW was also used for surfacing of worn out parts of large surface
area for different purposes such as reclamation, hard facing and cladding. The
typical application of submerged arc welding for weld surfacing includes
surfacing of roller barrels and wear plates. Submerged arc welding was widely
used for cladding carbon and alloy steels with stainless steel and nickel alloy
deposits
