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welding processes handbook pdf

Preface
Production of this guide to welding was prompted originally by a wish for an up-to-date
reference on applications in the field.

The content has been chosen so that it can be used
as a textbook for European welding courses in accordance with guidelines from the
European Welding Federation.

Over the last few years, an equivalent Swedish guide has
been used for courses on welding processes and equipment.

The author hopes that this
guide will serve as a useful reference book for those involved in welding.
In writing the book, there has been a conscious effort to ensure that both text and
illustrative material is clear, concentrating particularly on interesting and important
aspects.
Although the book has been written in Sweden, with input from Swedish experts, it
reflects technology and methods that are internationally accepted and used.

My thanks
are due to all those who have been involved in the work, with particular mention to:
Clues Olsson, HighTech Engineering, who wrote the chapter on design of welded
components.
Clues-Ove Pettersson, Sandvik, who edited the section on stainless steel.
Curt Johansson, SAQ, who wrote the chapter on quality management.
Gunnar LindLn, Air Liquide, who edited the chapter on welding costs.

1 Arc welding – an overview
1 .I History of welding
Methods for joining metals have been known for thousands of years, but for most of this
period the only form of welding was forge welding by a blacksmith.
A number of totally new welding principles emerged at the end of 19th century;
sufficient electrical current could then be generated for resistance welding and arc
welding.

Arc welding was initially carried out using carbon electrodes, developed by
Bemados, and was shortly followed by the use of steel rods.

The Swede Oskar Kjellberg
made an important advance when he developed and patented the coated electrode. The
welding result was amazing and formed the foundation of the ESAB welding company.

Terminology
Welding methods
Definitions of welding processes are given in IS0 857.

Reference numbers for the processes are defined in IS0 4063.

These numbers are then used on drawings (IS0 2553) or
in welding procedure specifications (EN 288) as references.

Joint types
Joint types are chosen with regard to the welding method and plate thickness. The ideal
joint provides the required structural strength and quality without an unnecessarily large
joint volume.

The weld cost increases with the size of the joint, and the higher heat input
will cause problems with impact strength and distortion.
Joint preparation can also be expensive; therefore it is preferable to use joint types where
the joint faces are parts of the workpiece.

This means that fillet welds are probably the
most commonly used joints.

I .3 Distortion
All fusion-welding methods produce the weld by moving a molten pool along the joint;
when the heated metal cools, the shrinkage introduces residual stresses and distortion in
the welded structure.

The stresses produce longitudinal and rotational distortion.
Longitudinal distortion.

“Shortens” the weld, but may in many cases not be a serious
problem.

An example of this type of distortion is a welded beam that can be bent if the
weld is not located symmetrically (in the centre of gravity of the cross section).

If more
than one weld is used, they must be symmetrical.

1.4 The welding arc
A welding arc is an electric discharge between two electrodes. The welding current is
conducted from the electrode to the workpiece through a heated and ionised gas, called
plasma.

The voltage drop and current in the arc give the amount of electric power that is
released, the heat of which, melts the electrode and the joint faces.
The power must also be high enough to keep the temperature of the arc sufficient for
the continued transport of the current. The temperature maintains ionisation of the gas,
i.e. it creates electrically charged particles that carry the current.

Spray arc
At high current, the resulting magnetic forces are directed downwards which helps the
droplet to be released from the surface tension at the electrode.

The droplet transfer is
characterised by a stream of small droplets.
Short arc
At lower current it has the opposite effect. The magnetic forces are smaller and are also
directed upwards.

The droplet hanging at the tip of the electrode tends to increase in size
and the process runs the risk of being unstable.

A way to overcome this problem is to
keep the arc length so short that the droplets will dip into the pool before they have
grown too much.

Surface tension will then start the transfer of the melted material and
the tail of the droplet will be constricted by the magnetic forces, the so-called “pinch
effect”.
No metal is transferred in the form of free droplets across the arc gap.

The stability of
the short circuiting transfer is very sensitive to variations in the shielding gas, the
chemical composition of the electrode and the properties of the power source and wire
feed system.

1.5 Shielding gases
The most important reason to use a shielding gas is to prevent the molten metal from the
harmful effect of the air.

Even small amounts of oxygen in the air will oxidise the
alloying elements and create slag inclusions.

Nitrogen is solved in the hot melted material but when it solidifies the solubility decreases and the evaporating gas will form
pores. Nitrogen can also be a cause of brittleness.

The shielding gas also influences the
welding properties and has great importance for the penetration and weld bead geometry

1.6 Power sources
The importance of the power source for the welding process
The main purpose of the power source is to supply the system with suitable electric
power.

Furthermore, the power source performance is of vital importance for the welding
process; the ignition of the arc, the stability of the transfer of the melted electrode material and for the amount of spatter that will be generated.

For this purpose it is important
that the static and dynamic characteristics of the power source is optimised for the particular welding process.

2 Gas welding
Gas welding is one of the oldest methods of welding and, for many years, was the most
widely used method of metal-melting; however, its use is a lot less common today.
Nevertheless, it is a versatile method, using simple and relatively cheap equipment.                                 It is suitable for repair and erection work, for welding pipesltubes and structures with a wall
thickness of 0.54 rnm in materials particularly prone to cracking, such as cast iron and
non-ferrous metals. It is also widely used for cladding and hardfacing.

The heat is generated by the combustion of acetylene in oxygen, which gives a flame temperature of about
3100 “C. This is lower than the temperature of an electric arc, and the heat is also less
concentrated. The flame is directed onto the surfaces of the joint, which melt, after which
filler material can be added as necessary.

The melt pool is protected from air by the
reducing zone and the outer zone of the flame.

The flame should therefore be removed
slowly when the weld is completed.

2.1 Equipment
A set of equipment (Figure 2.1) consists essentially of gas bottles, pressure regulators,
gas hoses, flashback arresters and welding torches.

Welding torches
One can distinguish between two types of welding torches: injector torches for low
pressure acetylene and high pressure torches.
In high pressure torches, the acetylene and oxygen flows are self-powered by the
pressure in their storage bottles, and mix in the mixing chamber section of the torch.
In low-pressure torches, the oxygen flows into the torch through a central jet,
producing an injection effect that draws in acetylene from the surrounding peripheral
connection.

From here, the gases continue to the mixing section prior to combustion.
Gas flames
The basic requirement for a good weld is that the size and type of the flame should be
suited to the type of work.
The size of the flame depends on the size of the torch nozzle and on the pressure of
the gases flowing through it.

This pressure should be maintained within certain limits. If
it exceeds the normal pressure, there will be a considerable jet effect and the flame will
become ‘hard’.

Below the correct pressure, the jet effect will be reduced and the flame
will be ‘soft’.
We distinguish between three different types of flames, depending on their chemical
effect on the melt pool: carburising, neutral, and oxidising.

The benefits of gas welding
Gas welding is very suitable for welding pipes and tubes, it is both effective and
economic for applications such as HVAC systems, for the following reasons:
The ability to even out the temperature in the weld at low temperatures. Slow heating
and cooling can avoid the risk of hardening.
Metal thicknesses up to about 6 mm can be welded with an I-joint.
Speed, as only one pass is needed.

Filler wires can be changed without having to
pause for grinding.
Good control of melting, as the welder can see at all times that he has the desired
pear-shaped opening in the bottom of the melt pool.
Root defects are avoided by taking care to ensure good burn-through.
Pipes and tubes often have to be welded in very confined spaces.

In such cases, gas
welding is often preferable, bearing in mind the less bulky protective equipment
required (goggles, as against a normal arc welding helmet or visor, and compact
torch) to perform the work.
The equipment is easy to transport and requires no electricity supply.
It is possible to use the light from the flame to locate the joint before welding starts.
The size of the HAZ can be reduced by surrounding the weld area with damp (fireproof!) material.

3 TIG welding
3.1 A description of the method
TIG welding (also called Gas Tungsten Arc Welding, GTAW) involves striking an arc
between a non-consumable tungsten electrode and the workpiece.

The weld pool and the
electrode are protected by an inert gas, usually argon, supplied through a gas cup at the
end of the welding gun, in which the electrode is centrally positioned.

3.2 Equipment
The following equipment is required for TIG welding:
welding gun
HF (= high-frequency) generator for ignition of the arc
a power source
shielding gas
control equipment

Striking the arc
A TIG welding arc is generally ignited with the help of a high-frequency generator, the
purpose of which, is to produce a spark which provides the necessary initial conducting
path through the air for the low-voltage welding current.

The frequency of this initial
ignition pulse can be up to several MHz, in combination with a voltage of several kV.
However, this produces strong electrical interference, which is the main disadvantage of
the method.
It is not good practice to strike the arc by scraping the electrode on the workpiece:
this not only presents risk of tungsten inclusions in the weld, but also damages the electrode by contaminating it with the workpiece material.
Another method of striking the arc is the ‘lift-arc’ method, which requires the use of a
controllable power source.

The arc is struck by touching the electrode against the workpiece, but in this case the special power source controls the current to a sufficiently low
level to prevent any adverse effects.

Lifting the electrode away from the workpiece
strikes the arc and raises the current to the pre-set level.
The power source
TIG welding is normally carried out using DC, with the negative connected to the electrode, which means that most of the heat is evolved in the workpiece.

However, when
welding aluminium, the oxide layer is broken down only if the electrode is connected to
the positive pole, this then results in excessive temperature of the electrode.

As a compromise, aluminium and magnesium are therefore generally welded with AC.
TIG power sources are generally electronically controlled, e.g.

in the form of an
inverter or a thyristor-controlled rectifier.

The open-circuit voltage should be about
80 V, with a constant-current characteristic.

4 Plasma welding

Classification of plasma welding methods
There are three different classes of plasma welding, depending on the current range:
Micro plasma (0.1-1 5 A).

The concentrated arc enables it to remain stable down to a
current of about 0.1 A, which means that the process can be used for welding metal
thicknesses down to about 0.1 mm.

This makes the process attractive to, for example,
the space industry.
Medium plasma welding (15-100 A). In this range, the method competes more
directly with TIG welding.

It is suitable for manual or mechanised welding and is
used in applications such as the automotive industry for welding thin sheet materials
without introducing distortion or unacceptable welded joints, as are produced by
MIG welding, or for the welding of pipes in breweries or dairies.
Keyhole plasma welding (>I00 A).

The third type of plasma welding is referred to as
keyhole plasma welding, taking its name from the ‘keyhole’ that is produced when
the joint edges in a butt weld are melted as the plasma jet cuts through them.

As the jet is moved forward, the molten metal is pressed backwards, filling up the joint
behind the jet.