Special welding process and NDT requirements for duplex stainless steel
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. Duplex stainless steel has good resistance to pitting corrosion, stress corrosion, intergranular corrosion, high strength and low linear expansion coefficient. It is mainly used in places with strong corrosiveness below 270 ℃.
The joint performance of duplex stainless steel is mainly in the welding heat affected zone, which is greatly affected by the welding heat cycle. The metallographic structure is good or bad, that is, the proportion of ferrite and austenite in this zone and the size of ferrite grains determine the performance of duplex 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.
Welding process of UNS S32205
According to the construction requirements of the project, according to the requirements of ASME IX and ASME B31.3, the welding process of UNS S32205 duplex stainless steel is developed, and the size of the welding joint for process qualification is shown in Figure 1.
Figure.1 UNS S32205 duplex stainless steel welding joint
Note: a = 70 ˚ B = 3.91mm C = 4.0mm d = 0.8mm
Base metal
The content of Cr and Ni in duplex stainless steel is 18% ~ 28% and 3% ~ 10% respectively. 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. UNS 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.
Welding material selection
Choosing suitable welding materials can control the proportion of ferrite and austenite in the post welding structure, make the proportion of two phases reasonable, and obtain the welded 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 material, 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.
Table 1 – Composition ranges for UNS S32205
|
Grade |
C |
Mn |
Si |
P |
S |
Cr |
Mo |
Ni |
N |
|
|
2205 (S31803) |
Min Max |
– 0.030 |
– 2.00 |
– 1.00 |
– 0.030 |
– 0.020 |
21.0 23.0 |
2.5 3.5 |
4.5 6.5 |
0.08 0.20 |
|
2205 (UNS S32205) |
Min Max |
– 0.030 |
– 2.00 |
– 1.00 |
– 0.030 |
– 0.020 |
22.0 23.0 |
3.0 3.5 |
4.5 6.5 |
0.14 0.20 |
Role of chemical components
Cr and Mo are the forming elements of ferrite, which promote the formation of ferrite. The ratio of alloy elements is large, so the duplex stainless steel is 100% ferrite at the beginning of solidification.
C. Ni, Mn, N, Cu are austenite forming elements. It can promote the precipitation of austenite from ferrite and ensure the precipitation of enough austenite structure from ferrite at a certain cooling rate.
Table 2 – Mechanical properties of UNS S32205
|
Grade |
Tensile Str |
Yield Strength |
Elongation |
Hardness |
|
|
Rockwell C (HR C) |
Brinell (HB) |
||||
|
2205 |
621 |
448 |
25 |
31 max |
293 max |
Physical performance analysis
In ideal duplex stainless steel, ferrite and austenite account for 50% respectively. In the process of material manufacturing and welding, the actual ratio of ferrite to austenite in duplex stainless steel is affected by alloy composition, cooling rate and other factors, resulting in two-phase imbalance. However, the minimum proportion of less phase is ≥ 40%, which can meet the performance requirements. The closer the ratio of two phases is to 50%, the better the comprehensive performance of the material is.
Duplex stainless steel combines the excellent corrosion resistance, plastic toughness and weldability of austenitic stainless steel with the high strength and chloride stress corrosion resistance of ferritic stainless steel, which makes it have the advantages of both austenitic and ferritic. Its comprehensive mechanical properties are good, not only has the high yield strength, but also has the good plastic toughness. Duplex stainless steel is not sensitive to intergranular corrosion, but it has good resistance to pitting and stress corrosion. The pitting corrosion resistance, crevice corrosion resistance, corrosion fatigue resistance and wear corrosion resistance of austenitic stainless steel are better than those of ordinary austenitic stainless steel under severe conditions.
Microstructure balance of duplex stainless steel
In duplex stainless steel, the content of ferrite forming elements is much higher than that of austenite forming elements, so 100% ferrite structure is formed in the early stage of solidification. The nucleation and growth of austenite can only begin at the temperature below the ferritic solid melting line.
Under equilibrium condition, the alloy elements will be segregated and the ferrite forming elements such as Cr and Mo will be concentrated in the ferrite phase by diffusion, while the austenite forming elements such as C, Ni, Mn, N and Cu will be concentrated in the austenite phase by diffusion.
With the decrease of temperature, the equilibrium composition of austenite and ferrite changes along the austenitic and ferritic solid fusion lines. At the same time, the diffusion speed decreases and the temperature continues to decline. Finally, the ferrite and austenite forming elements no longer diffuse and stabilize to form dual phase structure.
Therefore, the ratio of ferrite to austenite in the dual phase steel will be determined by the time that the temperature range exists.
Table 3 – Chemical composition of ER2594 welding wire
| C | Si | Mn | P | S | Cr | Mo | Ni | W | Cu | N | Nb |
| 0.02 | 0.44 | 0.89 | 0.020 | 0.001 | 25.9 | 3.8 | 9.6 | 0.45 | 0.50 | 0.26 | 0.03 |
Preheating, heat input and interpass temperature
The dual phase stainless steel depends on the reasonable dual phase ratio to develop its performance. After welding, it should ensure that the ferrite and austenite phases maintain a reasonable ratio. Generally, the welding method of small heat input and fast welding is used, which is easy to make the weld cooling speed too fast, and the transformation time of high temperature ferrite to austenite is too short, then the weld and the heat affected area will produce too much ferrite structure, while the austenite structure is insufficient, It 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 ℃.
Welding process
Gas tungsten arc welding (GTAW) is adopted. 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 proportion of ferrite and austenite phase, and the mixture of 1% ~ 5% nitrogen has better processability, especially the 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.
Figure.2 Welded joint
Table.4 Welding parameters of UNS S32205 stainless steel
Weld microstructure and mechanical properties
During the post weld solidification process of UNS 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.
Fig.3 Weld microstructure of UNS 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.
Special requirements for nondestructive testing
- (1) PT inspection is required after priming, and filling layer welding is required 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.
Figure.4 Pt detection
Figure.5 Determination of ferrite content
Conclusion
UNS 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 Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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