Процес на заваряване на плоча от супер дуплексна неръждаема стомана S32750

Using the combined welding method of GTAW+GMAW-P, the transverse and vertical welding positions of the 12 mm thick S32750 super duplex stainless steel plate were tested. The selection of welding methods, materials, grooves, process parameters, as well as the cleaning, preheating and interpass temperature control before welding were introduced in detail. After welding, tensile test, bending test, impact test and hardness test of the welded joint were carried out, the macro morphology of the joint was inspected, the ferrite content of the joint was measured, and the microstructure of the joint was observed. The results show that the adopted welding process has successfully passed the qualification requirements and can be used to guide the production implementation of the project.

S32750 super duplex stainless steel is composed of austenite phase and ferrite phase, so it combines the advantages of austenitic stainless steel and ferrite stainless steel. It not only has good toughness and strength, but also has excellent chloride corrosion resistance [1]. In addition, its high content of Cr, Mo and N elements extends the incubation period of pitting corrosion. Compared with other types of duplex stainless steel, it has better resistance to spot corrosion, crevice corrosion and stress corrosion [2]. It plays an important role in fields requiring high strength and corrosion resistance, such as petrochemical industry, shipbuilding and offshore oil and gas field exploitation [3]. For the project undertaken by our company, the welding procedure qualification of S32750 super duplex stainless steel is carried out in this paper.

1. Welding process test

According to the project requirements, project specifications and ISO15614-1 standards, the 12 mm thick S32750 super duplex stainless steel plate is used for welding procedure qualification at the horizontal welding position (PC) and vertical welding position (PF) respectively. See Table 1 and Table 2 for its chemical composition and mechanical properties.
Table 1 Chemical composition (mass fraction) of S32750 super duplex stainless steel (%)

Table.2 Mechanical Properties of S32750 Super Duplex Stainless Steel

1.1 Selection of welding methods
Except for electroslag welding, almost all fusion welding methods can weld super duplex stainless steel, such as SMAW, SAW, GTAW, GMAW, etc. [4]. In consideration of welding efficiency, on-site welding position and assembly quality, GTAW is selected for backing welding in process test. In order to control heat input and deformation, GMAW-P is selected for hot pass, filling and capping welding.
1.2 Selection of welding materials
A5.9ER2594 welding wire is selected, the ferrite content of deposited metal is 35% ～ 65%, PREN (Pitting Resistance Equivalent, N refers to nitrogen containing steel) ≥ 40, w (S) ≤ 0.010%. In the welding process test, the SMT-2594 welding wire with a diameter of 2.0mm is selected for backing welding, and the shielding gas is selected φ (Ar) 99.999% argon; Disk welding wire with diameter of 0.9mm shall be used for hot pass, filling and cover welding, and shielding gas shall be used φ (Ar)70%+ φ (He)30%。
To prevent oxidation of weld root, use φ (Ar) 99.999% argon for back protection. First of all, before welding, the front side of the groove is sealed with high temperature resistant tape, and the back side of the groove is set with stainless steel angle steel and high temperature resistant tape to set an air chamber. One end of the air chamber is inflated, and the other end is opened with a 4-7mm air outlet. The air inlet pipe should be set at a position lower than the air outlet. Secondly, use an oxygen detector to detect the gas in the air outlet before each welding, and the maximum oxygen content shall not exceed 0.05% (500 × 10-6)。 When welding the backing weld bead, tear the tape on the front of the groove for about 50 mm. After welding a section, tear another section. Do not tear all the tape on the front of the groove before welding. After the thickness of the deposited metal reaches 10mm, the back gas protection can be stopped.
1.3 Groove design
The groove form is shown in Figure 1, and the groove angle α 60 °± 5 °, the root gap R is 3 ～ 5mm, and the blunt edge f is 1 ～ 2mm. Plasma arc cutting is used for blanking, and the welding groove is prepared by cold processing. Before preparation, the groove side is removed by processing for about 2mm, and the heat affected zone of plasma arc cutting is removed.

Figure.1 Groove Form
1.4 Cleaning before welding
Before welding, use a special polishing disc to polish and trim the end face of the groove and the surface of the test panel within 25 mm from the edge of the groove, remove burrs, oxides, oil, paint, dirt, rust and other impurities on both sides of the groove until the metal luster is exposed, and use acetone or alcohol to wipe the test panel clean and degrease. Argon arc welding wire shall be placed in a special box, and each wire shall be cleaned and dried with acetone or alcohol before welding.
1.5 Determination of preheating and interpass temperature
The welding of super duplex stainless steel does not require preheating. However, in order to prevent the influence of moisture on the weld seam, oxygen acetylene large baking handle is used to heat and dehumidify the area about 75mm on both sides of the groove before welding. Do not use the cutter for preheating or dehumidification.
During welding, contact thermometer shall be used to detect the interpass temperature at the weld edge, and the interpass temperature shall not exceed 150 ℃. When the cooling is slow, dry compressed air can be used for auxiliary cooling.
1.6 Process parameters
The heat input of super duplex stainless steel shall not exceed 1.5kJ/mm, the thickness and width of each weld bead shall be controlled during welding, and the thickness shall not exceed 3mm, the weld bead width of GTAW shall not exceed 8mm, and the weld bead width of GMAW-P shall not exceed 12mm. The main welding process parameters of horizontal welding and vertical welding are shown in Table 3 and Table 4.
Table.3 Horizontal welding process parameters

Table.4 Vertical Welding Process Parameters

2. Performance test

After the completion of the welding process test, after passing the appearance inspection, X-ray flaw detection and surface dye penetrant flaw detection, the S32750 welding plate shall be subject to tensile test, bending test, impact test, hardness test, macro inspection, and metallographic examination according to the project specification and ISO15614-1 Specification and Qualification of Welding Processes for Metallic Materials – Welding Process Test Part 1: Electric Arc Welding and Gas Welding of Steel and Electric Arc Welding of Nickel and Nickel Alloys Ferrite content test and corrosion test.
2.1 Tensile test
Two full thickness transverse tensile specimens are taken from each welding test panel. Through testing, the transverse and vertical tensile specimens are fractured in the base metal. The measured tensile strength values are 843845845847MPa, which are higher than the minimum tensile strength of S32750 base metal. The tensile test results are qualified.
2.2 Bending test
Four lateral bending samples were taken from each welding test panel. The indenter diameter was 40mm, and the bending angle was 180 °. After the bending test, the external surface and side of the sample were inspected, and no cracks were found. The bending test result was qualified.
2.3 Impact test
According to the requirements of the project specification, take 3 groups of Charpy impact samples (10mm) from each welding plate × 10mm × 55mm), 3 for each group, the impact test temperature is – 46 ℃, and the impact notch is located at the weld center, weld fusion line and weld fusion line+2mm respectively. According to the test, the impact absorption energy of all samples reaches 45J required by the project. See Table 5 for specific data. The impact test results are qualified.
Table.5 Impact test results

2.4 Macro inspection and hardness test
One weld cross section is taken from each welding test panel. After polishing, it is corroded with ferric chloride hydrochloric acid solution. When the weld cross section clearly shows the distribution of weld, fusion line, heat affected zone and weld bead, macro inspection is carried out on the weld. After inspection, the welds are regular, without cracks, incomplete penetration, incomplete fusion and other welding defects, and the macroscopic inspection results are acceptable.
Vickers hardness test shall be carried out on the macro sample for the weld, heat affected zone and base metal of GTAW weld bead and GMAW-P weld bead. At least 3 points shall be tested in each area. The Vickers hardness of each area is lower than HV10340, meeting the requirements of the project specification, and the hardness test result is qualified.
2.5 Ferrite content test
Take one weld cross section from each welding test panel, and conduct hot corrosion with potassium ferricyanide+potassium hydroxide solution after polishing. The ferrite content of the weld and heat affected zone of the cover and root is calculated by using the grid point method, and the ferrite content is taken as the average value of the five visual fields. The ferrite content in each area is shown in Table 6, which is between 35% and 65%, meeting the requirements of the project specification.
Table 6 Ferrite content

2.6 Metallographic examination
One weld cross section is taken from each welding test panel, and after polishing, 40% water solution of w (KOH) is used for electrolytic corrosion. After corrosion, the metallographic structures of the cover and root welds, heat affected zone and base metal are observed. The microstructures of horizontal welding and vertical welding are shown in Fig. 2. Under 500 times, it is found that the microstructures of each part are austenite and ferrite with different distribution forms, no intergranular phase, carbide, nitride and other undesirable phases are found, and no obvious continuous precipitation phase is found at the grain boundary.

Figure 2. Microstructure
2.7 Corrosion test
Conduct corrosion test according to ASTM G48 method A to detect the weight loss rate and surface corrosion. The corrosion sample shall be 25mm along the weld length and 50mm vertically, and the weld shall be centered. After grinding, clean and decontaminate with alcohol solution, wash with clean water, dry in the air, and use 60 ℃ φ (HNO3)20%+ φ Pickle (HF) 5% solution for 5min, put it into w (FeCl3) 6% aqueous solution, keep the temperature at 40 ℃ for 24h, take it out for cleaning, drying, weighing and observation. The corrosion weight loss rate of horizontal welding sample and vertical welding sample is 0.094g/m2 and 0.140g/m2 respectively, the weight loss rate in accordance with BS EN ISO 17781 is less than 4.0g/m2, and the weight loss rate in accordance with the project specification is ≤ 1.0g/m2. The sample was magnified by 20 times and no pitting was found.

3. Заключение

Through the GTAW+GMAW-P welding process test and a series of performance tests on S32750 super duplex stainless steel, the results show that the properties of the transverse and vertical welding plates are qualified, meet the requirements of relevant standards, and successfully pass the welding process qualification test, which can be used to guide the production and construction of the project.

Authors: Liu Qin, Wu Fengmin, Wang Mao, Wu Jianxin, Xu Guangpei
Reference:

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