How to Select the Correct Welding Procedure for Duplex Stainless Steel Tubes to Avoid Loss of Corrosion Resistance?

A few minutes of poor welding can completely destroy the corrosion-resistant qualities of a duplex stainless steel tube. Every joint manufactured in the shop or in the field must carefully preserve the distinct two-phase microstructure that gives duplex its strength and resistance. Maintaining the alloy's hard-won corrosion resistance in harsh chloride and acid conditions is more important than just preventing leaks.

The Metallurgical Foundation: What Makes Duplex Unique
A balanced blend of roughly 50% austenite and 50% ferrite is found in duplex stainless steels (such as UNS S32205, S31803, or 2205). This two-phase structure offers:

High mechanical strength (about twice 316L's).

Outstanding resistance to stress corrosion cracking caused by chloride (SCC).

A PREN (pitting resistance equivalent number) of 34–36 indicates good resistance to pitting and crevice corrosion.

The material melts and quickly solidifies during the welding process. The final phase balance in the weld metal and the heat-affected zone (HAZ) is determined by the cooling rate and thermal history. Two detrimental things could happen if the process is done incorrectly:

Overproduction of ferrite: When ferrite predominates due to slow cooling or high heat input, toughness and corrosion resistance are diminished.

Intermetallic phase precipitation, particularly the sigma (σ) phase (FeCrMo) and chi (χ) phase, which develop between 600 and 900 °C. The development of these phases depletes the surrounding matrix of these essential alloying elements since they are rich in chromium and molybdenum. Severe embrittlement and a sharp decline in pitting corrosion resistance are the outcomes.

On the other hand, too much austenite may be produced by insufficient heat input or extremely quick cooling, which would decrease strength. As a result, a limited processing window needs to be adhered to.

Crucial Elements of a Duplex Stainless Steel Welding Process
The following factors are accurately controlled by an appropriate duplex stainless steel welding method tube corrosion prevention technique.

1. Selection of Filler Metal
To preserve phase equilibrium and corrosion resistance, the alloy content of the filler metal must equal or surpass that of the base metal. The standard filler for 2205 duplex is ER2209 (UNS S39209). Austenite production in the weld deposit is encouraged by its composition, which is somewhat over-alloyed in nickel and nitrogen relative to the base metal. This makes up for the ferrite-favoring effect of quick solidification.

Filler metal like ER2594 is utilized for super duplex grades (like 2507). On duplex base metal, never employ an austenitic filler (such as 308L or 316L); the resulting weld will have the incorrect corrosion characteristics and may crack.

2. Control of Heat Input
Travel speed, voltage, and current all affect heat input. The suggested heat input range for duplex stainless steels is usually between 0.5 and 2.5 kilojoules per millimeter. A heat input that is too high encourages sluggish cooling, which permits the formation of sigma phase and the coarsening of ferrite; a heat input that is too low runs the risk of incomplete fusion or excessive ferrite.

Gas tungsten arc welding (GTAW) of 2205 tubes typically aims at 0.5–1.5 kJ/mm. If the interpass temperature is closely regulated, somewhat larger ranges (up to 2.0 kJ/mm) might be acceptable for gas metal arc welding (GMAW) with pulsed spray transfer.

3. Temperature Interpass
It is necessary to keep the interpass temperature-the temperature of the weld area prior to the subsequent pass-below 150°C, and frequently below 100°C during the initial passes. Heat is accumulated by high interpass temperatures, which lengthens the cooling period and encourages sigma phase precipitation. Natural air cooling in between passes might be adequate for tubes with thin walls (e.g., 2–3 mm). Forced air cooling or even longer wait times between passes are necessary for thicker portions.

4. Back purging and shielding
Both the weld face and the root side of duplex stainless steel welds need to be shielded from ambient oxygen. Regarding tube welding:

Argon with 1-3% nitrogen is a common primary shielding gas. A tiny amount of nitrogen, an austenite stabilizer, added to the shielding gas aids in preserving the intended phase balance in the weld metal.

Back purging: To stop the root bead from oxidizing, pure argon (or argon with 2–5% nitrogen) is purged from the tube's interior. Chromium oxide scales, which are produced by oxygen pollution, lead to chromium depletion and a reduction in corrosion resistance.

5. Welding Method and Procedure
For duplex tubes, pulsed arc methods (pulsed GTAW or pulsed GMAW) are used because they

Use a high peak current followed by a low background current to reduce heat input.

Boost weld pool management and lower the chance of burn-through on narrow walls.

Create a microstructure that is more consistent and finer.

Instead of using wide weave beads, which increase heat input and slow cooling, stringer beads (straight pass without weaving) are utilized for manual welding.

Post-Weld Cleaning (Pickling and Passivation) is the Crucial Step
Heat tint (oxide scale) will appear on the surface of the weld and the surrounding HAZ even if the heat input is ideal. Because chromium interacts preferentially with oxygen, leaving the underlying metal with less chromium, this oxide layer is chromium-depleted. Pitting and crevice corrosion in chloride service are easily initiated in this deficient zone.

Cleaning after welding is required. There are two efficient methods:

Chemical Removal (Pickling)
Pickling paste or bath: The heat tint is dissolved and the chromium-depleted layer is removed using a solution of nitric acid (HNO₃) and hydrofluoric acid (HF). Pickling paste is applied to the weld and the heat-affected area of tubes, left for a predetermined amount of time (such as 15 to 60 minutes), and then completely rinsed with water.

Immersion pickling: Although it needs specific equipment, submerging small tube assemblies or spool parts in a heated pickling bath is more uniform.

Passivation After Mechanical Cleaning
Grinding or brushing: Fine abrasive pads or non-metallic brushes (such as stainless steel wire brushes, which are only used on duplex and never on carbon steel) can be used to remove the heat tint. Without leaving any imbedded iron particles, the entire tinted layer must be removed by grinding.

Passivation: To reinstate the passive chromium oxide coating, a nitric or citric acid solution is applied to the surface following mechanical cleaning. Grinding by itself can smear metal and produce a less protective surface, therefore this step is crucial.

After appropriate pickling, the weld and HAZ should have the same appearance and corrosion resistance as the parent tube since a good weld is impervious to corrosion. The weld will be the first point of failure if the heat tint persists.

Ferrite Measurement for Weld Quality Verification
A ferrite scope, a magnetic induction device, is used to measure the ferrite content of the weld metal and the HAZ in order to verify that the welding process has maintained the proper phase balance. For duplex stainless steels, a ferrite content of 30–70% (balancing austenite) is often acceptable. A lot of specs aim for 40–60%.

Strength is decreased by excessive austenite, which is indicated by values below 30%.

Excessive ferrite is indicated by values above 70%, which reduces toughness and may suggest the existence of sigma phase (sigma is non-magnetic, but the ferrite percentage decreases as sigma occurs; an unusually low or inconsistent ferrite reading can be a clue).

In critical applications, a welded coupon may undergo a microstructural analysis (metallography) to detect intermetallic phases. Within the ferrite grains, the sigma phase manifests as an acicular or blocky precipitate.

Typical Errors and How to Prevent Them
Preventing Mistake Consequences
Using a filler metal of 308L or 316LWeld is susceptible to cracking and has poor corrosion resistance.Use matching duplex filler (ER2209 for 2205) at all times.
Overheating (over 2.5 kJ/mm)Low PREN, embrittlement, and precipitation in the sigma phaseDetermine the heat input, employ a pulsed arc, and set a speed restriction.
Temperature interpass above 150°CHeat buildup encourages sigmaUse a thermocouple to measure; let it cool in between passes.
No tube root back purgeChromium depletion, root bead oxidation, and crevice corrosionUse argon or an argon-nitrogen combination to purge
Leaving the heat tint in placeAt the weld edge, localized corrosion begins.Pickle or passivate using a machine.
Using grinding wheels or carbon steel brushesGalvanic corrosion is caused by iron pollution.Use specialized stainless steel tools, ideally just for duplex.
Qualification and Welding Procedure Specification (WPS)
A formal Welding Procedure Specification (WPS) must be created and qualified in accordance with a code like ASME Section IX, AWS D1.6, or ISO 15614 for any industrial application. The following should be stated clearly in the WPS for duplex stainless steel tubes:

base metal thickness range and grade.

Classification of filler metal (e.g., ER2209).

GTAW, GMAW, or SMAW welding techniques.

composition and flow rate of shielding gases.

gas and technique for back purge.

Heat input range and electrical parameters (voltage, current, and travel speed).

exceed the temperature restriction, such as a maximum of 150°C.

Preheating is usually not necessary for duplex unless the ambient temperature is below 5 °C.

Pickling paste, immersion, or mechanical + passivation are methods of post-weld cleaning.

Ferrite content acceptance standards (e.g., 30–70% ferrite in weld metal).

Tensile and bend tests as well as corrosion tests (such as the ferric chloride pitting test according to ASTM G48) are examples of qualification testing. An eddy current inspection or hydrostatic test may also be necessary for tube welds.

Considerations for Field Welding
Because environmental control is more difficult, welding duplex tubes in the field (for tie-ins or repairs, for example) is more difficult than shop welding. Particular focus needs to be given to:

Utilizing gas lenses and screens to stop shielding gas disruption is known as wind shielding.

Temperature monitoring: Use contact pyrometers or infrared thermometers to measure the interpass temperature.

Purging: It may not be feasible to empty the full length of large-diameter tubes; instead, a limited purge dam made of inflatable rubber plugs can be utilized.

Pickling: To prevent spillage onto nearby equipment, field pickling pastes must be applied and neutralized carefully.

In conclusion, the quality of the material depends on the hands that work on it.
For duplex stainless steel tubes, choosing the right welding technique is crucial to preventing a disastrous loss of corrosion resistance. Excessive heat input, high interpass temperatures, mismatched filler metals, or the presence of chromium-depleted heat tint can all undermine the balanced austenite-ferrite microstructure that gives duplex its strength and pitting resistance. The weld zone maintains the same corrosion performance as the parent tube thanks to a methodical process that controls heat input (usually 0.5–2.5 kJ/mm), interpass temperature below 150°C, ER2209 filler for 2205 base metal, inert back purging, and rigorous post-weld pickling. Success is confirmed by ferrite measurement (30–70% ferrite). A well-done weld is impervious to corrosion; the material is only as good as the hands that join it.