2507 super duplex stainless steel welding and heat treatment process research
In order to obtain a good performance of 2507 super duplex stainless steel welded joints through mechanical, metallurgical, and corrosion testing means, the two weldings and heat treatment process of 2507 super duplex stainless steel welded joints organization and performance of the study. The results show that: with the use of mixed gas shielded welding process, after welding to 1050 ℃, heat treatment process after holding 13-15min, the welded specimen weld position of the third phase precipitation serious and the proportion of the two phases differ, resulting in increased weld brittleness, weld performance cannot meet the requirements of the relevant standards; pure argon gas protection, using a smaller line energy multi-layer multi-pass welding process, after welding to 1100 ℃, heat treatment process after holding 50-55min, welded specimens weld position of the phase ratio is relatively uniform, the performance of the weld is good, to meet the requirements of the relevant standards.
Duplex stainless steel is a solid solution of ferrite and austenite organization of about 50% of the stainless steel. According to GB/T21832 standard, duplex stainless steel ferrite and austenite in any phase should account for at least 40%, considering the performance of the two stainless steel materials, with excellent corrosion resistance and high strength.
The development and application of duplex stainless steel began in the 1930s and has developed over three generations. In the first generation of duplex stainless steel in the 1940s, the United States developed 329 steel as a representative; chromium and nickel content is high, and good local corrosion resistance, but the high carbon content (w (C) ≤ 0.1%), cannot guarantee its post-weld performance, in the application and development of restrictions. In the mid-1960s, Sweden developed 3RE60 steel to become the first generation of duplex stainless steel; instead of 304L, 316L stainless steel is widely used in the application of duplex stainless steel, 316L stainless steel is widely used in chloride ion corrosion-resistant conditions; since the 70s, with the emergence of secondary refining technology AOD and VOD and other methods and continuous casting technology, the unique effect of the use of nitrogen, as well as the popularity of ultra-low carbon steel, the development of 2205 steel as a representative of the ultra-low carbon nitrogen-containing new second-generation duplex stainless steel, is widely used in seawater heat exchangers and nitric acid equipment and other industries. In the late 80s, the development of 2507 steel as a representative of the super duplex stainless steel belongs to the third generation of duplex stainless steel, low carbon content (w (C) 0.01%-0.02%), high molybdenum, nitrogen (w (Mo) ≈ 4%, w (N) ≈ 0.3%), the ferrite volume fraction of 40%-45%, with excellent resistance to pore corrosion.
2507 super duplex stainless steel (from now on referred to as 2507 steel) as the object of study; through mechanical, metallurgical, corrosion, and other tests, the welding and heat treatment process of 2507 super duplex stainless steel was studied to provide theoretical support and technical support for the development of super duplex stainless steel welded pipe production process.
1. Test materials and methods
Table of Contents
1.1 Test material
Test material for 2507 steel hot-rolled steel plate, welding test plate size of 500mm × 300mm × 14mm (length × width × thickness), the number of 4 pieces. The shape and size of the welding test plate are shown in Figure 1.
Figure.1 2507 Stainless steel welding test plate shape and size
1.2 Chemical Composition and mechanical properties of the base material
The test metal material is a 2507 steel plate produced by Shanxi Taigang Stainless Steel Co., Ltd. Its main chemical composition is shown in Table 1, and its mechanical properties are shown in Table 2.
2507 steel is obtained based on 2205 steel by adjusting the content of alloying elements. As can be seen from Table 2, the yield strength of 2507 steel reached 666MPa (>550MPa), which is more than three times the austenitic stainless steel 316L containing Mo than 2304 duplex stainless steel high 150MPa.
Table.1 Main chemical composition of 2507 steel plate
| C | Cr | Ni | Mo | N | Others |
| 0.02 | 25 | 7 | 4 | 0.27 | S=0.001 |
Table.2 Mechanical properties of 2507 steel plate
| Grade | Tensile strength/MPa | Yield strength/MPa | Elongation rate/% | HRB |
| S32750 | 886 | 666 | 34.5 | 251 |
1.3 Parent material phase ratio
2507 steel (hot-rolled steel plate) after corrosion by FeCl3 metallurgical organization is shown in Figure 2. As can be seen from Figure 2, 2507 steel organization consists of ferrite and austenite (light gray for austenite, dark gray for ferrite), austenite distribution in the ferrite matrix, the overall strip, the proportion of the two phases of about 50% each, to meet the requirements of the duplex stainless steel standard phase ratio.
Figure.2 2507 steel metallographic organization after corrosion by FeCl3
1.4 Welding materials and equipment
Welding using stainless steel solid core wire, grade ER2594 (H03Cr25Ni9Mo3Cu1N), specifications Φ1.2mm. Wire chemical composition is shown in Table 3.
Table.3 ER2594 solid wire chemical composition %
Welding using plasma welding and tungsten arc welding, two welding methods, welding equipment using side beam double shot P + T longitudinal girth welding system TETRIX 522D-P, TETRIX 521TIG, power supply AC/DC 1000, welding position for flat welding.
2. Welding process of 2507 steel
2.1 Welding bevels and welding channel design
I am welding the specimen’s unilateral bevel shape and channel distribution, as shown in Figure 3. Figure 3 (a) shows the welding bevel for the Y-shaped, blunt edge size 4mm, unilateral bevel angle of 37.5 °. Figure 3 (b) in position 1 for the plasma does not need filler wire priming welding, penetrating the blunt bevel edge, to achieve single-sided welding double-sided molding; position 2-5 using argon arc welding filler wire welding, to complete the weld filler, cover welding.
Figure.3 Welding specimen single-sided bevel shape and welding channel distribution diagram
2.2 Pre-welding Preparation and welding process requirements
2.2.1 Pre-welding preparation
- (1) Bevel processing. They are machined to obtain the weld bevel shown in Figure 3(a).
- (2) Cleaning before welding. Use acetone or stainless steel wire brush to clean the bevel and both sides of the 30mm range of oil, rust, oxide, and other dirt.
- (3) Test plate grouping. Using manual TIG without filling wire welding method will test plate spot welding grouping, welding seam does not leave a gap.
- (4) Equipment inspection. Simulate torch walking; check the welding equipment, water supply, and gas supply device to ensure the normal operation of the welding equipment; check the wire grade and specifications to ensure that the material matches the parent material.
2.2.2 Welding process requirements
- (1) For duplex steel, the size of the welding line energy will affect not only the quality fraction of the weld alloying elements but also the ratio of the two phases in the weld metal, which in turn affects the performance of the weld. The welding process selected a smaller line energy welding and multi-layer multi-pass welding.
- (2) Strictly control the interlayer temperature ≤ 100 ℃.
- (3) Interlayer cleaning of the welding channel, requiring the use of angle sanders to clean the weld channel surface and both sides of the oxidation skin, oil, etc., so that the surface of the weld appears silvery white to ensure the quality of welding.
2.3 Welding process scheme and parameters
PAW and TIG are two welding methods; the welding process is welding center gas and shielding gas using two programs.
Program 1, the welding center gas using argon (Ar) and hydrogen (H2) gas mixture (H2 volume fraction of 3%-5%), the shielding gas is pure Ar.
Program 2, welding center and shielding gas, used pure Ar (purity ≥ 99.99%). Welding process parameters are shown in Table 4.
Table.4 Welding process parameters of 2507 steel under the two programs
| Welding and heat treatment process plan | Pass order | Welding method | Welding current/A | Welding voltage/V | Welding speed/(mm/min) | Wire feeding speed/(mm/min) | Ionic gas flow rate/(L/min) | Protective gas flow rate/(Lmin) |
| Option 1 | 1 | PAW | 260 | 26-28 | 200 | 5.0-6.0 | 25 | |
| 2 | TIG | 280 | 17-19 | 300 | 1400 | 25 | ||
| 3 | TIG | 300 | 17-18 | 260 | 2500 | 25 | ||
| 4 | TIG | 290 | 17-18 | 240 | 2800 | 25 | ||
| 5 | TIG | 300 | 17-18 | 240 | 2400 | 25 | ||
| Option 2 | 1 | PAW | 260 | 28-30 | 160 | 4 | 25 | |
| 2 | TIG | 280 | 17-19 | 150 | 1300 | 25 | ||
| 3 | TIG | 300 | 16-18 | 130 | 1800 | 25 | ||
| 4 | TIG | 300 | 16-17 | 120 | 1800 | 25 | ||
| 5 | TIG | 300 | 16-17 | 100 | 2000 | 25 |
Note: The last TIG welding process uses a transverse swing with a swing width of 20 mm.
2.4 Non-destructive inspection of the weld
After completing the test plate welding, an X-ray inspection visually inspects the weld. Requirements cannot appear porous, unfused, cracked, or other defects; the weld quality reaches the standard GB / T3323 Ⅰ.
3. Heat treatment process
3.1 2507 steel continuous cooling transformation process
2507 steel continuous cooling transformation curve shown in Figure 4. Figure 4 can be seen, 2507 steel heat treatment temperature shall not be less than 1050 ℃; in the 850-950 ℃ temperature range, σ phase precipitation rate is very fast; with the increase in temperature, σ phase precipitation rate gradually decreases when the temperature rises to about 1050 ℃, σ phase no longer precipitation.
Figure.4 2507 super duplex stainless steel continuous cooling transition (CCT) curve
3.2 Heat treatment process options
Two options are used for heat treatment.
- Option 1, heat treatment temperature of 1050 ℃, holding 13-15min, a cooling method for quenching water.
- Option 2, heat treatment temperature 1100 ℃, insulation 50-55min, cooling method is quenching water.
4. Test results and analysis
Mixed gas welding, heat treatment temperature 1050 ℃ process program one, take its specimens for the program one specimen; pure argon welding, heat treatment temperature 1100 ℃ process program two, take its specimens for the program two specimens.
4.1 Weld tensile test results
The use of steel research nak testing technology limited company production of tensile compressor weld transverse tensile test. The average width of the specimen is 20mm, the standard distance of 20mm, and the test implementation of GB/T228 standard. Test conditions: loading rate of 5mm/min, loading load 10kN. Weld tensile specimen macroscopic photos are shown in Figure 5; test results are shown in Table 5.
Figure.5 Two schemes under the 2507 steel weld tensile specimen photos
Table.5 Two schemes of 2507 steel weld transverse tensile test results
| Welding and heat treatment process | Yield strength/MPa | Tensile strength/MPa | Elongation rate/% | Fracture location |
| Option 1 | 0 | Weld line | ||
| Option 2 | 665 | 880 | 28 | Base material |
Figure 5 and Table 5 can be seen in mixed gas welding, heat treatment temperature of 1050 ℃ process option one, in the tensile test process, the tensile strength value instantly reached 879MPa with 926MPa, the specimen instantly fracture, elongation is almost zero, and fracture in the weld position. Pure argon welding, heat treatment temperature of 1100 ℃ process plan II, yield strength of 665MPa, greater than the parent material minimum 550MPa requirements; tensile strength of 880MPa, greater than the parent material minimum 795MPa requirements; elongation of 20%, greater than the parent material requirements of 15%. All properties meet the requirements of NB/T47014 standard, and the specimen fracture location is away from the base material at the weld. That pure argon welding, heat treatment temperature 1100 ℃ process program can meet the standard requirements.
4.2 Weld bending test results
Weld bending specimen size of 160mm × 38mm × 14mm (length × width × thickness), the test implementation of GB/T232 standard, specimen bending diameter 40mm, bending angle 180 °. Weld bending specimen photos are shown in Figure 6. Figure 6 can be seen, Figure 6 (a) bending specimen bending angle is almost zero, the specimen along the edge of the weld fusion line cracking; and Figure 6 (b) specimen, after bending 180 °, magnified 10 times to observe the specimen inside and outside the surface are no cracks, to meet the test requirements.
Under the gas mixture, the bending specimen of the weld state weld is shown in Figure 7. As can be seen in Figure 7, the face of the weld bending test in the welding state near the center of the crack, the crack length is about 3/4 of the width of the sample, the center of the weld grain is relatively rough, there is a trend of cracking. This indicates that the mechanical properties of 2507 steel weld can be reduced under mixed gas welding.
Figure.6 Two programs under the 2507 steel weld heat treatment state of the bending specimen photo
Figure.7 Bending specimen photos of 2507 steel weld state under the gas mixture welded seam
4.3 Impact test results
Weld impact specimen size of 55mm × 10mm × 10mm (length × width × thickness), the test temperature is room temperature, and the test implementation of GB/T229 standard. Weld impact specimen photos shown in Figure 8, Figure 8, from the top to the bottom of the three groups of specimens are the base material, heat-affected zone, and the impact of the weld zone specimens. Figure 8 (a) in the test process, the impact value of about 3J fracture, the fracture is neat, and the test value is lower than the standard minimum value of 31J; Figure 8 (b) in the specimen, the base material, weld, heat-affected zone impact work average of 319J, 160J, 216J, respectively, is greater than the standard minimum value of 31J. can be seen, pure argon welding, heat treatment temperature of 1100 ℃ process program The impact toughness of the second welded specimens can meet the standard requirements.
Figure.8 Two programs under the 2507 steel weld impact specimen photos
4.4 HAZ metallurgical analysis
HAZ metallurgical organization is shown in Figure 9. As seen in Figure 9 (a), the third phase precipitation is serious, showing small black lumps distributed in the austenite and ferrite grain boundaries, the volume fraction of ferrite in the organization is about 35%, the proportion of the two phases is large, is the direct result of the weld brittleness, high strength, and elongation, impact work decreased the main reason. As seen in Figure 9 (b), the left side near the base material area, island austenite distribution in the ferrite matrix, the base material hot-rolled strip organization is obvious; the right side near the weld area, austenite needle-shaped, with a certain growth direction, grain growth overall and weld organization shows similarity, the volume fraction of ferrite in the organization is about 55%, the two-phase organization is balanced, to meet the GB/T13305 standard dual-phase steel on Two-phase ratio of the requirements.
4.5 Weld intergranular corrosion test
Intergranular corrosion test implementation of the standard GB/T4334-2008 method E. Weld specimen size 40mm × 4mm × 80mm (width × thickness × length), the test using chemical etching method, the use of sulfuric acid – copper sulfate solution for corrosion.
Sulfuric acid – copper sulfate solution ratio program: 100g of pure copper sulfate (CuSO4-5H2O) dissolved in 700mL of distilled water or dechlorinated ionized water, then add 100mL of pure sulfuric acid, diluted to 1000mL with distilled water or dechlorinated ionized water, prepared into sulfuric acid – copper sulfate solution corrosion solution. The specimen will be boiled in sulfuric acid – copper sulfate solution for 16h; the specimen will be bent 180 ° after bending and magnified 10 times to observe that the specimen is not cracked on the outer surface; the specimen passed the test.
Figure.9 Two schemes under the 2507 steel weld HAZ corrosion after the metallographic organization of the photo
5. Conclusions
- (1) 2507 super duplex stainless steel test plate, the welding center gas is a mixture of Ar and H2 (H2 volume fraction of 3%-5%), the shielding gas is pure Ar welding process, after welding to 1050 ℃, the heat treatment process of holding 13-15min, welded specimens weld position of the third phase precipitation is serious. The proportion of the two phases is large, resulting in increased brittleness of the weld. The performance of the welded specimen does not meet the requirements of the relevant standards.
- (2) 2507 super duplex stainless steel test plate, pure argon gas (purity ≥ 99.99%) as the welding gas, the use of smaller linear energy multi-layer multi-pass welding process, after welding to 1100 ℃, holding 50-55min heat treatment process, the welded specimen weld position of the phase ratio is uniform, the performance of the welded specimen is good, to meet the requirements of the relevant standards.
Author: Yulan Feng
Source: China 2507 Forgings supplier: www.epowermetals.com
