Study on welding technology of super duplex stainless steel pipe

The welding process test of super duplex stainless steel pipe was carried out by manual argon tungsten arc welding. Through the analysis and summary of welding test data, the conclusion was drawn: under the premise of selecting suitable welding materials, reasonable setting of welding parameters and strict control of welding heat input and welding seam interlayer temperature can ensure the ferrite and austenite level of super duplex stainless steel weld It can meet the requirements of mechanical properties and corrosion resistance, and has guiding significance for the welding construction of super duplex stainless steel pipe in the future.

Super duplex stainless steel is used in the cargo handling system of a 18600t chemical tanker. Considering that the pipeline system will transport chemical liquid cargo with extremely strict environmental requirements in the future, the weld quality must pass the design requirements of 100% RT test. The company undertaking the construction of the chemical tanker is the first time to build this type of product, so it lacks the welding technology reserve in the early stage and the production technology management experience in the later stage. In order to solve this contradiction, the relevant personnel inquired a lot of data and carried out a lot of welding process tests to verify and determine the appropriate welding process parameters, so as to ensure the welding quality in the actual construction of later products.

Analysis of material properties

Compared with austenitic stainless steel in mechanical properties and chemical composition (see table.1 and table.2), it is found that Cr and Mo elements in super duplex stainless steel are significantly higher than those in austenitic stainless steel, which is beneficial to improve the corrosion resistance of steel. Ni and N are the promotive and stable elements of austenite. When combined with Cr, duplex stainless steel can be formed.

Table.1 Comparison of chemical composition between super duplex stainless steel and austenitic stainless steel (mass fraction) (%)

Element

Material

C Si Mn P S Cr Ni Mo N Cu
S32750 ≤0.030 ≤0.080 ≤1.200 ≤0.035 ≤0.020 24.000~26.000 6.000~8.000 3.000~5.000 0.240~0.320 ≤0.500
A312 TP304L ≤0.035 ≤1.000 ≤2.000 ≤0.045 ≤0.030 18.000~20.000 8.000~13.000
A312 TP316L ≤0.035 ≤1.000 ≤2.000 ≤0.045 ≤0.030 16.000~18.000 10.000~14.000 2.000~3.000

Table.2 Comparison of mechanical properties between super duplex stainless steel and austenitic stainless steel

Performance materials

Tensile strength/MPa

yield strength/MPa

S32750

≥800

≥550

A312TP304L

≥485

≥170

A312TP316L

≥485

≥170

The welding performance of super duplex stainless steel is excellent. On the premise of selecting suitable welding materials and reasonably controlling welding line energy, the sensitivity of welding cold crack and hot crack is low, and the weld has good corrosion resistance and mechanical properties. But in the welding process, the weld is not easy to be heated for many times, which is easy to damage the duplex stainless steel structure. The alloy content of this material is high, and the cooling rate is too slow during welding, which will lead to the formation of harmful phases. These conditions may have a direct impact on the corrosion resistance and mechanical properties of the welded joints of super duplex stainless steel. Therefore, it is necessary to take necessary measures in the welding process to ensure the welding quality.

Selection of welding materials and welding methods

Selection of welding materials

According to the material properties of welding test products, ER2594 and φ 2.4mm welding wire produced by a welding material factory is selected. The chemical composition of welding wire is shown in Table 3.

Table.3 chemical composition of welding materials (mass fraction) (%)

Weight% C Cr Ni Mo Mn Si P S N Cu W
ER 2594 0.03 max 24-27 8-10.5 2.5-4.5 2.5 max 1 max 0.03 max 0.02 max 0.20-0.30 1.5 max 1.0 max

With the enhancement of China’s independent R & D capability and the development of industrial manufacturing technology, industrial welding materials have been increasingly localized. The selected welding material factory has the production capacity of super duplex stainless steel wire ER2594. The localization of welding materials greatly reduces the procurement cost and transportation cycle. In addition, some domestic shipyards have adopted domestic ER2594 welding wire and used it in specific product construction. In conclusion, it is feasible to select this type of welding wire in the actual construction of later products.

Selection of welding method

According to the company’s experience in stainless steel welding, it is recommended to adopt TIG welding. The welding quality of this welding method is easy to control, the welding energy is relatively concentrated in the welding process, the protective effect of shielding gas on the molten pool is good, and the welding chemical composition is not easy to lose. Therefore, this welding method is also used in the welding of super duplex stainless steel, which can better ensure the welding quality of the weld.

Selection of welding parameters for super duplex stainless steel

Weld and groove selection

The base metal model of the test piece is UNS S32750, and the groove type is V-shaped groove, as shown in Fig.1. The model of welding wire is ER2594 and the specification is φ 2.4 mm. The welding process adopts single side welding and double side forming. This welding method requires very high quality of backing welding. If the current is too small, it is easy to cause slag inclusion or incomplete penetration; if the current is too high, it is easy to cause welding penetration.

20200721041800 26765 - Study on welding technology of super duplex stainless steel pipe
Fig.1 groove and weld bead

Selection of test parameters

With reference to the company’s experience in stainless steel welding, on the premise of the same welding material, welding shielding gas and welding method, several groups of tests were carried out by changing the size of weld groove, welding current, voltage and weld bead level distribution.

Welding precautions of super duplex stainless steel

The groove of weldment shall be carried out by beveling machine or machining method, and flame processing is strictly prohibited. Before assembling the weldment, the groove and the surface about 50 mm away from the groove shall be completely removed, and there shall be no impurities such as carbides (such as paint, rust, burr and moisture). The finished and semi-finished pipe fittings should be stacked in isolation to prevent iron from polluting the super duplex stainless steel. The isolation measures are shown in Figure.3.
20200721042922 39106 - Study on welding technology of super duplex stainless steel pipe

Figure.3 isolation measures

In the process of fabrication, transportation and assembly, the welding parts shall be free from iron ion pollution and bruise. The welding parts shall be processed in a special stainless steel workshop. The tools used in the process must be non-ferrous stainless steel special tools, such as electric grinder, stainless steel grinding wheel blade, stainless steel wire brush, etc. Before welding, clean the weldment with acetone or alcohol. Welding is strictly prohibited when the temperature is lower than 10 ℃. When the weldment is installed and butted, the concentricity of the weldment interface should be checked first to prevent installation misalignment. Welding parts shall not be assembled by force, and deformation prevention measures shall be taken. Pure argon of 99.95% should be used as protective gas.
Positioning tooling can be selected for butt positioning of weldments, or welding positioning can be made in weld bead groove. During the positioning welding of weldments, the same welding materials and welding process as backing welding shall be used. The positioning welding should adopt the method of uniform fixation (such as 4 points, 6 points or 8 points), and the height of the welding foot should not be higher than the height of the whole pipe wall, and should be located at about 2 / 3 of the thickness of the pipe wall. In the process of the positioning welding, the quality of the whole positioning welding should be ensured to be good without cracking. If any, it should be removed immediately.
In consideration of the following actual construction, the relevant welding process should be able to completely cover the large, medium and small pipe diameter, so in the welding test process, the selected relevant parameters should be strictly observed for welding test operation.
On the premise that the welding quality is fully guaranteed, low current, fast welding and multi-layer welding should be used. In the welding, try to use a small amount of welding, interlayer temperature is not more than 150 ℃.
The surface of the weld is sealed by aluminum foil tape, and the sponge plate and foam are used to make the blocking plate when the workshops are prefabricated. An air chamber is set on both sides about 300 mm away from the weld. One side of the gas chamber is used to fill argon, and a small hole is opened on the other side to exhaust air. Welding can be carried out only after the air in the chamber is completely exhausted and filled with argon. Before welding, a piece of aluminum foil tape about 40 mm long on the groove surface should be torn off, and then a section should be torn after welding. During the welding process, it is not allowed to tear off all the aluminum foil tape on the groove surface before welding, and during the whole welding process, the gas chamber must be filled with argon as protective gas.
Before welding the next layer, the upper layer of weld bead must be carefully inspected and cleaned, and all residual materials must be removed. During grinding, overheating zone shall be avoided to ensure good weld surface formation. If welding stay is required during welding, argon gas shall be stopped for several minutes after welding.
Arc striking is required in the groove. Current test or arc striking operation are strictly prohibited on the surface of base metal, and attention shall be paid to prevent arc from scratching the base metal. The welding method is the semi breakdown method. The welding wire melts in the form of water drops and transfers to the weld hole to form a welding pool. When it is necessary to start the arc again after the first welding, the arc shall be re started in the weld bead, and the arc starting point shall overlap about 15 mm with the last arc extinguishing place. After successful arc striking, the welding torch still needs to stay at the arc striking position for several seconds, so as to obtain the weld with the same width and luster as the previous weld, and then the arc can be moved out of the arc pit to continue the subsequent welding.
At the end of the whole welding process, the gas chamber will continue to be filled with argon to protect the molten pool, until the pool is completely solidified, the welding torch can be removed from the weld. After the end of the welding process, the weld surface must be carefully inspected to remove all residual impurities and defects. If any, the whole welding process can only be completed after the repair is qualified. After welding, pickling and passivation shall be carried out on the weld.

Welding performance test of super duplex stainless steel

Nondestructive testing

The weld surface was tested by 100% Pt, which met the requirements of JB/T4730.5-2005.i. 100% RT and X-ray inspection were used for the inside of the weld, which met the requirements of JB/T4730.2-2005.i.
The relationship between the color of super duplex stainless steel welding joint and the protection effect is shown in Table 4.

Destructive test

Two tensile tests, four bending tests, two corrosion tests and one metallographic test were carried out on the welded joint, and the test results all met the requirements of relevant standards. The test items and sample quantity are shown in Table 5.
Table 4 Relationship between color and protection effect of welded joint of super duplex stainless steel

Color of welded joint

Silver and gold

Blue

Red grey

Grey

Black

Protection effect

Best

Good

Preferably

Bad

Worst

Qualified

Unqualified

Table.5 requirements for destructive test

Number

Test items

Number of samples

1

Transverse pull up test

2

2

Bending test

Face bending specimen

2

Back bend specimen

2

3

Corrosion test

2

4

Metallographic test (base metal, heat affected zone, weld)

1

Conclusion

Under the premise of strict control of the temperature between welding seams, the corrosion resistance and phase balance of super duplex stainless steel can be well guaranteed by adopting multi-layer welding, multi pass and reasonable control of welding heat input. Both super duplex stainless steel and austenitic stainless steel have good weldability and corrosion resistance. However, the tendency of hot cracking of super duplex stainless steel is lower, and the chloride corrosion resistance is better than that of austenitic stainless steel. Therefore, super duplex stainless steel has been gradually used in the environment with extremely strict requirements on corrosion resistance, such as petrochemical equipment, etc It has broad application prospects.
Author: Wu Wei, Peng Wen, Jiang Qifeng

Source: China Super Duplex Stainless Steel PipManufacturer – 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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study on welding technology of super duplex stainless steel pipe - Study on welding technology of super duplex stainless steel pipe
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Study on welding technology of super duplex stainless steel pipe
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The welding process test of super duplex stainless steel pipe was carried out by manual argon tungsten arc welding.
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