Welding Defects MCQ Quiz in मल्याळम - Objective Question with Answer for Welding Defects - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Explanation:-

Stress corrosion cracking (SCC)- 

• Stress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses.
• SCC is an insidious form of corrosion; it produces a marked loss of mechanical strength with little metal loss; the damage is not obvious to casual inspection, and the stress corrosion cracks can trigger fast mechanical fracture and catastrophic failure of components and structures. 
• Fortunately, the occurrence of SCC depends on the simultaneous achievement of three requirements:-
  • A susceptible material,
  • An environment that causes SCC for that material.
  • Sufficient tensile stress to induce SCC.
• Several significant disasters have involved stress corrosion cracking, including the rupture of high-pressure gas transmission pipes, the explosion of boilers, and the destruction of power stations and oil refineries.

Explanation:

Non-destructive testing (NDT) is a testing and analysis technique used by industry to evaluate the properties of a material, component, structure, or system for characteristic differences or welding defects and discontinuities without causing damage to the original part.

Liquid penetrant testing: This is a type of non-destructive testing that involves the application of a small amount of fluorescent or visible dye solution to the surface of a nonporous workpiece. This liquid will seep into any surface-breaking defects.

Hardness testing: This is not a non-destructive method. Hardness testing involves pressing a hardened "indenter" into the surface of the material being tested with a specific force, causing an indentation. The hardness is determined by measuring the size of the indentation or the depth of the penetration. This process can cause minor damage to the material being tested and thus is considered a destructive testing method.

Leak testing: This is a non-destructive testing method where the presence of leaks, the leak rate, or total leaks in a system is determined. This could be done by pressurizing the system and observing if there's a change in pressure over time, or by using a gas detector, for example.

Visual testing: This is the most common non-destructive testing method. Visual testing includes direct visual testing (unaided eye), remote visual testing (using tools like mirrors, borescopes, charge-coupled devices (CCDs), and computer-assisted viewing), and translucent visual testing (using light table inspections).

Explanation:

• Slug inclusions are compounds such as oxides, fluxes, and electrode-coating materials that are trapped in the weld zone.
• If shielding gases are not effective during welding, contamination from the environment also may contribute to such inclusions.
• With control of welding process parameters, the molten slag will float to the surface of the molten weld metal and thus will not become entrapped.
• It is caused by:
  • Slag from previous runs not being cleaned away
  • Insufficient cleaning and preparation of the base metal before welding commences
• Slag inclusion can be prevented by cleaning the weld-bead surface by means of a wire brush (hand or power) or a chipper before the next layer is deposited.

Explanation:

Slag inclusion: 

• Slag Inclusions are a common welding defect that occurs when slag, a welding by-product, gets stuck inside the weldment. This is an un-favorable condition that creates performance issues down the line. In most cases, it is a result of bad welding techniques or improper selection of components and parameters.

• These slag inclusions frequently appear at the edges of a weld if the correct welding procedure is not used.

• The molten slag, being lighter than the weld metal, rises to the surface of the liquid pool, rapidly forming a blanket that covers the metal. Furthermore, the slag solidifies at a lower temperature than the metal. Thus, when an edge is not completely filled with metal and has a drooping contour, the slag blanket slips around this contour and settles in the corner. When the metal solidifies, more slag will settle in the edge to form a tightly adhering inclusion.

Main Causes Of Slag Inclusions: 

• Type Of Flux Used
• Welding Parameters
• Poor Welding Technique
• Bad Practices

Explanation:

A fault or defect 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. 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

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

3. 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

4. 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

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:

Lamellar tearing:

It is a cracking phenomenon that occurs beneath welds and is principally found in rolled steel plate fabrications. The tearing always lies within the parent plate, often outside the transformed (visible) heat-affected zone (HAZ), and is generally parallel to the weld fusion boundary.

It is caused due to the following reasons:

• Transverse strain - The shrinkage strains on welding must act in the short direction of the plate, i.e. through the plate thickness
• Weld orientation - The fusion boundary will be roughly parallel to the plane of the inclusions
• Material susceptibility - The plate must have poor ductility in the through-thickness direction.

Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction.

Additional Information

Shrinkage Void: 

It occurs in the arc welding of thick plates and is found near the surface. The cavity is formed due to the shrinkage of weld metal during solidification.

Incomplete Fusion: 

This defect is mainly due to low welding current and higher welding speed and hence control of these two parameters reduces the scope of the defect.

Rat Tail: 

This defect looks like a rat tail and is produced mainly on the flat surfaces of a casting due to the thermal expansion of the moulding sand.

• Rat tail is caused by the compression failure of the skin of the mould cavity because of the excessive heat in the molten metal, this high heat causes sand to expand and the mould wall moves backwards and when the walls give away the casting surface they have the marked lines.
• If the number of such failure increases the casting surface develops criss-crossing small lines.

• The main cause of this failure is the moulding sand has poor expansion properties and hot strength. The heat in the pouring metal is too high. It may also, result due to design problems such as providing a large flat surface on the mould.
• The facing sand is not able to provide the cushioning effect due to lack of carbonaceous material.
• Proper choice of facing sand and ingredients and the pouring temperatures are the measures to reduce the incidence of these defects.

Explanation:

Welding defects: Some of the welding defects are shown as follow:​

Important Points

Earring: ​Earring defect is related to sheet metal operations such as deep drawing. This generally occurs when the material is anisotropic, i.e. has varying properties in different directions. 

Explanation:

TIG Welding: 

• Tungsten Inert Gas (TIG) or Gas Tungsten Arc (GTA) welding is the arc welding process in which arc is generated between non-consumable tungsten electrode and workpiece.
• GTAW  a shielding gas is used to protect the weld area from atmospheric contamination. Since no flux is used in GTAW, no slag is generated.
• The tungsten electrode and the weld pool are shielded by an inert gas normally argon and helium.
• TIG welding is used for Aluminium, Magnesium and Titanium alloy.

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

Additional Information

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.

Gas metal arc welding (GMAW) or Metal inert gas arc welding (MIG)

• In this process, the arc is formed between a continuous, automatically fed, metallic consumable electrode and welding job in an atmosphere of inert gas, and hence this is called metal inert gas arc welding (MIG) process.
• In this process small droplets of molten metal from the electrode are transferred to the weld area, a ratio of several hundred droplets per second, the transfer is spattered free and very stable, high dc current, high voltage, and large diameter electrode are used along with the shielding gas.
• The shielding gases for MIG welding are mixtures of argon, oxygen, and CO2, and special gas mixtures may contain helium. 

Explanation:

Types of welding defects:

Weld porosity is due to atmospheric gases are trapped inside the liquid metal during the solidification of the metal.

Slag inclusion caused by trapping of the compound by oxides, fluxes and electrode coating materials in the weld zone.

Weld crack caused due to non-uniform cooling and internal stresses generated in weld Beed. If stress is more than the strength of the material cracks will be formed.

Weld spatter caused due to high welding current and low welding speed and arc blow.

Incomplete fusion caused due to insufficient heat and too fast/quick travel of torch or electrode.

Explanation:

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
• So, option 2 is correct.

Additional Information

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