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Effect of solution treatment temperature on microstructure and properties of 2507 stainless steel welded joint

In view of the deterioration of the properties of 2507 super duplex stainless steel multi-layer welded joint by plasma welding (PAW) + tungsten gas shielded welding (GTAW), the effects of solution heat treatment temperature on the microstructure and properties of 2507 super duplex stainless steel welded joint were studied by means of tensile test and microstructure analysis. The results show that there is brittleness in the microstructure when the solution temperature is 1050 ℃ σ The tensile and bending of the welded joint are brittle fracture, and the fracture occurs in the heat affected zone near the weld fusion line; When the solution temperature rises to 1100 ℃, brittleness decreases σ The phase disappears, the weld obtains a uniform austenite + ferrite dual-phase structure, the tensile and bending properties meet the requirements of the technical specifications, and the tensile fracture occurs in the base metal. The results show that plasma backing and multi-layer TIG welding can realize the welding of 2507 super duplex stainless steel; 1100 ℃ and holding for 15min can be used as the post weld solid solution treatment process of 2507 super duplex stainless steel.

In recent years, super duplex stainless steel has been widely studied and applied in petrochemical industry to solve the problem of CL corrosion. 2507 super duplex stainless steel is composed of austenite and ferrite, and the content of austenite accounts for about 40% ~ 60%, which makes it have both good mechanical properties of austenitic stainless steel and good corrosion resistance of ferritic stainless steel. Compared with the typical 2205 duplex stainless steel, 2507 is a nickel saving super duplex stainless steel. On the basis of 2205, the contents of Cr, Mo and Ni are increased, and a small amount of Cu is added, so that 2507 super duplex stainless steel has excellent mechanical and corrosion resistance [1-4]. After welding, the joint of 2507 super duplex stainless steel has the problem of uneven microstructure. In order to meet the application requirements in specific environment, it is necessary to improve the microstructure of the weld by adding post weld heat treatment process in actual production to improve its service performance.
This study takes 2507 super duplex stainless steel as the research object, analyzes the influence of post weld heat treatment process on weld microstructure and properties, and obtains a more reasonable heat treatment process through experiments, so as to provide technical support for the production process formulation of 2507 super duplex stainless steel welded pipe in the future.

Test materials and methods

Test materials

The test material is 2507 super duplex stainless steel hot rolled steel plate produced by Shanxi TISCO Stainless Steel Co., Ltd. its chemical composition and mechanical properties are shown in table.1 and table.2.
Table.1 2507 super duplex stainless steel chemical composition%
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Table.2 Mechanical properties of 2507 super duplex stainless steel
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Welding process and materials

2507 super duplex hot rolled stainless steel plate was processed into 450mm size × 150mm × 14mm welding test plate, the weld adopts Y-shaped groove, as shown in Figure.1 (a), the blunt edge size is 4mm, and the unilateral groove angle is 37.5 °. Plasma welding (PAW) backing + tungsten argon arc welding (GTAW) multi-layer and multi pass filling cover welding is adopted for welding, and the filler wire is adopted Φ The chemical composition of 1.2mm solid core welding wire er2594 is shown in Table 3, and the distribution of welding passes is shown in Figure.1 (b).
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Figure.1 Schematic diagram of 2507 stainless steel welding groove and welding sequence
Table.3 Chemical composition of ER2594 solid core welding wire%
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The welding test is completed on the side beam double gun P + T longitudinal circumferential seam welding system TETRIX 522D-P and ETRIX 521 TIG. The power supply is AC/DC1000 and the welding position is flat welding. See Table 4 for welding process parameters. After the test plate is welded, visual and X-ray real-time imaging inspection shall be carried out. The test results show that the weld has no defects and the quality meets the class I standard.
Table.4 Welding process parameters of 4 2507 stainless steel test plate
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Note: the last TIG welding adopts lateral swing process, with swing width of 25mm.

Solution heat treatment and performance test

After welding, the test plate shall be subject to solid solution treatment at 1050 ℃ and 1100 ℃ respectively, and water cooling shall be carried out quickly after holding for 15min. After solution treatment, the test plate is processed into tensile and bending samples according to GB/T228.1 [5] and GB/T232 [6] respectively. The tensile property test is completed on the GNT300 electronic tensile testing machine produced by steel yannak Testing Technology Co., Ltd., with a loading rate of 5mm/min. The diameter of the bending shaft in the bending test is 40mm and the bending angle is 180 °.

Test results and analysis

Mechanical properties

Table 5 shows the tensile test results of welded joints of 2507 stainless steel plate, and Figure.2 shows the macro photos of tensile fracture samples. After 1050 ℃ solution treatment, the tensile fracture occurs at the weld metal fusion line (Fig.2 (a)), the elongation is almost zero, the fracture stress is also low, and the test result of Rp0.2 is 0. After 1100 ℃ solution treatment, the tensile fracture is in the base metal, and its yield strength, tensile strength and elongation meet the requirements of GB/T3527 (Rp0.2 ≥ 550MPa, RM ≥ 795mpa, a ≥ 15%).
Table.5 Tensile test results of 2507 stainless steel plate welds

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Figure.2 macro photos of tensile specimens of 2507 stainless steel welded joints under different solution treatment processes
By comparing the surface colors of the samples treated with different solid solutions in Fig.2, it can be seen that the heat treatment temperature and holding time of the sample in Fig.2 (a) are insufficient, the color is light, which is close to the color of the original hot-rolled plate, and there is almost no oxide scale; Figure.2 (b) the color of the sample is the color of the normal heat treated surface. According to the test results in Table 5, the yield strength, tensile strength and elongation of the sample in Figure.2 (b) meet the requirements, while the sample in Figure.2 (a) breaks without extension, indicating poor toughness.
Fig.3 shows a macro photograph of the specimen after the bending test. When the solid solution temperature is 1050 ℃, the bending test breaks almost without deformation, and the fracture breaks along the metal edge of the fusion line. On the contrary, when the solution temperature is 1100 ℃, after 180 ° bending deformation, there are no cracks on the inner and outer surfaces of the sample, showing good plasticity.
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Figure.3 macro photos of bending specimens of 2507 stainless steel welded joints under different solution treatment processes

Microstructure analysis of welded joints

Fig.4 shows the microstructure of 2507 welded joint. Fig.4 (a) shows the microstructure of the fusion zone, heat affected zone and base metal of the welded joint when the solid solution temperature is 1050 ℃. According to the observation, the microstructure characteristics of the fusion zone are obvious, indicating that the solid solution treatment is not in place and the microstructure of the welded joint is not homogenized. The microstructure of the base metal zone is composed of austenite and ferrite. The austenite is distributed on the ferrite matrix in a strip shape. The two phases have uniform microstructure, accounting for about 50% each; There is a hot zone between austenite and austenite σ Phase exists (in the upper right corner of Fig.4 (a)), and the heat treatment fails to make σ The phase disappeared completely; Although the semi melting and semi condensing solid structure in the fusion zone is composed of austenite and ferrite, the two-phase structure is uneven and there are cracks at the grain boundary σ Phase precipitation, so its plastic toughness is poor. Fig.4 (b) shows the weld structure when the solution treatment temperature is 1050 ℃. Coarse ferrite and lamellar austenite can be obviously observed, so its plastic toughness is poor. According to Fig.4 (a) and Fig.4 (b), the poor plastic toughness of the heat affected zone and weld of the welded joint is the main reason for the fracture of the tensile test sample in the heat affected zone. Figure.4 (c) shows the microstructure of 1100 ℃ solution treated weld. Austenite is distributed in ferrite matrix in strips and blocks. The proportion of austenite in the weld increases and the structure is uniform, so its plastic toughness is good. Fig.4 (c) the reason for the improvement of microstructure is that the increase of solid solution temperature accelerates the transformation from ferrite to austenite, so that the austenite precipitated too late during post weld cooling transforms and precipitates during post weld solid solution treatment, and homogenizes the weld structure. The content of ferrite in two phases accounts for about 40%, so it shows good plastic toughness during tensile test [7-10].
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Figure.4 microstructure of 2507 stainless steel welded joints under different solution treatment processes

Microstructure of heat affected zone

In order to further analyze the relationship between microstructure and mechanical properties of welded joints, the microstructure of heat affected zone under different solution treatment processes was compared and analyzed again. Fig.5 (a) shows the microstructure of the heat affected zone of the welded joint after 1050 ℃ solution treatment. It can be seen that island austenite is distributed on the ferrite matrix, the hot-rolled banded structure is obvious, and the third phase is distributed in a continuous network along the austenite grain boundary, resulting in increased brittleness; Fig.5 (b) shows the microstructure of the heat affected zone of the welded joint after solution treatment at 1100 ℃. It can be seen that austenite is distributed on the ferrite matrix, and the proportion of austenite and ferrite is uniform. This is due to the high content of N element in the weld deposited metal and base metal, and the increase of N element in the weld and heat affected zone δ Opposite γ The phase transformation temperature shortens the time of grain coarsening [2], which prevents the grain coarsening of ferrite and enables the heat affected zone to obtain fine grain microstructure. In addition, at the solution treatment temperature of 1100 ℃, a part of it is obviously eliminated σ Phase, 1050 ℃ can not be completely eliminated σ During the tensile process, the sample is hard and brittle when plastic deformation occurs σ The phase exists at the grain boundary of austenite and ferrite, which hinders the movement of dislocations at the grain boundary. Therefore, dislocation stacking occurs at the grain boundary and dislocations gather at the grain boundary σ Stress concentration occurs around the phase, resulting in poor plastic toughness of welded joints during tensile test [11-12].

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Figure.5 microstructure of welding heat affected zone of 2507 stainless steel under different solution treatment processes

Conclusion

  • (1) Plasma welding backing and tungsten argon arc welding multi-layer welding can realize the welding of 2507 super duplex stainless steel without defects.
  • (2) Solid solution treatment at 1050 ℃ for 15min failed to completely eliminate the austenite grain boundary in the fusion zone and heat affected zone of the welded joint σ And the uneven distribution of austenite and ferrite is the main reason for the poor mechanical properties of 2507 welded joint.
  • (3) Solid solution treatment at 1100 ℃ and holding time of 15min completely eliminate the welding joint area σ Phase, the weld structure is relatively uniform, and the ferrite content of the two phases accounts for about 40%, showing good mechanical properties. This process can be used as the post weld solid solution treatment process of 2507 super duplex stainless steel.

Authors: Feng Yulan, Wu Zhisheng, Li Yan, Li Yajie, Wang Ruisen
Source: Network Arrangement – China 2507 Flange supplier: www.epowermetals.com
Reference:

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  • [9] SIEURIN H,SANDSTROM R. Austenite reformation in the heat-affected zone of duplex stainless steel 2205[J].Materials Science and Engineering A,2006,418(1-2):250-256.
  • [10] Zhang Deyuan. Study on welding process and properties of TP304/Q235 composite steel plate [D]. Chengdu: Xihua University, 2011
  • [11] GOU N N,ZHANG J X,WANG J L,et al. Butt Welding of 2205/X65 Bimetallic Sheet and Study on the In homogeneity of the Properties of the Welded Joint[J]. Journal of Materials Engineering and Performance, 2017,26 (4) :1801.
  • [12] Yang Li. Study on weldability between 2205 duplex stainless steel and Q345R steel [D]. Beijing: China University of mining and technology, 2013
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