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S. S. Nayak et al.
Solid-State Reactions
In fusion welding, the reactions helping to strengthen the weld metal are: solidstate phase transformations, delayed cracking [16], precipitation of secondary phases
during cooling, etc. These reactions notably change the microstructure and properties
of the weldment with respect to the BM.
Various Strengthening Mechanisms in Weld Metal
There are various strengthening mechanisms such as precipitation hardening, transformation hardening, solid solution strengthening and solidification grain structure.
Solidification grain structure strengthening mechanism applies to all types of welding
processes; the obtained microstructure is having fine dendrites because of rapid
freezing of the weld metal which blocks the plastic flow in case of a tensile test.
Therefore, in weld metals, the ratio of yield strength to tensile strength is high relative to BM. The weld metal is strengthened by the addition of various alloys (both
interstitial and substitution alloying) in case of solid solution strengthening mechanism, and it can be applied to any type of alloy. The grains form in this mechanism
is due to modulus effect and lattice distortion. A solid solution is formed due to
the addition of alloying element and sometimes it leads to non-uniformity in crystal
lattice. Precipitation hardening mechanism is accomplished by appropriate heat treatments and applicable for alloys of aluminium, magnesium, nickel, etc. In nickel, a
vast range of precipitations is found and many phases present in the interior of grain.
Precipitates act as obstacles to the dislocation motion, and stress increases to push the
dislocations leading to increase in strength. Transformation hardening mechanism is
applicable for alloys of steels [17]. Even if the austenite decomposition product is not
martensitic, the transformation hardening mostly occurs in ferrous steels. In precipitation hardening, the weld metals are strengthened with the ageing phenomenon and
different microstructures can be seen in it.
Metallurgical Events in HAZ
The microstructure of HAZ depends on the heat input, metals used, metal condition before welding, composition, etc. Various metallurgical reactions in HAZ
occur such as recrystallization, recovery, grain growth, precipitate formation, overageing/dissolution of precipitates, phase transformations, stress relaxation and
residual stress [18].
Recovery, Recrystallization and Grain Growth
During the recovery stage, rearrangement of dislocation occurs resulting in the formation of a cellular dislocation structure. This in turn produces strain-free regions due
to the reduction of internal energy. These regions help in the formation of new grains
by acting as nuclei. The strain-free nuclei grow in the recrystallization stage, and the
S. S. Nayak et al.
Solid-State Reactions
In fusion welding, the reactions helping to strengthen the weld metal are: solidstate phase transformations, delayed cracking [16], precipitation of secondary phases
during cooling, etc. These reactions notably change the microstructure and properties
of the weldment with respect to the BM.
Various Strengthening Mechanisms in Weld Metal
There are various strengthening mechanisms such as precipitation hardening, transformation hardening, solid solution strengthening and solidification grain structure.
Solidification grain structure strengthening mechanism applies to all types of welding
processes; the obtained microstructure is having fine dendrites because of rapid
freezing of the weld metal which blocks the plastic flow in case of a tensile test.
Therefore, in weld metals, the ratio of yield strength to tensile strength is high relative to BM. The weld metal is strengthened by the addition of various alloys (both
interstitial and substitution alloying) in case of solid solution strengthening mechanism, and it can be applied to any type of alloy. The grains form in this mechanism
is due to modulus effect and lattice distortion. A solid solution is formed due to
the addition of alloying element and sometimes it leads to non-uniformity in crystal
lattice. Precipitation hardening mechanism is accomplished by appropriate heat treatments and applicable for alloys of aluminium, magnesium, nickel, etc. In nickel, a
vast range of precipitations is found and many phases present in the interior of grain.
Precipitates act as obstacles to the dislocation motion, and stress increases to push the
dislocations leading to increase in strength. Transformation hardening mechanism is
applicable for alloys of steels [17]. Even if the austenite decomposition product is not
martensitic, the transformation hardening mostly occurs in ferrous steels. In precipitation hardening, the weld metals are strengthened with the ageing phenomenon and
different microstructures can be seen in it.
Metallurgical Events in HAZ
The microstructure of HAZ depends on the heat input, metals used, metal condition before welding, composition, etc. Various metallurgical reactions in HAZ
occur such as recrystallization, recovery, grain growth, precipitate formation, overageing/dissolution of precipitates, phase transformations, stress relaxation and
residual stress [18].
Recovery, Recrystallization and Grain Growth
During the recovery stage, rearrangement of dislocation occurs resulting in the formation of a cellular dislocation structure. This in turn produces strain-free regions due
to the reduction of internal energy. These regions help in the formation of new grains
by acting as nuclei. The strain-free nuclei grow in the recrystallization stage, and the