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Welding technology of 2205 duplex stainless steel

This paper analyzes the material characteristics and weldability of 2205 duplex stainless steel. Before construction, the welding process of 2205 duplex stainless steel plate is evaluated and the welding process is determined. In the actual welding process, by strictly implementing the welding process, not only the welding quality is guaranteed, but also the welding efficiency is improved, which meets the requirements of design, specification and construction unit.

In 2009, the eighth engineering company encountered the assembly and welding of 2205 circulating pipe during the installation of desulfurization wastewater advanced treatment project of Guangdong Heyuan power plant. The main material of the circulating pipe is 2205/Q235-B. in order to facilitate on-site welding, the construction unit requires the designer to change the material at the joint to pure 2205, so only the welding of pure 2205 is involved in the field construction. The project is the first advanced treatment project of desulfurization wastewater in China, and the ultimate goal is to achieve zero discharge of salt containing wastewater, which has great demonstration significance. The construction effect of installation engineering is directly related to the reputation of our company and the undertaking of follow-up projects. Among them, the installation and assembly welding of circulating pipe is one of the core technologies in construction.
Because the welding of 2205 material is rare in our company, in order to ensure the welding quality, the welding process of 2205 dual phase steel is specially studied. Here is the introduction.

Material properties

Composition characteristics

Austenitic ferritic stainless steel, referred to as duplex stainless steel. Dual phase steel is widely used in petroleum and natural gas industry, chemical and petrochemical processing industry, chemical fertilizer industry, transportation industry, papermaking and salt making light industry. The second generation duplex stainless steel is generally called standard duplex stainless steel. Its composition is characterized by ultra-low carbon and nitrogen, and its typical composition is 22% Cr + 5% Ni + 0.17% n. Compared with the first generation of duplex stainless steel, 2205 further increased the nitrogen content and enhanced the stress corrosion resistance and pitting corrosion resistance in acid medium with high chloride concentration. Nitrogen is a strong austenite forming element. Adding nitrogen to duplex stainless steel can not only improve the strength of steel, but also inhibit carbide precipitation and delay the formation of σ phase.

Organizational characteristics

At room temperature, austenite and ferrite account for about half of the solid solution of duplex stainless steel (the ferrite content of 2205 should be 30% ~ 55%, and the typical value is about 45%). It retains the characteristics of ferritic stainless steel, such as high thermal conductivity, small linear expansion coefficient, pitting corrosion resistance, crevice and chloride stress corrosion, and has the advantages of good toughness, low brittle transition temperature, intergranular corrosion resistance, good mechanical properties and welding properties of austenitic stainless steel.

Performance characteristics

The outstanding performance in performance is high yield strength and stress corrosion resistance.
The yield strength of duplex stainless steel is about one time higher than that of austenitic stainless steel. Under the same pressure level, the material can be saved. The coefficient of linear thermal expansion of austenitic stainless steel is lower than that of austenitic stainless steel and close to that of low carbon steel. It makes the welding of duplex stainless steel and carbon steel more suitable, which has great engineering significance. The formability of forging and cold stamping is not as good as that of austenitic stainless steel.

Weldability

2205 steel is an ultra-low carbon austenitic ferritic stainless steel. After solid solution treatment, the steel contains 45% – 70% austenite and 30% – 55% ferrite. The steel has good toughness, strength and weldability. Due to the proper ratio of Cr equivalent to Ni equivalent, a large amount of primary austenite and secondary austenite can be retained after high temperature heating The results show that the total amount of austenite phase in the steel is not less than 30% ~ 40%, so the steel has good corrosion resistance; because of the high content of N in the base metal, the single-phase ferrite zone will not be formed in the welding near seam area, and the austenite content is generally not less than 30%.
Duplex stainless steel 2205 has good weldability, and the sensitivity of cold crack and hot crack is small. Usually no preheating before welding and no heat treatment after welding. When the selection of welding material is reasonable and the control of welding line energy is appropriate, the welded joint has good comprehensive performance.

Hot crack

The sensitivity of 2205 steel to hot cracking is much less than that of austenitic stainless steel. This is because the content of nickel is not high, and it is easy to form low melting point eutectic impurities, and it is not easy to produce low melting point liquid film. In addition, there is no danger of rapid grain growth at high temperature.

Embrittlement in HAZ

The main problem of duplex stainless steel welding is not in the weld, but in the heat affected zone. Because the HAZ is in the non-equilibrium state of rapid cooling under the action of welding thermal cycle, more ferrite is always retained after cooling, which increases the corrosion tendency and the sensitivity of hydrogen induced cracking (embrittlement).

Ferrite embrittlement at 475 ℃

Duplex stainless steel contains about 50% ferrite and brittleness at 475 ℃, but it is not as sensitive as ferritic stainless steel.
During the welding process of duplex stainless steel, a series of changes have taken place in the weld metal and HAZ under the action of thermal cycle. At high temperature, the microstructure of all duplex stainless steels is composed of ferrite, and austenite precipitates during cooling. The amount of austenite precipitation is affected by many factors.
The mechanical properties and corrosion resistance of duplex stainless steel welded joint depend on whether the welding joint can maintain proper phase proportion. Therefore, the welding is carried out around how to ensure the dual phase structure. When the content of ferrite and austenite is close to 50%, the properties are better and close to the properties of base metal. The corrosion resistance and mechanical properties (especially toughness) of duplex stainless steel welded joint will be decreased if the ratio is changed. The optimum ferrite content of 2205 duplex stainless steel is 45%. Too low ferrite content (< 25%) will reduce the strength and stress corrosion cracking resistance; too high ferrite content (> 75%) will also damage the corrosion resistance and impact toughness.
The influence factors of phase ratio are as follows:
The equilibrium relationship between ferrite and austenite in welded joint is affected by the content of alloy elements in steel, filler metal, welding thermal cycle and process parameters.

Effect of alloying elements

According to the research and a lot of experiments, it is found that the nitrogen content in the base metal is very important. Nitrogen plays an important role in the formation of enough austenite in weld metal and heat affected zone after welding. Nitrogen, like nickel, forms austenite and expands austenite, but the ability of nitrogen is far greater than that of nickel. At high temperature, the ability of nitrogen to stabilize austenite is greater than that of nickel, which can prevent the appearance of single-phase ferrite after welding and prevent the precipitation of harmful metal phases.
Due to the effect of welding thermal cycle, when the composition of self fusion welding or filler metal is the same as that of base metal, the ferrite content of weld metal increases rapidly, even pure ferrite structure appears. In order to restrain the excessive increase of ferrite in the weld, it is the welding trend of duplex stainless steel to adopt the weld metal dominated by austenite. Generally, the two ways of increasing nickel or adding nitrogen in welding materials are adopted. Generally, the content of nickel is 2% – 4% higher than that of the base metal. For example, the nickel content of 2205 filler metal is as high as 8% – 10%. The results show that the effect of nitrogen containing filler material is better than that of nickel filler material. Both elements can increase the proportion of austenite phase and make it stable. However, nitrogen addition can not only delay the precipitation of intermetallic phase, but also improve the strength and corrosion resistance of weld metal. The filler metal has a higher Ni content than the base metal, which is mainly to balance the welding area and make the chemical composition of the weld area and the useful sample of the base metal.
At present, the filler materials are generally based on the increase of nickel, and then add nitrogen equivalent to the base metal content.

Influence of thermal cycle

The biggest characteristic of duplex stainless steel welding is that the welding thermal cycle has influence on the microstructure of the welded joint. Phase transformation occurs in both the weld and heat affected zone, which has a great impact on the performance of the welded joint. Therefore, multi-layer and multi pass welding is beneficial. The subsequent welding pass has heat treatment effect on the front layer weld bead, and the ferrite in the weld metal is further transformed into austenite, which becomes the two-phase structure dominated by austenite; the austenite phase in the heat affected zone adjacent to the weld is also increased correspondingly, which can refine the ferrite grain, reduce the precipitation of carbide and nitride from the grain boundary and grain boundary, so as to make the whole weld metal smooth The results show that the microstructure and properties of welded joints are improved significantly.

Influence of process parameters

The welding parameters, i.e. welding wire energy, also play a key role in the balance of dual phase structure. Because duplex stainless steel is 100% ferrite at high temperature, if the linear energy is too small, the cooling rate of heat affected zone is fast, and the austenite can not precipitate in time, the excess ferrite will be subcooled at room temperature. If the heat input is too high and the cooling rate is too slow, although enough austenite can be obtained, the ferrite grain growth in heat affected zone and the precipitation of σ equivalent harmful metal phase will also be caused, resulting in joint embrittlement.
In order to avoid the above situation, the best measure is to control the welding line energy and interlayer temperature, and use filler metal with higher nickel content than base metal and nitrogen content equivalent to base metal.

Welding procedure qualification

In August 2009, according to the actual situation of Heyuan project in Guangdong Province, a group of 2205 duplex stainless steel plates with size of 350 * 400mm and thickness of 12 were selected for welding procedure qualification.

Selection of welding position and welding method

The welding position is fixed horizontally. In order to ensure the internal quality, according to the construction requirements, TIG arc welding and manual arc welding are used to fill the cover.

Selection of welding materials

Argon arc welding wire ER2209 (Φ 2.4) produced by Avesta company in Switzerland is selected, and E2209 (Φ 3.2, Φ 4.0) produced by Sichuan Atlantic is selected as welding rod.
The composition comparison of base metal and selected welding material is as follows:
Table 3.2 selection list of welding materials

Material Chemical composition of base metal or weld deposited metal(%) Tensile strength(MPa) Elongation Yield point
C Mn Si S P Cr Ni Mo Cu N δ5(%) (MPa)
2205 0.02 0.07 0.4 0.001 0.025 22.4 5.8 3.3 0.16 700 35 515
ER2209 0.017 1.7 0.41 0.001 0.018 22.87 8.67 3.05 0.12 0.17 806 28 637
E2209 0.035 1.12 0.701 0.01 0.03 22.47 9.53 3.04 0.149 0.14 810 29
(Φ3.2)
E2209 0.024 1.08 0.74 0.009 0.024 22.64 9.38 2.97 0.054 0.132 795 28
(Φ4.0)

Groove form and size

According to the site conditions, the single-v-shaped groove (groove form and size are shown in the table below) shall be used for single-sided welding.
Table 3.3 list of groove form and size

Item Thickness T(mm) Groove name Groove form Groove size
Clearance c(mm) Blunt edge P(mm) Bevel anglea (º)
1 12 Ⅴ type 20200928073526 33191 - Welding technology of 2205 duplex stainless steel 0-3 0-3 55-65

Selection of welding parameters

The standard parameters selected for evaluation are as follows:
Table 3.4 list of welding parameters

Weld layer Welding power supply welding method Filler metal (Φ: mm) Welding current (A) Arc voltage (V) Welding speed (cm/min) Linear energy (kJ/cm)
Lay the bottom DC+ GTAW 2.4 75-95 19±2 11 8.809
Other layers DC- SMAW 3.2 95-115 20±2 12 10.5
DC- SMAW 4 120-140 22±2 15 11.44

The flow rate of argon is 12L / min on the outside and 4L / min on the inside.
During the welding process, the ferrite composition of the weld was determined by mp-30 ferrite analyzer after each welding layer. The results were between 40% and 50%, indicating that the phase proportion was qualified.
After the surface cleaning, visual inspection and pickling and passivation of the weld joint, the radiographic inspection is carried out, and the inspection results meet the class I standard of JB / t4730.2-2005.
According to the provisions of JB4708, the following tests shall be carried out on the test pieces

  • Tensile test (tensile test in accordance with the provisions of “metal tensile test methods” gb228, tensile strength is 720 MPa, 725 MPa) 2 pieces;
  • Four side bends (bending test in accordance with the provisions of “metal bending test method” gb232);
  • The impact energy of weld and HAZ (the impact test conforms to GB / t229, the impact energy of weld zone is 107j, 109J and 139j, with an average of 118j; the impact energy of HAZ is 282j, 282j, 276j, with an average of 280j);
  • Pitting corrosion (the pitting corrosion test conforms to the provisions of GB / t17897-1999, the corrosion amount of three samples are 2.329, 2.560 and 2.138mg/24h, with an average of 2.342 mg / 24h, and the mass loss is less than 0.1mg/cm2.

Results all the tests and tests were in accordance with the specifications and design requirements, which proved that the selected welding materials and welding process were correct. On this basis, the welding operation instruction was compiled and the technical disclosure was carried out.

Welding technology 

In late August 2009, on the basis of welding procedure qualification, the installation of circulating pipe officially began. The construction procedures are as follows:

The selection of welding methods and welding materials is shown in the following table:

Table 4.1 list of welding method and welding material selection

Item Welding steel grade Welding method Welding materials and specifications Remarks
1 2205 GTAW/SMAW Base:ER2209(Φ2.4)
Others:E2209(Φ3.2、Φ4.0)

Note: the above is applicable to butt weld; full manual arc welding is adopted for diagonal weld.
When the welding materials are put into storage, the certificate of conformity and quality certificate shall be carefully checked and put into use only after they meet the corresponding standards. The welding materials shall be stored in the warehouse with dry, well ventilated, temperature > 5 ℃ and air relative humidity < 60%; a secondary warehouse of welding materials shall be set up, and the keeper shall be responsible for the storage, baking, distribution and recycling of welding materials, and make various records; the welding rods shall be baked according to the requirements of the instructions before use, and then stored in a constant temperature box of 100 ~ 150 ℃ for taking as soon as possible; the welding rod shall be used for more than 4 hours Re bake, and the repeated baking shall not be more than two times; the welder shall receive the welding material with the material requisition sheet issued by the welding technician.

Groove processing

As the circulating pipe of the project is supplied by the equipment manufacturer and the blanking has been completed, only groove processing is required on site. The form and size of butt weld groove are shown in table 3.3.
Plasma arc cutting method is used to cut the groove on site, and the polishing machine is used for grinding. It is necessary to avoid splashing the cutting slag on the base metal surface, and the groove surface should be smooth and smooth.

Cleaning before welding

Before welding, mechanical method and organic solvent shall be used to remove oil, paint, rust, scale, burr, oxide film, etc. within the range of at least 20 mm inside and outside the welding groove, and the groove surface shall be free of cracks, interlayer and other defects; in multi-layer and multi pass welding, the slag and defects on the surface of the front weld must be removed. In order to prevent the welding spatter from polluting the stainless steel surface, lime water should be applied within the range of 100 mm on both sides of the groove.

Installation of internal argon filling device

In order to prevent the inner weld metal from being oxidized, the internal argon filling device should be placed before the assembly of circulating pipe joints.
According to the site conditions, we use high-density sponge, plywood, etc. to make plugs, which are placed at both ends of the groove before assembly, about 100 mm away from the groove. The valve core of the basketball is used as the intake valve and connected with the oxygen pipe as the internal argon filling device. In order to take out the argon filling device after welding, the iron wire should be fixed on the plug to facilitate pulling out. See the following diagram:
20200928075648 52369 - Welding technology of 2205 duplex stainless steel

Figure 4.4 schematic diagram of internal argon filling device

Pairing

In order to avoid the increase of internal stress and stress concentration, the inner and outer walls should be as flat as possible, and the misalignment of the inner wall should not exceed 10% of the wall thickness and should not be greater than 2mm. The welding joint should not be heated and corrected, and the strong force should not be used for butt joint.
Since the circulating pipe has been prepared by the manufacturer, it only needs to be grinded on site. However, during grinding, the length of pure 2205 material on both sides of the weld shall be controlled to ensure that the distance between the field weld and the manufacturer’s own weld shall not be less than 150 mm.

Tack welding

Welding must be carried out by qualified welders. The process is the same as that of formal welding. Pay attention to carefully check the quality of the weld. If there are cracks, pores and other defects, it is necessary to grind them off and re fix them. Each weld should ensure a sufficient number of spot welding seams. After the spot welding, it is not allowed to knock and transport the workpiece to avoid cracking. It is strictly forbidden to ignite electric arc, test current or weld temporary support on the surface of the workpiece to be welded.
The length, thickness and spacing of the tack weld shall ensure that the weld will not crack in the formal welding process. Manual arc welding can be used for positioning welding. In the process of backing with argon arc welding, the positioning weld will be gradually ground off.

Formal welding

The welding specification parameters of 2205 dual phase steel butt weld are shown in the following table:
Table 4.7 list of welding specification parameters for formal welding

Weld layer Welding power supply welding method Filler metal (Φ: mm) Welding current (A) Arc voltage (V) Welding speed (cm/min) Linear energy (kJ/cm)
Lay the bottom DC+ GTAW 2.4 75-95 19±2 11 8.809
Other layers DC- SMAW 3.2 95-115 20±2 12 10.5
DC- SMAW 4 120-140 22±2 15 11.44

The fillet weld is all manual arc welding, and the welding specification parameters are shown in the table “other layers”.
Welding precautions:
a. Meteorological management
1) During field operation, wind speed should be more than 2m / s for argon arc welding and 8m / s for manual arc welding.
2) The relative humidity within 1 meter range of welding arc is more than 90%. Welding shall be stopped in case of rain in outdoor operation.
3) Welding shall not be carried out when the surface of base metal is wet, or during rain and wind, and there are no protective measures for welders and weldments.
b. Welder code management
1) The welders participating in the construction of the project shall be numbered uniformly. Once the code is determined, it will not be changed in the construction process. After a person leaves the post, the corresponding code number will be vacant.
2) The welder shall clean up the welding junction after welding, and mark the weld with marking pen after passing the self inspection.
c. In order to prevent the oxidation and burning of alloy elements, reduce the welding residual stress, avoid intergranular corrosion, and prevent the generation of hot cracks, under the condition of ensuring good penetration and fusion, low current, short arc, fast welding speed, narrow pass and multi-layer and multi pass welding techniques should be selected appropriately, and the interlayer temperature should not exceed 100 ℃
d. Before backing, the joint gap shall be sealed with masking paper, and welding can be carried out after argon filling inside reaches the effect. Argon flow rate: 12 L / min for external and 4 L / min for internal. After welding, pull the plug out.
e. During the welding process, the quality of arc starting and arc stopping shall be ensured, and the arc pit shall be filled during arc extinguishing. The interlayer joints of multi-layer welding shall be staggered.
f. Attention should be paid to the welding sequence and symmetrical welding should be adopted as far as possible.
g. After welding, the welding slag, weld bead, spatter and other dirt on the surface of the weldment shall be carefully cleaned. The weld shall be partially trimmed if necessary.
h. Ferrite measurement: in order to ensure that the ferrite content in the weld meets the design requirements and specifications, the mp-30 ferrite content tester produced by Fischer company of Germany is used to measure the ferrite content of the weld. The measured ferrite content is 45-55%. The results show that the dual phase ratio of the weld is qualified.

Appearance inspection

Visual inspection or 4-10 times magnifying glass was used for appearance inspection, and the inspection rate was 100%.
The weld shape and geometric dimension shall meet the design requirements.
There shall be no cracks, pores, incomplete penetration, incomplete fusion, arc pit, slag inclusion, spatter and other defects on the surface of weld and heat affected zone; undercut is not allowed for weld because it is stainless steel.
The reinforcement of butt weld shall not exceed 3mm. The weld leg size of the fillet weld should be the thickness of the thinner part in the assembly joint, and the weld should be smoothly transferred to the base metal.

Pickling passivation

After the weld surface is cleaned, the weld surface should be acid washed and passivated.

Internal quality inspection

The internal quality inspection method, proportion and qualification level of weld shall be in accordance with the provisions of design documents.
For the defects found in the non-destructive inspection of welds, the causes shall be analyzed and the defects shall be eliminated before repair welding, and the repair welding position shall be inspected by the original specified method; the welding materials and processes shall be the same as those for the formal welding. The repaired weld shall be ground to be basically consistent with the original weld and inspected according to the original NDT requirements. When the same part is repaired more than twice, the repair measures shall be formulated and approved by the welding technical director. For the welds requiring local NDT, if no defects are found, the original method shall be used for extended inspection.
According to the requirements of the construction unit, ultrasonic wave is used for random sampling inspection in this project, and a total of 4 welds are detected, the detection ratio is 25%, and the evaluation results are JB / t4730.3-2005 grade I, and the first pass rate is 100%.
In order to ensure the quality, according to the requirements of the construction unit, the welding construction of the project strictly follows the following quality control point table:
Table 4 quality control points

Item Control point Level Item Control point Level
1 Qualification of welding personnel A(R) 9 Groove inspection C(R)
2 Welding equipment approval B(R) 10 Installation inspection of internal argon filling device C
3 Welding material inspection A(R) 11 Assembly dimension and cleaning inspection before welding C(R)
4 Welding procedure qualification A(R) 12 Welding specification parameters C(R)
5 Welding technical disclosure A(R) 13 Ferrite detection A(R)
6 Code management and material identification inspection B 14 Weld appearance inspection C(R)
7 Baking, distribution and recycling of welding rod C(R) 15 Pickling and passivation of weld C
8 Welding environment monitoring C(R)  16 Internal quality inspection of weld A(R)

Conclusion

This paper analyzes the weldability of duplex stainless steel 2205. Through welding procedure evaluation, the welding process is formulated. The welding process is successfully applied to the assembly and welding of circulating pipe in Guangdong Heyuan power plant desulfurization wastewater advanced treatment installation project, and the effect is good.
There are 24 welds of 2205 circulating pipe in the project, all of which are Φ 600 * 12, including 20 butt welds and 4 fillet welds. During the welding process, mp-30 ferrite tester imported from Fischer was used to detect each layer of each weld seam, and the measured values of ferrite in all welds were between 45-55%. After random sampling, four welds were tested for ultrasonic testing, which met the class I standard of JB / t4730.3-2005 nondestructive testing of pressure equipment, and passed the subsequent hydraulic test. Party A was very satisfied with this, This is also the first milestone node of our company in the construction site. The practice shows that the welding process of 2205 dual phase steel is appropriate and the welding quality is excellent, which not only effectively guarantees the construction period, but also enriches the welding performance of the company, improves the company’s image, and lays a solid foundation for continuing to undertake other similar projects of the unit.

Source: China 2205 Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.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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Summary
schematic diagram of internal argon filling device​ - Welding technology of 2205 duplex stainless steel
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Welding technology of 2205 duplex stainless steel
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This paper analyzes the material characteristics and weldability of 2205 duplex stainless steel.
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