Taishi Fujishiro

Evaluation on Dependence of Ductile Crack Propagation Resistance on Crack Velocity

Plastic strains were measured near a propagating ductile crack in drop-weight tear tests (DWTT). Plastic work evaluated from the plastic strains agreed with crack propagation energy evaluated from dynamic load versus displacement curve. Furthermore, plastic strains were measured near a propagating ductile crack in a full-scale burst tested pipes. Plastic work of the burst pipe was found to be much larger than that of the DWTT. Values of the plastic work of the DWTT and the burst pipe were plotted against crack velocity to construct crack resistance curve. Reason to the increased crack resistance with crack velocity was understood as a dependence of plastic strain distribution on crack velocity by elasto-plastic dymanic finite element analysis.Copyright

Effect of Plastic Deformation on Occurrence of Abnormal Fracture During DWTT

Demand for natural gas using LNG and pipelines to supply the world’s gas markets is increasing. Under the large demand for high-strength linepipe, crack arrestability is one of the most important properties. DWTT (Drop Weight Tear Test) is the major test method for evaluating crack arrestability. Generally, a DWTT shear area of 85% or higher is required as the acceptance criteria, such as those of the API (American Petroleum Institute). In high-toughness linepipe steels, the abnormal fracture frequently occurs in DWTT. Abnormal fracture is defined as a cleavage fracture on the hammer side. However, the mechanism for occurrence of the abnormal fracture during DWTT has not been fully clarified. This paper describes the effect of plastic deformation on occurrence of abnormal fracture during DWTT using various steels with different microstructures. Each DWTT was carried out at the same test temperature using 20 mm plates with approximately the same tensile strength. This paper describes the deformation during DWTT, which consists of deformation caused by hammer impact, bending compression, and bending tension. The deformation due to the impact of the hammer during DWTT on a 20 mm plate was limited, and the location affected by the hammer impact did not correspond to that where abnormal fracture occurred. Moreover, the equivalent plastic strain from bending deformation was dominant as compared with that of hammer impact regardless of the microstructure. This suggests that abnormal fracture occurred by exceeding the critical equivalent plastic strain due to the bending deformation.Copyright

Effect of Separation on Ductile Crack Propagation Behavior During Drop Weight Tear Test

The demand for natural gas using LNG and pipelines to supply the world gas markets is increasing. The use of high-strength line pipe provides a reduction in the cost of gas transmission pipelines by enabling high-pressure transmission of large volumes of gas. Under the large demand of high-strength line pipe, crack arrestability of running ductile fracture behavior is one of the most important properties. The CVN (Charpy V-notched) test and the DWTT (Drop Weight Tear Test) are major test methods to evaluate the crack arrestability of running ductile fractures. Separation, which is defined as a fracture parallel to the rolling plane, can be characteristic of the fracture in both full-scale burst tests and DWTTs. It is reported that separations deteriorate the crack arrestability of running ductile fracture, and also that small amounts of separation do not affect the running ductile fracture resistance. This paper describes the effect of separation on ductile propagation behavior. We utilized a high-speed camera to investigate the CTOA (Crack Tip Opening Angle) during the DWTT. We show that some separations deteriorate ductile crack propagation resistance and that some separations do not affect the running ductile fracture resistance.Copyright

Application of B-Added Low Carbon Bainite Steels to X80 UOE Line Pipe for Ultra Low Temperature Usage

Demand for high strength line pipes is increasing because of the reduction in natural gas transportation costs of pipelines. Low temperature toughness is required for high strength line pipes. Reduction in manufacturing cost of high strength linepipes is also required in an environment where alloying cost is increasing. To meet these requirements, boron (B) addition is extremely useful because the addition of very small amounts of B remarkably improves the strength and low temperature toughness. B-added low carbon bainite (LCB) line pipes with American Petroleum Institute (API) grade X60 to X80 have been developed for several decades [1–2]. B-added LCB steels have excellent low temperature toughness, however, it is challenging to achieve excellent crack initiation resistance and crack arrestability for ultra low temperatures such as −60°C. In particular, it is very difficult to achieve both excellent Drop Weight Tear Test (DWTT) properties of base metal, and excellent Charpy V-Notched (CVN) properties of seam welds in heavier wall thickness of X80 UOE linepipe. Metallurgical concepts such as the optimum chemical compositions, Thermo Mechanical Control Process (TMCP) conditions and seam weld conditions of B-added LCB steels with API grade X80 for ultra low temperature have been proposed in order to achieve the excellent mechanical properties even in a low manufacturing cost. Based on this concept, excellent DWTT properties of base metal and CVN properties of the seam welds of API grade X80 line pipe with 25mm thickness down to –60°C were obtained.Copyright

Metallurgical Design and Development of High-Grade Line Pipe

The application of high-strength line pipes has enabled pipelines to operate at high pressure, generating cost savings for both gas transportation and construction. In general, high-strength line pipes require crack initiation resistance and crack arrestability at low temperatures, as well as field weldability. High strength and deformability for strain-based design and excellent sour resistance are also required. Moreover, composite properties are often required for high-strength line pipes. This paper describes our progress in this field with regard to metallurgical design and development. Metallurgical design aimed at achieving a good balance between strength, low temperature toughness and deformability for strain-based design is also described from the perspectives of grain refinement, microstructure and chemical composition. Metallurgical design focused on a good balance between strength and sour resistance in limited low chemical composition is described from the perspectives of microstructure and control to chemical composition and center segregation. These efforts have led to the development of high-strength heavy wall line pipes of API X60 to X100 grades offering excellent low temperature toughness and high deformability for stain-based design, while API grades X65 to X70 with good sour resistance have also been developed.Copyright