Dong-Yeob Park

Evaluation of Fracture Toughness of X100 Pipe Steel Using SE(B) and Clamped SE(T) Single Specimens

J-resistance testing using a single-specimen unloading compliance technique has been performed on single-edge-notched tension (SE(T)) specimens of X100 pipe steel base material at room temperature and at −20°C, using a procedure developed at CANMET. J-resistance testing using single-edge-notched bend (SE(B)) specimens according to ASTM E1820 was also conducted for comparison. The specimens included two nominal through-thickness pre-crack aspect ratios (a/W = 0.25 and 0.5). The results show that shallow-cracked (a/W∼0.25) bend and tension specimens produce higher resistance curves than deeply-cracked (a/W∼0.5) specimens; ductile propagation was observed at both temperatures. Resistance curves are slightly higher at −20°C than at room temperature for both bending and tension, especially for shallow-cracked specimens. Crack length predicted from unloading compliance of crack mouth opening displacement for the SE(T) specimens was validated by optical measurement of initial crack length (ao ) and final crack extension (Δa>1.0 mm) after heat-tinting, as per ASTM E1820. Predicted crack growths show acceptable agreement with measured values in all cases. The effect of side-groove depth on the resistance curve and straightness of the crack front was briefly investigated. For both bending and tension, resistance curves for 10% (total) side-grooved specimens were close to those from plain-sided specimens when other testing conditions, such as precrack and testing temperature, were the same, whereas 20% (total) side-grooved specimens showed lower toughness. It was occasionally observed that the crack grew faster at the side for 20% side-grooved bend and tension specimens, resulting in a crack front of concave curvature. For 10% side-grooved specimens a rather straight crack front or slightly faster crack growth in the middle of the specimen (convex curvature) was observed.Copyright

Effect of Side Grooves on Compliance, J-Integral and Constraint of a Clamped SE(T) Specimen

The effect of side grooves on crack mouth opening displacement (CMOD) compliance, distribution of J-integral and crack-tip constraint parameters Q and A2 along the thickness of a clamped single-edge-notched tension (SE(T)) specimen were studied by finite element analysis (FEA). Focus was on the effect of depth of side grooves on J-integral and constraint parameters Q and A2 for shallow and deep cracks. The 3-D results were compared with those of SE(T) specimens in plane strain. The results show that the effective thickness equation used in ASTM E 1820 to evaluate compliance of side-grooved SE(B) and C(T) specimens can be used for clamped SE(T) specimens with reasonable accuracy. The results also suggest that the depth of the side grooves affects the distribution of the J-integral: the highest J-integral is at the center of the thickness for a SE(T) specimen with side grooves equal to or less than 10% of total thickness, and near the root of the side grooves for side grooves greater than 10% for a deeply-cracked specimen when the applied load P≥PY . The FEA results also show that the depth of side grooves affects the distribution of the constraint parameters: the crack-tip constraint is highest at the center of the thickness for a specimen with 0% side grooves (plain-sided), and near the root of the side grooves for side grooves equal to or greater than 10%. It was also found from FEA that the crack-tip constraint of a SE(T) specimen with 20% side grooves with shallow (a/W = 0.2) or deep (a/W = 0.5) crack is higher than that of a SE(T) specimen with the same crack depth in plane strain. As a result, the J-resistance of a SE(T) specimen with 20% side grooves may be lower than that of the same specimen in plane strain.Copyright

Fracture Toughness of X70 Pipe Girth Welds Using Clamped SE(T) and SE(B) Single-Specimens

Shallow-notched single edge-notched tension (SE(T) or SENT) and deep- and shallow-notched single edge-notched bend (SE(B) or SENB) specimens with notches positioned in the weld and the heat-affected zone were tested. Crack-tip opening displacement (CTOD) versus resistance curves were obtained using both a single and double clip gauge consolidated in a SE(T) single-specimen. Up until the peak load the resistance curves from both gauging methods yield approximately the same results; thereafter the curves deviate. Interrupted testing showed that the crack had initiated below 50% of the peak load, and in some cases had propagated significantly prior to reaching the peak load.Copyright

Low-Constraint Toughness Testing of Two SE(T) Methods in a Single Specimen

Single-edge notched tension (SE(T) or SENT) specimens has been increasingly proposed as a low-constraint toughness test to measure toughness of line pipe materials, as the crack tip constraint approximates a circumferential surface flaw in a pipe under loading. The clamped SE(T) single-specimen procedures recently developed by Shen and Tyson [1, 2] and Tang et al. [3] have in common the use of a clamped single-specimen of similar geometry and rely on unloading compliance technique for crack size estimation. In the former case, a single clip gauge is attached to the integral knife edge and the crack-tip opening displacement (CTOD) is estimated by means of a J-integral-to-CTOD conversion, similar to the procedure of ASTM E1820. The latter uses a pair of clip gauges mounted to an attachable raised set of knife edges to estimate CTOD at the original crack tip position by a triangulation rule. Consolidating these two sets of clip gauges in a specimen makes direct comparisons of two SE(T) methods on identical test conditions: material, specimen geometry, equipment, test temperature and operator [4]. In this study, SE(T) testing employing these two SE(T) methods on a single specimen was conducted on BxB shallow-cracked (a/W∼0.35) specimens of two x70 pipeline girth welds. This paper discusses the details of two SE(T) methods and techniques on the same specimen.© 2014 ASME

Fracture Toughness Characterization of High-pressure Pipe Girth Welds Using Single-edge Notched Tension [SE(T)] Specimens

The safety and reliability of large-diameter pipelines for the transport of fluid hydrocarbons is being improved by the development of high-strength steels, advanced weld technologies, and strain-based design (SBD) methodologies. In SBD, a limit is imposed on the applied strains rather than the applied stresses. For high-pressure pipelines, SBD requires an assured strength overmatch for the weld metal as compared to the base material, in order to avoid strain localization in the weldment during service. Achieving the required level of strength overmatch, as well as acceptable ductility and low-temperature fracture toughness, is a challenge as the pipe strength increases. Published studies show that low constraint geometries such as single-edge tension [SE(T)] or shallow-notched single-edge bend [SE(B)] specimens represent a better match to the constraint conditions of surface-breaking circumferential cracks in large-diameter pipelines during service (Shen, G., Bouchard, R., Gianetto, J. A., and Tyson, W. R., “Fracture Toughness Evaluation of High Strength Steel Pipe,” Proceedings of PVP2008, ASME Pressure Vessel and Piping Division Conference, Chicago, IL, July 27–31, ASME, New York, 2008). However, the SE(T) geometry is not included in any of the most widely used elastic-plastic fracture mechanics (EPFM) test standards. A procedure has been developed for performing and analyzing SE(T) toughness tests using a single-specimen technique that includes formulas for calculating the J-integral and crack-tip opening displacement, as well as for estimating crack size using rotation-corrected elastic unloading compliance. Here, crack-resistance curves and critical toughness values obtained from shallow-crack SE(T) specimens (a0/W ≈ 0.25) are compared to shallow-crack (a0/W ≈ 0.25) SE(B) specimens. We believe that the SE(T) methodology is mature enough to be considered for inclusion in future revisions of EPFM standards such as ASTM E1820 and ISO 12135, although additional work is needed to establish validity limits for SE(T) specimens.

Microstructural analysis of fracture in heat-affected zone of two ×70 pipeline steel weldments

ABSTRACT In the previous study, different crack propagation behaviours (ductile fracture and brittle cleavage fracture) were observed in two ×70 pipeline steel weldments (13.4 and 17.8-mm-thick) during single-edge notched bend testing. To further understand these two fracture behaviours, detailed microstructures of the base metal (BM), fine-grained heat-affected zone (FGHAZ), and coarse-grained heat-affected zone (CGHAZ) of these two ×70 pipeline steel weldments have been analysed. The results show that the initial structure of the two pipe BMs and different welding cooling rates owing to different thicknesses contributed to structural variations of the correlated sub-regions of the HAZ. For both weldments, the FGHAZ close to the BM has the highest fraction of the high-angle grain boundaries, the finest grain size, the lowest local strain levels, and the highest fraction of recrystallised ferrite grains. The CGHAZ of the 17.8-mm-thick pipe welds exhibits the lowest toughness with the highest hardness, a high frequency of deformed grains, the highest local strain level, and the highest density of preferred {100} cleavage planes than the other sub-regions in the HAZ. The high density of the {100}<011> texture components in the HAZ may cause the cleavage micro-cracks to propagate toward the BM at an approximate 45° angle to the original crack plane during bending tests.

TEM Characterization of HSLA Steels and Welds

Characterization of steel microstructure has always been considered challenging when it comes to transmission electron microscopy (TEM). There are a number of reasons behind that, such as magnetism, difficulty of TEM sample preparation, inherent complexity of the structure and the usual high density of dislocations. The microalloyed high strength low alloy (HSLA) steels are an important class of high strength steels extensively used in the pipeline industry. What gives the microalloyed steels their extra strength is the addition of elements such as Ti, Nb and V which can refine the microstructure by forming carbo-nitride precipitates of nanometric dimensions. Study of nano-precipitates within the steel microstructure is of great interest, and yet it is often difficult to directly characterize features of nano-scale dimensions within steel microstructure. The main techniques used for studying these precipitates are extraction replica and atom probe which have their own limitations.

Mechanical Properties and Microstructure of Weld Metal and HAZ Regions in X100 Single and Dual Torch Girth Welds

The main objectives of the current study were to further develop tensile and toughness testing protocols and to provide a better understanding of the factors that control both weld metal and HAZ microstructure and properties in pipeline girth welds. In this investigation, two series of rolled (1G) girth welds were made in X100 pipe of 36 in. diameter and 0.750 in. wall thickness using two pulsed-gas metal arc welding process variants: single and dual torch. The small-scale testing program included evaluations of all-weld-metal tensile strength, Charpy impact and standard fracture toughness measured by single-edge bend SE(B) tests, along with preliminary fracture toughness results using a single-edge tension SE(T) test developed at CANMET. Additional information was obtained from detailed microstructural characterizations of weld metal and HAZ regions along with microhardness testing. All-weld-metal tensile tests using round and strip tensile specimens showed variations with through-thickness location and in some case with clock position. Full stress-strain curves were generated, and 0.2% offset yield strength, flow stress, ultimate tensile strength, and uniform strain were measured and compared with pipe properties using calculated weld strength mismatch factors based on these properties. Charpy V-notch transition curves were generated for both weld metal and HAZ (notched within 0.5 mm of the fusion line). Fracture toughness of both weld metal and HAZ regions of single torch welds was assessed using standard SE(B) testing procedures with Bx2B preferred specimens notched through–thickness at the weld centerline and in the HAZ (within 0.5 mm of the fusion line). Full J-resistance curves were measured using SE(T) tests of surface-notched WM and HAZ specimens; the SE(T) test was designed to match the constraint of full-size pipeline girth welds.Copyright

Development of nanocomposite coatings with improved mechanical, thermal, and corrosion protection properties:

In this study, new composite coatings are fabricated and investigated for their applications as the metal coating. The studied coatings consist of two-layered composites with various nanoparticulates as fillers in a polymeric matrix (styrene acrylic). The first layer bonded to the steel plate uses a combination of zinc particles, multi-walled carbon nanotubes, and graphene nanoplatelets. For the second layer, hexagonal boron nitride with high electrical insulation properties is added to the matrix. The morphology of the nanoparticulates is conducted using a scanning electron microscope. The coefficient of thermal expansion, cathodic disbondment resistance, gas penetration, and scratch resistance of the coatings are evaluated. The corroded area on the cathodic disbondment test specimens reduced down up to 90% for the composite with zinc (20 wt%), multi-walled carbon nanotubes (2 wt%), and graphene nanoplatelets (2 wt%), compared to a specimen coated with a pure polymer. It is seen that the presence of nanoparticulates decreased gas permeation and thermal expansion of the matrix by 75% and 65%, respectively. The addition of nanoparticulates also enhanced scratch resistance of the coating composites.

Forming mechanism of delamination cracks observed during tensile and fracture toughness testing of X70 pipeline steel

In this study, we have investigated microstructures of the delamination cracks observed from tensile and fracture toughness test specimens using an API X70 pipeline steel. It is found that the delaminations observed from both tensile and toughness specimens are intergranular fractures. At the occurrence, characteristics of brittle fracture were observed, but it was found not to be a brittle fracture as the delaminations were induced by plastic deformation. It is shown that severe plastic deformation produced strain concentration around particles located along grain boundaries and caused decohesion between adjacent grains, resulting in intergranular fracture.