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

Explanation:

Lack of Penetration in Welding Defects

• Lack of penetration is a welding defect that occurs when the filler metal fails to completely penetrate into the root of the joint during the welding process. This defect is commonly encountered in welding operations and can significantly compromise the strength and integrity of the welded joint. It is critical to understand the causes, consequences, and preventive measures associated with this defect to ensure high-quality welds.
• Lack of penetration refers to the incomplete fusion or inadequate penetration of the filler metal into the root area of the joint. The root of the joint is the narrowest and deepest part of the weld, where the two pieces of base metal meet. Proper penetration ensures the welded joint is strong and can withstand mechanical stresses.

Causes of Lack of Penetration:

• Improper Welding Parameters: Using incorrect welding current, voltage, or travel speed can lead to insufficient heat generation, causing the filler metal to fail to penetrate into the root of the joint.
• Incorrect Joint Design: A poorly designed joint, such as an excessively narrow or deep root, can make it challenging for the filler metal to reach the root area.
• Insufficient Heat Input: Low heat input during welding can result in inadequate melting of the base metal, preventing the filler metal from penetrating properly.
• Improper Electrode Angle: Incorrect positioning or angling of the welding electrode can lead to incomplete penetration in the weld joint.
• Contaminated or Dirty Base Metal: Presence of impurities, grease, oil, or rust on the base metal can hinder the penetration of the filler metal into the root area.
• Inadequate Root Opening: A root opening that is too narrow prevents the filler metal from flowing into the joint effectively.

Consequences of Lack of Penetration:

• Reduced Joint Strength: The absence of adequate penetration results in weaker welds that are more prone to failure under mechanical stress.
• Structural Instability: Welded components with lack of penetration defects may compromise the structural integrity of the assembly, especially in critical applications.
• Crack Formation: Stress concentration at the root area can lead to the formation of cracks, further reducing the durability of the welded joint.
• Potential for Failure: Components with lack of penetration defects are at a higher risk of sudden failure, especially under dynamic or cyclic loading conditions.

Prevention of Lack of Penetration:

• Optimize Welding Parameters: Ensure proper selection of welding current, voltage, and travel speed to achieve sufficient heat input for complete penetration.
• Improve Joint Design: Design the joint with an appropriate root opening and bevel angles to facilitate effective penetration.
• Use Correct Electrode Angle: Position the welding electrode at the correct angle and maintain a consistent travel speed during welding.
• Clean the Base Metal: Thoroughly clean the base metal to remove any contaminants, rust, grease, or oil before welding.
• Perform Root Pass Welding: For thick joints, perform a root pass to ensure the filler metal penetrates deeply into the root area.
• Conduct Visual and Non-Destructive Testing: Inspect the welded joint using visual inspection or non-destructive testing techniques to identify lack of penetration defects.

Explanation:

Metallic gun type of metal spraying:

• The metallic gun type of metal spraying process is a thermal spraying method where molten metal is applied to a surface to create a protective or decorative coating. The primary function of the process is to melt the metal feedstock and project it onto the target surface. Among the given options, the oxy-acetylene flame is generally used for melting metal in the metallic gun type of metal spraying process. This is due to its ability to achieve high temperatures, which are sufficient to melt most metals used in spraying applications.

Why Oxy-acetylene Flame is Used:

• The oxy-acetylene flame is created by combining oxygen and acetylene gas in a specific ratio. When ignited, this mixture produces a high-temperature flame that can reach temperatures of up to 3,500°C (6,332°F). This temperature is adequate for melting metals like aluminum, zinc, and other alloys commonly used in thermal spraying.

Process Description:

In the metallic gun type of metal spraying process:

• The metal feedstock (typically in the form of wire or powder) is fed into the spray gun.
• An oxy-acetylene flame is used to melt the metal at the tip of the gun.
• Compressed air or another gas is then used to propel the molten metal particles onto the surface being coated.
• Upon impact, these particles solidify, forming a dense and adherent coating.

The oxy-acetylene flame provides a controlled and efficient heat source, ensuring that the metal is melted uniformly and sprayed effectively. This makes it the preferred choice for the metallic gun type of metal spraying process.

Applications:

The metallic gun type of metal spraying process using an oxy-acetylene flame is widely used in various industries, including:

• Corrosion protection: Applying coatings of zinc or aluminum to steel structures to prevent rust.
• Restoration: Repairing worn-out surfaces by adding layers of metal to restore dimensions.
• Decorative purposes: Adding metallic coatings for aesthetic appeal.
• Thermal insulation: Coating surfaces to improve thermal resistance.

Explanation:

Weldability of Mild Steel and Alloy Steel

• Weldability refers to the ease with which a material can be welded to form a strong, defect-free joint. The weldability of a material depends on several factors, including its chemical composition, physical properties, and the welding process being used. Mild steel and alloy steel are two commonly used materials in engineering and manufacturing, and their weldability differs significantly.

Mild steel, also known as low carbon steel, has a relatively low carbon content (typically below 0.25%). This low carbon content makes it highly weldable. Mild steel has the following characteristics that make it easier to weld compared to alloy steel:

• Low Carbon Content: The low carbon content in mild steel reduces the risk of forming brittle microstructures such as martensite during welding. This ensures that the welded joint remains ductile and strong.
• Lower Risk of Cracking: Mild steel has a lower risk of cracking during or after welding, as it is less susceptible to hydrogen embrittlement and thermal stresses.
• Wide Compatibility: Mild steel is compatible with a wide range of welding processes, including shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and gas tungsten arc welding (GTAW).
• Ease of Preparation: Mild steel requires minimal preheating and post-weld heat treatment, which simplifies the welding process and reduces the overall cost.

On the other hand, alloy steel contains additional alloying elements such as chromium, nickel, molybdenum, and vanadium, which are added to improve specific properties like strength, hardness, and corrosion resistance. However, these alloying elements also introduce challenges in welding:

• Higher Carbon Equivalent: Alloy steel typically has a higher carbon equivalent, which increases the risk of cracking and requires careful control of heat input and cooling rates during welding.
• Preheating and Post-Weld Heat Treatment: Many alloy steels require preheating before welding and post-weld heat treatment to relieve residual stresses and prevent cracking, which adds complexity and cost to the welding process.
• Formation of Brittle Microstructures: The presence of alloying elements can lead to the formation of brittle microstructures, such as martensite, if the welding process is not properly controlled.
• Specialized Filler Materials: Welding alloy steel often requires the use of specialized filler materials that match the composition of the base metal, further increasing the complexity of the process.

Explanation:

Lack of Fusion in Welding Defects

• Lack of fusion is a critical welding defect that occurs when the filler metal fails to properly fuse with the parent (base) metal or between layers of weld metal in a multi-pass weld. This defect compromises the structural integrity of the weld and can lead to failures under stress or load. Proper fusion is essential to ensure that the weld achieves the desired strength, durability, and performance characteristics.
• In welding, the process involves melting the base metal and filler material to create a strong bond. Lack of fusion typically arises when there is inadequate heat input, improper welding technique, contamination, or poor joint preparation. The defect is characterized by visible or subsurface separation at the weld interface, indicating that the materials did not bond effectively.

Causes of Lack of Fusion:

• Low Heat Input: Insufficient heat during the welding process prevents the base metal and filler material from reaching the appropriate temperature required for fusion.
• Improper Welding Technique: Incorrect manipulation of the welding torch, electrode, or filler material can result in poor bonding between the materials.
• Contamination: The presence of dirt, grease, rust, or other impurities on the welding surfaces can inhibit proper bonding.
• Incorrect Welding Parameters: Using incorrect current, voltage, or travel speed can lead to insufficient penetration and fusion.
• Improper Joint Preparation: Inadequate cleaning, poor fit-up, or incorrect joint design can contribute to lack of fusion.

Impact of Lack of Fusion:

• Reduction in the overall strength of the weld joint.
• Increased susceptibility to crack propagation and failure under load or stress.
• Compromised fatigue resistance, especially in dynamic or cyclic loading conditions.
• Potential safety hazards in critical applications, such as bridges, pipelines, and pressure vessels.

Detection of Lack of Fusion:

• Visual Inspection: Surface lack of fusion may be visible as a distinct line or separation at the weld interface.
• Non-Destructive Testing (NDT): Techniques such as ultrasonic testing, radiographic testing, or dye penetrant testing can identify subsurface or internal lack of fusion.

Prevention of Lack of Fusion:

• Use appropriate welding parameters, including correct current, voltage, and travel speed.
• Ensure proper cleaning and preparation of the base metal and filler material before welding.
• Adopt correct welding techniques and ensure proper torch or electrode manipulation.
• Increase heat input where necessary to achieve better penetration and fusion.
• Conduct regular training and skill development for welders to minimize defects due to human error.

Concept:

Arc welding

• Arc welding is the most commonly used welding type.
• It uses the heat-produced electric arc to weld metals together
• The arc occurs from the base material to the electrode, the welding rod or wire, and melts the metal. Then the welder can fuse the molten metal and craft it into a weld.
• In Arc Welding process, welding cables are used for conduction of current from the welding machine to the electrode holder.

The different types of arc welding is 

1. Shielded metal arc welding 

2. Gas metal arc welding

3. Flux-cored arc welding

4. Gas Tungsten Arc Welding (TIG welding)

5. Carbon arc welding

6. Submerged arc welding

7. Electroslag welding

       8. Drawn Arc (DA) stud welding

Gas Tungsten Arc Welding (TIG welding)

This method uses a non-consumable tungsten electrode and constant current power source to create a plasma arc between metals and can be conducted with or without filler material. Inert shielding gas protects the weld area and electrode from the atmosphere.

TIG welding can be difficult to learn and technically demanding. It requires more operator control than similar processes, but there are both manual and automatic methods available.

The process produces high-quality, clean, and strong welds but can be time-consuming. It’s primarily suitable for welding thin materials and non-ferrous metals but isn’t ideal for thicker metal joints.

Additional Information

Arc welding uses an electric arc to generate heat and join together two metals. The power supplied to the electric arc can be alternating current (AC) or direct current (DC). AC arc welders are often inexpensive while DC arc welders offer a smoother arc that works better on thin materials, however they are more expensive.

All arc welding processes use an electric arc to weld and have at least the following:

An electrode
An electrode cable
A work cable and clamp
Power supply
Metals to join

 The welding arc during the process of any type of arc welding will be around 3500°C. 

During the arc welding process, the welder works with two types of metal.

Parent Material: This is the metal parts that are joined together during the welding process. 

Consumables Material: This is the additional materials that are heated up in the arc and deposited over the joints to create a stronger bond. 

In a basic arc welding process, the power supply is switched on, and the electrode is brought near the base material. Then, intense heat is generated to produce the electric arc. The heat then melts the base metal, electrode core and flux coating. The flux coating then provides a shielding environment to weld. The molten metal is deposited between the two metal work pieces to join them together. Once this solidifies, it forms a strong bond between the two materials. Then, the metal work pieces are left to cool down. 

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.

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.