Abstract
In this research, stress-oriented hydrogen induced cracking (SOHIC) test was carried out on a 50 mm thickness of a commercial API 5L X70 steel plate. The evolution of microscopic features such as phase, boundary, interface, grain, and crystallographic data was analyzed before and after SOHIC, in order to comprehend the effect of crystallographic orientation on SOHIC propagation. Chemical composition and previous thermomechanical processing even finish rolling temperature and cooling rate determine the ferrite matrix microstructure. A recrystallized ultrafine ferrite grain with about 3–5% degenerated pearlite dispersed in the microstructure was characterized, called as-received specimen. The average lattice strain and dislocation density was calculated first using multiple Gaussian peak-fitting method from XRD pattern. Electrochemically charged combination mixed H2S-CO2 solution, constant hydrogen injection, and external loading were applied to tensile specimen, in order to simulate the H2S and CO2 environment. The results show that local misorientation and Taylor factor analyses predicted the possibility of hydrogen crack nucleation especially at boundaries and interfaces. Moreover, SOHIC crack propagation occurred along the mid-thickness of the cross section of steel plate along the ferritic boundaries, pearlitic colonies, and ferrite-cementite interfaces. Moreover, the crack propagated along distorted {110} and {001} grains, indicating a strong strain gradient towards the boundaries. The analysis of XRD patterns of SOHIC tested specimen by multiple Gaussian peak-fitting method estimated about 68% increment in micro-deformation and approximately 170% increase in dislocation density.
Highlights
An increasing demand for natural gas and oil products makes it necessary to use steel pipelines to carry oil and gas products over long distances from their source to refineries
Degenerated pearlite is considered as non-lamellar pearlite due to the lack of carbon and micro-segregation element alloys in hypo-eutectoid steels [22]
The results show that the crack propagation mainly depends on the loading direction
Summary
An increasing demand for natural gas and oil products makes it necessary to use steel pipelines to carry oil and gas products over long distances from their source to refineries. Haidemenopoulos et al [2] investigated the stress-oriented hydrogen-induced cracking (SOHIC) in riser steel operating under wet H2S service conditions. They showed that the cracks initiated from the welded zone and propagated along the rolling direction in the middle of the pipe thickness. Park et al [3] studied the effect of three different microstructures including ferrite (F), degenerated pearlite (DP), and bainite (B) on hydrogen-induced cracking (HIC) of a API X65 grade linepipe steel. According to their results, the hydrogen trapping efficiency is increased from DP to bainitic ferrite (BF) and acicular ferrite (AF). These researchers observed that HIC cracks nucleated form the local martensite/austenite (M/A) concentrated region with the steel microstructure of ferrite/acicular ferrite (F/AF) or ferrite/binite (F/B)
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