Takuya Hara

Conditions of Hydrogen-Induced Corrosion Occurrence of X65 Grade Line Pipe Steels in Sour Environments

Abstract Hydrogen-induced cracking (HIC) tests were performed on fourteen types of X65 line pipe steels in various environments in which the hydrogen sulfide (H2S) partial pressures (PH2S) were in the range from 1 × 10/5 MPa to 0.1 MPa and pH was in the range from 2 to 5. The conditions governing HIC occurrence were investigated and the following results were obtained: —Hydrogen permeability in steel ([PerFe]) was formulated as a function of PH2S and pH ([PerFe]) = 7.1 + 0.96 × (1.4logPH2S − 0.51 pH), where 0.1 MPa ≥ PH2S ≥ 1 × 10−3 MPa and 5 ≥ pH; [PerFe] = 3.3 + 0.75 × (0.3logPH2S − 0.51 pH), where 1 × 10−3 MPa > PH2S ≥ 1 × 10−5 MPa and 5 ≥ pH. —Critical hydrogen permeability in steel ([Percrit]) was expressed as a function of the critical length of inclusions (acrit) and the maximum hardness of the center-segregation zone (Hvmax) for the investigated thermomechanical-controlled process (TMCP) steels and the controlled rolled and air-cooled (CR) steels (log [Percrit] = [1.7 − 0.0030 Hv] − 1/4log[acrit])...

Effect of Flow Velocity on Carbon Dioxide Corrosion Behavior in Oil and Gas Environments

Abstract The effect of flow velocity on the corrosion rate of carbon steels and stainless steels in wet carbon dioxide (CO2) environments was investigated using a flow loop tester. The velocity-dep...

Effects of Microstructure and Texture on DWTT Properties for High Strength Line Pipe Steels

The crack arrestability for high strength line pipe steels with tensile strength of 650 to 850 MPa was evaluated using precrack DWTT (pc-DWTT). Moreover, the effects of microstructure and texture on pc-DWTT energy were investigated. The pc-DWTT energy was remarkably affected by tensile strength. The pc-DWTT energy of ferrite and bainite/martensite dual phase steels was much higher than that of bainite single phase steels in comparison with the same tensile strength. The {100} plane is a cleavage plane in iron, so the brittle crack mainly propagates along the {100} plane. Bainte single phase steels indicated a high intensity of the {100} on the plane rotated 40° from the rolling plane with the axis of the rolling direction. On the other hand, ferrite and bainite/martensite dual phase steels indicated not only a high intensity of the {100} plane rotated 40° from the rolling plane, but also a high intensity of the {100} plane parallel to the rolling plane. Slant fracture could be easily formed by the high intensity of the {100} on the plane rotated 40° from the rolling plane if local brittle areas such as martensite and austenite constituent (M-A constituent), which became the initiation point of brittle fracture, existed. In contrast, separation tended to be formed by the high intensity of the {100} plane parallel to the rolling plane that was caused by the formation of ferrite and bainte/martensite dual phase microstructure. Thus, pc-DWTT energy and shear area were remarkably affected by microstructure and texture. Therefore, to control microstructure and texture is vay important for the improvement of pc-DWTT properties.Copyright

Change of Mechanical Properties of High Strength Line Pipe by Thermal Coating Treatment

In strain-based design, the overmatch condition in the girth weld portion primarily must be maintained. The pipes may also be required to have a low yield to tensile (Y/T) ratio and a high uniform elongation (U.EL) in the longitudinal direction to achieve a high compressive buckling strain. However, change in the mechanical properties by heating during coating treatment has not been paid attention so much. Furthermore, how much the mechanical properties change is affected by production conditions is unclear. This study aims to clarify firstly the relation between the mechanical properties (Y/T ratio, U.EL etc.) and the microstructure and secondly the change in mechanical properties by thermal coating treatment. The Y/T ratio and U.EL are affected by the volume fraction of ferrite and the secondary phase, which are changed by thermomechanical control processing (TMCP) conditions. For example, use of dual phase microstructure is very effective for decreasing the Y/T ratio and increasing the U.EL as the pipe. On the other hand, yield strength (YS) rises and the U.EL does not change after coating. The increase in the YS after coating is influenced by the microstructure and TMCP conditions. Resultantly, dependence of the Y/T ratio on the microstructure and TMCP conditions is reduced for line pipes after thermal coating treatment.Copyright

Effect of Scratching on Galvanic Corrosion in Oil and Gas Environments

Galvanic corrosion behavior in stagnant oil and gas environments, when the metal specimens were mechanically scratched, was investigated by electrochemical methods using material combinations ranging from carbon steels to duplex stainless steels. Galvanic currents and coupled potentials increased rapidly upon mechanical scratching, immediately started to decrease, and finally returned to the values before scratching in sweet and sour environments. Therefore, an increase in the galvanic corrosion rate caused by scratching was negligibly small for the long term, and localized corrosion as a result of mechanical scratching did not occur in these stagnant solutions. The change in coupled potential immediately after scratching was small in sweet environments and large in sour environments. For rapidly forming corrosion films on less noble metals, the galvanic current after scratching decreased rapidly; however, for slowly forming corrosion films on less noble specimens, galvanic current after scratching decreased gradually.

Effects of alloying elements on carbon dioxide corrosion in 13% to 20% chromium-containing steels

Abstract Effects of alloying elements on corrosion rates were investigated for 13% to 20% chromium-containing steels in wet carbon dioxide (CO2) environments without wet hydrogen sulfide (H2S) gas ...

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

X100 Linepipe With Excellent HAZ Toughness and Deformability

Good low-temperature toughness of the base material (BM) and weld heat-affected zone (HAZ), and good deformability of the pipe body together with good field weldability are required for X100 linepipe to ensure the safety of pipelines and to facilitate field welding. It is, however, very difficult to attain these properties simultaneously because of the large addition of alloys. The technology of improving HAZ toughness by reducing carbon content through the reduction of M-A constituents harmful to low-temperature toughness was developed, and accelerated cooling after controlled rolling was applied to attain good low-temperature toughness of BM and high uniform elongation together with sufficient strength corresponding to X100. Two newly developed types of X100 linepipe, a “high HAZ toughness type” and a “high uniform elongation type”, exhibited excellent low-temperature toughness of the HAZ and high uniform elongation together with sufficient strength, respectively.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