Yaorong Feng

Crack arrest in rupturing steel gas pipelines

Failure incident investigations and full-scale experiments of pipelines rupturing indicate that the original methods in predicting arrest are no longer reliable when applied to high toughness pipelines. The cause of the phenomenon is analyzed in this paper through a macroscopic and phenomenological method by referring to the energy balance equation, by that is established the iterative algorithm used in finite element method (FEM) simulation of crack deceleration. This simulation, in combination with the two-specimen drop weight tear test (DWTT), provides a broad prediction of the dynamic fracture process. In addition, the crack tip opening angle (CTOA) criterion is consummated through the comparison between CTOA in FEM calculation and the critical value of (CTOA)C obtained in the two-specimen DWTT. Thus, the avoidance of both the dimensional effect and the dissipation of irrelevant energy common in small-scale tests are achieved.

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

Effect of Micro-Texture on Fracture Separation in X80 Line Pipe Steel

During tests on pipe and steel for the 2nd west-east gas pipeline project (WEPP) in china, fracture separation was found in X80 line pipe steel plates in tensile tests, charpy impact tests and DWTT tests. In this paper, fracture separation is investigated by using EBSD technique to analyze microstructure and compare it with microstructure transformation of the material after annealing. Results show that there are obvious texture of {111} and {110} in hot rolling plate, but a reduction of this two textures and a new texture of {110} are found after annealing at 700°C for 2 hours. It is found in charpy impact tests that {111} micro-texture as a main factor leading to fracture separation was eliminated in annealed specimens. The average grain size of specimen changes from 2.2687μm to 2.8362μm comparing with hot rolling plate because of the decomposition of Bainite. Results of Orientation Imaging display that the proportion of substructure and the distribution of misorientation of grain boundary of annealed specimens do not changed remarkably in contrast to the original ones. It is concluded that, X80 steel has well heat stability.Copyright

EBSD Analysis of Fracture Separation in X80 Line Pipe Steel

During testing pipe and steel for 2nd china west-east gas pipeline (WEGP), fracture separation was found in X80 line pipe steel plates in tensile tests, charpy impact tests and DWTT tests. In this paper, fracture separation is investigated by using EBSD technique to analyze microstructure and to compare it with microstructure of the material after annealing. Results show that there are obvious texture of {111} and {110} in hot rolling plate, but a reduction of this two textures and a new texture of {110} are found after annealing at 700°C for 2 hours. It is found in charpy impact tests that the {111} micro-texture, as a main factor leading to fracture separation was eliminated in annealed specimens. The average grain size of specimen changed from 2.2687μm to 2.8362μm comparing with hot rolling plate because of the decomposition of Bainite. The results of Orientation Imaging displayed that the proportion of substructure and the distribution of misorientation of grain boundary of annealed specimens did not changed remarkably in contrast to the original ones. It is concluded that, X80 steel has well heat stability.Copyright

The Toughness Index of X80 Mother Linepipe Steel and Its Optimum Microstructure

Large amount of experimental data indicated, that the index of CVN toughness and shearing area of mother pipe of X80 are (200J, 90%). And the results showed that when charpy toughness was closing to 200J, the shearing area would reach to 90%; when the charpy toughness increased continuously, the shearing area would not increase further more. So the selection of charpy toughness value as 200J for pipeline steels is reasonable, and also its corresponding shearing area. To gain thus component index, the optimum microstructure of X80 steel should be strip-like ferrite mastered, small amount of granular bainite contained-acicular ferrite type steel, and, in which the size of MA island should be less than 1μm. This kinds of optimum microstructure of X80 pipeline steel will exert its softening role in improving toughness and simultaneity not decrease its strength.Copyright

The Development of Large Diameter and Thickness X80 HSAW Linepipe

For the requirement of 2nd West-East Pipeline Project of China, X80 large diameter & thickness linepipe with helical seam submerged arc welded (HSAW) were developed, with 1219 mm OD and 18.4 mm WT. Acicular ferrite type and super-low carbon, high Niobium chemical composition pipeline steel was adopted for the base material. The very stringent requirements at −10 °C for toughness, i.e. 220J/170J for average/minimum for pipe body and 80J/60J for average/minimum for weld and HAZ were meet successfully. The yield strength loss due to Bauschinger effect was found lower than 20MPa, which benefited. The very low residual stress level was testified by cut-ring test which cuts a section pipe about exceed 100mm long, and then cut the section apart from welds 100mm along the longitudinal direction.Copyright

The Analysis to the Correlation between Cooling Medium and Cooling Result of DWTT Specimen

The cooling medium choice is restricted due to no detailed regulations for temperature control as the specimen with gaseous cooling medium in the API, ASTM and the standards or specifications of China. The analysis and calculating are made to the physical process of the cooling. The preservation hold time are provided for DWTT specimen at different temperature in the gaseous cooling medium. It provide primary sustaining base to control the cooling process of gaseous cooling medium. The essential point of the precise means of temperature testing is brought forward at the same time.

The Researches on Toughness Index of Reheated High Grade Linepipe Bend Pipes

In the development of high grade pipeline steels, the technologies on bend pipes are key points. Most reports are mainly on the cold bending techniques which allow a small deflection angle usually less than 10°. And induction heat is then introduced for higher deflection angle bend pipes. So it is very important to know the toughness index of reheated high grade linepipes. In current used standard in China for West-East Gas project, Charpy toughness is required to be ≥120J at −20°C, and corresponding shearing area is required to be ≥85%. Based on a large number of experimental data on high grade linepipes with heat treatment, it was found in the present study that the shearing area and Charpy toughness have a linear relationship and the slope coefficient is 0.34. As a result, the when shearing area is 85%, the corresponding Charpy toughness is 250J. and This is different from that of mother pipe. To find the reasons, the fractures and microstructure of these reheated linepipes are analysed and compared with that of mother pipes.Copyright

Full-Scale Fatigue Test on X60 Spiral Linepipe

Both fluctuation of operation pressure and transmission halt have obvious effect on the lifetime of the pipeline. When there exist flaws in the pipes, cyclic load caused by pressure fluctuation will promote the development of such defects especially as cracks. A full-scale fatigue test was performed on the X60 Spiral Submerged Arc-Welded pipes (Outside Diameter is 660mm), which was pre-manufactured with a small outside surface crack-like flaw. The test result indicated that both defect size and cyclic load magnitude had great effect on crack propagation rate and fatigue lifetime. During the test, the artificial shallow surface defect developed only in depth direction and finally caused the leakage. Comparison of test results between small-size and full-scale fatigue tests showed that small-size test can be used for lifetime prediction of the pipeline although there was some deviation.Copyright

Study on the Mismatched Girth Welds of X80 Line Pipes

X80 linepipes are going to be widely used for gas transmission pipelines in China. The welding procedure for girth weld is very critical to the safety of the X80 pipeline. In this paper, different welding procedures are employed to obtain three different mismatched girth welds, with which tests were carried out to analyze effect of mismatch on the properties of girth welds. Plastic deformation and critical defect size (δc ) is calculated and discussed for the different mismatched girth welds utilizing both FAD and FEM methods. Overmatched or evenmatched girth weld is recommended for high grade linepipes because overmatched weld has the advantages of improving limit load and enhencing fracture resistance of pipe girth welds. But the increase of weld strength may reduce the resistance of the weld to HIC and SCC, as well as make the field weld operation more difficult. The level of mismatch shouldn’t be too high because too much weld seam strength will not only be a way of waste, but also decrease weld resistance to HIC cracks.Copyright