Duplex Stainless Steel Welding: Heat Input, Interpass Temperature, and Filler Metal Selection

Master duplex stainless steel welding with this expert guide to heat input (0.5–2.5 kJ/mm), interpass temperature limits (100–150°C), and filler metal selection (ER2209, ER2594). Optimize phase balance, prevent sigma phase, and ensure corrosion resistance.

Duplex stainless steel welding succeeds or fails on three interlocking controls: heat input, interpass temperature, and filler metal selection. Get all three right and the weld retains the ~50/50 austenite-ferrite balance that gives duplex its strength and corrosion resistance. Get any one wrong and the joint turns brittle, pits prematurely, or cracks in service. The rules are simple enough to fit in one table:

What Is Duplex Stainless Steel and Why Does Welding Challenge It?

Duplex stainless steel is a two-phase alloy containing roughly equal parts austenite and ferrite, and welding disrupts this balance because the weld pool solidifies almost entirely as ferrite before austenite can re-form during cooling.

The word "duplex" refers to the material's microstructure, not its shape or surface. At room temperature, a well-annealed duplex grade like 2205 contains approximately 50% austenite and 50% ferrite. This balance is the source of every property that makes duplex valuable:

• Strength: Duplex yields at roughly twice the stress of 316L austenitic stainless steel, allowing thinner, lighter structures.
• Corrosion resistance: The combination of chromium, molybdenum, and nitrogen delivers pitting resistance equivalent numbers (PREN) of 35–40 for standard duplex and above 40 for super duplex.
• Stress corrosion cracking resistance: The ferrite phase resists chloride stress corrosion cracking far better than fully austenitic grades.

The problem is that welding melts the metal. When the weld pool cools, it solidifies first as nearly 100% ferrite (ferrite has a higher melting point than austenite). Austenite then nucleates and grows from the ferrite as the temperature drops through the 1200–800°C range. If cooling is too fast, austenite does not have time to form, leaving the weld and heat-affected zone (HAZ) excessively ferritic - brittle and corrosion-prone. If cooling is too slow, the metal lingers in the 700–1000°C window long enough to precipitate sigma phase, an intermetallic compound that destroys toughness and corrosion resistance.

This is why duplex welding lives inside a heat-input window: bounded below by the need to let austenite re-form, and bounded above by the need to escape sigma. Every parameter discussed in this article - heat input, interpass temperature, filler metal - exists to keep the weld inside that window.

How Does Heat Input Affect the Microstructure of Duplex Stainless Steel Welds?

Heat input controls the weld cooling rate, which in turn determines whether the joint re-forms enough austenite (good) or precipitates brittle intermetallic phases (bad); too little heat leaves excess ferrite, while too much heat grows sigma phase.

Heat input is the amount of energy deposited per unit length of weld, calculated as:

Heat Input (kJ/mm) = [Voltage (V) × Current (A) × 60] ÷ [Travel Speed (mm/min) × 1000]

This single number governs how long the weld and HAZ spend in the critical temperature ranges where microstructure is decided:

Two embrittlement mechanisms operate at different temperatures:

Sigma phase (σ): Forms between 600–1000°C, fastest near 850°C. In super duplex grades, sigma can appear in as little as 30–40 seconds at peak temperature. It is an intermetallic compound of chromium and iron that consumes chromium from the matrix, simultaneously reducing corrosion resistance and impact toughness.

475°C embrittlement (α′): Occurs between 300–525°C. A fine chromium-rich precipitate forms in the ferrite phase, causing hardening and loss of ductility. It can begin in as few as 7–10 minutes - well within the range of interpass cooling in thick sections.

Both mechanisms mean that the cooling time through the 800–500°C range (commonly called t8/5) must be neither too short nor too long. Heat input is the primary lever for controlling this cooling time.

What Is the Recommended Heat Input Range for Duplex Stainless Steel?

For standard duplex (2205), use 0.5–2.5 kJ/mm with a preferred target of 0.8–1.5 kJ/mm; for super duplex (2507), restrict to 0.5–2.0 kJ/mm with a preferred target of 0.5–1.0 kJ/mm; lean and hyper duplex grades have their own narrower windows.

The acceptable heat input range depends on the alloy's chromium and molybdenum content, because higher alloy content accelerates sigma-phase formation kinetics. The following ranges are drawn from leading producers and international welding guidelines (Acerinox, ESAB, TWI, and IMOA):

Note the trend: as PREN and alloy content increase, the acceptable heat input window narrows. This is because higher chromium and molybdenum content accelerates sigma-phase precipitation kinetics - super duplex grades can form sigma in seconds, not minutes.

Practical heat input guidance by pass type (for standard duplex 2205):

• Root pass: ~1.5 kJ/mm (enough heat for adequate penetration and austenite reformation)
• Hot pass: ~1.3 kJ/mm (slightly reduced to control grain growth)
• Fill and cap passes: 0.5 – 2.5 kJ/mm, adjusted for thickness and position

For super duplex, reduce each of these by roughly 20%: root ~1.2 kJ/mm, hot pass ~1.0 kJ/mm, fill 0.8–1.2 kJ/mm.

Always confirm the exact range through a qualified welding procedure specification (WPS) and verify the ferrite number on the procedure qualification coupon.

Why Is Interpass Temperature Control Critical for Duplex Stainless Steel?

Interpass temperature directly controls the starting temperature of each subsequent weld pass, which determines the cooling rate; if the joint is too hot when the next pass is laid, cooling slows and the weld spends too long in the 700–1000°C sigma-phase and 475°C embrittlement zones.

In multi-pass welding, each new pass is deposited onto a joint that is already warm from previous passes. The interpass temperature is the temperature of the base metal immediately before the next weld pass begins. A higher starting temperature means a slower cooling rate, because the temperature difference (gradient) between the weld pool and the surrounding metal is smaller.

Think of it like pouring hot coffee into a cold cup versus a warm cup. The cold cup cools the coffee faster. In welding, you actually want a relatively "cold cup" - fast enough cooling to escape sigma phase - but not so fast that austenite cannot re-form.

The danger of excessive interpass temperature is cumulative: each pass adds heat, and if that heat is not allowed to dissipate before the next pass, the joint gradually accumulates time in the embrittlement zone. For super duplex grades, which can precipitate sigma in 30–40 seconds at 850°C, even a modest increase in interpass temperature can push the cooling curve into the danger zone.

Key relationships:

• Higher interpass temperature → slower cooling → more time in sigma window → embrittlement
• Higher interpass temperature → slower cooling through 475°C zone → α′ embrittlement
• Higher alloy content (super duplex) → faster sigma kinetics → tighter interpass limit

What Are the Maximum Interpass Temperature Limits for Duplex Stainless Steel?

Limit interpass temperature to 150°C for standard duplex (2205) and 100°C for super duplex (2507); many project codes apply 150°C uniformly to all duplex grades, and hyper duplex is restricted to 100°C maximum.

Interpass temperature limits are set by the sigma-phase precipitation kinetics of each grade. The more highly alloyed the duplex, the faster sigma forms, and the lower the interpass ceiling must be. The following limits are drawn from producer data sheets and international fabrication guidelines:

Important practical notes:

• Preheat is generally not required. Duplex grades do not need preheating under normal conditions. The only exceptions are ambient temperatures below 5°C (where a light warm-up to 50–75°C prevents condensation) and very thick sections welded by submerged arc (where up to 100°C may be used).
• Avoid preheating above 100°C. Preheat slows cooling and increases risk of sigma formation.
• Forced cooling may be used on thick-wall vessels and pipes by blowing dry air through the bore after removing the purge, but this must be reflected in the procedure qualification test piece.
• Always check the project code. ASME, NORSOK, DNV, and client specifications may impose stricter limits than the base material producer recommends.

How Do You Select the Right Filler Metal for Duplex Stainless Steel Welding?

Select a filler metal that is over-alloyed in nickel by 2–3% relative to the parent metal, because nickel promotes austenite formation and compensates for the rapid cooling of the weld pool that would otherwise leave the deposit excessively ferritic.

When duplex stainless steel solidifies after welding, it solidifies as ferrite first. Austenite then forms from the ferrite as the metal cools through the 1200–800°C range. But weld pools cool much faster than a mill annealing furnace, so there is less time for austenite to form. Without intervention, the weld metal would end up with too much ferrite - often 70% or more - making it brittle and prone to corrosion.

The solution is to use a filler metal deliberately enriched in nickel. Nickel is the most powerful austenite stabilizer among the common alloying elements. By adding 2–3% more nickel than the parent metal contains, the filler ensures that austenite forms faster and more completely during the brief cooling window. For example:

• 2205 parent metal: ~5–8% nickel → ER2209 filler: ~8.5–10.5% nickel
• 2507 parent metal: ~7% nickel → ER2594 filler: ~8–10.5% nickel

The AWS classification system for duplex filler metals follows the pattern ER/E + composition:

• "ER" prefix (AWS A5.9): bare solid wire for TIG (GTAW) and MIG (GMAW) processes
• "E" prefix (AWS A5.4): covered electrodes for stick (SMAW) welding
• The numbers encode composition: e.g., ER2209 = 22% Cr, 9% Ni; ER2594 = 25% Cr, 9% Ni, 4% Mo

Two composition philosophies exist:

• Matching composition filler (with higher Ni): used for as-welded fabrication where no post-weld solution annealing is possible. This is the standard approach for the vast majority of duplex welding.
• Exactly matching filler (same composition as parent): used only when the weld will receive a full solution anneal after welding, which restores the phase balance regardless of the filler's nickel content.

What Filler Metals Are Recommended for Standard Duplex vs Super Duplex Grades?

Use ER2209 (wire) or E2209-16 (electrode) for standard duplex 2205; use ER2594 (wire) or E2594-16 (electrode) for super duplex 2507 and Zeron 100; for dissimilar joints to carbon steel or austenitic grades, use ER309LMo or nickel-based ERNiCrMo-3 (625) depending on service conditions.

The filler metal must match the base metal's chromium, molybdenum, and nitrogen content to maintain corrosion resistance, while being over-alloyed in nickel to ensure austenite reformation. The following selection guide covers the most common scenarios:

Detailed composition comparison of the two primary duplex fillers:

PREN (Pitting Resistance Equivalent Number) is calculated as: PREN = %Cr + 3.3 × %Mo + 16 × %N. A PREN ≥ 40 is the threshold that defines super duplex corrosion performance. The over-alloyed nickel in the filler does not appear in the PREN formula, but it is essential for phase balance - without it, the weld would be over-ferritic regardless of the PREN number.

How Do Heat Input, Interpass Temperature, and Filler Metal Selection Work Together?

These three parameters form an interdependent system: the over-alloyed filler compensates for rapid cooling that would otherwise leave the weld too ferritic, controlled heat input maintains the phase balance window, and interpass temperature limits prevent cumulative embrittlement across multiple passes - no single parameter works correctly without the other two.

Consider what happens if you get only one parameter right:

The interaction can be understood as a feedback loop:

• Filler metal sets the chemical potential for austenite formation. Without over-alloyed nickel, no amount of heat input control can produce a balanced weld - the weld will always be too ferritic.
• Heat input controls the cooling rate that determines whether that chemical potential is realized. Even with the right filler, too little heat means austenite cannot form fast enough; too much heat means sigma phase intervenes.
• Interpass temperature manages cumulative heat across multi-pass welds. Even with correct heat input and filler, successive passes raise the joint temperature; without interpass limits, later passes cool too slowly and embrittle.

A useful analogy: the filler metal is the recipe, heat input is the oven temperature, and interpass control is making sure each layer of a multi-layer cake cools before the next is added. Get all three right, and the result is structurally sound. Get any one wrong, and the whole thing collapses.

This is why welding procedure qualification (WPQ) for duplex stainless steel always includes ferrite measurement (by Ferritescope or metallography) on the test coupon. The ferrite number confirms that all three parameters worked together correctly. The target is typically 30–60 FN (Ferrite Number) for standard duplex and 35–50 FN for super duplex, corresponding to roughly 35–60% ferrite by volume.

What Are Common Welding Defects in Duplex Stainless Steel and How Can They Be Prevented?

The five most common duplex welding defects are excess ferrite, sigma-phase embrittlement, 475°C embrittlement, nitrogen loss, and hydrogen cracking - all preventable through controlled heat input, interpass limits, over-alloyed filler, nitrogen-bearing shielding gas, and proper joint cleanliness.

Each defect traces back to a specific violation of the three-parameter system:

Additional prevention measures include:

• Joint cleanliness: Degrease with acetone; remove all oil, paint, and moisture from at least 25 mm each side of the joint. Use dedicated stainless steel brushes and grinding discs - never tools shared with carbon steel, which transfer iron contamination and cause pitting.
• Back purging: Maintain oxygen levels below 50 ppm (0.005%) in the root purge. If oxygen exceeds 100 ppm, stop and adjust gas flow. Use Ar + 2% N₂ for the purge to prevent nitrogen loss from the root surface.
• Electrode storage: Keep covered electrodes in a heated quiver (300–350°C) to prevent moisture pickup, which is a hydrogen source.
• Avoid autogenous welding unless a full post-weld solution anneal is planned. Autogenous welds (no filler) freeze with excessive ferrite because there is no nickel-rich filler to promote austenite.

Does Duplex Stainless Steel Need Post-Weld Heat Treatment?

No - post-weld heat treatment is generally unnecessary and potentially harmful for duplex stainless steel; the only acceptable treatment is full solution annealing (1020–1100°C) followed by rapid water quenching, and any stress relief above 315°C should be avoided because it can precipitate embrittling phases.

Unlike carbon and low-alloy steels, where post-weld stress relief is common and beneficial, duplex stainless steel reacts poorly to intermediate-temperature heat treatment. The danger zones are:

Guidance for post-weld treatment:

• As-welded service: If the correct filler metal, heat input, and interpass controls were used, the weld is ready for service without any heat treatment. This is the standard and preferred approach.
• Solution annealing (when required): If the weld was made autogenously (no filler) or if the procedure was not controlled, a full solution anneal at 1020–1100°C followed by water quenching can restore the phase balance. Heating must be rapid, hold time is 5–30 minutes depending on section thickness, and water quenching must be immediate to prevent sigma reformation during slow cooling. Inert gas protection should be used because oxidation is severe at these temperatures.
• Never use stress relief: Conventional stress-relief heat treatment (550–650°C) is suitable for carbon steel but will embrittle duplex stainless steel. Do not apply it.

Frequently Asked Questions

Use argon with 2% nitrogen (Ar + 2% N₂) for both the shielding gas and the back purge in TIG and MIG welding. The nitrogen addition replaces nitrogen lost from the weld pool to the atmosphere and helps maintain the austenite-ferrite balance. Keep purge oxygen below 50 ppm. Avoid CO₂-rich gases for TIG, as carbon pickup can cause sensitization.

Can duplex stainless steel be welded autogenously (without filler)?

Autogenous welding is not recommended for duplex stainless steel except for thin-sheet butt joints using nitrogen-bearing shielding gas. Without the over-alloyed nickel filler, the weld solidifies with excessive ferrite and usually requires a post-weld solution anneal to restore properties. For any structural or pressure-bearing application, always use a matched over-alloyed filler.

What is the ideal ferrite percentage in a duplex weld?

The target is approximately 50% ferrite and 50% austenite, measured as 30–60 FN (Ferrite Number) for standard duplex and 35–50 FN for super duplex. Values below 25 FN indicate excessive austenite (rare); values above 70 FN indicate excessive ferrite, which reduces toughness and corrosion resistance and increases hydrogen cracking risk.

Is preheating required for duplex stainless steel?

No. Preheat is generally unnecessary and discouraged for duplex stainless steel. The only exception is when ambient temperature is below 5°C or condensation is present, in which case a light warm-up to 50–75°C is acceptable. Preheating above 100°C increases the risk of sigma-phase formation by slowing the cooling rate.

What is PREN and why does it matter for filler metal selection?

PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3 × %Mo + 16 × %N. It predicts resistance to chloride pitting corrosion. A PREN ≥ 40 defines super duplex performance. The filler metal must deliver a weld deposit PREN equal to or greater than the base metal, so the weld is never the weakest link. ER2209 deposits ~35 PREN; ER2594 deposits ~42 PREN.

What welding processes can be used for duplex stainless steel?

All conventional processes are suitable: TIG/GTAW (preferred for root passes and thin sections), MIG/GMAW (automated production), SMAW/MMA (field repairs), FCAW (offshore and construction), and SAW (heavy plate). Gas-shielded processes are preferred when high toughness or low design temperatures are required. For SAW, strictly control heat input and avoid high dilution rates.

How do I verify that my duplex weld has the correct phase balance?

Three methods are used: (1) Ferritescope measurement on the completed weld (non-destructive, gives a Ferrite Number); (2) metallographic examination of a cross-section (destructive, most accurate); (3) ASTM A923 corrosion testing on the procedure qualification coupon (detects detrimental intermetallic phases). For production welds, Ferritescope measurement is the standard field verification method.

Key Takeaways

Duplex welding lives in a heat-input window. Too little heat leaves the weld over-ferritic and brittle; too much heat precipitates sigma phase. Target 0.5–2.5 kJ/mm for standard duplex and 0.5–2.0 kJ/mm for super duplex.

Interpass temperature prevents cumulative embrittlement. Cap at 150°C for standard duplex and 100°C for super duplex. Preheat is unnecessary and generally harmful.

Over-alloyed filler is non-negotiable. ER2209 for 2205 and ER2594 for 2507 - the extra 2–3% nickel is what allows austenite to re-form during rapid weld cooling.

Control nitrogen loss. Use Ar + 2% N₂ shielding gas and keep purge oxygen below 50 ppm. Nitrogen stabilizes austenite and directly contributes to corrosion resistance.

Verify, do not assume. Measure the ferrite number on the qualification coupon. If it lands in the 30–60 FN band, all three parameters worked together correctly.

Never stress-relieve duplex. Post-weld heat treatment above 315°C can embrittle the joint. Only full solution annealing + water quenching is acceptable, and only when the weld was made without proper filler or control.