• Corresponing author Keyvan Seyedi Niaki,E-mail address: seyed_ebrahim_vahdat@yahoo.com • Doi: http://dx.doi.org/10.11127/ijammc2017.10.05Copyright@GRIET Publications. All rightsreserved

61

Advanced Materials Manufacturing & Characterization Vol. 7 Issue 2 (2017)

Advanced Materials Manufacturing & Characterization

journal home page: www.ijammc-griet.com

Optimizing Tensile Strength and Corrosion Resistance of API 5LX52 Steel

Pipe after Repair Using MMAW

Keyvan Seyedi Niaki1, Mohammad Karim Kheradmand Vojdan2, Seyed Ebrahim Vahdat3,*

1 MSc, Department of Mechanical Engineering, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran 2 MSc student, Department of Engineering, Bandar Abbas Branch, Islamic Azad University, Bandar Abbas, Iran 3 Assistant Professor, Department of Engineering, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran.

A B S T R A C T

In the present study, the effect of multiple repairs on the

microstructure and, thus, strength and corrosion resistance of API

5LX52 steel is investigated using manual metal arc welding

(MMAW). To this end, four sets of specimens were utilized for the

tensile and corrosion tests: control, once-, twice- and three-time-

repaired specimens. The result showed that strength was

decreased by repeating the repair. Also, the corrosion resistance

of the twice-repaired specimen was higher than that of other

repaired specimens. Therefore, considering maximum tensile

strength and corrosion resistance, the use of micro-alloy

electrodes with twice repair is recommended to achieve the best

result.

Keywords: Manual metal arc welding; Micro-alloy; Corrosion

1.Introduction The API 5LX52 pipe is one of the most frequently used pipes in the

oil, gas and petrochemical industry and is used for transferring oil

products. This steel is a low-carbon micro-alloy, the microstructure

of which is optimized using thermodynamic procedures. The

above-mentioned process results in the development of strong

sediments with various sizes and morphologies, which could

eventually enhance strength and corrosion resistance [1]. In oil

product transfer engineering, oil and gas pipeline fatigue is an

important challenge that results in a decrease in strength and an

increase in corrosion rate in the parts of the pipes. Pipe fatigue

must be resolved through the repair or replacement of the pipe.

Pipe replacement is highly time-consuming and costly [2].

Therefore, repair is usually performed.

The age hardening procedure at 250°C on X52 carbon steel pipe

changes the fracture type from brittle to ductile in welding metal

and weld-affected zone. It also increases grain size. The effect of

this procedure increases strength up to 500 hrs, but decreases it

afterwards. The increase in strength is attributed to the

precipitation hardening phenomenon and the decrease in strength

shows the increase in precipitation particle size [3-4]. Yield

strength and hardness are decreased in the A36 low-carbon steel

repaired using butt fusion TIG welding. This change in properties is

attributed to the increase in grain size and the area of weld-

affected zone [5].

The results of multi-pass welding on an API 5IX-70 steel pipe

demonstrate that tensile residual stress is created in the center of

welding metal along the circumference of the pipe’s outer surface.

Maximum stress equals 60% of welding yield strength. Thus,

tensile strength and impact resistance are decreased, which is

attributed to microstructure changes [6-7].

For X52 seamless carbon pipes which are repaired for multiple

times using manual metal arc welding (MMAW), a decrease is

reported in corrosion resistance (especially between the base

metal and heat-affected zone), yield strength, tensile strength and

malleability, which is ascribed to microstructure changes [8].

The above-mentioned studies have pointed to a decrease in API

X52 pipe properties after repair, which is attributed to

microstructures in all the cases. Although repairing the pipe is

faster and less costly, there is no precise evaluation of property

change in the API 5L X52 steel after the repair because its

relationship with microstructures is not yet clear. The main aim of

the present study is to determine and then evaluate the

62

relationship between microstructures and tensile strength and

corrosion resistance in the pipe after multiple repairs.

Materials and Method

An API 5LX52 steel seam pipe with 16” in diameter produced

through sub-powder welding was prepared from the 6th district

warehouse of gas transfer operations. This pipe is currently used in

high-pressure gas transfer pipelines. The mentioned steel is a low-

carbon micro-alloy with high strength. The chemical composition

was determined through light scattering spectrometry using the

equipment of Applied Research Laboratories (Switzerland)

according to the ASTM E415 [9]. The results are presented in Table

1.

Table 1 Chemical composition of St37

Nb Al Nb Ni Mn P S Si C

Fe

Element

0.01 0.03 0.01 0.02 1.09 0.013 0.001 0.30 0.16 Wt%

Zn Cr Mo Co Sn V Ti W Cu

balance

Element

0.001 0.03 0.003 0.004 0.001 0.002 0.01 0.005 0.02 Wt%

The pipe was repaired through MMAW using a Pars 630 welding

machine (Iran) with the maximum capacity of 400 A according to

the API 1104 [10]. The electrode types used in this study were

E6010 (3.25 mm in diameter) for the root pass and E7010 (4 mm in

diameter) for the other passes, and their chemical compositions

based on the ASME SEC II [11] are listen in Tables 2.

Table 2 Chemical composition Wt% of E7010 and E6010 electrodes

ASME SEC II [11]

V Mo Cr Ni S P Si Mn C Electrode

0.08 0.30 0.20 0.30 -------- 1.00 1.20 0.20 E6010

---- 0.4-0.65 ------- ≤0.03 ≤0.4 ≤0.6 ≤0.12 E7010

The tensile test was conducted according to the AWS D1.1 [12] on

4 sets of specimens: control, once-, twice- and three-time-

repaired; the control specimen was not repaired. The once-

repaired specimen was welded once; the twice-repaired specimen

was welded twice and the three-time-repaired specimen was

welded three times. Each set included4 specimens for the tensile,

1 for the metallographic and 1 for the corrosion tests. The

corrosion test was conducted according to the ASTM G3 and ASTM

G1 [13-14]. The yield strength and tensile strength of the specimen

under the single-axis tensile test were measured at ambient

temperature using the GOTECH tensile equipment. Mean yield

strength and tensile strength of the control specimen were 465

and 558 MPa, respectively.

Results and Discussion

The metallographic specimens were prepared according to the

ASTM E3 [15]. In Figs. 1 and 2, the light and dark phases are

pearlite and ferrite (background), respectively.

Figure 1 FESEM image of phase A

Figure 2 FESEM image of phase B

Based on energy dispersive spectroscopy (EDS), A and B particles

in Figs. 3-6 are the transfer-carbides because they contain 10.94-

19.32 A% of carbon.

63

Figure 3 EDS of phase A

Figure 4 EDS of phase A (second one repeated)

Figure 5 EDS of phase B

Figure 6 EDS of phase B (second one repeated)

Grain size was measured according to the ASTM E112 [16] and the

contents of ferrite and pearlite were calculated according to ASTM

E 883 [17], as presented in Table 3.

Table 3 Average of yield strength, tensile strength, pearlite

content, ferrite content, and grain size and corrosion rate for all

specimens

Yield

strength

Tensile

strength

Pearlite

content

Ferrite

content

Grain

size

Corrosion

rate Specimen

465 MPa 558 MPa 30 V% 70 V% 11.4

µm 13.15 mpy Control

417 MPa 552 MPa 37 V% 63 V% 11.8

µm 14.28 mpy 1 repair

425 MPa 550 MPa 32 V% 68 V% 11.2

µm 4.256 mpy 2 repairs

413 MPa 541 MPa 25 V% 75 V% 11.9

µm 28.52 mpy 3 repairs

Based on Table 3, the content increase in pearlite after the first

repair (from 30 to 37 V%) could be attributed to the fast cooling in

the welding metal. However, the content of pearlite in a ferrite

background decreased from 37 to 25 V% from the once- to the

three-time-repaired specimen. The reason is that the heat

entering in different welding passes from the once- to three-time-

repaired specimens increased the time, for which the piece was

maintained at grain-growth temperature. Thus, there was enough

time for the expansion of grains and decrease of grain size value.

The germination of the new phase in the melt occurred at the

grain boundary. The total grain boundary length decreased;

therefore, the content of pearlite had a decreasing trend with the

increase in the repair times.

Strength must increase with the increase in the amount of pearlite

and decrease in grain size [18-19]. For instance, based on Table 3,

the amount of pearlite was less (25 V%) and its grain size was

larger (11.9 μm) in the three-time-repaired specimen.

Consequently, its yield strength (413 MPa) and tensile strength

(541 MPa) were less than other specimens. The twice-repaired

specimen had less yield strength (425 MPa) and tensile strength

(550 MPa) than the control. Nevertheless, in this specimen, the

amounts of pearlite (32 V%) and grain size (11.2 μm) were less

than those of the control (30 V% and 11.4 μm, respectively).

As a result, there was no significant correlation between the

amount of pearlite or grain size, on the one hand, and yield and

64

tensile strength, on the other hand. To facilitate the comparisons,

the above-mentioned variables are compared in Fig. 7. We must

look for another variable to find a significant correlation.

Therefore, the available alloy elements and their total amount in

the welding metal are listed for all the specimens in Table 4.

Figure 7 Average of yield strength, tensile strength, pearlite

content, ferrite content and grain size for all specimens

Table 4 Alloy element inside the weld metal for all specimens

Specimen Mo Ni Cu V Cr Nb Sum

Control 0.003 0.35 0.02 0.004 0.02 0.002 0.399

1 repair 0.27 0.36 0.03 0.002 0.02 ---- 0.682

2 repairs 0.32 0.38 0.04 0.01 0.04 0.004 0.794

3 repairs 0.32 0.38 0.04 0.004 0.02 0.003 0.767

According to ASTM G3 and ASTM G1 [13-14], the results of the

corrosion test in the control and repaired specimens are listed in

Table 3 and shown in Fig. 8 for the comparison.

Figure 8 Average of yield strength, tensile strength, corrosion rate

and the content of alloy elements inside the weld metal for all

specimens

Based on Figs. 8, the decreasing trends in the final strength and

yield strengths of the specimens were similar. We expected the

strength trend to be decreasing with repeating the repair.

Nevertheless, the yield strength of the twice-repaired specimen

was slightly (less than 2%) more than that of the once-repaired

specimen, the reason for which is ascribed to the error of the

tensile test equipment (allowed up to 3%).

The corrosion rates of the specimens are compared in Fig. 8. It was

increased compared with the control one with once- and twice-

repair. Nevertheless, it was decreased in the twice-repaired

specimen (at least 3 times) compared with the other specimens.

The MMAW procedure at ambient temperature and in laboratory

conditions created thermal residual stress in the welding piece.

Therefore, high-energy points increased corrosion rate. However,

the thermal residual stress may be adjusted in the twice-repaired

piece, which needs further reason to confirm.

Based on Fig. 8, the total amount of alloy elements, i.e. copper,

nickel, molybdenum, chromium, vanadium and niobium, which

increased corrosion resistance was higher in the twice-repaired

specimen compared to the others. In addition, yield strength and

tensile strength were highest in the twice-repaired specimen

compared to other repaired specimens since the total amount of

alloy elements was the highest. According to Table 2, these

elements were resulted from the electrodes.

0.00

100.00

200.00

300.00

400.00

500.00

600.00

0 1 2 3

Perlite V%

× 10

Grain size

(micron) ×

20

Tensile

stregth

(Mpa)

Yield

Strength

(Mpa)

Number of repairing

Para

met

ers

0

100

200

300

400

500

600

0 1 2 3

Tensile

stregth

(Mpa)

Yield

Strength

(Mpa)

Corrosion

rate (mpy)

× 11

Sum of

alloy

content ×

470

Par

amet

ers

Number of repairing

65

Conclusion

The present study was aimed to investigate the effects of multiple

welding using MMAW on tensile strength and corrosion resistance

in an API 5LX 52 steel pipe in order to repair oil and gas transfer

pipelines. To this end, four sets (four specimens each) were

utilized. The findings suggested that in addition to tensile strength,

corrosion resistance was higher (at least 3 times) in the twice-

repaired specimen than other repaired ones because the amount

of alloy elements was maximized with twice repair. Therefore,

considering maximum tensile strength and corrosion resistance,

the use of micro-alloy electrodes with twice repair is

recommended to achieve the best result.

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