Heating And Welding MCQ [Free PDF] – Objective Question Answer for Heating And Welding MCQ Quiz

Ques.51. In a synchronous welding control, the welding current begins at

  1. Later than the power factor angle
  2. Before the power factor angle
  3. At the power factor angle
  4. None of these

Answer.3. At the power factor angle

Explanation:-

Time and sequence control

Weld timers may have a non-synchronous or synchronous form.

A non-synchronous timer starts and stops the current flow by contactor at instants unrelated to the A.C waveform, as a result, the current may vary, and the time of repetition of a preset sequence be subject to an inaccuracy of plus or minus one-half period of the A.C supply.

Synchronous timers:- A synchronous timer provides an accurate timing period in that it closes and opens the primary of the welding transformer at the same point of the supply voltage in the making of each weld It reduces or eliminates electrical transients.  Therefore the synchronous control timer operates at the power factor angle. Where quality is important, and particularly for such materials as aluminum and its alloys, synchronous timers are usually regarded as essential. Modern ‘digital solid-state’ controls provide precise and repeatable control of the welding operation.

This is mostly used with sophisticated spot welding machines. There is a rotating dial that can very precisely select various lengths of time. The time periods are selected as per the requirement of a particular weld. This type of timer ensures that current flows for complete cycles and not for fraction of a cycle. If a particular weld starts with a positive half-cycle it always ends with the negative half cycle. A new weld can never start with the same kind of half-cycle that has ended In the previous weld. This is an essential condition to be fulfilled because, otherwise, the magnetism retained by the welding transformer from the final half-cycle of the previous weld will affect the current and hence will affect the heat of the next weld.

 

Ques.52. The function of SCR contactor in resistance welding machine is

  1. To provide an accurate weld time for each weld
  2. To connect the large power supply to welding by closing a small switch
  3. To provide full-wave rectification of the welding current
  4. To avoid saturation of transformation core

Answer.2. To connect the large power supply to welding by closing a small switch

Explanation:-

SCR contractors are electronic control devices designed to handle large amounts of current. SCRs are triggered or turned on by pulses supplied by the timing or sequencing circuits of the welder. The SCRs are usually cooled by circulating water. 

During the welding process, a large amount of current is required for short periods of time. Silicon-controlled rectifiers (SCRs) are commonly used to control the starting and stopping of the large currents associated with electric welders. The current rating of these devices must be very high, sometimes in the range of 1000 to 100,000 amperes, and the power distribution equipment must be able to handle these high currents. 

SCR Contactor

The two main functions of SCR is in phase control mode with transformers and in converter configuration.

The common SCR phase-controlled machines are three-phase machines in either constant current or constant voltage modes. Because of electronic control of output characteristics, automatic line voltage compensation is easily achieved; this allows the machine to set the precise welding power, independent from the variations of input power. An SCR can also serve as a secondary contactor that allows the welding current to flow only when the control allows the SCRs to conduct. This is a very useful feature in spot welding and tack welding operations, in which rapid cycle operation is required. An SCR contactor does not provide electrical isolation; this requires that a circuit breaker or similar device would be provided for electrical safety.

 

Ques.53. Which of the following is the unit for thermal conductivity?

  1. W/m °K
  2. W/m2 °C
  3. W2/m °K
  4. W2/m2 °K

Answer.1. W/m °K

Explanation:-

Thermal conductivity is the property of a material to conduct heat. In the International System of Units (SI), thermal conductivity is measured in watts per meter kelvin (W/(m⋅K))

 

Ques.54. Which is not the type of butt welding process?

  1. Upset butt Welding
  2. Downset butt welding
  3. Flash butt welding
  4. Percussion butt welding

Answer.2. Downset butt welding

Explanation:-

Butt welding

Butt welding is similar to spot welding; however, the only difference is, in butt welding, instead of electrodes the metal parts that are to be joined or butted together are connected to the supply. In this case, the two workplaces are brought into contact end-to-end and the butted ends a heated by passing a heavy current through the joint. 

This process is useful where parts have to be joined end-to-end or edge-to-edge. i.e. for welding pipes, wires and rods. It is also employed for making continuous lengths of chains.

Type of butt welding

The three basic types of the butt welding process are:

(a) Upset butt welding.

(b)Flash butt welding.

(c) Percussion butt welding.

 

Ques.55. The main purpose of the light coated electrode is to

  1. Increase arc stability
  2. Reduce Voltage
  3. Reduce Current
  4. Any of the above

Answer.1. Increase arc stability

Explanation:-

Lightly coated electrodes

These electrodes are coated with a thin layer of coating material up to less than 1 mm. This coating usually consists of lime mixed with soluble glass which serves as a binder. ‘These electrodes are considered as an improvement over bare electrodes.

The main purpose of using the light coating layer on the electrode is to increase the arc stability, so they are also called as stabilizing electrodes. The mechanical strength of the weld increased because the slag layer will not be formed on the molten weld. For this reason, lightly coated electrodes may only be used for welding nonessential workpieces.

 

Ques.56. The highest value of thermal conductivity is for

  1. Solid ice
  2. Melting ice
  3. Water
  4. Steam

Answer.1. Solid Ice

Explanation:-

In heat transfer, the thermal conductivity of a substance, k, is an intensive property that indicates its ability to conduct heat.

The thermal conductivity of the ice is highest and it is 0.050–0.250(W·m−1·K−1).

The thermal conductivity of water is 0.5918(W·m−1·K−1).

 

Ques.57. The type of electrode used in the seam welding is

  1. Bare wire Rods
  2. Roller electrodes
  3. Heavily coated electrode
  4. Lightly covered electrode

Answer.2. Roller electrodes

Explanation:-

Welding electrode. A component of the welding circuit through v hick current is conducted and that terminates at the arc, molten conductive slag, or base metal. See also arc welding

Electrode (arc welding)—a “stick” or ~wire” from which an arc is struck. It may include the bare metal electrode, carbon electrode, composite electrode, covered electrode, lightly coated electrode, and the heavy coated electrode.

Electrode (resistance welding)—that part or parts of a machine through which the welding current and in most cases, pressure, are applied directly to the work. The electrode may be in the form of a rotating wheel, bar, cylinder, plate, clamp.

Seam Welding

In principle, this process is similar to spot welding except for the shape of the electrodes. In seam welding, continuously rotating disc-type electrodes. are used whereas in spot welding pointed electrodes are used. In seam, welding pressure is applied. In seam welding pressure is applied continuously along the whole length of the lap joint to produce a seam weld. The metallic pieces a held in two copper disc roller electrodes and are passed through them to complete the joint.

Seam welding is accomplished by using roller-type electrodes in the form of wheels that are 6 in. (152 mm) to 9 inches. (229 mm) or more in diameter. These roller-type electrodes are usually copper alloy discs 3/8 inch (10 mm) to 5/8 inch (16 mm) thick. Cooling is achieved by a constant stream of water directed to the electrode near the weld.

img.1 1

Welding is done either with the roller electrodes in motion or while they are stopped for an instant. If continuous motion is used, the rate of welding usually varies between 1 ft and 5 ft (30 cm to 152 cm) per minute. The greatest welding speed is obtained on the thinnest materials. An indexing mechanism can be used when the wheels are to be stopped for each weld.

Electric timing equipment is used when it is necessary to provide the precise control required for the highest quality welds. Spot welds can be positioned at almost any interval desired by simply adjusting the timing and rate of electrode motion.

It is possible to change the welding current and electrode pressure to control the surface condition and width of the weld. The seam width should be about two times the thickness of the sheet plus 1/16 inch (2 mm).

Types of Resistance Seam Welds

The figure illustrates the type of seams used in most seam welding processes. The lap seam is the most common of these seams. The tops and bottoms of containers are usually fastened together with a Hanged seam. For a metal thickness of 1/16 inch (2 mm), the mash seam is often used. The electrode face should be wide enough to cover the overlap by approximately one and a half times the thickness of the sheet.

img.2 1

High-Frequency Resistance Seam Welding

This process is similar to RSEW in some ways. TO major differences are that the welding current is supplied as supplied frequency, 200 to 500 kHz as opposed to DC 50-cycle AC. The high-frequency power provides very localized heating. This heating is a result of the resistance to the current induced in the metal and not to the resistance between parts. As a consequence of this localized heating, pipe and tubing can be welded with little loss of power flowing around the backside of the joint.

High-Frequency Resistance Seam Welding is used in the production of welded pipe tubing, and structural shapes. It works very well for tip fabrication of l-beams, H-beams, channels, and so on. The welded structural shapes are lighter and stronger than the roll-formed counterparts.

 

Ques.58. Which of the following outlines the features of D.C welding over A.C welding?

1.1 The cost of equipment is higher

2. It is less safe as the no-load voltage is high

3. The efficiency is lower due to the presence of the rotating part

2.1. The cost of equipment is higher

2. it is safer

3. The efficiency is higher due to the absence of a rotating part

3.1 The cost of equipment is cheaper

2. It is less safe as the no-load voltage is high

3. The efficiency is higher due to the absence of a rotating part

4.1. The cost of equipment is cheaper

2. It is less safe as the no-load voltage is high

3. The efficiency is lower due to rotating the part

Answer.1. The cost of equipment is higher

2. It is less safe as the no-load voltage is high

3. The efficiency is lower due to the presence of the rotating part

Explanation:-

Both D.C. and A.C. equipment are employed in arc welding. D.C. arc welding machines are D.C. generators driven by an electric motor or an internal combustion engine. A.C. welding machines are transformers for stepping down the main supply voltage to the voltage (20 to 40V) and current (150 to 1000 A) suitable for arc welding. The Comparision between A.C welding and DC welding are discussed below:-

[ninja_tables id=”28565″]

 

Ques.59. The quantity of heat required to change the temperature of 1 gm of ice from −6°C to −5°C is known as

  1. Latent heat of fusion
  2. Temperature coefficient
  3. Specific heat
  4. Latent heat of vaporization

Answer.3. Specific heat

Explanation:-

Specific Heat:- The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. Here the temperature is increasing from −6°C to −5°C i.e +1° C.

The latent heat of fusion (or meltings) of a substance is the heat energy required to convert a unit mass of the substance from the solid to the liquid state, without chant of temperature.

The latent heat of vaporization (or boiling) of a substance is the heat energy required to convert a unit mass of the substance from the liquid to the gaseous stats without a change of temperature. 

A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature

 

Ques.60. A body at the temperature T K radiates heat in proportion to

  1. T2
  2. 1/T2
  3. T4
  4. 1/T4

Answer.3. T4

Explanation:-

According to Stephen’s Law, the total rate at which a black body emits heat radiation is proportional to the 4th power of its absolute temperature. If E is the rate of emission of heat radiations T is the absolute temperature, then

E ∝ T4

or

E = σT4

Here σ is the proportionality constant and known as Stefan’s constant and its value is 5.672×10−2 W/m2/K4

Scroll to Top