6 Microstructure and Texture in Welding: A Case Study on Friction Stir Welding
201
Gas–Metal Reactions
Gas–metal reactions happen because of absorption of the reactive gases like nitrogen,
oxygen or hydrogen from arc/flame into the weld pool, and it has an impact on
microstructure. In few cases, the entrapped gas dissolves in the liquid metal. In fluxcored arc welding (FCAW) and shielded metal arc welding (SMAW), a main source
for hydrogen is chemically bonded with water/moisture in the electrode coating or
core and it causes embrittlement [7]. To stabilize the arc, argon along with oxygen
is used in gas metal arc welding (GMAW) [8]. The weld metal microstructure is
observed to be changing due to variation in oxygen activity in case of high-strength
low-alloy steels. Oxide inclusion acts as nucleation sites during cooling, which
promotes decomposition of austenite [9].
In few cases, liquid metal and the gas reaction lead to formation of a chemical
compound. This compound, if it is soluble, it causes embrittlement of the welded
joint. However, if it is insoluble, then produces slag which obstructs the formation
of the weld pool. Therefore, the use of excessive gas during welding is not desirable,
and flux can be used to dissolve the so formed chemical compound, as found in
submerged arc welding process (SAW) [10]. The microstructure in the weldment of
ferritic stainless steel depends on types of shielding gas mixture used. If the amount
of carbon dioxide (oxidizing gas) increases, it will increase the carbon content in
the weld and also there will be an increase in the martensite content at the grain
boundary. If a gas dissolves in the liquid weld pool, it tries to come out during
cooling in the form of bubbles. Otherwise, the weld becomes porous in nature, if the
gas gets entrapped. In order to avoid this situation, deoxidizers are utilized [11].
Nitrogen helps in grain refining and is a powerful austenite stabilizer with an
ability to reduce the ferrite formation [12]. Nitrides and oxides help in reducing both
notch toughness and ductility of steel weld metal. The availability of hydrogen in
excess amount during solidification is not desirable because it causes porosity. It has
been reported that nucleation gets affected by the shielding gas pressure and can be a
reason to suppress the weld porosity [13]. The diffusion of gas into BM is one more
important gas–metal reactions which helps in altering the microstructure.
Liquid–Metal Reactions
One of the most important liquid–metal reactions is the formation of non-metallic
liquid phases (e.g. slag layers) which interacts with the molten weld pool produced in
electro slag welding, SAW and SMAW [14]. The flux layers used in these processes
absorb those contaminated and deoxidation products. These products are initially
silicates of iron, manganese and aluminium floating on top surface of molten pool
which get changed into slag. Some of them reside inside the weld metal, which are
known as inclusions. The second type of liquid–metal reaction is hot cracking. If
the interdendritic liquid has a low freezing temperature in comparison to formerly
solidified metal, hot cracking occurs, which in turn produces shrinkage stress in
the liquid portion, forming microcracks within the dendrite [15]. Both slag layer
formation and hot cracking have significant impact on altering weld microstructure.
201
Gas–Metal Reactions
Gas–metal reactions happen because of absorption of the reactive gases like nitrogen,
oxygen or hydrogen from arc/flame into the weld pool, and it has an impact on
microstructure. In few cases, the entrapped gas dissolves in the liquid metal. In fluxcored arc welding (FCAW) and shielded metal arc welding (SMAW), a main source
for hydrogen is chemically bonded with water/moisture in the electrode coating or
core and it causes embrittlement [7]. To stabilize the arc, argon along with oxygen
is used in gas metal arc welding (GMAW) [8]. The weld metal microstructure is
observed to be changing due to variation in oxygen activity in case of high-strength
low-alloy steels. Oxide inclusion acts as nucleation sites during cooling, which
promotes decomposition of austenite [9].
In few cases, liquid metal and the gas reaction lead to formation of a chemical
compound. This compound, if it is soluble, it causes embrittlement of the welded
joint. However, if it is insoluble, then produces slag which obstructs the formation
of the weld pool. Therefore, the use of excessive gas during welding is not desirable,
and flux can be used to dissolve the so formed chemical compound, as found in
submerged arc welding process (SAW) [10]. The microstructure in the weldment of
ferritic stainless steel depends on types of shielding gas mixture used. If the amount
of carbon dioxide (oxidizing gas) increases, it will increase the carbon content in
the weld and also there will be an increase in the martensite content at the grain
boundary. If a gas dissolves in the liquid weld pool, it tries to come out during
cooling in the form of bubbles. Otherwise, the weld becomes porous in nature, if the
gas gets entrapped. In order to avoid this situation, deoxidizers are utilized [11].
Nitrogen helps in grain refining and is a powerful austenite stabilizer with an
ability to reduce the ferrite formation [12]. Nitrides and oxides help in reducing both
notch toughness and ductility of steel weld metal. The availability of hydrogen in
excess amount during solidification is not desirable because it causes porosity. It has
been reported that nucleation gets affected by the shielding gas pressure and can be a
reason to suppress the weld porosity [13]. The diffusion of gas into BM is one more
important gas–metal reactions which helps in altering the microstructure.
Liquid–Metal Reactions
One of the most important liquid–metal reactions is the formation of non-metallic
liquid phases (e.g. slag layers) which interacts with the molten weld pool produced in
electro slag welding, SAW and SMAW [14]. The flux layers used in these processes
absorb those contaminated and deoxidation products. These products are initially
silicates of iron, manganese and aluminium floating on top surface of molten pool
which get changed into slag. Some of them reside inside the weld metal, which are
known as inclusions. The second type of liquid–metal reaction is hot cracking. If
the interdendritic liquid has a low freezing temperature in comparison to formerly
solidified metal, hot cracking occurs, which in turn produces shrinkage stress in
the liquid portion, forming microcracks within the dendrite [15]. Both slag layer
formation and hot cracking have significant impact on altering weld microstructure.