Classification of Welding MCQ Quiz - Objective Question with Answer for Classification of 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:

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:

MIG Welding

Definition: Metal Inert Gas (MIG) welding, also known as Gas Metal Arc Welding (GMAW), is a welding process in which an electric arc forms between a consumable wire electrode and the workpiece metals, causing them to melt and join. The process uses a shielding gas to protect the weld pool from contamination.

Components and Setup:

• Consumable Electrode: MIG welding uses a consumable wire electrode that is continuously fed through the welding gun. This wire electrode melts during the welding process to form the weld joint.
• DC Supply: MIG welding typically uses a Direct Current (DC) supply. The polarity used is usually DC Electrode Positive (DCEP), which provides a stable arc, good weld penetration, and better control of the weld bead.
• Shielding Gas: An inert or semi-inert gas, such as Argon, Carbon Dioxide, or a mixture, is used to shield the weld area from atmospheric contamination, such as oxygen and water vapor.

Working Principle: In MIG welding, the welding gun is connected to a DC power source. The consumable wire electrode is fed through the welding gun to the weld area. When the trigger is pulled, an electric arc is created between the wire electrode and the workpiece. The heat generated by the arc melts the wire electrode and the base metal, forming a weld pool. The shielding gas flows through the gun and protects the weld pool from contaminants. As the electrode melts, it is continuously fed to maintain a consistent arc and weld bead.

Advantages:

• High welding speed due to the continuous feed of the consumable electrode.
• Produces clean and high-quality welds with minimal spatter and slag.
• Suitable for welding a wide range of materials, including steel, aluminum, and stainless steel.
• Easy to learn and use, making it accessible for both novice and experienced welders.

Disadvantages:

• Requires a shielding gas supply, which adds to the cost and complexity of the equipment.
• Less effective in outdoor or windy conditions where the shielding gas can be dispersed.
• Not suitable for welding thick materials as efficiently as some other welding processes.

Applications: MIG welding is widely used in various industries, including automotive, construction, and manufacturing. It is commonly employed for welding thin to medium thickness materials, fabrication of metal structures, and repair and maintenance work.

Explanation:

Welding of Thick Metal Pieces

Suitable Processes: To weld thick metal pieces, certain welding processes are preferred due to their ability to generate deep penetration, strong welds, and manage significant amounts of heat effectively. Among these processes, Submerged Arc Welding (SAW), Thermit Welding, and Electroslag Welding are highly suitable.

Submerged Arc Welding (SAW):

Definition: SAW is a common arc welding process that utilizes a continuously fed consumable solid or tubular (flux-cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux.

Working Principle: In SAW, an electric arc is formed between a continuously fed electrode and the workpiece, creating a pool of molten metal. The arc and molten weld pool are submerged under a layer of flux, which prevents spatter and sparks, minimizes the exposure of the molten metal to atmospheric contamination, and stabilizes the arc.

Advantages:

• High deposition rates and deep penetration, making it ideal for thick sections.
• Good quality welds with minimal defects.
• High welding speeds and productivity.

Disadvantages:

• Limited to horizontal or flat positions due to the use of gravity in flux application.
• Requires significant equipment and setup, making it less flexible for on-site welding.

Thermit Welding:

Definition: Thermit welding is a process that employs an exothermic chemical reaction between a metal oxide and aluminum powder to produce the necessary heat for welding. This reaction generates molten metal and slag, which can be used to join metal parts.

Working Principle: The thermit reaction involves the reduction of a metal oxide by aluminum powder, producing a significant amount of heat. The molten metal produced by the reaction is used to fill the joint between the metal pieces, creating a strong weld upon solidification.

Advantages:

• Capable of welding very thick sections, such as rail tracks and heavy machinery parts.
• Produces high-quality welds with excellent mechanical properties.
• Does not require an external power source, making it suitable for remote or field operations.

Disadvantages:

• Limited to specific applications due to the complexity of the process and the need for precise control of the thermit reaction.
• Requires preheating and proper joint preparation to ensure successful welds.

Electroslag Welding (ESW):

Definition: ESW is a welding process that uses an electric current to melt the filler metal and the edges of the workpieces to be joined. The molten metal is contained by a flux, which forms a slag that covers the weld pool and helps to shape the weld bead.

Working Principle: In ESW, an electric arc is struck between a consumable electrode and the workpieces. The heat generated by the arc melts the electrode and the base metal, creating a molten weld pool. The molten metal is contained by a layer of flux, which solidifies to form slag that protects the weld pool and assists in shaping the weld.

Advantages:

• Highly efficient for welding thick sections, such as in heavy structural components and pressure vessels.
• Produces strong, high-quality welds with excellent mechanical properties.
• Minimizes distortion and residual stresses due to the controlled heat input.

Disadvantages:

• Limited to vertical or near-vertical positions due to the need for gravity to assist in flux containment.
• Requires significant setup and equipment, making it less suitable for field operations.

Analysis of Other Options:

The correct option for welding thick metal pieces includes all three processes mentioned: Submerged Arc Welding (SAW), Thermit Welding, and Electroslag Welding. Each of these processes has distinct advantages that make them suitable for welding thick sections. Here's why the other options are not as comprehensive:

Option 1: (i) and (ii)

This option includes Submerged Arc Welding (SAW) and Thermit Welding. While both these processes are indeed suitable for welding thick metal pieces, excluding Electroslag Welding (ESW) overlooks a highly efficient method that is specifically designed for vertical welding of thick sections. ESW's capability to handle thick sections in a controlled manner is a significant advantage, especially for applications involving heavy structural components.

Option 2: (i) and (iii)

This option includes Submerged Arc Welding (SAW) and Electroslag Welding (ESW). While these processes are effective for welding thick metal pieces, omitting Thermit Welding means missing out on a process that is particularly useful for field operations where external power sources are not available. Thermit Welding's ability to weld very thick sections, such as rail tracks, makes it indispensable for certain applications.

Option 3: (ii) and (iii)

This option includes Thermit Welding and Electroslag Welding (ESW). While these processes are suitable for welding thick metal pieces, excluding Submerged Arc Welding (SAW) ignores a highly productive and efficient method that is widely used in industrial applications. SAW's high deposition rates and deep penetration make it ideal for welding thick sections in a controlled environment.

Conclusion:

The correct option is option 4, which includes all three processes: Submerged Arc Welding (SAW), Thermit Welding, and Electroslag Welding. Each of these processes has unique advantages that make them suitable for welding thick metal pieces in various applications. By considering all three methods, one can ensure the selection of the most appropriate welding process for a given application, taking into account factors such as the environment, equipment availability, and specific requirements of the weld joint.

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.

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:

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

Concept:

Flash butt welding

• In flash welding (FW), also called flash butt welding, heat is generated very rapidly from the arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint.
• After the proper temperature is reached and the interface begins to soften, an axial force is applied at a controlled rate and a weld is formed by plastic deformation of the joint.
• The mechanism is called hot upsetting, and the term upset welding (UW) also is used for this process.
• Some molten metal is expelled from the joint as a shower of sparks during the process-hence the name flash welding. 
• It is used for joining of HSS drill bit to the carbon steel shank.

The advantages of flash butt welding are:

1) Less requirement of power

2) When the surfaces being joined, it requires only less attention.

3) Weld obtained is so clean and pure; due to the foreign metals appearing on the surfaces will burn due to flash or arc.

Soldering:

• Soldering is a non-fusion and non-pressure welding operation.
• The mechanism by which the joint formation taking place is wetting and surface alloying.
• The filler material used having a melting temperature less than 427°c
• Borax is used as Flux material.
• The filler material used is an alloy of lead and tin known as solder.
• Filler material entered into the workpiece by means of capillary action.

Brazing:

• It is also a non-fusion and non-pressure welding operation. 
• Filler material- Alloy of Cu and Zn, Cu and Ag, Cu and Al
• Flux material- Borax
• Filler material entered into the workpiece by means of capillary action.
• Filler material melting temperature greater than 427°c and less than the melting point of the base material

Braze welding:

• It is also a non-fusion and non-pressure welding operation.
• Filler material – an alloy of Cu and Tn (Bronze).
• The strength of the joint is more than brazing and soldering.
• Filler material entered into the workpiece by gravity force.

Explanation:

Welding technology

• The stability of a DC arc with a consumable electrode depends largely on how the molten metal is transferred in the arc.
• One can be distinguished essentially between two different types of arcs, depending on material transport.
  1. Spray arc
  2. Short arc

Short arc welding

• The heat input from Short arc welding is low, which makes the process suitable for welding in thinner materials.
• The drops from the electrode dip into the weld pool.
• This can be repeated up to 200 times per sec.
• If the short circuit current is too high, it has a considerable effect on the pinch-off forces, causing weld spatter.
• Some means of limiting the short-circuit current must therefore be provided in the power unit, e.g. through the use of an inductor coil.
• It is not easy, with Short arc welding, to achieve a completely stable arc.
• The objective is to achieve a consistent, high short-circuiting frequency, resulting in small droplets being transferred to the workpiece and spatter droplets being so fine that they do not adhere to the workpiece.

Explanation:

TIG Welding: 

Gas Tungsten Arc Welding (GTAW), also known as tungsten inert gas (TIG) welding is a process that produces an electric arc maintained between a non-consumable tungsten electrode and the part to be welded.

Inert Gas in TIG Welding

The heat-affected zone, the molten metal, and the tungsten electrode are all shielded from atmospheric contamination by a blanket of inert gas fed through the GTAW torch.

Inert gas is inactive or deficient inactive chemical properties. The shielding gas serves to blanket the weld and excludes the active properties in the surrounding air. Inert gases, such as Argon and Helium, do not chemically react or combine with other gases.

The principle of the tungsten inert gas welding process is shown below

Explanation:

Types of flames in gas welding:

• Neutral flame:
  • ​The neutral flame has a 1:1 ratio of oxygen and acetylene by volume. Structurally it consists of two parts namely the inner cone and the outer envelope.
  •  It has a clear, well-defined, or luminous inner cone indicating that the combustion is complete. Such a flame makes a hissing sound and is the most used type of flame for welding metals.
  • It normally does not affect the chemistry of the weld metal and usually produces a clean-looking weld having properties comparable to the base metal. It is most often used for welding low-carbon structural steel and aluminum.​
• Carburizing Flame:
  • ​The carburizing flame has a 0.85:0.95 ratio of oxygen and acetylene by volume. 
  • The inner zone has white color, the intermediate zone which is red in color and the outer cone has a blue color. The inner cone temperature is about 2900° Centigrade. This flame is used to weld medium carbon steel, nickel, etc.
• Oxidizing Flame:
  • The oxidizing flame has a 1.15:1.5 ratio of oxygen and acetylene by volume. 
  • The inner zone has a very bright white color and has a temperature of about 3300 degrees centigrade. The outer flame has blue in color. This flame is used to weld oxygen-free copper alloys like brass, bronze, etc.

Explanation:

Resistance spot welding occurs in four steps. They are as follows:

Squeeze time:

• The time required for the electrodes to align and clamp the work-piece together and provide necessary electrical contact.

Weld time:

• Time the current flows through the work-piece till they are heated to melting temperature.

Hold time:

• Time till the pressure is maintained, without the current, wherein the pieces are expected to get forge weld.

Off time:

• When the pressure of the electrode is taken off so that plates can be positioned for the next spot.

Resistance welding:

• This process makes use of the electrical resistance for generating heat that is required for melting the work-piece.
• Generally used to and join thin plate structures.
• Also considered as a green process since it does not generate gases and flames as in metal arc welding and gas welding.
• The heat generated in Resistance welding is given by H = I²Rt

H = Heat generated, I = Current, R = Resistance of joint, t = Time of flow of current.

• Resistance depends upon:
  • work-piece to be joined
  • electrode used
  • gap resistance

Types:

Spot welding

• Individual weld is produced by momentary application of pressure and resistance into the work-piece.

Seam welding

• It can produce continuous fast and leak-proof weld mainly used for thin metallic sheets, galvanized roofing, small tanks, etc

Projection welding

• A dimple is embossed into one of the work-piece at the locations where the weld is desired.

Explanation:

Submerged arc welding: In submerged arc welding the arc is completely submerged into the granular flux powder and forming a blanket.

Tungsten inert gas welding: In this type of welding non-consumable tungsten electrode will be used to generate the arc. A gas shield is provided around the welding.

Electro slag Welding: Welding is started by generating the electric arc and completed by resistance heating effect of slag material and if shielding gas is provided it is called as Electro Gas Welding.

Explanation:

Metal inert gas welding or gas metal arc welding:

• it uses continuous solid wire consumable electrode.
• the electrode is heated and fed into the weld pool from the welding gun.
• MIG welding is done in an inert gas atmosphere.
• The shielding gases for MIG welding are mixtures of argon, oxygen and carbon dioxide and a special gas mixture may contain helium.
• Excessive speed or irregularity in wire feed results in the welding spatter. Spatter occurs when the filler material enters the weld pool. 
• Direct current power supply is used to weld the workpiece.