Lecture Notes: Duplex Stainless Steels

Introduction

• Speaker: Dan (retired in 2007)
• Location: Chapel Hill, NC
• Main Topic: Duplex Stainless Steels and Chloride Stress Corrosion Cracking (CSCC)

Chloride Stress Corrosion Cracking (CSCC)

• Susceptible Alloys: Austenitic stainless steels like 304L, 316L
• Resistant Alloys: Ferritic stainless steels
• Duplex Stainless Steels: Combine properties of both austenitic and ferritic
• Pitting vs. CSCC: Real issue is CSCC, not pitting
• CSCC Conditions: Requires chlorides and tensile stress, often found around welds

Duplex Stainless Steels

• Composition: Approx. equal amounts of ferrite and austenite
• Advantages: Resistant to CSCC, good fabrication properties, double yield strength
• Solidification Process: Liquid to ferrite -> Liquid + ferrite -> 100% ferrite -> Ferrite + austenite
• Generations of Duplex Steels: Four generations, with improvements over time in nitrogen content and nickel adjustments

Microstructural Analysis

• Base Material vs. Weld Metal: Different partitioning of elements after welding
• Important Elements: Chromium, nickel, molybdenum, and nitrogen
• Nitrogen Partitioning: Almost complete partitioning to austenite
• Heat Treatment: Different effects at high and low temperatures

Historical Development & Generations

1. First Generation: 1930s, CD4MCu (forerunner to 255), poor properties as-welded
2. Second Generation: 1980s with Argon Oxygen Decarburization (AOD), improved weldability
3. Third Generation: Higher nitrogen content, UNS 32205 redefinition, improved heat-effect zone properties
4. Fourth Generation: Super duplex steels like 2507, higher pitting resistance index (PREN)

Welding Considerations

• Welding Challenges: Autogenous welds prone to poor mechanical properties
• Techniques: Adding nitrogen to the shielding gas, higher nickel filler metals
• Common Weld Materials: 2209 for 2205, adjusting based on application (cast vs. as-welded)
• Heat Input Control: Controlling heat input to balance phase content and avoid brittle phases
• Hydrogen Embrittlement: Keeping filler metals dry, avoiding hydrogen in shielding gas

Precipitation Phases

• Alpha Prime: Forms between 400-540°C, causes brittleness and corrosion susceptibility
• Sigma Phase: Forms between 500-1070°C, similar effects
• Heat Treatment Strategies: Steps involving higher initial temperatures followed by lower temperatures to stabilize phases

Practical Applications & Considerations

• Restraint in Welding: Duplex steels develop strength rapidly, need strong restraint to prevent distortion
• Use in Seawater: Not stainless in seawater, requires specific alloy compositions
• Modern Applications: Includes seawater applications, fume gas desulfurizers, and low-alloy folded structures

Summary

• Differences in Welding Duplex: Must follow specific rules to achieve desirable properties
• Restraint Techniques: Strong restraint required, as material otherwise moves a lot due to strength development

Q&A

• Open floor for questions from attendees.