234
D. Sen et al.
Process parameters such as arc voltage, welding speed, line power, wire feed rate and
nozzle to plate distance influence the weld bead parameters, particularly the bead
penetration, width of the bead, reinforcement and dilution. Hence, it is important to
find out an optimum relationship among the above-mentioned bead parameters and
process parameters to control the bead shape to produce a quality welded tubes [18,
19].
In a SAW process, higher heat input is induced in the welded zone because the
heat losses are less as the welding is being done inside the flux. This allows welding
of thicker tubes without any contamination in the welded zone. High heat generation
affects the microstructure and results in coarser grain structures in the welded zone,
which in turn modifies the properties of the tubular structure. Properties, such as the
strength and toughness determining the suitability of tubular structures in various
applications, get deteriorated in the nugget zone (NZ) and the HAZ due to these
microstructural changes. Hence, these zones form the weakest link in the tube where
failure can occur easily [17, 20].
Further, in SAW, one of the unavoidable limitations is the generation of residual
stresses in the weldments because of the complex thermal cycle produced. Heat
transfer feature of the fusion zone and the HAZ which leads in contraction of the
weld bead during solidification process induces residual stress and strain near the
weld bead [21]. This residual stress affects the mechanical performance of the welded
tubes by increasing its susceptibility to fracture and fatigue. Moreover, the presence
of inclusions in the welded zone leads to crack growth. This crack gets accelerated
with the existence of residual stresses in the welded zone along the weld line which
leads to an early failure of the tube [22]. Similarly, the presence of residual stresses in
SAWed joint accelerates the occurrence of corrosion cracks and notably increases the
stress corrosion cracking susceptibility of the tube [23]. Thus, even though SAWed
tubes are easily manufactured with very less cost, these cannot be used in places
where failure of tubes can lead to a catastrophic disaster. Therefore, to fabricate tubes
with minimum residual stress and enhanced properties, another welding technique,
known as the electrical resistance welding (ERW), is being used by the manufacturers
widely.
7.2.2 Electric Resistance Welding (ERW)
In ERW of tubular structures, the edges of the rolled sheet or plate are held together
and high current is passed through them. Due to skin and proximity effect, the
current gets localized at the edges and heats them up to the melting point by Joule
heating. The current feeding in ERW can be done either by contact method or by the
induction method. Contact method is preferred to weld large diameter tubes where
copper based shoes are applied close to the strip edges. It allows the current to flow
into the edges and melt. Further, to weld small diameter tubes, induction method is
preferred where high-frequency induction coils are used to concentrate the current
in the edges and heat it for welding. In order to complete the welding of the tubes,
D. Sen et al.
Process parameters such as arc voltage, welding speed, line power, wire feed rate and
nozzle to plate distance influence the weld bead parameters, particularly the bead
penetration, width of the bead, reinforcement and dilution. Hence, it is important to
find out an optimum relationship among the above-mentioned bead parameters and
process parameters to control the bead shape to produce a quality welded tubes [18,
19].
In a SAW process, higher heat input is induced in the welded zone because the
heat losses are less as the welding is being done inside the flux. This allows welding
of thicker tubes without any contamination in the welded zone. High heat generation
affects the microstructure and results in coarser grain structures in the welded zone,
which in turn modifies the properties of the tubular structure. Properties, such as the
strength and toughness determining the suitability of tubular structures in various
applications, get deteriorated in the nugget zone (NZ) and the HAZ due to these
microstructural changes. Hence, these zones form the weakest link in the tube where
failure can occur easily [17, 20].
Further, in SAW, one of the unavoidable limitations is the generation of residual
stresses in the weldments because of the complex thermal cycle produced. Heat
transfer feature of the fusion zone and the HAZ which leads in contraction of the
weld bead during solidification process induces residual stress and strain near the
weld bead [21]. This residual stress affects the mechanical performance of the welded
tubes by increasing its susceptibility to fracture and fatigue. Moreover, the presence
of inclusions in the welded zone leads to crack growth. This crack gets accelerated
with the existence of residual stresses in the welded zone along the weld line which
leads to an early failure of the tube [22]. Similarly, the presence of residual stresses in
SAWed joint accelerates the occurrence of corrosion cracks and notably increases the
stress corrosion cracking susceptibility of the tube [23]. Thus, even though SAWed
tubes are easily manufactured with very less cost, these cannot be used in places
where failure of tubes can lead to a catastrophic disaster. Therefore, to fabricate tubes
with minimum residual stress and enhanced properties, another welding technique,
known as the electrical resistance welding (ERW), is being used by the manufacturers
widely.
7.2.2 Electric Resistance Welding (ERW)
In ERW of tubular structures, the edges of the rolled sheet or plate are held together
and high current is passed through them. Due to skin and proximity effect, the
current gets localized at the edges and heats them up to the melting point by Joule
heating. The current feeding in ERW can be done either by contact method or by the
induction method. Contact method is preferred to weld large diameter tubes where
copper based shoes are applied close to the strip edges. It allows the current to flow
into the edges and melt. Further, to weld small diameter tubes, induction method is
preferred where high-frequency induction coils are used to concentrate the current
in the edges and heat it for welding. In order to complete the welding of the tubes,