Drop Weight Tear Test Research Articles

New experimental set‐up to approach pipeline fracture behaviour by three‐point bending specimens

ABSTRACTA new experimental set‐up for drop weight tear tests is proposed. Its principal characteristic is the adoption of inclined supports driving the hinges connected to the specimen. This choice accomplishes the need that the specimen was subjected to a persistent stress field in the ligament, thus approaching the stress field experienced by a crack propagating in a pipeline. This set‐up requires a proper processing of the data that is accurately explained in the paper. The tests are conducted under a quasi‐static loading but the method has been designed to apply in dynamic tests.

Microstructure and Mechanical Properties Analysis of X70 Pipeline Steel with Polygonal Ferrite Plus Granular Bainite Microstructure

Microstructure and mechanical properties of X70 pipeline steel with polygonal ferrite plus granular bainite were characterized using tensile tests, Charpy V-notch impact tests, drop weight tear tests, hardness tests and scanning electron microscopy. The results of experiment indicated that X70 pipeline steel with polygonal ferrite plus granular bainite showed an excellent combination of high strength and toughness. The base metal with polygonal ferrite plus granular bainite microstructure exhibited perfect mechanical properties in terms of the transverse yield ratio of 0.81, elongation of 46%, an impact energy of 335 J at -10 °C and a shear area of 90% at 0 °C in the drop weight tear test. The heat affected zone contained coarse grain zone and fine grain zone, which exhibited good low temperature toughness of 216 J at -10 °C. The weld metal primarily consisted of intragranularly nucleated acicular ferrites which led to the high strength and toughness.

Effects of finish rolling temperature on inverse fracture occurring during drop weight tear test of API X80 pipeline steels

In this study, drop-weight tear tests (DWTT) were conducted on API X80 pipeline steels fabricated with different finish rolling temperatures in order to analyze abnormal fracture appearance, i.e., inverse fracture, occurring in the region impacted by a hammer. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, DWTT absorbed energy, and strain hardening of the hammer-impacted region. As the finish rolling temperature decreased, the volume fraction of fine-grained acicular ferrite increased, while that of large-grained upper bainite or granular bainite decreased. According to the DWTT results, the absorbed energy tended to increase with increasing volume fraction of acicular ferrite (with decreasing finish rolling temperature). A large area of inverse fracture of cleavage type was found in the hammer-impacted region of the steels fabricated with high finish rolling temperatures, but the area fraction of inverse fracture was reduced in the steels fabricated with low finish rolling temperatures. Since the area fraction of inverse fracture was closely related with strain hardening of the hammer-impacted region, it could be successfully reduced by lowering strain hardening and by promoting the formation of acicular ferrite via low finish rolling temperatures.

Crack Tunnelling and Crack Tip Opening Angle in Drop-Weight Tear Test Specimens

Interrupted drop-weight tear tests (DWTT) of typical low-strength and high-strength pipe steels were performed at quasi-static and impact loading rates, to quantitatively characterize crack tunnelling and crack-tip opening angle (CTOA). Crack front tunnelling was observed in all interrupted DWTT specimens, but the extent of tunnelling varied with the steel grade and the loading rate. CTOA values measured optically at the surface were higher than those measured at a cross-section close to mid-thickness. This is attributed to the effect of crack tunnelling, i.e. the effect of constraint, on CTOA. CTOAs derived from load vs. load-line displacement, using the simplified single-specimen method, are in good agreement with values measured at the mid-thickness.

Fractography of burst-tested linepipe

The fracture mechanism of fast running ductile crack was investigated from the viewpoint of microscopic fracture appearance. Fracture surfaces were sampled from burst linepipes after full-scale burst tests, and examined by scanning electron microscope (SEM). In spite of sufficient thickness reduction due to local necking, the macroscopic appearance of the burst-tested specimens was a slanted fracture in the wall thickness direction. The microscopic observations revealed an equiaxed dimple pattern on the fracture path in the thickness direction tilted to the maximum shear stress direction. The Charpy impact test, drop weight tear test (DWTT), plate tensile test and round bar tensile test were also performed with the same materials as in the full-scale burst tests. The size and frequency of the primary dimples on the fracture surface of the laboratory specimens were compared with those of the full-scale burst linepipe. The stress state at the running crack tip in the full-scale burst test was also investigated based on the dimple morphology.

Effect of fracture tunneling in DWT Tests

The work deals with the effective shape of the fracture during a Drop Weight Tear Test (DWT). An inexpensive technique, able to determine the geometrical shape of the fracture, is discussed and presented. The technique is capable to perform reliable measurements and offers a mathematical description of the fracture profile. In few words, the fracture tunneling measurement and its correlated effects are the objectives of this work. The technique is based on a copy of the fracture by a silicone mold, followed by a digital analysis of the tomographic images. The attention is finally focused on two different steels used in inshore gas piping. The tunneling measurement is exploited in a finite element simulation of the fracture propagation, thus opening the opportunity to update the kinematic models applied for Crack Tip Opening Angle assessment.

Microstructure and properties of pipeline steel with a ferrite/martensite dual-phase microstructure

Abstract In order to satisfy the transportation of the crude oil and gas in severe environmental conditions, a ferrite/martensite dual-phase pipeline steel has been developed. After a forming process and double submerged arc welding, the microstructure of the base metal, heat affected zone and weld metal was characterized using scanning electron microscopy and transmission electron microscopy. The pipe showed good deformability and an excellent combination of high strength and toughness, which is suitable for a pipeline subjected to the progressive and abrupt ground movement. The base metal having a ferrite/martensite dual-phase microstructure exhibited excellent mechanical properties in terms of uniform elongation of 7.5%, yield ratio of 0.78, strain hardening exponent of 0.145, an impact energy of 286 J at − 10 °C and a shear area of 98% at 0 °C in the drop weight tear test. The tensile strength and impact energy of the weld metal didn't significantly reduce, because of the intragranularly nucleated acicular ferrites microstructure, leading to high strength and toughness in weld metal. The heat affected zone contained complete quenching zone and incomplete quenching zone, which exhibited excellent low temperature toughness of 239 J at − 10 °C.

Effects of Specimen Thickness and Notch Shape on Fracture Modes in the Drop Weight Tear Test of API X70 and X80 Linepipe Steels

This study is concerned with effects of specimen thickness and notch shape on drop weight tear test (DWTT) properties and fracture modes of API X70 and API X80 low-carbon microalloyed linepipe steels. Detailed fractographic analysis of broken DWTT specimens showed that the fracture initiated in an initial cleavage mode near the specimen notch and that some delaminations occurred at the center of the fracture surface. The chevron notch (CN) DWTT specimens had broader initial cleavage areas than the pressed notch (PN) DWTT specimens. The larger inverse fracture areas (i.e., cleavage areas close the hammer impact side) appeared in the PN DWTT specimens, because their higher fracture initiation energy at the notch allowed a higher strain hardening in the hammer-impacted region. The number and length of delaminations were larger in the CN DWTT specimens than in the PN DWTT specimens, and increased with increasing specimen thickness due to the plane strain condition effect. As the test temperature decreased, the tendency of delaminations increased, but delaminations were not found when the cleavage fracture prevailed at very low temperatures. The DWTT test results such as upper shelf energy (USE) and energy transition temperature (ETT) were discussed with relation to microstructures and fracture modes including initial cleavage fracture, ductile fracture, inverse fracture, and delaminations.

New Development of High Grade X80 to X120 Pipeline Steels

An overview of the manufacturing technology for high-strength pipeline steels was introduced. The microstructure and mechanical properties of X80, X100, and X120 grade steel plates and pipes were analyzed and discussed. Through the UOE simulator, the changes of mechanical properties after pipe-forming from plate to pipe were evaluated. The microstructure of X80 steel was composed of acicular ferrite containing M/A (Martensite/Austenite Constituent) phase. Tested X80 steel plate and pipe were found to have excellent drop weight tear test (DWTT) property. DWTT 85% SATT of X80 steel was about −40°C in pipe. The deformation capacity of X80 pipeline was evaluated by large-scale deforming machine operating under the loading of bending and axial compression force. It was found that the developed X80 pipeline was within the specification of DNV and API codes in terms of buckling capacity. In case of X100 steel, bainitic ferrite exhibiting lath and granular type morphology were major phase, and M/A existed as second phase. It was shown that the developed X100 steel can be realized to have appropriate properties for UOE pipe. DWTT 85% SATT of X00 steel pipe was shown below −40°C. Also, the development of X120 grade pipeline steel was tried. The microstructure of X120 steel was composed of bainitic ferrite and acicular ferrite. The tensile properties of developed X120 steel plate and pipe fully satisfied the property target required in the current research. DWTT 75% SATT of the developed X120 steel plate and pipe was below −30°C.

Microstructure homogenization control of Nb-bearing X65 pipeline steel by the CSP process

In order to get the controlled methods of microstructure homogenization and high strengthening-toughening combination by compact strip production (CSP) rolling, the dynamic recrystallization characteristics of each pass were obtained during CSP rolling using a Gleeble-1500 thermal mechanical simulator. Then, the CSP process was simulated by laboratory rolling experiment. Through thermal mechanical simulation experiment and laboratory rolling experiment, a design idea of Nb-bearing pipeline steel by CSP can be obtained as the following: the level of dynamic recrystallization behavior should be increased through the reasonable balance of high deformation temperature and deformation amount in F1 and F2 passes, and F3-F7 passes should be controlled in the austenite non-recrystallization zone. Finally, X65 pipeline steel with microstructure uniformity and good properties was produced by CSP. The yield strength is up to 497 MPa, the tensile strength is up to 563 MPa, the elongation is 30% on average, and the toughness is very good whose Charpy impact value is 110 J on average and drop-weight tear test shearing areas are all 100% at −60, −40, −20, 0, and 20°C.

Effect of the Volume Fraction and Grain Size of Polygonal Ferrite on the Fracture Properties of Drop Weight Tear Test of Pipeline Steel

In the study on the brittle fracture arrest of natural gas pipeline,the shear-area percentage(SA) of fracture is regarded as an important measurement parameter in drop weight tear test(DWTT) which is used to evaluate the ability of brittle fracture arrest of pipeline steel,can accurately reflect the actual stress-strain state of pipeline,and show the true state of fracture and the ability to prevent fracture.X70 pipeline steels are hot rolled under different conditions of industrial production,and then the drop weight tear test,fatigue test,tensile test and impact test are carried out.Effect of the volume fraction and grain size of polygonal ferrite on the SA of DWTT fracture of X70 pipeline steel is investigated.Results indicate that the SA of fracture increases and then decreases with the increase of polygonal ferrite volume fraction when the polygonal ferrite exists as a second phase;and it is mainly affected by polygonal ferrite grain size when the polygonal ferrite exists as main matrix microstructure,and the shear-area percentage increases with the finer of the grain.

Microstructure and Mechanical Properties of X80 Pipeline Steels in Different Cooling Schedules

The microstructures and mechanical properties of three X80 pipeline steel plates were investigated in this study. The plates were hot rolled by two stages, and cooled by three different cooling schedules to obtain different microstructures and mechanical properties. Transmission electron microscope (TEM) was used to observe the microstructure. The results indicated that the amount of polygonal ferrite (PF) and quasi-polygonal ferrite (QPF) decreased with decreasing of the finish cooling temperature (FCT), while the amount and size of M-A islands were larger. Relaxation promoted the Charpy V-notch (CVN) impact toughness at −20 ◦C, but not the drop weight tear testing (DWTT) property at −15 ◦C, and decreased the strength of the steel.

Analysis of Carbide Precipitates in API X80 Medium-Thickness Plate

In order to discuss the abnormal fracture in drop weight tear test (DWTT) of API X80 medium-thickness plate, some unqualified samples in DWTT are characterized by SEM, AES, electrolysis and thermal simulation. SEM results show that there are many spherical precipitates in the fracture surface. EDS and AES results show that the abnormal spherical precipitates are carbides including Fe, Ni, Mo, Ti, etc. These precipitates can be extracted by neutral electrolyte method and their EDS results further demonstrate they are carbides. The formationmechanismof the precipitates is discussed based on thermal simulation experiments. When the cooling rate of medium-thickness plate is not enough high, especially in carbide formation area in Fe-C diagram, abnormal spherical carbides will occur and they are harmful for DWTT.

Mechanical Properties and Microstructure of X120 Grade High Strength Pipeline Steel

Abstract A new ultrahigh strength pipeline steel with high yield strength and high impact toughness was fabricated in this work, and mechanical properties and microstructure of the steel were investigated. The steel exhibited outstanding mechanical properties with yield strength levels of up to 951 MPa and tensile strength levels up to 1023 MPa. The sharp notch toughness with absorbed energy values of 227 J/cm 2 at —30 °C and shear area of up to 95% in drop weight tear test (DWTT) at temperature of —20 °C were achieved. It was found that microstructure of the steel comprises a majority of low-carbon lath bainite with different sublaths and sub-sublaths, meanwhile there is a high density of dislocation between laths and the dispersed film-like martensite-austenite (M-A) constituents. Most of the precipitates in this steel are duplex type containing Nb and Ti with characterized morphology of cap with the range of precipitation size from a few to tens nanometers.

Effects of Specimen Thickness and Notch Shape on Fracture Mode Appearing in Drop Weight Tear Test (DWTT) Specimens of API X70 and X80 Linepipe Steels

Effects of Specimen Thickness and Notch Shape on Fracture Mode Appearing in Drop Weight Tear Test (DWTT) Specimens of API X70 and X80 Linepipe Steels

Influences of microstructure and texture on crack propagation path of X70 acicular ferrite pipeline steel

The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD). The microstructure presents a typical acicular ferrite characteristic with fine particles of martensite/austenite (M/A) constituent, which distributes in grains and at grain boundaries. The bulk textures of the pipeline steel plate are {112}〈110〉 and 〈111〉 fibers, respectively, and the {112}〈110〉 component is the favorable texture benefiting for drop weight tear test. Moreover, low angle boundaries and low coincidence site lattice boundaries are inactive and more resistant to fracture than high energy random boundaries.

Mastering the production of K70 skelp for large-diameter pipe on a 5000 mill

A technology for making a new generation of skelp in thicknesses from 15 to 27 mm for large-diameter pipes (LDP) has been developed on the 5000 mill at the Severstal Cherepovets Metallurgical Combine with the use of modern methods and metallographic techniques. The quality of the rolled product meets the requirements of the corresponding TU standards for steel of strength class K70 with respect to its strength and toughness-ductility characteristics, including the amount of tough component in the fracture of specimens in drop-weight tear tests (DWTT). Controlled rolling with accelerated cooling in an ACU (with possible subsequent tempering of the rolled product) was chosen as the most suitable technology.

Effect of microstructure on drop weight tear properties and inverse fracture occurring in hammer impacted region of high toughness X70 pipeline steels

In the present study, the effect of microstructure on drop weight tear test (DWTT) properties and inverse fracture occurring in the hammer impacted region of high toughness API X70 pipeline steels was investigated. Six kinds of steel specimens were fabricated under varying hot rolling conditions. The DWTT was conducted on the hot rolled steel specimens and the results were discussed in comparison with the Charpy V-notch (CVN) impact test data. The DWTT results indicated that all the specimens showed excellent DWTT properties as the per cent shear area well exceeded 85% at –20°C or lower, irrespective of microstructures. The specimens rolled in the single phase region had the lower 85% shear appearance transition temperature (SATT) than the specimens rolled in the two phase region, which was similar to the CVN test results. This 85%SATT had a good correlation with the energy transition temperature (ETT) obtained from CVN test. Inverse fracture started to occur at –20°C and its size increased with decreasing test temperature. The strain hardened hammer impacted region after DWTT became thicker and harder with decreasing temperature, thereby deteriorating the toughness of the specimens.

Dynamic fracture study by an experiment/simulation method for rich gas transmission X80 steel pipelines

The drop weight tear tests (DWTTs) are conducted to obtain the material toughness (CTOA) C (crack tip opening angle) for X80 pipeline steel, and the empirical formulas for X70 steel are modified because of the inaccuracy in the toughness evaluation of X80 steel. Based on the experimental data of the full-scale burst test, the dynamic crack propagation of the rich gas transmission pipeline is simulated using the finite element method. A multi-parameter state equation is applied in the simulation and the three-stage linear decompression model of rich gas is put forward. The different factors affecting the dynamic crack propagation are also discussed. The reliability for rich gas pipeline engineering is evaluated though the experiment and simulation in this paper.

Crack tip opening angle optical measurement methods in five pipeline steels

Crack tip opening angle (CTOA) is becoming one of the more widely accepted properties for characterizing fully plastic fracture. In fact, it has been recognized as a measure of the resistance of a material to fracture in cases where there is a large degree of stable-tearing crack extension during the fracture process. Our current pipeline research uses the CTOA concept as an alternative or an addition to the fracture energy characterizations provided by the Charpy V-notch (CVN) and drop weight tear test (DWTT). A test technique for direct measurement of CTOA was developed by use of a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate CTOA. In this report, different optical measurement methods are compared, three using the crack edges adjacent to the crack tip (defined in the ISO draft standard and ASTM standard) and one using the specimen surface grid lines. Differences in CTOA resulting from the various measurement methods are evaluated. The CTOAs for five different grades of gas pipeline steel are reported, and the effect of microstructure on CTOA is discussed.