Materials Science Forum Vol. 850

Abstract: In this paper, the optical microscope, scanning electron microscope (SEM), X-ray Diffraction (XRD) and simulation test were utilized to study the corrosion failure of L360 pipeline. The results show that the corrosion pits distributed around the pipe inner surface. The pipe mechanical properties, chemical constituents and inclusions grade satisfied the requirements of the technical conditions. Inclusions were composed of Ca, S, Al and O. Corrosion products consisted of the Fe₃O₄, Fe₂O₃ and FeO (OH). The compositions of corrosion products within and around the pits were similar to the inclusions. Due to the combined effects of water and high pressure air sealed in the pipeline, the pits formed around inclusions, and developed into occluded cells which accelerating the corrosion and leading to the failure finally.

Abstract: In this paper, an overview of experimental research and production of 40.5mm DNV 485FDU heavy wall thickness pipeline steel for deep water trunkline project was introduced and analyzed systematically. Alloy design of the steel was introduced. Continuous cooling phase transformation behavior of the steel under deforming condition was investigated. The microstructure of the steel at different cooling rates was observed by the microscope. An optimized cooling process (OCP) after finish rolling was developed in Shouqin Steel Co., Ltd. (SQS). The optimized cooling process guaranteed the steady control of the final bainitic microstructure with optimum polygonal ferrite (PF) for heavy gauge DNV 485FDU plates. The plates produced by this process achieved good tensile strength and excellent lower temperature toughness. The pipes were produced via the JCOE pipe production process and had favorable forming properties and good weldability. Plate mechanical properties successfully met the required final pipe mechanical properties due to the ideal microstructure from the alloy/OCP design.

Abstract: Two kinds of industry trial X90 pipeline steels which have different chemical composition were chosen as test objects, and the grain coarsening, microstructural characteristics and the variation rules of low-temperature impact toughness in weld CGHAZ of this two steel under different welding heat input were studied by physical thermal simulation technology, SEM, optical microscope and Charpy impact tests. The results showed that the microstructure in weld CGHAZ of 1# steel was mainly bainite ferrite (BF) and most of the M/A constituents were blocky or short rod-like; the grains of 2# steel were coarse and there was much granular bainite (GB). Meanwhile M/A constituents became coarser and their morphology changed from block to long bar; alloy content of X90 pipeline steel under different weld heat input had great effect on the grain size of original austenite. When heat input was lower than 20KJ/cm, the impact toughness in CGHAZ of lower alloy content pipeline steel was good; as heat input increased, impact toughness in CGHAZ of 1# steel increased to the values between 260J and 300J when heat input was between 20KJ/cm and 25KJ/cm and the dispersion of impact energy was small. The impact toughness of 2# steel decreased gradually and the impact energy had the obvious dispersion.

Abstract: Corrosion defect in oil tubes is one main threat for the safety production of oil field. The corrosion defect in the tube will cause stress concentration and even lead to failure. The injection of CO₂ widely used during the recent years in oil fields, making the corrosion problems more severe. In this study a finite element model for the residual strength analysis of oil tube with corrosion defect was established by the programing language APDL. Based on the actual data of one oil well in Tahe oil field in China, the sensitivities of the axial length, circumferential width, radial depth of the corrosion defect, the internal pressure and axial force on the residual strength of the tube was examined in detail. The proposed method can be referred in the safety evaluation for oil tubes with corrosion defect.

Abstract: There is a potential for major damage to the pipelines crossing faults, therefore the strain-based design method is essential for the design of buried pipelines. Finite element models based on soil springs which are able to accurately predict pipelines’ responses to such faulting are recommended by some international guidelines. In this paper, a comparative analysis was carried out among four widely used models (beam element model; shell element model with fixed boundary; shell element model with beam coupled; shell element model with equivalent boundary) in two aspects: differences of results and the efficiency of calculation. The results show that the maximum and minimum strains of models coincided with each other under allowable strain and the calculation efficiency of beam element model was the highest. Besides, the shell element model with beam coupled or equivalent boundary provided the reasonable results and the calculation efficiency of them were higher than the one with fixed boundary. In addition, shell element model with beam coupled had a broader applicability.

Abstract: Grain boundary engineering (GBE) was carried out on 316L austenitic stainless steel with Thermo-mechanical processing (TMP), which was performed by unidirectional compression and subsequent annealing. The effect of TMP parameters including the strain and annealing time on grain boundary character distribution (GBCD) and the corresponding mechanism was investigated in the study. The results showed that high fraction of low-Σ coincident-site lattice (CSL) grain boundaries (about 55%) associating with interrupted network of random boundaries was obtained through TMP of 5% cold compression followed by annealing at 1000 °C for 45 min. The fraction of low-Σ boundaries increased with increasing the annealing time under all the experiment strain, but the mechanisms were different between the low and medium above levels of strain. Grains rotation and reaction of migratory boundaries might be the reasons of low-Σ boundaries growth in the strain of 5% and in the strain greater than or equal to 10%, respectively.

Abstract: Aimed at the serious problem of N80 tubing failure in the drainage of CBM (Coal bed methane) well, it can define the failure into two types: eccentric failure wear and the corrosion failure, through analyzing the well space structure, the stress and the chemical reaction on the tube in the coal bed water. The main reason is well deviation, instability of rod string and rod vibration. It makes sucker rod contacts with the N80 tube, when the sucker rod is up and down, the friction exists between the sucker rod and the tube, which leads to the tube wear out. Corrosion failure may happened in the condition that there are some acid anions in the coal seam water, such as HS^-, S^2-, Cl^-, HCO₃^-, SO₄^2-, CO₃^2-, etc. These ions react with N80 tubing that corrodes and destroys the N80 tube. In this paper, we suggest to replace the N80 tube with the UHMWPE (ultra-high molecular weight polyethylene) liner tube. This is capable to reduce the wear and corrosion rate effectively. The field tests show that it is still useful over 600 days, which means that the serving life of UHMWPE liner tube can be increased over 50 percent than others. Therefore, it has a significant meaning to popularize UHMWPE liner tube in the CBM well.

Abstract: In this paper, the failure analysis was conducted on a non-occupation refined-oil pipeline, in which corrosion perforation occurs when the pipeline was filled with 1.2 MPa air for more than 1 year, and the material of the corrosion pipeline was L245. To find out the reason of the corrosion, some tests such as XRD analysis for corrosion products, corrosion pits analysis, inclusions and microstructure inspection, as well as corrosion simulation trial were conducted. The experimental results showed that the failure was caused by electro-chemical corrosion because of the dissolved oxygen, while the existence of 1.2 MPa inner air and non-metallic inclusions accelerated the corrosion.

Abstract: Austenitic stainless steels widely used in cryogenic pressure vessel construction, and heads are formed by cold or warm spinning and stamping. Because of large deformation at excessive circular arc and at the straight parts of head during fabrication, the possibility of damage in these two positions during service is very high. In order to solve this problem, the commonly used S304408, S30403, S31608 steel plates processed under different temperature, different deflection and deformation rate were comparative studied. The evolution of the microstructure and mechanical properties were investigated to quantify the driving characteristics of the martensitic transformation by temperature and mechanical deformation. Consequently, the intergranular corrosion resistance of different microstructure was studied. The results of the research provide valuable reference for the selection and the processing technology of the head steel. Meanwhile, this study also provides a simple method for estimating the material properties after forming.

Abstract: The effect of finish cooling temperature on microstructure and mechanical properties of X100 pipeline steels were investigated through SEM, TEM and mechanical tests. The results showed that the effect of finish cooling temperature on tensile strength of studied steels were slight in the temperature range of 180°C~500°C. The yield ratio, yield strength and impact toughness all presented peak value when the finish cooling temperature was 360°C, however, both the volume fraction and particle size of MA component at 360°C finish cooling temperature had the minimum values. As a brittle hardening phase in bainite the increased number and enlarged size of the MA component were the key factor of the decrease of the yield ratio and toughness. Therefore, rational choosing finish cooling temperature could optimize the morphology and distribution of MA component to make sure the required strength and toughness for the X100 pipeline steel production.