236
D. Sen et al.
[26, 27]. It was observed that these defects were unavoidable because of the alloying
content present in the tube material which significantly decreased the properties of
the welded tube, such as ductility and corrosion resistance. In this context, it was also
reported that the weld zone exhibits severe localized corrosion due to microstructural
changes. These microstructural changes were because of faster cooling in the weld
zone. Thus, change in microstructure and the variance of alloying elements between
the welded metal and the parent metal act as a potential corrosion sites in the tubular
structure. To mitigate these potential corrosion sites, heat treatment of the tubes is
suggested by the researchers [28].
7.2.3 High Energy Beam Welding
High energy beam welding of tubular components are used by industries where very
precise welded tubes are required with high performance. This welding technique
have small beam size which helps in getting deep penetration, thus making it possible
for welding thicker tubular components with less heat input. Welding processes, such
as laser welding and electron beam welding (EBW), fall in the category. Among all,
laser beam welding (LBW) is widely used for manufacturing of tubes.
LBW is an advanced technique which provides high precision and flexibility in
welding of tubular components. In LBW for tubular components, the welded joint
is formed by melting the edges of the rolled sheet or plate by a laser beam. Energy
required for welding tube is focused into a very small area allowing it to weld with
a high energy density and a very less heat input, resulting in a narrow fusion zone
with small HAZ. Due to less heat input, the cooling rate becomes high resulting
in low residual stress, fine grain structure, low thermal distortion maintaining good
dimensional accuracy and good surface quality. LBW results in high productivity
and produces deep penetration and narrow weld bead with fine grain structure [29].
In laser welding, owing to high density and reduced heat input, rapid solidification
of the molten metal occurs at the welded zone. This allows for microstructural grain
refinement and chemical homogeneity of the joint, which enhances the mechanical
properties of the overall LBWed tube [8]. Researchers compared the performance of
LBWed and conventionally welded (GTAWed) austenitic steel tubes and observed
that LBWed tube offers better metallurgical properties and exhibits near-seamless
quality [30].
LBW, for fabrication of tubular structures, mainly depends on various factors
which can be classified into three categories, namely material properties, process
parameters and system parameters, as shown in Fig. 7.4. Even though LBW has got
many advantages, the efficiency and the quality of LBWed tubular structure depend
on the material of the tube to be welded and its thickness. For a thick tube, multi-pass
welding is used with an addition of filler material. However, with an addition of the
filler metal, many of the advantages associated with LBW either get reduced largely,
or substantially get offset by other complications, such as residual stress generation,
formation of defects and intermetallics [31]. Moreover, properties of the material of
D. Sen et al.
[26, 27]. It was observed that these defects were unavoidable because of the alloying
content present in the tube material which significantly decreased the properties of
the welded tube, such as ductility and corrosion resistance. In this context, it was also
reported that the weld zone exhibits severe localized corrosion due to microstructural
changes. These microstructural changes were because of faster cooling in the weld
zone. Thus, change in microstructure and the variance of alloying elements between
the welded metal and the parent metal act as a potential corrosion sites in the tubular
structure. To mitigate these potential corrosion sites, heat treatment of the tubes is
suggested by the researchers [28].
7.2.3 High Energy Beam Welding
High energy beam welding of tubular components are used by industries where very
precise welded tubes are required with high performance. This welding technique
have small beam size which helps in getting deep penetration, thus making it possible
for welding thicker tubular components with less heat input. Welding processes, such
as laser welding and electron beam welding (EBW), fall in the category. Among all,
laser beam welding (LBW) is widely used for manufacturing of tubes.
LBW is an advanced technique which provides high precision and flexibility in
welding of tubular components. In LBW for tubular components, the welded joint
is formed by melting the edges of the rolled sheet or plate by a laser beam. Energy
required for welding tube is focused into a very small area allowing it to weld with
a high energy density and a very less heat input, resulting in a narrow fusion zone
with small HAZ. Due to less heat input, the cooling rate becomes high resulting
in low residual stress, fine grain structure, low thermal distortion maintaining good
dimensional accuracy and good surface quality. LBW results in high productivity
and produces deep penetration and narrow weld bead with fine grain structure [29].
In laser welding, owing to high density and reduced heat input, rapid solidification
of the molten metal occurs at the welded zone. This allows for microstructural grain
refinement and chemical homogeneity of the joint, which enhances the mechanical
properties of the overall LBWed tube [8]. Researchers compared the performance of
LBWed and conventionally welded (GTAWed) austenitic steel tubes and observed
that LBWed tube offers better metallurgical properties and exhibits near-seamless
quality [30].
LBW, for fabrication of tubular structures, mainly depends on various factors
which can be classified into three categories, namely material properties, process
parameters and system parameters, as shown in Fig. 7.4. Even though LBW has got
many advantages, the efficiency and the quality of LBWed tubular structure depend
on the material of the tube to be welded and its thickness. For a thick tube, multi-pass
welding is used with an addition of filler material. However, with an addition of the
filler metal, many of the advantages associated with LBW either get reduced largely,
or substantially get offset by other complications, such as residual stress generation,
formation of defects and intermetallics [31]. Moreover, properties of the material of