Analysis and selection of material characteristics of expansion bellows

Liu Xiaodan 1, 2 Tao Xinghua 1 Niu Xinming 1

(1. China Institute of Petrochemical Engineering and Technology, Beijing10010/;

2. Youshi University, China, Beijing 102249)

Based on the research and analysis of the material characteristics of expandable solid pipe by predecessors, this paper focuses on the matching of different material components and performance requirements of expandable bellows in combination with field requirements. This paper analyzes the key factors restricting the strength improvement of expansion bellows from the microscopic point of view of materials, and puts forward the requirements for controlling the material composition of expansion bellows. On this basis, the applicability and superiority of ferrite-martensite dual-phase steel material in manufacturing expansion bellows are analyzed. This is of great enlightening significance for further exploring new materials suitable for expansion bellows, and also makes basic preparations for expanding the application scope of expansion bellows in the next step.

Expansion bellows material; Chemical composition and mechanical properties; dual-phase steel

Performance Analysis of Expandable Corrugated Liner Materials

And explore new applicable materials.

Liu Xiaodan 1, 2, Tao Xinghua 1, Niu Xinming 1

(1. China Petrochemical Petroleum Engineering Research Institute, Beijing10010/,

China; 2. China Shiyou University, Beijing 102249, China)

In this paper, the relationship between the composition and properties of expandable corrugated lining materials is studied in detail on the basis of the previous research on solid expansion pipe materials and the needs of the site. The key factors restricting the strength improvement of expandable corrugated liner are analyzed. At the same time, how to control the material composition to obtain higher casing strength is given. Therefore, this also proves the applicability and advantages of ferrite-martensite dual-phase steel in manufacturing expandable corrugated lining. This will help to further explore new applicable materials. This also prepares for the wide application of expandable liner.

Keywords expandable corrugated board; Materials; Chemical composition; Mechanical properties; dual-phase steel

Expansion bellows technology is one of the earliest casing subsidy technologies in Russian petroleum engineering field. The cross section of the pipe body is corrugated or plum petal-shaped. The round pipe is processed by cold pressing to reduce the outer diameter of the pipe section, so as to ensure that the target well section can be lowered through the upper casing or naked eyes during operation, and then it can be restored into a round pipe through hydraulic and mechanical expansion, so as to achieve the purpose of repairing the damaged outer casing, improving the bearing capacity of the formation and plugging the complex formation [1]. With the gradual expansion of the application field of expansion bellows technology, it is necessary to develop expansion bellows with high strength, high plasticity and high stability. It is considered that tubing is one of the important factors affecting the strength and performance of expanded bellows. In this paper, the matching between material composition and performance requirements is studied. By analyzing the main control factors of expansion bellows material, the interaction between key factors is studied, and the basic principles of material selection of expansion bellows are given. On this basis, the applicability of ferrite-martensite dual-phase steel in manufacturing expansion bellows is preliminarily explored. It provides reference for further exploring new applicable materials of expansion bellows and inspires new research ideas.

Material performance analysis and composition control of 1 expansion bellows

Up to now, great progress has been made in the engineering application of expansion bellows, but it still cannot meet the needs of oil exploration and development in complex geological and engineering environments. Due to the limitation of material selection, welding technology and processing equipment, it is difficult to obtain higher performance expansion bellows. Therefore, it is necessary to analyze the performance requirements of expansion bellows from the perspective of processing and construction technology, and put forward the requirements of material composition control.

1. 1 material characteristics analysis of expansion bellows

At present, the expansion bellows in China are all cold-pressed into circular tubes, which need to be lowered into the target layer and then expanded into circular tubes by hydraulic and mechanical means. Therefore, the pipe should meet the following requirements: (1) has good plastic deformation ability; (2) High tensile strength; (3) Low yield strength; (4) Higher work hardening index. In addition, because the expansion bellows need to be welded in the construction site, the pipeline is required to have good on-site welding performance. Data analysis shows that the C equivalent needs to be controlled within 0.46% [2]. Therefore, under the current construction process conditions, the material of expansion bellows is generally low-alloy high-strength steel or microalloyed steel [3]. Fine grain strengthening is the main strengthening method of low alloy high strength steel and microalloyed steel. The specific methods are as follows: in the aspect of alloying, a small amount of strong carbide forming elements such as Nb, V and Ti are microalloyed; Forming technology mainly includes: (1) refining austenite recrystallization grains and cooling ferrite grains by controlling finishing rolling temperature and cooling rate after rolling; (2) obtaining ultrafine grains through cyclic heat treatment; (3) Rapid austenitizing multistage heat treatment, etc. The strengthening effect of grain refinement can be expressed by Hall-Petch equation [4]:

Oil and gas accumulation theory and exploration and development technology (5)

Where σy is the yield stress; σ0 is lattice friction; K is a constant; D is the particle diameter.

According to the test results of low carbon steel, grain refinement will reduce the work hardening index n, and the relationship is as follows:

Oil and gas accumulation theory and exploration and development technology (5)

It shows that grain refinement improves the strength, yield ratio and work hardening index of materials. According to the special performance requirements of the above expansion bellows, it is necessary to strictly control the chemical composition of the material and the rolling and heat treatment process of the pipe to ensure that the performance of the pipe is controlled within a reasonable range.

1.2 material composition control requirements for expansion bellows

Through the analysis of the special properties of the above-mentioned expansion bellows materials, it is considered that reasonable control of the composition of bellows is the key factor to control its performance requirements. As mentioned above, the material of expansion bellows is generally low-alloy high-strength steel or microalloyed steel, and the carbon content is generally controlled within 0.2%. At this time, the steel pipe has good toughness and high elongation, which is suitable for cold pressing forming. Microscopically, manganese is a weak carbide-forming element, a little dissolved in cementite and most dissolved in ferrite, which has obvious solid solution strengthening effect on ferrite, can improve the strength of steel, weaken and eliminate the adverse effects of sulfur, improve the hardenability of steel and reduce the yield ratio of materials. When the manganese content is low (generally less than 1% ~ 1.5%), the impact toughness of steel increases with the increase of manganese content. When the manganese content is greater than 1% ~ 1.5%, the impact toughness decreases significantly with the increase of manganese content. In addition, too high manganese content will increase the cold crack sensitivity index of steel and reduce the weld toughness [3]. When the carbon content is less than 0.2%, the manganese content is in the range of 1% ~ 2%, which will not significantly affect the weldability of steel. Therefore, the manganese content in the expanded bellows material should be controlled within 2% as far as possible. Silicon is an element with obvious solid solution strengthening effect. Increasing the Si content in the material can reduce the yield ratio of the material, but too high Si content will significantly reduce the weldability of steel, so the Si content needs to be controlled within a certain range. In addition, by adding appropriate alloying elements such as Cr and Mo, the hardenability of steel can be improved and the yield ratio of steel can be reduced. Sulfur and phosphorus are harmful impurity elements in steel, which reduce the plasticity and toughness of steel and worsen the weldability. It is necessary to control the content of S and P in steel in a low range.

2 Applicability analysis of expansion bellows made of dual-phase steel

2. Advantages and application of1dual-phase steel material

Dual-phase steel has good strong-plastic matching and cold deformation properties. Compared with the commonly used low-alloy high-strength steel, dual-phase steel has lower yield ratio, higher elongation and higher work hardening rate at the same strength grade. These characteristics make it have good formability, especially suitable for cold forming such as cold drawing, cold rolling and cold stamping [5]. Compared with conventional low-alloy high-strength steel, dual-phase steel has high tensile strength, low yield ratio and no obvious yield platform. This is because the general low-alloy high-strength steel is composed of ferrite, pearlite, carbide and nitride precipitation in ferrite, and its strengthening mechanism is generally solid solution strengthening and fine grain strengthening which hinder dislocation movement, thus producing high yield strength and obvious yield platform. For dual-phase steel, in the process of partial austenitizing in dual-phase region, alloying elements such as C and Mn gather in austenite, and ferrite becomes pure matrix. After austenite transforms into martensite, the volume expands by 3% ~ 8%, which induces a large number of high-density movable dislocations near the interface between martensite and ferrite. These movable dislocations can move under the action of lower external stress, which makes the material yield. The existence of strong martensite can coordinate the stress concentration at the plug end, so dual-phase steel. In addition, the alloy content of dual-phase steel is low, so it has good weldability. At the same time, dual-phase steel has simple production process and low cost, and has broad development space as the material of expandable bellows.

Dual-phase steel has attracted great interest of petroleum engineering researchers because of its superior mechanical properties. In 2007, Yamazaki et al. published an American patent that used dual-phase seamless steel pipe as a solid expansion pipe, in which hot-rolled seamless steel pipe adopted conventional heat treatment or dual-phase heat treatment process [6]. Table 1 gives the chemical composition of the test steel tube, and Table 2 gives the microstructure and mechanical properties of the test steel tube after different heat treatment processes. As can be seen from Table 2, after dual-phase heat treatment and conventional heat treatment, the yield ratio of steel after dual-phase heat treatment decreases, and the uniform elongation and elongation after fracture increase.

Table 1 chemical composition mass fraction of test steel

Table 2 Mechanical properties of test steel tubes

Figure 1 Schematic diagram of dual-phase heat treatment process

2.2 Expansion bellows for straight weld of dual-phase steel

Compared with seamless steel pipe, straight welded pipe has the characteristics of uniform wall thickness and high ovality, and is more suitable for making expansion bellows in structure. According to the existing reports, there are two production processes to obtain dual-phase steel straight welded pipe at present. One process is to directly curl and weld dual-phase steel plates into steel pipes. In this process, it needs to be curled into a circular tube before welding. In the welding process, heat will change the structure of the welding heat affected zone, resulting in changes in mechanical properties, increased strength of the welding zone, decreased plasticity, and discontinuous microstructure and mechanical properties around the welded pipe. In addition, the plastic deformation process of rolled steel plate will not only produce work hardening effect on welded pipe, but also reduce the uniform elongation of welded pipe relative to the original plate, and the residual stress produced in the deformation process will also adversely affect the performance of welded pipe. Another process is to weld ordinary ferrite-pearlite steel plate into thin-walled tube, and then heat treat it to obtain dual-phase structure. E.J.Pavlina and others have successfully developed dual-phase steel pipes with ferrite and martensite by using the latter process. In the experiment, the author firstly rolled and welded1019 (Fe-0.19c) alloy sheet (microstructure is ferrite-pearlite) into a thin-walled tube with an outer diameter of 44.5 mm and a wall thickness of1.6 mm; ; Then the steel tube is completely annealed, and then the steel tube is heated to the critical temperature zone by an induction coil for isothermal quenching to obtain a ferrite-martensite dual-phase structure. The heat treatment process is shown in figure 1, and the microstructure of the weld and main area of the steel pipe after heat treatment is shown in figure 2. The analysis shows that the microstructure and properties of the welding affected zone are basically the same as those of other parts of the steel pipe. In addition, the author also carried out the laboratory test of hydraulic bulging of the trial-produced two-phase straight seam steel pipe. When the water pressure in the pipe reaches 54.4MPa, the steel pipe body explodes, but the weld is intact, which shows that the mechanical properties of the straight-seam steel pipe body are better than those of the steel pipe body [7].

Fig. 2 Dual-phase microstructure of Fe-0. 19c straight-seam steel pipe after dual-phase heat treatment.

2.3 Bauschinger Effect Analysis

Compared with other low-alloy high-strength steels, the expansion process of dual-phase steel obviously inhibits the Bauschinger effect of materials. The magnitude of Bauschinger effect is related to the composition, microstructure, strength and plastic deformation of materials. Therefore, Hitoshi Asahi and others have studied the influence of microstructure on the Bauschinger effect of straight welded pipe in detail [8]. In this experiment, a straight welded pipe with an outer diameter of 194mm and a wall thickness of 9.6mm was produced by hot rolling, and then heat treatment was carried out after rolling. Table 3 shows the chemical composition of the tested steel, and Table 4 shows the microstructure, Bauschinger effect ratio and collapse resistance pressure of straight welded pipes produced by different processes. The analysis shows that the larger the Bauschinger effect ratio (the difference between steel pipe 1 # and steel pipe 2 # is as high as 22%), the smaller the Bauschinger effect is. As can be seen from Table 4, the (4 #) Bauschinger effect of steel pipes with the same chemical composition is the most obvious, followed by ferrite-pearlite structure (2 #), and the Bauschinger effect of ferrite-martensite dual-phase structure is the smallest. The collapse resistance tests of 1 # and 4 # steel pipes were also carried out. Because of the small Bauschinger effect and high work hardening rate, the collapse strength of straight welded pipes with ferrite-martensite dual-phase structure is obviously higher than that of welded pipes with martensite structure. This experiment fully proves that the expanded tube with ferrite-martensite dual-phase structure is less affected by Bauschinger effect and has higher external extrusion strength after expansion. It is of great value to apply it to the manufacture of expansion bellows to improve the expansion process performance and the strength of the expansion bellows body.

Table 3 Mass fraction of chemical composition of test steel

Table 4 Microstructure and Mechanical Properties of Straight Welded Pipe

3 Conclusion

1) According to the special requirements in the process of processing and using expansion bellows, the pipe requires high tensile strength and plastic deformation capacity, low yield strength, high work hardening index and good welding performance. Among them, the weldability is one of the key factors affecting the improvement of the comprehensive properties of expanded bellows materials.

2) According to the performance requirements of expansion bellows, the carbon content of expansion bellows generally needs to be controlled within 0.2%, the Mn content within 2%, and the S and P contents within a lower range.

3) Ferrite-martensite dual-phase steel meets the requirements of chemical composition and mechanical properties of expanded bellows materials. Ferrite-martensite dual-phase structure can be obtained by heat treatment in critical zone for ordinary hot rolled steel pipes.

4) Compared with ordinary steel pipes, ferrite-martensite dual-phase steel pipes have the advantages of low yield ratio, high uniform elongation and high extrusion strength after expansion. Dual-phase steel has a broad application prospect as an expansion bellows material.

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