Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 891-895
Published Online September 2012 (http://www.SciRP.org/journal/jmmce)
Correlation between Process Variables in Shielded
Metal-Arc Welding (SMAW) Process and Post Weld Heat
Treatment (PWHT) on Some Mechanical Properties of
Low Carbon Steel Welds
J. O. Olawale*, S. A. Ibitoye, K. M. Oluwasegun, M. D. Shittu, R. C. Ofoezie
Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria
Received May 4, 2012; revised June 10, 2012; accepted June 29, 2012
ABSTRACT
This investigation was conducted to correlate process variables in shielded metal-arc welding (SMAW) and post weld
heat treatment on some mechanical properties of low carbon steel weld. Three hundred and sixty pieces of weld samples
were prepared. The samples were welded together using AWS E6013 electrodes with DC arc welding process. Varying
welding curr ents of 100 A, 120 A, 140 A were used with a ter minal voltage of 80 V. Th e weld samples were prepared
for hardness, tensile and impact test. The prepared samples were then subjected to normalising heat treatment operation
at temperatures of 590˚C, 600˚C, 620˚C, 640˚C, 660˚C, 680˚C, and 700˚C. It was observed that increase in welding
current led to an in crease in hardness and ultimate tensile strength values of as-weld samples while impact strength de-
creases. After post heat treatment operation the hardness and ultimate tensile strengths decreases while i mpact strength
increases. From this outcome we conclude that there is correlation between the welding current and mechanical proper-
ties of weld metal on one hand and normalising temperatures and mechanical properties on the other hand. As the cur-
rent increases the hardness and strength increases but impact strength reduces, while hardness and strength continuou sly
reduces but impact strength increases as normalising temperatures increases.
Keywords: Post-Welding; Normalising; Current; Steel; Hardness; Strength
1. Introduction
Welding is one of the most important technological proc-
esses used across numerous branches of industry such as
industrial engineering, shipbuilding, pipeline fabrication
to name but a few.
The welding process generally involves melting and
subsequent cooling, and the result of this thermal cycle is
distortion if the welded item is free to move or residual
stress if the item is securely held [1]. There comes a
point when the amount of residual stress can create po-
tential problems, either immediately or during the life of
the welded structure, and it needs to be reduced or re-
moved. Post weld heat treatment is the most widely used
form of stress relieving on completion of fabrication of
welded structures.
High level residual stresses can occur in weldment due
to restraint by the parent metal during weld solidification
[2]. The stresses may be as high as the yield strength of
material itself. When combined with normal load stresses
these may exceed the design stresses. These stresses can
be relief by post-weld heat treatment. In concept, post-
weld heat treatment can encompass many different po-
tential treatments; however, in steel fabrication, the two
most common procedures used are post heating and
stress relieving [3]. The heat treatment consists of the
stressrelief, annealing or solution annealing depending
upon the requirements. For instance annealing and tem-
pering have been found to have significantly improve the
mechanical properties of welded steel [4,5].
Also, to consistently produce high quality of welds,
arc welding requires experienced welding personnel. One
reason for this is the need to properly select welding pa-
rameters for a given task to provide a good weld quality
which identified by its micro-structure and the amount of
spatter, and relied on the correct bead geometry size.
Therefore, the use of the control system in arc welding
can eliminate much of the “guess work” often employed
by welders to specify welding parameters for a given task
[6]. Investigation into the relationship between the weld-
ing process parameters and bead geometry began in the
*Corresponding author.

J. O. OLAWALE ET AL.
892
mid 1900s when regression analysis was applied to
welding geometry research by Lee and Raveendra [7-9].
Many efforts have been carried out for the development
of various algorithms in the modeling of arc welding
process [10]. In the early days, arc welding was carried
out manually so that the weld quality can be totally con-
trolled by the welder ability. McGlone and Chadwick [11 ]
have reported a mathematical analysis correlating proc-
ess variables and bead geometry for the submerged arc
welding of square edge close butts. Similar mathematical
relationship between welding variables and fillet weld
geometry for gas metal arc (GMA) welding using flux
cored wires have also been reported [12]. Chandel [13]
first applied this technique to the GMA welding process
and investigated relationship between process variables
and bead geometry. These results showed that arc current
has the greatest influence on bead geometry, and that
mathematical models derived from experimental results
can be used to predict bead geometry accurately.
In their study Kumar and Murugan [14] find out that
depth of penetration, bead width and height of reinforce-
ment of weld increases with increase in welding current
but decreases with increase in welding speed. However,
depth of penetration and bead width decreases with in-
crease in nozzle-to-plate distance and electrode angle
whereas height of reinforcement increases with increase
in nozzle-to-plate distance and electrode angle. Nearly
90% of welding in the world is carried out by one or the
other arc welding process [15]; therefore it is imperative
to study the effects of welding parameters on the some
mechanical properties of the welds during arc welding.
Hence, this study is conducted to correlate process
variables in shielded metal-arc welding (SMAW) and
post weld heat treatment on some mechanical properties
of low carbon steel weld. Low carbon steel was used for
this study because it accounts for about 90% of total
plain carbon steel and is widely applied due to their eco-
nomic value, excellent weld ability, plus good mechanical
and physical properties acceptable to many applications
[16]. For these reasons, this study looks into the influ-
ence of welding current and normalizing temperatures on
the tensile strength, impact strength and hardness of
welded low carbon steel.
2. Experimental Procedure
2.1. Materials and Methods
The materials used are 12 mm low carbon steel round
bars with chemical composition as presented in Table 1,
and E6013 consumable electrodes. Round bars of diame-
ter 12 mm by 50 mm length were sectioned from the bars.
Three hundred and six ty pieces of samples were pr epar ed
in all. Each of these samples was welded together using
Shielded Metal-Arc Welding (SMAW) process. AWS
E6013 electrodes were used with DC arc welding process.
Varying welding current of 100 A, 120 A and 140 A
were used with a terminal voltage of 80 V.
2.2. Machining
The welded samples were machined into standard test
samples for tensile and impact test as presented in Fig-
ures 1 and 2 respectively. The standard test samples for
tensile test is according to dimensions of Instron cylin-
drical specimen (Figure 1) while impact test samples
were prepared according to Izod specimen (Figure 2).
The Izod specimen has a round cross-section with a
V-shaped notch. The depth of the notch is 2 mm and in-
cluded angle is 45˚.
2.3. Post Weld Heat Treatment (PWHT)
The machined samples were subjected to normalising
heat treatment operation. Seven different normalising
temperatures were used. Three hundred and fifteen of
machined samples were heated to temperature of; 590˚C,
600˚C, 620˚C, 640˚C, 660˚C, 680˚C and 700˚C respec-
tively in a muffle furnace. After reaching a temperature
of 590˚C the samples were soaked for 20 min in accor-
Table 1. Chemical composition of the low carbon steel bar
(in wt%).
Element Composition (%) Element Composition (%)
C 0.2301 Ca 0.0007
Si 0.2217 Zn 0.0057
Mn 0.8227 Al 0.0050
P 0.0494 Pb 0.0046
S 0.0464 Sn 0.0162
Cr 0.1191 As 0.0060
Mo 0.0209 Co 0.0091
Ni 0.1169 W 0.0044
Cu 0.2193 Fe 98.0683
V 0.0034
Figure 1. Dimensions of Instron cylindrical specimen.
Figure 2. Notched Izod spec i men.

J. O. OLAWALE ET AL.
893
dance with the ASTM soaking time requirement of 3 - 4
min/mm thickness at each normalising temperature. After
this period of soaking time forty five samples were taken
out of furnace and allowed to cool in air. The remaining
samples were then heated further to 600˚C, soaked for 20
min and another forty five samples removed and cooled.
The procedure was repeated for 620˚C, 640˚C, 660˚C,
680˚C and 700˚C.
2.4. Mechanical Testing
2.4.1. Hardness Testi ng
The hardness values of fifteen as-welded samples and
one hundred and five normalised samples were measured;
five sample each for each of welding current of as-
welded samples, and five samples for each of welding
current and normalised temperature. For each of the
categories the average of measured values were taken
and presented as in Table 2.
2.4.2. Tensile Testing
The as-welded and heat-treated samples were tested for
tensile strength using Instron Electromechanical Testing
System Model 3369. Fifteen as-welded samples and one
hundred and five normalised samples were tested. The
average ultimate tensile strength of five samples for each
of the categories of correlation of welding current and
normalised temperature is as presented in Table 3.
2.4.3. Impact Testing
The impact strength of fifteen as-welded samples and
one hundred and five normalised samples were measured
using Izod impact test. In this test the samples were sub-
jected to sudden load. A hammer is made to swing from
a fixed height and strike the test specimen held as a ver-
tical cantilevered beam with the notch faces the hammer.
The pendulum hits the specimen and breaks it. The en-
ergy lost by the pendulum in the breaking process is
equated with the energy absorbed by the test specimen.
This absorbed energy is expressed in energy lost per unit
cross-sectional area at the notch (J/m²) and is an indica-
tion of a materials impact resistance. The average values
for five samples for each of the categories of correlation
of welding current and normalised temperature is as pre-
sented in Table 4.
3. Results and Discussion
3.1. Results
The hardness, tensile strength and impact strength for
each value of welding current for different normalise
temperature are presented in Tables 2-4 respectively.
While the effects of correlation between the welding
current and various normalise temperature on these me-
chanical properties are presented in Figures 3-5 respec-
tively.
Table 2. Brinell hardness values for the low carbon steel welds for various normalising temperatures.
Welding current
(A) As-welded (HB) Normalised at 590˚C (HB) Normalised at 600˚C (HB)Normalised at 620˚C (HB)Normalised at 640˚C (HB)Normalised at 660˚C (HB) Normalised at 680˚C (HB) Normalised at 700˚C (HB)
100 398 354 298 269 257 211 209 207
120 415 359 302 272 266 231 226 226
140 457 409 373 333 306 278 266 260
Table 3. UTS values for the low carbon steel welds for various normalising temperatures.
Welding current
(A) As-welded (MPa) Normalised at 590˚C (MPa) Normalised at 600˚C (MPa)Normalised at 620˚C (MPa)Normalised at 640˚C (MPa)Normalised at 660˚C (MPa) Normalised at 680˚C (MPa) Normalised at 700˚C (MPa)
100 581.89 447.61 443.84 432.74 410.55 402.23 385.59 360.62
120 666.70 542.03 533.61 497.38 451.15 428.43 415.45 399.22
140 742.50 613.64 609.39 577.11 556.93 534.64 520.61 488.32
Table 4. Impact strength values for the low carbon steel welds for various normalising temperatures.
Welding current
(A) As-welded (J/m2) Normalised at 590˚C (J/m2) Normalised at 600˚C (J/m2)Normalised at 620˚C (J/m2)Normalised at 640˚C (J/m2)Normalised at 660˚C (J/m2) Normalised at 680˚C (J/m2) Normalised at 700˚C (J/m2)
100 65.67 71.89 77.66 80.65 93.43 99.84 109.21 111.80
120 50.27 60.32 70.38 75.41 78.42 81.44 97.01 98.52
140 40.47 47.75 54.63 58.28 64.75 67.99 79.46 80.53

J. O. OLAWALE ET AL.
894
Figure 3. Variation of Brinell hardness with normalising
temperature.
Figure 4. Variation of ultimate tensile strength with nor-
malising temperature.
Figure 5. Variation of impact strength with normalising
temperature.
3.2. Discussion
Increase in welding current led to an increase in hardness
and ultimate tensile strength values of as-weld samples
(Tables 2 and 3) while impact strength decreases (Table
4). This is due to the fact that increasing current brings
about greater heat input. In traditional arc welding,
power and heat input are controlled directly by the inde-
pendent variables, i.e., welding current, voltage, and
travel speed [17]. Power and heat input have a linear re-
lationship with these independent variables, e.g., increa-
se with increasing welding voltage and current, and de-
ceasing welding speed. As the heat input increases the
temperature gradient of the solidifying weld metal in-
creases. Also, the shape and size of weld pool is signifi-
cantly affected by heat input, the weld pool becomes
elongated as the heat input increases. This favours the
formation of columnar dendrites in the weld metal and
also increases the level of residual stress in the weld
metal [18].
The development of residual stresses can be explained
by considering heating and cooling under constraint. This
is because compressive and tensile stresses are produced
in weld metal and they increase with temperature. When
the heating stops and the weld metal are allowed cooling
off, its thermal contraction is restrained by the adjacent
base metal father away from the weld metal. Conse-
quently, after cooling to the room temperature, residual
tensile stresses exist in the weld metal and adjacent base
metal, while residual compressive stresses exist in the
areas father away from the weld metal.
After post heat treatment operation it was observed
that the hardness and ultimate tensile streng ths decreases
(Tables 2 and 3) while impact strength increases (Table
4). This is due to the reduction in residual stresses de-
velop during welding operation. After the normalising
operation both the tensile and compressive stresses that
were built-up during welding operation were relief. As
the normalising temperatures is increasing these stresses
are reducing as well as hardness and strength (Figures 3
and 4) while impact strength is increasing (Figure 5).
4. Conclusions
The following conclusions can be made from this inves-
tigation:
1) There is correlation between the welding current
and mechanical properties of weld metal. As the current
increases the hardness and strength increases but impact
strength reduces.
2) Post welding normalising heat treatment operation
reduces the weld metal hardness and strength but in-
creases the impact strength. As the normalising tempera-
tures increases the hardness and strength continuously
reduces while impact strength increases.
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