Welding Practice - The Hong Kong Polytechnic University
IC LEARNING SERIES
Welding Practice
The Hong Kong Polytechnic University
Industrial Centre
IC LEARNING SERIES
Welding Practice
Suitable for the following learning modules offered by the Industrial Centre:
TM0212 Ducting and Welding Practice
TM0402 Fabrication Processes Appreciation
TM0412 Fabrication and Welding Practice
TM1213 Structural Concrete and Steelwork
IC2113 IC Training I (TSE)
IC235 Integrated Practical Training
IC253 Introduction to Product Prototyping and Fabrication Processes
IC348 Appreciation of Manufacturing Processes
Last updated: June 2012
Copyright reserved by Industrial Centre, The Hong Kong Polytechnic University
Welding Practice
Welding Practice
Objectives:
To understand the basic principles of common welding technologies
To be able to select and apply among different kinds of weld.
1. Introduction
Welding is a permanent joining of two materials, mainly metals, through
localized coalescence, resulting from a suitable combination of temperature,
pressure, and metallurgical conditions. In the last fifty years, welding technology
has been developed extensively and it is now the case that its use may often
result in the saving of time and money when compared with some other
methods of manufacturing. Today, welding is used in a wide range of
applications, both in jobbing production (one off) as well as in the mass
production industries. Typical applications are found in shipbuilding, the aircraft
industry, civil engineering and construction, automobile manufacturing, and
many consumer product manufacturing industries. With the advancement of
automatic welding techniques and equipment, welding is no longer a skill-ofhand-dependent
activity nor is it necessarily a costly process. Nowadays, welding
equipment is often highly automated, including the increasing use of industrial
robots leading to good system flexibility as to use coupled to a high degree of
accuracy and quality. Developments of this kind allow welding now to be used
for work with high precision requirements previously considered impossible and
uneconomical.
1.1 FUNDAMENTAL PRINCIPLES OF WELDING
A weld is defined as a localized coalescence of metals wherein coalescence is
produced by heating the metal to suitable temperatures, with or without the
application of pressure and with or without the use of any filler metal.
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Details of a Weld
In order to obtain coalescence between two metals, there must be a combination
of proximity of surfaces and sufficient activity between the molecules of the
pieces being joined to cause the formation of common metallic crystals. Such
proximity and activity are often impeded by an oxide layer or a thin layer of
absorbed gas on the oxide surface. These contaminant layers must be removed
by mechanical or chemical means in order to obtain satisfactory welds.
In summary, in order to obtain satisfactory welds it is necessary to have a
satisfactory heat and/or pressure source, a means of protecting or cleaning the
metal, and avoidance of, or compensation for, harmful metallurgical effects.
2. Welding Processes
The field of welding can be divided into two main classifications fusion and nonfusion.
Fusion welds are usually made without the assistance of mechanical
pressure. Non- fusion welds generally necessitate the assistance of mechanical
pressure to bring the heated surfaces into intimate contact.
When parts are joined by fusion the method is very similar to a casting process
since the surfaces being welded are heated until they melt and run together. If a
filler is added this is melted along with the metals being joined and run into the
space which it is necessary to fill.
2.1 Gas Welding
Gas welding comprises the group of welding processes in which the heat
necessary for welding is obtained from the combustion of a fuel gas oxygen
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mixture. The fuel gases which may be used include acetylene, butane, coke gas.
ethylene hydrogen, natural gas, propane, and other petroleum derivatives or
mixtures. Because acetylene in combination with oxygen produces the hottest
flame, oxy-acetylene welding is the most commonly used of these processes. The
equipment required for gas welding is simple and compact, making it especially
useful for maintenance or site work where portability is an advantage. However
the rate of welding is slower with gas than with arc welding and the lower heat of
gas welding limits its use. For production purposes, it is ideally suited to lightgauge
metals work, as in aircraft and automotive assemblies of sheet and tube,
or to the welding of metals which have low melting points, such as copper and
its alloys.
2.1 Arc Welding
Arc welding is a fusion process in which the welding rod is cast into the
previously fused space between the metals to be joined. The heat necessary to
melt the metal and welding rod is obtained from an electric arc struck between
the rod (electrode) and the work, a temperature of 3500-4000°C being obtained
near the crater (Fig 3) of the arc.
There are many types of arc welding processes, some of the common ones
are outlined below.
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Shielded metal-arc welding is probably the most widely used welding process.
For the most part, welding with this process is done manually. It may be used in
all positions: flat, vertical, horizontal, and overhead. Suitable electrodes are
available for shielded metal-arc welding of plain carbon steel, low-alloy steel,
stainless steel, copper 'and its alloys, aluminium and its alloys, and cast iron. A
diagram of shielded metal-arc welding is as shown in Fig 3. Submerged-arc
welding can be used in a fully automatic system set-up with the electrode and
granular flux (see Fig 4) feed-controlled and either the welding head traversing
the work or the work moving under a fixed head. Because it is necessary that the
flux remains on the joint, this welding method is restricted to the flat position
except for horizontal fillet welds.
Inert gas arc welding employs either consumable electrodes (see Fig 5) or nonconsumable
electrodes, (see Fig 6) with the arc drawn between the electrode and
the work piece. Inert gas such as carbon dioxide, helium, or argon is used to
provide shielding of the arc and the molten metal. Consumable electrode inert
gas arc welding is commonly referred to as Metal-arc Inert Gas welding (MIG)
whilst non-consumable electrode inert gas arc welding is referred to as Iungstenarc
Inert Gas welding (TIG) because tungsten electrodes are used in the process.
In the case of TIG filler metal, where needed, is provided by a welding rod
inserted into the arc without forming a part of the welding circuit.
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Inert gas arc welding was originally developed for welding light metals,
aluminium and magnesium. Its use has been extended to the welding of stainless
steels, mild steels, copper and nickel and their alloys, and the more recent high
temperature metals such as titanium and zirconium.
2.1 Brazing
The brazing process differs from welding processes in that the non ferrous filler
metal used has a melting point below that of the metal to be brazed, but higher
than 450°C, and further, the flow of filler metal through the joint is by capillary
attraction. To accomplish this action, the joint should have a clearance of 0.05 to
about 0.25 mm depending upon the shape of the joint.
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Brazing and Braze Welding
Joint preparation for brazing
Brazing is mainly employed for sheet steel and tubular work, for bronze
surfacing, and for joining dissimilar metals. The main advantage of the process is
that the joint surfaces are heated only to the melting point of the welding rod,
which is several hundred degrees lower than that of the iron or steel.
3. Special Welding Processes - Cutting Operations
Cutting process is a method which brings removal of metals. This can be done by
machining, melting and chemical reaction, nowadays oxygen acetylene flame
cutting and plasma arc cutting metals process are widely used in industry fields.
The two cutting methods may be done by manual or with mechanized
equipment. In manual cutting process, the operator manipulates a cutting torch
over the area to be cut. In machine cutting process, the torch is guided entirely
by automatic controls.
3.1 Oxy-acetylene Flame Cutting
Cutting metal by oxy-acetylene process is done by means of hand cutting torch
or by more complicated, automatically controlled cutting machine. (Fig 10 and
11). When a piece of ferrous metal is heated by hot flame until red-hot and then
exposed to pure oxygen, a chemical reaction takes place between the heated
metal and the oxygen. This reaction causes rapid burning and oxidation of the
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metal. By this continuous process of oxidation, the metal can be cut through very
rapidly.
3.2 Plasma Arc Cutting
Plasma arc cutting can be used for cutting all electrically conductive materials,
including stainless steel and aluminium. In this process, when an arc is struck
between the electrode in the torch and the workpiece, gas is heated to a high
temperature, and change into positive ions, neutral atoms and negative
electrons. When matter passes from one state to another, latest heat is
generated and for. a high velocity plasma gas jet, which can melt the metal and
blow it away to form a kerfs The basic arrangement and terminology for a
plasma arc torch are shown below. (see Fig 11)
4. Quality Control and Testing of Welds
Two main methods are employed in the quality control and testing of welds,
namely, destructive testing and non-destructive testing. The former method is
mainly used in a mass production situation where it is acceptable to lose a few
pieces of a workpiece from a large batch so as to obtain sufficient information
concerning the quality of the welds. Non-destructive testing, however, is aimed
at checking the quality without causing any damage to the workpiece, and this is
often employed on work such as in shipbuilding and civil engineering steel
structures.
Destructive testing is essentially in determining the mechanical properties of the
weld. Non-destructive testing is for spotting internal defects that impair the
soundness of the weld during production. The degree of acceptance of quality
thus has to be carefully worked out.
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4.1 Weld Defects
Several types of weld defects are commonly found in practice. They are porosity,
burn-through blow-holes, slag inclusions, poor penetration, undercuts, and
cracks. Fig 12 illustrates these defects.
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4.2 Non-destructive Testing Methods
The Magnetic Particle method is used for detecting flaws and cracks in
paramagnetic materials, principally iron and steel. The principle of the method
involves magnetizing ferromagnetic parts in such a direction that the magnetic
flux will produce north and south poles at opposite edges of a discontinuity, then
applying finely divided magnetic particles to the vicinity. The particles are
attracted and held by the 'leakage' fields between poles. The readily visible
accumulation of these particles indicates and outlines the discontinuity.
4.2.1 Dye penetration
The Dye penetration method provides a means of detecting discontinuities which
are open to the surface. Penetrants are applied to the surface and are drawn into
the discontinuities by capillary action. The excess penetrant is removed and. a
developer is applied to the surface. The penetrant remaining in the
discontinuities will be drawn back by blotting action of the developer and will
colour or stain the developer to form a visible indication from which one can
interpret the characteristics of the discontinuity.
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4.2.2 Ultrasonic testing
Ultrasonic testing utilizes mechanical vibration in detecting imperfections in
materials. At very high frequencies such as 1 to 10 megacycles, mechanical
vibration can be generated in the form of well defined beams of small cross
section. These beams can be directed into work pieces on internal examination
somewhat in the manner of X rays. Like other wave motions, the beams are
subject to the laws of reflection and refraction when they encounter changes in
the physical properties governing the propagation of vibrational waves. Thus any
unhomogeneity can be detected as this represents a substantial change in either
the elastic modulus or the density of the material. In practice, commercial
instruments do not measure these parameters directly; instead they indicate one
or more-of the factors involved in sound-wave propagation; velocity of travel,
attenuation of beam, and reflection of energy from discontinuities.
4.2.3 Radiographic
Radiographic method of inspection utilizes the shadow pattern resulting from
penetrating radiation to determine a material's homogeneity. This inspection
process requires a source of penetrating radiation such as radioisotopes or X-ray
generators, and recording devices such as film or fluoroscopic screens.
Inspection is accomplished by registration of discontinuities in the material on
the observing medium, and relating them to the material's physical properties.
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References
The Procedure Handbook of Arc Welding, The Lincoln Electric Company.
ASM Handbook Volume 6, 6A
American Welding Society
Last Updated: Jun 2012
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