Lingkang Ji

Influence of Dual-Phase Microstructures on the Properties of High Strength Grade Line Pipes

Abstract The influence of dual-phase microstructures on mechanical properties of X70, X80, and X90 line pipes is investigated. It is found that the line pipes with dual-phase microstructures possess both larger uniform elongation and higher hardening exponent, especially for high grade steel X90. The tensile deformation of dual-phase line pipe does not follow the trend predicted by the Hollomon formula, and a stable strain-hardening exponent is not found. This stress-strain behavior is different from the normal line pipe. In the initial stage of plastic deformation, the strain-hardening capacity of dual-phase line pipe increases rapidly. However, it reaches a stable stage after 2.0% total strain. The dual-phase pipeline steel is composed of soft phase (polygonal ferrite) and hard phase (bainite), and thus the relatively soft ferrite is good for its deformability. Besides, the fraction of large angle grain boundaries in the dual-phase microstructures is greater than that of the normal line pipe, which is proven to be critical for improving the resistance to plastic deformation and crack propagation.

The Micro Structural Characteristic Parameters of High Grade Pipeline Steel and its Mechanical Properties

The microstructure of high grade pipeline steels, including X65, X70, X80, X100, were studied by SEM and EBSD, respectively. It was found that the microstructures of high grade pipeline steels were composed of lower bainite, granular bainite and acicular ferrite. The phases of kinds of pipeline steels were composed of Fe3 C, retained austenite and ferrite. And their percentage content, grain size and its distribution were studied respectively also. These micro structural parameters were correlated to the mechanical properties of kinds of pipeline steels. Furthermore, all kinds of angles of grain boundaries were studied, and the relationship between the angles of grain boundaries and mechanical properties was obtained. It was shown that as the improving of the steel grade, the grain boundary including small angle and large angle increased. And only when grain boundary was greater than 15 degree, it was effective to the toughness behavior.© 2006 ASME

Comparison of BTC, RBTC and HLP Models in the Calculation of the Dynamic Ductile Fracture Propagation Velocities Based on the X80 Full-scale Burst Test

Many efforts have been paid to solve the running ductile fracture (RDF) arrest issue in the gas pipeline since RDF could cause a serious disaster. Battelle two-curve model (BTCM), reformulated BTCM (RBTCM) and HLP Model (HLPM) seem to be the promising way to solve the RDF arrest issue. In order to clarify the validity of BTCM, RBTCM and HLPM in the calculation of RDF velocity for Chinese modern X80 pipes, a full scale burst test was carried out. And, the RDF velocities were calculated based on BTCM, RBTCM, and HLPM, respectively. By comparing the calculation results with the experimental ones, it was found that the RBTCM shows the best fitting with the experimental results. The possible reasons were discussed in this Paper.

Deformation Behavior Prediction of X80 Steel Line Pipe and Implication on High Strain Pipe Specification

The use of strain based design in pipeline technology has been widely discussed during the last decade for pipelines in harsh environment. In such cases pipelines should be designed based on strain criterion. Strain based design poses a number of challenges, particularly on pipe size and material properties. This paper presents preliminary studies on prediction of buckling strain and buckling mode for X80 high-strain line pipe by finite element methods based on full-scale test. The effects of several parameters such as internal pressure, material properties pipe size and geometric imperfection, were investigated to predict the critical strain for 48″ diameter line pipe under compression and pure bending with 12MPa internal pressure. Material parameters of a specification for high strain line pipe were analyzed to promote its application in the 2nd West-East pipeline of China National Petroleum Corporation.Copyright

Microstructure and Deformation Mechanism of High Strain Line Pipe

Strain-hardening mechanism of high strain line pipe was investigated and discussed in terms of microstructure and grain orientation. It is concluded that X70 high strain line pipe does not fit to the Hollimon formula in the entire deforming process, but the normal X70 line pipe does. At the beginning of plastic deformation, the strain-hardening capacity of X70 high strain line pipe is excellent. It decreases rapidly as the stain increases, then becomes stable after 2.0% total strain. The high strain line pipe is composed of soft phase (polygonal ferrite) and hard phase (bainite). When plastic deformation initiates, the strain-hardening capacity of high strain line pipe is better because of the ferrite deformation firstly ahead of the bainite, due to strain-hardening capacity of the ferrite is much better than that of the bainite. With ferrite deformation decreasing and bainite deformation increasing, the strain-hardening capacity declines until the micro-deformation is homogeneous. Besides, it is obvious that the large angle grain boundaries of more than 15° are in the majority of the microstructure of high strain line pipe without deformation, so the strain-hardening capacity is highest at the beginning of plastic deformation. As strain

Investigation on Axial Deformation of High Strain Line Pipe

A systematic investigation of the effect of material inhomogeneity on pipe buckling is presented in this paper. The material properties of the pipe were obtained by a series of tensile tests. Numerical simulations were carried out to evaluate the effect of material inhomogeneity on pipe buckling. Post-buckling behavior was also simulated. This paper also explains the mechanism of cracking.The authors show how pipe buckling can be caused by weakened sections, and how both internal pressure and the ratio of diameter to thickness can significantly affect pipe buckling.

Evaluation Method of Large Inclusions in High-Grade Pipeline Steel

Good internal quality of steel pipes is a basic guarantee for the safe and efficient operation of pipeline system. While presence of large inclusions has adverse effect on the mechanical properties, weldability and corrosion resistance of the steel pipes, and then it would bring huge risk to the safe operation of oil and gas transmission pipeline. In order to reduce the risk to a minimum extent, it is quite necessary to analysis and study the characteristics of large inclusions in high grade pipeline steel, and then it would provide evidence for the formulation of level-determination standard of large inclusions during steel pipe acceptance. Optimized production technology such as electromagnetic stirring is put forward accordingly to prevent the occurrence of large inclusions in pipeline steel.Copyright

Effect of Cold Bending Process and Strain-Age on Properties of X80 Linepipe

Through the field cold bending and mechanical properties tests of Φ1219 mm × 22 mm X80 longitudinal submerged-arc welding line pipe, the effects of different bend radii consisting of 20D, 28D and 47D (D is the diameter of steel pipe) on properties of X80 linepipe were studied. The tests results indicate that the change of the transverse tensile property, roughness, hardness and bend performance of cold bends manufactured by various radius of curvature compare with the mother pipe is relatively small. When the radius of curvature is decreased, the transverse yield strength and yield ratio in intrados of bend increase slightly, but those in extrados of bend decrease in a little range. The changes of longitudinal tensile properties are bigger. When the radius of curvature is decreased, the yield strength and yield ratio in intrados of bend decrease obviously, but those in extrados of bend increase. The longitudinal tensile curves in different position of bend exhibit a rather large difference. After the strain-age for mother pipe and bends, the effects of material deformation in the pipe production process appear which result in that the transverse tensile strength and yield ratio increase obviously. The longitudinal tensile strength and yield ratio in different position of bend also present a increase tendency.

Strain-Hardening Capacity and Microstructure Analysis of X70 High Strain Line Pipe

Strain-hardening capacity is the most effective way to improve the deformability of line pipe. In this paper, four kinds of new X70 high strain line pipe have been produced by four manufactures respectively. Microstructure, strain-hardening capacity, and characterization of the X70 high strain line pipe have been analyzed. It is concluded that X70 high strain line pipe steel does not fit to the Hollimon formula in the entire deforming process, and a stable strain-hardening exponent is not available. At the beginning of plastic deformation, the strain-hardening capacity of X70 high strain line pipe steel is excellent. It decreases rapidly as the stain increases then becomes stable after 2.0% total strain. The stress ratios of Rt₁.₅/Rt₀.₅, Rt₂.₀/Rt₁.₀ and Rt₅.₀/Rt₁.₀ are proved to characterize the strain-hardening capacity of X70 high strain line pipe steel. When Rt₁.₅/Rt₀.₅≥1.070, Rt₂.₀/Rt₁.₀≥1.025, and Rt₅.₀/Rt₁.₀≥1.050, the average strain over the gauge length of 2 times the outer diameter, centered at the wrinkle on intrados peak, is above 1.3%.

Production of Grade X80 High Strain Linepipes for Seismic Region Application

In order to achieve safety and reliability of long-distance gas transmission pipeline installed in seismic region while obtaining economical benefit by reducing material and construction cost, it is essential to apply the high-strength linepipes with sufficient strain capacity against buckling and weld fracture by seismic ground movement. At the same time, it is quite important to develop appropriate material requirement for strain capacity depending on the pipe dimension and strain demand of the region where the pipeline is installed. Grade X80 heavy gauge linepipes with excellent deformability were mass produced by applying advanced plate manufacturing technologies. These linepipes exhibit low Y/T and high uniform elongation in the longitudinal direction even after pipe coating. Strain capacity of the pipe against bending deformation with internal pressure was verified by conducting full scale pipe bending testing. In this paper, production results of high strain X80 linepipes for the application in long-distance pipelines in seismic region and full scale pipe bending and hydraulic burst test results were introduced.Copyright