A Study on the Metallurgical and Mechanical Characteristics of the Weld Joint of X80 Steel

Choong-Myeong Kim, Jong-Bong Lee, Jang-Yong Yoo

Technical Research Laboratories, POSCO Pohang, Kyungbuk, KOREA

ABSTRACT In order to respond the needs for high strength linepipe steel on these

natural gas pipeline projects, POSCO has developed API-X80 grade steels with enhanced low temperature toughness. This paper describes the design concept and qualities of both base materials and weld joints of POSCO’s API-X80 steel pipes.

Until recently X70 steel has been the generalized grade for linepipe steel. However, there is a need to use higher grade steels such as X80 or X100 for high transmission efficiency and lower construction costs.As X80 steel is manufactured by higher alloy addition and severe rolling reduction, weldability of X80 steel seems to be a more important issue than with previously-used lower grade steels. Several mill trials have been performed to optimize the production processes for X80 steel at POSCO. Field manufactured API-X80 grade plates with a thickness of 15.6mm were found to have very high strength and toughness. It is thought that the high strength and enhanced low temperature toughness of developed plates are attributed to the acicular ferrite and very fine polygonal ferrite microstructure of the material. The yield strength of API-X80 grade plates with acicular ferrite microstructure increased after pipe forming. X80 plates have high impact toughness in seam weld heat affected zones (HAZ) and sufficient strength without cold cracking in girth weld joints.

STEEL MANUFACTURING Alloy design concept of API-X80 Low-grade steel, API-X70, has a polygonal ferrite – pearlite microstructure. However, the polygonal ferrite – pearlite microstructure cannot guarantee the combination of strength and toughness required for API-X80 steel (Vito, 1995). API-X80 steel was designed to have an acicular ferrite microstructure of carbide-free cells of bainite grouped in domains with an isle-like dispersed Martensite/Austenite (MA) constituent. It forms by decreasing the bainite transformation temperature (Bs) (Leslie, 1982; Tamura, 1988), as expressed in Equation 1. The decrease of Bs makes easy transformation of acicular ferrite. This microstructure is responsible for the strength of X80 steel. Adding nickel, manganese, or molybdenum is effective in the formation of acicular ferrite. From the viewpoint of steel chemistry, manganese is the least expensive element that retards the formation of polygonal ferrite, which can be supplemented with molybdenum. To obtain high toughness, the carbon content is kept at about 0.07% or less by weight. Table 1 shows the typical chemical composition of X80 steel plate.

KEY WORDS: API-X80, linepipe, seam welding, girth welding, HAZ toughness, DWTT. INTRODUCTION The material quality requirements of the gas and oil transmission pipelines have steadily increased over the past few decades. The gas and oil industry demands more economical transportation due to longer transportation lines and active exploitation in areas of severe weather conditions.This transport efficiency can be achieved by constructing pipelines with high strength steels, allowing higher operating pressures and gas transmission rates. API-X80 steels have been developed and applied to the several pipeline projects during the last decade. Further, the market needs for the more economical and safe transportation of oil and gas in hostile environments require that X80 steel has better toughness than the previously developed steels. Recently, large-scale natural gas pipeline projects are being constructed and planned in North America and Northeast Asia (Klatt, 2000; Asakura, 2000). These pipelines will be operated under cold climate conditions that require API-X80 steel for the economical construction.

BS = 830 - 270(%C) – 540(%Mn/6) – 555(%Ni/15) – 350 (%Cr/5) – 415(%Mo/5) (1) Table 1 Chemical compositions of API-X80 steel (wt. %)

C Si Mn P S Mo Others Pcm Ceq

0.07 0.33 1.78 ≤0.015 ≤0.003 0.24 Cr+Mo+NiCu≤0.8 0.20 0.45

Rolling and Cooling Conditions Acicular ferrite can be obtained by rapid cooling from the austenite region, while the formation of polygonal ferrite is retarded by the effect of alloying elements. The derived rolling temperature is about Ar3 + 30ºC and the finishing cooling temperature is below 500ºC. The fast cooling rate is over 10ºC/s. The steel was manufactured in oxygen converters and then cast to slabs with a thickness of 250mm. The slabs were then rolled into plates of 15.6mm thickness x 2,900mm (W). Based on the given chemical composition and rolling conditions, the X80 steel exhibits a fine microstructure, as shown in Figure 1. Notice that the microstructure contains fine acicular ferrite, polygonal ferrite, and MA.

Figure 1 Microstructure of X80 steel plate

BASE METAL PROPERTIES Tensile Properties API-X80 steel plates of 15.6mm thickness were formed as pipe by three-roll bending and then cold expanded into 914mm diameter pipe. The amount of cold expansion was about 1%. Figure 2 shows the changes of yield strength from the plates to the roll bended pipes. Yield and tensile strength of the base plates are greater than the specification requirements of API-X80 steel. After pipe forming, yield strength actually increased in transverse and longitudinal directions by 10~20MPa. The increase of strength after pipe forming is attributed to the main phase of X80 steel plate being composed of acicular ferrite.

300

400

500

600

700

800

YS (Tran) YS(Long.) TS (Trans) TS(Long.)

Strength

(M

Pa)

Plate Pipe

Figure 2 Tensile properties of the X80 steel plate and pipe

Impact Toughness Fig.3 shows the impact test results of base plate and pipes. The Charpy

V-notch impact toughness of the plates and pipes shows very high toughness level that exceeds 300 joule at -60°C. Toughness levels are similar in both plate and pipe until a temperature of about -80°C. These toughness levels could be a result of the fine accicular ferrite structures established in alloy design.

Figure 3 Charpy V-notch impact toughness of X80 steel plate and pipe Drop Weight Tear Test Properties Figure 4 shows the drop weight tear test (DWTT) properties of both plate and pipe. As shown in the figure, the 85% shear area transition temperature of X80 plate is about -55°C and that of pipe is about -45°C at the 90° location from the weld seam. Together with its impact toughness, DWTT properties show that X80 plate has sufficient toughness for use in low temperature environments.

Figure 4 DWTT properties of the X80 steel plate and pipe

Table 2 shows summary of the mechanical properties of X80 steel plates. As shown in the table, X80 steel meets or exceeds all of the required property targets. Table 2 Summary of mechanical properties of X80

Design YS (MPa) TS (MPa) YR (%) DWTT SA at -40°C(%)

CVN Energy at -40°C (J)

Target 552~670 620~827 ≤ 93 ≥ 85 ≥ 110

Results 571~591 681~724 81~87 96~99 300~350

Tensile and Bending Properties of SAW Joint SEAM SAW JOINT PROPERTIES To ensure that 15.6mm thick X80 steel plate has the adequate seam weld properties needed to X80 steel pipe, X80 steel was welded by 2-pole submerged arc welding (SAW). SAW conditions used for seam welding are listed in Table 3. Heat inputs for inside and outside welding were about 2.7kJ/mm. SAW wire and flux manufactured by a domestic welding consumables maker were equivalent to the grade of AWS A5.23 F8A4-EA3-G. Wire diameter was 4.0mm. The macrograph of weld section is shown in Figure 5. The full thickness of 15.6mm could be fully joined with the applied heat input.

Figure 7 shows the fractured appearance of tensile test specimens of the seam SAW joint, with the fractures occurring at the weld metal. This may have been caused by the relatively low hardness of weld metal. It is expected that fractures of tensile specimens would not occur at the weld metal if the weld reinforcement was not removed. Although the fractures occurred at the weld metal, the tensile strength of the weld joint is sufficiently high enough to meet strength requirements, as shown in Figure 8. The average tensile strength was about 712 MPa. Note that elongation of the SAW joint is lower than expected. This appears to be because of the narrow width of weld metal that has a lower hardness than the base metal.

Table 3 Seam SAW conditions Although elongation in tensile test was low, the bending properties of

SAW joint shows high resistance to the failure from bending deformation as shown in Figure 9. Side Polarity Current

(A) Voltage

(V) Speed

(cm/min)

Heat Input

(kJ/mm)

Interpass temp. (°C)

L DC 700 35 Inside

T AC 600 35 100 2.73

L DC 685 35 Outside

T AC 585 38 102 2.72

Max. 150

* SAW wire: A-3(φ4.0), flux: S-777MXH

Figure 5 Macrograph of seam weld section Hardness Distribution of SAW Joint Figure 6 shows the hardness distributions across the base metal, heat affected zone, and weld metal of seam SAW joint. There was no hardening near fusion line in the HAZ, though there was a slight softening at the fine grained HAZ. The maximum hardness of the HAZ is below Hv 250. The weld metal hardness was not higher than that of the base metal hardness.

Figure 6 Hardness distributions of seam SAW joint

Figure 7 Fractured appearances of tensile test specimens

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T.S.> 620MPaY.S.> 552MPa

p

T.S. Y.S. El.T

.S.,

Y.S

. (M

Pa)

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ngat

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(%)

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Weld Metal HAZ Base Metal

15.6 mmtHar

dnes

s (H

v, 1

0kg)

Distance from Fusion Line (mm)

ID OD

Figure 8 Tensile properties of SAW joint

Figure 9 Bended specimens of SAW joint

Impact Toughness of SAW Joint Figure 10 shows the Charpy V-notch impact toughness of SAW joints. As demonstrated in well-known previously published papers, the lowest impact toughness of the weld joint is at the fusion line in the HAZ. However, the impact toughness of the fusion line of X80 steel was about 129J at -20°C. Even at -40°C, the fusion line has an impact toughness of about 90J. These toughness levels are adequate for most linepipe projects. Weld metal formed with base metal, welding wire, and flux also has high impact toughness at low temperatures. From these results, the welding consumables used in this test were deemed suitable to use with X80 steel. The authors performed several weld tests regarding applicable welding heat input. The results indicate that a heat input range of 2.5~3.0kJ/mm yields sufficient impact toughness for 15.6mm-thick X80 steel.

-70 -60 -50 -40 -30 -20 -10 0 10 20 30

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300 15.6 mmt

CV

N Im

pact

Abs

orbe

d En

ergy

(jou

le)

Temperature (oC)

WeldMetal FusionLine F.L.1mm F.L.3mm F.L.5mm

-70 -60 -50 -40 -30 -20 -10 0 10 20 30

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pact

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ergy

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WeldMetal FusionLine F.L.1mm F.L.3mm F.L.5mm

Figure 10 Charpy V-notch impact toughness of SAW joint

IRTH GMAW JOINT PROPERTIES

o confirm that the X80 steel plate has adequate girth weld properties

Table 4 Mechanized GMAW conditions elding Conditions

G Tfor X80 pipe, test plate was welded by mechanized gas metal arc welding (GMAW). GMAW conditions used for girth welding are listed in Table 4. 1.2mm diameter GMAW wire, equivalent to the grade of AWS A5.29 E91T1-K2, was used to make the test welds. The macrograph of a GMAW weld section is shown in Figure 11.

Welding Parameters WWelding Method GMAW (FCAW)

Welding Wire SC-91K2 Cored Shielding Gas 100% CO2 as Flow (l/min. 20

Polarity D C+emperatu 20

Arc Voltage (V) 26 elding Current (A 230

elding Speed (cm/min. 32 Heat Input (kJ/mm) 1.12

G )

Initial T re (°C)

W ) W )

Figure 11 Macrograph of girth GMAW joint

ardness Distribution of GMAW Joint

MAW joints. Due to a

ensile and Bending Properties of GMAW Joint

specimens of

H

igure 12 shows the hardness distribution of GFrelatively low heat input compared to SAW, GMAW HAZ shows more hardening near the fusion line. However, softening at the base metal side of HAZ does not occur. Final weld runs should be carefully controlled as the maximum hardness is above Hv300 with a heat input of 1.12kJ/mm. The weld metal hardness is slightly higher than that of the base metal.

Figure 12 Hardness distribution of GMAW joint

-6 -4 -2 0 2 4 6 8 10 1250

100

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250

300

Fusion Line

Weld Metal HAZ Base Metal

15.6 mmtHar

dnes

s (H

v, 1

0kg)

Distance from Fusion Line (mm)

T

igure 13 shows the fractured appearance of tensile test Fgirth GMAW joints. Fractures occurred at the base metal due to its relatively low hardness. The average tensile strength was about 659MPa, satisfying specification requirement, as shown in Figure 14. Similar to seam SAW joints, girth GMAW joints also have good resistance to failure under bending deformation. There were no cracks on the bent surfaces in Figure 15.

Figure 13 Fractured appearances of tensile test specimens

Figure 14 Tensile properties of GMAW joint

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Y.S.> 552MPa

T.S. Y.S. El.T

.S.,

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Pa)

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-50 -40 -30 -20 -10 0 10 20 30

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CVN

Impa

ct A

bsor

bed

Ener

gy (j

oule

)

Temperature (oC)

WeldMetal FusionLine F.L.1mm F.L.3mm F.L.5mm

pact Toughness of GMAW Joint

pact toughness of GMAW ints. The impact toughness of the fusion line was about 134J at -20°C

ONCLUSIONS

olled steel plates with a thickness of 15.6mm have een developed at POSCO. POSCO’s X80 steel plates satisfy the API

ughness of GMAW Joint

pact toughness of GMAW ints. The impact toughness of the fusion line was about 134J at -20°C

ONCLUSIONS

olled steel plates with a thickness of 15.6mm have een developed at POSCO. POSCO’s X80 steel plates satisfy the API

Figure 15 Bended specimens of GMAW joint

Figure 16 Charpy V-notch impact toughness of GMAW joint Figure 16 Charpy V-notch impact toughness of GMAW joint

and pipe are -55℃ and -45℃ respectively. An acceptable combination of high strength and enhanced toughness of POSCO’s X80 steel plates is attributed to an acicular ferrite and very fine polygonal ferrite microstructure. Seam SAW joints and girth GMAW joints have sufficiently high tensile strength and impact toughness at low temperatures. Both weld joints also have high resistance to failure under bending deformation.

and pipe are -55℃ and -45℃ respectively. An acceptable combination of high strength and enhanced toughness of POSCO’s X80 steel plates is attributed to an acicular ferrite and very fine polygonal ferrite microstructure. Seam SAW joints and girth GMAW joints have sufficiently high tensile strength and impact toughness at low temperatures. Both weld joints also have high resistance to failure under bending deformation.

Im Figure 16 shows the Charpy V-notch imFigure 16 shows the Charpy V-notch imjojo and 82J at -40°C. HAZ toughness in GMAW appeared to be nearly same as with SAW joints. These toughness levels in GMAW joint also are more than sufficient for most linepipe projects. However, weld metal toughness is relatively lower than HAZ toughness, about 68J at -20°C. This difference appears to be caused by using different welding consumables. The GMAW heat input range that makes for a sufficient toughness level in the HAZ is approximately 0.5~1.5kJ/mm.

and 82J at -40°C. HAZ toughness in GMAW appeared to be nearly same as with SAW joints. These toughness levels in GMAW joint also are more than sufficient for most linepipe projects. However, weld metal toughness is relatively lower than HAZ toughness, about 68J at -20°C. This difference appears to be caused by using different welding consumables. The GMAW heat input range that makes for a sufficient toughness level in the HAZ is approximately 0.5~1.5kJ/mm.

REFERENCES REFERENCES Asakura, K. (2000), “Asian demand growth driving global gas trade

outlook”, Oil and Gas Journal, Vol.98, May 15, p.8 Asakura, K. (2000), “Asian demand growth driving global gas trade

outlook”, Oil and Gas Journal, Vol.98, May 15, p.8 Klatt, T.J. (2000), “Responding to a Nothern Pipeline Challenge”, Proc.

of the 2000 International Pipeline Conference, ASME, Vol.1, p.91. Klatt, T.J. (2000), “Responding to a Nothern Pipeline Challenge”, Proc.

of the 2000 International Pipeline Conference, ASME, Vol.1, p.91. Leslie, W.C. (1982), The Physical Metallurgy of Steels, McGRAW-

HILL International Book Company, p.201. Leslie, W.C. (1982), The Physical Metallurgy of Steels, McGRAW-

HILL International Book Company, p.201. CC Tamura, I., Sekine, H., Tanaka, T. and C. Ouchi (1988),

Thermomechanical Processing of High-strength Low-alloy Steels, Butterworth & Co., p.162

Tamura, I., Sekine, H., Tanaka, T. and C. Ouchi (1988), Thermomechanical Processing of High-strength Low-alloy Steels, Butterworth & Co., p.162

API-X80 grade hot rAPI-X80 grade hot r

Vito, A.D. et al, (1995), “From standard production X70 towards the development of X80”, Pipeline Technology (edited by R. Denys), Vol.2, Elsevier Science B.V. p.565.

Vito, A.D. et al, (1995), “From standard production X70 towards the development of X80”, Pipeline Technology (edited by R. Denys), Vol.2, Elsevier Science B.V. p.565.

bbrequirements for strength and toughness for commercial applications. The DWTT transition temperatures of 15.6mm thick X80 steel plate

requirements for strength and toughness for commercial applications. The DWTT transition temperatures of 15.6mm thick X80 steel plate