Experimental Research on Submerged Multi-Arc … · Experimental Research on Submerged Multi-Arc...

Experimental Research on Submerged Multi-Arc Welding

of API-5L-70X Steel

1LUIGI RENATO MISTODIE,

2EMIL CONSTANTIN,

1COSTICA VOICU,

1VIRGIL TEODOR

1Department of Manufacturing Engineering

“Dunarea de Jos” University of Galati

47 Domneasca St., 800008 - Galati 2Romanian Welding Society

30 Mihai Viteazu Blv., 300222 - Timisoara

ROMANIA

[email protected] http://www.if.ugal.ro

Abstract: - The research focuses on the behaviour of API-5L-X70 steel, a high strength steel used in pipelines

manufacturing, which was subjected to submerged mono and multi-arc welding. Due to the advantages of the

welding process in comparison with other manufacturing processes, welding process is more and more applied

in pipelines manufacturing field. Combining the number of wires and electrical arcs many variants of the

submerged welding process were developed. It is obvious that multi-arc welding leads to a high productivity

and efficiency. On the other hand, welded joints free flaws are required, in order to increase the safety of gas

supplying and to reduce the possible incidents which could affect the environment and working conditions.

That is why, an investigation of the welding process effects on the thermal, metallurgical and mechanical

changes in the base material is compulsory. Two welding process variants were applied during the experimental

programme. One variant consists of a single electrical arc and the other welding variant includes two arcs and

three wires. A comparative study on mechanical properties of the welded joints performed by the both welding

variants is deeply described in this paper. Tensile strength, yield strength, toughness and hardness testing were

made in order to assess the behaviour of this steel grade during the welding process. Finally, the experimental

results are graphically processed and comparatively discussed. Based on the results of the investigation,

important conclusions are drawn in the end of the paper.

Key-Words: - API-5L-X70 steel, submerged mono-arc and multi-arc welding, mechanical properties, testing

1 Introduction Due to the advantages of the welding process in

comparison with other manufacturing processes,

welding process is more and more applied in

pipelines fabrication. Presently, the submerged

welding process is preferred by the welding

technologies designers and technologists for the

achievement of the pipelines longitudinal welds.

Many variants of the submerged welding processes,

combining the number of wires and arcs, are

described in the literature [1] - [2]. It is obvious that

multiarc welding has a positive impact on the

process productivity. Nevertheless, an investigation

of the welding effects on the thermal, metallurgical

and mechanical changes in the base material is

compulsory.

When multiarc welding is applied, the total heat

is a sum of heats developed by each arc which has a

specific role and influence on the welded joint

properties and appearance (shape, dimensions). For

instance, when three electric arcs welding is applied,

the role of each arc is described below [3], [4]:

� the first arc is responsible for the weld root

achievement. A good correlation between

primary welding parameters is required in order

to avoid the defects occurrence;

� the second arc is needed to fill the gap between

sheets. Usually, the amperage is lower than that

used at first electric arc, in order to obtain a

suitable width of the weld and to keep the

penetration previously achieved;

� the third arc finalizes the weld shape and its

dimensions. The wire comes into the melted

metal pool performed by the first and second

electric arcs. Therefore, lower amperage and a

higher voltage are set so that an appropriate weld

width is obtained.

Two multiarc welding variants of the three

electric arcs welding are applied in the metallic

structures industry: DC-AC-AC in Europe and AC-

Recent Advances in Industrial and Manufacturing Technologies

ISBN: 978-1-61804-186-9 227

AC-AC in USA and Japan. Taking into account the

amperage used for the weld root, a correlation

between the amperages is recommended in Japanese

literature [5]. Also, there are correlations between

the pipe thickness on one side and the melting

coefficient, heat input and welding speed on the

other hand [6] - [8].

The equipment welding producers focussed on

the productivity increasing. For example, ESAB is

one of the leaders on the equipment welding market

and developed many multi-wires welding

technologies such as twin welding, tandem welding,

one arc welding with warm wire and twin-tandem

welding with warm wire. Applying multi-wires

welding with six simultaneous arcs, the deposition

ratio could reach 100 kg/h [10].

Combining the advantages of different welding

technologies, then productivity, deposition rate,

penetration and base metal dilution are monitored

and controlled. The optimal welding technology will

be established, as far as the diameters of wires,

welding parameters and optimal distance between

electric arcs. A high performance of the mechanical

properties of welded joints can be achieved, using

adequate welding materials and several welding

passes, simultaneously performed in separate

welding pools. The result is a thermal self-

treatment, with an important effect on the quality of

longitudinal welded joints of pipeline sections. A

special welding flux will be used to increase the

number of crystallizing centres in the welding pool

and to refine the final structure of the weld. Finally,

applying the multiarc welding procedure to the

pipelines manufacturing, an improvement of

plasticity and a better control of the brittle

compounds, which could occur in the weld and in

HAZ, are expected.

2 Experimental program The experimental program, including mono arc and

multiarc submerged welding, was conducted using

the welding installation presented in the figure 1.

The advantages of then welding system are related

to the following performances:

� high productivity;

� high deposition rate;

� welding of medium and large components

thickness;

� high heat generated by the electric arcs which

involves higher penetration and lower cooling

speed;

� different diameters and chemical composition of

the filler material can be combined within the

welding process;

� AC power supplies are cheaper, robust and easy

maintenance;

� the successive heating and cooling produce

extensions and the slag can be easily detached;

� reduced stresses and strains of the welded joint.

Still, high productivity of the process is achieved

only if the metallic structures are large or very large.

Besides, the heat introduced in the base metal

produces welding pool with large dimensions that

tends to flow. Because of this phenomenon,

ceramic, copper or flux backing is needed in order

to stop the melted pool flow.

One or two wires can be used when monoarc

submerged welding procedure is applied. When

double arc submerged welding is applied, distinct

welding pools are formed and higher deposition rate

and penetration are achieved. The distance between

arcs is set at 100 mm. The schematic representation

of the welding system, presented below, describes

the principle of mono and multiarc submerged

welding. Depending on the welding variant, one or

two welding torches, one to three wires feed

systems, one to three welding wire spools, one or

two welding hoppers and one or two power supplies

are combined in order to carry out the experiments.

Fig.1. Schematic representation of the welding

installation [9]

Welding in AC current, energy savings and a

better stability of the electrical arc are obtained. As

it can be noticed, two AC power supplies are

included in the welding system. The first power


ISBN: 978-1-61804-186-9 228

supply (1) is a three-phase AC one, which is

connected to the base metal and two wires, ensuring

high heat input and therefore a proper penetration.

The second power supply (2) is a mono-phase one

connected to the base metal and the third wire,

ensuring a suitable deposition and a good root

filling. At multiarc submerged welding, the filler

metal is supplied from distinct wire feed systems.

Due to the wire feed system design different wire

diameters can be used. Each electric arc is protected

by the flux provided through distinct welding flux

hopper. A welding flux vacuum cleaner was

positioned behind the welding arcs in order to

recover the welding flux and to ensure its

continuous recirculation.

The experimental program focused on an

investigation of each welding process influence on

the mechanical changes of the welded joint. Two

thickness values (17.5mm and 19.1mm) of API-5L-

X70 steel sheets have been investigated. OE-S2Mo

welding wire and OP119 welding flux were used in

the research. A set of three samples was performed

for each combination of welding variant and plate

thickness. According to the API-5L standard, X-

groove configuration has been prepared before

welding the sheets. The root was continuously

welded by gas metal arc welding (GMAW)

procedure. Then, longitudinal butt welds were made

on the both sides of the joint.

The welding conditions, including each welding

variant, plate thickness (s) and regimes parameters

(amperage – I, voltage – U, welding speed – sw), are

presented in the tables 1 and 2. The values of I1 and

I2, U1 and U2 are characteristic to the first welding

arc, respectively the second welding arc. The

symbols of d1 and d2 represent the selected wires

diameter for the first welding arc, respectively the

second welding arc.

Table 1.The welding parameters corresponding to the mono welding process

Welding

variant

Plate thickness

s [mm]

Wire

diameter

d [mm]

Sample Side I

[A]

U

[V]

Sw

[cm/min]

Monoarc

welding

17.5 d = 4

P1 Top 755 36 45

Bottom 830 36 45

P2 Top 780 35 40

Bottom 820 36 45

P3 Top 800 37 50

Bottom 840 38 45

19.1 d = 4

P1 Top 800 37 45

Bottom 900 38 40

P2 Top 800 37 40

Bottom 800 37 30

P3 Top 800 37 45

Bottom 900 38 40

Table 2.The welding parameters corresponding to the multiarc welding process

Welding

variant

Plate

thickness

[mm]

Wire

diameter

[mm]

Sample Side I1 [A]

I2 [A]

U1

[V]

U2

[V]

Sw

[cm/min]

Double arc

welding

17.5 d1 = 3.2

d2 = 4.0

P1 Top 680 510 31 32 60

Bottom 780 600 33 35 60

P2 Top 800 650 34 35 80

Bottom 880 720 33 35 80

P3 Top 780 550 33 34 62

Bottom 830 620 33 34 62

19.1 d1 = 3.2

d2 = 4.0

P1 Top 800 640 36 37 60

Bottom 850 550 36 36 60

P2 Top 850 850 39 38 100

Bottom 850 850 39 38 90

P3 Top 750 600 34 34 60

Bottom 850 680 34 36 70


ISBN: 978-1-61804-186-9 229

After the welding phase, the samples were

examined by non-destructive test (NDT) and those

with defects were excluded from the investigation.

The defect-free specimens were mechanically

machined – transversally to the welding direction –

and subject to tensile strength, toughness, yield

strength and hardness tests.

3 Results and discussion In order to make an analysis of the mono and

multiarc welding process influence on the

mechanical proprieties changes of the material

supposed to be weld, initial tensile strength, yield

strength, toughness and hardness of the base

material (BM) have to be known. The mechanical

characteristics of API-5L-X70 base metal are

presented in table 3.

The comparative study between monoarc and

multiarc (double) submerged welding variants

reveals that the behaviour of this steel is different

when one or double electric arcs act on the parent

metal. The first electric arc seems to have a

preheating role on the base metal, leading to a less

impact of the welding process on it.

To have a general view on the mechanical

properties changes, the average results of the tensile,

yield strength, toughness and hardness tests are

comparatively presented in the tables 3 and 4. The

tests were done according to ASTM standards. All

samples were fractured in the base material, not in

weld or HAZ, proving that the welding technologies

were rightly designed and the couple wire-flux

correctly selected. The tests for toughness testing

were performed at 0oC on specimens with notches

located in different regions of the joint (weld and

HAZ). As a conclusion, better results of the

mechanical characteristics, meaning a less impact of

the welding process, have been achieved when

double arc submerged welding was applied.

Table 3. API-5L-X70 steel proprieties according to the analysis report

Rm [N/mm2] Rp02 [N/mm

2] KV [J/cm

2] HV10

617 485 245 209

Table 4. Experimental results when monoarc welding is applied

Plates thickness [mm] Rm [N/mm2] Rp02 [N/mm

2] KV [J/cm

2] Tensile testing

17.5 615 465 Weld – 152 / HAZ – 166 Fracture → BM

19.1 616 470 Weld – 148 / HAZ –154 Fracture → BM

Table 5. Experimental results when double arc welding is applied

Plates thickness [mm] Rm [N/mm2] Rp02 [N/mm

2] KV [J/cm

2] Tensile testing



3.1. Tensile Strength Testing This destructive test is used to measure the strength

of a welded joint. The usual requirements for the

tensile strength of welds are that the specimen shall

pull not less than 90 percentage of the base metal

tensile strength. The samples of dimensions

350x50x17.5mm and 350x50x19.1mm were

subjected to the tensile strength testing. Few of the

samples are presented in the figure 2. To obtain a

suggestive image of the welding process influence

on the material behaviour from the mechanical point

of view, a calculus to obtain the average values. All fractures were located in the parent metal as

presented in the tables above.

Fig.2. Tensile strength vs. thickness


ISBN: 978-1-61804-186-9 230

That is why the average obtained values are

approximately equal with tensile strength of the

parent metal, no matter thickness or welding variant.

The average values of tensile strength and yield

strength vs. thickness and welding variant are

illustrated in the charts shown in the figure 2 and

figure 3.

A slight decrease of yield strength was noticed in

the welded joints performed by the both welding

variants. Still, the influence of the welding process

is less when double arc welding is applied. In this

case a decrease from 485N/mm2 to 472N/mm

2

(when s=17,5mm) or to 480N/mm2 (when

s=19,1mm) is observed. When monoarc submerged

welding process is used the decrease is lightly

higher from 485N/mm2 to 465N/mm

2 (when

s=17,5mm) or to 470N/mm2 (when s=19,1mm).

Fig.3. Yield strength vs. thickness

3.2. Toughness Testing

The test is used to determine the energy absorption

on the surface unity in fracturing the specimen and

consists of impact loading a three-point bend bar

which contains a relatively blunt notch. This

specimen type characterises very well the

propagation effect of the cracks in the welded joint.

The tests for toughness testing were performed at

0ºC on specimens of 100x15x17.5 mm and

100x15x19.1 mm dimensions, with notches located

in different regions of the joint (weld and HAZ) as

figure 4 shows. The lowest impact notch value from

the joint is achieved in the weld and that is due to

the casting structure obtained in the weld. Besides,

overheating effect is responsible the decrease of

plasticity propriety in HAZ of the joint.

Fig.4. Samples subjected to toughness testing

A high decrease of toughness was noticed in the

welded joints performed by the both welding variant

(Fig.5). Still, the influence of the welding process is

less when double arc welding is applied. In this case

a decrease from 245J/cm2 to 178J/cm

2 in weld and

to 180J/cm2 in HAZ (when s=17,5mm), or to

165J/cm2 in weld and to 171J/cm

2 in HAZ (when

s=19,1mm) is observed. When monoarc submerged

welding process is used, the decrease is higher from

245J/cm2 to 152J/cm

2 in weld and to 166J/cm

2 in

HAZ (when s=17,5mm), or to 148J/cm2 in weld and

to 154J/cm2 in HAZ (when s=19,1mm) is observed.

Fig.5. Toughness vs. thickness

3.3. Hardness Vickers HV 10 Testing

Vickers hardness HV10 measurements were carried

out using a NEOPHOT 32 microscope to assess the

metallurgical modifications in the fusion zone,

HAZ, and BM areas, according to the indentation

pattern illustrated in figure 6. The Vickers HV10

hardness has the highest values in the fusion weld

area, no matter thickness or welding variant (Fig.7).

An increasing of hardness is registered in HAZ too,

but still lower than in the weld. The metallurgical

process caused by welding involves structural

modifications and overheating important influences

on the mechanical performances of the welded joint.

Fig.6. Vickers hardness HV10 indentation pattern


ISBN: 978-1-61804-186-9 231

Fig.7. Vickers hardness HV10 vs. thickness

4 Conclusions Based on the conducted work, the following main

conclusions can be drawn:

� When multiarc welding is applied, the total heat

is a sum of heats developed by each arc which

has a specific role and influence on the welded

joint properties and its appearance:

• the first arc is responsible for the weld root

achievement. An accurate and good correlation

between primary welding parameters is required

in order to avoid the defects occurrence;

• the second arc is needed to fill the gap

between sheets. Usually, the amperage is lower

than that used at first electric arc, in order to

obtain a suitable width of the weld and to keep

the penetration previously achieved;

• the third arc finalizes the weld shape and its

dimensions. The wire comes into the melted

metal pool performed by the first and second

electric arcs. Therefore, lower amperage and a

higher voltage are set so that an appropriate weld

width is obtained.

� The comparative study between monoarc and

multiarc (double) submerged welding variants

reveals that the behaviour of API-5L-X70

pipelines steel is different when one or double

electric arcs act on the parent metal. The first

electric arc seems to have a preheating role on

the base metal, leading to a less impact of the

welding process on its;

� Better results of the mechanical characteristics,

meaning a less impact of the welding process,

have been achieved when double arc submerged

welding was applied.

Finally, it is obvious that multiarc welding has la

positive impact on the productivity and the

investigation reveals the less impact of the multiarc

submerged welding on the mechanical properties of

the base material.

Acknowledgement This work was supported by the Romanian National

Authority for Scientific Research, CNDI –

UEFISCDI, through grant 27/2012, project number

PN-II-PT-PCCA-2011-3.1-1057.

References:

[1] Babu S. P. K., Natarajan S., Influence of heat

input on high temperature weldment corrosion in

submerged arc welded power plant carbon steel,

Materials & Design, 29(5), 2008, pp. 1036–1042.

[2] Datta, S., Bandyopadhyay, A. and Pal, P. K.,

Application of Taguchi philosophy for

parametric optimization of bead geometry and

HAZ width in submerged arc welding using a

mixture of fresh flux and fused flux, International

Journal of Advanced Manufacturing Technology,

36, 2008, pp. 689-698.

[3] Gunaraj, V. and Murugan, N., Prediction and

Comparison of the Area of the Heat Affected

Zone for the Bead-no-Plate and Bead-on-Joint in

Submerged Arc Welding of Pipes, Journal of

Materials Processing Technology, 95(1-3), 1999,

pp. 246-261.

[4] Joarder, A., Saha, S. C. and Ghosh, A. K., Study

of Submerged Arc Weld Metal and Heat Affected

Zone Microstructure of Plain Carbon Steel,

Welding Journal, 1991, pp. 151-157.

[5] NN, Pipes and tubes of nippon steels, Nippon

Steel News, br. 326, 2005, http://www.nsc.co.jp.

[6] Datta S., Sundar M., Bandyopadhyay A., Pal P.

K., Nandi G., Roy, S. C., Statistical modeling for

predicting bead volume of submerged arc butt

welds, Australasian Welding Journal, 51(2),

2006, pp. 39–47.

[7] Kanjilal P., Pal T.K., Majumdar S.K., Combined

effect of flux and welding parameters on

chemical composition and mechanical properties

of submerged arc weld metal, Journal of

Materials Processing Technology, 171, 2006, pp.

223-231.

[8] Karaoglu S., Secgin A., Sensitivity analysis of

submerged arc welding process parameters,

Journal of Material Processing Technology,

202(3), 2008, pp. 500-507.

[9] Mistodie L.R., Rusu C.C., Constantin E.,

Scutelnicu E., Voicu C., Comparative

experimental study on mono and multiarc

submerged arc welding of API 5L-X70 steel,

Proceedings of the European Federation for

Welding - EWF EUROJOIN 8 Conference, 24-

26 May 2012, Pola-Croatia, pag. 125-132,

ISBN978-953-7518-02-8

[10] NN, Pipeline catalogue, ESAB.


ISBN: 978-1-61804-186-9 232

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