How to prevent phase imbalance during welding of duplex steel tube

Duplex stainless steel tube is a stainless steel material with a dual-phase structure of austenite and ferrite, with a typical structure ratio of 50% austenite and 50% ferrite. This structure gives it high strength, high toughness and excellent corrosion resistance, especially in chloride stress corrosion environment. However, during the welding process, improper operation will lead to phase imbalance, which will seriously affect the mechanical properties and corrosion resistance of the pipe.

Causes of phase imbalance in welding
The welding heat cycle will affect the microstructure of the parent material and the weld area. The main causes include:
Too high or too low heat input;
Improper welding speed;
Poor control of preheating temperature and interlayer temperature;
Too fast or too slow cooling speed;
Incorrect selection of welding materials and shielding gas.
The above factors may cause the austenite phase to fail to form fully, or induce the precipitation of harmful secondary phases (such as σ phase and χ phase), causing the microstructure of the weld area to deviate from the ideal ratio of 50:50.

Controlling heat input is a key measure
Maintaining appropriate heat input is the core means to prevent phase imbalance. It is generally recommended to control the heat input between 0.5–2.5 kJ/mm. If the heat input is too high, it will promote the precipitation of σ phase or other brittle phases; if the heat input is too low, the weld metal may cool too fast, the austenite phase cannot be fully precipitated, the ferrite ratio increases, and the toughness decreases.
Using multi-layer multi-pass welding and narrow weld technology can effectively reduce the heat input of a single pass and reduce the formation of unfavorable structures.

Choose a suitable welding method
Different welding methods have a significant impact on the control of the structure. Common welding methods include:
Gas tungsten arc welding (GTAW/TIG): suitable for root welding, controllable heat input, which is conducive to the regulation of the structure;
Gas metal arc welding (GMAW/MIG): suitable for filling and capping welds, and good structures can be obtained by adjusting the parameters appropriately;
Laser welding and plasma arc welding: The heat-affected zone is narrow, and proper control can reduce the deviation of the structure.
The use of pulsed arc welding can achieve more precise heat input control and promote the formation of austenite phase.

Correct selection of welding materials
The composition of the filler material must ensure that the austenite content in the weld can reach the target. Usually, a welding wire or electrode with a slightly higher nickel content than the base material is used. For example, the filler material for UNS S32205 base material can be ER2209 welding wire, which has a nickel content of 8.5%-9.5%, which is higher than the base material, to promote austenite regeneration after welding.
In addition, the impurity content of phosphorus, sulfur and other impurities in the filler material should be avoided to reduce the possibility of forming harmful inclusions.

Gas shielding quality is crucial
During TIG welding or MIG welding, the purity and composition of the shielding gas play an important role in microstructure control. High-purity argon or argon/nitrogen mixed gas should be selected. The right amount of nitrogen can promote the formation of austenite phase and help improve pitting resistance. Usually, a mixed gas with 1-2% nitrogen added has a significant effect on microstructure optimization.
Air infiltration must be avoided during welding to prevent the formation of oxide interlayers or grain boundary oxide zones.

The cooling rate should be moderate
Cooling too fast will prevent austenite from precipitating in time, resulting in excessive ferrite. Cooling too slowly may lead to precipitation of σ phase. The ideal cooling method is natural cooling in the air, avoiding forced air cooling or water cooling.
For thick-walled pipes, temperature control blankets or post-weld insulation measures can be used appropriately to ensure that the cooling curve is gentle and the microstructure transformation is sufficient.

Control interlayer temperature
In multi-pass welding, interlayer temperature control is one of the key steps to prevent phase imbalance. It is generally recommended that the interlayer temperature should not exceed 150°C. Excessive interlayer temperature will cause heat accumulation, increase the grain boundary diffusion rate, and induce the precipitation of brittle phases. Using an infrared thermometer to monitor the temperature in real time can improve the controllability of the welding process.

Post-weld heat treatment and metallographic testing
For Duplex Steel Tubes for special purposes, such as those used in key areas such as marine engineering and oil and gas equipment, it is recommended to perform post-weld solution annealing (generally at 1050–1120°C) and then rapidly cool to restore the ideal duplex structure ratio and dissolve harmful precipitates.

After welding, a metallographic microscope should be used to check the phase ratio of the weld area, or a ferrite content detector (such as a magnetic induction instrument) should be used for quantitative analysis to ensure that the austenite content is between 35% and 65%.