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

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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

Email: *[email protected]

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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