Views: 0 Author: Site Editor Publish Time: 2026-11-09 Origin: Site
The strength and corrosion resistance of duplex stainless steel derive from a microstructure in which ferrite and austenite each account for approximately half. Welding fundamentally alters this state: the molten pool solidifies with ferrite as the stable phase at high temperature, and austenite can only re-precipitate through solid-state transformation during cooling. The cooling rate determines whether austenite can "make it back in time" – too fast, and austenite nucleation and growth are limited, leaving excessive ferrite in the weld along with nitride precipitation, degrading toughness and corrosion resistance; too slow, and the weld lingers too long in the intermediate temperature range, causing harmful intermetallic phases such as σ-phase to precipitate, embrittling the joint and sharply degrading corrosion resistance.
Therefore, every parameter in duplex welding – filler metal, heat input, interpass temperature, shielding gas – acts on the same outcome: the phase ratio after cooling. Two welds may both pass visual and radiographic inspection, yet one remains intact after a year of service while the other suffers pitting perforation at the weld – the difference lies in whether phase balance was measured and controlled.
Both materials belong to the duplex stainless steel family, but 2507 has higher alloy content and a narrower process window, so quality control requirements must be upgraded accordingly.
Control Dimension | 2205 (Standard Duplex) | 2507 (Super Duplex) | Quality Implication |
|---|---|---|---|
Filler metal | ER2209 / E2209 | ER2594 / E2594 | Both are "over-alloyed" designs with nickel content higher than the base metal |
Heat input window | 0.5–2.5 kJ/mm (0.5–1.5 recommended) | ≤1.6 kJ/mm (stricter specs ≤1.5; Sandvik gives 0.2–1.5) | The 2507 ceiling is lower, and the window is significantly narrower |
Interpass temperature | ≤150°C | ≤100°C (some specifications tighten further) | Heat accumulation in multi-pass welding poses a greater threat to 2507 |
Preheat | Not required, generally harmful | Not required | Preheating prolongs intermediate-temperature residence and promotes σ-phase |
Shielding gas | Ar or Ar+2%N₂ | Ar or Ar+2%N₂; back purge oxygen <0.05% | Nitrogen compensation for the weld's austenite-forming ability |
Post-weld heat treatment | Generally not performed | Generally not performed; if solution treatment is needed, ≥1050°C, avoiding the rapid σ-phase precipitation range of 850–950°C | Full solution treatment is often impractical for large structures |
Data notes: For 2205 at a heat input of approximately 1.0–1.5 kJ/mm, the δ/γ ratio in the weld and heat-affected zone is closest to that of the base metal; when heat input exceeds 2.5 kJ/mm, the ferrite ratio can rise above 70% and impact performance declines. Engineering practice for 2507 generally caps linear energy at 1.6 kJ/mm and strictly controls interpass temperature within 100°C.
The design logic for duplex filler metals is fundamentally different from that for austenitic stainless steels. The weld composition cannot equal the base metal composition, for three reasons:
① Solidification segregation: The weld solidifies with ferrite as the primary phase, and the austenite transformation depends on diffusion – but welding cooling is far faster than the cooling rate during base metal solution treatment, so residual ferrite is inevitably on the high side. Raising the filler metal's nickel content provides greater driving force for the austenite transformation. ER2209 has a nickel content of 8.5–10.5%, whereas the 2205 base metal has only 4.5–6.5%.
② Nitrogen loss: Nitrogen solubility in the molten pool drops sharply at high temperature, and nitrogen readily escapes into the arc atmosphere. Nitrogen simultaneously stabilises austenite and improves pitting resistance, so adding approximately 2% nitrogen to the shielding gas is a common compensation measure.
③ No opportunity for solution treatment: The base metal is solution-treated at 1040–1100°C and rapidly cooled before delivery, "locking in" the structure; welded joints are usually not given a full solution treatment afterwards, so the weld can only achieve the target ratio in the as-solidified state through composition design + heat input control.
The hard rule that follows: When welding duplex to duplex, never use 308L, 316L, or 309L; use ER2209/E2209 for 2205 and ER2594/E2594 for 2507.
The essential role of heat input is to indirectly control the cooling path, and thereby control the degree of austenite re-precipitation.
Upper limit side: For 2205 above 2.5 kJ/mm, the ferrite ratio rises abnormally and impact performance declines; the ceiling for 2507 is tighter, generally requiring ≤1.6 kJ/mm, with some specifications at ≤1.5 kJ/mm. The essence of the upper limit is preventing σ-phase precipitation caused by excessively slow cooling – σ-phase forms rapidly in the approximately 600–1000°C range, embrittling the joint and destroying corrosion resistance.
Lower limit side: Too low a heat input is equally unacceptable. Cooling that is too fast prevents austenite from precipitating in time, leaving the weld with excessive ferrite and possibly precipitating chromium nitrides, impairing toughness and corrosion resistance. The recommended lower limit for 2205 is approximately 0.5 kJ/mm; Sandvik data for 2507 gives a lower limit of 0.2 kJ/mm, but engineering practice generally does not go below 0.5 kJ/mm to ensure process stability.
Practical implication: Heat input is not a parameter where "lower is better" – it is a window with both a lower and an upper bound that must be constrained in both directions. The WPS should specify both minimum and maximum heat input, and control should be exercised during welding through the combination of current, voltage, and travel speed.
A controlled heat input for a single pass does not mean the multi-pass joint is controlled. Interpass temperature is the "cumulative amplifier" of heat input: even if the linear energy of each pass is within the window, if successive passes are welded without allowing cooling between them, the actual intermediate-temperature residence time experienced by the joint will far exceed the single-pass level, and the risk of σ-phase precipitation rises accordingly.
The interpass temperature ceiling is 150°C for 2205 and 100°C for 2507. This difference is not arbitrary: 2507 has higher alloy content and greater driving force for σ-phase precipitation, so a lower interpass temperature must be used to compress the intermediate-temperature residence window. Interpass temperature must be measured, not judged by feel – waiting for cooling to reach the target before welding the next pass is an indispensable discipline in 2507 welding.
Shielding gas for duplex welding is not merely a means of "excluding air" – it is also a chemical adjustment tool for phase balance.
Front-side shielding: Either Ar or Ar+2%N₂ can be used; the purpose of adding nitrogen is to compensate for nitrogen loss from the molten pool and help austenite formation. However, the amount of nitrogen added must be controlled – excessive nitrogen can itself increase porosity tendency.
Back purge: For pipes and enclosed structures, the oxygen content on the root side must be strictly controlled; 2507 practice requires oxygen content below 0.05%. Back-side oxidation not only forms heat tint but also consumes nitrogen and chromium near the surface, destroying root corrosion resistance. Using 90%N₂+10%H₂ Formier gas as the back purge gas can raise the critical pitting temperature of the root by at least 10°C, making it an effective means of improving root corrosion resistance.
CO₂ prohibited: CO₂ should not be used in duplex steel shielding gas, because the oxidizing nature of CO₂ interferes with the molten pool's nitrogen balance and increases the risk of carbon pickup.
Welding procedure qualification for duplex steels should follow ISO 15614-1 (or ASME Section IX, depending on the project specification). The special requirements for duplex qualification are:
Impact testing: One set of specimens should be taken from the weld centre, the fusion line, and the heat-affected zone 2 mm from the fusion line; classification society rules (such as CCS) require a test temperature of -20°C and impact energy of not less than 27 J. This requirement directly verifies whether phase balance and harmful phase precipitation have been controlled within an acceptable range.
Ferrite content measurement: This should be included as a qualification item, measured on the deposited weld metal.
Corrosion testing: Performed in accordance with ISO 17781 or the project specification to verify the joint's pitting/crevice corrosion resistance.
The acceptance range for ferrite content in duplex stainless steel welded joints is typically 30–70%, with 35–65% being more common. The ferrite content requirement for 2205 base metal is generally 40–60%, and the weld root may be relaxed to 30–70%.
Measurement methods: Ferrite content can be measured with a ferrite scope in accordance with EN ISO 8249 or GB/T 1954, or determined quantitatively by metallographic point counting per ASTM E562.
A blind spot that must be guarded against: The ferrite scope measures the content of the magnetic phase. σ-phase is non-magnetic – a weld already embrittled by σ-phase precipitation may still read a "passing" number on the ferrite scope. Therefore, acceptable ferrite content cannot substitute for metallographic examination and impact testing; acceptance must be a combined judgment of chemical composition, phase ratio, impact toughness, and corrosion performance, not a single reading.
Control Level | 2205 | 2507 | General Principle |
|---|---|---|---|
Filler metal | ER2209/E2209 | ER2594/E2594 | Over-alloyed, high nickel; austenitic stainless fillers prohibited |
Heat input | 0.5–2.5 kJ/mm | ≤1.6 kJ/mm | WPS specifies both upper and lower limits |
Interpass temperature | ≤150°C | ≤100°C | Measured, not judged by feel; multi-pass welds must cool to target |
Shielding gas | Ar or Ar+2%N₂ | Ar or Ar+2%N₂; back purge O₂<0.05% | CO₂ prohibited; nitrogen compensates austenite formation |
Preheat / PWHT | No preheat; PWHT usually not performed | No preheat; if solution treatment is needed ≥1050°C, avoid 850–950°C | PWHT cannot substitute for process control |
Qualification & acceptance | ISO 15614-1; ferrite + impact + corrosion | Same, but impact and ferrite requirements are stricter | Ferrite scope has a σ-phase blind spot; metallography and impact testing are needed for cross-verification |
The core principles can be summarised in three sentences:
Phase balance is the objective; heat input and interpass temperature are the means – all parameters ultimately serve the ferrite-austenite ratio after cooling.
2507 is not a "more corrosion-resistant 2205" – it is a material with a narrower window and greater sensitivity to heat accumulation; the process discipline for 2205 cannot simply be transplanted to 2507.
Acceptance cannot rely on the ferrite number alone – the "magnetic blind spot" of σ-phase means that metallography and impact toughness must form a cross-verification, otherwise a "passing reading" may conceal an already embrittled joint.
The difficulty of duplex welding is not "whether you can weld it," but "whether the microstructure after welding is still within the window" – and this can only be guaranteed through the systematic coordination of filler metal, heat input, interpass temperature, and the acceptance system.
