206
S. S. Nayak et al.
medium-alloy steel (pearlite + small pearlite) and plain carbon steels. All these steels
are specifically designed with several factors such as section thickness, preheat,
hydrogen content and heat input and the formation of undesirable transformation
products, such as martensite to have good weldability to be used in many applications.
During welding of a steel component or structure, it is important to select a grade
that has good weldability and meets the mechanical requirements of the design. One
example of a specially designed steel is ASTM A 216, Grade WCB, which is used
for pressure vessel, valve and pump castings. Its ability to be welded and repaired
makes it as a popular material [24].
Stainless steels are one of the important commercial alloy steels that contain more
than 12% chromium and other alloying elements like nickel, manganese, phosphorous and silicon. The four classes of stainless steel are duplex, ferritic, austenitic
and martensitic. Because of chromium, steel forms a tiny layer of chromium oxide
which is non-porus, coherent and protective to BM. The austenitic alloys are used
in applications where good toughness, high temperature resistance, good corrosion
resistance are needed. They do not harden in the HAZ and do have a good weldability.
But few problems associated with welding these steels are solidification cracking
of weld metal and other problems such as weld decay/sensitization, sigma phase
formation, hair like cracking in HAZ. The chromium percentage in ferritic stainless
steels is 12–27%. It also has a small amount of austenite-forming elements. These
steels have good formability. Martensitic stainless steels have the lowest percentage
of chromium. Due to this, it shows hardenability. Due to this property, it is used in
material where sharp edges need to be maintained, e.g. cutlery, surgical equipment’s,
etc. Necessary care such as preheating and post-heating must be taken during welding
of these steels, as the martensitic HAZ is inclined towards cracking. Duplex stainless
steels are chosen for their improved corrosion resistance [25]. They do not lead to
the formation of martensitic transformation product during welding. Most stainless
steels are easily welded by welding method such as resistance spot (RSW), laser
beam, electron beam, arc (plasma arc, SMAW, gas tungsten arc welding (GTAW),
etc.) and friction welding processes. In the case study section, FSW is used to weld
steel along with aluminium.
6.2.4 Microstructure of Aluminium and Its Weldability
The alloys of aluminium have face-centred cubic (FCC) crystal lattice structure. It
is not possible to design aluminium’s microstructure by using phase transformation. They have properties such as: (a) low density (b) good corrosion resistance (c)
capability to form oxide film which is insoluble in the molten pool of aluminium
and (d) high electrical and thermal conductivity. Aluminium has a very less melting
point. Therefore, it is easy to produce the fusion weld joints of the aluminium alloys
from the melting point of view. But the problem is thermal expansion coefficient
of aluminium. It expands too much because of the high expansion coefficient (25E6/°C) as compared to iron. It expands much and contracts leading to high residual
S. S. Nayak et al.
medium-alloy steel (pearlite + small pearlite) and plain carbon steels. All these steels
are specifically designed with several factors such as section thickness, preheat,
hydrogen content and heat input and the formation of undesirable transformation
products, such as martensite to have good weldability to be used in many applications.
During welding of a steel component or structure, it is important to select a grade
that has good weldability and meets the mechanical requirements of the design. One
example of a specially designed steel is ASTM A 216, Grade WCB, which is used
for pressure vessel, valve and pump castings. Its ability to be welded and repaired
makes it as a popular material [24].
Stainless steels are one of the important commercial alloy steels that contain more
than 12% chromium and other alloying elements like nickel, manganese, phosphorous and silicon. The four classes of stainless steel are duplex, ferritic, austenitic
and martensitic. Because of chromium, steel forms a tiny layer of chromium oxide
which is non-porus, coherent and protective to BM. The austenitic alloys are used
in applications where good toughness, high temperature resistance, good corrosion
resistance are needed. They do not harden in the HAZ and do have a good weldability.
But few problems associated with welding these steels are solidification cracking
of weld metal and other problems such as weld decay/sensitization, sigma phase
formation, hair like cracking in HAZ. The chromium percentage in ferritic stainless
steels is 12–27%. It also has a small amount of austenite-forming elements. These
steels have good formability. Martensitic stainless steels have the lowest percentage
of chromium. Due to this, it shows hardenability. Due to this property, it is used in
material where sharp edges need to be maintained, e.g. cutlery, surgical equipment’s,
etc. Necessary care such as preheating and post-heating must be taken during welding
of these steels, as the martensitic HAZ is inclined towards cracking. Duplex stainless
steels are chosen for their improved corrosion resistance [25]. They do not lead to
the formation of martensitic transformation product during welding. Most stainless
steels are easily welded by welding method such as resistance spot (RSW), laser
beam, electron beam, arc (plasma arc, SMAW, gas tungsten arc welding (GTAW),
etc.) and friction welding processes. In the case study section, FSW is used to weld
steel along with aluminium.
6.2.4 Microstructure of Aluminium and Its Weldability
The alloys of aluminium have face-centred cubic (FCC) crystal lattice structure. It
is not possible to design aluminium’s microstructure by using phase transformation. They have properties such as: (a) low density (b) good corrosion resistance (c)
capability to form oxide film which is insoluble in the molten pool of aluminium
and (d) high electrical and thermal conductivity. Aluminium has a very less melting
point. Therefore, it is easy to produce the fusion weld joints of the aluminium alloys
from the melting point of view. But the problem is thermal expansion coefficient
of aluminium. It expands too much because of the high expansion coefficient (25E6/°C) as compared to iron. It expands much and contracts leading to high residual