Based on the increasing demand for natural gas to be extracted under severe environmental conditions, requirements for thicker sour service pipe steels with high strength (X65 grades or more) are increasing. In order to achieve both a resistance to hydrogen induced cracking (HIC) and better mechanical properties, it is important to obtain a homogeneous microstructure. For this purpose, we manufactured API X65 and X70 heavy wall (up to 37mm in thick) UOE linepipe for sour service utilizing an advanced thermo mechanical control process (TMCP) employing the theoretical maximum cooling rate with water (the ‘ultimate cooling rate’) and homogeneous temperature distribution in accelerated cooling. For applications in deeper water, higher pipe thickness to diameter ratio (t/D) is required. However, during pipe forming, these high ratio pipes suffer higher plastic strain in the vicinity of the surface. This plastic strain causes increasing surface hardness and HIC resistance may deteriorate. It is important to evaluate the effect on strain systematically based on the difference of microstructure morphologies. Therefore, the effect of bending strain on HIC properties was investigated through a simulated laboratory bending test with a strain of up to 8.9%. It was found that homogeneous bainitic microstructure can prevent HIC even under a higher bending strain. On the other hand, crack area ratio (CAR) was almost doubled when 5% strain was applied near the surface area in the material in which inclusion morphologies were not optimized.
Recently, X80 grade UOE pipes have been planned to apply to steam injecting oil sand recovery systems to increase the volume of steam to be injected and lowering installation cost. The pipes for systems are subjected to high temperature for a long time, such as 350-C, for 20 years. Before real applications of the pipes, it is important to ensure the reliability of the pipes during and after long-term operations. In this study, in order to establish simulation conditions for 350-C × 20 years of operation, the change in microstructure and resulting mechanical properties of X80 grade pipes after a long-term exposure at elevated temperatures were investigated. Then, mechanical properties of the pipes subjected to the established simulated condition were examined. Change in the microstructure was quite small after exposure of 400-C and lower temperatures. Tensile strengths of the base metal and seam weld after up to 400-C of heat treatment can be arranged with the Larson-Miller parameter composed with temperature and holding time of the heat treatments. Therefore, heat treatments at 400-C for shorter than 20 years can be simulation conditions for the operation condition of the systems. As a result of mechanical tests simulating long-term exposure, satisfied performance of X80 grade pipes can be obtained.
Higher grade linepipes such as grade X80 have been developed and applied to long distance pipelines in order to reduce the cost of pipeline construction by using thinner pipes than is possible with conventional grades. Service pressures have also been increased in recent years for efficient gas transportation. In addition to the requirement of higher strength, running ductile fracture should be prevented in long distance and high pressure pipelines. Resistance to ductile fracture, as evaluated by Charpy energy, is an important material property for higher grade linepipes. It has been reported that bainite single-phase steel tends to show higher Charpy energy than ferrite-bainite or bainite-MA (martensite-austenite constituent) dual-phase steels, since void nucleation is suppressed in single-phase steels compared with dual-phase steels. However, in higher grade steels with a bainite single phase, a small amount of MA grains generally remains due to the chemical stability of MA. Therefore, further reduction of MA is key to improving Charpy energy for higher grade linepipe steels. In order to achieve high Charpy energy by MA formation control, the optimum conditions of the plate manufacturing process were investigated. As a result, a high Charpy energy was achieved by the combination of controlled rolling and precise control of the accelerated cooling conditions, by which the MA phase was minimized. Based on the above investigation, grade X80 high Charpy energy linepipes were trial-produced by applying JFE Steel’s optimized accelerated cooling (ACC) system with a high cooling rate and homogeneous temperature profile. Stable higher Charpy energy was achieved by minimizing MA formation and achieving a homogeneous microstructure by advanced cooling control.
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