(SEM-VIII) THEORY EXAMINATION 2018-19 ADVANCED WELDING TECHNOLOGY

SECTION A

(Attempt all – Short but complete explanations)
 

a) Types of welded joints used in welding

The commonly used welded joints are butt joint, lap joint, tee joint, corner joint and edge joint. Butt joints are used when plates lie in the same plane, while lap joints are used for overlapping plates. Tee and corner joints are mainly used in structural fabrication.
 

b) Meaning of cladding and surfacing

Cladding is the process of depositing a corrosion-resistant or wear-resistant metal layer on a base metal to improve surface properties.

Surfacing is used to restore worn-out surfaces or to increase hardness and wear resistance of components.
          

c) Definition of welding and comparison with other joining processes

Welding is a permanent joining process in which two or more metal parts are joined by fusion, with or without filler material and pressure.

Unlike riveting or bolting (mechanical joining), welding provides high joint efficiency, rigidity, leak-proof joints and reduced weight, but it is non-detachable.
 

d) Welding symbols

Welding symbols are standardized graphical representations used in drawings to specify type of weld, size, length, location and finishing. They consist of a reference line, arrow, basic weld symbol, and supplementary symbols.
 

e) Arc blow in welding

Arc blow is the deflection of welding arc from its normal path due to magnetic forces, especially in DC welding. It causes uneven weld beads and defects. It can be reduced by changing electrode angle, using AC supply, or proper grounding.
 

f) Weld distortion and its prevention

Weld distortion is dimensional change due to uneven heating and cooling during welding.
Prevention methods include proper joint design, balanced welding sequence, use of fixtures, presetting and controlled heat input.
 

g) Heat Affected Zone (HAZ)

HAZ is the region adjacent to the weld metal that does not melt but undergoes metallurgical changes due to welding heat. It often has altered mechanical properties and may be prone to cracking.
 

h) Types of brazing techniques

Common brazing techniques include torch brazing, furnace brazing, dip brazing, induction brazing and resistance brazing.
Torch brazing uses a gas flame to melt the filler metal and is widely used due to simplicity and low cost.
 

i) Physics of arc welding

Arc welding works on the principle of electric arc formation between electrode and workpiece. The arc generates intense heat (about 6000°C) which melts base metal and filler metal to form a weld joint.
 

j) Reclamation welding

Reclamation welding is used to restore worn, cracked or damaged machine parts by depositing metal on worn surfaces, thereby increasing service life and reducing replacement cost.
 

SECTION B

(Attempt any three – Long descriptive answers)
 

a) Comparison between Laser Beam Welding (LBW) and Electron Beam Welding (EBW)

Laser Beam Welding uses a highly focused laser beam as a heat source and can be performed in atmospheric conditions. It is suitable for automation and high-speed welding.

Electron Beam Welding uses a focused beam of high-velocity electrons and must be carried out in a vacuum chamber. EBW offers deeper penetration and very narrow welds but involves high equipment cost and size limitation.
 

b) Types of underwater welding and their working
 

Underwater welding is classified into wet welding and dry welding.
In wet welding, the welder and electrode are directly exposed to water, making it simple and economical but less reliable.

Dry welding is performed inside a sealed chamber filled with gas, producing high-quality welds with better control and safety.
 

c) Weld defects and explanation of any two

Common weld defects include porosity, slag inclusion, cracks, lack of fusion and undercut.
Porosity occurs due to trapped gases forming cavities in weld metal.
Cracks are serious defects caused by high residual stresses and improper cooling rates.

d) Effect of alloying elements on weldability

Carbon increases hardness but reduces weldability.
Manganese improves strength and reduces hot cracking.
Sulphur and phosphorus reduce weld quality by increasing brittleness.
Nickel improves toughness, while chromium increases hardness and corrosion resistance.

e) Use of transformer, rectifier and generators in welding

Transformers are used in AC welding to step down voltage.
Rectifiers convert AC into DC for stable arc welding.
Generators are used where power supply is not available and provide smooth DC output.

SECTION C

(Attempt any one from each question – Long answers)

Q3(a) Heating and cooling rate and its effect on weld properties

Heating and cooling rate refers to the speed at which temperature rises and falls during welding. Fast cooling increases hardness and brittleness, while slow cooling improves ductility but may reduce strength. Proper control is essential to avoid cracking and distortion.

Q4(a) Metalizing and hard facing

Metalizing is a thermal spray process where molten metal is sprayed onto a surface for corrosion protection.
Hard facing involves depositing a hard alloy on a base metal to improve wear resistance.
Advantages include extended component life and cost savings. Applications include shafts, gears and cutting tools.

Q5(b) Cracking of weld – hot and cold cracking

Weld cracking is the fracture of weld metal or HAZ.
Hot cracking occurs at high temperatures during solidification due to impurities.
Cold cracking occurs after cooling due to hydrogen embrittlement and residual stresses.
Cracking can be avoided by proper material selection, preheating and controlled cooling.

Q6(b) FCAW welding principle and MIG vs FCAW

Flux Cored Arc Welding (FCAW) uses a flux-filled tubular electrode that provides shielding gas.
Compared to MIG, FCAW gives deeper penetration and is suitable for outdoor welding.
Weld quality is affected by current, voltage, wire feed speed and shielding gas.

Q7(a) Numerical – Arc length and maximum power

Given:
V = 24 + 4L
Open circuit voltage = 80V
Short circuit current = 600A

Using power source characteristics and arc equation, optimum arc length is obtained at maximum power condition, and maximum power occurs when arc voltage equals half of open-circuit voltage.
Thus, maximum power ≈ 12 kW, and optimum arc length ≈ 6 mm.