UNS S32750 Super Duplex Welding: Filler Metal, Heat Input and Testing

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

UNS S32750 welding requires tighter process control than ordinary austenitic stainless steel and even standard duplex grades. S32750, commonly known as 2507, is a super duplex stainless steel whose ferritic-austenitic microstructure gives it high strength and excellent chloride corrosion resistance. But that same microstructure is sensitive to the welding thermal cycle, and a weld made without attention to filler metal, heat input, and cooling can end up with degraded phase balance, toughness, or corrosion resistance — even when the base metal is perfectly good.

This article explains why S32750 welding needs careful control, what happens during the weld, how filler metal and heat input influence the result, and how testing confirms the joint is fit for service. It is written for welding engineers, materials engineers, pressure equipment and pipe fabricators, and oil and gas or marine equipment buyers who need a decision-oriented overview rather than a full welding procedure specification.

1. Why S32750 Welding Needs Tight Control

UNS S32750 is a super duplex stainless steel, meaning it is a duplex grade with a higher alloy content — particularly chromium, molybdenum, and nitrogen — than standard duplex grades such as 2205. Its microstructure is a balance of ferrite and austenite, and this balance is what delivers the grade’s combination of strength and corrosion resistance.

Welding applies intense, localized heat and then cools the material. This thermal cycle changes the ferrite/austenite balance in the weld metal and heat-affected zone (HAZ). The goal of controlled S32750 welding is not simply to produce a strong joint, but to restore and maintain a suitable microstructure and corrosion resistance after welding. A joint can pass a tensile test and still have an unacceptable phase balance or reduced localized corrosion resistance.

2. What Happens During S32750 Welding

During welding, several distinct regions form, and each behaves differently:

• Weld metal: The melted and re-solidified region, whose chemistry comes from both the base metal and the filler metal.
• Fusion zone: The boundary region where weld metal and base metal mix.
• Heat-affected zone (HAZ): The base metal adjacent to the weld that is heated but not melted, and whose microstructure changes with the thermal cycle.
• Ferrite/austenite transformation: As the material cools from high temperature, austenite re-forms from ferrite. The amount of austenite that re-forms depends heavily on the cooling rate and the chemistry.
• Cooling rate: The speed at which the weld cools through the transformation temperature range.

Both too-fast and too-slow cooling can produce an undesirable microstructure. Cooling too quickly can leave excessive ferrite because there is not enough time for austenite to re-form. Cooling too slowly, or holding at intermediate temperatures for too long, can promote grain growth or the formation of undesirable intermetallic phases. The aim is a controlled cooling path that yields an acceptable phase balance.

3. Filler Metal Selection

Filler metal selection is one of the most consequential decisions in S32750 welding, because the filler directly shapes the chemistry of the weld metal and therefore its phase balance and corrosion resistance.

• Why filler chemistry matters: The weld metal is a mix of base metal and filler. If the filler does not compensate for the loss of alloying elements during welding, the weld metal can end up with the wrong phase balance.
• Promoting austenite: A filler with a suitably adjusted composition — commonly with a higher nickel content than the base metal — helps ensure that enough austenite re-forms during cooling.
• Nickel and phase balance: Nickel is an austenite stabilizer. Adjusting the nickel level in the filler is a key way to steer the weld metal toward an acceptable ferrite/austenite balance.
• Weld metal corrosion resistance: The filler must also be selected so the weld metal retains adequate pitting and crevice corrosion resistance for the service.

There is no single filler metal that is correct for every S32704 or S32750 weld across all processes, positions, and thicknesses. The filler must be chosen based on the specific welding process, the base metal, the applicable AWS/ASME specification, and the qualified welding procedure (WPS) supported by a procedure qualification record (PQR). A filler that works well for GTAW may not be the correct choice for SAW.

4. Heat Input and Cooling Rate

Heat input and cooling rate are closely related and are controlled through the welding parameters. Heat input depends on the welding current, voltage, and travel speed; a slower travel speed or higher current generally raises heat input, which slows the cooling rate.

• Current and voltage: Together with travel speed, these determine the energy delivered to the joint.
• Travel speed: Slower travel speed increases heat input for a given current and voltage.
• Heat input: The energy per unit length of weld, typically expressed in kJ/mm or kJ/in.
• Cooling rate: Higher heat input generally means slower cooling; lower heat input means faster cooling.

The concern runs in both directions. Very low heat input and rapid cooling can leave excessive ferrite in the weld metal and HAZ, because austenite does not have time to re-form. Very high heat input and slow cooling can cause grain growth, undesirable phase formation, or a reduction in some properties. For this reason, no single "universal" heat-input value applies across all thicknesses, positions, and processes. The acceptable range must come from the qualified WPS and the applicable code.

5. Interpass Temperature and Welding Sequence

Beyond heat input, the thermal management of a multi-pass weld also depends on interpass temperature and the sequence of passes.

• Interpass temperature: The temperature of the weld area before the next pass is deposited. For super duplex grades, the interpass temperature is usually kept controlled and limited to avoid excessive thermal exposure.
• Pass sequence: The order and placement of passes affects how heat builds up in the joint.
• Cooling between passes: Allowing the joint to cool between passes prevents heat from accumulating and driving the microstructure in an undesirable direction.
• Preheating: Preheating is generally not applied in the same way as for carbon steel. Whether any preheat is used depends on the specific material, thickness, and code requirements.
• Shielding gas: Correct shielding protects the weld pool from nitrogen loss and oxidation, both of which can harm the final microstructure and surface condition.

The key point is that super duplex welding requires deliberate control of the whole thermal cycle — not just the arc parameters in isolation.

6. Phase Balance and Corrosion Resistance

The reason phase balance matters is that it links directly to performance:

• Ferrite/austenite balance: A balanced two-phase structure delivers the intended combination of strength, toughness, and corrosion resistance.
• Pitting and crevice corrosion: Localized corrosion resistance depends on the chemistry and phase balance of the weld metal, not just the base metal.
• Chloride SCC: The duplex structure resists chloride stress corrosion cracking, but only if the phase balance is maintained.
• Toughness: Excessive ferrite can reduce toughness, especially at low temperature.

A welded joint’s phase balance can differ from the base metal, so the base metal’s PREN or MTC alone does not describe the weld. If the weld metal is too ferritic, both toughness and localized corrosion resistance can suffer. At the same time, the goal is not simply "ferrite as low as possible" — a healthy duplex weld needs an appropriate balance, not the elimination of ferrite.

7. Welding Processes

S32750 can be welded with the common arc processes, each with different characteristics:

• GTAW / TIG: Offers good control over heat input and weld pool, widely used for root passes and precise work.
• GMAW / MIG: Higher deposition and productivity, but requires careful parameter control to manage heat input.
• SMAW: Useful for field work; electrode selection and welder skill are critical for maintaining shielding and chemistry.
• SAW: High productivity for heavy sections, but the higher heat input means cooling rate and interpass control must be carefully managed.

Each process influences heat input, shielding, productivity, and weld quality differently. The process choice should be part of the welding plan and validated through the WPS/PQR, not treated as an afterthought.

8. Testing and Inspection

Testing confirms that the finished weld actually meets the intended microstructure and mechanical requirements. Common elements include:

• Visual inspection: Surface condition, weld profile, and freedom from visible defects;
• Dimensional / weld inspection: Conformance to the specified weld size and geometry;
• NDE: Non-destructive examination such as radiography or ultrasonic testing where required;
• Ferrite measurement: A check on phase balance, used as one part of verification rather than the sole quality metric;
• Tensile testing: Confirmation of joint strength;
• Impact testing: Where required by the code or service;
• Corrosion testing: Where specified, to confirm localized corrosion resistance;
• PMI / traceability: Positive material identification and heat-number traceability where applicable.

The exact test list is not identical for every project. It must be defined by the applicable code, product standard, project specification, and the WPS/PQR. Ferrite measurement is useful but should not be treated as the only indicator of weld quality; mechanical and, where relevant, corrosion testing complete the picture.

9. Common S32750 Welding Problems

10. How to Specify S32750 Welding in an RFQ

A clear RFQ removes ambiguity and protects the project. When specifying S32750 welding, include at minimum:

• UNS S32750: Stated explicitly, not just "2507";
• Base-metal specification: The applicable product standard for the base material;
• Product form: Pipe, plate, fitting, or other form;
• Welding process: GTAW, GMAW, SMAW, SAW, or a combination;
• Filler metal: The specified filler and its applicable specification;
• WPS/PQR: Requirement for qualified welding procedures and qualification records;
• Heat-input control: The required heat-input range;
• Interpass temperature: The required interpass temperature control;
• Shielding gas: The specified gas and back-purge requirements;
• Ferrite / phase-balance requirements: Any specified phase-balance or ferrite limits;
• NDE: Non-destructive examination requirements;
• Mechanical testing: Tensile, and impact where required;
• Corrosion testing: Where required by the service;
• Welder qualification: Requirement for qualified welders;
• MTC and heat-number traceability: Material test certificates and traceability to heat number.

11. Common Purchasing Mistakes

• Treating S32750 welding like ordinary duplex welding;
• Focusing only on tensile strength and ignoring microstructure;
• Ignoring filler metal chemistry;
• Failing to control heat input;
• Ignoring interpass temperature;
• Checking only the base metal and not the welded joint;
• Using ferrite measurement as the only quality indicator;
• Proceeding without a WPS/PQR;
• Overlooking NDE or corrosion testing requirements;
• Failing to confirm the applicable code and acceptance criteria.

FAQ

Q1: Is UNS S32750 difficult to weld?
It is not inherently difficult, but it demands tighter process control than ordinary austenitic or standard duplex grades. The weld thermal cycle changes the ferrite/austenite balance, so filler, heat input, cooling, and shielding must all be managed carefully.

Q2: What filler metal is used for S32750 welding?
The filler is selected for the specific process, base metal, and service, and is commonly over-alloyed with nickel to promote austenite re-formation. There is no single filler correct for every process, so selection must follow the applicable AWS/ASME specification and the WPS/PQR.

Q3: Why is heat input important for 2507 welding?
Heat input determines the cooling rate, which in turn controls how much austenite re-forms from ferrite. Too little heat input can leave excess ferrite; too much can cause microstructural changes. The correct range comes from the qualified WPS.

Q4: What happens if S32750 cools too quickly after welding?
Rapid cooling can leave excessive ferrite because austenite does not have enough time to re-form. This can reduce toughness and localized corrosion resistance in the weld metal and HAZ.

Q5: What happens if heat input is too high?
Excessive heat input and slow cooling can cause grain growth or the formation of undesirable phases, potentially reducing some properties of the joint.

Q6: How is ferrite/austenite balance checked in a 2507 weld?
It is typically checked by ferrite measurement, used as one part of phase-balance verification together with mechanical testing and, where required, corrosion testing. Ferrite measurement alone is not the sole quality judgment.

Q7: Does S32750 welding require preheating?
Preheating is generally not applied in the same way as for carbon steel. Whether any preheat is used depends on the specific material, thickness, and applicable code requirements.

Q8: What tests are required for S32750 welds?
The test list is project-specific and may include visual inspection, NDE, ferrite measurement, tensile testing, impact testing where required, corrosion testing where specified, and PMI/traceability. Requirements follow the applicable code, standard, and WPS/PQR.

Q9: Can S32750 be welded using TIG?
Yes. GTAW/TIG is commonly used for super duplex welding, particularly for root passes, because it offers good control over heat input and the weld pool.

Q10: What should an S32750 welding RFQ include?
Include UNS S32750, base-metal specification, product form, welding process, filler metal, WPS/PQR, heat-input control, interpass temperature, shielding gas, phase-balance requirements, NDE, mechanical and corrosion testing where required, welder qualification, and MTC/heat-number traceability.

Related Reading

• UNS S32205 Duplex Stainless Steel Welding: Heat Input and Phase Balance
• UNS S32750 vs S32205: Super Duplex and Duplex Selection
• 2507 Super Duplex Stainless Steel Chloride Resistance
• Duplex Stainless Steel Welding: A Practical Overview