Complete Guide to Welding Duplex Stainless Steels (P No. 10H)

Introduction

With increasing demand in the Oil & Gas, water purification, and chemical processing industries, Duplex Stainless Steels (DSS) have become essential materials. Their exceptional corrosion resistance, mechanical strength, and toughness make them ideal for high-performance applications.
However, welding DSS is not difficult—but it is different. This article combines practical industry guidance and technical depth to give fabricators, engineers, and welders a reliable source of knowledge to weld DSS correctly.

What Are Duplex Stainless Steels?

Duplex stainless steels are iron-chromium-nickel (Fe-Cr-Ni) alloys with additions of molybdenum (Mo), nitrogen (N), copper (Cu), and tungsten (W). Their structure is a balanced mix of austenite (fcc) and ferrite (bcc).

Key Advantages:

High resistance to stress corrosion cracking (SCC)

Better toughness and ductility than ferritic steels

Good yield strength and fatigue resistance

Excellent resistance to pitting and crevice corrosion

Classification of Duplex Stainless Steels (DSS)

Duplex Stainless Steels are classified based on their microstructure, alloying content, and corrosion resistance into four main types:

1. Standard DSS
2. Lean DSS
3. Super DSS
4. Hyper DSS

1. Lean DSS (Lean Duplex Stainless Steel)

• Description:
  Lean DSS have a lower content of alloying elements like nickel and molybdenum compared to Standard DSS. They provide good corrosion resistance and strength but at a more economical cost. Mainly used for moderate corrosive environments.
• Typical Grades:
  • ASM Handbook: 2304
  • AWS D10.18: 2304
  • API 582 / 938-C: S32304

2. Standard DSS

• Description:
  Standard DSS grades have a balanced microstructure of austenite and ferrite, with moderate amounts of alloying elements. They offer higher strength and better corrosion resistance than Lean DSS, suitable for many industrial applications like chemical and petrochemical industries.
• Typical Grades:
  • ASM Handbook: 2205
  • AWS D10.18: 2205
  • API 582 / 938-C: S32205

3. Super DSS

• Description:
  Super DSS grades contain higher amounts of alloying elements such as molybdenum and nitrogen, enhancing resistance to pitting, crevice corrosion, and stress corrosion cracking. These steels are used in more aggressive environments, such as offshore and marine applications.
• Typical Grades:
  • ASM Handbook: 2505 or 2507 (some overlap)
  • AWS D10.18: 2507
  • API 582 / 938-C: S32750

4. Hyper DSS

• Description:
  Hyper DSS are the highest alloyed grades with maximum corrosion resistance and mechanical strength, typically used in the most demanding environments, including harsh chemical processing and highly corrosive media.
• Typical Grades:
  • ASM Handbook: 2507 (sometimes classified here)
  • AWS D10.18: No specific designation (dash “-” indicates not listed)
  • API 582 / 938-C: S32707

Summary Table

Notes:

• 2304: Lean DSS with moderate corrosion resistance and strength.
• 2205: Most widely used Standard DSS with balanced properties.
• 2505/2507: Super DSS with improved corrosion resistance.
• S32750/S32707: Super and Hyper DSS grades as per API, with high molybdenum and nitrogen content.
• AWS D10.18 mainly lists common grades used in welding and fabrication.

PREN – Pitting Resistance Equivalent Number

Formula (per API 582/938-C):

PREN = %Cr + 3.3 × (%Mo + 0.5×%W) + 16 × %N

Welding Challenges & Best Practices

Before Welding

• – Keep DSS separate from carbon and low-alloy steels
• – Avoid carbon arc gouging unless followed by grinding
• – Clean plasma-cut edges to remove HAZ
• – Post-fabrication cleaning is essential

Interpass Temperature Control

Interpass temperature is very important in welding of Duplex Stainless steel (DSS), As per our experience we have suggested interpass for different thickness in below table but always refer code, specification and application before applying interpass temprature.

PWHT is not recommended due to risk of embrittlement.

Shielding and Back Purging Gases

• – Use argon or argon + nitrogen mixtures
• – Back purge with Ar + ≥2% N₂, O₂ < 0.10%
• – Use GTAW or GMAW for root passes with back purging up to 6 mm

Ferrite Content Management

• – Target 30–70% ferrite in welds
• – Avoid excess delta ferrite (>60%) or austenite (>50%)
• – Use WRC-92 or Ferritescope for ferrite prediction

Filler Metal Recommendations (API 582)

Welding Techniques and Sequence

• – Use cold pass (~75% heat input) after open root
• – Stringer beads for horizontal; weaving for vertical-up
• – For dissimilar welding: butter carbon steel with E309L, PWHT, then weld DSS

Joint Preparation

• – Avoid small bevel angles; prefer X-joint for reduced distortion
• – Maintain 2–2.5 mm root gap with 60–70° bevel

Post-Weld Considerations

• – Inert gas backing for TIG/MIG root passes
• – Perform post-weld cleaning by pickling or grinding
• – Store filler wires/electrodes in dry, temperature-controlled areas

Applications

Duplex stainless steels are used in:

• – Civil structures: bridges, barriers, floodgates
• – Pressure vessels and exchangers
• – Chemical and pulp industries
• – Storage tanks and trailers
• – Coastal and marine structures

5 Keys to Welding DSS Successfully

1. Choose the right filler metal for structure compatibility

2. Use the correct welding method per application

3. Ensure proper joint prep, fit-up, and cleanliness

4. Train welders thoroughly and follow WPS

5. Perform post-weld cleaning properly

References

– AWS D10.18, AWS D1.6

– API 582 (2023), API 938-C (2015)

– ASM Handbook – Stainless Steels