Strength and Bauschinger Effect in TMCP Line Pipe Steels

I.Yu. Pyshmintsev1, D.A. Pumpyanskyi2, L.G. Marchenko2, V.I. Stolyarov2

(1. Russian Research Institute for the Tube and Pipe Industries, Russia; 2. TMK, Russia)

Abstract: Loss of strength in TMCP microalloyed low carbon linepipe steels with ferrite-pearlite microstructure due to Bauschinger

effect (up to 70-80MPa of yield stress reduction) has been studied. Mechanical behavior of the steels has been investigated using

tests of full-section flattened specimens taken from both 15,7 or 18,7mm plates and 1420mm spiral welded pipe with specified

minimum yield stress of 461 MPa. The dependence of the microstructure and yielding behavior on position in wall section has been

studied using sub-size 3 mm thickness specimens machined from sub-surface and central parts. Change of tensile curves of surface

and internal layers during forming, flattening and protective polymer coating have been studied using sub-size samples subjected to

three or four-point bending, flattening and annealing. The studies have revealed strong dependence of properties on test methods and

the difference in yielding can be decreased significantly by low temperature annealing.

Key words: TMCP high strength steel, Bauschinger effect, forming, testing, annealing, simulations

1 Introduction Increase in yield stress enables increase in operating pressure and, thus in gas pipeline capacity without weight growth which initiates a developing high strength steels. Past decades of the studies pointed out significant differences in results of yield stress measurements for these steels using transverse round bar and flattened full-section specimens. The reasons of the difference are usually associated with work hardening, Bauschinger effect and residual stresses. While an opinion adopted in the 70th concerning deficiency of softening during pipe forming using new high strength (API Х70 and higher) steels with considerable amount of bainite or martensite in the structure now needs to be corrected[1-5]. Nowadays discussion of issues of yield stress measurement for pipe steels is of special interest. The purpose of this article is to generalize the known data and experimental results of the effect during forming and tests of high strength spiral welded pipes. 2 “Loss” of Strength due to Forming There are three known methods to determine strength properties in hoop direction by tension of specimens: flattened full-section, round bar and ring. There were found significant differences in the results of measuring the yield stress by these methods for high strength steels[1,2,5]. Stress and strain gradients form in the wall section

during pipe forming. Flattening the specimen for a tensile strength test introduces additional tension-compression cycle into external and internal fibers of metal. As a result the material layer-by-layer acquires different properties. It is fundamental that the bending provides a heterogeneous deformation in section that determines the presence of high residual stresses. Historically loss of linepipe strength had become a problem during application of control rolled high strength steels, in which about a half of the yield stress increment due to grain refinement, substructure and precipitations was lost during pipe making. Nowadays it is well known[6] that the type and place of sampling have a governing influence on results of yield stress measurement. The wall thickness to diameter ratio t/D and strain hardening rate during cold expansion influence on the effect value and it’s sign. The general tendency is the “softening” growth in new higher strength steels that is stipulated by higher value of residual stresses. Nevertheless the value of the effect at equal strength is determined as well by the type of microstructure (specific value of each of the strengthening mechanisms resulted in strain hardening behaviour). The published data prove[7] that the thermal cycle of polymeric coating of pipes may lead to significant or even full relief of “softening” in high strength materials due to strain age-hardening and other effects. The problem of “strengthening” has a great practical

importance both relatively widely used low pearlitic steels of X60-X70 type and advanced higher strength materials. The research was aimed at peculiarities of the effect relating to technology of manufacturing 1420 mm spiral welded pipes. 3 Experimental Industrial low-carbon control rolled plate steels and spiral welded 1420 mm diameter pipes of 15.7 and 18.7 mm wall thickness were used in this study. The range of the chemical composition of used heats for each wall thickness is specified in Table 1.

Table 1 Chemical composition of steels Content of elements. wt %

C Mn Si- Nb V Ti Mo N

t, mm

х100 х100 х100 х1000 х1000 х1000 х1000 х100015,7 5-6 150-157 26-29 49-60 71-72 3-9 186-226 7-8

18,7 6-9 155-156 19-23 50-52 69-73 8-16 220-220 6-9

S-0,003…0,005%; P-0,007…0,016%; Al – 0,028…0,041%

Flattened full-section specimens for tension tests were machined from plates in transverse and longitudinal directions. The specimens of the same type were cut out of pipes along and across the axle that corresponded the diagonal direction in the plates. The simulation of pipe forming and testing on strength properties were carried out by bending and flattening full-section specimens up to deformation corresponding to the pipe forming process. The influence of the microstructure type and the level of strength properties on the effect for different layers of the plates were studied using flattened micro-specimens with 3x10 mm cross section cut out of pipes and original plates at different depth down from the surface. The orientation of micro-specimens was similar to full-section specimens. The simulation of the “forming - testing” cycle was carried out using a three-point or four-point bend of the specimens. The strain of external fibers of micro-specimens during three-point bend varied in the value of the residual deflection from 0.5 to 2.2% and was calculated by the formula:

%10062lft

=δ,

where f is residual deflection; t is specimen thickness; l is distance between supports. At four-point bend the radius R was calculated using deflection and the strain was determined:

%1002Rt

=δ.

4 Results and Discussion The research showed the occurrence of three microstructure zones in wall thickness. The 100 mkm thick surface layer has a very fine grain structure (3-5 mkm) of ferrite. Deeper on the depth up to 2-2.5 mm microstructure with various grain ferrite 3 to 20 mkm and some amount of pearlite was observed. The biggest grains are elongated in the rolling direction. Acicular and low-temperature products were not observed in the structure. The medium size grain is higher in the biggest central area of the plate and grains are sized 8-30 mkm. Separate bands and elongated islands of pearlite are observed here in which elongated pearlite particles are separated by needle like ferrite which is diagonally oriented relatively rolling direction. Thus, plates have a natural gradient of microstructure and properties that provides as well for the difference in the stress-strain properties and hardening response. Figure 1 gives the results of yield stress measures as well as the tensile strength in the plates 18.7 mm thick and pipes using full-section transverse specimens. The results of it were compared with the plate mill certificate data. The difference of results of all the three data groups is obvious. The dependence of properties on the direction is more clearly observed in pipes. The yield stress and tensile strength measured at the longitudinal specimens from pipes are noticeably higher than at the transverse ones that is probably caused by the differences in the treatment. Tensile curve analysis showed (Fig.2), that due to anisotropy the yield stress measured on the transverse specimens from plate are, as a rule, higher by 25-30 MPa than in longitudinal direction. The yield stress for diagonal direction has an intermediate value what corresponds to the known data[8]. Yielding plateau on the curves for specimens from pipes was not observed and yield stress measured for residual strain of 0.2% is

reduced by 40-70 MPa relatively to plates. The “softening” effect is lower for the diagonal pipe specimens.

480 520 560 600 640

480

520

560

600

640

T.S.

Y.S.

/ transverce/ longitudial

plate mill/ pipe mill

Stre

ss (

for p

late

spe

cim

ens)

МPа

Stress (for transverce pipe specimen), МPа Fig.1. Stresses measured using 18.7 mm full-section

specimens

300

350

400

450

500

550

0,2%

4321

Engi

neer

ing

stre

ss, М

Pа

Engineering strain

Fig.2 Typical tensile curves for full section flat specimens

of 18.7 mm thick plate (1,2) and pipe (3,4). 1-transverse, 2-

longitudinal, 3–along the direction of sheet rolling (diagonal

in pipe); 4-transverse (in pipe).

The study of the yield stress change during treatment simulation on the full-section flat specimens indicated different behavior of 15.7 mm steel in sheets 1 and 2 (fig.3). Despite the far lower yield stress owning to features of microstructure the sheet 2 “softens” more

560 600 640 680

540

560

580

600

620

640

660

680

Т L D

Yiel

d st

ress

(afte

r ben

ding

), M

Pa

Yield stress (in plate), MPa

а

440 480 520 560 600

440

480

520

560

600

Т L D

Yiel

d st

ress

(afte

r ben

ding

), M

Pa

Yield stress (in plate), MPa

b Fig.3 Yield stress in the “plate” and “pipe” at simulation of

forming and testing of full-section specimens from sheets 1

(a) и 2(b) 15.7 mm thick. intensely, resulting in substantial strength reduction by 60-90 MPa. Figure 4 shows typical tensile curves at small strains for micro-specimens cut off the pipes at different depth from surface. Difference in deformational behavior of these layers is evident. The surface layer has the maximum yield strength at the presence of the plateau at a stress of 600 MPa. The intermediate layer has showed lower yield stress. Central area has the lowest yield stress, and the yield plateau is minimal though transition to plastic yield is clear. Micro-specimens cut from the “pipes” (without bending) indicates the behavior different from those observed on transverse full-size specimens of pipes. Tensile curves of micro-specimens cut from pipes and sheets differ slightly. One can suppose that absence of apparent “softening” during measuring on flat micro-specimens can be caused by the absence of unbending prior to testing, lower fiber strains and change of residual stress during machining both in relation tofull section specimen and initial pipe.

0 1 2 3 4300

400

500

600

700

Surface

middle

1,5 mm from surface

En

gine

erin

g st

ress

, MPa

Engineering strain, %

Fig.4 Tensile curves parts for micro-specimens, cut from

pipes in diagonal direction.

Figure 5 shows the correlation of the strength properties, measured with use of full-size specimens and micro-specimens cut from various pipe layers and 18.7 mm plate. The simulation of the cycle “forming-testing” prior to testing was carried out using three-point bending up to plastic yield of outer fibers 1-2%. It is apparent that strength, measured with use of micro-specimens cut from central and pre-surface layers, is in good correspondence with properties measured at full section. The use of micro-specimens

500 520 540 560 580 600 620 640500

520

540

560

580

600

620

640

660

680

T.S.

Y.S.

3

2

1

3

2

1

plate before bending plate after bending pipe before bending

Stre

ss fo

r sub

-siz

e sp

ecim

en, M

Pa

Stress for full size specimen, MPa

Fig.5. Correlation of tension in the initial state and upon

forming simulation with the use of three point-bending of

micro-specimens, 1 – surface, 2- pre-surface, 3 - central

allows to quite accurately simulate the cycle “forming-testing”, as a result the effect of 20-25 MPa is observed, that is close to practice. It is to be noted that “softening” effect was observed in pipes before coating. The simulation of thermal cycle of polymer coating by annealing showed that as a result coating the restoration of yield stress with formation of the plateau can be found for all microstructures. The increment of yield stress after annealing can reach 40-60 MPa. Tensile curves for various layers after “bending- unbending” cycle with strain of 1,5-2%, is similar to full size specimens. After the cycle the yield peak disappears and the yield stress decreases. From the data in Table 2, one can be seen that the “softening” depends on the yield stress and maximum was found in the surface layers with highest yield stress. The annealing corresponding to the thermal cycle of coating considerably changes strain hardening behaviour at small strains. As result the upper yield stress and yield plateau can be found on the curves.

Table 2 The effect of deformation and annealing on

microspecimens yield stress, MPa Upon bending –unbending Plate

before annealing After annealing 200 оС, 10 min

Spec

imen

Y.S. Y.S.(0,2) Y.S.−Y.S.(0,2) Y.S.′ Y.S.−Y.S.′ 1 615 565 50 600 15 2 550 531 19 555 -5

Pla

te

3 520 525 -5 550 0 1 600 521 79 590 10 2 555 495 60 554 1

Pip

e

3 525 500 25 525 0

5 Conclusions One can consider that the traditional evaluation of reliability of high grade steel pipes by yield stress value, measured with the use of flat full section transverse specimens is conditional. It is proved by yield stress measurement with the use of micro-specimens cut off various microstructural zones in the pipe wall section which demonstrates no considerable metal “softening”. It is experimentally showed that the observed “softening” upon deformation cycle “bending-unbending” is practically fully relieved after even short annealing at 200оС, corresponding to the cycle of polymer coating. The strength measured with the use of relatively small flat or round bar specimens to the greater extent

reflects the real pipe strength rater than flattened full-size specimens. The use of flattened full-size specimens for high strength steels can lead to reduced measurement result, and as a consequence, to increase of pipelines weight and cost. To a certain extent the observed yield stress “reduction”, resulting in decrease of its ration to tensile strength, can be considered positive in terms of criteria based on pipeline strain limit. However this effect shall be considered mainly as a measurement result rather than physical phenomenon. References: [1] Glover A., et al. Yield strength and plasticity of high strength pipelines. 4th Int. Conf. on Pipeline Techn., Oostende. 2004. V.1.: 65-79. [2] Millwood N.A., et al. The influence of tensile testing method on the measured properties of high strength steel linepipe. 4th Int. Conf. on Pipeline Techn., Oostende. Belgium, 2004. V.4.:1857-1879. [3] Shoemaker A.K. The effect of plate stress-strain behaviour and pipemaking variables on the yield strength of large diameter DSAW linepipe/ Eng.Mater.Techn., 1984.106(20):119-126.

[4] Ratnapuli R.C. et al. A method of calculating Bauschinger effect in API linepipe steels. Mech. and steel processing. 28th MWSP. Conf. ISS, AIME, 1987. [5] Saikaly W.E. et al. Comparison of ring expansion vs flat tensile testing for determination linepipe yield strength/ Int. Pipeline Conf. Calgary. 1996. vol.1.:209-213. [6] Liessem A. et al. Influence of heat treatment on mechanical properties of UOE linepipe. M.K.Graef, G.Knauf, U.Marewski// 4th Int. Conf. on Pipeline Techn., Oostende. 2004. V.3. pp.1262-1281. [7] Fluess P., Schwinn V., Bush K. Production and development of pipes for conductors and risers with strength level X80 and X100 without pipe expansion /4th International Conference on Pipeline Techn., Oostende. 2004. V.2. :809-822. [8] Baczynski G.J. et al. The influence of rolling practice on notch toughness and texture development in high-strength linepipe. Met&Mat Trans., 1999. 30A.: 3045-3054. [9] Streisellberger A. Correlation of pipe to plate properties – Model Calculations and Application in design of X80 linepipe steels. Int. Conf. of pipeline reliability, Calgary. vol.1., 1992.