1.2 Manual Metal Arc Welding Process

TRADE OF

Pipefitting



PHASE 2




Module 2




Thermal Processes



UNIT: 3




Manual Metal Arc Welding
































Produced by

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In cooperation with subject matter expert:



Finbar Smith



© SOLAS 2014


Table of Contents

Unit Objective 1
Learning Outcome 2
1.0 MMA Welding for Pipefitting 3
1.2 Manual Metal Arc Welding Process 3
2.0 Equipment Used in MMA Welding 5
2.1 MMA Welding Power Supplies 5
2.2 Types of Power Supply for Arc Welding 6
2.3 Polarity Settings for Welding Electrodes 6
2.4 The Advantages and Disadvantages MMA Welding 7
2.5 The Advantages and Disadvantages of AC and DC Welding Plants 7
2.6 MMA Welding Cables 8
2.7 Setting Up a MMA Welding Plant 9
3.0 Hazards and Safety Precautions Associated With MMA Welding 10
3.1 Protection for the Operator 10
3.2 Protection for Others 11
3.3 General Safety Precautions 11
3.4 Safety Precautions While MMA Welding 11
4.0 Welding Techniques 13
4.1 Current Too Low 13
4.2 Current Too High 13
4.3 Correct Current 14
4.4 Arc Length 14
4.5 Speed of Travel 15
5.0 MMA Welding Electrodes 16
5.1 Welding Electrodes 16
5.2 Functions of the Electrode Coating 16
5.3 Classification System for Welding Electrodes 17
5.4 Care of Electrodes 18
6.0 Identify Weld Defects and Their Causes 19
6.1 Weld Defects and their Causes 19
6.2 Control of Distortion 22
7.0 Weld Symbols and Terminology Used On Drawings 24
7.1 Weld Symbols on Drawings 24
7.2 Welding Standards Drawing Notation 25
7.3 Explanation for Standard Weld Symbols 26
7.4 Intermittent Welds 29
Exercises 30
Additional Resources 30
Unit Objective

There are seven Units in Module 2. Unit 1 focuses on Introduction to
Thermal Process and safety, Unit 2; Introduction to Oxy-acetylene welding,
Unit 3; Manual Metal Arc welding, Unit 4; Metal Active Gas welding, Unit 5;
Tungsten Active Gas welding, Unit 6; Oxy-fuel cutting and Unit 7 Plasma arc
cutting.

In this unit you will be introduced to Manual Metal Arc (MMA) welding and
the safety precautions required when using MMA equipment.





Learning Outcome

By the end of this unit each apprentice will be able to:

. Define the MMA welding process and applications in the pipefitting
industry
. Identify the different types of Manual Metal Arc (MMA) plant,
equipment and their functions
. Set up MMA welding equipment to complete welding exercises
. State the risks and hazards associated with the MMA welding process
. State the safety precautions and Personal Protective Equipment (PPE)
applicable to MMA Welding
. Identify electrodes by metric size and coating and interpret
classifications according to EN standards
. Identify weld defects and their causes
. Identify weld symbols and terminology used on pipefitting drawings

1.0 MMA Welding for Pipefitting

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1.2 Manual Metal Arc Welding Process

Arc welding processes uses an electrical power supply to create and
maintain an electric arc between an electrode and the base material to melt
metals at the welding point. They can use either direct (DC) or alternating
(AC) current, and consumable or non-consumable electrodes. The welding
region is protected by some type of inert or semi-inert gas, known as a
shielding gas.

The manual metal arc process occurs when two wires which form part of an
electrical circuit are brought together and then pulled slowly apart, an
electric spark is produced across their ends. This spark, or arc as it is
called, has a temperature of up to 3,600°C. As the arc is confined to a
very small area it can melt metal almost instantly. If one of these wires
is connected to the job and the other to a wire rod or electrode, as it is
usually called, the heat of the arc melts both the metal of the job and the
point of the electrode. The molten metal from the electrode mixes with that
from the job and forms the weld. It is important to realize that tiny
globules of the molten metal from the electrode are forced through the arc
(they do not fall by gravity). If this were not so it would be impossible
to use this process for overhead welding.

To create the arc for welding, a voltage between 60 and 100 Volts is
required to create the arc, but once it has been established, 20-40 Volts
is required to maintain it. The following stages occur when creating an
arc:

1. With the welding plant switched on, and before welding commences, no
current passes through the leads and the ammeter reads zero. A voltage
has been applied to the circuit, however, and the voltmeter will read
the open-circuit or no-load voltage (i.e. between 60 and 100 V).
2. When the electrode is brought into contact with the job a large current,
called The Short-circuit Current, passes through the leads, and the
ammeter will deflect a large amount. While this is happening, however,
the voltage drops almost to nothing. The tip of the electrode becomes
hot because of the resistance created between it and the job.
3. If the electrode is slightly withdrawn an arc is formed between the
electrode and the job. The air between the two conducts the welding
current. As the arc is formed the voltage rises to between 20 and 40 V
and the current falls to the value to which it has been set (i.e. the
welding current). The arc is then in the normal welding condition. The
heat generated by the arc melts both the work piece and the electrode,
and metal is deposited in the weld pool. During the depositing of the
weld metal, variations in both the voltage and current of the arc can
occur and the welding plant must be capable of coping with these
changes.

2.0 Equipment Used in MMA Welding

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The principal equipment used in manual metal-arc welding includes:

. Welding power source
. Cables
. Electrode holder
. Earth clamp.

Manual metal arc power source and welding leads


2.1 MMA Welding Power Supplies

There are two types of welding power source used to supply current for
metal-arc welding.

. Alternating Current (AC) type.
. Direct current (DC) type.
The AC power source

This power source takes its power directly from the main electricity
supply. It uses a transformer to supply the correct voltage to suit the
welding conditions. A special device in the transformer allows the current
in the secondary coil to be adjusted. The primary coil is connected to the
electricity power supply and the secondary coil is connected to the earth
clamp and the electrode holder.

The DC power source

There are two types of DC welding plant in use:

. DC generator
. Transformer-rectifier.
The DC generator uses a motor (electric, petrol or diesel powered) to
generate electricity. The generator provides DC current for the arc.

A Transformer-rectifier is basically a transformer with an electrical
device for changing the alternating current into a direct current output.
This device is known as a rectifier. The transformer-rectifier has the
advantage that it can be made to supply AC or DC.


2.2 Types of Power Supply for Arc Welding

To supply the electrical energy necessary for arc welding processes, a
number of different power supplies can be used. The most common
classification is constant current power supplies and constant voltage
power supplies. In arc welding, the voltage is directly related to the
length of the arc, and the current is related to the amount of heat input.
Constant current power supplies are most often used for manual welding
processes such as gas tungsten arc welding and shielded metal arc welding,
because they maintain a relatively constant current even as the voltage
varies. This is important because in manual welding, it can be difficult to
hold the electrode perfectly steady, and as a result, the arc length and
thus voltage tend to fluctuate. Constant voltage power supplies hold the
voltage constant and vary the current, and as a result, are most often used
for automated welding processes such as gas metal arc welding, flux cored
arc welding, and submerged arc welding. In these processes, arc length is
kept constant, since any fluctuation in the distance between the wire and
the base material is quickly rectified by a large change in current. For
example, if the wire and the base material get too close, the current will
rapidly increase, which in turn causes the heat to increase and the tip of
the wire to melt, returning it to its original separation distance.


2.3 Polarity Settings for Welding Electrodes

The type of current used in arc welding also plays an important role in
welding. Consumable electrode processes such as shielded metal arc welding
and gas metal arc welding generally use direct current, but the electrode
can be charged either positively or negatively. In welding, the positively
charged anode will have a greater heat concentration, and as a result,
changing the polarity of the electrode has an impact on weld properties. If
the electrode is positively charged, it will melt more quickly, increasing
weld penetration and welding speed. Alternatively, a negatively charged
electrode results in more shallow welds. Non-consumable electrode
processes, such as gas tungsten arc wel