UNS S31803 Duplex Stainless Steel Welded Tee Cracking Failure Analysis

Literature Overview

This paper by Yang Yashan and Zhang Zhipeng, published in Hot Working Technology (2020, Vol. 49, No. 13), presents a detailed failure analysis of a UNS S31803 duplex stainless steel welded tee that experienced through-thickness cracking during service in a marine natural gas field wellhead riser pipeline. The work was conducted by CNOOC (China) Zhanjiang Branch, indicating its relevance to offshore oil and gas operations in the South China Sea. This case study provides valuable insights into the complex interplay between residual stress, microstructural evolution, and corrosion mechanisms in duplex stainless steel weldments.

Service Background and Failure Description

The failed component was a welded tee used in a wellhead riser pipeline system on an offshore natural gas platform. Riser pipelines connect the subsea wellhead to the platform processing facilities and are subject to cyclic loading, thermal cycling, and continuous exposure to corrosive environments. The tee was fabricated from UNS S31803 (equivalent to 2205 duplex stainless steel) pipe using a welding process, and the failure manifested as a through-thickness crack that caused a natural gas leakage incident.

The failure mode is particularly significant because:

• Duplex stainless steels are specifically selected for their combination of high strength and excellent corrosion resistance in chloride-containing environments
• The failure occurred in a welded joint, which is the most vulnerable region of a duplex stainless steel component due to microstructural changes in the heat-affected zone (HAZ)
• The failure resulted in a safety incident involving flammable gas release in an offshore environment

Materials Characterization

Base Metal Composition and Properties

UNS S31803 is a 22% Cr, 3% Mo duplex stainless steel with the following typical composition:

The duplex microstructure consists of approximately 50% austenite and 50% ferrite in the ideal condition, providing balanced mechanical properties and corrosion resistance. The elevated nitrogen content (0.14-0.20%) compared to conventional 2205 (UNS S32205) provides additional strength and improved pitting resistance.

Microstructural Analysis of the Failed Tee

The failure analysis employed a comprehensive suite of characterization techniques:

• Optical Emission Spectroscopy (OES): Confirmed the chemical composition of the base metal and weld metal
• Vickers Hardness Testing: Revealed hardness variations across the weld cross-section
• Tensile Testing: Evaluated the mechanical properties of the weldment
• Optical Metallography: Identified microstructural features in the base metal, HAZ, and weld metal
• Scanning Electron Microscopy (SEM): Examined the fracture surface morphology
• Energy Dispersive X-ray Spectroscopy (EDS): Mapped elemental distribution at the crack location
• X-ray Diffraction (XRD): Identified secondary phases present in the material

Failure Mechanism Analysis

Primary Failure Mechanism: Stress Concentration-Induced Corrosive Cracking

The fundamental failure mechanism identified in this case is a synergistic interaction between stress concentration and corrosion. This is not a simple fatigue failure or a pure stress corrosion cracking (SCC) event, but rather a more complex scenario where:

1. Stress concentration at the weld toe or weld root created localized regions of elevated tensile stress
2. Corrosive environment (chloride-containing seawater or process fluid) provided the chemical driving force for crack initiation
3. Microstructural susceptibility in the HAZ, particularly in regions containing sigma (σ) phase, lowered the material's resistance to crack initiation and propagation

Crack Initiation and Propagation Sequence

The failure analysis reveals a clear crack evolution sequence:

Sigma Phase and Its Role in Failure

The presence of sigma phase (σ phase, Cr₂N or Cr₂₅N₆) in the duplex stainless steel is a critical factor in this failure. Sigma phase is a brittle, Cr-rich intermetallic compound that forms during prolonged exposure to temperatures in the range of 450-850°C. Its formation has several detrimental effects:

• Depletion of Cr from the ferrite matrix: Sigma phase precipitation locally reduces the chromium content of the surrounding ferrite below the passive film stability threshold (approximately 12% Cr), creating susceptibility to intergranular corrosion
• Loss of toughness: The brittle sigma phase acts as a crack initiation site and provides preferential crack propagation paths
• Disruption of the duplex balance: Sigma phase formation shifts the microstructure away from the ideal 50:50 austenite-ferrite ratio, reducing overall toughness and ductility

Fracture Surface Analysis

The SEM examination of the fracture surface revealed distinct morphological features corresponding to different propagation stages:

• Intergranular fracture in the sigma-phase-rich regions, indicating corrosion-assisted crack propagation along grain boundaries
• Transgranular fracture in the HAZ, suggesting that the crack propagated through the grains where the microstructure was more homogeneous
• Possible evidence of corrosion products at the crack tips, confirming the active role of the corrosive environment in crack growth

Welding Process Considerations

The failure in a welded tee highlights several critical welding considerations for duplex stainless steels:

Heat Input Control

Duplex stainless steels are highly sensitive to welding heat input because:

• Excessive heat input promotes sigma phase formation and coarse grain growth in the HAZ
• Insufficient heat input can lead to incomplete fusion and cold cracking susceptibility
• The optimal heat input range for 2205 duplex stainless steel is typically 0.5-1.5 kJ/mm, depending on thickness

Weld Metal Composition

The weld metal must be designed to maintain the duplex microstructure and resist secondary phase formation:

• Filler metal should have a slightly higher Ni content than the base metal to compensate for preferential Ni dissolution into the austenite phase
• Typical filler metal for S31803 includes ER31903 or equivalent compositions
• The PREN (Pitting Resistance Equivalent Number) of the weld metal should match or exceed that of the base metal

Post-Weld Heat Treatment

Post-weld solution annealing at 1050-1100°C followed by rapid cooling is essential for:

• Dissolving any sigma phase formed during welding
• Restoring the optimal austenite-ferrite ratio
• Relieving residual stresses
• Homogenizing the microstructure

Standards and Code Requirements

The fabrication and inspection of duplex stainless steel welded components in offshore applications must comply with relevant standards:

Engineering Practice Implications

This failure case provides several important lessons for engineers designing and fabricating duplex stainless steel welded components:

1. Heat input control is non-negotiable: Strict adherence to maximum heat input limits during welding is essential to prevent sigma phase formation and HAZ embrittlement. Multi-pass welding with individual pass heat input below 0.5 kJ/mm is recommended for thick sections.
2. Post-weld heat treatment is mandatory: Solution annealing after welding is not optional for critical offshore components. The PWHT must be performed within the specified temperature window and with appropriate cooling rates to avoid re-precipitation of detrimental phases.
3. Residual stress management: Stress relief through PWHT or mechanical methods (such as low-frequency vibration stress relief) should be considered for components subject to cyclic loading.
4. Corrosion protection: Even with proper welding practices, duplex stainless steel components in offshore environments require additional corrosion protection measures, including cathodic protection, coating systems, and regular inspection.
5. Inspection requirements: Enhanced NDE requirements should be applied to duplex stainless steel weldments, including:

• 100% visual examination
• 100% dye penetrant testing (PT) for surface defects
• 100% ultrasonic testing (UT) for volumetric defects
• Consideration of phased array UT (PAUT) for weld root examination
• Regular in-service inspection of critical welds

Key Questions and Reflections

Several aspects of this failure case warrant further consideration:

1. Sigma phase quantification: What was the volume fraction of sigma phase in the failed component, and at what location was it most concentrated? The sigma phase distribution along the weld cross-section would provide valuable insight into the thermal history.
2. Welding process parameters: Were the actual welding parameters (current, voltage, travel speed, interpass temperature) within the recommended ranges for S31803? Deviations from optimal parameters could accelerate sigma phase formation.
3. Service life and sigma phase evolution: How long had the component been in service before failure? The time-temperature exposure history during service could contribute to sigma phase formation even if the initial welding was performed correctly.
4. Stress level assessment: What was the actual stress level at the failure location compared to the material's yield strength? The stress concentration factor at the weld geometry plays a critical role in determining the crack initiation threshold.
5. Corrosive environment characterization: What was the exact composition of the corrosive environment (chloride concentration, pH, temperature, dissolved oxygen, H₂S content)? The severity of the environment directly influences the crack growth rate.

Summary and Implications

The failure of the UNS S31803 duplex stainless steel welded tee provides a clear demonstration of how stress concentration, microstructural degradation, and corrosion interact to cause premature failure in offshore piping components. The transition from transgranular to intergranular fracture, with the intergranular propagation occurring preferentially through sigma-phase-rich regions, highlights the critical importance of controlling sigma phase formation throughout the component's life cycle. For engineers working with duplex stainless steels in corrosive environments, this case underscores the necessity of rigorous welding procedure qualification, mandatory post-weld heat treatment, comprehensive residual stress management, and systematic in-service inspection programs. The lesson is clear: the inherent advantages of duplex stainless steels can only be fully realized when all aspects of material processing, fabrication, and maintenance are carefully controlled.