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S. S. Nayak et al.
significant recovery of the properties can be seen. Tensile strength reduced significantly and ductility increased at this stage. Finally, when each of the grains begins to
enlarge at high temperature, a small amount of additional recovery of the properties
can be seen. The vast majority difference occurs during the recrystallization stage
when each of the deformed grains is given the opportunity to reform its crystalline
structure.
Allotropic Phase Transformation
Allotropy signifies the existence of different crystallographic forms of a metal
at different temperature values. Due to allotropic phase transformation, various
microstructures get evolved in steel. Austenite is formed at elevated temperature
in HAZ; however with cooling, various other transformational products like ferrite,
bainite and martensite are also evolved. Sometimes, the phase transformation is
taken into consideration to optimize the strength of a material. In some of the welding
processes, the temperature gradient is intentionally decreased by preheating the alloy
steels to form martensite or bainite, and finally, to avail a microstructure consisting
of both tempered martensite and bainite.
Precipitation reactions are another most important metallurgical event which cause
microstructural change in HAZ.
6.2.2 Microstructure in Solid-State Welding
In solid-state processes, the formation of metallic bonds occurs at the atomic levels.
As stated above, melting and solidification do not take place in these welding
processes. Instead, heat and deformation play a vital role at weld interface. A few of
the examples are mentioned in the introduction section. This section starts with the
regions of solid-state welding processes, followed by the influence of metallurgical
processes influencing these welding processes.
6.2.2.1 Regions of a Solid-State Weld
The BM, HAZ, and heat and deformation zone are the three regions found in
these welding processes. Heat and deformation zone refers to the weld zone, where
dynamic recrystallization occurs due to forging action by various means which in
turn results in extremely fine grains, which thereby provides higher strength to this
zone. The properties of HAZ are significantly different from BM (which as usual
stays unaffected during these welding processes).
S. S. Nayak et al.
significant recovery of the properties can be seen. Tensile strength reduced significantly and ductility increased at this stage. Finally, when each of the grains begins to
enlarge at high temperature, a small amount of additional recovery of the properties
can be seen. The vast majority difference occurs during the recrystallization stage
when each of the deformed grains is given the opportunity to reform its crystalline
structure.
Allotropic Phase Transformation
Allotropy signifies the existence of different crystallographic forms of a metal
at different temperature values. Due to allotropic phase transformation, various
microstructures get evolved in steel. Austenite is formed at elevated temperature
in HAZ; however with cooling, various other transformational products like ferrite,
bainite and martensite are also evolved. Sometimes, the phase transformation is
taken into consideration to optimize the strength of a material. In some of the welding
processes, the temperature gradient is intentionally decreased by preheating the alloy
steels to form martensite or bainite, and finally, to avail a microstructure consisting
of both tempered martensite and bainite.
Precipitation reactions are another most important metallurgical event which cause
microstructural change in HAZ.
6.2.2 Microstructure in Solid-State Welding
In solid-state processes, the formation of metallic bonds occurs at the atomic levels.
As stated above, melting and solidification do not take place in these welding
processes. Instead, heat and deformation play a vital role at weld interface. A few of
the examples are mentioned in the introduction section. This section starts with the
regions of solid-state welding processes, followed by the influence of metallurgical
processes influencing these welding processes.
6.2.2.1 Regions of a Solid-State Weld
The BM, HAZ, and heat and deformation zone are the three regions found in
these welding processes. Heat and deformation zone refers to the weld zone, where
dynamic recrystallization occurs due to forging action by various means which in
turn results in extremely fine grains, which thereby provides higher strength to this
zone. The properties of HAZ are significantly different from BM (which as usual
stays unaffected during these welding processes).