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TRANSCRIPT
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Shielded Metal Arc Welding (SMAW):
Technology Overview
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
1 NCIRI RACHED, D.Eng.
1. Objectives
At the end of this technology overview, the student must be able to:
• Assimilate the principle of the SMAW.
• Know the coated electrode, the roles of the coating and the types of coating.
• Know the welding parameters and aspects.
• Know the types of welding position and joint.
• Determine the appropriate diameter of the electrode and the intensity of the current.
• Choose the appropriate welding polarity.
• Know the SMAW risks and their associated safety precautions.
2. Prerequisite
• No specific prerequisite.
3. Technology overview
3.1. Principle of the SMAW
The principle of the SMAW [1,2] is based on the production of heat from electrical energy
by the Joule effect. The produced heat is used to fuse the edges of the workpieces to be
assembled. The arc welding power source produces a high electrical current (AC or DC
current) that will pass through both electrode (connected to one terminal of the power source)
and workpiece (connected to the other terminal of the power source). Electric cables,
electrode holder and workpiece connector (clamp) are used to ensure the electrical circuit.
When the electrode is close enough to the workpiece (almost 2 mm to 3 mm), the high
electrical current will be able to pass through the air gap between the electrode and the
workpiece. The air, initially considered as a highly insulating medium, becomes conductor
under the influence of the high electrical current: an electric arc is created. This electric arc is
simply the passage of electric current through the air. The air crossed by the electric current
releases heat thanks to the Joule effect. Hence, the fusion of the workpiece (base metal) edges
and the coated electrode (filler metal) is created: it is the weld bead. The following figure [3]
depicts the different components of an SMAW machine.
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
2 NCIRI RACHED, D.Eng.
The following figure [4] depicts the welding phenomena.
3.2. Types of welding position and joint
The following figure [7] depicts the different welding positions and joints:
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
3 NCIRI RACHED, D.Eng.
3.3. Coated electrode
The coated electrode consists of 2 parts as depicted by the following figure:
• Electrode: it is a metallic rod intended to conduct the electric current. It is generally of
the same nature as that of the workpiece.
• Coating: it is an adhesive chemical composition that covers the electrode, and it is
intended to play 3 principal roles [2]:
o Metallurgical roles: protection of the molten weld pool against Oxygen and
Nitrogen form the ambient air, harmful to the mechanical properties of the
weld bead. This protection is ensured by producing a protective layer, called a
slag, which will envelop the molten weld bead.
o Electrical roles: facilitating the strike (ignition) of the electric arc and maintain
its stability.
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
4 NCIRI RACHED, D.Eng.
o Mechanical roles:
- Channeling and concentration of the electric arc.
- Improvement of the mechanical properties of the weld bead.
3.4. Types of coating
The following table summarizes the most used types of coating [5,6] as well as their
chemical characteristics, welding applications and eventual precautions to consider.
Type of
coating
Chemical
characteristics Welding applications Precautions
Basic (B)
High proportion of
calcium carbonate
and fluoride
- Limestone slag (desulfurizing
effect)
-Suitable for welding ferritic
steels
- Reduced risk of hot cracking of
the deposited metal
- Direct current / reverse polarity
(electrode at terminal +)
Steaming at 350 ° for
2 hours.
(unless otherwise
specified by the
supplier)
Cellulosic
(C)
High proportion of
cellulose
- Little slag
-Suitable for rapid welding in
downward position
- Improved penetration
- Direct current / reverse polarity
(electrode at terminal +)
Open circuit voltage
≥60V
Rutile (R) High proportion of
titanium dioxide
- Slag is easily eliminated
-Suitable for welding in all
positions
-Metal deposited with good
mechanical characteristics when
steels have limited carbon and
sulfur contents
- Direct / alternating current
No special precautions
Acid (A) High proportion of
acidic matter
- Very fluid and abundant slag
- Suitable for flat, horizontal
(Corner joint and T-joint)
welding. Not suitable for vertical
(Up and down) and overhead
welding.
- Requires good weldability of
the base metal otherwise joint
sensitive to hot cracking.
No special precautions
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
5 NCIRI RACHED, D.Eng.
3.5. Parameters and aspects of the SMAW
There are several parameters for SMAW:
• Diameter of the electrode.
• Intensity of the electrical current.
• Travel speed (speed of the movement of the electrode during the welding operation).
• Distance between the electrode and the workpiece (air gap dimension).
• Welding positions (flat, horizontal (Corner joint and T-joint) and vertical: up and
down).
• Welding polarity.
The variation of these parameters mainly affects the following welding aspects:
• Size and shape of the weld bead.
• Penetration of the filler metal within the base metal.
• Fusibility of the coated electrode.
The principal weld bead aspects are depicted by the following figure:
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
6 NCIRI RACHED, D.Eng.
3.6. Determination of the appropriate electrode diameter
The diameter of the electrode eφ is determined according to the thickness t of the
workpiece [8]. Comply with the indications on the supplier's technical data sheets or use the
following general formula:
te
≤φ
3.7. Determination of the appropriate welding current
The intensity of the welding current I depends mainly on the diameter of the electrodee
φ [1,8]. Comply with the indications on the supplier's technical data sheets or use the general
formula for butt joint:
)(I)mm(
e
)A(
150 −×= φ
For corner joint, the value of the current intensity is reduced by 20%.
For T-joint, the value of the current is increased by 20%.
3.8. Welding polarity
When the Direct Current (DC) is used, the SMAW welding polarity may be 2 types [9]:
• Straight polarity: it is a direct current with a negatively charged electrode and a
positively charged workpiece (DCEN). With a straight polarity, 2/3rd
of the arc heat is
generated near the workpiece while the remaining 1/3rd
is generated at the electrode
tip. The melting rate of the workpiece increases and the filler deposition rate of the
electrode decreases. The width of the weld bead is decreased while the penetration
(depth) is increased.
• Reverse polarity: it is a direct current with a positively charged electrode and a
negatively charged workpiece (DCEP). With a reverse polarity, 2/3rd
of the arc heat is
generated at the electrode tip while the remaining 1/3rd
is generated near the
workpiece. The melting rate of the electrode increases and the filler deposition rate of
the electrode increases. The width of the weld bead is increased while the penetration
is decreased.
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
7 NCIRI RACHED, D.Eng.
It is to note that when the alternating current is used, the polarity will change almost 100
times per second. An even heat distribution is created, so, between the electrode and the
workpiece. Thus, a balance between bead penetration and electrode filler deposition rate
(bead width) is provided.
3.9. Safety precautions
When practicing the SMAW, some safety precautions must be taken into consideration.
The following table [2] summarizes the encountered risks and their associated safety
precautions:
Risks Safety precautions
High electrical current: electric shock. -Checking electrical cables.
-Safety gloves.
Ultraviolet rays: Micro wounds in the eyes. Welding mask (Safety mask).
Smokes released :
- Inhalation poisoning
- Eye burns
-Smoke suction circuit for indoor welding.
-Outdoor welding.
Significant heat release:
Burns of the hands and possibly the body
-Safety gloves.
-Safety apron.
4. References
[1] Houldcroft, P. T. (1973) [1967], "Chapter 3: Flux-Shielded Arc Welding". Welding
Processes. Cambridge University Press. p. 23. ISBN 978-0-521-05341-9.
[2] Nciri, R. (2019). Atelier Procédés et Méthodes I-Soudage à l’arc avec électrode enrobée,
Higher Institute of technological Studies of Gafsa-Tunisia, Digital courses,
http://www.isetgf.rnu.tn/ENS/uploads/nciri_rached/AtelierProcedes_et_Methodes_I_TP_Rac
hed_Nciri.pdf, Access date: 01/09/2020; 07:50 AM
[3]Welding321 (2018), Stick Welding, https://www.weldingis.com/smaw-stick-welding/,
Access date: 08/23/2020; 11:42 AM
[4]U.S. Army (1967), Operators manual welding theory and application, Figure 5-26, p.72 at
https://archive.org/details/TM9-237/page/n71/mode/2up, Access date: 01/09/2020; 07:40 AM
[5] The International Organization for Standardization (2016), ISO/TR 25901-4:2016(fr)
Soudage et techniques connexes
[6] David, H. (2018), Les électrodes enrobées pour le soudage : leurs caractéristiques et leurs
choix, https://www.soudeurs.com/site/les-electrodes-enrobees-pour-le-soudage-leurs-
caracteristiques-et-leurs-choix-444/ , Access date: 08/23/2020; 12:40 PM
[7] Mechanical Engineering blog (2019), Welding position for plate (Figure), 1G, 2G, 3G,
4G, 5G, 6G, 1F, 2F, 3F, 4F pipe and plate welding position,
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Higher Institute of Technological Studies of Gafsa-Tunisia Workshop: Processes and Methods of Production
Mechanical Engineering Department Shielded Metal Arc Welding (SMAW)_Technology overview
8 NCIRI RACHED, D.Eng.
http://www.mechanicalengineerblog.com/2019/03/30/1g-2g-5g-6g-pipe-welding-position/,
Access date: 08/23/2020; 12:44 PM
[8] Centre National de Ressources-Structures Métalliques, Procédé soudage 111-Réglage de
l’intensité de soudage (2012),
http://cnrsm.fr/c_ressources_cnrsm/5_divers/501_Dossier_machines/08_Fiches%20securite%
20machines/Version_2/34_procede_111_catomatec_3058_09_04_2012.pdf, Access date:
08/23/2020; 12:49 PM
[9] Jeffus, L. (2016), Welding: Principles and Applications (8th
ed.), Boston, MA 02210:
Cengage Learning, ISBN 10: 1305494695, ISBN 13: 9781305494695
Thanks a lot
Good luck!