Shielded Metal Arc Welding (SMAW) Technology overvie de procédés et... · calcium carbonate and...

Shielded Metal Arc Welding (SMAW):

Technology Overview

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.




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:




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.




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




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:




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.




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,




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!

Shielded Metal Arc Welding (SMAW) Technology overvie de procédés et... · calcium carbonate and...

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