Question 1. State two precautions associted with
oxyacetylene welding.
• Oxygen should never be used instead of compressed
air.
• Special care must always be taken when using
pressurised and flammable gases.
• You must always wear protective clothing.
• Goggles must be worn to protect your eyes from
harmful rays.
• All equipment must be checked regularly
Question 2.
Explain the two stage combustion process in
Oxyacetylene welding.
• Stages of combustion
• Primary Combustion – Acetylene & Oxygen burn in the
primary combustion zone to give off Carbon Monoxide and
Hydrogen.
•
C2H2+O2
2CO+H2
• Secondary Combustion – oxygen is sucked in from the weld
pool and is burned with Carbon Monoxide & Hydrogen to give
off Carbon Dioxide and Water vapour.
•
CO+H2+O2
CO2+H2O
Question 3.
Explain the difference between a carburising
and oxidising flame.
The Oxidising Flame
This is a result of excess Oxygen. Approx 1.5 parts oxygen to 1 part acetylene. If
used with steel the excess oxygen would contimnate the joint, so it is used for
copper and some bronzes. It has a temp. of about 3500ºC
max temp zone
inner cone
secondary combustion zone
Question 3.
Cont.
The Carburising Flame
This is as result of excess Acetylene. Approx 0.9 parts oxygen to 1 part acetylene. Has
a temp of approx 3150ºC and is used for welding alloy steels and aluminium alloys
max temp zone
inner cone
secondary combustion zone
C
Acetylene
Feather
Question 4.
State two functions of a flux in Oxyacetylene
welding.
• It protects the weld pool from oxygen and
nitrogen.
• It prevents blowholes from forming in the
weld.
Question 5.
State two functions of the electrode coating in
manual metal arc welding
• It facilitates the striking of the arc and enables it to burn
steadily.
• It picks up impurities and brings them to the surface to form
slag. The slag reduces the cooling rate.
• It provides a gas shield to keep oxygen and nitrogen away
from the weld pool.
• It increases the rate of melting.
• It enables the use of AC for welding.
• It gives good penetration.
• It can increase or decrease fluidity for special functions.
Question 6. State two factors to be considered when
preparing a joint for welding
• Ensure that the joint is clean and free from
grease.
• Edges may also be bevelled to allow good
penetration and fusion.
Question 7.
Describe with the aid of a diagram, a manual metal arc
welding circuit operating from a 220V ac power supply.
Explain how such a circuit operates when supplying dc for welding
Transformer
Rectifier
D1
Smoothing
Capacitor
D4
AC
D3
AC High
Voltage
AC
Low
D4
DC
DC at Low
Voltage
Question 7.
Cont.
• The function of the transformer is to step down the voltage without
changing it’s frequency. It is done by two coils of wire a primary and
secondary. The current is induced from one to the other, causing the
voltage drop. This in turn causes the current to increase.
•
•
•
•
•
Converting ac to dc uses the rectifier.
This is a Single-Phase bridge converter.
It uses 4 Diodes.
During one half-cycle of the Ac supply diodes D1 and D3 are conducting.
During the next half-cycle D2 and D4 are conducting.
Question 8.
Describe with the aid of a diagram Metal Arc
Gas Shielded welding.
• Metal Inert Gas
• Semi-automatic Process
• Consumable Electrode fed
automatically.
• Argon gas protects the weld
pool like a flux.
• No slag produced.
• Suitable for steel.
Q9 Describe with the aid of a
diagram Seam Welding
Question 10. State three functions of a heat source in welding.
• Must generate enough heat to melt the parent
metals.
• Must be able to melt the filler rod.
• The heat source must be able to be controlled.
Question 11. Explain the special precautions necessary foe
welding in confined spaces.
• Good ventilation if possible.
• Breathing equipment in special cases.
• No flammable materials in the area.
Question 12. Why is it more difficult to weld aluminium that
mild steel?
It is more difficult to weld aluminium
because on the application of heat there is
a build up of a hard layer on the surface of
the aluminium, which makes it difficult to
weld.
Q13. describe with the aid of a
diagram Tungsten Arc Gas Shielded
Filler Rod
• Tungsten Inert Gas
• Non –consumable
Electrode.
• Filler rod used.
• Argon gas protects the
weld pool like a flux.
• Suitable for Aluminium
& Stainless Steel.
Q14 Describe with the aid of a diagram Electro-Slag
Welding
Question 15. State two important functions of the slag
produced in manual metal arc welding.
• It helps shape the weld.
• It slows down the cooling rate.
• It protects the weld from contamination as it cools.
Question 16. State two precautions which should be taken into
consideration to eliminate hazards associated with mains
operated welding equipment.
•
•
•
•
Properly earthed.
Correct cables
Cables in good condition.
Correct fuse.
Question 17. In manual metal arc welding explain, using
suitable diagrams, the principles of operation when using:
(i)ac (ii)dc.
• AC
Both types of MMA welding
processes use step down
transformers. These transformers
use primary and secondary coils
to change the voltage from 220240V to 80-100V which gives a
much higher current for welding
220V
• DC
To supply DC for welding a
rectifier is used. It consists of four
diodes and a smoothing
capacitor. A bridge Rectifier
converts Ac to DC.
80V
Rectifier
D1
D4
AC in
AC High
Voltage
AC
Low
Smoothing
Capacitor
D3
D4
DC out
Question 18 Explain the function and advantage of each of the
following:
• Multi Runs.
These are less brittle than single runs as successive layers will anneal the
hard zones on the previous run.
• Shielded Arc
The weld pool must be protect at all times from the atmosphere, this can
be done with an inert gas or the burning of flux. This can stop slag
inclusion and other problems.
• Edge Preparation.
The pieces to be welded must be clean a free from grease. This will ensure
that no foreign matter in the weld lowering quality.
• Slag
this helps to shape the weld and it also slows down cooling which reduces
brittleness.
Question 19. Explain the function of each part of a welding
regulator.
• Pressure gauge 1- This indicates the pressure of the
contents of the bottle.
• Pressure Gauge 2- This indicates the pressure of the
gas leaving the bottle.
• Valve- This opens and closes the bottle.
Question 20. Describe the following defects and suggest a
remedy in each case.
• Slag Inclusion.
Foreign matter entrapped in the weld caused by dirt or contamination
from the atmosphere.
Remedy-Weld on clean surfaces with correct filler rod and flux.
• Porosity
Gas entrapped in the weld due to excessive moisture.
Remedy-Dry electrodes and dry work piece.
• Lack of Penetration.
Welding current set too low or incorrect diameter rod.
Remedy- Check the above.
Question 21. Describe the principles, functions and application
of Spot Welding.
Question 22. Describe the principle, function and application of
Friction Welding.
Friction Welding
A
B
• When A & B come into
contact the friction causes
them to fuse together.
• This is a result of A rotating
a high speeds and B brought
into contact with a large
force.
• This process is used for
joining shafts.
Q23 Describe Submerged arc Welding
Question 24. What is meant by dissolved Acetylene?
• Acetylene cannot be compressed as it would
explode. It is absorbed in the cylinder by a substance
called acetone. Acetone can absorb 25 times it’s own
volume of acetylene, this is dissolved acetylene.