CHAPTER 2

PREPARED BY A
COMMITTEE CONSISTING
OF:
D. R. Amos, Chairman

Westinghouse Electric Corp.

D. A. Fink
Lincoln Electric Co.

SHIELDED
METAL ARC
WELDING

J. R. Hannahs

Midmark Corporation

R. W. Heid

Newport News Shipbuilding

A. R. Hollins

Duke Power Co.

J. E.

Mathers
Welding Consultants, Inc.

L. C. Northard*

Tennessee Va/fey Authority

M. Parekh
Hobart Brothers Co.

A. Pollack
------------------------------ Consultant
M. S. Sierdzinski

- Alloy Rods Co.

44 M. ]. Tomsic
Fundamentals of the Process
Plastronic Inc.

Equipment

HANDBOOK
47 WELDING
COMMITTEE MEMBER:

----------------------------- D.R.Amos

52 Westinghouse Electric Corp.

Materials
------------------------------ * Deceased
Applications
56

Joint Design and Preparation

57

Welding Procedures

61

Quality of the Weld

68

Safety Recommendations

70

Supplementary Reading List

71

44
ARC

SHIELDED

METAL

WELDING

, CHAPTER 2

SHIELDED METAL
ARC WELDING
FUNDAMENTALS OF THE PROCESS
DEFINITION AND GENERAL DESCRIPTION
SHIELDED METAL ARC welding (SMAW) is an arc welding
process in which coalescence of metals is produced by heat
from an electric arc that is maintained between the tip of a
covered electrode and the surface of the base metal in the
joint being welded.
The core of the covered electrode consists of either a
solid metal rod of drawn or cast material or one fabricated
by encasing metal powders in a metallic sheath. The core
rod conducts the electric current to the arc and provides
filler metal for the joint. The primary functions of the elec
trode covering are to provide arc stability and to shield the
molten metal from the atmosphere with gases created as
the coating decomposes from the heat of the arc.
The shielding employed, along with other ingredients
in the covering and the core wire, largely controls the
mechanical properties, chemical composition, and metal
lurgical structure of the weld metal, as well as the arc char
acteristics of the electrode. The composition of the elec
trode covering varies according to the type of electrode.

PRINCIPLES OF OPERATION
SHIELDED METAL ARC welding is by far the most widely
used of the various arc welding processes. It employs the
heat of the arc to melt the base metal and the tip of a
consumable covered electrode. The electrode and the
work are part of an electric circuit illustrated in Figure 2.1.
This circuit begins with the electric power source and in
cludes the welding cables, an electrode holder, a work
piece connection, the workpiece (weldment), and an arc
welding electrode. One of the two cables from the power
source is attached to the work. The other is attached to the
electrode holder.

Welding commences when an electric arc is struck be

tween the tip of the electrode and the work. The intense
heat of the arc melts the tip of the electrode and the sur
face of the the work close to the arc. Tiny globules of mol
ten metal rapidly form on the tip of the electrode, then
transfer through the arc stream into the molten weld
pool. I In this manner, filler metal is deposited as the elec
trode is progressively consumed. The arc is moved over the
work at an appropriate arc length and travel speed, melting
and fusing a portion of the base metal and continuously
adding filler metal. Since the arc is one of the hottest of the
commercial sources of heat [temperatures above 9000F
(5000C) have been measured at its center], melting of the
base metal takes place almost instantaneously upon arc ini
tiation. If welds are made in either the flat or the horizon
tal position, metal transfer is induced by the force of grav
ity, gas expansion, electric and electromagnetic forces, and
surface tension. For welds in other positions, gravity works
against the other forces.
The process requires sufficient electric current to melt
both the electrode and a proper amount of base metal. It
also requires an appropriate gap between the tip of the
electrode and the base metal or the molten weld pool.
These requirements are necessary to set the stage for co
alescence. The sizes and types of electrodes for shielded
metal arc welding define the arc voltage requirements
(within the overall range of 16 to 40 V) and the amperage
requirements (within the overall range of 20 to 550 A). The
current may be either alternating or direct, depending
upon the electrode being used, but the power source must
be able to control the level of current within a reasonable
range in order to respond to the complex variables of the
welding process itself.
1. Metal transfer across the welding arc is described in Chapter 2, "Phys
ics of Welding," Welding Handbook, Vol. 1, 8th ed., pp. 5054.

S H I ELD E D

M E TA L

ARC

W ELDING

45

ELECTRODE HOLDER
AC OR DC POWER SOURCE
AND CONTROLS

WORKPIECE LEAD

WORK

ELECTRODE LEAD

Figure 2.1-Elements of a Typical Welding Circuit for Shielded Metal Arc Welding

Covered Electrodes
IN ADDITION TO establishing the arc and supplying filler
metal for the weld deposit, the electrode introduces other
materials into or around the arc, or both. Depending upon
the type of electrode being used, the covering performs
one or more of the following functions:
(1) Provides a gas to shield the arc and prevent
excessive atmospheric contamination of the molten filler
metal.
(2) Provides scavengers, deoxidizers, and fluxing agents
to cleanse the weld and prevent excessive grain growth in
the weld metal.
(3) Establishes the electrical characteristics of the
electrode.
(4) Provides a slag blanket to protect the hot weld metal
from the air and enhance the mechanical properties, bead
shape, and surface cleanliness of the weld metal.
(5) Provides a means of adding alloying elements to
change the mechanical properties of the weld metal.
Functions 1 and 4 prevent the pickup of oxygen and
nitrogen from the air by the molten filler metal in the arc
stream and by the weld metal as it solidifies and cools.
The covering on shielded metal arc electrodes is applied
by either the extrusion or the dipping process. Extrusion is
much more widely used. The dipping process is used pri
marily for cast and some fabricated core rods. In either
case, the covering contains most of the shielding, scaveng
ing, and deoxidizing materials. Most SMAW electrodes
have a solid metal core. Some are made with a fabricated
or composite core consisting of metal powders encased
in a

metallic sheath. In this latter case, the purpose of some or

even all of the metal powders is to produce an alloy weld
deposit.
In addition to improving the mechanical properties of
the weld metal, electrode coverings can be designed for
welding with alternating current (ac). With ac, the welding
arc goes out and is reestablished each time the current re
verses its direction. For good arc stability, it is necessary to
have a gas in the arc stream that will remain ionized during
each reversal of the current. This ionized gas makes possi
ble the reignition of the arc. Gases that readily ionize are
available from a variety of compounds, including those
that contain potassium. It is the incorporation of these
compounds in the electrode covering that enables the elec
trode to operate on ac.
To increase the deposition rate, the coverings of some
carbon and low alloy steel electrodes contain iron powder.
The iron powder is another source of metal available for
deposition, in addition to that obtained from the core of
the electrode. The presence of iron powder in the covering
also makes more efficient use of the arc energy. Metal
powders other than iron are frequently used to alter the
mechanical properties of the weld metal.
The thick coverings on electrodes with relatively large
amounts of iron powder increase the depth of the crucible
at the tip of the electrode. This deep crucible helps to con
tain the heat of the arc and permits the use of the drag
technique (described in the next paragraph) to maintain a
constant arc length. When iron or other metal powders are
added in relatively large amounts, the deposition rate and
welding speed usually increase.

46

SHI E LO E D

M E TA L

A RC

W E LDING

Iron powder electrodes with thick coverings reduce the

level of skill needed to weld. The tip of the electrode can
be dragged along the surface of the work while maintain
ing a welding arc. For this reason, heavy iron powder elec
trodes frequently are called drag electrodes. Deposition
rates are high, but, because slag solidification is slow, these
electrodes are not suitable for out-of-position use.

ELECTRODE COVERING

Arc Shielding
THE ARC SHIELDING action, illustrated in Figure 2.2, is es
sentially the same for all electrodes, but the specific
method of shielding and the volume of slag produced vary
from type to type. The bulk of the covering materials on
some electrodes is converted to gas by the heat of the arc,
and only a small amount of slag is produced. This type of
electrode depends largely upon a gaseous shield to prevent
atmospheric contamination. Weld metal from such elec
trodes can be identified by the incomplete or light layer of
slag which covers the bead.
For electrodes at the other extreme, the bulk of the cov
ering is converted to slag by the heat of the arc, and only a
small volume of shielding gas is produced. The tiny glob
ules of metal being transferred across the arc are entirely
coated with a thin film of molten slag. This molten slag
floats to the surface of the weld puddle because it is lighter
than the metal. The slag solidifies after the weld metal has
solidified. Welds made with these electrodes are identified
by the heavy slag deposits that completely cover the weld
beads. Between these extremes is a wide variety of elec
trode types, each with a different combination of gas and
slag shielding.
Variations in the amount of slag and gas shielding also
influence the welding characteristics of covered elec
trodes. Electrodes which produce a heavy slag can carry
high a.mperage and provide high deposition rates, making
them ideal for heavy weldments in the flat position. Elec
trodes which produce a light slag layer are used with lower
amperage and provide lower deposition rates. These elec
trodes produce a smaller weld pool and are suitable for
making welds in all positions. Because of the differences in
their welding characteristics, one type of covered elec
trode usually will be best suited for a given application.

--DIRECTION OF WELDING--..

Figure 2.2-Shielded Metal Arc Welding

(3) Auxiliary gas shielding or granular flux is not

required.
(4) The process is less sensitive to wind and draft than
gas shielded arc welding processes.
(5) It can be used in areas of limited access.
(6) The process is suitable for most of the commonly
used metals and alloys.

SMAW electrodes are available to weld carbon and low

alloy steels, stainless steels, cast irons, copper, and nickel
and their alloys, and for some aluminum applications. Low
melting metals, such as lead, tin, and zinc, and their alloys,
are not welded with SMAW because the intense heat of the
arc is too high for them. SMAW is not suitable for reactive
metals such as titanium, zirconium, tantalum, and colum
bium because the shielding provided is inadequate to pre
vent oxygen contamination of the weld.
Covered electrodes are produced in lengths of 9 to
18 in. (230 to 460 mm). As the arc is first struck, the
current flows the entire length of the electrode. The
amount of current that can be used, therefore, is limited
by the electrical resistance of the core wire. Excessive
amperage overheats the electrode and breaks down the
covering. This, in turn, changes the arc characteristics
and the shielding that is obtained. Because of this limita
PROCESS CAPABILITIES AND LIMITATIONS tion, deposition rates are generally lower than for a weld
SHIELDED METAL ARC welding is one of the most widely ing process such as GMAW.
Operator duty cycle and overall deposition rates for
use.cl processes, particularly for short welds in production,
maintenance and repair work, and for field construction. covered electrodes are usually less than provided with a
continuous electrode process such as FCAW. This is be
The following are advantages of this process:
cause electrodes can be consumed only to some certain
minimum
length. When that length has been reached, the
(1) The equipment is relatively simple, inexpensive, and
welder must discard the unconsumed electrode stub and
portable.
insert a new electrode into the holder. In addition, slag
Themetal
fillerfrom
metal,
and the
means of
protecting
it and
the(2)eld
harmful
oxidation
during
welding,
are usually must be removed at starts and stops and before
provided by the covered electrode.
depositing a weld bead next to or onto a previously depos
ited bead.

SHIELDED

EQUIPMENT
POWER SOURCES
Type of Output Current
EITHER ALTERNATING CURRENT (ac) or direct current (de)
may be employed for shielded metal arc welding, depend
ing upon the current supplied by the power source and the
electrode selected. The specific type of current employed
influences the performance of the electrode. Each current
type has its advantages and limitations, and these must be
considered when selecting the type of current for a specific
application. Factors which need to be considered are as
follows:
Voltage drop in the welding cables is
lower with ac. This makes ac more suitable if the welding
is to be done at long distances from the power supply.
How ever, long cables which carry ac should not be
coiled be cause the inductive losses encountered in such
cases can be substantial.

Voltage Drop.

Low Current. With small diameter electrodes and low

welding currents, de provides better operating characteris
tics and a more stable arc.

Striking the arc is generally easier with

de, particularly if small diameter electrodes are used. With
ac, the welding current passes through zero each half cycle,
and this presents problems for arc starting and arc stability.
Arc Starting.

Arc Length. Welding with a short arc length (low arc

voltage) is easier with de than with ac. This is an important
consideration, except for the heavy iron powder elec
trodes. With those electrodes, the deep crucible formed by
the heavy covering automatically maintains the proper arc
length when the electrode tip is dragged on the surface of
the joint.

Alternating current rarely presents a prob

lem with arc blow because the magnetic field is constantly
reversing (120 times per second). Arc blow can be a signifi
cant problem with de welding of ferritic steel because of
unbalanced magnetic fields around the arc.2

METAL

ARC

WELDING

47

Metal Thickness. Both sheet metal and heavy sec

tions can be welded using de. The welding of sheet metal
with ac is less desirable than with de. Arc conditions at low
current levels required for thin materials are less stable on
ac power than on de power.
Review of a welding application will generally indicate
whether alternating or direct current is most suitable.
Power sources are available as de, ac, or combination
ac/dc units. The power source for the SMAW process
must be a constant-current type rather than a constant
voltage type, because it is difficult for a welder to hold the
constant arc length required with constant-voltage power
sources.

Significance of the Volt-Ampere Curve

FIGURE 2.3 SHOWS typical volt-ampere output characteris
tics for both ac and de power sources. Constant-voltage
power sources are not suitable for SMAW because with
their flat volt-ampere curve, even a small change in arc
length (voltage) produces a relatively large change in am
perage. A constant-current power source is preferred for
manual welding, because the steeper the slope of the volt
ampere curve (within the welding range), the smaller the
change in current for a given change in arc voltage (arc
length).
For applications that involve large diameter electrodes
and high welding currents, a steep volt-ampere curve is
desirable.
Where more precise control of the size of the molten
pool is required (out-of-position welds and root passes of
joints with varying fit-up, for example), a flatter volt-am
pere curve is desirable. This enables the welder to change
the welding current within a specific range simply by
changing arc length. In this manner, the welder has some
control over the amount of filler metal that is being depos
ited. Figure 2.4 portrays these different volt-ampere curves
for a typical welding power source. Even though the differ
ence in the slope of the various curves is substantial, the
power source is still considered a constant-current power
source. The changes shown in the volt-ampere curve are
accomplished by adjusting both the open circuit voltage
(OCV) and the current settings on the power source.

Arc Blow.

Direct current is somewhat better

than ac for vertical and overhead welds because lower am
perage can be used. With suitable electrodes, however, sat
isfactory welds can be made in all positions with ac.
Welding Position.

2. The influence of magnetic fields on arcs and arc blow is discussed in

Chapter 2, "Physics of Welding," \Ve/ding Handbook, Vol.1, 8th ed., pp.

47-49.

Open Circuit Voltage

OPEN CIRCUIT VOLTAGE, WHICH is the voltage set on the
power source, does not refer to arc voltage. Arc voltage is
the voltage between the electrode and the work during
welding and is determined by arc length for any given elec
trode. Open circuit voltage, on the other hand, is the volt
age generated by the welding machine when no welding is
being done. Open circuit voltages generally run between

48

S H I E L DE D

META L

100

A RC

W E LDI NG

-- --

- - - CONSTANT CURRENT PERFORMANCE

'\.'........
\ '\ <,

80

en

CONSTANT VOLTAGE PERFORMANCE

60

<,

'"-\..

'

<,

""' -,

\ "......... . . . <, \

>

40

-,

\
0

<,

-,

<,

'\." <,

-,

\\ \ \\ x-,
\ -,

20

<,

-,

<,

-,

<,

-,

-,

\
\

100

200

300

400

500

\
600

CURRENT.A

Figure 2.3- Typical Volt-Ampere Curves for Constant Current and Constant Voltage
Power Sources
SO and 100 V, whereas arc voltages are between 17
and
40 V. The open circuit voltage drops to the arc voltage
when the arc is struck and the welding load comes on the
machine. The arc length and the type of electrode being
used determine just what this arc voltage will be. If the arc
is lengthened, the arc voltage will increase and the welding
current will decrease. The change in amperage which a
change in arc length produces is determined by the slope of
the volt-ampere curve within the welding range.
Some power sources do 'not provide for control of the
open circuit voltage because this control is not needed for
all welding processes. It is a useful feature for SMAW, yet it
is not necessary for all applications of the process.

Power Source Selection

SEVERAL FACTORS NEED to be considered when a power
source for SMAW is selected:
(1)
(2)
(3)
(4)

The
The
The
The

type of welding current required

amperage range required
positions in which welding will be done
primary power available at the work station

Selection of the type of current, ac, de, or both, will be

based largely on the types of electrodes to be used and the
kind of welds to be made. For ac, a transformer or an alter
nator type of power source may be used. For de, trans
former-rectifier or motor-generator power sources are
available. When both ac and de will be needed, a single
phase transformer-rectifier or an alternator-rectifier power
source may be used. Otherwise, two welding machines will
be required, one for ac and one for de.
The amperage requirements will be determined by the
sizes and types of electrodes to be used. When a variety
will be encountered, the power supply must be capable of
providing the amperage range needed. The duty cycle must
be adequare.I
The positions in which welding will be done should also
be considered. If vertical and overhead welding are
planned, adjustment of the slope of the V-A curve proba
bly will be desirable (see Figure 2.4). If so, the power supply
must provide this feature. This usually requires controls
for both the output voltage and the current.

3. See Chapter 1, pp. 14-15, for an explanation of duty cycle.

SHIE LDE D

A supply of primary power is needed. If line power is

available, it should be determined whether the power is
single-phase or three-phase. The welding power source
must be designed for either single- or three-phase power,
and it must be used with the one it was designed for. If line
power is not available, an engine-driven generator or' alter
nator must be used.

M E TA L

A RC

W E LDI NG

ACCESSORY EQUIPMENT
Electrode Holder
AN ELECTRODE HOLDER is a clamping device which allows

the welder to hold and control the electrode, as shown in

Figure 2.5. It also serves as a device for conducting the

100

MAXIMUM OCV

(!J

:.....i

50

32 --

LONG ARC -----

27 --NORMAL ARC LENGTH-+---,"-' --22 --SHORT ARC -----t---'"

100
CURRENT, A

49

-J

15

40

W.

200

_J

NOTE: LOWER SLOPE GIVES A GREATER CHANGE IN WELDING CURRENT

FOR A GIVEN CHANGE IN ARC VOLTAGE.

Figure 2.4- The Effect of Volt-Ampere Curve Slope on Current Output With a Change
in Arc Voltage

50

S H I E LDE D

M E TA L

A RC

W E LDI NG

Figure 2.5-Welding a Structure With the Shielded MetI Arc Welding Process

52

S H IE L DE D

M E TA L

A RC

W E LDI NG

SHIELDED

welding current from the welding cable to the electrode.

An insulated handle on the holder separates the welder's
hand from the welding circuit. The current is transferred
to the electrode through the jaws of the holder. To assure
minimum contact resistance and to avoid overheating of
the holder, the jaws must be kept in good condition. Over
heating of the holder not only makes it uncomfortable for
the welder, but also it can cause excessive voltage drop in
the welding circuit. Both can impair the welder's perfor
mance and reduce the quality of the weld.
The holder must grip the electrode securely and hold it
in position with good electrical contact. Installation of the
electrode must be quick and easy. The holder needs to be
light in weight and easy to handle, yet it must be
sturdy
enough to withstand rough use. Most holders have insulat
ing material around the jaws to prevent grounding of the
jaws to the work.
Electrode holders are produced in sizes to accommo
date a range of standard electrode diameters. Each size of
holder is designed to carry the current required for the
largest diameter electrode that it will hold. The smallest
size holder that can be used without overheating is the best
one for the job. It will be the lightest, and it will provide
the best operator comfort.

METAL

ARC

WELDING

51

Welding Cables
WELDING CABLES ARE used to connect the electrode
holder and the ground clamp to the power source. They
are part of the welding circuit (see Figure 2.1). The cable is
constructed for maximum flexibility to permit easy manip
ulation, particularly of the electrode holder. It also must
be wear and abrasion resistant.
Welding cable consists of many fine copper or alumi
num wires stranded together and enclosed in a flexible,
insulating jacket. The jacket is made of synthetic rubber or
of a plastic that has good toughness, high electrical resis
tance, and good heat resistance. A protective wrapping is
placed between the stranded conductor wires and the in
sulating jacket to permit some movement between them
and provide maximum flexibility.
Welding cable is produced in a range of sizes (from
aboutAWG 6 to 4/04). The size of the cable required fora
particular application depends on the maximum amperage
to be used for welding, the length of the welding circuit
(welding and work cables combined), and the duty cycle of
the welding machine. Table 2.1 shows the recommended
size of copper welding cable for various power sources and
circuit lengths. When aluminum cable is used, it should be
two AWG sizes larger than copper cable for the applica
tion. Cable sizes are increased as the length of the welding
circuit increases to keep the voltage drop and the attend
ant power loss in the cable at acceptable levels.
If long cables are necessary, short sections can be joined
by suitable cable connectors. The connectors must pro
vide good electrical contact with low resistance, and their
insulation must be equivalent to that of the cable. Lugs, at
the end of each cable, are used to connect the cables to the
power source. The connection between the cable and a
connector or lug must be strong with low electrical resis
tance. Soldered joints and mechanical connections are

Workpiece Connection
A WORKPIECE CONNECTION is a device for connecting the
workpiece lead to the workpiece. It should produce a
strong connection, yet be able to be attached quickly and
easily to the work. For light duty, a spring-loaded clamp
may be suitable. For high currents, however, a screw clamp
may be needed to provide a good connection without
overheating the clamp.

4. American wire gage sizes.

Table 2.1
Recommended Copper Welding Cable Sizes

Power Source
Duty Cycle, %
Size in Amperes
20

100

180
200
200
250
300
400
500
600

20-30
60
50
30
60
60
60
60

* Use two 3/0 cables in parallel.

Oto 50 ft
(0 to 15 m)
6

4
2
3
3
1/0
2/0
2/0
2/0

Awg Cable Size for Combined Length

of Electrode and Ground Cables
50 to 100 ft
100 to 150 ft
150 to 200 ft
(15 to 30 m)
(30 to 46 m)
(46 to 61 m)
4

4
2

3
2

2
1

1/0
2/0
2/0
2/0

1/0
2/0
3/0
3/0

2/0
3/0
3/0
4/0

200 to 250 ft
(61 to 76 m)
1
1

1/0
1/0
1/0
3/0
4/0
4/0
*

used. Aluminum cable requires a good mechanical connec

tion to avoid overheating. Oxidation of the aluminum sig
nificantly increases the electrical resistance of the connec
tion. This of course, can lead to overheating, excessive
power loss, and cable failure.
Care must be taken to avoid damage to the jacket of the
cable, particularly for the electrode cable, Contact with
hot metal or sharp edges may penetrate the jacket and
ground the cable.

Helmet
THE PURPOSE OF the helmet is to protect the welder's eyes,
face, forehead, neck, and ears from the direct rays of the
arc and from flying sparks and spatter. Some helmets have
an optional "flip lid" which permits the dark filter plate
over the opening in the shield to be flipped up so the
welder can see while the slag is being chipped from the
weld. This protects the welder's face and eyes from flying
slag. Slag can cause serious injury if it strikes a person, par
ticularly while it is hot. It can be harmful to the eyes
whether it is hot or cold.

Helmets are generally constructed of pressed fiber or

fiberglass insulating material. A helmet should be light in
weight and should be designed to give the welder the great
est possible comfort. The welder in Figure 2.5 has a helmet
on. The observer is using a hand shield.

Miscellaneous Equipment
CLEANLINESS IS IMPORTANT in welding. The surfaces of the
workpieces and the previously deposited weld metal must
be cleaned of dirt, slag, and any other foreign matter that
would interfere with welding. To accomplish this, the
welder should have a steel wire brush, a hammer, a chisel,
and a chipping hammer. These tools are used to remove
dirt and rust from the base metal, cut tack welds, and chip
slag from the weld bead.
The joint to be welded may require backing to support
the molten weld pool during deposition of the first layer of
weld metal. Backing strips or nonmetallic backing materi
als are sometimes used, particularly for joints which are
accessible from only one side.

MATERIALS
.BASE METALS
THE SMAW PROCESS is used in joining and surfacing appli
cations on a variety of base metals. The suitability of the
process for any specific base metal depends on the avail
ability of a covered electrode whose weld metal has the
required composition and properties. Electrodes are avail
able for the following base metals:
(1)
(2)
(3)
(4)
(5)
(6)
(7)

Carbon steels
Low alloy steels
Corrosion resisting steels
Cast irons (ductile and gray)
Aluminum and aluminum alloys
Copper and copper alloys
Nickel and nickel alloys

Electrodes are available for application of wear, impact,

or corrosion resistant surfaces to these same base metals.

COVERED ELECTRODES
COVERED ELECTRODES ARE classified according to the re
quirements of specifications issued by the American Weld
ing Society. Certain agencies of the Department of Defense
also issue specifications for covered electrodes. The AWS
specification numbers and' their electrode classifications

are given in Table 2.2. The electrodes are classified on the

basis of the chemical composition or mechanical proper
ties, or both, of their undiluted weld metal. Carbon steel,
low alloy steel, and stainless steel electrodes are also classi
fied according to the type of welding current they are
suited for and sometimes according to the positions of
welding that they can be used in.

Carbon Steel Electrodes

IN ANSI/ AWS A5.1, Specification for Covered
Carbon Steel Arc Welding Electrodes, a simple numbering

system is used for electrode classification. In 6010, for

example,

Table 2.2
AWS Specifications for Covered Electrodes
Type of Electrode

Carbon steel
Low alloy steel
Corrosion resistant steel
Cast iron
Aluminum and aluminum alloys
Copper and copper alloys
Nickel and nickel alloys
Surfacing

AWS Specification

A5.1
A5.5
A5.4
A5.15
A5.3
A5.6
A5.11
A5.13 and A5.21

54

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ARC

WELDING

the E designates an electrode. The first two digits (60) sig

nify the minimum tensile strength of the undiluted weld
metal in ksi, in the as-welded condition. The third digit
represents the welding position (1, in this case, refers to all
positions). The last digit refers to the covering type and
type of current with which the electrode can be used.
There are two strength levels of carbon steel electrodes:
the 60 series and the 70 series. The minimum allowable
tensile strength of the weld metal for the 60 series is 62 ksi
(427 MPa), although additional elongation may allow
some of these to go as low as 60 ksi (414 MPa). For the 70
series, it is 72 ksi (496 MPa) and, again, some of these may
go as low as 70 ksi (483 MPa), with additional elongation.
Maximum chemical composition limits for significant ele
ments are provided within the applicable AWS specifica
tions for most electrode classifications. Charpy V-notch
iml?act requirements are given for some electrodes in both
series.
Certain of the carbon steel electrodes are designed to
operate only on de. Others are for either ac or de. Polarity
on de usually is reverse (electrode positive), although a few
of the electrodes are intended for straight polarity. Some
of these may be used with either polarity.
Most electrodes are designed for welding in all posi
tions. However, those which contain large amounts of
iron powder or iron oxide in the coatings are generally
restricted to groove welds in the flat position and horizon
tal fillet welds. The coverings on these electrodes are very
heavy, which precludes their operation in the vertical and
overhead positions.
Several electrodes of the 70 series are low hydrogen
type. Their coatings are formulated with ingredients that
are low in moisture and cellulose and, hence, in hydrogen
content. Hydrogen is responsible for low ductility and for
underbead cracking sometimes encountered in highly re
strained welds. For this reason, low hydrogen electrodes
are used to weld hardenable steels. They are also used for
high sulfur steels and to provide weld metal having good
low temperature notch toughness.
The specification does not set a limit on the moisture
content of these electrodes, but less than 0.6 percent is
recommended. To control moisture, proper storage and
handling are required. Typical storage and baking condi
tions are given in ANSI/AWS AS.l.

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53

metal strength levels range from 70 to 120 ksi (480 to 830

MPa) minimum tensile strength, in 10 ksi (70 MPa) incre
ments. In this specification, weld metals that are com
monly used in the as-welded condition are classified on the
basis of their properties in that condition. Similarly, those
that are commonly used in the stress-relieved condition
are classified on the basis of their properties after a stress
relief heat treatment.
In this connection, it should be noted that the stress
relief called for in ANSI/AWS AS.S consists of holding
the test assembly at
temperature
for one hour.
Fabricators using holding times that are significantly
different from one hour at temperature may have to be
more selective in the elec trodes they use, and may be
required to run tests to demon strate that the selected weld
metal mechanical properties will be adequate after a
specific heat treatment temperature and holding time.
Radiographic quality standards for depos ited weld metal
and notch toughness requirements are in cluded for many
SMAW electrode classifications.
The military specifications for low alloy steel electrodes
sometimes use designations that are similar to those in the
AWS specification. Also, some electrodes are produced
that are not classified in AWS specifications, but which are
designed for specific materials or which broadly match
standard AISI low alloy steel base metal compositions,
such as 4130. The AS.5 specification sets limits on the
moisture content of low hydrogen electrodes packaged in
hermetically sealed containers. These limits range from 0.2
percent to 0.6 percent by weight, depending on the classifi
cation of the electrode. The higher the strength level, the
lower the limit on the moisture content. The reason for
this is that moisture is a primary source of hydrogen, and
hydrogen can produce cracking in most low alloy steels,
unless high preheats and long, slow cooling cycles are em
ployed. The higher the strength of the weld and the base
metal, the greater the need for low moisture levels to avoid
cracking. Exposure to high humidity (in the range of 70
percent relative humidity or higher) may increase the mois
ture content of the electrode in only a few hours.

Corrosion Resisting Steel Electrodes

COVERED ELECTRODES FOR welding corrosion resisting
steels are classified in ANSI/AWS AS .4, Specification for

Covered Corrosion-Resisting Chromium and Chromium

Nickel Steel Welding Electrodes. Classification in this speci
fication is based on the chemical composition of the undi
ANSI/AWS AS.S, Specification for Low Alloy Steel Cov luted weld metal, the positions of welding, and the type of
ered Arc Welding Electrodes, classifies low alloy steel cov welding current for which the electrodes are suitable. The
ered electrodes according to a numbering system which is classification system is similar to the one used for carbon
similar to that just described for carbon steel electrodes. It and low alloy steel electrodes. Taking E310-15 and E310uses, in addition, a suffix such as Al to designate the chem 16 as examples, the prefix E indicates an electrode.
ical composition (alloy system) of the weld metal. Thus, a The
complete electrode classification is E7010-Al. Another is first three digits refer to the alloy type (with respect to
E8016-C2. Alloy systems into which the electrodes fall are chemical composition). They may be followed by a letter
carbon-molybdenum steel, chromium-molybdenum steel, or letters to indicate modification, such as E310Mo-15.
nickel steel, and manganese-molybdenum steel. Weld The last two digits refer to the position of welding and the
type of current for which the electrodes are suitable. The

Low Alloy Steel Electrodes

-1 indicates that the electrodes are usable in all positions

through 5/32 in. (4 mm) diameter. The number 5 indicates
that the electrodes are suitable for use with dcrp (electrode
positive). The number 6 means that electrodes are suitable
for either ac or dcrp (electrode positive). Electrodes over
5 /32 in. (4 mm) diameter are for use in the flat and
hori
zontal positions.
The specification does not describe the covering ingre
dients, but -15 coverings usually contain a large proportion
of limestone (calcium 'carbonate). This ingredient provides
the CO and C02 that are used to shield the arc. The binder
which holds the ingredients together in this case is sodium
silicate. The -16 covering also contains limestone for arc
shielding. In addition, it usually contains considerable tita
nia (titanium dioxide) for arc stability. The binder in this
case is likely to be potassium silicate.
Differences in the proportion of these ingredients result
in differences in arc characteristics. The -15 electrodes
(lime type coverings) tend to provide a more penetrating
arc and to produce a more convex and coarsely rippled
bead. The slag solidifies relatively rapidly so that these
electrodes often are preferred for out of position work,
such as pipe welding. On the other hand, the -16 coverings
(titania type) produce a smoother arc, less spatter, and a
more uniform; finely rippled bead. The slag, however, is
more fluid, and the electrode usually is more difficult to
handle in out-of-position work.
Stainless steels can be separated into three basic types:
austenitic, martensitic, and ferritic. The austenitic group
(2XX and 3XX) is, by far, the largest one. Normally, the
composition of the weld metal from a stainless steel elec
trode is similar to the base metal composition that the
electrode is designed to weld.
For austenitic stainless steels, the composition of weld
metal differs from the base metal compositions slightly in
order to produce a weld deposit which contains ferrite
(i.e., which is not fully austenitic) to prevent fissuring
or hot cracking of the weld metal. The amount of
ferrite common to the various welding electrodes is
discussed in ANSI/ AWS A5.4 in some detail. In
general, a minimum ferrite content in the range of 3 to 5
ferrite number is suffi cient to prevent cracking. Ferrite
content as high as 20 FN may be acceptable for some
welds when no postweld heat treatment is employed. The
Schaeffler diagram, or the De long modification of a
portion of that diagram, can be used to predict the
ferrite content of stainless steel weld
metals. Magnetic instruments are available to make direct
measurements of the ferrite content of deposited weld
metal. (See ANSI/AWS A4.2, Standard Procedures for Cal

Appropriate welding procedures can also be used to re

duce fissuring and cracking. Low amperage, for example, is
beneficial. Some small amount of weaving, as a means of
promoting cellular grain growth, may be helpful. Proper
procedures in terminating the arc should be used to avoid
crater cracks.
ANSI/AWS AS .4 contains two covered electrode classi
fications for the straight chromium stainless steels
(4XX
series). One contains 11 to 13.5 percent chromium, the
other, 15 to 18 percent. The carbon content of both is 0.1
percent maximum. Both weld metals are air hardening and
their weldments require preheat and postheat treatment to
provide the ductility which is necessary in most engineer
ing applications.
The specification also contains three electrode classifi
cations that are used to weld the four to ten percent chro
mium-molybdenum steels. These materials, too, are air
hardening, and preheat and postheat treatment are re
quired for sound, serviceable joints.

Nickel and Nickel Alloy Electrodes

COVERED ELECTRODES FOR shielded metal arc welding of
nickel and its alloys have compositions which are generally
similar to those of the base metals they are used to join,
and some have additions of elements such as titanium,
manganese, and columbium to deoxidize the weld metal
and thereby prevent cracking.
ANSI/AWS A5.ll, Specification for Nickel and Nickel
Alloy Covered Welding Electrodes, classifies the electrodes
in
groups according
to their principal alloying
elements.
The letter "E" at the beginning indicates an electrode, and
the chemical symbol "Ni" identifies the weld metals as
nickel base alloys. Other chemical symbols are added to
show the principal alloying elements. Then, successive
numbers are added to identify each classification within its
group. ENiCrFe-1, for example, contains significant chro
mium and iron in addition to nickel.
Most of the electrodes are intended for use with dcrp
(electrode positive). Some are also capable of operating
with ac, to overcome problems that may be encountered
with arc blow (when nine percent nickel steel is
welded, for example). Most electrodes can be used in all
positions, but best results for out-of-position welding are
achieved using electrodes of 1/8 in. (3.2 mm) diameter
and smaller.
The electrical resistivity of the core wire in these elec
trodes is exceptionally high. For this reason, excessive am
perage will overheat the electrode and damage the cover
ing, causing arc instability and unacceptable amounts of
spatter. Each classification and size of electrode has an op
timum amperage range.

ibrating Magnetic Instruments to Measure Delta Ferrite

Content of Austenitic Stainless Steel Weld Metal.)
Certain austenitic stainless steel weld metals (Types 310,
320, and 330, for example) do not form ferrite because
their nickel content is too high. For these materials, the
phosphorus, sulfur, and silicon content of the weld metal Aluminum and Aluminum Alloy Electrodes
is limited or the carbon content increased as a means of ANSI/AWS AS.3, Specification for Aluminum and
minimizing fissuring and cracking.
Alumi num Alloy Electrodes for Shielded Metal Arc
Welding, con tains two classifications of covered
electrodes for the

welding of aluminum base metals. These classifications are

based on the mechanical properties of the weld metal in
the as-welded condition and the chemical composition of
the core wire. One core wire is commercially pure alumi
num (1100), and the other is an aluminum-five percent sili
con alloy (4043). Both electrodes are used with dcrp (elec
trode positive).
The covering on these electrodes has three functions. It
provides a gas to shield the arc, a flux to dissolve the
alumi num oxide, and a protective slag to cover the weld
bead. Because the slag can be very corrosive to aluminum,
it is important that all of it be removed upon completion
of the weld.
The presence of moisture in the covering of these elec
trodes is a major source of porosity in the weld metal. To
avoid this porosity, the electrodes should be stored in a
heated cabinet until they are to be used. Those electrodes
that have been exposed to moisture should be recondi
tioned (baked) before use, or discarded.
One difficulty which may occur in welding is the fusing
of slag over the end of the electrode if the arc is broken. In
order to restrike the arc, this fused slag must be removed.
Covered aluminum electrodes are used primarily for
noncritical welding and repair applications. They should
be used only on base metals for which either the 1100 or
4043 filler metals are recommended. These weld metals do
not respond to precipitation hardening heat treatments. If
they are used for such material, each application should be
carefully evaluated.

Copper and Copper Alloy Electrodes

ANSI/AWS AS.6, Specification for Covered Copper and
Copper Alloy Arc Welding Electrodes, classifies copper and
copper alloy electrodes on the basis of their all-weld-metal
properties and the chemical composition of their undi
luted weld metal. The designation system is similar to that
used for nickel alloy electrodes. The major difference is
that each individual classification within a group is identi
fied by a letter. This letter is sometimes followed by a num
ber, as in ECuAl-A2, for example. The groups are: CuSi for
silicon bronze, CuSn for phosphor bronze, CuNi for cop
per-nickel, and CuAl for aluminum bronze. These elec
trodes, generally, are used with dcrp (electrode positive).
Copper electrodes are used to weld unalloyed copper
and to repair copper cladding on steel or cast iron. Silicon
bronze electrodes are used to weld copper-zinc alloys,
copper, and some iron base materials.' They are also used
for surfacing to provide corrosion resistance.
Phosphor bronze and brass base metals are welded with
phosphor bronze electrodes. These electrodes are also
used to braze weld copper alloys to steel and cast iron. The
phosphor bronzes are rather viscous when molten, but
their fluidity is improved by preheating to about 400F
(200C). The electrodes and the work must be dry.
Copper-nickel electrodes are used to weld a wide
range of copper-nickel alloys and also copper-nickel

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cladding on 5555
steel. In general, no preheat is necessary for
these materials.
Aluminum bronze electrodes have broad use for weld
ing copper base alloys and some dissimilar metal combina
tions. They are used to braze weld many ferrous metals and
to apply wear and corrosion resistant bearing surfaces.
Welding is usually done in the flat position with some
preheat.

Electrodes for Cast Iron

ANSI/AWS AS.15, Specification for Welding Electrodes
and Rods for Cast Iron, classifies covered electrodes for
welding cast iron. The electrodes classified in AS.15 are
nickel, nickel-iron, nickel-copper alloys, and one steel al
loy. Preheat is recommended when welding iron castings,
particularly if the steel electrode is used. The specific tem
perature depends on the size and complexity of the casting
and the machinability requirements. Small pits and cracks
can be welded without preheat, but the weld will not be
machinable. Welding is done with low amperage dcrp
(electrode positive) to minimize dilution with the base
metal. Preheating is not used in this case, except to mini
mize the residual stresses in other parts of the casting.
Proprietary nickel and nickel alloy electrodes may also
be used to repair castings and to join the various types of
cast iron to themselves and to other metals. The hardness
of the weld metal depends on the amount of base metal
dilution.
Phosphor bronze and aluminum bronze electrodes are
used to braze weld cast iron. The melting temperature of
their weld metals is below that of cast iron. The casting
should be preheated to about 400F (200C), and welding
should be done with dcrp (electrode positive), using the
lowest amperage that will produce good bonding between
the weld metal and groove faces. The cast iron surfaces
should not be melted.

Surfacing Electrodes
MOST HARD SURFACING electrodes are designed to meet
ANSI/AWS AS.13, Specification for Solid Surfacing Weld
ing Rods and Electrodes, or ANSI/ AWS AS.21, Specifica

tion for Composit Surfacing Welding Rods and Electrodes.

A wide range of SMAW electrodes is available (under these
and other AWS filler metal specifications) to provide wear,
impact, heat, or corrosion resistant layers on a variety of
base metals. All of the covered electrodes specified in
AS.13 have a solid core wire. Those specified in AS.21
have a composite core. The electrode designation system
in both specifications is similar to that used for copper
alloy electrodes with the exception of tungsten carbide
electrodes. The E in the designation for these electrodes is
followed by WC. The mesh size limits for the tungsten
carbide granules in the core follow these to complete the
designation. The core, in this case, consists of a steel tube
filled with the tungsten carbide granules.

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Surfacing with covered electrodes is used

buttering, buildup, and hard surfacing. The
in these applications is intended to provide
of the following for the surfaces to which
plied:

for cladding,
weld deposit
one or more
they are ap

(1) Corrosion resistance

Metallurgical control
Dimensional control
Wear resistance
Impact resistance

(2)
(3)
(4)
(5)

Covered electrodes for a particular surfacing application

should be selected after a careful review of the required
properties of the weld metal when it is applied to a specific
base metal.

Electrode Conditioning
SMAW ELECTRODE COVERINGS are hygroscopic (they
readily absorb and retain moisture). Some coverings are
more hygroscopic than others. The moisture they pick up
on exposure to a humid atmosphere dissociates to form
hydrogen and oxygen during welding. The atoms of hy
drogen dissolve in the weld and the heat-affected zone and
may cause cold cracking. This type of crack is more preva
lent in hardenable steel base metals and high strength steel

weld metals. Excessive moisture in electrode coverings can

cause porosity in the deposited weld metal.
To minimize moisture problems, particularly for low hy
drogen electrodes, they must be properly packaged,
stored, and handled. Such control is critical for electrodes
which are to be used to weld hardenable base metals. Con
trol of moisture becomes increasingly important as the
strength of the weld metal or the base metal increases.
Holding ovens are used for low hydrogen electrodes once
those electrodes have been removed from their sealed con
tainer and have not been used within a certain period of
time. This period varies from as little as half an hour to as
much as eight hours depending on the strength of the elec
trode, the humidity during exposure, and even the specific
covering on the electrode. The time which an electrode
can be kept out of an oven or rod warmer is reduced as the
humidity increases.
The temperature of the holding oven should be within
e range of 150 to 300F (65 to 150C). Electrodes
that
have been exposed too long require baking at a substan
tially higher temperature to drive off the absorbed mois
ture. The specific recommendations of the manufacturer
of the electrode need to be followed because the time and
temperature limitations can vary from manufacturer to
manufacturer, even for electrodes within a given classifica
tion. Excessive heating can damage the covering on an
electrode.

APPLICATIONS
MATERIALS
THE SMAW PROCESS can be used to join most of the com
mon metals and alloys. The list includes the carbon steels,
the low alloy steels, the stainless steels, and cast iron, as
well as copper, nickel, and aluminum and their alloys.
Shielded metal arc welding is also used to join a wide range
of chemically dissimilar materials.
The process is not used for materials for which shielding
of the arc by the gaseous products of an electrode covering
is unsatisfactory. The reactive (Ti, Zr) and refractory (Cb,
Ta, Mo) metals fall into this group.

THICKNESSES
THE SHIELDED METAL arc process is adaptable to any mate
rial thickness within certain practical and economic limita
tions. For material thicknesses less than about 1/16 in.
(1.6 mm), the base metal will melt through and the molten
metal will fall away before a common weld pool can be
established, unless special fixturing and welding proce
dures are employed. There is no upper limit on thickness,
but other processes such as SAW or FCAW are capable of

providing higher deposition rates and economies for most

applications involving thicknesses exceeding 1-1/2 in.
(38 mm). Most of the SMAW applications are on thick
nesses between 1/8 and 1-1/2 in (3 and 38 mm), except
where irregular configurations are encountered. Such con
figurations put an automated welding process at an eco
nomic disadvantage. In such instances, the shielded metal
arc process is commonly used to weld materials as thick as
10 in. (250 mm).

POSITION OF WELDING
ONE OF THE major advantages of SMAW is that welding
can be done in any position on most of the materials for
which the process is suitable. This makes the process use
ful on joints that cannot be placed in the flat position.
Despite this advantage, welding should be done in the flat
position whenever practical because less skill is required,
and larger electrodes with correspondingly higher deposi
tion rates can be used. Welds in the vertical and overhead
positions require more skill on the welder's part and are
performed using smaller diameter electrodes. Joint designs

for vertical and overhead welding may be different from

those suitable for flat position welding.

LOCATION OF WELDING
THE SIMPLICITY OF the equipment makes SMAW an
ex tremely versatile process with respect to the location
and environment of the operation. Welding can be
done in-

SHIELDED
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WELDING

doors or outdoors, on a production line, a ship, a bridge, a

building framework, an oil refinery, a cross-country pipe
line, or any such types of work. No gas or water hoses are
needed and the welding cables can be extended quite some
distance from the power source. In remote areas, gasoline
or diesel powered units can be used. Despite this versatil
ity, the process should always be used in an environment
which shelters it from the wind, rain, and snow.

JOINT DESIGN AND PREPARATION

TYPES OF WELDS
WELDED JOINTS ARE designed primarily on the basis of the
strength and safety required of the weldment under
the service conditions imposed on it. The manner in which
the service stresses will be applied and the temperature of
the weldment in service must always be considered. A
joint required for dynamic loading may be quite different
from one permitted in static loading.
Dynamic loading requires consideration of fatigue
strength and resistance to brittle fracture. These, among
other things, require that the joints be designed to reduce
or eliminate points of stress concentration. The design
should also balance the residual stresses and obtain as low
a residual stress level as possible. The weld must produce
adequate joint strength.
In addition to service requirements, weld joints need to
be designed to provide economy and accessibility for the
welder during fabrication. Joint accessibility can improve
the ability of the welder to meet good workmanship and
quality requirements, and can assist in control of distor
tion and reduction of welding costs. The effect of joint
design on some of these considerations is discussed below.

Groove Welds
GROOVE WELD JOINT designs of different types are used.
Selection of the most appropriate design for a specific ap
plication is influenced by the following:
(1)
(2)
(3)
(4)

Suitability for the structure under consideration

Accessibility to the joint for welding
Cost of welding
Position in which welding is to be done

A square groove is the most economical to prepare. It

only requires squaring-off of the edge of each member.
This type of joint is limited to those thicknesses with
which satisfactory strength and soundness can be ob
tained. For SMAW, that thickness is usually not greater
than about 1 /4 in. (6 mm) and then only when the joint is

to be welded in the flat position from both sides. The type

of material to be welded is also a consideration.
When thicker members are to be welded, the edge of
each member must be prepared to a contour that will per
mit the arc to be directed to the point where the weld
metal must be deposited. This is necessary to provide fu
sion to whatever depth is required.
For economy as well as to reduce distortion and residual
stresses, the joint design should have a root opening and a
groove angle that will provide adequate strength and
soundness with the deposition of the least amount of filler
metal. The key to soundness is accessibility to the root and
sidewalls of the joint. ]-groove and U-groove joints are de
sirable for thick sections. In very thick sections, the savings
in filler metal and welding time alone are sufficient to more
than offset the added cost of this joint preparation. The
angle of the sidewalls must be large enough to prevent slag
entrapment.

Fillet Welds
WHERE THE SERVICE requirements of the weldment per
mit, fillet welds frequently are used in preference to groove
welds. Fillet welds require little or no joint preparation,
although groove welds sometimes require less welding. In
termittent fillet welding may be used when a continuous
weld would provide more strength than is required to
carry the load.
A fillet weld is often combined with a groove weld to
provide the required strength and reduce the stress con
centration at the joint. Minimum stress concentration at
the toes of the weld is obtained with concave fillets.

WELD BACKING
WHEN FULL PENETRATION welds are required and welding
is done from one side of the joint, weld backing may be
required. Its purpose is to provide something on which to
deposit the first layer of metal and thereby prevent the
molten metal in that layer from escaping through the root
of the joint.

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Four types of backing are commonly used:

(1) Backing strip
(2) Backing weld
(3) Copper backing bar
(4) Nonmetallic backing

, ,_., P"
(A) BACKING STRIP

Backing Strip
A BACKING STRIP is a strip of metal placed on the back of
the joint, as shown in Figure 2.6(A). The first weld pass ties
both members of the joint together and to the backing
strip. The strip may be left in place if it will not interfere
with the serviceability of the joint. Otherwise, it should be
removed, in which case the back side of the joint must be
accessible. If the back side is not accessible, some other
means of obtaining a proper root pass must be used.
The backing strip must always be made of a material
that is metallurgically compatible with the base metal and
the welding electrode to be used. Where design permits,
another member of the structure may serve as backing for
the weld. Figure 2.6(B) provides an example of this. In all
cases, it is important that the backing strip as well as the
surfaces of the joint be clean to avoid porosity and slag
inclusions in the weld. It is also important that the backing
strip fit properly. Otherwise, the molten weld metal can
run out through any gap between the strip and the base
metal at the root of the joint.

Copper Backing Bar

A COPPER BAR is sometimes used as a means of supporting
the molten weld pool at the root of the joint. Copper is
. used because of its high thermal conductivity. This high
conductivity helps prevent the weld metal from fusing to
the backing bar. Despite this, the copper bar must have
sufficient mass to avoid melting during deposition of the
first weld pass. In high production use, water can be passed
through holes in the bar to remove the heat that accumu
lates during continuous welding. Regardless of the method
of cooling, the arc should not be allowed to impinge on the
copper bar, for if any copper melts, the weld metal can
become contaminated with copper. The copper bar may
be grooved to provide the desired root surface contour
and reinforcement.

(8) STRUCTURE
BACKING

Figure 2.6-Fusible Met.al Backing for a Weld

Refractory type backing consists of a flexible, shaped
form that is held on the back side of the joint by clamps or
by pressure sensitive tape. This type of backing is some
times used with the SMAW process, although special weld
ing techniques are required to consistently produce good
results. The recommendations of the manufacturer of the
backing should be followed.

Backing Weld
A BACKING WELD is one or more backing passes in a single
groove weld joint. This weld is deposited on the back side
of the joint before the first pass is deposited on the face
side. The concept is illustrated in Figure 2.7. After the
backing weld, all subsequent passes are made in the groove
from the face side. The root of the joint may be ground or
gouged after the backing weld is made to produce sound,
clean metal on which to deposit the first pass on the face
side of the joint.
The backing weld can be made with the same process or
with a different process from that to be used for welding
the groove. If the same process is used, the electrodes
should be of the same classification as those to be used for
welding the groove. If a different process such as gas tung
sten arc welding is used, the welding rods should deposit
weld metal having composition and properties similar to

GROOVE WELD MADE

AFTER WELDING OTHER SIDE

Nonmetallic Backing
NONMETALLIC BACKING OF either granular flux or refrac
tory material is also a method that is used to produce a
sound first pass. The flux is used primarily to support the
weld metal and to shape the root surface. A granular flux
layer is supported against the back side of the weld by
some method such as a pressurized fire hose. A system of
this type is generally used for production line work, al
though it is not widely used for SMAW.

BACK WELD

Figure 2.7-A Typical Backing Weld

60

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W ELDING

59

uniform. This can cause problems when the finished

those of the SMAW weld metal. The backing weld must dimensions have been predicated on the basis of uniform,
be large enough to support any load that is placed on it. controlled shrinkage.
This is especially important when the weldment must be
Misalignment along the root of the weld may cause lack
reposi tioned after the backing weld has been deposited of penetration in some areas or poor root surface contour,
and be fore the groove weld is made.
or both. Inadequate root opening can cause lack of com
plete joint penetration. Too wide a root opening makes
welding difficult and requires more weld metal to fill the
FIT-UP
joint. This, of course, also involves additional cost. In thin
JOINT FIT-UP INVOLVES the positioning of the members of members, an excessive root opening may cause excessive
the joint to provide the specified groove dimensions and melt-through on the back side. It may even cause the edge
alignment. The points of concern are the root opening and of one or both members to melt away.
the alignment of the members along the root of the weld.
Both of these have an important influence on the quality of
the weld and the economics of the process. After the joint TYPICAL JOINT GEOMETRIES
has been properly aligned throughout its length, the posi THE WELD GROOVES shown in Figure 2.8 illustrate typical
tion of the members should be maintained by clamps or designs and dimensions of joints for shielded metal arc
tack welds. Finger bars or U-shaped bridges can be placed welding of steel. These joints are generally suitable for eco
across the joint and tack welded to each of the members. nomically achieving sound welds. Other joint designs or
If the root opening is not uniform, the amount of weld changes in suggested dimensions may be required for spe
metal will vary from location to location along the joint. cial applications.
As a result, shrinkage, and, hence, distortion, will not be

\xi

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l
c::=:::::Jc::=:::::JL T .Jc:::=::jLJ
T-it-

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DIM. T
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1/8 TO 1/4

1-R

DIM. R

DIM. R

POSITIONS

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20
12

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1/2
1/2

ALL
F,V,O
F

JOINT RECOMMENDED FOR

HORIZONTAL POSITION

SINGLE V-GROOVE JOINTS WELDED FROM ONE SIDE WITH BACKING

T/2 MAX
60 1/16MAX
MIN..L_

SQUARE GROOVE JOINTS WELDED FROM BOTH SIDES

-j t-TMIN
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JOINT RECOMMENDED FOR

HORIZONTAL POSITION
DOUBLE V-GROOVE JOINTS WELDED FROM BOTH SIDES

SQUARE GROOVE JOINTS WELDED FROM ONE SIDE WITH BACKING

1/8 MAX
1/16 MAX
1 /16 TO 1/8

LJ'

/'\.
60 -,
1/8 MAX

1/4
MIN

IN

450A.._1/16

,Q I jl"

i{t1/4

10 TO 15
JOINT RECOMMENDED FOR
HORIZONTAL POSITION

SINGLE V-GROOVE JOINTS WELDED FROM ONE OR BOTH SIDES

450
rMIN

60
\Ml

114

10 TO 15

T 3/16 MAX

60
\MIN/

45
MIN
1/4

MIN
5 TO 10

TO
1/4

1/16 MAX

,JC.i1
1/16 MAX

45
IN'

45
rMIN

b]

1/8 TO 1/4

1/16
MAX

l/8JL
TO
1/4

3/16 MIN

SINGLE BEVEL-GROOVE JOINTS WELDED FROM ONE OR BOTH SIDES

ALL DIMENSIONS IN INCHES EXCEPT ANGLES

(Al

Figure 2.8-Typical Joint Geometries for Shielded Metal Arc Welding of Steel

rx)'

\1:4/

;wDIM. R

POSITIONS

1/4
3/8
3/8

ALL
ALL
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ANGLE X
45

krJ

POSITIONS
ALL
F,V,O

3/8

20
12

j'c:Jl

45f"':r1 /16 TO 3/8

M

IN

L ?

1/4t1.L

T3/16MAX
5 TO 10
JOINT RECOMMENDED FOR
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SINGLE U-GROOVE JOINTS WELDED FROM ONE OR BOTH SIDES

45 T1/16 TO 3/8
t:v; 1/16 TOMIN3/811;40/

l<

Ill\

DIM. R
POSITIONS

ANGLE X

t:1

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!
...Jl-3/16 MAX
45
35

ALL
ALL

1/4
3/8

SINGLE BEVEL-GROOVE JOINTS WELDED FROM

ONE SIDE WITH BACKING

JD
451
MIN

45o -,

MAX

POSITIONS

45
20

ALL
F,V,O

1/16 MAX

45
'<MIN I

1 /8
1/4

l. c:=:=:::==i, 1 /16
MAX
l--118 TO 1/4
1/8 TO 1/J

DOUBLE BEVEL-GROOVE JOINTS WELDED FROM BOTH SIDES

(B)

R=l/2

16

3L{:f-\-=-1-/2
1/16T03/16

3/16 MAX
ANGLE

36 MIN
25

10
JOINT RECOMMENDED FOR
HORIZONTAL POSITION

DOUBLE U-GROOVE JOINTS WELDED FROM BOTH SIDES

f1
MIN

1/

TO

ANGLE X

MIN

.
3/16

c==;J;"f,6
MAX

POSITIONS
ALL
F,V,O

1116
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3116
3116

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DOUBLE J-GROOVE JOINTS WELDED FROM BOTH SIDES

DIM. T

DIAM

UNDER 1/8
1/8 TO 1/2
OVER 1/2

1/4 MIN
2TMIN
T+ 1/2

JOINT FOR PLUG WELD

DIM. T

DIM.W

UNDER 1/8
1/8AND OVER

2 TMIN
1-1/2 T MIN

JOINT FOR SLOT WELD

ALL DIMENSIONS IN INCHES EXCEPT ANGLES

(Cl

Figure 2.8-(Continued)- Typical Joint Geometries for Shielded Metal Arc Welding of Steel

62

SH IE LDE D

M E TA L

A RC

WELD ING

RUNOFF TABS

SH IE L DE D

MET A L

ARC

WELDING

61

IN SOME APPLICATIONS, it is necessary to completely fill out

the groove right to the very ends of the joint. In such cases,
runoff tabs are used. They, in effect, extend the groove
beyond the ends of the members to be welded. The weld is
carried over into the tabs. This assures that the entire
length of the joint is filled to the necessary depth with
sound weld metal. A typical runoff tab is shown in Figure
2.9. Runoff tabs are excellent appendages on which to
start and stop welding. Any defects in these starts and
stops are located in areas that later will be discarded.
RUN-OFF TAB
Selection of the material for runoff tabs is important.
The composition of the tabs should not be allowed to ad
versely affect the properties of the weld metal. For exam
Figure 2.9-Runoff Tab at End of a Weld Joint
ple, for stainless steel which is intended for corrosion ser
vice, the runoff tabs should be of a compatible grade of
stainless steel. Carbon steel tabs would be less costly, but
fusion with the stainless steel filler metal would change the welder's discomfort increases with higher preheats and
composition of the weld metal at the junction of the car tends to reduce the quality of the work. Preheat tempera
bon steel tab and the stainless steel members of the joint. tures employed would be based upon welding code re
The weld metal at this location probably would not have quirements, competent technical evaluation, or the results
adequate corrosion resistance.
of tests. In general, the temperature will depend on the
material to be welded, the electrodes to be used, and the
degree of restraint in the joint.
PREHEATING
Hardenable steels, high-strength steels, and use of elec
HEATING THE AREA to be welded before and during weld trodes other than low hydrogen types normally require ap
ing is required in order to achieve desired properties in the plication of preheat.
Preheat is sometimes used when welding materials hav
weld or the adjacent base metal, or both. Unnecessary pre
heat should be avoided as it takes time and energy. Exces ing high thermal conductivity, such as copper and alumi
sive preheat temperatures are not cost effective and could num alloys, to reduce the welding amperage required, im
degrade the properties and the quality of the joint. A prove penetration, and aid in fusing the weld metal to the
base metal.

WELDING PROCEDURES
ELECTRODE DIAMETER
THE CORRECT ELECTRODE diameter is one that, when used
with the proper amperage and travel speed, produces a
weld of the required size in the least amount of. time.
The electrode diameter selected for use depends largely
on the thickness of the material to be welded, the position
in which welding is to be performed, and the type of joint
to be welded. In general, larger electrodes will be selected
for applications involving thicker materials and for weld
ing in the flat position in order to take advantage of their
higher deposition rates.
For welding in the horizontal, vertical and overhead po
sitions, the molten weld metal tends to flow out of the
joint due to gravitational forces. This tendency can be con
trolled by using small electrodes to reduce the weld pool
size. Electrode manipulation and increased travel speed
along the joint also aid in controlling weld pool size.
Weld groove design must also be considered when elec
trode size is selected. The electrode used in the first
few

passes must be small enough for easy manipulation in the

root of the joint. In V-grooves, small diameter electrodes
are frequently used for the initial pass to control melt
through and bead shape. Larger electrodes can be used to
complete the weld, taking advantage of their deeper pene
tration and higher deposition rates.
Finally, the experience of the welder often has a bearing
on the electrode size. This is particularly true for out-of
position welding, since the welder's skill governs the size
of the molten puddle that the welder can control.
The largest possible electrode that does not violate any
pertinent heat input limitations or deposit too large a weld
should be used. Welds that are larger than necessary are
more costly and, in some instances, actually are harmful.
Any sudden change in section size or in the contour of a
weld, such as that caused by overwelding, creates stress
concentrations. It is obvious that the correct electrode size
is the one that, when used with the proper amperage and
travel speed, produces a weld of the required size in the
least amount of time.

WELDING CURRENT
SHIELDED METAL ARC welding can be accomplished with
either alternating or direct current, when an appropriate
electrode is used. The type of welding current, the polar
ity, and the constituents in the electrode covering influ
ence the melting rate of all covered electrodes. For any
given electrode, the melting rate is directly related to the
electrical energy supplied to the arc. Part of this energy is
used to melt a portion of the base metal and part is used to
melt the electrode.

Direct Current
DIRECT CURRENT ALWAYS provides a steadier arc and
smoother metal transfer than ac does. This is because the
polarity of de is not always changing as it is with ac. Most
covered electrodes operate better on reverse polarity (elec
trode positive), although some are suitable for (and even
are intended for) straight polarity (electrode negative). Re
verse polarity produces deeper penetration, but straight
polarity produces a higher electrode melting rate.
The de arc produces good wetting action by the molten
weld metal and uniform weld bead size even at low amper
age. For this reason, de is particularly suited to welding
thin sections. Most combination ac-dc electrodes operate
better on de than on ac, even though they are designed to
operate with either type of current.
Direct current is preferred for vertical and overhead
welding and for welding with a short arc. The de arc has
less tendency to short out as globules of molten metal are
transferred across it.
Arc blow may be a problem when magnetic metals (iron
and nickel) are welded with de. One way to overcome this
problem is to change to ac.

Alternating Current
FOR SMAW, ALTERNATING current offers two advantages
over de. One is the absence of arc blow and the other is the
cost of the power source.
Without arc blow, larger electrodes and higher welding
currents can be used. Certain electrodes (specifically, those
with iron powder in their coverings) are designed for oper
ation at higher amperages with ac. The highest welding
speeds for SMAW can be obtained in the drag technique
with these electrodes on ac. Fixturing materials, fixture de
sign, and workpiece connection location may not be as
critical with ac.
An ac transformer costs less than an equivalent de
power source. The cost of the equipment alone should not
be the sole criterion in the selection of the power source,
however. All operating factors need to be considered.

Amperage
COVERED ELECTRODES OF a specific size and classification
will operate satisfactorily at various amperages within

some certain range. This range will vary somewhat with the
thickness and formulation of the covering.
Deposition rates increase as the amperage increases. For
a given size of electrode, the amperage ranges and the re
sulting deposition rates will vary from one electrode classi
fication to another. This variation for several classifica
tions of carbon steel electrodes of one size is shown in
Figure 2.10.
With a specific type and size of electrode, the optimum
amperage depends on several factors such as the position
of welding and the type of joint. The amperage must be
su.fficient to obtain good fusion and penetration yet per
mit proper control of the molten weld pool. For vertical
and overhead welding, the optimum amperages would
likely be on the low end of the allowable range.
Amperage beyond the recommended range should not
be used. It can overheat the electrode and cause excessive
spatter, arc blow, undercut, and weld metal cracking. Fig
ure 2.ll(A), (B), and (C) show the effect of amperage on
bead shape.

ARC LENGTH
THE ARC LENGTH is the distance from the molten tip of the
electrode core wire to the surface of the molten weld pool.
Proper arc length is important in obtaining a sound welded
joint. Metal transfer from the tip of the electrode to the
weld pool is not a smooth, uniform action. Instantaneous
arc voltage varies as droplets of molten metal are trans
ferred across the arc, even with constant arc length. How
ever, any variation in voltage will be minimal when welding
is done with the proper amperage and arc length. The latter
requires constant and consistent electrode feed.
The correct arc length varies according to the electrode
classification, diameter, and covering composition; it also
varies with amperage and welding position. Arc length in
creases with increasing electrode diameter and amperage.
As a general rule, the arc length should not exceed the
?iameter
the core
wireelectrodes
of the electrode.
The coverings,
arc usually
is
shortrofthan
this for
with thick
such as iron powder or "drag" electrodes.
Too short an arc will be erratic and may short circuit
during metal transfer. Too long an arc will lack direction
and intensity, which will tend to scatter the molten metal
as it moves from the electrode to the weld. The spatter may
be heavy and the deposition efficiency low. Also, the gas
and flux generated by the electrode covering are not so
effective in shielding the arc and weld metal. This can re
sult in porosity and contamination of the weld metal by
oxygen or nitrogen, or both.
Control of arc length is largely a matter of welder skill
involving the welder's knowledge, experience, visual per
ception, and manual dexterity. Although the arc length
does change to some extent with changing conditions, cer
tain fundamental principles can serve as a guide to the
proper arc length for a given set of conditions.

S H I E L D E D M E TA L A R C W E L D I N G
S H I E L D E D M E TA L A R C W E L D I N G
6363

11
10

E7024

i-

9 -

s:

63

8 -

<,

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ul

a:

7 -

-:

j:::

en

0
e,

w
c

65

4
3

E7018

.c

<,

14

12

6011

E6013

E6010 (de)
E

1
125

175

225

200

250

300

275

225

325

200

250

300

WELDING CURRENT, A

Figure 2. 10-The Relationship Between Deposition Rate and Welding Current for Various Types
of 3/16 in. (4.8 mm} Diameter Carbon Steel Electrodes

For downhand welding, particularly with heavy elec

trode coverings, the tip of the electrode can be dragged
lightly along the joint. The arc length, in this case, is auto
matically determined by the coating thickness and the
melting rate of the electrode. Moreover, the arc length is
uniform. For vertical or overhead welding, the arc length is
gaged by the welder. The proper arc length, in such cases,
is the one that permits the welder to control the size and
motion of the molten weld pool.
For fillet welds, the arc is crowded into the joint for
highest deposition rate and best penetration. The same is
true of the root passes in groove welds in pipe.
When arc blow is encountered, the arc length should be
shortened as much as possible. The various classifications
of electrodes have widely different operating characteris
tics, including arc length. It is important, therefore, for the
welder to be familiar with the operating characteristics of
the types of electrodes the welder uses in order to recog-

nize the proper arc length and to know the effect of differ
ent arc lengths. The effect of a long and a short arc on bead
appearance with a mild steel electrode is illustrated in Fig
ures 2.11(D) and (E).

TRAVEL SPEED
TRAVEL SPEED IS the rate at which the electrode moves
along the joint. The proper travel speed is the one which
produces a weld bead of proper contour and appearance,
as shown in Figure 2.ll(A). Travel speed is influenced by
several factors:
(1)
(2)
(3)
(4)
(5)

Type of welding current, amperage, and polarity

Position of welding
Melting rate of the electrode
Thickness of material
Surface condition of the base metal

6464 S H I E L D E D M E T A L A R C
N G(6) Type of joint
(7) Joint fit-up
(8) Electrode manipulation

W E LDI

When welding, the travel speed should be adjusted so

that the arc slightly leads the molten weld pool. Up to a
point, increasing the travel speed will narrow the weld
bead and increase penetration. Beyond this point, higher
travel speeds can decrease penetration, cause the surface of
the bead to deteriorate and produce undercutting at the
edges of the weld, make slag removal difficult, and entrap
gas (porosity) in the weld metal. The effect of high travel
speed on bead appearance is shown in Figure 2.11 (G).
With low travel speed, the weld bead will be wide and
convex with shallow penetration, as illustrated in
Figure
2.11 (F). The shallow penetration is caused by the arc
dwelling on the molten weld pool instead of leading it and
concentrating on the base metal. This, in turn, affects dilu
tion. When dilution must be kept low (as in cladding), the
travel speed, too, must be kept low.
Travel speed also influences heat input, and this affects
the metallurgical structures of the weld metal and the heat-

S H IELD ED

M E TA L

ARC

W ELD ING

affected zone.6464Low travel speed increases heat input and

this, in tum, increases the size of the heat-affected zone and
reduces the cooling rate of the weld. Forward travel speed is
necessarily reduced with a weave bead as opposed to the
higher travel speed that can be attained with a stringer bead.
Higher travel speed reduces the size of the heat-affected
zone and increases the cooling rate of the weld. The in
crease in the cooling rate can increase the strength and hard
ness of a weld in a hardenable steel, unless preheat of a level
sufficient to prevent hardening is used.

ELECTRODE ORIENTATION
ELECTRODE ORIENTATION, WITH respect to the work and
the weld groove, is important to the quality of a weld. Im
proper orientation can result in slag entrapment, porosity,
and undercutting. Proper orientation depends on the type
and size of electrode, the position of welding, and the ge
ometry of the joint. A skilled welder automatically takes
these into account when the orientation to be used for a
specific joint is determined. Travel angle and work
angle are used to define electrode orientation.

Figure 2. 11-The Effect of Welding Amperage, Arc Length, and Travel Speed; (A) Proper
Amperage, Arc Length, and Travel Speed; (B) Amperage Too Low; (C) Amperage Too High; (D) Arc
Length Too Short; (E) Arc Length Too Long; (F) Travel Speed Too Slow; (G) Travel Speed Too Fast

Travel angle is the angle less than 90 degrees between the

electrode axis and a line perpendicular to the weld axis, in
a plane determined by the electrode axis and the weld axis.
Work angle is the angle less that 90 degrees between a line
perpendicular to the major workpiece surface and a plane
determined by the electrode axis and the weld axis. When
the electrode is pointed in the direction of welding, the
forehand technique is being used. The travel angle, then, is
known as the push angle. The backhand technique involves
pointing the electrode in the direction opposite that of
welding. The travel angle, then, is called the drag angle.
These angles are shown in Figure 2.12.
Typical electrode orientation and welding technique for
groove and fillet welds, with carbon steel electrodes, are
listed in Table 2.3. These may be different for other materi
als. Correct orientation provides good control of the mol
ten weld pool, the desired penetration, and complete
fusion with the steel base.
A large travel angle may cause a convex, poorly shaped
bead with inadequate penetration, whereas a small travel
angle may cause slag entrapment. A large work angle can
cause undercutting, while a small work angle can result in
lack of fusion.

WELDING TECHNIQUE
THE FIRST STEP in SMAW is to assemble the proper equip
ment, materials, and tools for the job. Next, the type of
welding current and the polarity, if de, need to be deter
mined and the power source set accordingly. The power
source must also be set to give the proper volt-ampere
characteristic (open circuit voltage) for the size and type of
electrode to be used. After this, the work is positioned for
welding and, if necessary, clamped in place.
The arc is struck by tapping the end of the electrode on
the work near the point where welding is to begin, then
quickly withdrawing it a small amount to produce an arc
of proper length. Another technique for striking the arc is
to use a scratching motion similar to that used in striking a
match. When the electrode touches the work, there is a
tendency for them to stick together. The purpose of the
tapping and scratching motion is to prevent this. When the

Type of Joint
Groove
Groove
Groove
Groove
Fillet
Fillet
Fillet

electrode does stick, it needs to be quickly broken free,

Otherwise, it will overheat, and attempts to remove it from
the workpiece will only bend the hot electrode. Freeing it
then will require a hammer and chisel.
The technique of restriking the arc once it has been bro
ken varies somewhat with the type of electrode. Generally,
the covering at the tip of the electrode becomes conduc
tive when it is heated during welding. This assists in restrik
ing the arc if it is restruck before the electrode cools. Arc
striking and restriking are much easier for electrodes with
large amounts of metal powders in their coverings. Such
coverings are conductive when cold. When using heavily
covered electrodes which do not have conductive coat
ings, such as E6020, low hydrogen, and stainless steel elec
trodes, it may be necessary to break off the projecting cov
ering to expose the core wire at the tip for easy restriking.
Striking the arc with low hydrogen electrodes requires a
special technique to avoid porosity in the weld at the point
where the arc is started. This technique consists of striking
the arc a few electrode diameters ahead of the place where
welding is to begin. The arc is then quickly moved back,
and welding is begun in the normal manner. Welding con
tinues over the area where the arc originally was struck, re
fusing any small globules of weld metal that may have re
mained from striking the arc.
During welding, the welder maintains a normal arc
length by uniformly moving the electrode toward the work
as the electrode melts. At this same time, the electrode is
moved uniformly along the joint in the direction of weld
ing, to form the bead.
Any of a variety of techniques may be employed to
break the arc. One of these is to rapidly shorten the arc,
then quickly move the electrode sideways out of the cra
ter. This technique is used when replacing a spent elec
trode, in which case welding will continue from the crater.
Another technique is to stop the forward motion of the
electrode and allow the crater to fill, then gradually with
draw the electrode to break the arc. When continuing a
weld from a crater, the arc should be struck at the forward
end of the crater. It should then quickly be moved to the
back of the crater and slowly brought forward to continue
the weld. In this manner, the crater is filled, and porosity

Table 2.3
Typical Shielded Metal Arc Electrode Orientation and Welding Technique
Travel Angle, Deg
Technique of Welding
Position of Welding for Carbon
WorkSteel
Angle, Electrodes
Deg
Flat
Horizontal
Vertical-Up
Overhead
Horizontal
Vertical-Up
Overhead

90
80-100
90
90
45
35-55
30-45

* Travel angle may be 10 to 30 for electrodes with heavy iron powder coatings.

5-10*
5-10
5-10
5-10
5-10*
5-10
5-10

Backhand
Backhand
Forehand
Backhand
Backhand
Forehand
Backhand

PUSH ANGLE FOR

FOREHAND WELDING
DRAG ANGLE
FOR BACKHAND
WELDING

AXIS OF WELD

(A) GROOVE WELD

PUSH ANGLE FOR

FOREHAND WELDING
DRAG ANGLE FOR
BACKHAND WELDING

(B) FILLET WELD

Figure 2. 12-0rientation of the Electrode

and entrapped slag are avoided. This technique is particu

larly important for low hydrogen electrodes.

(4) Ionization of the arc path from the electrode to the

work
(5) Manipulation of the electrode

SLAG REMOVAL

The first two factors are related to the design and oper
ating characteristics of the power source. The next two are
functions of the welding electrode. The last one represents
the skill of the welder.
The arc of a covered electrode is a transient arc, even
when the welder maintains a fairly constant arc length. The
welding machine must be able to respond rapidly when the
arc tends to go out, or it is short circuited by large droplets
of metal bridging the arc gap. In that case, a surge of
current is needed to clear a short circuit. With ac, it is
important that the voltage lead the current in going
through zero. If the two were in phase, the arc would be
very unstable. This phase shift must be designed into the
welding machine.
Some electrode covering ingredients tend to stabilize
the arc. These are necessary ingredients for an electrode to
operate well on ac. A few of these ingredients are titanium
dioxide, feldspar, and various potassium compounds (in
cluding the binder, potassium silicate). The inclusion of
one or more of these arc stabilizing compounds in the cov
WORKPIECE CONNECTION
ering provides a large number of readily ionized particles
PROPER CONNECTING OF the worklead is a necessary con and thereby contributes to ionization of the arc stream.
sideration in shielded metal arc welding. The location of Thus, the electrode, the power source, and the welder all
the lead is especially important with de welding. Improper contribute to arc stability.
location may promote arc blow, making it difficult to con
trol the arc. Moreover, the method of attaching the lead. is
important. A poorly attached lead will not provide consis ARC BLOW
tent electrical contact, and the connection will heat up. ARC BLOW, WHEN it occurs, is encountered principally
This can lead to an interruption of the circuit and a break with de welding of magnetic materials (iron and nickel). It
ing of the arc. A copper contact shoe secured with a may be encountered with ac, under some conditions, but
C-clamp is best. If copper pickup by this attachment to the those cases are rare, and the intensity of the blow is always
base metal is detrimental, the copper shoe should be at much less severe. Direct current, flowing through the elec
tached to a plate that is compatible with the work. The trode and the base metal, sets up magnetic fields around
plate, in turn, is then secured to the work. For rotating the electrode which tend to deflect the arc from its in
work, contact should be made by shoes sliding on the tended path. The arc may be deflected to the side at times,
work or through roller bearings on the spindle on which but usually it is deflected either forward or backward
the work is mounted. If sliding shoes are used, at least two along the joint. Back blow is encountered when welding
shoes should be employed. If loss of contact occurred with toward the workpiece connection near the end of a joint
only a single shoe, the arc would be extinguished.
or into a corner. Forward blow is encountered when weld
ing away from the lead at the start of the joint, as shown in
Figure 2.13.
ARC STABILITY

THE EXTENT TO which slag is removed from each weld bead

before welding over the bead has a direct bearing on the
qual ity of a multiple pass weld. Failure to thoroughly clean
each bead increases the probability of trapping slag and,
thus, pro ducing a defective weld. Complete and efficient
slag removal requires that each bead be properly contoured
and that it blend smoothly into the adjacent bead or base
metal.
Small beads cool more rapidly than large ones. This
tends to make slag removal from small beads easier. Con
cave or flat beads that wash smoothly into the base metal
or any adjoining beads minimize undercutting and avoid a
sharp notch along the edge of the bead where slag could
stick. Finally, it is most important that welders be able to
recognize areas where slag entrapment is likely to occur.
Skilled welders understand that complete removal of slag
is necessary before continuing a weld.

A STABLE ARC is required if high quality welds are to be

produced. Such defects as inconsistent fusion, entrapped
slag, blowholes, and porosity can be the result of an unsta
ble arc.
The following are important factors influencing arc
stability:
(1) The open circuit voltage of the power source

(2) Transient voltage recovery characteristics of the

power source
(3) Size of the molten drops of filler metal and slag in the

Figure 2. 13-The Effect of Workpiece

Connection Location on Magnetic Arc Blow

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Arc blow can result in incomplete fusion and excessive

weld spatter and may be so severe that a satisfactory weld
cannot be made. When welding with iron powder elec
trodes and electrodes which produce heavy slag, forward
blow can be especially troublesome. It permits the molten
slag, which normally is confined to the edge of the crater,
to run forward under the arc.
The bending of the arc under these conditions is caused
by the effects of an unbalanced magnetic field. When there
is a greater concentration of magnetic flux on one side of
the arc than on the other, the arc always bends away from
the greater concentration. The source of the magnetic flux
is indicated by the electrical rule which states that a con
ductor carrying an electric current produces a magnetic
flux in circles around the conductor. These circles are in
planes perpendicular to the conductor and are centered on
the conductor.
In welding, this magnetic flux is superimposed on the
steel and across the gap to be welded. The flux in the plate
does not cause difficulty, but unequal concentration of
flux across the gap or around the arc causes the arc to bend
away from the heavier concentration. Since the flux passes
through steel many times more readily than it does
through air, the path of the flux tends to remain within the
steel plates. For this reason, the flux around the electrode,
when the electrode is near either end of the joint, is con
centrated between the electrode and the end of the plate.
This high concentration of flux on one side of the arc, at
the start or the finish of the weld, deflects the arc away
from the ends of the plates.
Forward blow exists for a short time at the start of a
weld, then it diminishes. This is because the flux soon
finds an easy path through the weld metal. Once the mag
netic flux behind the arc is concentrated in the plate and
the weld, the arc is influenced mainly by the flux in front of
it as this flux crosses the root opening. At this point, back
blow may be encountered. Back blow can occur right up
to the end of the joint. As the weld approaches the end, the

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flux ahead of the arc becomes more crowded, increasing

the back blow. Back blow can become extremely severe
right at the very end of the joint.
The welding current passing through the work creates a
magnetic field around it. The field is perpendicular to the
path of current between the arc and the workpiece connec
tion. The flux field around the arc is perpendicular to the
one in the work. This concentrates the magnetic flux on the
worklead side of the arc and tends to push the arc away.
The two flux fields mentioned above are, in reality, one
field. That field is perpendicular to the path of the current
through the cable, the work, the arc, and the electrode.
Unless the arc blow is unusually severe, certain corrective
steps may be taken to eliminate it or, at least, to reduce its
severity. Some or all of the following steps may be necessary:
(1) Place worklead connections as far as possible from
the joints to be welded.
(2) If back blow is the problem, place the workpiece
connection at the start of welding, and weld toward a
heavy tack weld.
(3) If forward blow causes trouble, place the workpiece
connection at the end of the joint to be welded.
(4) Position the electrode so that the arc force counter
acts the arc blow.
(5) Use the shortest possible arc consistent with good
welding practice. This helps the arc force to counteract the
arc blow.
(6) Reduce the welding current.
(7) Weld toward a heavy tack or runoff tab.
(8) Use the backstep sequence of welding.
(9) Change to ac, which may require a change in elec
trode classification.
(10) Wrap the worklead around the workpiece in a direc
. tion such that the magnetic field it sets up will counteract
the magnetic field causing the arc blow.

QUALITY OF THE WELD

A WELDED JOINT must possess those qualities which are
necessary to enable it to perform its expected function in
service. To accomplish this, the joint needs to have the
required physical and mechanical properties. It may need a
certain microstructure and chemical composition to meet
these properties. The size and shape of the weld also are
involved, as is the soundness of the joint. Corrosion resis
tance may be required. All of these are influenced by the
base materials, the welding materials, and the manner in
which the weld is made.
Shielded metal arc welding is a manual welding process,
and the quality of the joint depends on the skill of the

welder who makes it. For this reason, the materials to be

used must be selected with care, the welder must be profi
cient, and the procedure the welder uses must be correct.
Welded
joints,
by their nature,
contain
discontinuities of various types and sizes. Below some
acceptable level, these are not considered harmful.
Above that level, they are considered defects. The
acceptance level can vary with the severity of the service to
be encountered, or more com monly will be based on
fabrication contract requirements or a specific code or
specification.
The following discontinuities are sometimes encoun
tered in welds made by the SMAW process:

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(1)
(2)
(3)
(4)
(5)

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Porosity
Slag inclusions
Incomplete fusion
Undercut
Cracks

POROSITY
THIS TERM IS used to describe gas pockets or voids in the
weld metal. These voids result from gas that forms from
certain chemical reactions that take place during welding.
They contain gas rather than solids, and, in this respect,
they differ from slag inclusions.
Porosity usually can be prevented by using proper
amper
age and holding a proper arc length. Dry electrodes are
also helpful in many cases. The deoxidizers which a
covered electrode needs are easily lost during deposition
when high
amperage or a long arc is used. This leaves a supply which
is insufficient for proper deoxidation of the molten metal.

SLAG INCLUSIONS
THIS TERM IS used to describe the oxides and nonmetallic
solids that sometimes are entrapped in weld metal, be
tween adjacent beads, or between the weld metal and the
base metal. During deposition and subsequent solidifica
tion of the weld metal, many chemical reactions occur.
Some of the products of these reactions are solid nonme
tallic compounds which are insoluble in the molten metal.
Because of their lower specific gravity, these compounds
will rise to the surface of the molten metal unless they be
come entrapped within the weld metal.
Slag formed from the covering on shielded metal arc
electrodes may be forced below the surface of the molten
metal by the stirring action of the arc. Slag may also flow
ahead of the arc if the welder is not careful. This can easily
happen when welding over the crevasse between two par
allel but convex beads or between one convex bead and a
side wall of the groove. It can also happen when the weld
ing is done downhill. In such cases, the molten metal may
flow over the slag, entrapping the slag beneath the bead.
Factors such as highly viscous or rapidly solidifying slag or
insufficient welding current set the stage for this.
Most slag inclusions can be prevented by good welding
practice and, in problem areas, by proper preparation of
the groove before depositing the next bead of weld metal.
In these cases, care must be taken to correct contours that
are difficult to adequately penetrate with the arc.

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Incomplete fusion may be caused by failure to raise the

base metal ( or the previously deposited bead of weld
metal) to the melting temperature. It may also be caused by
failure to dissolve, because of improper fluxing, any oxides
or other foreign material that might be present on the sur
faces which must fuse with the weld metal.
Incomplete fusion can be avoided by making certain
that the surfaces to be welded are property prepared and
fitted and are smooth and clean. In the case of incomplete
root fusion, the corrections are to make certain that the
root face is not too large; the root opening is not too
small; the electrode is not too large; the welding current
is not too low; and the travel speed is not too high.

UNDERCUT
THIS TERM IS used to describe either of two situations. One
is the melting away of the sidewall of a welding groove at
the edge of the bead, thus forming a sharp recess in the
sidewall in the area in which the next bead is to be depos
ited. The other is the reduction in thickness of the base
metal at the line where the beads in the final layer of weld
metal tie into the surface of the base metal (e.g., at the toe
of the weld).
Both types of undercut usually are due to the specific
welding technique used by the welder. High amperage and
a long arc increase the tendency to undercut. Incorrect
electrode position and travel speed also are causes, as is
improper dwell time in a weave bead. Even the type of
electrode used has an influence. The various classifications
of electrodes show widely different characteristics in this
respect. With some electrodes, even the most skilled
welder may be unable to avoid undercutting completely in
certain welding positions, particularly on joints with re
stricted access.
Undercut of the sidewalls of a welding groove will in no
way affect the completed weld if the undercut is removed
before the next bead is deposited at that location. A well
rounded chipping tool or grinding wheel will be required
to remove the undercut. If the undercut is slight, however,
an experienced welder who knows just how deep the arc
will penetrate may not need to remove the undercut.
The amount of undercut permitted in a completed weld
is usually dictated by the fabrication code being used, and
the requirements specified should be followed because ex
cessive undercut can materially reduce the strength of the
joint. This is particularly true in applications subject to fa
tigue. Fortunately, this type of undercut can be detected by
visual examination of the completed weld, and it can be
corrected by blend grinding or depositing an additional
bead.

INCOMPLETE FUSION

CRACKS

THIS TERM, AS it is used here, refers to the failure to fuse

together adjacent beads of weld metal or weld metal and
base metal. This condition may be localized or it may be
extensive, and it can occur at any point in the welding
groove. It may even occur at the root of the joint.

CRACKING IN WELDED joints can be classified as either hot

or cold cracking. Cracking can occur in the weld metal,
base metal, or both. If cracking is observed during welding,

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S H I E L D E D M E TA L A RC W ELD I N G
the cracks
should be removed prior to further welding,
(3) Changing the welding technique/procedure by
because weld metal deposited over a crack can result in modifying the preheat and interpass temperatures and re
continuation of that crack into the newly deposited weld ducing the welding current
metal.
Hot cracking is a function of chemical composition.
Cold cracking is the result of inadequate ductility or the
The main cause of hot cracking is constituents in the weld presence of hydrogen in hardenable steels. This condition
metal which have a relatively low melting temperature and is caused by inadequate toughness in the presence of a me
which accumulate at the grain boundaries during solidifi chanical or metallurgical notch and stresses of sufficient
cation. A typical example is iron sulfide in steel. The cracks magnitude. These stresses do not have to be very high in
are intergranular or interdendritic. They form as the weld some materials-large grained ferritic stainless steel, for
metal cools. As solidification progresses in the cooling instance.
weld metal, the shrinkage stresses increase and eventually
To prevent cold cracking in hardenable steels, the use of
draw apart those grains which still have some liquid at their
dry
low hydrogen electrodes and proper preheat is re
boundaries. Coarse-grained, single-phase structures have a
quired.
Preheat is also required for those materials which
marked propensity to this type of cracking. Solutions to
are
naturally
low in ductility or toughness. Materials
cracking problems include:
which are subject to extreme grain growth (28 percent
chromium steel, for instance) must be welded with low
(1) Changing the base metal (for instance, use a steel heat input and low interpass temperatures. Notches need
with manganese additions, or one produced to provide a to be avoided.
fine grained structure)
More information on the quality of welded joints can be
(2) Changing filler metal (using filler metal with suffi found in the Welding Handbook, Chapter 5, Volume 1, 7th
cient ferrite when welding austinetic stainless steel, for Edition and Chapter 6, Section 1, 6th Edition. Welding In
instance)
spection, published by AWS, also is a good reference.

SAFETY RECOMMENDATIONS
THE OPERATOR MUST protect eyes and skin from radiation
from the arc. A welding helmet with a suitable filter lens
should be used, as well as dark clothing, preferably wool,
to protect the skin. Leather gloves and clothing should be
worn to protect against burns from arc spatter.
Welding helmets are provided with filter plate windows,
the standard size being 2 by 4-1/8 in. (51 by 130 mm).
Larger openings are available. The filter plate should be
capable of absorbing infrared rays, ultraviolet rays, and
most of the visible rays emanating from the arc. Filter
plates that are now available absorb 99 percent or more of
the infrared and ultraviolet rays from the arc.
The shade of the filter plate suggested for use with elec
trodes up to 5 /32 in. (4 mm) diameter is No; 10. For 3 /
16 to 1/4 in. (4.8 to 6.4 mm) electrodes, Shade No. 12
should be used. Shade No. 14 should be used for
electrodes over
1/4 in. (6.4 mm).
.
The filter plate needs to be protected from molten spat
ter and from breakage. This is done by placing a plate of
clear glass, or other suitable material, on each side of the
filter plate. Those who are not welders but work near the
arc also need to be protected. This protection usually is
provided by either permanent or portable screens. Failure
to use adequate protection can result in eye burn for the
welder or for those working around the arc. Eye burn,
which is similar to sunburn, is extremely painful for a pe
riod of 24 to 48 hours. Unprotected skin, exposed to the

arc, may also be burned. A physician should be consulted

in the case of severe arc burn, regardless of whether it is of
the skin or the eyes.
If welding is being performed in confined spaces with
poor ventilation, auxiliary air should be supplied to
the
welder. This should be done through an attachment to the
helmet.
The method used must not restrict the welder's manipu
lation of the helmet, interfere with the field of vision, or
make welding difficult. Additional information on eye
protection and ventilation is given in ANSI Z49.1, Safety
in Welding and Cutting, published by the American Weld
ing Society.
From time to time during welding, sparks or globules of
molten metal are thrown out from the arc. This is always a
point of concern, but it becomes more serious when weld
ing is performed out of position or when extremely high
welding currents are used. To ensure protection from
burns under these conditions, the welder should wear
flame-resistant gloves, a protective apron, and a jacket (see
Figure 2.5). It may also be desirable to protect the welder's
ankles and feet from slag and spatter. Cuffless pants and
high work shoes or boots are recommended.
To avoid electric shock, the operator should not weld
while standing on a wet surface. Equipment should be ex
amined periodically to make sure there are no cracks or
worn spots on electrode holder or cable insulation.

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SUPPLEMENTARY READING LIST

American Society for Metals. "Welding, brazing, and Sol
dering." Metals Handbook, Vol. 6, 9th Ed., 75-95. Met
als Park, Ohio: American Society for Metals, 1983.
Barbin, L. M. "The new moisture resistant electrodes."
Welding Journal 56(7): 15-18;July 1977.
Chew, B. "Moisture loss and gain by some basic flux cov
ered electrodes." Welding Journal 55(5): 127s-134s;
May 1976.
Gregory, E. N. "Shielded metal arc welding of galvanized
steel." Welding journal 48(8): 631-638; August 1969.
Jackson, C. E. "Fluxes and slags in welding." Bulletin 190.
New York: Welding Research Council, December 1973.

Lincoln Electric Company. The procedure handbook ofarc

welding, 12th Ed. Cleveland: Lincoln Electric Company,
1973.
Silva, E. A. and Hazlett, T. H. "Shielded metal arc welding
underwater with iron powder electrodes." Welding
Journal 50(6): 406s-415s; June 1971.
Stout, R. D., and Doty, W. D. We/dability of Steels, 2nd
Ed., Ed. Epstein, S., and Somers, R. E. New York: Weld
ing Research Council, 1971.