The influence of boss welding on the performance of HFW welded pipe
In order to solve the problem that the improper position of the boss welder affects the overall performance of the HFW welded pipe during the installation of valves, instruments and flanges, the influence of the direct welding of the boss on the original residual stress and microstructure of the HFW welded pipe was studied. Studies have shown that the overall residual stress value on the HFW resistance welded pipe is relatively small, and the residual stress value in the straight weld area is lower than the pipe body area; after boss welding, the axial residual stress on the straight weld near the cross weld and the circumferential residual on the pipe body The stress value increased sharply by 2.5 and 3.8 times, 444 MPa and 433 MPa, respectively; the intrusion width of the boss welding to the upper surface of the HFW welded pipe was 15.167 mm and the thickness was 3.376 mm, and chain bainite appeared in the intrusion area. In summary, it is recommended to use a smaller welding heat input during boss welding.
Stations and valve rooms are an indispensable part of oil and gas storage and transportation. In order to meet the installation needs of valves, instruments and flanges, the design of stations and valve chambers often encounters the situation of opening holes on the main pipe and welding bosses (branch seats) to connect the branch pipes [1-5]. The opening position of the boss should generally be offset 100mm from the longitudinal weld or spiral weld of the main manifold itself . However, in actual operation, because the welds of HFW resistance welded pipes are difficult to distinguish with the naked eye, the installation positions of some bosses are very close to or even overlap with the HFW welds.
When the boss installation weld and the HFW straight weld form a cross weld, the complex residual stress distribution near the intersection of the weld and the transformation of the structure of the pipe body and the heat affected zone of the weld after the multi-pass welding thermal cycle will affect the overall welded pipe Performance [7-9]. In this study, the small hole detection method was adopted. Aiming at the HFW resistance welded pipe commonly used in the station/valve room of the second West-East Gas Pipeline, the influence of the boss welding on the straight weld of the HFW welded pipe on the original residual stress of the pipeline was analyzed, and the boss was analyzed. The effect of welding on the organization of the pipe body and the heat-affected zone, the research results have a certain theoretical guidance for the correct design and guidance of the boss welding on the construction site.
Test materials and test methods
At present, the steel grade of HFW resistance welded pipe commonly used in the main pipelines of the first and second lines of the West-East Gas Pipeline is L415MB, with a specification of Φ406.4mm×12.5mm. The test welded pipe was taken from the HFW resistance welded pipe of the same steel grade and the same specification produced by Baosteel. Its chemical composition is shown in Table 1, and the tensile and impact properties of the material are shown in Table 2. The test boss selects a welding boss with a working pressure of 12MPa, material A350LF2, and specifications of DN400mm×25mm.
Table.1 Chemical composition of L415MB resistance welded pipe%
Table. 2 Mechanical properties of L415MB resistance welded pipe
Note: The yield strength is 0.5% EUL.
A small hole of Φ25mm is made by thermal cutting at the position of the straight weld of the HFW resistance welded pipe. According to the welding process specification of the station/valve chamber of the second West-East Gas Pipeline, the boss is welded by the welding method of argon tungsten arc welding (GTAW) for bottoming and electrode arc welding (SMAW) for filling the cover surface. The specific welding process is shown in Table 3. .
CM-1L-32 static resistance strain gauge and BE120-2CA-K strain rosette were used to measure the residual stress changes of HFW welded pipe before and after boss welding. During the test, combined with the specific size of the pipeline, a more reasonable arrangement was made for the position of the blind holes to ensure that the distance between the measuring points is above 12mm. Use OLS4100 laser confocal microscope, MEF4M metallurgical microscope and image analysis system to observe the influence of thermal cutting and multi-pass welding on the base metal and weld structure of HFW welded pipe.
Table.3 Boss welding process
|Weld bead||Welding material model||Welding material specification/mm||DC polarity||Welding current/A||Welding voltage/V||Welding speed/(cm/min)||Argon flow/(L/min)||Tungsten electrode extension length/mm||Nozzle aperture/mm||Argon arc length/mm|
Note: DCEN means that the electrode connected to the welding material is associated with the negative electrode of the power source; DCEP means that the electrode is associated with the positive electrode of the power source.
The influence of boss welding on the residual stress of HFW welded pipe
The original residual stress distribution of HFW resistance welded pipe is shown in Figure 1. It can be seen from Figure 1 that the axial residual stress is higher than the circumferential residual stress on the HFW resistance welded pipe, and the maximum axial and circumferential residual stress are 203MPa and 152MPa. There is no stress concentration in the straight weld area of HFW resistance welded pipe. The maximum axial and circumferential residual stress peaks on the straight weld are 178MPa and 114MPa, which are far lower than the yield strength of the material, which are 34% and 22% of the base material of the welded pipe. Compare the residual stress on the straight weld of the HFW welded pipe and the residual stress at a distance of 100mm from the straight weld, as shown in Figure 1(a). The axial and circumferential residual stress values in the area 100mm away from the straight weld are slightly larger than the residual stress on the straight weld. The maximum axial residual stress difference is 47MPa, and the maximum circumferential stress difference is 45MPa. It can be seen from Figure 1(b) that the residual stress is more evenly distributed at different distances from the straight weld. The overall rule is still that the axial residual stress is higher than the circumferential residual stress. The difference between the two is 45~55mm from the weld. The distance is smaller, but as the distance continues to increase, the difference between the two increases.
The boss welding changes the original distribution of residual stress on the HFW welded pipe, as shown in Figure 2. In Figure 2(a), the residual stress value on the straight weld of the HFW welded pipe near the boss rises sharply after the boss is welded, and the increase in the axial residual stress is much larger than the circumferential residual stress. The amplitudes of axial residual stress and hoop residual stress are 444 MPa and 201 MPa, respectively, which are about 84% and 38% of the yield strength of the base material, which are 2.5 and 1.4 times higher than the original peak distribution of HFW resistance welded pipe. In Figure 2(b), as the distance from the boss increases, the increase in residual stress gradually decreases. The range of stress concentration on the straight weld is about 92mm. Consistent with the change pattern on the HFW straight weld, the residual stress value of the pipe body at the edge of the boss also rises rapidly after boss welding. However, the increase of the circumferential residual stress in the welded pipe body is much larger than the axial residual stress. The amplitudes of axial residual stress and hoop residual stress are 256 MPa and 433 MPa, respectively, which are about 48% and 82% of the yield strength of the base material. Compared with the original peak distribution of HFW resistance welded pipe, the peak distribution has increased by 1.4 and 3.8 times. The range of hoop stress concentration is about 51mm.
Fig.1 The original residual stress distribution of HFW resistance welded pipe
Fig. 2 Residual stress distribution of HFW resistance welded pipe after boss welding
The influence of boss welding on the microstructure of HFW welded pipe
The body structure of HFW resistance welded pipe is a mixture of polygonal ferrite, pearlite and a small amount of granular bainite (PF+P+B grains), with a grain size of 10.6, as shown in Figure 3(a). The structure of the straight weld of the HFW welded pipe is a mixture of polygonal ferrite and a small amount of pearlite (PF+P), with a grain size of 10, as shown in Figure 3(b).
Fig. 3 Microstructure of HFW resistance welded pipe
After the boss is welded, the width of the upper surface of the HFW welded pipe is 15.167mm and the thickness is 3.376mm, which accounts for about 27% of the total wall thickness. After the boss is welded, the original tube body and straight weld structure in the invaded area disappears, and the structure is mainly transformed into the following three types: a small amount of surfacing weld structure, coarse heat-affected zone structure and fine-grained area structure, as detailed Shown in Figure 4.
Fig. 4 Organizational changes in the invasion area after boss welding
The weld structure of surfacing welding is coarse columnar crystals directly nucleated and grown on the base metal. The first eutectoid white ferrite outlines the grain morphology along the slender columnar grain boundary, and the grains are fine needles. The ferrite structure is shown in Figure 4(a). The coarse-grained structure of the HFW welded pipe body and the straight weld are respectively coarse granular bainite (as shown in Figure 4(b)), a mixture of a small amount of polygonal ferrite and coarse granular bainite (as shown in Figure 4 ( c) as shown). The intermittently distributed massive M-A at the grain boundaries of granular bainite are connected into a line, and a chain structure appears. The structure of the fine-grained region of the HFW welded pipe body and the straight weld is basically the same, which is a mixture of polygonal ferrite PF, quasi-polygonal ferrite QF and a small amount of pearlite at the grain boundary, as shown in Figure 4(d).
After the HFW welded pipe is welded, the structure and performance of the straight weld and the heat-affected zone are improved by online heat treatment. This results in the residual stress value of the straight weld and its surrounding area being lower than the pipe body, and the straight weld area of the HFW welded pipe does not exist Stress concentration in conventional welds. However, when the boss is opened and welded directly near the straight weld, the axial residual stress of the straight weld near the cross weld and the circumferential residual stress of the pipe body increase sharply by 2.5 and 3.8 times, respectively. That is to say, in the actual operation process, the straight weld in the 93mm area near the cross weld after the boss welding bears the internal pressure, but also bears the axial and circumferential tensile stress, which is extremely unfavorable to the actual operating safety of the pipeline [10 -13].
The original structure of HFW welded pipe is uniform and small, and the pipe body and weld seam have high impact toughness. After the boss welding thermal cycle, there are three types of microstructures in the 3.376mm thickness area. Except for the average microhardness of the fine-grain zone, which is the same as the base material, which is 180HV1, the hardness values of the other microstructures are all higher than those of the HFW welded pipe. 203HV1. The grain size of granular bainite and primary austenite in the coarse-grained area of the HFW tube body and the straight weld area is basically the same, but the shape, number and size of M-A are different. The chain-like M-A islands at the grain boundaries of the granular bainite grains in the coarse-grained region are very easy to form crack sources and crack propagation channels, which have a greater impact on toughness, especially low temperature toughness. Therefore, it is recommended to use a smaller welding heat input during boss welding and increase the welding cooling rate. This measure can avoid the formation of chain structure in the HFW welded pipe intrusion area [14-15].
(1) The overall residual stress value on the HFW resistance welded pipe is small, and the maximum axial and circumferential residual stress are 203MPa and 152MPa respectively. The residual stress value of the straight weld and its adjacent area is lower than the pipe body area.
(2) After the boss welding, the axial and circumferential residual stresses of the straight weld near the cross weld sharply increased by 2.5 and 3.8 times to 444 MPa and 433 MPa, respectively.
(3) After the boss is welded, the intrusion width of the upper surface of the HFW welded pipe is 15.167mm and the thickness is 3.376mm. The microhardness of the invaded area is higher than that of the original tube body, and the chain-like bainite structure appears in the coarse-grained area. It is recommended to use a smaller welding heat input during welding.
Author: HU Meijuan1, MIN Xihua2, LUO Jinheng1, SHAO Chunming2, ZHANG Jie2
Source: Network Arrangement – China Welded Pipe 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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