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Influence of thermal treatment on mechanical properties of UOE linepipe

A. Liessem, M.K. Graef

Europipe GmbH, Ratingen, Germany

G. Knauf, U. Marewski

Mannesmann Forschungsinstitut GmbH, Duisburg, Germany

Abstract UOE-linepipes are normally coated for corrosion protection. The coating is applied at temperatures of approximately 200°C. It is known from the literature that thermally induced processes can be activated at relatively low temperatures and may change the properties of materials that have been subjected to cold deformation. Certain applications can benefit from the material modification, for example the collapse resistance can increase and the material properties can become more homogeneous. The main aim of the work was to verify the influence of thermal treatment (coating simulation) on the mechanical properties of linepipe materials. The investigation was focussed on a comparison of impact toughness, yield strength and tensile strength and ductility characteristics. Furthermore full scale burst test were carried out with special attention to the influence of the coating process on yield and ultimate burst pressure and also on circumferential plastic deformation. KEY WORDS: UOE linepipe, pipeline steels, material properties, coating, ageing, burst tests, tensile testing, Y/T ratio, circumference elongation Introduction

It is known from the literature [1] that the mechanical and technological properties of certain steels are capable of changing within certain limits under exposure to even

fairly low temperatures (T≅200° C). This thermally and time depending induced process is consciously exploited under the term "bake hardening" in the automotive industry, for example, in order to increase the yield strength – and, simultaneously, the load-bearing capacity – of bodyshell metal during painting. The influence of aging on the resultant material behaviour is also known in the case of pipes produced from

cold-formed plate [2-4], since the "Bauschinger effect" occurring as result of deformation is reduced. Very frequently, pipes are coated at temperatures of around 200° C at the end of the production process. Despite the fact that these temperatures act on the pipe only for a few minutes during the coating process, it can nonetheless be assumed that the mechanical and technological properties of the pipe will have changed, after coating, compared to their initial condition. Since ageing is a temperature-dependent and time-controlled process, it – and a change in the mechanical and technological properties of a pipe – must be anticipated even at low temperatures, such as normal ambient temperature, for example, with extreme long exposure times (e.g. several years).

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The target of the investigation program performed was that of describing, in terms of the resultant changes in mechanical and technological properties, the effect of strain aging caused by cold deformation during pipe forming and, in particular, due to expansion followed by exposure to moderate heat during PE coating of the pipe. Experimental Program The investigations were carried out on cold expanded SAW pipe of grades X60, X65 sour, X70 and X80. To compare the mechanical properties in the non-aged "as received" and the "PE-aged" condition, the pipes were cut into two pieces of approximately the same length. One of the sections of each grade was heat treated in the coating yard either by coating or simulation of coating (210°C/2’/water). In order to characterize the material properties present in the pipe, all the tests described below were performed for both the "as-received" and "PE-aged" states. The scope of the mechanical and technological tests included tensile tests in conformity with API 5L and, in addition, tensile tests performed on unflattened round specimens. Comparative assessment of the material properties found in unflattened tensile specimens is of particular interest in the context under examination here, since only unflattened specimens are capable of describing adequately the component properties of a pipe. The flat API tensile-test specimens taken from the circumferential direction of pipes are, on the other hand, capable of illustrating, in particular, modified materials properties, as a result of the straightening process and

the Bauschinger effect thus also imparted [5]. The toughness properties of the materials used are also of decisive importance for the safety of pipelines. Charpy V-notch impact energy curves were also drafted, and DWT tests were performed, in order to check these properties, which are necessary, in particular, for the avoidance of unstable crack propagation. Full-scale tests performed on pipes in both "as-received" and "PE-aged" condition formed the central emphasis in the test program conducted. Yield pressures and rupture pressures, and also local and integral circumferential plastic deformations, were determined in the context of these tests. Material Property Tests Table 1 shows a summary of the characteristics data established during the tensile tests performed on the test material. The tensile-test specimens were in each case taken from various circumferential directions of the pipes. The mean values of the properties from the circumferential direction of the pipes are therefore shown.

It can be seen from the table that an increase in yield strengths (σt0.5), in particular, has occurred as a result of the heat treatment applied. The increase in tensile strengths is negligibly low. The yield strength ratio in "PE-aged" condition is therefore greater than in "as-received" condition. As was expected, comparative assessment of elongations at fracture against uniform elongations indicates that somewhat reduced elongations occur in "PE-aged" condition. Figure 1 shows by way of

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example a stress/strain diagram drafted in "as-received" and "PE-aged" condition for 40" dia. X 16.9 mm wt. Grade X70 pipe. It is apparent that a significant increase in the material's yield strength has occurred as a result of heat treatment, whereas tensile strength has remained virtually unchanged. Slightly reduced uniform elongations and elongations at fracture are also observable in "PE-aged" state, due to the raised yield strength ratio. Plotting of the characteristics data determined in the tensile test over the circumferential direction of the pipes (Figure 2) indicates similar variation of material properties for both conditions. Figures 3 and 4 show, using the example of pipemaking grades X65 sour and X70, results obtained from V-notch bending tests. The data points given are the average values of three individual tests at each test temperature. Comparison of the "as-received" and "PE-aged" states indicates practically no differences at the impact energy values determined only a slight shift in and the transition temperature observed. The influence of aging is therefore not detectable in the context of CVN notch tests. DWT tests can be used to realistically predict tough or brittle component behaviour in the case of fracture. Figure 5 again shows a comparative assessment of the "as-received" and "PE-aged" states in pipemaking grades X65, X70 and X80. As in the V-notch impact tests, practically no difference is observable between aged condition and the original material state. Full-scale Tests

One pipe in each case (length ≅ 12 m) was cut in two prior to performance of the burst tests, and one half exposed in a coating system to simulated coating. This procedure was intended to ensure that the results obtained for the "as-received" and "PE-aged" states in the test were truly representative. Pipe sections of a length of 5 to 6 m were therefore available for performance of the burst tests. The test sections for the burst tests (as received and aged condition) were converted into pressure vessels by welding caps to the pipe ends. The circumference of the vessels were measured at 21 locations along the pipe length before and after they were pressurized with water up to burst. Figure 6 gives the experimental set up and instrumentation for the burst tests. The records of internal pressure vs. increase in volume during the burst tests are shown for Grade X65sour by way of example for both test conditions in Figure 7. Figures 8 and 9 show pipes after performance of the burst test. Local strains were determined by means of wire strain gauges and chains for measurement of circumferential strains at selected positions on the pipe during the tests (see Figure 6). In addition, circumferential measurements were also performed across the entire length of the pipe during the burst test, in order to determine the residual mean

circumferential plastic deformation (εcM) of the ruptured pipe. The two outermost measuring points were not taken into account in determination of this factor, since significant clamping effects occur here as a result of the caps welded on to each

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end. Figure 10 shows by way of example a summary of the results obtained after a burst test.

The factor εcM defined above is a characteristic value determined on the component and describes deformation capacity in the circumferential direction of a pipe The non-proportional elongation of the pipe as a result of reduction of cross-section at fracture is only included in this factor to an extremely limited extent, since the majority of the measuring points are located outside the fracture zone and, in addition, the total circumference of the pipe is extremely great in relation to the

reduced-cross-section fracture zone. Factor εcM can, therefore, set against to the characteristics data from the uniaxial tensile test, best be compared against uniform elongation AG. The fact that the burst tests performed generate multi-axial stress states in the component, whereas the tensile test generates a uniaxial load, must be taken into account in this context, however. Table 2 shows a summary of the results from the full-scale burst tests performed. The burst pressures and internal pressures determined at an total strain of 0.5 %

(yield pressures) are plotted. In addition, the factor εcM described above (integral circumferential plastic deformation) is also stated. The relationship between internal pressure and the yield strength can be by the “Barlow” equation:

tD

tp

a

tT−

=2

5,0σ /1/

The corresponding stresses σt0.5 (stress calculated as per /1/ at 0.5 % total strain) and Rm (stress calculated as per /1/ at maximum pressure) are therefore additionally plotted in the table for the purposes of comparability. It is apparent, analogously to the tensile tests performed on round bar specimens in the transverse direction, that the yield pressures at 0.5 % total strain on the pipe are – particularly in the case of the aged pipes – extremely close to the maximum pressures measured. In pipes in "as-received" condition, flow also occurs on the pipe later than in the case of the characteristics data determined using the round bar tensile specimen. This behaviour can be explained by the multi-axial stress state present in the component test. In addition, significant flow in a vessel only occurs when not only the most load-exposed inner surface of the pipe wall has started to flow but rather only once plastification of the entire wall thickness has occurred. Comparative assessment of the measured uniform elongation on the pipe against the corresponding uniform elongation in the round specimen taken from the circumferential direction (see Table 1) shows that in all cases significantly lower strains were observed in the full-scale test. In addition, the pipes in "PE-aged" condition also exhibit lower uniform elongations than pipes in "as-received" condition. The correlation shown in Figure 11 results if the yield strength ratio determined on round specimens (circumferential direction) is plotted against the uniform elongation in the pipes achieved in the burst test. Further results from the SZMF burst test data base are also shown in this view, in addition to the test results examined here. The tests performed in the context of the program described here are within the scatter band of these data taken from burst tests on UOE as well as HFI pipes.

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It is clearly apparent that the residual uniform elongation determined on the pipe decrease significantly as yield strength ratio increases. Uniform elongation on the pipe of around 2 % are achieved at a yield strength ratio of 93 to 94 %. Comparison of Results to Established Equations There are several classic strength theories that can be used to analyze material failure behaviour. The so-called Barlow equation /1/ is used in the majority of standards for estimation of the yield and fracture performance of pipes and vessels. The Barlow equation is based on a uniaxial stress state in pipes, with stresses occurring only in the circumferential direction. In a vessel exposed to internal pressure, however, a tensile stress is also present in the longitudinal direction, due to the end caps fitted. Stress intensity hypotheses have been developed for assessment of material stressing under exposure to multi-axial load, one of the best known of these hypotheses being the "Maximum Distortion Energy" theory.

( ) ( ) ( )2

13

2

32

2

212

1σσσσσσ −+−+−=YS /2/

In the Maximum Distortion Energy theory, failure by yielding occurs when the distortion energy per unit volume in a state of combined stress becomes equal to that associated with yielding in a simple tensile test. The yield pressures calculated using Equations /1/ and /2/ are compared against those determined in the tests described here in Figure 12. Further test results also obtained in burst tests have been added here to improve comparability. This figure illustrates that the "Maximum Distortion Energy" theory in most cases predicts the start of flow more accurately than the "Barlow formula", which very frequently produces excessively low yield pressures in the case of testing on a vessel. The PE-aged pipes exhibit no conspicuous differences compared to pipes in "as-received" condition. There exist in the relevant literature models for the prediction of complete failure of a vessel under exposure to internal pressure (burst), of which the most important models are described in [6]. The hypotheses below exhibited the best accordance with the data material examined here.

Maximum shear stress criterion:

+

−⋅⋅=

1

12

κ

κTSpH /3/

i

a

D

D=κ

Criterion by Turner [7]: ( )κln⋅= TSpH /4/

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Wellinger and Uebing [8] ( )κln'4.21

'1.1

G

HA

TSp

+

⋅= /5/

( )GG AA += 1ln' ( ) TSATS G ⋅+= 1'

In addition, the Barlow equation /1/ described above manifests very good accordance if tensile strength TS is used in the equation instead of the material's yield strength (Figure 13). The "Maximum Distortion Energy" theory examined above generally produces excessively high burst pressures compared to the test data when tensile stress is used in the calculation. It becomes apparent that the burst pressures of PE-aged pipes are also within the normal scatter band and that an adequately accurate calculation with respect to burst is possible.

More recent theoretical works based on the "Finite Strain" theory [6] demonstrate that, in the case of burst tests on pipes and the instability case (burst), the uniform elongation in the body of the pipe must be significantly lower than the uniform elongation at maximum force in the tensile test. The reasons for this are the negligibly low strain in the longitudinal direction of the pipe also demonstrated in the

test [9-11] and the incompressibility of the material (ν = 0.5) in the plastic range. A large number of results from tensile tests (circumferential direction) determined on large-diameter linepipe steels are shown in Figure 14 to permit direct comparison against the results from the component tests found in Figure 11. Uniform elongation is plotted here against yield strength ratio. As can be derived from Figures 11 and 14, a practically proportional correlation

(εcM ≅ AG /2.5) exists between the uniform elongation determined in the tensile specimen (circumferential specimen) and the integral circumferential plastic deformation found in the component. Conclusions In the scope of an Europipe project work a test programme was carried out to evaluate the effect of PE-ageing on the mechanical properties of cold expanded SAW linepipe. Tensile, Charpy, DWT and burst tests were carried out on X60, X65(sour), X70 and X80 SAW pipe. The results of tests performed on Charpy and DWT specimens indicated that there is no expressed difference between PE-aged and non-aged pipe material under sudden exposure to load. The – compared to the result of tensile test - relatively low residual circumferential plastic deformations in the pipe generally found in burst tests are the result of multi-dimensional loads and the associated obstruction of strain. Data obtained from a component test can therefore be compared only indirectly with results obtained in the tensile test.

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As expected, cold deformation during pipe forming and expansion, and subsequent heat treatment during the PE coating operation, generally resulted in an increase in yield strength and Y/T ratio, and in a decrease in strain and elongation values after PE aging. This effect was observed in both tensile and burst tests. Furthermore, it can be assumed that aging of the material has occurred in older pipelines which have already been in operation for a long time, even where these have not been coated. While the toughness properties in terms of Charpy and DWTT are not affected, a certain amount of decrease of the circumferential elongation of cold formed pipes has to be accepted as a physical response. For typical API linepipe grades up to X80, values down to 2% or even lower can occur. But up to now, no cases of failure which could be attributed to aging effects have been reported. Further research should be focussed defining safe limits for minimum circumferential elongation in pipes with regard to pipeline design conditions. Nomenclature AG = uniform elongation AG‘ = true strain at uniform elongation A2“ = elongation after fracture (Lo=2“) A5 = elongation after fracture (Lo=5do) Da = outside diameter of pipe Di = inside diameter of pipe pH = maximum pressure of pipe pT = pressure at yield point Rm = tensile strength t = wall thickness TS = tensile strength TS‘ = true stress at tensile strength YS = yield strength Y/T = yield-to-tensile strength ratio

εcM = uniform elongation of the pipe

κ = Da/Di

σ1,2,3 = principal stresses

σt0.5 = stress at 0.5% total limit

ν = poisson’s contraction ratio

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References

[1] Müsken G., Kaiser H.J., 1984. Auswirkungen des Bauschinger-Effektes auf das Bauteilverhalten hochfester Stähle. Thyssen Technische Berichte. Issue 2/84

[2] Hillenbrand H.G., Graef, M.K., Groß-Weege, J., Knauf G., Marewski U., 2002. Development of Line Pipe for deep-water applications. ISOPE. The 12

th

International Offshore and Polar Engineering Conference & Exhibition. Kyushu Kitakyushu. Japan. May 26-31.

[3] Gresnigt, van Foeken, Shilin Chen, 2000. Collapse of UOE Manufactured Steel Pipes. Proceedings of the Tenth (2000) international Offshore and Polar Engineering Conference Seattle. USA. May 28-June 2., 2000

[4] Al-Sharif, A.M., Preston, R. 1996. Improvement in UOE Pipe Collapse Resistance by Thermal Ageing. Proceedings of the 28

th Annual Offshore

Technology Conference. OTC 8211. Houston May 1996. 579-588

[5] Hohl G., Knauf G., Knoop F.M., 2001. The Effect of Specimen Type on Tensile Test Results and ist Implications for Linepipe Testing. 3R international, Nr.10-11 2001

[6] Christopher T., Rama Sarma B.S.V., Govindan Potti P.K., Nageswara Rao B., Sankarnarayanasamy K. 2002. A comparative study on failure pressure estimations of unflawed cylindrical vessels. International Journal of Pressure Vessels and Piping 79 (2002) 53-66

[7] Turner L.B., 1910. The stresses in a thick hollow cylinder subjected to internal pressure. Trans Camb. Phil. Soc. 1910. 21.

[8] Wellinger K., Uebing D., 1960. Festigkeitsverhalten dickwandiger Hohlzylinder unter Innendruck im vollplastischen Bereich. Mitteilungen der Vereinigung der Großkesselbetreiber. Heft Juni 1960

[9] Crossland B., Bones JA. 1955. The ultimate strength of thick walled cylinders subjected to internal pressure. Engineering (London) 1955. 179. 80-3

[10] Costantine CJ., 1965. The strength of thin-walled cylinders subjected to dynamic internal pressures. Trans ASME J. Appl. Mech. 1965. 32. 104-8

[11] Margetson J. 1978. Burst pressure predictions of rocket motors. AIAA paper No. 78-1569. AIAA/SAE 14

th Joint Propulsion Conference, Las Vegas. NV

USA, July 1978

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Tensile test according to part 1 of DIN EN 10002

Strip specimen API Spec. 5L Round bar spec. B12x60 DIN 50125

Identification σt0.5

(MPa)

Rm

(MPa)

σt0.5/R

m

(%)

A2”

(%)

σt0.5

(MPa)

Rm

(MPa)

σt0.5/R

m

(%)

AG

(%)

A5

(%)

X60 30“x14.4mm „as-received” 475 567 84 - 484 578 84 - 23.8

X60 30“x14.4mm „PE-aged” - - - - 522 585 89 - 22.5

X65sour 36“x19.1mm „as-received” 483 543 89 39.6 473 536 88 8.2 26.8

X65sour 36“x19.1mm „PE-aged” 491 553 89 41.9 508 545 93 7.2 24.8

X70 40“x16.9mm „as-received” 518 634 81 33.1 565 645 88 8.06 20.6

X70 40“x16.9mm „PE-aged” 543 641 85 34.5 606 652 93 7.27 19.9

X80 48“x18.3mm „as-received” 599 741 81 35.4 640 742 86 8.7 23.0

X80 48“x18.3mm „PE-aged” 636 766 83 34.2 714 769 93 6.8 21.1

Table 1: Comparison of strength- and ductility-values measured on transverse strip and round bar specimen

Full-scale-test Identification PT

(bar)

PH

(bar)

εcM

(%)

σt0.5

(MPa)

Rm

(MPa)

X60 30“x14.4mm „as-received” 183 209 4.05 501 543

X60 30“x14.4mm „PE-aged” 209 219 3.11 543 568

X65sour 36“x19.1mm „as-received” 228 236 3.29 534 553

X65sour 36“x19.1mm „PE-aged” 240 243 2.12 562 570

X70 40“x16.9mm „as-received” 217 231 4.27 641 683

X70 40“x16.9mm „PE-aged” 226 235 3.30 668 695

X80 48“x18.3mm „as-received” 195 219 3.55 640 719

X80 48“x18.3mm „PE-aged” 218 227 2.36 715 744

Table 2: Comparison of strength- and ductility-values of unaged and PE-aged pipes determined in full-scale-tests

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Figure 1:Comparison of the stress-strain curves in the as-received and PE-aged conditions (transverse round bar specimen)

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Figure 3: Transverse Charpy V-notch impact energy curves for the pipe body material

Figure 4: Transverse Charpy V-notch impact energy curves for the pipe body material