Welding MCQ Quiz - Objective Question with Answer for Welding - Download Free PDF

Explanation:

Thermit Welding in Railway Track Joining

Definition: Thermit welding is a process that utilizes the exothermic reaction between a metal oxide and aluminum powder to produce intense heat for welding. This process is particularly advantageous for joining large sections of metal, such as railway tracks, due to its ability to generate high temperatures and melt the steel, creating a strong, homogenous joint.

Working Principle: In thermit welding, a mixture of iron oxide and aluminum powder is ignited to initiate a highly exothermic reaction. The reaction can reach temperatures of about 2500°C (4500°F), which is sufficient to melt steel. The molten steel produced by the reaction flows into the mold prepared around the joint, filling the gap between the railway tracks. As the molten steel cools and solidifies, it forms a robust weld that is integral to the tracks.

Procedure: The thermit welding process typically involves the following steps:

• Preparation: The ends of the railway tracks to be joined are cleaned and aligned. A mold is then placed around the joint area to contain the molten steel.
• Ignition: The thermit mixture is placed in a crucible and ignited using a special ignition device. The reaction starts, producing molten steel.
• Pouring: Once the reaction is complete, the crucible is tilted, and the molten steel is poured into the mold, filling the gap between the track ends.
• Cooling: The molten steel is allowed to cool and solidify, forming a strong weld. The mold is then removed, and any excess material is ground away to ensure a smooth joint.

Advantages:

• Ability to produce high-quality welds that are strong and durable.
• Suitable for welding large sections of metal, such as railway tracks.
• Portable and can be performed in the field without the need for extensive equipment.

Disadvantages:

• The process generates extremely high temperatures, which require careful handling and safety precautions.
• The initial setup and preparation can be time-consuming.

Applications: Thermit welding is extensively used in the railway industry for joining and repairing railway tracks. Its ability to produce strong and durable welds makes it ideal for ensuring the structural integrity of rail networks.

Analysis of Other Options:

1) Electron Beam Welding:

Definition: Electron beam welding (EBW) is a fusion welding process where a beam of high-velocity electrons is applied to the materials to be joined. The kinetic energy of the electrons is converted into heat upon impact, melting the materials and forming a weld.

Advantages:

• High precision and control over the welding process.
• Ability to weld a wide range of materials, including refractory metals.

Disadvantages:

• Requires a vacuum environment, which can limit its application in the field.
• High equipment and operational costs.

Applications: Electron beam welding is used in industries requiring high precision and control, such as aerospace, automotive, and electronics manufacturing. It is not typically used for railway track joining due to the need for a vacuum environment and high costs.

2) Ultrasonic Welding:

Definition: Ultrasonic welding is a solid-state welding process that uses high-frequency ultrasonic vibrations to create a weld. The vibrations generate heat through friction, causing the materials to fuse without melting.

Advantages:

• Rapid welding process with low energy consumption.
• Suitable for welding thermoplastics and some metals.

Disadvantages:

• Limited to thin materials and certain types of metals and plastics.
• Not suitable for welding large sections or thick materials like railway tracks.

Applications: Ultrasonic welding is commonly used in the electronics, automotive, and medical device industries for joining small components and assemblies. It is not suitable for railway track joining due to its limitations in handling large and thick materials.

4) Laser Beam Welding:

Definition: Laser beam welding (LBW) is a welding technique that uses a focused laser beam to melt and join materials. The high energy density of the laser allows for deep penetration and precise control over the welding process.

Advantages:

• High precision and control, allowing for fine and intricate welds.
• Ability to weld a wide range of materials with minimal distortion.

Disadvantages:

• High equipment costs and the need for precise alignment.
• Not suitable for field applications where portability is required.

Applications: Laser beam welding is used in industries requiring high precision and control, such as aerospace, automotive, and electronics manufacturing. It is not typically used for railway track joining due to the high costs and need for precise alignment.

Explanation:

1) Thermit Welding:

Definition: Thermit welding is a process that uses the exothermic reaction of a thermite mixture (usually aluminum powder and a metal oxide) to produce molten metal that is used to join workpieces.

Working Principle: The thermite mixture is ignited producing molten metal at temperatures higher than the melting point of the workpieces. This molten metal is poured into a mold surrounding the joint fusing the workpieces together.

Analysis: In thermit welding the workpiece is not part of an electric circuit since the process relies on a chemical reaction to produce the heat needed for welding.

Atomic Hydrogen Welding (AHW):

Definition: Atomic hydrogen welding (AHW) is a welding process that uses an electric arc between two tungsten electrodes in an atmosphere of hydrogen. The hydrogen is used as a shielding gas and also plays a role in the welding process. The workpiece in AHW is not part of the electric circuit.

Working Principle: In atomic hydrogen welding an electric arc is established between two tungsten electrodes. Hydrogen gas is passed through the arc dissociating into atomic hydrogen. When the hydrogen atoms recombine into molecular hydrogen on the workpiece a significant amount of heat is released. This heat is used to melt the base metal and filler metal creating a weld joint. The workpiece itself does not conduct electricity as it is not part of the electric circuit; the arc is solely between the tungsten electrodes.

Advantages:

• The use of hydrogen as a shielding gas prevents oxidation and contamination of the weld area resulting in highquality welds.
• The process can achieve very high temperatures making it suitable for welding materials with high melting points.

Disadvantages:

• Requires specialized equipment such as tungsten electrodes and hydrogen gas supply.
• More complex and expensive compared to some other welding processes.

Applications: Atomic hydrogen welding is used in applications that require highquality welds and where the prevention of contamination is crucial. It is often used in the aerospace industry for welding highstrength steels and other materials that are difficult to weld with conventional methods.

Analysis of Other Options:

2) TIG Welding:

Definition: Tungsten Inert Gas (TIG) welding also known as Gas Tungsten Arc Welding (GTAW) uses a nonconsumable tungsten electrode to produce the weld.

Working Principle: An electric arc is established between the tungsten electrode and the workpiece. The workpiece is part of the electric circuit and a shielding gas (usually argon) is used to protect the weld area from contamination.

Analysis: In TIG welding the workpiece is part of the electric circuit as the arc is established between the tungsten electrode and the workpiece.

3) MIG Welding:

Definition: Metal Inert Gas (MIG) welding also known as Gas Metal Arc Welding (GMAW) uses a consumable wire electrode and a shielding gas to produce the weld.

Working Principle: An electric arc is established between the consumable wire electrode and the workpiece. The workpiece is part of the electric circuit and the wire electrode melts to form the weld joint while being shielded by an inert gas.

Analysis: In MIG welding the workpiece is part of the electric circuit as the arc is established between the wire electrode and the workpiece.

In Conclusion: The correct answer is option 4) Atomic hydrogen welding where the workpiece is not part of the electric circuit. This distinction is crucial for understanding the various welding processes and their applications as it influences factors such as the quality of the weld the equipment required and the types of materials that can be welded.

Explanation:

Non-Consumable Electrodes in Welding

Definition: Non-consumable electrodes are those that do not melt or get consumed during the welding process. Instead, these electrodes serve as a conductive medium for the arc and may also help in transferring filler material (if any) to the weld pool. The primary function of non-consumable electrodes is to establish an arc and sustain it, without adding material to the weld.

Processes Using Non-Consumable Electrodes:

Among the given options, the processes that use non-consumable electrodes are:

(i) Atomic Hydrogen Welding (AHW): In this process, two tungsten electrodes are used, and an arc is struck between them in an atmosphere of hydrogen gas. The hydrogen gas disassociates into atomic hydrogen due to the high temperature of the arc. When the atomic hydrogen recombines into molecular hydrogen on the surface of the workpiece, it releases a large amount of heat, which is used to weld the materials. Tungsten electrodes are non-consumable as they do not melt or get consumed during the welding process.

(iii) Plasma Arc Welding (PAW): This welding process uses a non-consumable tungsten electrode to create a plasma arc. The arc is formed between the tungsten electrode and the workpiece or between the tungsten electrode and a constricting nozzle. The plasma arc is highly focused and has high energy density, making it suitable for precision welding. The tungsten electrode remains intact and does not get consumed during the welding process.

Therefore, the correct option is 3, which includes (i) Atomic Hydrogen Welding and (iii) Plasma Arc Welding.

Other Processes and Electrodes:

To analyze why the other processes mentioned in the options are not correct:

(ii) MIG Welding (Metal Inert Gas Welding): Also known as Gas Metal Arc Welding (GMAW), this process uses a consumable wire electrode that is continuously fed through the welding gun. The wire electrode melts and becomes part of the weld pool, making it a consumable electrode process.

(iv) SAW (Submerged Arc Welding): This welding process uses a consumable wire electrode that is fed into the weld zone under a blanket of granular flux. The wire electrode melts and contributes to the weld pool, classifying it as a consumable electrode process.

Conclusion:

In summary, the processes that use non-consumable electrodes among the given options are Atomic Hydrogen Welding and Plasma Arc Welding, making option 3 the correct answer. This differentiation is crucial in welding technology as the choice of consumable versus non-consumable electrodes impacts the welding process's efficiency, application, and outcome.

Explanation:

Submerged Arc Welding (SAW)

• Submerged Arc Welding (SAW) is an arc welding process that produces coalescence of metals by heating them with an arc between a bare metal electrode and the workpiece.
• The arc and molten metal are shielded by a blanket of granular fusible flux which is fed through a tube directly over the weld zone. This flux not only shields the arc but also stabilizes it and prevents spatter and sparks as the arc is completely submerged under the flux.
• In SAW a continuouslyfed consumable electrode is used which creates an arc between the electrode and the workpiece. The arc is submerged beneath a layer of flux which melts to form a protective slag and gas shield around the weld pool.
• This prevents contamination by atmospheric gases ensuring a clean and highquality weld. The flux also serves several additional functions such as deoxidizing the weld area and aiding in the formation of the weld bead.

Advantages:

• High deposition rates making it suitable for thick materials and long welds.
• Minimal welding fumes and spatter due to the submerged arc.
• Excellent weld quality with deep penetration and uniformity.
• High welding speeds can be achieved improving productivity.

Disadvantages:

• Limited to horizontal or flat welding positions due to the flow characteristics of the flux.
• Not suitable for thin materials as the high heat input can cause warping.
• The process setup and equipment can be more complex and costly compared to some other welding methods.

Applications: SAW is widely used in industries requiring high productivity and highquality welds such as shipbuilding pressure vessel fabrication structural steel construction and large diameter pipes manufacturing.

Explanation:

FluxCored Arc Welding (FCAW) combines the advantages of SMAW and MIG

Definition: FluxCored Arc Welding (FCAW) is a semiautomatic or automatic arc welding process. FCAW requires a continuouslyfed consumable tubular electrode containing a flux which provides the necessary shielding from the atmosphere. This process combines the advantages of Shielded Metal Arc Welding (SMAW) and Metal Inert Gas (MIG) welding.

Working Principle: In FCAW the welding equipment feeds a continuouslyfed tubular wire containing flux to the weld area. The flux inside the core of the wire provides the necessary shielding gas when the arc is created. This shielding gas protects the weld from contamination. The process can be used with or without an external shielding gas depending on the specific requirements and the type of flux in the wire.

Advantages:

• High deposition rates compared to SMAW making it faster for larger welds.
• Suitable for welding thick materials due to deep penetration capabilities.
• Less cleanup required postwelding as the flux helps in reducing spatter.
• Can be used outdoors due to the flux providing adequate shielding in windy conditions unlike MIG welding which relies on external shielding gas that can be blown away.

Disadvantages:

• More complex equipment compared to SMAW leading to higher initial costs.
• Requires a power source that can provide consistent feeding of the wire which may not be ideal for all environments.
• Potential for slag inclusion if not properly handled and cleaned.
• Learning curve for operators transitioning from SMAW or MIG to FCAW due to different techniques and equipment.

Applications: FCAW is widely used in construction shipbuilding and heavy equipment manufacturing due to its capability to weld thick materials and its adaptability to outdoor environments.

Explanation:

Nomenclature of butt and fillet weld:

Throat thickness: The distance between the junction of metals and the midpoint on the line joining the two toes.

Leg length: The distance between the junction of the metals and the point where the weld metal touches the base metal ‘toe’.

The length of the leg is the distance from the root of the weld to the toe of the weld.

The theoretical throat is the perpendicular distance between the root of the weld and the hypotenuse joining the two ends of the length. It is the shortest distance from the root to the face.

Root: The parts to be joined that are nearest together.

Root gap: It is the distance between the parts to be joined.

Root face: The surface formed by squaring off the root edge of the fusion face to avoid a sharp edge at the root.

Reinforcement: Metal deposited on the surface of the parent metal or the excess metal over the line joining the two toes.

The toe of weld: The point where the weld face joins the parent metal.

Weld face: The surface of a weld seen from the side from which the weld was made.

Root penetration: It is the projection of the root run at the bottom of the joint.

Explanation:

Following table represents the weld symbols:

Explanation:

Grey cast iron is welded by gas welding.

In welding grey cast iron Neutral flame is used. Sometimes slightly oxidized flame can also be used for grey cast iron welding.

The grey iron castings are widely used for machine tool bodies, automotive cylinder blocks, heads, housings, fly‐wheels, pipes, and pipe fittings, and agricultural implements.

The grey cast iron is designated by the alphabet ‘FG’ followed by a figure indicating the minimum tensile strength in MPa or N/mm². For example, ‘FG 150’ means grey cast iron with 150 MPa or N/mm² as minimum tensile strength.

Explanation:

Submerged arc welding:

• Submerged arc welding is an arc welding process in which heat is generated by an arc which is produced between bare consumable electrode wire and the work-piece.
• The arc and the weld zone are completely covered under a blanket of granular, fusible flux which melts and provides protection to the weld pool from the atmospheric gases.
• The molten flux surrounds the arc thus protecting arc from the atmospheric gases.
• The molten flux flows down continuously and fresh flux melts around the arc.
• The molten flux reacts with the molten metal forming slag and improves its properties and later floats on the molten/solidifying metal to protect it from atmospheric gas contamination and retards cooling rate.
• A process of submerged arc welding is illustrated in Figure.

Heat Affected Zone (HAZ):  

• The area of the base material of metal which is affected by the heat of the welding process. Melting of the base material does not occur here only microstructure is changed.
• Heat affected zone may range from small to large depending on the rate of heat input. A process with low rates of heat input will result in a large HAZ.
• The size of HAZ also increases as the speed of the welding process decreases.

\({\rm{Size\;of\;HAZ}}\; \propto \frac{1}{{speed\;of\;welding}}\)

So, order of welding processes in increasing speed is

Arc welding → Submerged Arc welding → MIG welding → Laser Beam welding

Therefore, the order of size of the heat affected zone in increasing sequence is

Laser Beam welding → MIG welding → Submerged Arc welding → Arc welding 

Important Points

Butt welding:  Joining of metal by its whole cross section side by side.

Explanation:

• The selection of an optimum value of OCV (Open circuit voltage) depends on the type of base metal, the composition of electrode coating, type of welding current and polarity, type of welding process etc.
• It generally varies from 50 V - 100 V.

Concept:

Power in welding is given as

P = V × I

where V = voltage (V), I = current (A)

Calculation:

Given:

V = 25 V, I = 250 A

Power required is:

P = V × I

P = 25 × 250 = 6250 W = 6.25 kW

Explanation:

TIG Welding: 

• Tungsten Inert Gas (TIG) or Gas Tungsten Arc (GTA) welding is the arc welding process in which an arc is generated between a non-consumable tungsten electrode and workpiece.
• The tungsten electrode and the weld pool are shielded by an inert gas normally argon and helium.
• The principle of tungsten inert gas welding process is shown below

Explanation:

A fault is an imperfection in the weld which may result in failure of the welded joint while in service.

The following faults occur commonly in gas welding.

1. Undercut: A groove or channel formed in the parent metal at the toe of the weld is called undercut.

Cause: 

• When the current setting is too high
• When welding speed is too fast
• By overheating of the job due to continuous heating
• Due to faulty electrode motion
• When electrode angle is wrong

2. Incomplete Penetration: Failure of the weld metal to reach the root of the joint is known as incomplete penetration.

Cause:

• Too narrow edge penetration
• Excessive welding speed
• When the current setting is low
• When a larger diameter electrode is used
• Due to inadequate cleaning or gouging before depositing sealing run

3. Porosity or blow-hole:  A group of pin-holes in a weld (porosity) or a larger hole in the weld (blow-hole) are caused by the gas being entrapped.

Cause:

• Presence of contaminants on the job or electrode surface
• Presence of high sulphur in the job or electrode material
• Moisture trapped between joining surfaces
• Freezing of weld at a faster rate

4. Spatters: An unintentional deposit of weld metal, in the shape of small globules on the job surface along the weld is known as spatters.

Cause:

• A too high current setting
• Use of moisture affected electrode
• Wrong polarity
• Use of a long arc
• Arc-blows

5. Overlap: Metal flowing onto the surface of the base metal without fusing it.

Cause:

• Improper welding technique
• High welding current
• By using large electrodes

6. Lack of fusion: If there is no melting of the edges of the base metal at the root face or on the side face or between the weld runs, then it is called lack of fusion.

Cause:

• It occurs because of the low heat input
• Incorrect electrode and torch angle
• Low welding current
• High welding speed

Explanation:-

• The electromagnetic force is a type of physical interaction that occurs between electrically charged particles.
• It acts between charged particles and is the combination of all magnetic and electrical forces.
• The electromagnetic force can be attractive or repulsive.
• A pinch welding gives narrow and long flame which is concentrated on the desired part, it is achieving by an induction coil, which results in electromagnetic forces.

Important Points

Arc blow is the undesirable effect of arc, during arc welding.