Study on low temperature tensile properties of thick wall X80 pipeline at different wall thicknesses
Pipeline transportation is one of the most economical and safe ways for long-distance transportation of crude oil and natural gas. In order to improve transportation efficiency, reduce energy consumption and construction cost, the development of pipeline steel aims to achieve high strength and superior weldability. In recent years, with the increasing demand of global oil and gas resources, the research and construction of oil and gas pipelines in China has entered a new peak. Large diameter, thick wall and high-grade steel pipes have become one of the main options for pipeline construction [1-2]. X80 steel not only meets the requirements of material economy, but also has high strength, good toughness and superior corrosion resistance. The exploitation of oil and gas resources and pipeline construction in the world tend to the high and cold area with rich oil and gas resources, the transmission pressure of pipeline also increases gradually, and the requirements for low-temperature mechanical properties of pipeline steel are more stringent.
With the requirements of pipeline engineering and the improvement of pipeline steel smelting technology, the content of C element in pipeline steel is gradually reduced, and micro alloy elements such as Nb and Ti are added to improve the strength and toughness of pipeline steel, and extend the safe operation period of pipeline [3-5]. Tensile property is one of the important mechanical property indexes of pipeline steel, and it is also the basic index of pipeline design and safety assessment . In order to ensure that the actual mechanical properties of the pipeline are consistent with the design performance, it is necessary to systematically analyze the mechanical properties of the pipeline. This study takes Φ 1422 mm × 32.1 mm X80 pipeline as an example to simulate and evaluate the applicability of X80 pipeline in low temperature environment, focusing on the relationship between microstructure and tensile properties at different thicknesses of X80 pipeline, and to explore X80 The reason for the change of microstructure and mechanical properties of pipeline steel in low temperature environment is expected to provide data support and guidance for the material selection of high strength pipeline steel with large wall thickness in low temperature environment.
Test materials and test methods
The test material is Φ 1422 mm × 32.1 mm X80 pipeline produced by a domestic manufacturer, and the sample is taken at the base metal of the steel pipe for chemical composition analysis. See Table 1 for the main chemical composition of the base metal of the pipeline.
Table.1 Main chemical composition% of base metal of X80 pipeline
It can be seen from table 1 that w (c) ＜ 0.05% and w (MN) is between 1.5% and 2.0%. The reduction of C content is beneficial to improve the weldability and ductility of steel, but at the same time, it also reduces the strength of steel. In general, Mn is used instead of C by solution strengthening to make up for the loss of material strength caused by the decrease of C content, and to reduce the transformation temperature of austenite, which is helpful for grain refinement, so as to improve the toughness of materials and reduce the transformation temperature of toughness and brittleness. The w (NB) in steel is 0.075%, which also plays an important role in grain refinement of steel [7-8].
The tensile property test refers to the standard GB / T 13239-2006 “low temperature tensile test method for metallic materials”. The flame cutting machine is used to take the transverse samples of the outer layer, the central layer and the inner layer of the steel pipe wall thickness along the axis direction at the position 90 ° from the pipe body weld seam for the test, and the specific sampling position is shown in Figure 1. The wire cut layered sampling is adopted, and the round bar tensile sample is processed by numerical control machine, and the clamping section of the sample is processed into thread head. During sampling, overheating and work hardening shall be prevented to avoid affecting tensile properties.
The size and shape of the round bar tensile specimen are shown in Figure 2. The diameter of the sample is 6.25 mm, the transition radius r of the sample is 10 mm, and the length of the test section l0 is 65 mm. The tensile test is carried out on mts810-15 electronic universal testing machine with ultra-low temperature test chamber and thread tensile fixture. In order to use the cold source more economically and shorten the cooling time as much as possible, the samples in the test chamber are cooled and controlled by spraying liquid nitrogen. The cooling time of the test is controlled at 30 min, and the test temperature is respectively set as: 20 ℃, 0 ℃, – 20 ℃, – 40 ℃ and – 60 ℃, and the tensile strain rate is 2 mm / min.
Fig.1 Schematic diagram of sampling position of pipe body
Fig.2 Dimension and shape of round bar tensile specimen
The metallographic samples were cut from the outer layer, central layer and inner layer of the wall thickness of the steel pipe. After 180 × 400 × 600 × 800 × 1000 × 1500 × sandpaper was polished and mechanically polished step by step, 3% nitric acid alcohol solution was used for corrosion, and then 75% alcohol solution was used to wash the surface of the metallographic samples. The metallographic structure of the corroded samples was observed on the mef4m metallographic microscope.
Test results and analysis
The test material is a typical low carbon NB Ti MICROALLOYED X80 pipeline. The change rule of tensile properties of X80 pipeline under different temperature environment is shown in Figure 3. From Fig. 3 (a) and Fig. 3 (b), it can be seen that when the temperature drops from 20 ℃ to – 60 ℃, the tensile strength and yield strength of the three groups of samples all increase, and the lowest tensile strength and yield strength of the outermost samples increase by 11% and 12%, respectively. At the same temperature, the order of strength of three groups of samples is: outer layer > inner layer > central layer. It can be seen from Fig. 3 (c) that the elongation after fracture of the three groups of samples increases with the decrease of temperature, but the numerical change range is not large. It can be seen that at – 60 ℃, the tensile strength and yield strength of the specimen increase while the plasticity does not decrease.
Generally, with the decrease of temperature, there will be embrittlement tendency in ferritic alloy steel, that is, the change range of yield strength with temperature is larger than that of tensile strength [9-10]. However, from Figure 3, it can be seen that the change range of yield strength with temperature is weaker than that of tensile strength, especially when the test temperature is lower than – 20 ℃, which indicates that the possibility of embrittlement tendency of the test material in the low-temperature service environment is small, and it has good low-temperature tensile performance.
Fig.3 Change rule of tensile properties of X80 Pipeline Steel under different temperature environment
The microstructure of different thickness of X80 steel tube and the banded structure morphology of X80 steel plate are shown in Fig. 4. Fig. 4 (a) to Fig. 4 (c) show the microstructure of the outer layer, center and inner layer of X80 steel respectively. From Fig. 4, it can be seen that the microstructure of the sample is typical ferrite + granular bainite, and the morphology distribution of the island in granular bainite (M-A) has an important influence on its mechanical properties. The acicular ferrite has high dislocation density, which has a good strengthening effect on the matrix. In addition, the structure grain size in the center of the sample is larger than that in the outer layer and inner layer, which is mainly due to the thick wall of the tube. In the rolling process, the rolling pressure is difficult to penetrate into the center of the steel plate, so that the grain size in the center of the plate is relatively coarse, resulting in the poor overall uniformity of the microstructure in the center.
Some studies [11-14] show that the majority of banded structure in high grade pipeline steel is M-A island. During the continuous cooling transformation of pipeline steel, carbon will gradually accumulate in the retained austenite during the formation of ferrite. Because the transformation temperature of low carbon steel is high and the driving force of transformation is small, the transformation can not be completely carried out, a small amount of austenite will remain and distribute on the lath and grain boundary in irregular island shape . The content of alloy elements in the composition of M-A island is similar to that of matrix, mainly the enrichment of carbon. However, the enrichment degree of the island is not enough to precipitate carbide, so it will become some carbon rich austenite island. In the subsequent cooling process, these austenitic islands will be transformed into different products due to different composition and cooling speed. When the composition segregation in the steel reaches a certain degree, these M-A islands will be divided into strips, forming strip structure. Generally, the thicker the steel plate is, the more obvious the carbon segregation is and the more serious the banded structure is. M-A island belongs to hardenable structure, its strength and hardness are generally higher than that of matrix. When the material deforms, the deformation of banded structure and matrix is not synchronous, and brittle cracks are easy to occur at the interface of intersection, which is unfavorable to the dynamic crack arrest toughness of material.
Fig.4 Microstructure of different wall thickness of X80 steel tube and strip structure of X80 steel plate
- (1) When the temperature drops from 20 ℃ to – 60 ℃, the tensile strength, yield strength and elongation after fracture of three groups of X80 pipelines increase with the decrease of temperature, but the increase is small. The effect of temperature on the tensile properties is not obvious, which shows that the material has good low-temperature tensile properties, and the outer layer has relatively good low-temperature tensile properties.
- (2) Because the tube wall is thick and the grain size at the center of the plate is relatively large, the M-A island which forms the banded structure belongs to the hardenable structure. When the material deforms, it is easy to produce brittle cracks at its phase interface, resulting in brittle fracture of the test material at low temperature.
Source: China X80 Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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