Welding Terms MCQ Quiz in বাংলা - Objective Question with Answer for Welding Terms - বিনামূল্যে ডাউনলোড করুন [PDF]

Explanation:-

Gas welding is the process in which a gas flame is used to raise the temperature of the metals to be joined. The metals are heated up to melt. The metal flows and on cooling, it solidifies.

Filler metal may be added to the flowing molten metal to fill up the cavity made during the end preparation. Many combinations of gases are used in gas welding. But the most common of these is oxygen and acetylene.

The process of oxy-acetylene welding can be used for almost all metals and alloys for engineering purposes. A high-temperature flame (3200°C) can be produced by this method.

Oxy-acetylene welding is particularly used for sheet metal work.

All the metals can be welded with proper filler metals. 

A torch is a device used to mix acetylene and oxygen in the correct proportion and the mixture flows to the tip of the torch.

For different types of jobs, different tips are used.

The size of the tip is specified by the outlet hole diameter. More than one hole is also provided in tips. The tip is screwed or fitted on the front end of the torch.

Concept:

In arc welding, penetration is minimum for DC – Electrode Positive.

Types and importance of polarity in arc welding

Polarity indicates the direction of current flow in the welding circuit.

Kinds of polarity

• Straight polarity or electrode negative (DCEN)
• Reverse polarity or electrode positive (DCEP)

Straight polarity: In straight polarity the electrode is connected to the negative and the work to the positive terminal of the power source.

Reverse Polarity: In reverse polarity the electrode is connected to the positive and the work to the negative terminal of the power source.

DCSP (Electrode Negative)—Maximum penetration.

AC—Moderate penetration.

DCRP (Electrode Positive)—Minimum penetration

Explanation:

• Welding arcs give off radiation over a broad range of wavelengths - from 200 nm (nanometers) to 1,400 nm (or 0.2 to 1.4 µm, micrometers).
• This includes ultraviolet (UV) radiation (200 to 400 nm), visible light (400 to 700 nm), and infrared (IR) radiation (700 to 1,400 nm).
• This is the reason that eyes need to be protected against both UV and Infra-Red rays.

Explanation:

Electron Beam Welding

• Electron Beam Welding is a fusion welding in which coalescence is produced by heating the workpiece due to impingement of the concentrated electron beam of high kinetic energy on the workpiece.
• As the electron beam impinges the workpiece, the kinetic energy of the electron beam converts into thermal energy resulting in melting and even evaporation of the work material. 
• Electron beam welding joins metals by bombarding a specific confined area of the base metal with high-velocity electrons.
• It is performed in a vacuum without shielding gas to prevent the reduction of electron velocity. 
• Neither an electrode nor a filler rod is used.
• Electron beam welding is focused on the weld spot using the magnetic lens.

• It allows fusion welds of great depth with a minimum width because the beam can be focused and magnified.

Concept:

Melting efficiency \({\eta _m} = \frac{{{H_m}}}{{{H_s}}}\) 

Where H_m is heat required for melting & H_s is heat supplied.

We know that, \({H_s} = \frac{{VI}}{{{A_b} \times v}} \times {\eta _h}\;\;J/m{m^3}\)

Where, H_S = Heat input per unit Volume, V = Arc voltage, I = Current supplied, η_h = Arc or thermal efficiency, v = welding speed, A_b is weld bead area

For heat input in per unit length:  \({H'_s} = \frac{{VI}}{v} \times {\eta _h}\;\;J/mm\)

\(\Rightarrow {\eta _m} = \frac{{{H_m}}}{{{H_s}}} = \frac{{{H_m}}}{{\frac{{VI{\eta _h}}}{{{A_b} \times v}}}}\)

Calculation:

H’_s = 1000 J/mm, η_m = 0.45, H_m = 20 J/mm³

\({\rm{\;}}\frac{{VI}}{v} \times {\eta _h} = 1000\;J/mm\)

\(\therefore {\eta _m} = \frac{{{H_m}}}{{\frac{{1000}}{{{A_b}}}}}\)

\(\Rightarrow 0.45 = \frac{{20}}{{\frac{{1000}}{{{A_b}}}}} \Rightarrow {A_b} = 22.5\;m{m^2}\)

Points to remember:

• If heat input is asked in per unit length, use \({H_S} = \frac{{VI}}{v} \times {\eta _h}\)
• If heat input is asked in per unit volume, use \({H_S} = \frac{{VI}}{{{A_b} \times v}} \times {\eta _h}\) where A_b is weld bead area

Explanation:

Based on their connections polarity is defined.

Explanation:

In arc welding, the flux is coated on the electrodes. The flux forms a protective coating of slag over the weld metal and creates a non-oxidising atmosphere.

Functions of the flux coating:

During welding, because of the heat of the arc the flux coating on the electrode melts and perform the following function:

• It stabilizes the arc
• Removes oxide, scale etc. and cleans the surfaces to be welded
• It forms a gaseous shield around the arc which protects the molten weld pool from atmospheric contamination
• It compensates the losses of certain elements which are burnt out during welding
• It retards the rate of cooling of the deposited metal by covering with slags and improves its mechanical properties
• It helps to give a good appearance to the weld and controls penetration
• Both AC and DC can be used for the welding

Concept:

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 of Resistance Welding:
  • Spot welding
  • Seam welding
  • Projection welding
  • Flash butt welding
  • Stud welding
  • Percussion welding

Concept:

The duty cycle is the percentage of time a welding machine can be used continuously. A 60% duty

cycle means that out of 10 minutes, the machine can be used for a total of six minutes at the

 maximum rated current. When providing power at this level, it must be cooled off four minutes out of every 10 minutes. The duty cycle increases as the amperage is lowered & decreased for higher amperage. The relation is given by:

I²D = constant

Where, D: duty cycle

Calculation:

(I)² × 1 = (600)² × 0.6

I = 464.7580 A

\(\% \;decrease = \frac{{600 - 464.7580}}{{600}} \times 100\)

Explanation:

Pure tungsten electrode offers shorter life than coated electrodes because of the rapid wear and tear of pure tungsten electrode owing to thermal damage caused by their low current carrying capacity.

Pure tungsten electrodes are frequently coated with oxides of Th, Zr, La & Ce.

In general, Thorium is the best material coated on pure tungsten electrode to enhance its current-carrying capacity, in the gas tungsten arc welding process.

These oxides are expected to perform two important functions

• Increasing arc stability.
• Increasing the current carrying capacity of the electrodes i.e. increases the melting point temperature of electrode.