determination of quasistatic fracture toughness and ...brittle zones (lbz) in coarse-grain haz...

$: Determination of Quasistatic Fracture Toughness and ...brittle zones (LBZ) in coarse-grain HAZ (CGHAZ) are addressed for relatively poor resistance to brittle fracture initiation of$
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Determination of Quasistatic Fracture Toughness and Fracture Resistance Curves for On-Shore and Off -Shore Pipeline Systems

Uygar Tosun¹, Koray Yurtışık², Süha Tirkeş²

¹Ion Industrial Metallurgy Research and Development Limited, Ankara²Middle East Technical University, Welding Technology and Non-destructive Testing Research and Application Center, Ankara, TR

AbstractCompatitive reserves of oil and gas are generally remotely located from the end-users. Reducing transmission costs, thin wall pipelines made of high strength low alloy (HSLA) steel plates offer an additional mechanical and economical advantages. Pipeline materials are generally subjected to considerable plastic strains during installation and operation. In addition to this, microstructural alterations during girth welding operations and possible weld imperfections are also threatening factors for the structural integrity of pipeline systems. Instead of conventional workmanship criteria, engineering critical assessment (ECA) emerges to provide more effective control methodologies on possible fracture failures during construction and operation of pipelines. Tests to determine fracture toughness and resistance curves provide necessary data for such assessments, as well as geometrical factors and stress-strain analyses. Contribution of the studies of elastic-plastic fracture mechanics to qualification of the line pipe girth welding operations, the important fracture mechanics and testing terms and definitions that pipeline construction, welding and quality engineers should be familiar with, and the progress in development of quasi static elastic-plastic fracture toughness test methodologies are introduced and reviewed in the present proceeding manuscript.

Keywords: Pipeline, plastic strain, line pipe steels, fracture toughness, fracture resistance, fitness-for-service, engineering critical assessment

1. Weldability concerns in pipeline construction

The construction of a pipeline involves joining sections of pipes with circumferential welds known as girth welds employing automated Gas Metal Arc Welding (GMAW) and partly automated Flux Cored Arc Welding (FCAW) procedures (Figure 1), as well as manual Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc Welding (SMAW) techniques. The girth welding is a rate limiting process in pipeline construction. This dedicates special concern for weldability and efficiency in girth welding operations.

The purpose of pipeline construction standards is to specify methodologies for the production of reliable weldments employing validated welding procedures. The standards conventionally introduce acceptance conditions that are based on empirical criteria for workmanship and place primary importance on imperfection size and morphology.

Such conventional criteria have provided a satisfactory record of

reliability in pipeline service for many years. Besides specifying maximum tolerable flaw sizes, minimum tolerable Charpy energy is also considered to sustain structural integrity. However, these criteria do not take fracture toughness and fracture resistance properties of as-welded line pipe materials under consideration. Thus, they provide over-safe assessments and consequently weld imperfection tolerances are quite tight.

Figure 1 On-shore pipeline girth-welding operation (top). A platform employed for off-shore pipeline construction (bottom)

API 5L [1] specifies the manufacture of two product specification levels, namely PSL 1 and PSL 2, for linepipe steels in the petroleum and natural gas transportation. Annex H and J of the standard are referred for sour service and off-shore service applications (Figure 1). EN 10025-4 [2] and DNVGL-OS-B101 [3] also specifies linepipe steel grades. As-rolled, normalized, normalizing rolled and quenched and tempered grades are available. The main function of such

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processes and alloying are strengthening of ferrite through grain refinement, solid solution and precipitation hardening. High strength could be resulted with a decrease in toughness, which can damage the structural integrity by increasing the risk of crack propagation [4, 5]. Micro-alloying and thermo-mechanical control processing are optimized in order to obtain optimum strength, ductility and toughness properties [6, 7].

Figure 2 Macro section of a weldment

Original ferritic microstructures of the linepipe steels are polygonal ferrite (PF), quasi-polygonal or massive ferrite (QF), granular bainitic ferrite or granular bainite (GB) and bainitic or acicular ferrite (AF) [8, 9]. These microstructures are transferred to other ferritic and austenitic phases at proximity of the fusion zone, namely heat affected zone (HAZ), due to welding thermal cycles (Figure 2). Local brittle zones (LBZ) in coarse-grain HAZ (CGHAZ) are addressed for relatively poor resistance to brittle fracture initiation of as-welded high-strength low alloy (HSLA) steels [10, 11, 12, 13]. Sequence of weld passes and heat inputs control size and distribution of local brittle zones (LBZ) [14]. Especially the microstructure of intercritically reheated (IR) CGHAZ, where M-A constituents are more detrimental in terms of toughness, has to be controlled by welding engineers (Figure 3).

Welding processes may also yield strength mismatch in the material. Mismatch in the mechanical properties between the fusion zone and the parent metal is inevitable and in fact intentionally optimized to the benefit of the fusion zone (Figure 4). This engineering optimization is technically called strength over-matching. However, it should be noted during this optimization that over-matching generally contributes to the toughness deterioration at the proximity of fusion line.

Figure 3 Microhardness contour map of the weldment (top) and SEM micrograph of IRCGHAZ (bottom)

Figure 4 An example of engineering stress-strain curves obtained by all weld metal and base metal tensile tests

2. An alternative criteria based on FFS

21st edition (2013) of API Standard 1104 [15] additionally provides an alternative method that is based on fracture mechanics analyses and fitness-for-service (FFS) criteria for the evaluation of both size and location of the discontinuity. However, using FFS requires tests to determine fracture toughness parameters or sometimes generate fracture



resistance curves in addition to conventional mechanical tests, as well as stress analyses and inspection using state-of-art techniques. Performing analyses based on the principles of FFS is commonly termed Engineering Critical Assessment (ECA). ECA presents more realistic scenarios for elastic-plastic fracture behavior of the material. Thus this assessment provides more generous allowable discontinuity dimensions. There are other standards and codes available for several industrial infrastructures, in which the discontinuity evaluation methodologies are based on ECA, like DNVGL-ST-F101 [16] for off-shore pipelines or RCC-M [17], R5 for high-temperature damage and R6 for fracture avoidance assessment procedures for nuclear power plants.

Introducing the strength and toughness testing data and comprehensive stress-strain analyses to ECA, tolerable flaw sizes were determined to be larger than the ones from conventional workmanship criteria, consequently quite low repair rates and consequently quite high production rates have been achieved in TANAP project. TEKFEN Construction Co., one of the contractors of the project, reached 140 joints (Ø 56’’, WT 19.45 mm) per day, which corresponds to highest number of joints commissioned in a working day during a pipeline construction in the world.

3. Fracture toughness and resistance

Modern standards such as BS 7910 present extensive evaluation methods for a standardized ECA and limit fracture toughness parameters [18]. Stresses acting on the region containing the flaw, size, orientation to the weld and position of the flaw and tensile and toughness properties of the related region is required to carry out an ECA.

One of the most vital models that structural integrity assessment built around is weak link model. This model is an assumption that inhomogeneous components contain randomly distributed small regions of low toughness [19]. Once a crack front close enough to nearest weak, stress concentration shifts into the weak link guiding crack front to the link. Crack propagates rapidly and the component “fails” because of a chain reaction connecting weak links. Weak link is referred to as LBZ in weldments of HSLA steels. IRCGHAZ region neighboring the fusion line is a potential host for LBZ with lower toughness [20]. Consequently, a fracture toughness analysis targeting that specific region is required.

Figure 5 CTOD occurance after a force applied to specimen with a crack

Fracture toughness is the resistance of material under stress to crack extension [21]. Measurement of fracture toughness by standardized parameters plays an imperative role in engineering applications of fracture mechanics to structural integrity assessment. Stress intensity factor (K) is the first parameter widely accepted and it defines the stress intensity of elastic crack-tip fronts in Linear Elastic Fracture Mechanics (LEFM) where a linear crack growth without any plastic resistance is expected [22]. LEFM is inadequate in realistic assessments of linepipe steels where a higher resistance to fracture is obtained by the plastic region occurs on crack tip. Crack Tip Opening Displacement (CTOD) is an Elastic-Plastic Fracture Mechanics (EPFM) parameter proposed in 1963 to serve as an engineering parameter [23]. CTOD occurrence after a force applied to a specimen with crack is shown in Figure 5. The J-integral is introduced in 1968 and widely accepted as a measure of the crack driving force and measures the energy release associated with crack growth or tearing [24]. CTOD is commonly used in United Kingdom, where it has been developed. J was originally developed in the United States of America and more common to use in there [25].

4. Testing for single values and resistance curves

Standards such as ASTM E1820 [26], ISO 12135 [27], DNVGL-RP-F108 [28] and BS 8571 [29] offer specimen preparation guidance, testing procedures, validity evaluations, calculations for fracture toughness parameters and conversion formulas between the parameters. Testing procedures has an extended range of variety from different loading types to different conditioning methods, which grants different assessments to be carried out. Typical specimen configurations are presented in Figure 6. For pipeline reliability and structural integrity assessments, generally single-edge notched bending or tensile specimens are tested.

Figure 6. Typical fracture toughness and resistance curve testing specimens; (top) single edge notched bending/tensile, (bottom left) straight-notch compact and (bottom right) stepped-notch compact specimens [27].

Due to the variation of the welding heat input through the pipe circumference during girth welding, sampling is also important. Location and quantity of the specimens are also



specified in production standards such as API 1104 [15]. A sampling and sectioning plan for a main-line procedure qualification test pipe coupon is available in Figure 7. Following the NADCAP (National Aerospace and Defense Contractors Accreditation Program) testing and specimen designation system, A, AA, N, PC, PJ, XN and XL stand for transverse (cross-weld) tensile, all-weld metal tensile, Charpy (V-notch) impact toughness, fracture toughness with CTOD definition, fracture toughness with J-integral definition and bend testing and macro-section examination specimens, respectively.

5. Recent developments in testing

The demand on SENT testing to generate fracture toughness data is increasing since higher fracture toughness results are obtained compared to SENB testing. Crack tip constraint which is a function of the load type, material property and specimen geometry is lower in SENT test specimens than in SENB test specimens [30]. The constraint is modelled as Q parameter with elastic-plastic finite element analysis [31] and also indirectly presented in terms of micro hardness increment zone caused by strain hardening in the strain field at the proximity of crack tip [32]. The differences between SENT and SENB testing is the predominant load type and different applicable notch ranges. BS 8571 [29] for the SENT test require the a/W range to be between 0.2 and 0.5 and ISO 12135 [27] for SENB test requires the a/W range to be between 0.45 and 0.70. All fracture toughness parameter calculation formulas are applicable only in the defined ranges of the standards.

Figure 7 Pipe sectioning to 8 arcs and specimen designation with respect to the position

There is limited experience on SENT specimen test since the standardization of the test and the assessments of the results are relatively new. DNV recently released a recommended practice for flaw assessments [28] and BS 7910 [18] briefly mentions the use of SENT data with a constraint correction method in Annex N.

Figure 8 Fracture surface of an as-tested fracture toughness specimen and its features. A macro-view at top and higher resolution views obtained under scanning electron microscope (SEM)

Current standards are developing as advanced finite element models are validated by empirical data. In BS 8571 [29], J equations for clamped specimens will be replaced with the CANMET equations [33] and a/W range will be reduced between 0.3 and 0.5 in the next revision of BS 8571 [29]. There are still quite a lot of testing aspects to be investigated. Some of these parameters are: the test speed which is copied from ISO 12135 [27] as stress intensity rate between 0.2 and 3.0 MPa m1/2 s-1, effect of strain hardening and other environmental parameters on specimens, quantitative measurement of stretch zone width and depth to determine the initiation fracture toughness.



Figure 9. Measurement of stretch zone dimensions using a 3D imaging interface integrated to SEM [34].

Figure 10. Mean R-curves generated using SENB and SENT testing data.

Acknowledgement: Supports of Trans Anatolian Pipeline (TANAP) administration is sincerely appreciated. We especially would like to thank Mr. Alper Toprak, Mr. Varol Erdat, Mr. Semih Öncül, Mr. Bülent Yılmaz and Mr. Arash Shadmani.

6. References

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determination of quasistatic fracture toughness and ...brittle zones (lbz) in coarse-grain haz...

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