High Grade Pipeline Steel Research Articles

Determination on the fracture toughness of the welded joints of X80 pipeline steels based on small punch test

For the sake of design and integrity assessment of the fracture toughness of high-grade pipelines, it is necessary to carry out accurate fracture toughness assessment of the welded joints. The study of more concise and effective fracture toughness testing methods has important theoretical and engineering value. At present, the fracture control of girth welding joints is mainly based on J integral, CTOD and CTOA. However, it is hard to carry out standard fracture toughness tests at some local narrow areas of the welded joints such as the heat affected zone for the difficulty of sampling. On this basis, this paper aims to propose a method to determine the fracture initiation toughness JIC of the welded joints of high-grade pipeline steels based on the micro-sample test method - small punch test. Firstly, small punch specimen with through-thickness notch was designed. Secondly, the finite element model of the notched specimen was established, and verified by the notched specimen tests of X80 steel. Furthermore, the elastic–plastic material properties of different zones of the welded joint were obtained by the empirical correlation method of small punch test and further used for the determination of J contour integral. The GTN damage parameters were calibrated by the three-point bending tests and further used for the determination of the crack initiation displacement. Finally, the fracture initiation toughness JIC(SPT) of different zones of the welded joint were obtained by the SPT method and verified by the three-point bending test. The results show that the fracture initiation toughness JIC(SPT) of each region of the welded joint obtained by notched SPT specimen were all less than the SENB test results. But there is a good empirical correlation between the two results so that the SPT method can be further modified by establishing an empirical correlation model. The SPT method has good engineering application prospect for the accurately characterization of the different distribution of fracture toughness in different areas of the welded joints.

Microstructure Characterization and Strengthening Mechanism Analysis of X100 Pipeline Steel

The strengthening mechanism of X100 high steel grade pipeline steel, including grain boundary strengthening, solution strengthening, precipitation strengthening, dislocation strengthening, and texture strengthening, was analyzed by the technics of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD), physicochemical phase analysis, and so on. The results showed that the tested steel had a mixed structure of granular bainite and lath bainite, the average effective grain size was refined to about 1 μm by severe hot plastic deformation, the dislocation density was as high as 1.74 × 1015/m2, and the second phase precipitation was mainly (Ti, Nb)(C, N) in submicron scale and nanoscale NbC. Through orientation distribution function (ODF) orientation analysis, it was found that the tested steel had obvious anisotropy and had a strong rolling direction (RD) texture along the rolling direction compared with the 30° direction: {115}<110> and {113}<110>. After the quantitative analysis of strengthening mechanisms, it was found that the highest strengthening increment was caused by high-density dislocations in bainite of about 268 MPa, while the lowest strengthening increment was induced by precipitation particles of about 31 MPa.

The Flow Stress-Strain and Dynamic Recrystallization Kinetics Behavior of High-Grade Pipeline Steels.

The hot deformation behavior of high-grade pipeline steels was studied in the strain rate range of 0.001~0.1 s−1 and the temperature range of 1050~1200 °C by using hot compression tests on a Gleeble 3500 thermomechanical simulator. The flow stress increases with the increase in strain rate and the decrease in deformation temperature, and the deformation activation energy is about 358 kJ/mol. The flows stress–strain behavior of the work-hardening and dynamic recovery (DRV) was calculated using the Estrin–Mecking equation, and the kinetics model of the dynamic recrystallization (DRX) was established based on the Avrami equation through characteristic strains. Furthermore, the flow stress–strain behavior of high-grade pipeline steels was predicted by the established model based on the coupling effects of DRV and DRX. The corresponding predicted results are in good agreement with the experimental results according to standard statistical parameters analysis. Finally, the economic strain (ε3) is proposed by the third derivative of the given kinetic model. Based on these calculation results, when the economic strain (ε3) is reached, uniform and refined DRX grains can be obtained, the energy consumption reduced, and the production costs controlled, which is of great significance to actual factory production.

Determination of the true stress-strain relations of high-grade pipeline steels based on small punch test correlation method

The small punch test (SPT) has been widely used to evaluate the mechanical properties of small regions in pipeline steels. However, its application has thus far been limited to the determination of elastoplastic mechanical properties such as the ultimate tensile strength, and few works on estimating he true stress-strain curves. This paper aims to propose a new method for determining the true stress-strain relations of high-grade pipeline steels based on the SPT results, and apply it to the welded joints of X80 grade pipeline steel where the traditional mechanical characterizations cannot be carried out. Mechanical properties of several high-grade pipeline steels were obtained by empirical formulas that were determined by the correlation relationships between the conventional tensile tests and SPT results. Then the Ramberg-Osgood(R–O) power law which can describe the stress-strain relations of high-grade pipeline steels was simplified and expressed by four determined mechanical property parameters so that the true stress-strain curves could be generated using these parameters. Meanwhile, the SPT correlation method was used to estimate the true stress-strain curves of the heat-affected zones and weld zones of the welded joints of X80 steel and the accuracy was verified by numerical simulations. The results demonstrate that the true stress-strain curves determined by the SPT correlation method are in good agreement with the uniaxial tensile results for base materials of high-grade pipeline steels and the predicted results of different areas of the welded joints of X80 steel are reliable.

Relationship between microstructure and corrosion behavior of high-grade pipeline steel in a low-temperature environment

Relationship between microstructure and corrosion behavior of high-grade pipeline steel in a low-temperature environment

Unified correlation of constraints with crack arrest toughness for high-grade pipeline steel

There is much controversy surrounding using laboratory specimens to predict the fracture behavior of high-grade pipeline steel due to their different constraints. Since there is a lack of unified parameters to quantitatively characterize the constraint effects, a model for the transformation of crack arrest toughness between different specimens and pipes has not been formed. In order to solve this problem, the evolution law of the plastic zone of the crack tip with the crack tip opening degree under different in-plane and out-of-plane constraints is explored using the finite element simulation technology. On this basis, a constraint parameter η which is based on the area APEEQ surrounded by the isoline of equivalent plastic strain εp at the crack tip is proposed. The feasibility of this parameter to characterize both in-plane and out-of-plane constraints is discussed, and a quantitative correlation model between it and the crack arrest toughness, critical crack tip opening angle (CTOAC), is established. The results show that the proposed parameter can be used to characterize the overall constraints caused by different specimens with different loading forms and geometries. The normalized arrest toughness, CTOAC/CTOAref, has a linear relationship with η1/2, and the correlation line is only related to the material. The quantitative correlation model has universal applicability for different specimens and is expected to predict the transformation of crack arrest toughness between laboratory specimens and pipelines.

Experimental Research on Fatigue Properties of X80 Pipeline Steel for Synthetic Natural Gas Transmission

In recent years, many synthetic natural gas demonstration projects have been put into operation all over the world, and hydrogen is usually contained in synthetic natural gas. X80 is the most commonly used high-grade pipeline steel in the construction of natural gas pipelines. The compatibility between high-grade pipeline steel and hydrogen directly affects safety and reliability of long-distance pipelines. Therefore, in order to study the effect of hydrogen content on fatigue properties of high-grade pipeline steel, fatigue specimens were taken from base metal, spiral welds, and girth weld of submerged arc spiral welded pipes, respectively. Specifically, the total pressure was 12 MPa and hydrogen content was from 0 to 5vol%. Experimental results indicate that the hydrogen significantly increases the fatigue crack growth rate for both base metal, spiral weld, and heat-affected zone of X80 pipeline steel for about ten times compared with reference environment nitrogen, hydrogen would greatly reduce the fatigue life of the X80 pipeline steel, and the fatigue lifetime would decrease with the increase in hydrogen volume fraction. In order to ensure the safe operation of SNG pipeline, the hydrogen content should be controlled as low as possible.

Research on the influence of heat treatment temperature on bearing capacity of high steel grade pipeline during in-service welding

During the welding of in-service pipeline, natural gas is continuously transported in the pipeline, which maintains a high gas pressure. Therefore, the welding process is completed under strong cooling conditions, and welding delay crack is easy to occur. Preheating before welding and heat treatment after welding can effectively control the hardened microstructure, reduce the residual stress and ensure the welding quality. In this study, the influence of heating temperature on the bearing capacity of high steel grade pipe during in-service welding repair was studied. The high temperature tensile test was used to simulate the bearing capacity of the pipe under heating and high temperature environment. It is found that when the heating temperature is below 400°C, the pipe strength remains at the original level. With the increase of heating temperature, when the test temperature is higher than 400°C, the yield strength and tensile strength of the pipe decrease significantly. When the test temperature is 450°C, the yield strength and tensile strength of the material decrease by 15.8% and 11.1%, respectively, compared with the normal temperature, which are lower than the pipe standard requirements. Therefore, it is suggested that when the heat treatment temperature is higher than 400°C during in-service welding repair, it is necessary to consider reducing the pipeline pressure. At the same time, the box furnace heat treatment method was adopted to heat treat the pipe, and the tensile properties of the pipe after heat treatment were tested to analyze the bearing capacity of the pipe after heat treatment. It is found that when the heating temperature is higher than 700°C, the tensile properties of high grade pipeline steel pipe decrease sharply. It is suggested that when the heat treatment temperature is higher than 700°C, the risk assessment of the service safety of the heat treated pipeline should be carried out.

External Stress Corrosion Cracking Risk Factors of High Grade Pipeline Steel

The influence weights of various sensitive factors, such as steel grade, applied potential, temperature and soil environment, on stress corrosion cracking behavior of high-grade pipeline steel were studied by means of orthogonal test at mixed level and slow strain rate test. The stress corrosion behavior of X80 pipeline steel in near neutral and high pH simulated soil solution under overprotection potential was observed by SEM. The results showed that the applied potential has the greatest weight on stress corrosion index of ISSRT, followed by temperature, steel grade and soil environment. X80 pipeline steel exhibits high stress corrosion sensitivity in near-neutral and high pH simulated soil solutions at applied potential of -1500mV, and its SCC mechanism is hydrogen embrittlement (HE). This study can provide guidance for practical engineering application of pipeline steel.

Determination of Johnson–Cook parameters and evaluation of Charpy impact test performance for X80 pipeline steel

Controlling the Charpy impact toughness of materials is the key to the ductile fracture arrest design of pipelines. The finite element method based on Johnson–Cook (J-C) model has become an effective method to assist the experimental study of material impact toughness. In this paper, model parameters of J-C constitutive relation and damage parameters of J-C failure model for X80 pipeline steel have been determined experimentally from quasi-static uniaxial tension tests and Split Hopkinson Pressure Bar (SHPB) tests. Charpy impact test has been simulated using J-C constitutive and failure models with the determined parameters. Reasonable agreements between the simulation and experimental results have been achieved. On this basis, the effects of the pendulum velocity, specimen width and striker radius on Charpy impact test results are studied. The results show that pendulum velocity has negligible influence on Charpy impact test results; the maximum force and impact absorbed energy both have a highly linear relationship to specimen width; Charpy impact test is sensitive to striker geometry, the results measured by 2 mm-striker are obviously smaller than those of 8 mm-striker. Based on the above researches, the impact absorbed energy correlation model between sub-size specimens and standard full-size specimens, the test results transformation rules of the two strikers are established. The methods and results presented in this paper will provide references for dynamic behavior study, impact dynamic design, pipeline safety operation and risk assessment of high-grade pipeline steel.

Study on Microstructure Control of X80 Pipeline Steel and Precipitation of Nb Element

In this paper, X80 grade test steel was trial-produced in the laboratory, the mechanical properties, microstructure and precipitate morphology of the trial-produced test steel were analyzed and studied, and the precipitation law of Nb element in high-grade pipeline steel was studied by thermal simulation method. The results show that the X80 grade test steel has excellent mechanical properties, yield strength of 660 MPa, tensile strength of 720MPa and yield ratio of 0.91. The impact energy is greater than 295 under -20°C test conditions and the cross-section rate of the DWTT fiber reached 90% under -15°C test conditions, and the test results completely meet the API.5L standard requirements. By the thermal simulation experiment, it is found that the precipitation process of Nb particles in the test steel is the nucleation growth process of controlled by diffusion. The tip temperature of the PTT curve is about 900°C, and the Nb element in the test steel is found at 810°C by solid solubility calculation can be completely precipitated.

Influence of Effective Grain Size on Low Temperature Toughness of High-Strength Pipeline Steel.

In this study, the series temperature Charpy impact and drop-weight tear test (DWTT) were investigated, the misorientation angles among structural boundaries where the cleavage crack propagated were identified, and angles of {100} cleavage planes between adjacent grains along the cleavage crack propagated path were calculated in five directions (0°, 30°, 45°, 60°, and 90° to the rolling direction) of high-grade pipeline steel. Furthermore, the effective grain size (grain with misorientation angles greater than 15°) was redefined, and the quantitative influences of the redefined effective grain size on Charpy impact and DWTT is also discussed synthetically. The results showed that the microstructure presented a typical acicular ferrite characteristic with some polygonal ferrite and M-A islands (composed of martensite and retained austenite), and the distribution of the high-angle grain boundaries were mainly distributed in the range of 45°–65° in different directions. The Charpy impact energy and percent shear area of DWTT in the five directions increased with refinement of the redefined effective grain size, composed of grains with {100} cleavage planes less than 35° between grain boundaries. The ductile-to-brittle transition temperature also decreased with the refining of the redefined effective grain size. The redefined effective grain boundaries can strongly hinder fracture propagation through electron backscattered diffraction analysis of the cleavage crack path, and thus redefined effective grain can act as the effective microstructure unit for cleavage.

Experimental Study on the Fracture Toughness of Welded Joints of API X90 High-grade Pipeline Steels Using Single-Edge-Notched Tension Specimens

The fracture toughness and R-curves of welded joints of API X90 pipeline steels were experimentally studied. Single-edge- notched tension (SENT) specimens and a normalization method were adopted. Joints welded by using three technologies (A, B, and C) were investigated. The results show that the SENT specimen could be used to test the fracture toughness and R-curves of the welded joint; the welded joint of welding method B presented the poorest fracture toughness; and the R-curves and fracture toughness of the heat-affected zone were seriously affected by the weld zone and base metal. A new idea to test the fracture toughness and R-curves of the heat-affected zone was also proposed.

Fracture Toughness of Different Locations in API X80 Pipeline Steel on Low Constraint SENT Specimens

With the considerable use of high-grade pipeline steel in onshore and offshore project, welded joints are recognized as the weak link in pipeline because of the non-uniform microstructural regions induced by welding heat input. At first, the microstructural of different regions in API X80 pipeline welded joints was characterized and quantified by SEM, which indicate that the pipeline steel is a typical acicular ferrite steel. In this paper we investigated the J-integral resistance curve (J-R curve) in different locations of API X80 pipeline welded joints through low constraint SENT specimens with side grooves at room temperature. The effect of notch orientation (longitudinal-radial (L-R) and transverse-radial (T-R)) on resistance curve were investigated in base metal, which reveal the orientation almost have no effect on resistance curve. As the welded joints adopted in this study is two-pass steel arc welds, so the J-R curves of the inner surface, the outer surface and through-thickness surface notches specimens in the weld metal were investigated. The inner surface sample have the highest toughness through three samples because of the effect of second pass welding process. The effect of constraint on resistance curve was conduct between low constraint SENT specimen and high constraint SENB specimen, which found that the lower constraint corresponding to the higher resistance curve. After finishing the test, crack advancing plan of different positions were etched and observed by OM to demonstrate that the crack path always in the region which we would like to test.

Effect of effective grain size and grain boundary of large misorientation on upper shelf energy in pipeline steels

X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very important for exploring even higher grade pipeline steels. We established the relationship between toughness and crystallographic parameters of high grade pipeline steels by studying the crystallographic parameters of X65, X70, and X80 using EBSD and analyzing Charpy CVN of X65, X70 and X80. The results show that the effective grain size, the frequency distribution of grain boundary misorientation and the ratio of high angle grain boundary to small angle grain boundary are important parameters. The finer the effective grain size, and the higher the frequency distribution of grain boundaries (≥ 50°), the more excellent toughness of high grade pipeline steels will be.

Study of Thickness Effect on Fracture Toughness of High Grade Pipeline Steel

The critical fracture toughness decreases when thickness of specimens increases and stress-strain field in crack tip start changing to plane strain state from plane stress state. In this paper, fracture toughness tests were carried. Based on the analysis of stress-strain field in crack tip and fracture toughness test results, a fracture toughness-thickness empirical model was established and the plane strain fracture toughness and critical thickness of X80/X100 were calculated. Then the validity of the empirical model was discussed and verified. The analytical results indicate that the safety of thick wall pipelines is worth of attention.

A new specimen for high-grade pipeline steels CTOA test

In order to accurately measure CTOA of high-grade pipeline steels, a new modified compact tension (MCT) specimen was designed to alleviate the drawbacks of standard tests specimens and MDCB specimen. CTOA of X80 pipeline steel was measured using DIC technology with MCT specimen which behaves steady-state propagation in tearing process. FE method was adopted to verify the accuracy of results acquired from experiment tests. Results from DIC technology agree well with those from FE method. Based on the results of tests, FE simulation and comparison with MDCB specimen test previously conducted with the same material, it shows that MCT specimen is available for high grade pipeline steel CTOA test.

An energy based regression method to evaluate critical CTOA of pipeline steels by instrumented drop weight tear tests

An energy based regression method to estimate critical crack-tip-opening-angle (\(\hbox {CTOA}_\mathrm{C}\)) of high strength and toughness pipeline steels has been established derived from the Martinelli-Venzi ductile fracture model. Key curve method was applied onto the load-displacement curves of standard pressed-notch drop-weight-tear-test specimens to evaluate the dynamic crack extension, providing a way to verify the correlation between the load and remaining ligament width. In the meanwhile, the material based parameter (\(\hbox {A}^{*} \upsigma _\mathrm{f}\)), usually required by other \(\hbox {CTOA}_\mathrm{C}\) estimation algorithm, could also be determined experimentally. As a result, \(\hbox {CTOA}_\mathrm{C}\) of a typical high grade pipeline steel plate was acquired as a constant over steady-state stage during crack propagation, independent of specimen geometry.

电感耦合等离子体原子发射光谱法测定高钢级管线钢中多种元素

采用电感耦合等离子体原子发射光谱(ICP-AES)分析技术，同时测定了高钢级管线钢中的硅、锰、磷、铬、镍、钼、铌、钒、钛、铜、铝等元素。对试样溶解方法、元素分析谱线、仪器分析参数等因素进行了研究，选择了最佳的分析谱线及合适的仪器工作条件。方法应用于钢铁标准样品中各元素的测定，测定值与认定值吻合，精密度小于3%。方法应用于实际样品分析，测定了X90、X100管线钢中的元素含量，结果满意。In this paper, by using inductively coupled plasma atomic emission spectrometry(ICP-AES) analysis, a method for simultaneous determination of Si, Mn, P, Cr, Ni, Mo, Nb, V, Ti, Cu and Al in high grade pipeline steel have been studied. The experiment factors including the method for dissolving sample, spectra of analyzed element and instrument parameters have been investigated. This method was applied to determine the elements in steel standard sample and the results were con-sistent with the certified values. The relative standard deviations (RSDs, n = 10) for all elements were smaller than 3%. The proposed method was successfully applied to X90 and X100 high grade pipeline steel practical samples determination with satisfactory results.

Three-dimensional Finite Element Analysis for Flying Shearing of X100 Hot-rolled Steel Plate

Increasing demands for transporting oil and natural gas promote the developing and production of high-grade pipeline steel. The hot rolling is a vital process to produce the steel plate of pipe fabrication. In a hot rolling line, a flying shear is used to shear the leading and trailing edges of a rough rolled plate. In the present research, three-dimensional FE model of flying shear was built on ABAQUS/Explicit, considering the motions of blades. The feeding rollers and shear blades are considered as rigid material, and the steel plate adopts elastic-plastic material model. Ductile damage criterion for X100 steel was built to simulate the material damage and fracture during the shear processing. By the simulation, the plastic deformation, the damage initiation and evolution, the fracture of steel plate can be obtained, which are coincided to production practise. The stroke-force curves of blades were generated from the FE simulation. The shearing force varying with the blade stroke was analyzed. FE simulation shows that the calculated force from FE model is much lower than that from empirical equation.