Ø1219mm, Ø1422mm X80 steel induction bends
At present, China’s major domestic pipeline projects basically use X80 steel grade Ø1219mm large wall thickness bends, and X80 steel grade Ø1422mm hot simmering bends have high steel grades, large diameters, large wall thicknesses, and complex manufacturing processes, filling the domestic gap. In the development, the technical problems of chemical composition design and strength control of thick-walled bends have been solved, and the internal and external synchronous cooling technology has been innovated. The overall heating and simmering technology and the final tempering heat treatment process ensure the overall performance of the bend. The bend has high strength and toughness, good weldability, and long service life. The average yield strength of the pipe body exceeds 580MPa, the average tensile strength exceeds 650MPa, the yield ratio is less than 0.90, and the average impact toughness of base metal at -20℃ exceeds 150J, -20℃ The impact toughness of the weld is over 100J on average, and the performance and quality have reached the international advanced level.
At present, X80 steel grade Ø1219mm large wall thickness induction bends are widely used in major pipeline engineering projects. X80 steel grade Ø1422mm processing and manufacturing technology is still blank in China, and there is no relevant mature technology abroad.
In order to meet the needs of engineering construction, the Pipe Fittings Branch of CNPC. took the lead in developing X80 steel grade Ø1422mm bends, which are currently the highest grade, largest wall thickness, and largest diameter bend product.
During the development process, the chemical composition of the special longitudinal submerged arc welded pipe for induction bends was successfully designed by studying the bend mother pipes of various steel mills, and the reasonable parameter range of the simmering forming process was determined by the control variable method. The internal and external synchronous cooling technology has developed the overall heating and simmering technology and the final tempering heat treatment process to ensure the overall performance of the induction bend.
Induction Bending is a controlled means of bending pipes through the application of local heating using high frequency induced electrical power.
Originally used for the purpose of surface hardening steels, induction technology when used in pipe bending consists basically of an induction coil placed around the pipe to be bent. The induction coil heats a narrow, circumferential section of the pipe to a temperature of between 850 and 1100 °C (dependant on the material to be formed). As the correct bending temperature range is reached, the pipe is moved slowly through the induction coil whilst the bending force is applied by a fixed radius arm arrangement.
Induction bends are formed in a factory by passing a length of straight pipe through an induction bending machine. This machine uses an induction coil to heat a narrow band of the pipe material. The leading end of the pipe is clamped to a pivot arm.
As the pipe is pushed through the machine, a bend with the desired radius of curvature is produced. The heated material just beyond the induction coil is quenched with a water spray on the outside surface of the pipe. Thermal expansion of the narrow heated section of pipe is restrained due to the unheated pipe on either side, which causes diameter shrinkage upon cooling.
The induction bending process also causes wall thickening on the intrados and thinning on the extrados. The severity of thickening/thinning is dependant on the bending temperature, the speed at which the pipe is pushed through the induction coil, the placement of the induction coil relative to the pipe (closer to the intrados or extrados), and other factors.
Most induction bends are manufactured with tangent ends (straight sections) that are not affected by the induction bending process. Field welds are made or pipe pup sections are attached to the unaffected tangent ends, allowing for fitup similar to that found when welding straight sections of pipe together.
Induction bends come in standard bend angles (e.g. 45° bend, 60° bend, 90° bend, etc.) or can be custom made to specific bend angles. Compound bends (out-of-plane) bends in a single joint of pipe can also be produced. The bend radius is specified as a function of the diameter. For example, common bend radii for induction bends are 3D, 5D and 7D, where D is the nominal pipe diameter. (from http://www.wermac.org/fittings/hot_induction_bends.html)
|Hot induction bending of 48 inch line pipe at Mannesmann||View on induction coil and heated zone during bending|
Benefits of Induction Bends
- Large radii for smooth flow of fluid.
- Cost efficiency, straight material is less costly than standard components (e.g. elbows) and bends can be produced faster than standard components can be welded.
- Elbows can be replaced by larger radius bends where applicable and subsequently friction, wear and pump energy can be reduced.
- Induction bending reduces the number of welds in a system. It removes welds at the critical points (the tangents) and improves the ability to absorb pressure and stress.
- Induction bends are stronger than elbows with uniform wall thickness.
- Less non-destructive testing of welds, such as X-ray examination will save cost.
- Stock of elbows and standard bends can be greatly reduced.
- Faster access to base materials. Straight pipes are more readily available than elbows or standard components and bends can almost always be produced cheaper and faster.
- A limited amount of tools is needed (no use of thorns or mandrels as required in cold bending).
- Induction bending is a clean process. No lubrication is needed for the process and water needed for the cooling is recycled.
The tensile properties of the bend are shown in Table 1. The Charpy impact performance (-45～60℃) of the bent pipe is shown in Table 2.
X80 steel grade Ø1219mm, Ø1422mm induction bend series products adopt integral heating and simmering technology and tempering heat treatment process to ensure the overall performance of the bend. It has the characteristics of high strength and toughness, good weldability, and long service life. The bend body The average yield strength is more than 580MPa, the average tensile strength is more than 650MPa, the yield ratio is less than 0.90, the average impact toughness of the base metal at -20℃ exceeds 150J, and the average impact toughness of the weld at -20℃ exceeds 100J.
Table.1 Tensile performance of bend
|Sample position||Yield strength (MPa)||Tensile strength (MPa)||Flexion ratio||Elongation(%)|
|Bend body||555 ～ 705||≥ 620||≤ 0.94||≥ 16.0|
Table.2 Charpy impact performance of bend
|Sample position||Fracture shear area (SA%)||Impact power (J)|
|The minimum value of a single sample||Average of 3 samples||The minimum value of a single sample||Average of 3 samples|
|Bend body||For reference||For reference||≥ 65||≥ 90|
|Weld and hot zone||For reference||For reference||≥ 60||≥ 80|
Main technical achievements
Design of chemical composition of bend main pipe
The chemical composition and mechanical properties of the X80 pipeline steel materials selected for the test are shown in Table 3 and Table 4.
Table 3 Chemical composition of test steel
|Test material number||C||Mn||Si||Nb||V||Ti||Al|
|Test material number||N||Cu||Cr||Mo||Ni||CEpcm||CE Ⅱ w|
Table 4 Mechanical properties of test steel
|Test material number||R t0.5/(MPa)||R m/(MPa)||R t0.5 / R m||Elongation A (%)||-20℃|
|Akv (J)||SA (%)|
It can be seen from Table 3 that this X80 coil belongs to low carbon, Nb, Ti microalloyed high Mo controlled rolled steel, also known as HTP steel. In order to delay the recrystallization in the austenite region when the steel is heated, the austenite is deformed into a cake shape, and fine ferrite and bainite grains are formed after quenching, and an appropriate amount of Nb is added; in order to suppress the austenite when the structure is heated The excessive growth of the austenite grains makes the austenite grains smaller before the phase transformation, which lays the foundation for obtaining a fine structure after quenching, and an appropriate amount of Ti is added; Mn, as the main alloying element of pipeline steel, can cause solid solution strengthening, Reduce the g-a transformation temperature of steel, refine the grain size of α, and change the microstructure after phase transformation. Studies have shown that adding 1% to 1.5% of Mn can reduce the g-a phase transition temperature by 50°C.
The Mn content of X80 grade pipeline steel is increased to 1.65% to 1.8%, which can further reduce the phase transition temperature and promote the transformation of acicular ferrite. It not only increases the strength of X80 pipeline steel, but also improves the toughness of the steel and reduces the ductile-brittle transition temperature of the steel. X80 pipeline steel has a high content of Mo, and an appropriate increase in the content of Mo can enhance the strength and hardenability of the steel, reduce the phase transformation temperature, and make the steel have an obvious deformation strengthening effect, thereby compensating for the Bauschinger effect. The strength loss. Mo can increase the precipitation strengthening effect of Nb and shift the C curve to the right during phase transformation to increase the amount of bainite and increase the strength and toughness. In addition, Mo can improve the tempering stability of steel. When Mo coexists with elements such as Cr and Mn, it can reduce or inhibit the tempering stability caused by other elements.
The optical metallographic structure of X80 pipeline steel base material is shown in Figure 1 and Figure 2. Its main structure is the combination of acicular ferrite (AcicularFerrite) and fine dispersed granular bainite (GranularBainite) and a small amount of banded organization. Acicular ferrite has the structural characteristics of small equivalent grain size, fine substructure and high density of movable dislocations. This structure can maximize the strength of the material and the yield ratio is close to 0.85.
Figure 1 The microstructure of the base metal center of X80 steel pipe
Figure 2 The surface microstructure of X80 steel pipe base metal
Integral heating and simmering technology of bend
Through a series of intermediate frequency induction heating temperature tests, a series of water pressure tests and a series of simmering speed tests, the process parameters such as the heating temperature, cooling water pressure and simmering speed of the bend were determined, and a test bend was simmered accordingly; After optimizing the intermediate frequency heating frequency, a test bend was simmered; after adopting the internal and external synchronous cooling technology, the test bend was simmered again. The test results are shown in Table 5.
For the test tubes of the above three simmering schemes, two rod-shaped specimens of f12.7mm were processed on the outside, middle and inside of the sample plate, respectively, and the tensile test was performed again. The results are shown in Table 6. Based on the above table, it can be seen that before the intermediate frequency heating frequency is optimized, the various indexes of the test tube meet the requirements, but the yield strength margin is small, and the tensile strength of the bent tube gradually decreases from the outer wall to the inner wall. The reasons for this situation are: one is that the temperature of the inner and outer walls of the bend is uneven due to the high heating frequency; the other is that the cooling of the bend is transmitted from the outer wall to the inner wall, and the outer wall is cooled sufficiently, and the cooling close to the inner wall is seriously insufficient, so The strength near the outer wall is higher, and the strength near the inner wall is lower. After optimizing the intermediate frequency heating frequency, the difference in strength between the inner and outer walls is reduced. After adopting internal and external synchronous cooling technology, the strength of the inner and outer walls is basically the same.
Table 5 Tensile test results
|Sample position and direction||Tensile test after determining the basic process parameters||Tensile test after optimizing intermediate frequency heating frequency||Tensile test after adopting synchronous cooling technology|
|Yield strength (MPa)||Tensile strength (MPa)||Yield strength (MPa)||Tensile strength (MPa)||Yield strength (MPa)||Tensile strength (MPa)|
|Left transition zone||Outer arc side||Longitudinal||580||670||590||690||580||670|
|Right transition zone||Outer arc side||Longitudinal end bend||585||680||585||680||585||680|
|Bending zone||Inner arc side||Vertical||580||680||575||670||580||680|
|Outer arc side||Vertical||590||685||585||680||590||685|
|Vertical weld horizontal||670||680||670|
|Induction for pipeline engineering||Bend body||≥ 555||≥ 620||≥ 555||≥ 620||≥ 555||≥ 620|
|Technical conditions of heating bend||Horizontal weld||—||≥ 620||—||≥ 620||—||≥ 620|
Table 6 Tensile test results of repeated samples
|The location of the sample||Tensile test after determining the basic process parameters||Tensile test after optimizing intermediate frequency heating frequency||Tensile test after adopting synchronous cooling technology|
|Yield strength (MPa)||Tensile strength (MPa)||Yield strength (MPa)||Tensile strength (MPa)||Yield strength (MPa)||Tensile strength (MPa)|
|To the outside||600||690||610||690||600||690|
The production process of the bend is a closed-loop system. The controlled variable method is adopted to study the influence of the tempering process of the X80 bend on its performance, that is, set the bending temperature, speed and cooling water pressure unchanged, and conduct a series of temperature tempering tests. To determine the law. First, the test pipe section is heated to 1050°C with medium frequency induction heating equipment and quenched, and then tempered at different temperatures at 470-650°C. The strength, toughness and hardness test results of the bend after 1h are shown in Table 7. It can be seen that after tempering at different temperatures, the mechanical properties of X80 bends change greatly. A careful analysis of the test results can reveal:
- (1) Compared with the untempered sample, the strength and toughness of the bend after tempering have been significantly improved, and the hardness has decreased significantly.
- (2) Tempering between 470～600℃, as the tempering temperature increases, the yield strength of the bend increases, the tensile strength decreases, the hardness decreases to a certain extent, and the impact toughness improves . Tempering between 540°C and 560°C can obtain a better combination of strength and toughness, and the comprehensive mechanical properties of the bend are good.
- (3) When tempering above 600℃, the strength and toughness of the bend will decrease significantly.
Table 7 X80 bend mechanical properties under different tempering temperatures
|Tempering temperature (℃)||R t0.5 (MPa)||R m (MPa)||R t0.5/R m||Elongation A (%)||-20℃||Average hardness (HV10)|
|A kv（J）||S A（%）|
Indoor experiment and field application
Based on the successful trial production of a single bend, a small batch trial production of X80 steel grade Ø1219mm and Ø1422mm induction bends was carried out.
In accordance with the requirements of “Technical Conditions for Induction Heating Bends for the Second West-East Gas Pipeline Project” (Q/SYGJX0129—2008) and “Technical Conditions for OD1422mm IB555 Induction Heating Bends for Natural Gas Transmission Pipelines“, a comprehensive quality inspection of trial-produced bends , And sent to the China Petroleum Pipe Research Institute for testing, the testing proved that all performances meet the standard requirements.
The pipe fittings branch has a production capacity of 2×104t/a, and has supplied 5,605 X80 steel grade induction bends, totaling 10471t, with sales of 154.4 million yuan and a profit of 23 million yuan.
With the development of the national economy and the implementation of the energy strategy, the development of large-scale gas transmission pipeline projects with an annual gas transmission capacity of more than 400×108 m3 is imminent. Technical innovations such as increasing the steel grade of pipes, increasing the transmission pressure and the diameter of steel pipes are effective ways to reduce the cost of long-distance natural gas transportation. X80 steel grade Ø1219mm induction bends were successfully applied to the second West-East Gas Pipeline project, which not only ensured the smooth progress of pipeline construction, but also realized the localization of bends for engineering construction, saving a lot of foreign exchange for the country.
X80 steel grade Ø1422mm induction bend is the first in China and is at the international advanced level. Its successful development has enabled my country’s bend manufacturing level to achieve a historic leap, and its application prospects are broad.
Author: Fu Yanhong, Bai Fuliang, Peng Lishan, Zhao Zhiwe, Li Zhengkun
Source: Network Arrangement – China Induction Bends 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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