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Development Development Development Development and production and production and production and production of linepipe steels in of linepipe steels in of linepipe steels in of linepipe steels in

grade grade grade grade X100 and X120X100 and X120X100 and X120X100 and X120

Hans-Georg Hillenbrand EUROPIPE GmbH, Mülheim, Germany

Andreas Liessem EUROPIPE GmbH, Mülheim, Germany

Karin Biermann Salzgitter Mannesmann Forschung, Duisburg, Germany

Carl Justus Heckmann Salzgitter Mannesmann Forschung, Duisburg, Germany

Volker Schwinn AG der Dillinger Hüttenwerke, Dillingen, Germany

Seminar of X120 grade high performance pipe steels, Technical Conference

July 28 - 29, 2005 Bejing, China

TP66

EUROPIPE. The world trusts us.

Andreas Liessem EUROPIPE GmbH, Germany 1 X120 Seminar Bejing, July 2005

DEVELOPMENT AND PRODUCTION OF LINEPIPE STEELS IN GRADE X100 AND X120

Hans-Georg Hillenbrand,

Europipe GmbH,

D-45405 Mülheim, Germany

Andreas Liessem,

Europipe GmbH,

D-45473 Mülheim, Germany

Karin Biermann,

Salzgitter Mannesmann Forschung,

D-47259 Duisburg, Germany

Carl Justus Heckmann,

Salzgitter Mannesmann Forschung,

D-47259 Duisburg, Germany

Volker Schwinn,

AG der Dillinger Hüttenwerke,

D-66748 Dillingen, Germany

ABSTRACT The ever-increasing demand for natural gas will further

influence the type of its transportation in the future, both from the strategic and economic point of view. Long-distance pipelines are a safe and economic means to transport the gas

from production sites to end users. The energy scenario has been changing quickly in recent

years. International studies forecast that the demand for natural gas will be nearly doubled by 2030. The distance between gas

production sites and end users increases, implying the need for the construction of complex gas transportation pipeline networks, when the use of LNG tankers is impossible or

uneconomical. This will make the high pressure natural gas transportation via pipelines increasingly interesting.

The use of grade X 80 linepipe has already proven to result in substantial cost savings and its development is considered as

completed for onshore pipelines. But the economic transport of gas over very long distances requires additional cost cuts. The use of grade X100 and/or X120 could be a solution. Therefore, the benefits of using high-strength linepipe and the current status of the development of such high strength steels are addressed in this paper. Laboratory and production results on high-strength large-diameter pipes are presented to describe the

materials properties as well as some aspects of the service behaviour.

INTRODUCTION The development of high-strength steels is intensified

world-wide to use the economic advantages. As the development of grade X80 is finished this grade is state of the art for high pressure gas pipelines. High-strength steels in grade X80 are nowadays state of the art. Grade X100 was recently

developed but up to now only used in small demonstration lines. EUROPIPE started its development of X100 already in

1995. Since that time different approaches with regard to chemical composition and rolling parameters have been investigated in order to find the best balance between strength, toughness and weldability.

In order to increase the knowledge necessary for the utilization of X100 steel pipes EUROPIPE participated in an ECSC-Demonstration project (DemoPipe) /1/, partially sponsored by the EPRG.. Within the scope of this project the most important questions arising in the technical field of a high pressure, high strength onshore gas pipeline have been examined like pipe manufacture, field welding and bending but

also, maybe as the most important point for the pipeline safety, the fracture propagation behaviour. In this paper the development of X100 is shortly summarized and an overview on the most interesting mechanical test results of the pipes produced by EUROPIPE for the DemoPipe Project is given.

The use of materials with even higher strength such as grade X 120 is the challenging next step as further cost savings for the gas transport companies seem to be possible. This paper gives an overview of the development of grade X120 pipe material. The experiences made during this development will

also be addressed in this paper. Special attention is focused on the development of pipe forming and modified welding technologies which had to be adapted from the existing and well established processes. The production of pipes in grade X120 has just started with the first trial production so that a definite statement on the above mentioned possible cost savings has to be made when this investigation is finished.


PROJECT COST REDUCTION

Project cost reduction may be a result of the sum of the

different benefits that can be derived by using high-strength steels /2/, even when the price per tonne of the pipe increases as the material grade increases. The benefits include:

• reduced quantity of steel required

• lower pipe transportation costs

• lower construction costs. The use of grade X 80 linepipe in the construction of the

first Ruhrgas X80 pipeline led to a material saving of about

20 000 t, compared with grade X 70 pipes (Figure 1), through a reduction of the wall thickness from 20.8 mm for X 70 to 18.3 mm for X 80. This resulted also in a reduction of the pipelaying costs because of reduced pipe transportation costs

and greatly reduced welding costs through reduced welding times needed with thinner walls. The use of materials with still higher strength, such as grade X 100 or grade X 120 could lead

to further material savings as it is further illustrated in Figure 1.

165000

145000

126000

101000

50000

60000

70000

80000

90000

100000

110000

120000

130000

140000

150000

160000

170000

Pip

e lin

e w

eig

ht

[t]

X70 X80 X100 X120

API steel grade

Figure 1: Possible material savings through the use of high strength material

A preliminary economic evaluation /3/ highlighted that high pressure X 100 pipelines could give investment cost savings of about 7% compared with grade X 80 pipeline. This study claims cost savings of up to 30 % when X 70 and X 100 are compared. Given that, in a complex pipeline network operating at high pressure, the capital expenditure is very high,

it becomes understandable how much more attractive the high strength steel option could be.

On the other hand, it becomes clear from Figure 2 that the

reduction in the manufacturing cost per tonne of the pipe at a given transport capacity of a pipeline is enhanced not just by the increase in the material grade of the steel but also by the

reduction in pipe wall thickness. From the point of view of pipe manufacturers, reduction of pipe wall thickness is not a preferred option. A reduction in pipe diameter at constant pipe thickness and a simultaneous increase in pipeline operating

pressure would represent, in our opinion, a more favourable solution to the problem.

450

550

650

750

850

950

1050

1150

1250

X60 X80 X100 X120

API steel grade

Man

ufa

ctu

rin

g c

ost

per

metr

ic t

on

ne

19.1 mm

25.4 mm

12.7 mm

15.9 mm

Figure 2: Manufacturing cost per tonne of pipe for different steel grades and wall thicknesses to be used at a constant transportation capacity

DEVELOPMENT OF HIGH-STRENGTH STEEL GRADES

The improved processing method for currently used high-

strength steels like X 80 and higher, consists of thermomechanical rolling (emerged in the 1980s) followed by accelerated cooling. By this method, it has become possible to produce high-strength NbTi microalloyed material, having a

reduced carbon content and thereby excellent field weldability. Additions of molybdenum, copper and nickel enable the strength level to be raised to that of grade X 100, when the steel is processed to plate by thermomechanical rolling plus modified accelerated cooling. The development of high strength steel for pipes in grade X 120 consists of further optimisation of the thermomechanical treatment and the use of niobium, titanium and boron as microalloying elements. First results of this development as regards the mechanical properties of the new material are very encouraging.

DEVELOPMENT OF GRADE X100

To cope with the market requirements for enhanced pipe strength, EUROPIPE put its effort to the development of grade X 100. No technological breakthroughs in TM rolling and

accelerated cooling had been necessary. Only optimisation of the existing technology was required for the production of grade X 100 plate. As a result, the production window became narrower. Heat treatment of plate or pipe was obviously not

necessary. Since 1995 six small scale production runs have been completed in a wall thickness range from 12,7mm up to

25,4mm and diameters between 30” and 56”.

EUROPIPE has developed different approaches to produce

high strength materials /4/ in order to find the best balance between strength, toughness and weldability. As can be seen in

Figure 3, three different approaches are generally possible in selecting the chemical composition and plate rolling conditions.

Approach A (Table 1), which employs a relatively high carbon equivalent, at 0.49, has the disadvantage that the crack arrest

toughness properties are not good and therefore requirements to prevent long-running cracks may not be fulfilled. Moreover, this approach is also detrimental, e.g. to field weldability. A


typical result of this approach was as follows:

Figure 3: Different approaches to reach the strength level of grade X 100 /7/

Table 1: Approach A for plate in API grade X100

Approach B (Table 2), which adopts a carbon equivalent of only 0.43 and which is used in combination with fast cooling rates in the plate mill down to a very low cooling-stop temperature, results in the formation of large fractions of martensite in the microstructure, which has a detrimental effect on the toughness properties of base metal. This effect cannot be adequately compensated for by using extremely low carbon

contents. In addition, softening of the heat affected zone was

observed.

Table 2: Approach B for plate in API grade X100

Experience gained meanwhile indicates that Approach C (Table 3) is the best choice. This approach enables the desired

property profile to be achieved through an optimised two-stage rolling process in conjunction with a medium carbon content, a medium carbon equivalent and an optimised cooling process. The special potential of the existing rolling and cooling facilities contributes significantly to the success of this approach.

The medium carbon content employed in Approach C ensures excellent toughness as well as fully satisfactory field weldability, despite the relatively high carbon equivalent, at

about 0.46. The chemical composition should therefore be considered acceptable for the purpose of current standardisation.

EUROPIPE has already produced hundreds of tonnes of grade X 100 pipe adopting Approach C. Recent trials have covered the wall thickness range between 12.7 and 25.4 mm. It was demonstrated that the same steel composition could be used and only slight changes in the rolling conditions would be necessary.

Table 3: Approach C for plate in API grade X100

As can be seen in Figure 4, the results on production pipes

show uniform strength properties for all the wall thicknesses tested. Tensile tests were performed using round bar specimens. Yield-to-tensile ratios were still relatively high. The elongation values are lower than those known for grade X 70. The impact

energy (CVN) measured was in excess of 200 J in all cases. It seems to be impossible to guarantee values in excess of 300 J at low temperatures on a production basis. In Figure 5, the DWT test results at –20°C are shown for the different wall thicknesses. Typically, the shear area values are higher for thin-wall X 100 material. Because of the relatively high carbon equivalent and the high strength level, the toughness of the

longitudinal seam weld metal and the HAZ is limited. The X 100 material produced responds favourably to manual and mechanized field welding, a finding which can be attributed to its reduced carbon content /5, 6/.

Heatpipe size

OD X WTC Mn Si Mo Ni Cu Nb Ti N CEIIW PCM

I 30" x 19.1 mm 0.08 1.95 0.26 0.26 0.23 0.22 0.05 0.018 0.003 0.49 0.22

Approach A

Heat

I

CVN

(20°C)

DWTT-

transition

temperature

739 MPa 792 MPa 0.93 18.4% 235 - 15 °C

* transverse tensile tests by round bar specimens

yield strength

Rt0.5

*

tensile strength

Rm

*

yield to

tensile ratio

Rt0.5

/ Rm

*

Elongation

A5 *

Heatpipe size

OD X W TC M n Si M o Ni Cu Nb Ti N CEIIW PCM

II

Approach B

Heat

II

CVN

(20°C)

DW TT-

transition

tem perature

755 M Pa 820 MPa 0.92 17.1 % 240 - 25 °C

30" x 15.9 m m 0.07 1.89 0.28 0.15 0.16 - 0.05 0.015 0.004 0.43 0.19

yield strength

Rt0.5

*

tens ile strength

Rm

*

yield to

tensile ra tio

Rt0.5

/ Rm

*

E longation

A5 *

Heatpipe size

OD X WTC Mn Si Mo Ni Cu Nb Ti N CEIIW PCM

III

Heat

III

yield strength

Rt0.5

*

tensile

strength

Rm

*

yield to

tensile ratio

Rt0.5

/ Rm

*

Elongation

A5 *

CVN

(20°C)

DWTT-

transition

temperature

737 MPa 800 MPa 0.92 18 % 200 J - 20 °C

56" x 19.1 mm 0.07 1.90 0.30 0.17 0.33 0.20 0.05 0.018 0.005 0.46 0.20

IV 36" x 16.0 mm 0.06 1.90 0.35 0.28 0.25 - 0.05 0.018 0.004 0.46 0.19

IV 752 MPa 816 MPa 0.92 18 % 270 J ∼∼∼∼ - 50 °C

200-

270 J

* transverse tensile tests by round bar specimens

** -60°C for WT 12.7mm -10°C for WT 25mm

Approach C


Figure 4: Tensile properties of X 100 pipes with different wall thicknesses

Figure 5: Influence of wall thickness on DWT test results at –20°C (X 100 pipes)

Figure 6: Charpy test results on 36” OD x 16 mm WT grade X 100 pipe (as delivered and aged conditions)

For reasons of technical feasibility and cost-effective production, it is necessary in the context of grade X 100 to re-assess and re-define some of the requirements for mechanical

properties, giving consideration to anticipated service conditions.

The pipes produced were subjected to different tests to

evaluate the service behaviour. Figure 6 shows the influence of an ageing treatment on the Charpy transition curve. There was

only a slight decrease in toughness properties after a thermal treatment for 30min at 250°C.

Field cold-bending trials were also completed with satisfactory results.

Figure 7 shows photographs of full scale burst tests, which

were conducted by CSM as part of an ECSC funded research project /7/. So far, our experience has shown that it is impossible to install a grade X100 pipeline in arctic regions

without the use of crack arrestors. EUROPIPE offers different types of crack arrestors to the industry /8/.

Figure 7: View of full scale burst tests carried out on 56” x 19.1 mm and 36” x 16.0 mm grade X 100 pipes

DEVELOPMENT OF X120 PIPE MATERIAL

In the current line pipe standards a grade X120 with a minimum yield strength of 827 MPa is not yet specified.

Therefore, the development of a steel with this SMYS in combination with a minimum tensile strength of 931 MPa was decided to be the main target. A Charpy impact toughness of at least 231 J at a testing temperature of –30 °C was the crack arrest criterion. In the Battelle drop weight tear test the transition temperature for a shear area of 75 % should be lower than –20 °C. The requirements on grade X120 are summarised

in Table 4 /9,10/.

Parameter Value

Yield strength 827 MPa

Tensile strength 931 MPa

CVN toughness @ -30 °C 231 J

CTOD @ -20 °C 0.14 mm

DBTT < -50°C

BDWTT (SA%) @ -20 °C 75 %

Yield-to-tensile ratio 93 %

Wall thickness 16 mm

Table 4: Requirements on grade X120

DESIGN OF MICROSTRUCTURE AND CHEMICAL COMPOSITION

The required properties of a steel in grade X120 are only

reachable with a mainly baintic microstructure that predominantly consists of lower bainite. Due to the combination of a high dislocation density and a very fine scale

substructure, so called domains, this lower bainitic


microstructure is reasonable option for an ultra-high-strength level along with sufficient toughness properties.

To produce such a microstructure especially with a low C and low PCM steel the chemical composition had to be designed very carefully. The basic alloying system contains CuNiCrMo and the microalloying elements VNbTi. Besides these the microalloying element boron should also be effective. To improve the hardenability of the austenite the analysis containes sufficient effective boron. Boron has a strong

retarding effect on the transformation of austenite to ferrite and with this supports the formation of the required bainitic microstructure. To intensify this effect the attention during the production of the mill heats had to be turned to a low carbon content in general. In particular the carbon content was within a certain scatter band. In the end this led to variation in the strength levels of the plates. Furthermore, the combination of

boron with nitrogen and oxygen had to be avoided. With a manganese content of about 1.90 % the carbon equivalent CEIIW of the chemical composition used in initial investigations was in a range of 0.50 % up to 0.55 %. The PCM

value was approximately 0.23 %. The basic chemical compositions of the steel is summarised in Table 5.

C Si Mn Nb Ti N PCM others

0.06 0.23 1.91 0.042 0.017 0.004 0.23 Cu Ni Cr Mo V B

Table 5: Chemical composition of base material (weight-%)

PLATE ROLLING PROCESS PARAMETERS To reach the best possible combination of strength and

toughness properties the grain size of the microstructure should be very fine. The production of such a microstructure was done by a careful choice of the plate rolling process parameters namely the reheating and the finish rolling temperatures as well as the deformation ratios and the conditions for the accelerated

cooling. An optimum reheating temperature leads to an best

possible initial austenite grain size which is the starting point for the further control of the mechanical properties during the

rolling process. Furthermore, an as high as possible deformation ratio during the first rolling stage is of great importance for a first grain refining of the recrystallising

austenite. The finish rolling has to be done close to the Ar3 transition point for a proper control of pancaking of the non-recrystallising austenite. This extremely pancaked austenite has a high dislocation density and transforms into a fine grained

lower bainite after the accelerated cooling. For a highly effective accelerated cooling process after finish rolling a cooling rate of above 20 K/s and a cooling stop temperature below 400 °C are the main process parameters.

PLATE PRODUCTION AND PROPERTIES

Based on these considerations for a fundamental concept of alloy design and choice of process parameters a series of heavy

plates was rolled in a trial production. The mechanical-technological properties met the targets concerning yield strength, tensile strength, Charpy impact toughness, BDWT properties and microstructure, as can be seen in Table 6. A more detailed description of the plate is given in /11/.

By using narrow temperature ranges for the individual rolling stages, which were based on precisely measured Ar3 temperatures, a very high strength level could be achieved. The

yield strength reached values of ≥ 840 MPa and the values of

the tensile strength were ≥ 1000 MPa. Furthermore, impact

energy values ≥ 240 J were measured at –30°C.

Parameter Mean values transverse

Yield strength Rt0.5 843 MPa

Yield strength Rp2.0 1087 MPa

Tensile strength Rm 1128 MPa

Yield-to tensile ratio Rt0.5/Rm 75 %

Yield-to tensile ratio Rp2.0/Rm 96 %

Elongation A5 14.3 %

CVN toughness @ –30 °C 250 J

Table 6: Mechanical technological properties of plate material in grade X120

MODIFICATION OF PIPE FORMING PARAMETERS One of the great challenges in the pipe forming of ultra-

high strength plate with a relatively small wall thickness is the large spring back due to the greater elastic range of a material with high yield stress. Especially the U-forming step is affected

by this spring back. It could lead to shells that cannot be inserted into the O-press.

Finite element calculations were carried out to support the optimisation of the U-ing process and the shape of the U-ing

punch. The results of the FEM calculations are provided by Figure 8. The large spring back after an U-ing with a conventional U-press tool is visible here. Such a plate could not be inserted into the O-press.

In addition to this the shape of the U-press was modified in the calculations, as can be seen from Figure 9, to find the best possible parameters for an adapted UOE-process. The newly

calculated shape of the U-Press will lead to a reduction of the spring back. The plate could be inserted into the O-Press. Furthermore, it was assured that the ovality and peaking after O-ing were as well set to optimal value to avoid problems with the welding and expansion steps of the pipe production.


Figure 8: Spring back after U-ing with conventional U-ing punch

Figure 9: Shape of the modified U-Press tool

ASPECTS TO BE SOLVED WITH RESPECT TO WELDING OF THE LONGITUDINAL SEAM

The multi-wire submerged-arc welding process used

universally to deposit the two-pass longitudinal seam weld in pipe is associated with a high heat input and leads to aspects that cannot be underestimated in the case of the grade X 120 as described below.

The first aspect is the softening of the base material adjacent to the longitudinal seam weld. This problem is existent to some extent also in the case of materials in grades X 80 and X 100. To handle this aspect the X120 material contained a

certain amount of vanadium to use its precipitation hardening effect.

The second aspect is associated with the continuing use of the proven submerged-arc welding and achieving adequate strength and toughness for the weld metal of the two-pass longitudinal seam weld in the highest strength material X 120. This problem had to be resolved by selecting a matching chemical composition for the weld metal alone and additionally by reducing the heat input per pass.

The average heat input per pass, which is at 2 kJ per cm of the weld and per mm of the pipe wall thickness, needed to be reduced considerably (e.g. to a value ≤ 1.5 kJ per cm of the weld and per mm of the pipe wall thickness). Production experience available today in this connection is not sufficient to permit an assessment of the softening that occurs in the base material adjacent to the weld. This depends also on the pipe

wall thickness. Finally, such an approach is limited by the need for a sufficiently overlapped welding and therefore adequate production safety.

DEVELOPMENT OF NEW WELDING CONSUMABLES AND LOW HEAT INPUT WELDING TECHNOLOGY

Extensive experimental work was done to develop a new

wire and an optimal wire - flux combination for the longitudinal weld seam. For this, numerous seam welds were made in the laboratory with a wide range of welding conditions using X120 heavy plate material. Table 7 shows the main chemical compositions of the different variants of the seam weld chemistry that were produced during the investigation. The first laboratory weld seams were conducted with a standard wire/flux combination for longitudinal weld seams (A, B).

Wire/Flux

combination

C Si Mn Cr Ni Mo PCM

1 0.06 0.30 1.93 0.98 1.57 0.83 0.33 C

2 0.06 0.32 1.94 1.00 1.60 0.83 0.33

1 0.07 0.47 1.98 0.35 1.04 0.40 0.27 B

2 0.07 0.50 2.01 0.35 1.12 0.41 0.28

1 0.05 0.26 1.95 0.21 0.22 0.52 0.25 A

2 0.05 0.26 1.99 0.20 0.21 0.56 0.22

Table 7: Chemical compositions of different weld metals (weight-%) (1: Inside pass; 2: Outside pass)

The existing welding technology was modified and

optimised by reducing the heat input of each pass. A low heat input welding procedure leads to a minimisation of the softening of the heat affected zone in combination with an improvement of its toughness. Especially combinations of current and voltage were examined to establish the optimum conditions for an adopted welding procedure.

The transverse weld tensile strength of these laboratory

weld seams were tested. Some of the specimens were tested with the weld reinforcement removed. Independent from removing the weld reinforcement all specimens tested broke either in the weld metal or in the HAZ. Table 8 illustrates the

results that were reached within the laboratory investigation


with different wire-flux combinations.

Wire/Flux combination

Tensile strength Fracture position

1024-1025 Mpa HAZ C

956-986 Mpa HAZ*

976-1013 Mpa HAZ B

914-918 Mpa WM*

A 860-865 Mpa HAZ

777-779 Mpa WM*

Table 8: Transverse weld tensile strength of different weld seams (* Weld reinforcement removed)

PIPE PROPERTIES – BASE MATERIAL

EUROPIPE produced pipes of 30” outer diameter with

16.1 mm wall thickness. With the intention to reach both the upper and the lower limit of a mass production in this trial plates with the highest as well as the lowest possible strength

level were chosen on purpose for pipe forming. The highest possible level reached up to 100 MPa more than the target values. The lowest possible level was very close to the target values. This is an appropriate measure to mark out the boundaries for a new material.

Due to the ultra-high strength level of the plate material the pipe forming was actually one of the most challenging steps of the pipe production. The pipe forming was carried out in different ways that led to a variation in the peaking of the individual pipes after the O-forming. Some of the pipes failed

during the expansion along the longitudinal seam weld especially in the case of the highest possible strength level and a not optimum peaking. But in most of the cases the pipe forming including the expansion of the pipes was successful.

The results of these first production tests for the base material of grade X120 are shown in Tables 9 and 10. Table 9

summarises the results of the ultra high strength variants. The strength values were determined using round bar and flat bar tensile specimens. For comparison the tensile properties were both measured in transverse and in longitudinal direction. In both directions the measured tensile strength values of both specimen types fully conformed to the specification requirements in all cases. The standard deviation for the yield and tensile strength values was very low.

The yield strength measured in this case was Rp0.2 as for

high strength steels like X120 the usually measured Rt0.5 results in values that do not seem to reflect the actual strength level. The use of Rt0.6 or even Rt0.65 instead of Rt0.5 appears to be more appropriate for high strength steels. Table 7 provides the results

Parameter Target Mean values

values Transverse

Yield strength Rp0.2 827 MPa 865 MPa

Tensile strength Rm 931 MPa 944 MPa

Yield-to-tensile ratio Rp0.2/Rm <93 % 92 %

Elongation A2” 24.3 %

Elongation Au 7.4 %

CVN toughness @ -30 °C 231 J 267 J

DBTT <-50 °C <-50 °C

BDWT @ -20 °C 75 % ≥65 %

Table 10: Mechanical properties of medium high strength variant (flat bar specimens)

of the variants with the somewhat lower strength level. The

tensile properties that were measured using transverse flat bar specimens as well safely met the target values.

As also given in Tables 9 and 10, the yield-to-tensile ratios are lower than 93 %. The A5 fracture and uniform elongation

are lower than known for grades X80 to X100. Especially the uniform elongation of the ultra high strength variant showing values lower than 2 % appears to be one limitation for the pipe forming in particular and the production of pipes in grade X120 in general.

Parameter Target values Mean values Specimen

Transverse Longitudinal

Yield strength Rp0.2 827 MPa 963 MPa 976 MPa Round bar

Yield strength Rp0.2 966 MPa 961 MPa Flat bar

Tensile strength Rm 931 MPa 1139 MPa 1081 MPa Round bar

Tensile strength Rm 1150 MPa 1062 MPa Flat bar

Yield-to-tensile ratio Rp0.2 / Rm <93 % 85 % 90 % Round bar

Yield-to-tensile ratio Rp0.2 / Rm 85 % 90 % Flat bar

Elongation A5 13.1 % 15.5 % Round bar

Elongation A5 10.3 % 11.3 % Flat bar

Elongation Au 1.8 % 2.6 % Round bar

Elongation Au 1.5 % 1.3 % Flat bar

CVN toughness @ -30 °C 231 J 262 J

DBTT <-50 °C -60 °C

BDWT @ -20 °C 75 % ≥70 %

Table 9: Mechanical properties of the ultra-high strength variants


The toughness properties are provided in general in Tables 9 and 10. The Charpy impact energy values in particular are given in Figure 10. They were measured on Charpy V-notch impact specimens in a temperature range of -10 °C down to –70 °C. At a testing temperature of –30 °C the values resulted in an average of ≥ 260 J. All individual Charpy toughness values were in excess of 231 J at this temperature.

For the ultra-high strength variant the results of the Battelle Drop Weight Tear (BDWT) test led to transition temperatures to

75% shear fracture area of 0°C, as can be seen from Figure 11. In addition to the shear area and illustrated in Figures 12 and 13, the specific total energies and the specific energies for crack propagation during the BDWT test were measured. First results show that for X120 pipe material especially the specific crack propagation energies are lower than for X100 pipes. Therefore, the results of the BDWT tests have to be improved in the next

trial production.

Figure 10: Charpy-V impact test results of pipe base material

Figure 11: Results of the Battelle Drop Weight Tear (BDWT) test PIPE PROPERTIES – LONGITUDINAL WELD SEAM

The results of the laboratory investigation were

successfully transferred to the pipe mill for the large-diameter

pipe production. The main chemical composition of the submerged arc welds of the first prototype pipes is summarised in Table 11.

Figure 12: Total specific energy of the Battelle Drop Weight Tear (BDWT) test

Figure 13: Specific energy for crack propagation of the Battelle Drop Weight Tear (BDWT) test

C Si Mn Cr Ni Mo CEPC

M

Outside 0.06 0.29 1.88 0.9 1.3 0.82 0.32

Inside 0.06 0.30 1.87 0.8 1.3 0.75 0.32

Table 11: Main chemical composition of SAW prototype pipe welds

The strength of the seam weld was checked by means of

flattened transverse weld specimens, with the weld

reinforcement removed by machining as well as with the weld reinforcement not removed. The fracture positions do vary but in most of the cases the specimens failed in the heat affected zone. In contrast to the findings of the laboratory investigation

the actual fracture initiation did not appear in the softened area of the HAZ but rather close to the fusion line. The mechanical properties of the welds are given in Table 12.

YS TS Y/T El CVN @ –20°C

780 MPa 941 Mpa 83 % 18.5 % 64 J

Table 12: Mechanical properties of prototype pipe welds


The strength of the weld metal was measured with tensile specimens that were taken from the longitudinal weld. The all-weld metal tensile properties confirm to the required tensile strength of the base material. However, the yield strength of the weld metal did not meet the required value for grade X120. Thus, all the tensile strength values measured reflect the strength of the base material and were above the specified minimum value of 931 MPa.

Due to the unexpected failure of the flattened transverse

weld tensile specimen a HV2 hardness measurement was performed from the weld metal to the HAZ. The results of this measurement are given in Table 13. The lowest hardness values were found in the HAZ area close to the fusion line. The results of the transverse weld tensile test were confirmed by this.

Location Hardness HV 2

WM 301 / 311

HAZ near to FL 271 / 265

HAZ 296 / 295

Table 13: Distribution of hardness in WM and HAZ near to the fusion line

An example for the hardness distribution of the weld is

given in Figure 14. As can be seen here all values of the hardness in the base metal as well as in the HAZ and the weld metal are below 350 HV10.

Figures 15 and 16 illustrate the toughness properties of the longitudinal weld seam. The notch location was 25/50/25. The target of 84 J at a testing temperature of –20 °C was not consistently met. Due to the high strength level, the toughness of the longitudinal weld seam and the HAZ is limited.

CONCLUSION

The predicted growth in energy consumption in the coming

decades necessitates severe efforts for transporting large amounts of natural gas to end users economically. Large-diameter pipelines serve as the best and the safest means of transport. The use of grade X 80 linepipe has already proven to result in substantial cost savings and its development is considered as completed for onshore pipelines. But the economic transport of gas over very long distances requires additional cost cuts. The use of grade X100 and/or X120 could

be a solution. Several pipelines installed in Europe and North America in

the past two decades show that the use of X 80 linepipe causes no problems with respect to mechanical properties and welding. The development work performed by EUROPIPE during the last decade led to the conclusion that grade X 100 mechanical properties can be reliably achieved on a large scale basis. After

several trial productions where the first priority of development has been to find the best balance between strength, toughness and weldability, the last production lot of X100 was delivered for the TAP project, where a full pipeline lifetime will be

simulated during a two year test service. Crack arrest properties

for several pipe sizes have been verified in full scale burst tests. The results of these tests, in terms of arrest/propagation properties, have shown that the applicability of the Batelle equations and their direct extrapolation from X80 to X100 operating at a very high hoop stress level (>500 Mpa) is highly questionnable /1/. EUROPIPE therefore developped glass fibre reinforced crack arrestors which performance was proven by a full scale burst test /8/. Crack arrestors could therefore be a solution

The initial results of the work directed to developing grade X 120 are encouraging with respect to the properties of the base material. During this current development the technologies for heavy plate rolling and pipe production as well as the welding process for the longitudinal weld seam were modified or even

Spot

marks Hardness Spot

marks Hardness

1 348 12 294

2 330 13 303

3 326 14 306

4 273 15 299

5 317 16 302

6 292 17 279

7 297 18 276

8 291 19 268

9 289 20 333

10 290 21 332

11 294 22 348

Figure 14: Distribution of Hardness HV10 (BM-HAZ-WM- HAZ-BM) of pipe grade X120

completely new developed with respect to the new high

strength steel grade X120. The main objectives of this development were the design of low Carbon, low PCM and VNbTiB microalloyed steel and the re-calculation of the main

aspects of UOE-forming to optimise especially the U-forming step. Furthermore, new welding consumables and a low heat input welding technology were developed.

A first prototype production of pipes in grade X120 led to

promising results with respect to the mechanical-technological properties, the weldability and the pipe forming. However, further optimisation of certain aspects is still necessary.


Especially the pipe forming of X120 has not reached the stage of production safety yet, but specially designed tools will improve the pipe forming process. The toughness properties of weld metal and heat affected zone do not fully meet the target values as the low heat input welding technology still needs further improvement. This could be achieved with modern welding tools and optimised welding consumables. Furthermore, the results of the BDWT test need to be improved with respect to a possible crack arrest. Therefore, the use of

crack arrestors is currently recommended. Further investigations are planned for the near future.

Figure 15: Charpy-V impact test results of weld metal

Figure 16: Charpy-V impact test results of HAZ

With regard to the economical aspect, at present the

manufacturing cost per tonne of grade X120 material can only be specified on basis of the prototype production and with its

production parameters. But a number of cost increasing aspects have to be taken into consideration. The alloying cost will be high compared to standard high strength material like X70. For a safe pipe forming new tools for the UOE-process have to be developed to reduce the risk of pipe failures during expansion. The welding with a low heat input appears to be mandatory with regard to the mechanical properties but it decreases the productivity in the pipe mill. As the range of wall thicknesses should be from 16 mm up to 20 mm this product could not be

produced with high flexibility. For a safe pipe line operation the use of crack arrestors will be necessary.

A first roughly estimation of cost resulted in a price per tonne for X120 in the dimensions presented in this paper that is in excess of 250 US$ above the price for pipes in grade X70 with comparable dimensions. Therefore, from all these points of view further study and evaluation are needed to make an

optimistic statement about the possible use of grade X120 in the near future.

REFERENCES /1/ G. Demofonti, G. Manucci, M. Di Biagio, H.-G.

Hillenbrand, D. Harris: “Fracture propagation resistance evaluation of X100 TMCP steel pipes for high-pressure gas transportation pipelines using full-scale burst tests”, 4

th Int.

Conf. on Pipeline Technology, Ostend, Belgium, May 2004 /2/ M. K. Gräf and H.-G. Hillenbrand: “High Quality Pipe

– a Prerequisite for Project Cost Reduction”, 11th PRCI-EPRG Joint Technical Meeting, Arlington, Virginia, April 1997

/3/ L. Barsanti, H.-G. Hillenbrand, G. Mannucci, G.

Demofonti, and D. Harris: “Possible use of new materials for high pressure linepipe construction: An opening on X100 grade steel”, International Pipeline Conference, Calgary, Alberta, September 2002

/4/ V. Schwinn, P. Fluess and J. Bauer: "Production and progress work of plates for pipes with strength level of X80 and above", International Conference on the Application and

Evaluation of high-grade linepipes in hostile Environments, Yokohama, Japan, November 2002

/5/ L. Barsanti, G. Pozzoli and H.-G. Hillenbrand: “Production and Field Weldability Evaluation of X100

Linepipe”, 13th

Joint Meeting PRCI-EPRG, New Orleans, USA 2001

/6/ H.-G. Hillenbrand, A. Liessem, G. Knauf, K. A. Niederhoff and J. Bauer: “Development of large-diameter pipe in grade X100 – State of the art report from the manufacturer’s point of view“, International pipeline technology conference, Brugge, Belgium, May 2000

/7/ G. Demofonti, G. Mannucci, D. Harris, H.-G. Hillenbrand and L. Barsanti: “Fracture behaviour of X100 gas pipeline by full scale tests”, International Conference on the Application and Evaluation of high-grade linepipe in hostile Environments, Yokohama, Japan, November 2002

/8/ H.-G.. Hillenbrand, H. Brauer, G. Knauf: „Crack arrestors“, 4

th Int. Conf. on Pipeline Technology, Ostend,

Belgium, May 2004 /9/ J. Y. Koo, N. V. Bangaru, R. A. Petkovic et al.:

“Metallurgical design of ultra-high strength steels for gas

pipelines”, Proceedings of the 13th

Int. Offshore and Polar Eng. Conf., Hawaii, USA, May 2003


/10/ S. Okaguchi, H. Makino, M. Hamada et al.: “Development and mechanical properties of X120 linepipe”, Proceedings of the 13

th Int. Offshore and Polar Eng. Conf.,

Hawaii, USA, May 2003 /11/ V. Schwinn, S. Zajac, P. Fluess and K.-H. Tacke:

“Bainitc steel plates for X100 and X120 ”, 4th

Int. Conf. on Pipeline Technology, Ostend, Belgium, May 2004

development development and production and … development and production and production and...

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