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Materials Science and Engineering A 528 (2011) 1648–1655

Contents lists available at ScienceDirect

Materials Science and Engineering A

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m s e a

Strength–hardness statistical correlation in API X65 steel

S.H. Hashemi ∗

Department of Mechanical Engineering, The University of Birjand, POBOX 97175-376, Birjand, Iran

a r t i c l e i n f o

Article history:

Received 9 September 2010

Received in revised form 24 October 2010

Accepted 26 October 2010

Keywords:

API X65 steel

Weld microstructure

Weldment properties

Gas transportation pipeline

Probability density function

a b s t r a c t

Accurate determination of pipe yield strength (as an indication of material resistance to plastic collapse)

is of great importance to pipeline engineers. In this research,Vickers hardness data from 100tested pipes

(APIX65) wasused to derive strength–hardnessstatisticalcorrelation. First, hardnessdata weremeasured

in different sub-zones of weldment (i.e. weld metal, HAZ, and base metal). Next, tensile properties of base and weld metal were measured on flat tensile strips. The minimum, maximum, mean and standard

deviation of test data were calculated then for statistical variation and difference of mean value in each

test zone. All test data were described by probability density function (PDF), and zones with the largest

variancewere determined. It wasshown that hardness data could be used foryieldstrengthprediction in

APIX65 steelwith reasonable accuracy.Discussionon weldmentmicrostructurecorrelation withstrength

and hardness data concluded the paper.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The increasing demand for natural gas as clean energy has led

to mass production of high-strength low-alloy (HSLA) steels in

recent years.These steels areused in constructionof long-distance,high-pressure gas transportation pipelines and networks. The pipe

technicalspecifications aregivenby standard codes,such as API[1].

The APIX80 (80ksi yield strength) for large-diameter gas pipelines

is currently in use [2], and high toughness materials (X100 and

X120) have reached the stage of full-scale testing [3]. The apparent

benefits of such pipe steels are reduction in overall project cost as

a result of reduced quantity of steel, lower pipeline transportation

and laying cost, and lower welding time and cost due to thinner

wall thicknesses [4].

The API steels offer combined strength and toughness, coming

from their TMCR (thermo-mechanical controlled rolling) pro-

cess. This process favours the formation of acicular ferrite-based

microstructure, which is the preferred microstructure for pipe

steels [5]. The main objective is to obtain the best possible combi-nation of strength and toughness, for pipe steels experiencing high

internal pressures of 10 MPa and even above [6]. These properties

(strength and toughness) are vitalfor pipeline structures, which are

vulnerableto plastic collapse and to ductile crackpropagation. Typ-

ically, the latter is evaluated either on Charpy or drop weight tear

test (DWTT) samples [7,8], whereas flat-strip tensile specimens are

used for measurements of material strength [9].

∗ Corresponding author. Tel.: +98 561 2502516; fax: +98 561 2502515.

E-mail addresses: [email protected], s h [email protected]

In this research, tensile test on API X65 steel was conducted

to assess its mechanical characteristics. Moreover, Vickers hard-

ness test [10] was performed on this steel, to ensure no hard spots

existed in test material, and to use hardness data for strength

prediction. In total, 100 spiral pipes with similar heats and manu-facturing processes were used. Material characterisation was done

in different sub-zones of weldment in tested steel. The obtained

data were analysed statistically, and their mean value in vari-

ous weldment sub-zones were calculated and compared to service

requirements setby APIindustrystandard. Theestablishment of an

empirical relationship for prediction of yield strength in pipe base

metal (as an indication of structure resistance to plastic collapse)

from measured hardness data and detailed discussion on the rela-

tionship between weldment microstructure and experimental data

concluded the paper.

2. Characteristics of test pipes

The material under investigation was API X65 (gas pipe steel)

with 1219 mm outside diameter (OD) and 14.3mm wall thickness

(WT). Its average chemical composition (wt.%) was 0.071C, 0.209Si,

1.515Mn,0.018Cu, 0.011Ni,0.003Mo, 0.044Nb,0.042V, 0.017Ti,and

0.157Cr. It is worth noting that the sum of Nb and V contents

should not exceed 0.06% for welded pipes, according to API stan-

dard. The sum of Nb (0.044) and V (0.042) contents (i.e. 0.086%)

in this research was higher than the specified value from standard

code, which is agreed between pipe purchaser and manufacturer

[1].

The API X65 pipes were formed by spiral welding technique.

The original coil used for pipe manufacture was produced by

0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.

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S.H. Hashemi / Materials Science and Engineering A 528 (2011) 1648–1655 1649

Fig. 1. Location and orientation of flat strip tensile samples in spiral pipe.

thermo-mechanical control-rolled (TMCR) process. The API X65

steel, together with API X70 are the most commonly used pipe

materials in Iran high-pressure gas transportation pipelines andnetworks. The typical working pressure of these pipelines in Iran

is around 750–1000psi (equivalent to 5–7 MPa). It should be noted

that hoop stresses caused by the internal pressure in the natural

gas pipelines confined to 0.4–0.8 SMYS (specified minimum yield

strength) of the used pipe steel [11]. As the SMYS of the API X65

steel is 448 MPa, its hoop stress ranges from 180MPa (0.4SMYS) to

360 MPa (0.8SMYS).

3. Tensile experiments

Gas pipe steels are subjected to high internal pressures and

should have high strength and toughness levels to withstand sever

loading conditions. Accordingly, tensile tests are the very first

(destructive) mechanical experiments, which are conducted onpipe materials. In this research, flat strip tensile specimens were

used to measure mechanical properties of API X65 steel, as recom-

mended by API code [1]. In total, 100 pipes with similar heats and

manufacturing processes were analysed. The general mechanical

characteristics of base and weld metal were determined for each

pipe, and compared then to industry requirements set by API 5L

standard code.

3.1. Tensile test standard and sample preparation

Tensile test procedure conformed to the requirements of API

5L [1]. Flat test samples were cut from each pipe in the specified

position and direction, as shown in Fig. 1.

Ascanbeseeninthisfigure,basemetal(BM)specimenswerecutin hoop orientation (parallel to the direction of maximum stress),

whereas weld metal (WM) specimens were machined perpendic-

ular to spiral weld [1].

The design geometry of tensile test specimens (for base metal)

is demonstrated in Fig. 2. Flat test samples had gauge width, thick-

ness, and length of 38.1, 14.3, and 50mm, respectively.

According to API standard, the transverse tensile test pieces

for seam weld experiments shall be approximately 38mm (1.5 in.)

wide, and shall have the butt weld at the mid-length of the test

piece. Furthermore, the weld reinforcement shall be removed from

both faces [1].

It should be noted that the thickness of flat strip samples was

equal to pipe full-thickness (t = 14.3mm) to accurately capture

material properties. The initial test plates for specimen prepara-

Fig. 2. Dimensions of flat strip tensile specimens (used for base metal testing).

tion were extracted from each pipe body by flame cutting first,

and flattened then by hydraulic pressing machine to remove the

initial curvature. This process introduced slight pre-strains in test

samples, which is tolerated by standard code.

3.2. Tensile tester specifications

A 600 kN Zwick tensile testing machine with hydraulic clamps

and computer data logger was used in this research. All tensile

experiments were conducted at room temperature under displace-

ment control with ramdisplacement of 0.05 mm/s. In each test, theapplied load and specimen elongation were measured. An exten-

someter with 50 mm gauge length was used to monitor specimen

axial strains. The computer software gave yield strength (at 0.5%

total elongation according to API 5L), ultimate tensile strength

(UTS),and specimen elongation (in50 mmgaugelength) atfracture

point for each test.

3.3. Tensile test results

In total, 100 tensile data were obtained in each test set from

tensile testing of two different specimens (i.e. base and weld

metal). The yield strength (YS), ultimate tensile strength (UTS),

and specimen elongation were obtained from base metal experi-

ments, whereas only UTS was measured for weld metal specimensaccording to test standard recommendations [1].

Tensile test data, like other statistical quantities, can be

described by their mean, standard deviation, and distribution type.

The probability density is defined as the number of occurrences

divided by the total sample number. If the tensile test data are

given in the form of Gaussian or normal distribution, the proba-

bility density function (PDF) is calculated for each set, as follows

[12]:

f ( x) = 1

SD√

2exp

−1

2

x−mean

SD

2

(1)

where SD is the standard deviation of test data in each set, x is the

measured strength (in MPa), and mean is the average strength (inMPa) in each set. Table 1 contains alltest results from tensile exper-

iments, together with target values given in API 5L [1]. As can be

seen from this data, the material tensile properties fulfilled the API

specifications (448MPa< YS< 598 MPa, 531 MPa < UTS < 758 MPa)

for API X65 steel.

Fig. 3 demonstrates the frequency distribution of yield strength

for API X65 (base metal). As can be seen in this figure, the

most occurrence of yield strength (30%) was associated with yield

strength level of 540–560MPa. This was close to average yield

strength of API X65 steel of 538 MPa (±26 MPa SD), see Table 1.

In Figs. 4 and 5, the cumulative probability and probability den-

sity function of material strength are demonstrated. As can be seen

in these plots, an average ultimate tensile strength of 624MPa and

638 MPa were obtained for base and weld metal, respectively.



1650 S.H. Hashemi / Materials Science and Engineering A 528 (2011) 1648–1655

Table 1

Tensile data of API X65 pipes measured on flat strip.

Property Min. Max. Mean PDF (%) SD

Base metal yield strength (MPa) 479 589 538 1.6 26

Target (MPa, API 5L standard) 448 598 – – –

Base metal tensile strengtha(MPa) 582 672 624 2.2 18

Target (MPa, API 5L standard) 531 758 – – –

YS/UTS 0.77 0.89 0.86 29 0.02

Target (API 5L standard) – 0.93 – – –

Base metal elongation (%) 30 38 34 24 1.7Target (API 5L standard) 24 – – – –

Weld metal tensile strengthb (MPa) 549 676 638 2.5 16

a Measured in pipe circumferential direction.b Measured perpendicular to spiral weld.

Fig. 3. Frequency versus yield strength in API X65 steel.

Fig. 4. Cumulative probability versus strength in API X65 steel.

4. Hardness experiments

Gas pipe steels should have a minimum hardness (resistance to

indentation and deformation) to resist surface scratch and dents.

On the other hand, their average hardness level should be less than

Fig. 5. Probability density function versus strength in API X65 steel.

a maximum value corresponding to unfavourable hard spots. High

hardness of weld metal generates weak spots for crack initiation.

The APIstandardstates that anyhard spot of hardness level greater

than or equal to 35 HRC (327 HB or 345 HV10) shall be rejected.

Therefore, hardness experiment is required to control pipe accept-

able hardness level and to ensure its safetyand structural integrity.

In this research, Vickers hardness test was used to determinethe hardness levels of API X65 steel in four different sub-zones of

weldment (i.e. weld metal, HAZ(1) in the left side of fusion line,

HAZ(2) in the right side of fusion line, and base metal). In total, 100

welded joints were tested for hardness measurement.

4.1. Hardness test standard and sample preparation

The hardness test did not include in older API 5L standard

(43rd edition) [1], and this experiment was conducted on weld-

Fig. 6. Photograph of hardness tester and test samples for hardness measurement.




Fig. 7. Schematicof weldment cross-sectiondemonstratingdifferent sub-zones for

hardness measurement.

ment according to the more recent API 5L standard (44th edition)

[13]. This standard required that samples for hardness tests should

contain a section of the helical seam at their centre.

Each weld joint included indentation on the weld metal (WM),

HAZ (both in the left side and right side of fusion line), and base

metal (BM). The Vickers method HV10 (transverse weight of 10kg)

was used. In Fig. 6, hardness test samples, cut from test pipes aredemonstrated.

Before hardness test, the transverse weldment cross-section

was prepared and polished using different grades of emery papers

and diamond paste. The specimen then was etched with Nital 2%,

and examined by lightopticalmicroscopy, and by scanning electron

microscopy.

In total, 43 test points were examined in each test sample on

the cross-section of API X65 weldment, as shown in Fig. 7.

Indentations were made from one base metal side to the other,

below the surface and in the horizontal centre line, on either side

of the weld. From this, the hardness of test material was deter-

mined for each pipe, and compared then to industry requirements

set by standard code. It should be noted that while the control of

all 43 data points was required for hard spots examination, onlythree test points (out of 43) was used for average hardness mea-

surement in each zone. This is common industry practice, based on

pipe delivery conditions. The data points used for hardness evalu-

ation were 10,23,36 (in vertical centreline of seam weld) for weld

metal, 7,21,33 for HAZ(1) in the left side of fusion line, 13,25,39 for

HAZ(2) in the right side of fusion line, and 3,18,29 for base metal,

respectively.

4.2. Hardness tester specifications

Generally, a square-base diamond pyramid is used in Vickers

hardness testing as the indenter with the included angle between

opposite faces of the pyramid of 136◦. The loads (ranging from 1 to

120 kg, depending on the hardness of test material) are applied for10–15 s. The Vickers hardness number (HV) is defined as the load

divided by the surface area of the indentation [10]. In this research,

a Vickers HTMhardness testerwith10 kg(constant) transverse load

was used for hardness measurement (see Fig. 6).

4.3. Hardness test results

Table 2 summarises all test results from hardness experiments,

together with target value given in API 5L [13]. As can be seen, all

measured hardness data conformed to the API specifications.

Fig.8 demonstrates thedistribution of hardness in APIX65 (base

metal). As can be seen from this data, the maximum occurrence of

hardness valueswas 27%correspondingto hardness level of 221HV

in base metal.

Table 2

Hardnessdata(HV10) of 100pipes measured in four different sub-zonesof APIX65

weldment.

Sp ecimen V icker s har dnes s

Min. Max. Mean P DF ( %) S tan dard deviation

Weld metal 230 239 235 27.5 1.44

HAZ(1) 213 215 214 39.7 0.98

HAZ(2) 213 218 215 35.4 1.00

Base metal 217 228 221 19.7 1.99

API 5L standard – 345 – – –

Fig. 8. Frequency versus hardness data in API X65 steel (base metal).

Fig. 9. Variation of hardness and tensile strength in weldment cross-section of API

X65 steel.

In Fig. 9, the variation of hardness and tensile strength data in

weldment cross-section is demonstrated. As can be seen from this

plot, the weldmetal had the highest level of hardness (235HV) and

tensile strength (638 MPa). On thecontrary, theHAZ(1) andHAZ(2)

sub-zones had the lowest hardness level, with an average value of

215HV.

In Fig. 10, the cumulative probability of hardness values for API

X65 base metal, HAZ and weld metal are demonstrated. As can be

seen in these plots, an average hardness level of 235 and 221 HV

were obtained for weld and base metal, respectively. The HAZ(1)

andHAZ(2) sub-zoneshad close hardness levels with average value

of 214 and 215 HV, respectively.

Fig. 10. Cumulative probability versus hardness data in API X65 weldment.




Fig. 11. Plot of base metal yield strength versus hardness data in API X65 steel.

4.4. Relationship between hardness and yield strength

Tensile strength measurement in pipematerials is timeconsum-

ing, and requires flame cutting of test plate, specimen flattening,

machining and preparation, and finally test set up and experiment.

On the other hand, hardness experiment is rather easy and quick,as it requires specimen preparation and indentation. Thus, it is

favourable to use hardness data and predict material strength level

whenever possible. In this research, the measured hardness data

were used to obtain an empirical equation for yield strength cal-

culation in pipe base metal. Fig. 11 shows the variation of yield

strength versus measured hardness data in API X65 (base metal).

As can be seen in this plot, the scatter in the measured data

made it difficult to derive a linear relationship (as is common for

steel specimens) between material yield strength and hardness.

The hardness data were in the range of 217 and 228, with an aver-

age value of 221 HV (all of which conformed to API 5L). On the

other hand, yield strength fell in the range of 479 and 589 MPa,

with an average value of 538MPa (again all of which conformed

to API 5L). However, the yield strength data for pipe base metalhad different values even for the same hardness level. For exam-

ple, yield strength between 488 and 570MPa was observed for the

same hardness level of 221 HV (see Fig. 11). The probable reason

for this inconsistency is that yield strength measurement was car-

ried out on full-thickness flat strip specimens in hoop direction.

The thick tensile specimen could accurately capture the average

tensile strength of the bulk material. Hardness measurement how-

ever, was conducted on different specimen (though from the same

pipe), cut in a plane perpendicular to the seam weld. This is nec-

essary as different sub-zones in weldment cross section should be

analysed for hard spots. Moreover, hardness value in each pipe is

thealgebraic average of 3 discrete pointsin thebase metal on weld-

Fig. 12. Linear correlation between yield strength and hardness data in API X65

steel.

ment cross section (see Fig. 7). Despite this, the experimental data

can be used to reasonably correlate hardness and yield strength of

base metal, as shown in Fig. 12.

The graph shows a linear relationship of yield strength with

hardness, with low correlation coefficient (R2 = 0.2), indicating rel-ative scattering. This linear relationship was obtained by the use

of average yield strength in each hardness data point. For example,

27 tensile specimens had yield strength between 488 and 570 MPa,

with equal hardness level of 221 HV. The average 538 MPa yield

strength of these 27 specimens was calculated, and all discrete

points were shown in the plot of Fig. 12 by a single point with

538MPa yield strength, and 221 HV hardness level. This was con-

ductedfor allothertestdatain this graph,fromwhich thefollowing

equation was obtained for yield strength prediction in base metal:

YS = 2× HV+ 105 (2)

The comparison between measured yield strength data and

those predicted by Eq. (2) is given in Table 3.Ascanbeseenfromthistable,theminimumandmaximumerror

(absolute values in percent) between the measured yield strength

and the predicted one was 0.2 and 13.8, respectively. The calcu-

lated error of 13.8% was associated with the point of hardness level

of 220 HV with 479 MPa minimum measured yield strength in the

whole100 measured data.The useof prediction equation gave yield

strength level of 545MPa forthis point (with 220HV),equivalent to

66 MPastrength overestimation.As the 545MPa predictedstrength

still fell in API 5L recommendations, and as this value is close to

564MPa (538MPa average yield strength plus 26 MPa standard

deviation), it can be concludedthat the linear Eq. (2) can be applied

for strength prediction in tested steel with reasonable accuracy.

Table 3Measured and predicted yield strength of API X65 base metal.

HV Predicted YS

(MPa) from Eq.

(2)

Measured

minimum YS

(MPa)

Absolute error

(%)

Measured

maximum YS

(MPa)

Absolute error

(%)

Measured

average YS

(MPa)

Absolute error

(%)

217 539 521 3.5 550 2.0 536 0.7

218 541 528 2.5 528 2.5 528 2.5

219 543 504 7.7 572 5.1 550 1.2

220 545 479 13.8 589 7.5 529 3.1

221 547 488 12.1 570 4.0 539 1.5

222 549 490 12.0 570 3.7 538 2.2

223 551 506 8.9 580 5.0 550 0.2

224 553 491 12.6 550 0.5 518 6.8

225 555 527 5.3 554 0.2 540 2.8

226 557 537 3.7 547 1.8 542 2.8

228 561 573 2.1 573 2.1 573 2.1




Fig. 13. Linear correlation between tensile and yield strength in API X65 steel.

4.5. Relationship between tensile and yield strength

In this research, yield strength was estimated from hardness

data. A similar relationship between tensile strength and hardness

can be found in Table 2A and B ofASTM 370 [14]. However, there is

no conversion table from hardness to yield strength. On the other

hand, it can be argued that plastic deformation is produced on the

surface of hardness test specimen when the indenter is applied

to specimen. Hardness value is calculated then from indented size

of tested specimen, implying that hardness is more related with

tensile strength. This point can be addressed properly with respect

to test information in Fig. 13.

This figure demonstrates a linear relationship between tensile

and yield strength of API X65 (base metal), with relatively low

scatter (R2 = 0.8). It shows rising trend in tensile strength with an

increase in material yield strength, as expected. Figs. 12 and 13

demonstrate thatboth yieldand tensile strength arelinearlyrelated

to hardness data. This also facilitates conversion from yield to ten-

sile strength and vice versa, provided that hardness data of test

material is available beforehand. The combination of test infor-

mation from Figs. 12 and 13 resulted in the following relationshipbetween tensile strength and hardness, which is comparable with

similar equation from Ref. [14]:

UTS = 1.3 × HV+ 344 (3)

4.6. Microstructural features

The microstructure of BM, HAZ and WM was examined by

optical microscopy and SEM. This was conducted to correlate

mechanical properties of tested steel to its microstructure. The

standard metallurgical analysis was carried out through mount-

ing, grinding, polishing, and etching of test specimens in nital 2%.

From this, different zones in the weldment were revealed. Fig. 14

demonstrates the microstructure of different sub-zones in welded joint, including WM, HAZ and BM [15].

5. Discussion

5.1. Statistical considerations of tensile data

From the probability density function of the experimental

data (see Figs. 4 and 5), the minimum, maximum and average

yield strength of 479MPa, 589 MPa and 538MPa, respectively

(with ±26MPa SD and 1.6% PDF) were found for API X65 base

metal. The measured average elongation of base metal at frac-

ture point was 34% (±1.7 SD). Similarly, the average ratio of yield

to tensile strength for base metal was obtained as 0.86 (±0.02

SD). As expected, the API X65 weld metal had higher tensile

strength (638±16MPa SD) compared to base metal (624±18MPa

SD).

Statistical analysis of all tensile datademonstratedthat even not

a single value exceeded the lowest or highest standard targets, and

all measured parameters conformed to API 5L test specifications.

5.2. Statistical considerations of hardness data

From the probability density function of the experimental data

(see Fig. 10), an average hardness level of 235(±1.44SD), 214(±0.98

SD), 215(±1.00 SD), 221(±1.99 SD)HV, corresponding to 50% prob-

ability was obtained for weld metal, HAZ(1), HAZ(2), and base

metal, respectively. The probability density function (PDF) of the

hardness experimental data gave values of 28%, 40%, 35%, and 20%

for the four tested sub-zones, respectively. As can be seen, a gra-

dient of hardness values observed in the weldment. The average

ratio of weld metal hardness to base metal hardness was 106%,

demonstrating hardness increase in weld metal centreline. A slight

HAZ softening (compared to base metal) was observed in the HAZ

weldment.

Statistical analysis of all hardness data demonstrated that even

nota singlevalueexceeded thehighest standard target(345 HV10),

and all measured parameters conformed to API 5L test specifica-

tions.

5.3. Sources of strength and hardness variation

In general, the variation in mechanical properties is a conse-

quence of a number of factors including material re-crystallization,

grain refinement and growth, phase composition, and precipitates

[16]. The variation of strength and hardness data in this research

canbe expalined by SEMobservations from APIX65 weldment (see

Fig. 14). This figure shows gradient of microstructure from proeu-

tectoid and Widmanstattenferrite (in weldmetal centreline)to fine

acicular microstructures (in the unaffected base metal).

5.3.1. Weld metal hardness and strength level

Theweld metal hadthe highest hardness level (235 HV)and ten-sile strength (638MPa) in the weldment cross-section, as shown

in Fig. 9. The average ratio of weld metal to base metal hardness

was 106%. Similarly, the average ratio of tensile strength of weld

metal to base metal was 102%. This demonstrated hardness and

strength increase in weld metal centreline (compared to API X65

base metal). It should be noted that tensile strength overmatch

in WM is favourable in natural gas pipelines for in-service safety.

Another importantpointto emphasisehere is that none of theweld

metal tensile specimens (with the butt weld at their mid-length,

and removed weld reinforcement from their both faces) was broke

at the weld. Instead, test specimens fracture occurred at the base

metal, where tensile strength is lower than that of the weld metal.

This suggested the useof (round or flatstrip) all-weld tensile speci-

mens to get accurate tensile properties from weld metal joint. Suchtest data were not available for this research, and conventional flat

striptest data (suggested byAPI standard)wereused forweldmetal

characterisation.

From microstructural point of view, the higher hardness and

strength level of theWM canbe attributed toits cast microstructure

and presence of grain boundary phases in the micorsructure (i.e.

lower temperature transformation products such as Widmanstat-

ten ferrite, see Fig. 14) [17,18].

During the welding process, WM was melted and re-solidified.

The weld microstructure was inherently coarse-grained. The

coarse directionally solidified grains in the WM can be seen in

Fig. 14A and B. The average grain size within the solidified grains

of WM was around 2–4m. Fig. 14A also demonstrated that the

WM microstructure mainly consisted of acicular ferrite and grain




Fig. 14. Micrographs of welded joint and microstructure of different sub-zones: (A) centre of the weld metal; (B) near the end of the weld metal; (C) coarse-grained HAZ;

(D) fine-grained HAZ; (E) unaffected base metal [15].

boundary phases (i.e. proeutectoid and Widmanstatten ferrites).

Such products exhibited high level of strength and hardness.

5.3.2. HAZ hardness and strength level

The hardness level of HAZ (i.e. 214 HV for HAZ(1)and 215 HV for

HAZ(2)) was slightly lower than that of the BM (221 HV), and not-

icably lower than that of the WM (235HV). This can be attributed

to HAZ microstructure (bainite, polygonal and acicular ferrite), as

shown in Fig. 14.

During the welding process, the HAZ did not experience melt-

ing, but its microstructure changed due to phase transformation.

Fig. 14C and D illustrates the coarse-grained (CG) and fine-grained

(FG) HAZ microstructure, respectively. These zones were consisted

of bainite,polygonal andacicularferrite.The grain size of HAZgrad-

ually varied with distance from fusion line. The coarseness of the

microstructure was due to high heat input of SAW process (around

11.4 kJ, obtained by multiplication of average welding parameters;

I =750A, V = 32 V, weld speed S = 1.2 m/min and arc weld efficiency

of 0.95) [15].

The CGHAZ experienced subsequent thermal cycles in this

high energy welding, and then the grains in this zone tendedto grow and become relatively coarse. The lower hardness level

of HAZ compared to BM hardness, demonstarted slight HAZ

softening in API X65 weldment [19]. Considerable HAZ soften-

ing reported for similar steel (API X65, 762mm O.D, 17.5mm

W.T) [20]. The HAZ softening effect in this reference attributed

to decomposition of martensite by over-tempering. The high

temperature from the welding process altered the hard low-

temperature transformation products to soft high-temperature

products [20]. This further tempering resulted in lower hardness

and strength levels in the HAZ. It should be noted that tensile

properties of HAZ were not tested in the current research, and

was not addressed in API standard [1]. Determination of strength

levels of HAZ is notably difficult and requires special testing

setup. Interestingly, Ref. [20] proposed micro-tensile experiments




to measure HAZ tensile properties (both for longitudinal seam

and girth weld HAZ). An average yield and tensile strength of

396MPa and 567MPa were found for seam weld HAZ, respec-

tively.

5.3.3. Base metal hardness and strength level

The average hardness, yield strength and ductility of base metal

were 221 HV, 538 MPa, and 34%, all of which fulfiled API standard.

The excellent mechanical prperties of API X65 (base metal) can

be attribiuted to BM fine grianed microstrcuture (bainite–acicular

ferrite), produced via TMCR process [21–23]. During the welding

process, the base metal region remained unaffected. As can be

seen in Fig. 14E, the BM had a very fine-grained microstructure.

The average ferrite grain size of the BM was around 3–7m. The

improved cooling rate and reduced cooling interrupt temperature

led to the complete change of the final microstructure of this steel

from traditional ferrite–pearlite structure to bainite–acicular fer-

rite microstructure [5].

6. Conclusions

Variation of mechanical properties in base and weld metal of

100 test pipes (API X65 steel) were measured, and compared to

API 5L standard specification to qualify the test steel performance

under design conditions. Standard full-thickness flat strips were

used for tensile testing on 14.3mm thick, 1220 mm outside diam-

eter spiral pipes, from which yield strength, tensile strength and

maximum elongationwere determinedfor eachtest pipe.The hard-

ness properties data were measured too in different sub-zones of

pipe weldment(i.e.weld metal,heataffectedzone, andbasemetal).

The minimum, maximum, mean and standard deviation of mea-

sured mechanical properties were calculated then for statistical

variationand differenceof mean value in each test pipe. Alltestdata

were described by probability density function (PDF), and zones

with the largest variance were determined. The probability density

function of the experimental data gave an average yield strength

of 538MPa (

±26 MPa SD) with 1.6% PDF for base metal. Similarly,

the average hardness with 50% probability was obtained for weldmetal (235±1.44 SD), HAZ(1) (214±0.98 SD), HAZ(2) (215±1.00

SD), and base metal (221±1.99 SD), respectively. The sources of

tensile and hardness variation in different sub-zones of API X65

weldment were correlated then to material microstructure in each

testedzone.Finally,an empiricalrelationship wasproposed to pre-

dict yield strength in pipe base metal from available hardness data.

This relationship was found to be reasonably linear. Comparison

between the obtained andpredicted results andindustrycode con-

firmed that API 5L requirements were fulfilled for the tested steel.

From this, it can be concluded that the obtained experimental data

from this research were sufficient to meet the final properties on

pipe safely in accordance to API 5L specifications.

Acknowledgements

Supplyof rawhardness andtensile data of APIX65 steel bySadid

Pipe and Equipment Company (via Mr. N. Pourkia), and financialsupport by The University of Birjand is greatly acknowledged. The

author would like to thank Mr. D. Mohammadyani (Materials and

Energy Research Center, Tehran, Iran) for his help and comments

on SEM analysis of API X65 weldment.

References

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[email protected]

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