No abstract

The influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H 2 S content sour corrosion environment was investigated. These experiments were conducted with steel for pipelines, and electrochemical methods were used. The results showed that external stress increases the amount of corrosion weight loss and trench depth by promoting the anodic dissolution reaction, and stress concentration was proven to be one of the driving forces for trench formation due to localized corrosion. Based on the experimental findings, the mechanism of trenching was discussed from the viewpoint of promotion of the anodic dissolution reaction by dislocations.

TMCP (thermo-mechanical controlled process) linepipes have been long used for severe sour environment, but recently sulfide stress cracking (SSC) caused by local hard zones has become a concern. In order to clarify the hardness threshold that leads to SSC, four-point bend (4PB) SSC tests as per NACE TM0316 were conducted under several H 2 S partial pressure conditions. For 1 bar and higher H 2 S partial pressure conditions, the surface hardness threshold (at 0.25 mm from surface) observing 4PB SSC specimens without SSC cracking was approximately correlated to a maximum acceptable hardness level of 250 HV0.1. By suppressing the hard lath bainite (LB) and obtaining the soft granular bainite (GB) microstructure, stable low surface hardness of 250 or less HV0.1 was achieved, resulting in superior SSC-resistant property. On the other hand, it was found that SSC crack propagated when the surface hardness increased with increasing the volume fraction of LB microstructure. In the case of 16 bar H 2 S partial pressure condition, the crack growth rate increased in the sour environment, and hydrogen embrittlement by H 2 S was promoted. However, in the 4PB SSC test at 16 bar, since the shape of localized corrosion is semicircular due to low localized corrosivity, it was considered that the stress concentration and transition to crack were suppressed. This may be the reason why the SSC susceptibility was similar to 1 bar condition, especially in the 4PB SSC test using the samples with lower surface hardness level of 250 or less HV0.1.

Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H 2 S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04-0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.

Linepipes installed in permafrost ground or seismic region, where larger strains can be expected by ground movement, are required to have sufficient strain capacity in order to prevent local buckling or girth weld fracture. On the other hand, strain capacity of linepipes usually degreases with increasing strength, and this is one of the reasons for preventing wider use of high-grade linepipe for high strain application. Furthermore, external coating is necessary for corrosion resistance of pipe, but coating heat can cause strain-aged hardening, which results in increased yield strength and Y/T. Therefore, there is a strong demand for developing high strength linepipe for a high strain application with resistance to strain-aged hardening. Extensive studies to develop Grade X100 high strain linepipe have been conducted. One of the key technologies for improving strain capacity is dual-phase microstructural control. Steel plate with the microstructure including bainite and dispersed martensite-austenite constituent (MA) can be obtained by applying accelerated cooling followed by heat treatment online process (HOP). HOP is the induction heating process that enables rapid heating of the steel plates. Variety of microstructural control, such as fine carbide precipitation and MA formation, can be utilized by this newly developed heating process. One of the significant features of the HOP process is to improve resistance to strain-aged hardening. Increase in yield strength by coating can be minimized even for the Grade X100 linepipe. Trial production of X100 high strain linepipe with the size of 36″ OD and 15mm WT was conducted by applying the HOP process. Microstructural characteristics and mechanical properties of developed X100 linepipe are introduced in this paper. In order to evaluate compressive strain capacity of the developed pipe, full-scale pipe bending test was carried out by using the trial X100 high strain linepipe after external coating. Full scale bending test of developed X100 linepipe demonstrated sufficient compressive strain capacity even after external coating.