Automatic TIG welding process of duplex stainless steel welded pipe
S22053 Φ 89 mm × Taking 6 mm duplex stainless steel welded pipe as an example, the automatic tungsten argon arc welding process is introduced. This process can realize 360 ° continuous welding in circumferential direction. The welding quality is guaranteed by adopting multi-layer and multi pass welding and controlling interlayer temperature. Visual inspection, nondestructive testing, mechanical property test, harmful interphase test, ferrite content test and chloride stress corrosion cracking resistance test are carried out on the welded joints. The test results can meet the standard requirements. The results show that the automatic TIG welding process can ensure the welding quality of duplex stainless steel welded pipe joint, reduce the labor intensity of welders, reduce the pause in the welding process and improve the construction efficiency.
Duplex stainless steel is a kind of stainless steel whose microstructure is composed of ferrite and austenite, and the ratio of two phases is close to 1:1. Due to the dual phase microstructure of ferrite and austenite, duplex stainless steel combines the advantages of ferrite and austenite stainless steel. It is a structural and functional integration material with high strength and good corrosion resistance. It is widely used in petroleum, chemical industry, marine, energy, construction and other industries, and has become a research hotspot in the field of corrosion resistant alloys in recent years .
Duplex stainless steel can be divided into economical (low alloy) duplex stainless steel (2101, 2304), standard duplex stainless steel (2205) and super duplex stainless steel (2507) according to its alloy composition and properties. Among them, the research and development of standard duplex stainless steel material has been relatively mature. This material not only has the advantages of high thermal conductivity, low linear expansion coefficient, pitting corrosion resistance and stress corrosion resistance of ferritic stainless steel, but also has the advantages of plasticity and toughness, intergranular corrosion resistance, good mechanical properties and welding properties of austenitic stainless steel .
The welding of duplex stainless steel is mainly manual TIG welding and electrode arc welding. However, when the welding heat input is too large, the adjacent base metal will change color, which will seriously affect the corrosion resistance of duplex stainless steel, thus affecting the operation safety and service life of duplex stainless steel welded pipeline. The heat input of manual welding is greatly affected by operation and is difficult to control. The heat input is mostly controlled by cooling after welding for a period of time, which seriously affects the construction efficiency. With the rapid popularization and application of automatic welding process in recent years, the research on automatic welding process of duplex stainless steel is also carried out. Taking s22053 as an example, this study introduces the automatic tungsten argon arc welding process of duplex stainless steel.
The test material is s22053 (022cr23ni5mo3n) standard duplex stainless steel, with yield strength ≥ 485 MPa and tensile strength ≥ 655 MPa. See Table 1  for its chemical composition.
Table.1 chemical composition of s22053 duplex stainless steel%
According to the principle of equal composition, ER2209 solid welding wire with Ferritic and austenitic cladding metal structure and similar chemical composition to s22053 duplex stainless steel is selected as the welding material, with tensile strength ≥ 690 MPa. See Table 2  for chemical composition.
Table.2 chemical composition of er2209 welding wire%
The welding material used in the test is Φ 89 mm × 6 mm s22053 duplex stainless steel welded pipe. The pipe end groove shall generally be machined, and the equipment for machining the groove shall be cleaned before use to prevent the pollution of harmful substances to the material. When the machining method cannot be used, plasma cutting can be used, but effective protective measures must be taken for the material surface. The groove shall be polished smooth with grinding wheel, and the oxide layer shall be removed at least 1 mm. Liquid penetrant flaw detection was performed on the groove, and there was no relevant indication . The finished groove size is shown in Figure 1.
Fig.1 Schematic diagram of groove form and size
The assembly clearance shall be kept within the range of 0 ~ 0.5 mm. During assembly, the amount of misalignment shall be reduced as much as possible. The misalignment shall be evenly distributed along the circumference, and the hammer method shall not be used to correct the misalignment. The tools used in the assembly process shall prevent the pollution of copper, iron ions and other low melting point metals near the weld.
Generally, preheating is not required for the welding of duplex stainless steel, but when the welding ambient temperature is lower than 10 ℃, the range of 100 mm on both sides of the groove shall be preheated to 50 ~ 80 ℃ . When flame preheating is adopted, oxidation flame shall be adopted to prevent carbon pollution at the welding part. Before welding, the air in the pipe shall be completely replaced by argon filling, and the argon filling state shall be maintained throughout the welding process. Before welding, the welding equipment shall be adjusted and the welding process parameters shall be set. Automatic tungsten argon arc welding shall be adopted for circumferential uninterrupted welding. The welding process requirements shall be strictly followed. Multi layer and multi pass welding shall be adopted. The temperature between layers (passes) shall not be greater than 100 ℃ [7-8]. The welding passes are shown in Figure 2. Before each weld pass is welded, the spatter, slag and surface defects on the surface of the previous weld pass must be removed. The tools used for weld bead cleaning (including flat shovel, grinding wheel and wire brush) shall be made of stainless steel and shall not be mixed with carbon steel, low alloy steel or other non-ferrous metal materials .
Figure. 2 Φ 89 mm × Schematic diagram of welding passes of 6 mm duplex stainless steel welded pipe
use Φ For 3.2 mm cerium tungsten electrode welding, pulse current mode is adopted for root welding to ensure good weld back shape, and constant current mode is adopted for filling cover welding. See Table 3 for welding process parameters.
Table 3. Welding process parameters of automatic TIG welding of s22053 duplex stainless steel welded pipe
Appearance inspection and NDT
The weld surface shall be visually inspected, and the weld shall be well formed, and the surface shall be free of cracks, pores, slag inclusions, craters, undercuts, pits, incomplete welding and other defects. Smooth transition between weld and base metal, and the reinforcement of weld surface is 1.0 ~ 1.5 mm. NDT is carried out in accordance with Nb / T 47013-2015 nondestructive testing of pressure equipment. The test results are grade I and meet the standard requirements (not lower than grade II).
According to the requirements, sample and process  from the flat welding position and overhead welding position respectively, and complete the tensile test as required. See Table 4 for the test results. It can be seen from table 4 that the tensile test results meet the standard requirements.
Table.4 tensile test results
According to the requirements, samples shall be taken from the 45 ° position of the pipe circumference, and the back bend and face bend samples  shall be processed. The tensile test shall be completed as required. See Table 5 for the test results. It can be seen from table 5 that there is no obvious defect in the bending sample, which meets the standard requirements.
Table 5 bending test results
|Style number||Inspection items||Inspection results||Standard requirements|
|F1||Face bending||No obvious deficiency is found||There shall be no single opening defect with a length greater than 3mm along any direction in the tensile surface weld and heat affected zone.|
|F2||Face bending||No obvious deficiency is found|
|R1||Back bending||No obvious deficiency is found|
|R2||Back bending||No obvious deficiency is found|
Take the hardness sample of the whole weld section as required. After the sample is polished, corrode the weld cross section with corrosive agent to determine the position of the weld, fusion line and heat affected zone . The position of the hardness test point is shown in Figure 3. The hardness test results are shown in Table 6. The maximum hardness value is 286hv10, which does not exceed 300hv10 specified in the standard.
Fig.3 location of hardness test
Table.6 hardness test results
Harmful interphase detection (pitting test)
After the samples were tested in FeCl3 solution at 22 ℃ for 24 hours , the corrosion rates of the three parallel samples were 0.90 MDD, 9.25 MDD and 1.49 MDD respectively, and the average corrosion rate was 3.88 MDD, which was less than 10 MDD required by the standard, and there was no pitting corrosion on the surfaces of the three samples. See Table 7 for the test results.
Table.7 test results of harmful interphase detection
Determination of ferrite content
Observe the metallographic structure of base metal, heat affected zone and weld of three parallel samples (as shown in Fig.4), and no obvious harmful phase is found. The ferrite content of base metal, heat affected zone and weld is measured by quantitative metallographic image analysis software . The measurement results are shown in table 8, which meets the requirement that the ferrite content of weld metal is 30% ~ 60% The ferrite content in heat affected zone shall be 30% ~ 70%.
Table 8 ferrite content determination test results
Fig.4 metallographic structure of weld zone
Chloride stress corrosion cracking resistance test
After degreasing and drying the sample, measure the size of the sample, load the sample on the fixture and load it to 242.5 MPa. Prepare 25% MgCl2 of the test solution and heat it to complete boiling, put in the stressed sample, and keep the solution boiling throughout the test . After 96 h test, no fracture occurred to all samples and no crack was found on the tensile surface. The macro morphology of the samples after the test is shown in Fig.5.
Fig.5 macro morphology of sample after chloride stress corrosion cracking resistance test
Through the appearance inspection, nondestructive testing, mechanical property test and corrosion resistance test of welded joints, the results show that the automatic tungsten argon arc welding process can ensure the welding quality of duplex stainless steel welded pipe joints, reduce the labor intensity of welders, reduce the pause in the welding process, and improve the construction efficiency. It has the value of popularization and application.
Authors: Shao Hongbo, Chen Jiaxing, Zhang Xilei
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry (www.pipelinedubai.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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