Welding Practice - The Hong Kong Polytechnic University

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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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