Special welding process and NDT requirements for duplex stainless steel

1. Overview
Duplex stainless steel is a kind of stainless steel with dual phase microstructure of ferrite and austenite. The proportion of ferrite and austenite in solution structure is about 50%, and the content of less phase is more than 30%. The metallographic structure determines that the performance of the Ferritic Austenitic duplex stainless steel is between ferritic stainless steel and austenitic stainless steel. It has the advantages of both stainless steels. It not only has good plasticity, toughness, corrosion resistance and weldability, but also has stronger intergranular corrosion resistance than other kinds of stainless steel. Therefore, it has many advantages in the fields of energy, chemical industry, pharmaceutical industry, papermaking, seawater desalination, etc It is widely used. Faced with the high standards and strict requirements of the owner Aramco, the Saudi Jizan project of China Power Construction Nuclear Power Co., Ltd. applies duplex stainless steel to the public water and fire water systems, so that the pipeline system can work for a long time under the conditions of poor service environment and high corrosion resistance requirements.
Bidirectional stainless steel is the stainless steel that points to metallographic structure to have ferrite and austenite double phase structure to plant, in solid solution structure ferrite and austenite occupy the proportion of about 50% respectively, the content of less phase commonly also is in 30% above. Microstructure determines the ferrite to austenite of duplex stainless steel performance between ferritic stainless steel and austenitic stainless steel, the advantages of both two kinds of stainless steel, not only has good plasticity and toughness, corrosion resistance and weldability, and is more stronger than other types of stainless steel intergranular corrosion resistant ability, so in energy, chemical, pharmaceutical, paper making, water desalination, and other fields has been widely used. Faced with the high standards and strict requirements of the owner aramco, the Saudi jizan project of China power construction nuclear power plant applies duplex stainless steel to the public water and fire water system, so that the pipeline system can work for a long time under the conditions of poor service environment and high anti-corrosion requirements.
2. Welding process of S32205

According to the construction requirements of the project, according to the requirements of ASME IX and ASME B31.3, the welding process of S32205 duplex stainless steel is developed, and the size of the welding joint for process qualification is shown in Figure 1.

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Figure 1 S32205 duplex stainless steel welding joint
Note: a = 70 ˚ B = 3.91mm C = 4.0mm d = 0.8mm
(1) base metal: the content of Cr and Ni in duplex stainless steel is 18% ~ 28% and 3% ~ 10% respectively under the condition of low C content. Some steels also contain Mo, Cu, Nb, Ti, N and other alloy elements. In this paper, UNS S32205 steel pipe with thickness of 3.91mm and diameter of 2in (1in = 25.4mm) is used.
Its chemical composition and mechanical properties are shown in Table 1 and table 2 according to ASTM A790. S32205 duplex stainless steel with carbon content less than 0.03% belongs to ultra-low carbon stainless steel. Ultra-low carbon content can improve the weldability of the material, reduce the tendency of carbide precipitation at the grain boundary, and improve the intergranular corrosion resistance. The addition of nitrogen can improve the corrosion resistance of the metal after welding, improve the mechanical properties of the weld, promote the formation of two-phase structure, balance the proportion of two phases, high content of chromium and molybdenum can improve the corrosion resistance of steel.

(2) selection of welding materials: select appropriate welding materials to control the proportion of ferrite and austenite in the post welding structure, so as to make the proportion of two phases reasonable, and obtain the welding joint with mechanical properties greater than or equal to the base metal. After comparison, the er2594 welding wire of Bohler company is used as the welding materials, with the diameter of 1.6-2.4mm. The main reason for selection is that the content of nickel element in er2594 welding wire chemical composition is relatively higher than that in the base metal, which can promote the formation of austenite and stabilize the two-phase ratio in the process of rapid cooling after welding. If only the welding material with the same composition as the base metal is selected, the content of ferrite in the weld is higher. The chemical composition of welding materials is shown in Table 3.

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(3) preheating, heat input and interpass temperature: dual phase stainless steel performs by reasonable dual phase ratio. After welding, the ferrite and austenite should be kept at a reasonable ratio. Generally, small heat input and rapid welding are used for welding, which is easy to make the cooling speed of weld too fast. If the transformation time of high temperature ferrite to austenite is too short, too much iron will be produced in weld and heat affected area The lack of ferrite structure and austenite structure will reduce the corrosion resistance of duplex stainless steel and the toughness of welded joints. Therefore, the preheating temperature of the base metal shall be ≥ 10 ℃, and the room temperature under the perennial high temperature environment of Saudi Jizan project is enough. If a larger heat input is selected, the cooling rate is too slow, the ferrite grains will be coarse, and the intermetallic phase will be produced, which also reduces the joint toughness and corrosion resistance. When multi pass welding is carried out, the interlayer temperature shall be controlled. If the interlayer temperature is too high, the heat will accumulate, the heated area will increase, the heat affected area will be widened, the grain will be coarse, the strength and toughness will be reduced, and the interlayer temperature of the weld bead shall not exceed 58 ℃.
(4) welding process: gas shielded tungsten argon arc welding (GTAW) is used, and the shielding gas is a mixture of 98% AR and 2% N2. The nitrogen element in the molten pool will escape when the pure argon shielded welding wire is used for melting. The nitrogen element mentioned above can increase the content of austenite phase, balance the ratio of ferrite and austenite phase, and the mixture of 1% ~ 5% nitrogen has better processability, especially It is root weld, nitrogen protection is particularly important. When the nitrogen content in the mixed gas is more than 5%, the tungsten is easy to burn, resulting in the instability of the arc. Therefore, 98% Ar + 2% N2 gas tungsten arc welding is selected. During welding, keep the back continuously filled with protective gas, and the oxygen content of the back filled with protective gas shall be less than 0.05%.
The welding current is 70 ~ 110A, the arc voltage is 10 ~ 16V, the welding speed is 40 ~ 95mm / min, and the interlayer temperature is less than 58 ℃. The welding wire of ϕ 2.4mm shall be used as the backing of the root weld bead of the welding joint, and the welding wire of ϕ 1.6 ϕ 2.4mm shall be used as the backing of the filler and cover weld bead. The heat input shall be small and shall not exceed the requirements of the process specification. The heat input during filling and capping shall not be higher than the heat input during bottoming.

The welding parameters are shown in Table 4, and the welded joints are shown in Figure 2.

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Figure 2 welded junction

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(5) weld structure and mechanical properties: during the post weld solidification process of S32205 stainless steel, the ferrite structure solidifies first, and then decreases with the temperature. Some ferrites begin to transform into austenite at the grain boundary and grow into ferrite crystal. The post weld corrosion test structure is shown in Figure 3.

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Fig. 3 weld microstructure of S32205 duplex stainless steel weld corrosion test results
The results show that the ferrite phase ratio is 42.5% ± 7%, and the microstructure ratio meets the requirements. The test results of mechanical properties of the weld and heat affected zone show that the minimum tensile strength is 858mpa, the bending test result is qualified, the maximum hardness is 265.2hbw, the impact test at – 20 ℃ is kV = 35 ~ 45j, the heat affected zone is kV = 48 ~ 55j, and the mechanical properties are superior to the base metal and meet the standards.
3. Special requirements for NDT

  • (1) PT test shall be conducted after priming, and filling layer welding shall be conducted after qualification (see Fig. 4).
  • (2) the ferrite content shall be measured within 30% ~ 60% of the completed weld center (see Figure 5).
  • (3) take 20% of all welds after welding for hardness test of weld area and heat affected area, and the hardness value shall not exceed 285hbw.

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Figure 4 Pt detection

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Figure 5 Determination of ferrite content
4. Conclusion
S32205 duplex stainless steel has good weldability and mechanical properties, which can be used in power plant. In this paper, a reasonable welding process is developed, and the special requirements for welding and NDT of duplex stainless steel in the process of construction are put forward to ensure the quality of field welding construction.

Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – 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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Nondestructive Testing (NDT) plays an important role in assuring that structural and mechanical components perform their function in a safe, reliable, and cost-effective manner. NDT technicians perform the necessary tests to locate the indicators and discontinuities that may cause failures or shut downs in such systems. These tests are performed in a manner that does not affect the future usefulness of the object or material – hence, the name “nondestructive.” NDT allows for careful and thorough materials evaluation without the need for deconstruction or damage. NDT is typically used at various points in a part’s life cycle. NDT can be used prior to the use of a component for the sake of quality control. NDT is also employed while components are in use to detect service related conditions caused by wear, fatigue, corrosion, stress, or other factors which affect reliability.
NDT Technologies Include:
Visual and Optical Testing (VT)
Visual Examination can be an effective way to recognize surface imperfections that could adversely affect a part or component. Visual Examiners use knowledge of how a part is manufactured, the function of the human eye, lighting requirements, and precise measuring tools to evaluate materials. Computer controlled camera systems and optical aids such as borescopes may also be used to recognize and measure features of a component.
Radiography (RT)
Radiographic Examination involves using radioactive isotopes (gamma rays) or X-rays on materials to peer qualitatively for indications the same way a doctor looks for fractures or other conditions within the body. Radiation is directed through a part and projected onto film or a digital detection device leaving an image which can be examined by the qualified Radiographer.
Ultrasonic Testing (UT)
Ultrasonic Examination uses high-frequency sound waves which are transmitted into a material to detect discontinuities or locate changes in material characteristics. Sound is introduced into the object being examined and reflections from internal imperfections, areas of acoustic impedance, or varying geometrical surfaces are returned to a receiver.
Magnetic Particle Testing (MT)
Magnetic Particle Examination is accomplished by inducing a magnetic field into a ferromagnetic material and applying iron particles to the surface of the item being examined. Surface and near-surface discontinuities affect the flow of the magnetic field within the part causing the applied particles to gather at locations of flux leakage, thus producing a visible indication of the irregularity on the surface of the material.
Penetrant Testing (PT)
Penetrant Examination is performed with a dye solution. Once applied to the surface, the dye will effectively penetrate any surface-breaking cavity. Excess solution is removed from the object. A developer is then applied to draw out any penetrant that remains unseen. With fluorescent dyes, ultraviolet light is used to make the “bleed-out” fluoresce brightly, allowing imperfections to be readily seen. With visible dyes, a color contrast between the penetrant and developer makes the “bleed-out” easy to see.

Why NDT is required?

NDT or NDE can be used to find, size and locate surface and subsurface flaws and defects. NDT plays a crucial role in everyday life and is necessary to assure safety and reliability. … NDT techniques can then be used to monitor the integrity of the item or structure throughout it’s design life.

What is PT test in NDT?

Dye Penetrant Inspection (DPI) also called as Liquid Penetrant Inspection (LPI) or Penetrant Test ( PT) is fast, economical and widely used non destructive test method to detect surface-breaking discontinuities in all non-porous materials (metals, plastics, or ceramics).

What are the different types of welding process?

More than 30 different types of welding exist, and they range from simple oxy-fuel to high-tech processes such as laser beam welding. However, only four welding types are used commonly, and they are MIG, TIG, Stick and Flux- Cored arc welding.

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