Microstructure and Properties of Submerged Arc Welding Joints of Duplex Stainless Steel S22053

Literature Overview

Duplex stainless steel (DSS) has gained significant attention in the pressure vessel and piping industry due to its excellent combination of mechanical strength, corrosion resistance, and resistance to stress corrosion cracking. The grade S22053 (equivalent to UNS S31803 / EN 1.4462) is one of the most widely used DSS grades in marine, oil and gas, and chemical processing applications. Submerged arc welding (SAW) remains a preferred process for thick-section DSS fabrication owing to its high deposition rate and excellent weld quality. This study note examines the metallurgical behavior, microstructural evolution, and mechanical performance of SAW joints in S22053, drawing connections to engineering practice in clad pressure vessel fabrication.

Core Technical Content

The fundamental metallurgical challenge in welding DSS lies in maintaining the ferrite-austenite (F-A) phase balance in the weld metal and the heat-affected zone (HAZ). The parent material typically contains 40-60% delta ferrite by volume fraction, which is critical for resistance to chloride stress corrosion cracking (Cl-SCC). During SAW, the thermal cycle can cause austenite formation in the HAZ due to high peak temperatures, while rapid cooling can lead to excessive ferrite or the formation of brittle intermetallic phases such as sigma (σ) and chi (χ) phases.

Key findings from the literature include:

• The weld metal microstructure is predominantly composed of acicular ferrite and austenite, with the ferrite content influenced by the dilution ratio with the parent material and the thermal input.
• The HAZ exhibits a gradient of microstructural changes: the coarse-grained HAZ (CGHAZ) adjacent to the fusion line shows increased austenite fraction due to solid-state phase transformation, while the fine-grained HAZ retains a closer resemblance to the parent microstructure.
• Welding thermal input is the most critical process parameter. Excessive thermal input (>30 kJ/mm) promotes σ-phase precipitation in the HAZ, reducing ductility and toughness. Insufficient thermal input leads to excessive hardness and increased susceptibility to cold cracking.

Interpretation of Technical Points

Phase Balance Control

The ferrite number (FN) is the primary metric for evaluating phase balance in DSS welds. For S22053, the target FN in the weld metal should be in the range of 35-50 FN (approximately 40-55% ferrite by volume). The Schaeffler diagram is used to predict the phase balance based on the chromium equivalent (CrEQ) and nickel equivalent (NiEQ) of the weld metal composition after accounting for dilution.

The dilution ratio in SAW joints of DSS is typically 20-40%, depending on the groove geometry, number of passes, and wire diameter. This dilution shifts the weld metal composition toward the parent material, which means the filler metal must be enriched in nickel to compensate. ER3190 (UNS E3190) is the standard filler for S22053, with a Ni content of approximately 18-22%, compared to 22-25% in the parent material.

Intermetallic Phase Precipitation

One of the most significant degradation mechanisms in DSS welds is the precipitation of intermetallic phases, particularly σ-phase (Cr,Ni,Fe-rich) and χ-phase (Cr-rich). These phases form preferentially in the ferrite phase and are detrimental to both mechanical properties and corrosion resistance. The precipitation kinetics follow the C-curve behavior:

• σ-phase: nucleates at 600-870°C, with a minimum precipitation time of 1-10 hours at 700-750°C
• χ-phase: nucleates at 700-800°C, with a minimum precipitation time of 0.1-1 hour

The welding thermal cycle can inadvertently traverse the critical precipitation temperature range during multi-pass welding. This is why strict interpass temperature control (≤150°C) is essential in DSS SAW fabrication.

Mechanical Properties

The mechanical properties of SAW joints in S22053 are characterized by:

• Tensile strength: 550-650 MPa (parent material: 550-800 MPa)
• Yield strength: 450-550 MPa (parent material: 450-650 MPa)
• Elongation: 25-35% (parent material: 25-35%)
• Charpy V-notch impact energy: 100-200 J at -40°C (parent material: 150-250 J)
• Hardness: 220-280 HV (parent material: 220-280 HV)

The weld metal typically exhibits slightly lower strength than the parent material due to the higher nickel content of the filler metal and the finer grain structure. The HAZ may show localized hardening due to grain growth and phase transformation, but this is generally within acceptable limits when thermal input is properly controlled.

Corrosion Resistance

The corrosion resistance of the weld joint is evaluated through:

• Ferric sulfate intergranular corrosion test (ASTM A923 Practice A): the weld metal and HAZ should show no intergranular attack
• Potentiodynamic polarization testing: the weld metal should exhibit a passivation potential within 50 mV of the parent material
• Salt spray testing: minimum 1000 hours without pitting in 5% NaCl solution

The key factor governing corrosion resistance is the phase balance. Excess austenite leads to susceptibility to Cl-SCC, while excess ferrite or intermetallic phases lead to pitting and crevice corrosion.

Standards and Codes Analysis

The fabrication and qualification of SAW joints in DSS are governed by several standards:

The NB/T 47014 standard for welding procedure qualification in China requires specific attention to thermal input limits and interpass temperature control for DSS. The standard mandates that the thermal input shall not exceed 30 kJ/mm for single-layer single-pass welding and 25 kJ/mm for multi-layer multi-pass welding when the plate thickness exceeds 20 mm.

Engineering Practice Integration

In the fabrication of clad pressure vessels using S22053 as the overlay material, several practical considerations must be addressed:

Base Metal Compatibility

When S22053 is used as an overlay on carbon steel or low-alloy steel base plates, the dissimilar weld joint presents unique challenges. The base metal dilution can be as high as 50-60% in the first pass, significantly altering the weld metal composition. This necessitates the use of a transition layer or the selection of a filler metal with higher nickel content for the root pass.

The recommended welding sequence for DSS overlay on carbon steel base plates is:

1. Root pass: Use ER3190 or a high-nickel filler (e.g., ER3290) to establish adequate ferrite content
2. Fill passes: Continue with ER3190, monitoring thermal input to prevent overheating
3. Cap pass: Apply a final pass with careful thermal management to ensure proper phase balance at the surface

Multi-Pass Welding Strategy

For thick-section DSS plates (>25 mm), multi-pass SAW is required. The welding strategy should follow these principles:

• Use a step-back welding sequence to minimize the number of times the last-pass weld is reheated through the critical temperature range
• Maintain interpass temperature at or below 150°C, with mandatory cooling when exceeded
• Use a thermal input of 15-25 kJ/mm for fill passes and 10-15 kJ/mm for the cap pass
• Apply a back-gas protection of argon with 2-5% CO₂ to prevent nitrogen pickup in the root side

Post-Weld Heat Treatment Considerations

Post-weld heat treatment (PWHT) for DSS welds is generally avoided because the required temperature range (700-800°C) overlaps with the intermetallic phase precipitation range. When PWHT is necessary (e.g., for stress relief in thick sections), the following approach is recommended:

• Use a solution heat treatment at 1050-1100°C with rapid cooling (water quench or forced air cooling)
• This dissolves any intermetallic phases and restores the phase balance
• However, solution heat treatment may cause dimensional changes and residual stresses, requiring careful engineering assessment

Key Questions and Reflections

Several questions emerged during the study of this topic that warrant further investigation:

1. Optimal thermal input for different thickness ranges: While the literature provides general guidelines, the optimal thermal input for SAW of DSS varies with plate thickness, groove geometry, and welding position. A systematic parametric study correlating thermal input with microstructural evolution and mechanical properties across the full thickness range (6-100 mm) would be valuable.
2. Effect of flux composition on weld metal quality: The shielding flux plays a critical role in SAW by controlling slag viscosity, deoxidation, and nitrogen pickup. However, the influence of specific flux compositions on the phase balance and corrosion resistance of DSS welds is not well documented in the literature.
3. Long-term service behavior: Most studies focus on as-welded or solution-treated conditions. The long-term behavior of DSS SAW joints under cyclic loading, elevated temperature, and corrosive environments requires further investigation, particularly regarding the kinetics of intermetallic phase precipitation under service conditions.
4. Welding procedure qualification for clad plates: The qualification of SAW procedures for DSS overlay on carbon steel base plates is more complex than for homogeneous DSS welds. The dilution ratio varies significantly between the root pass and subsequent passes, requiring a more nuanced qualification approach than currently prescribed by standards.

Study Insights and Implications

The study of SAW joints in S22053 reveals that the successful fabrication of DSS weldments hinges on three interrelated factors: thermal input control, filler metal selection, and interpass temperature management. The phase balance is not a static property but evolves with the welding sequence, and each pass introduces a new thermal cycle that can alter the microstructure of previously deposited layers.

From a practical standpoint, the most common failure mode in DSS welds is not mechanical failure but corrosion failure due to intermetallic phase precipitation. This insight has profound implications for welding procedure design: the primary objective of the welding procedure is not merely to achieve adequate mechanical properties but to maintain the microstructural integrity of the ferrite-austenite balance throughout the entire weld volume.

The concept of "thermal budget" is particularly relevant here. Each pass in a multi-pass weld adds to the cumulative thermal exposure, and the total thermal budget must be managed to prevent excessive reheating. This is analogous to the thermal budget concept used in austenitic stainless steel welding, but the critical temperature range is different: for DSS, the concern is not sensitization (as in austenitic stainless steels) but intermetallic phase precipitation.

In the context of bimetal pressure vessel fabrication, where DSS is often used as an overlay on carbon steel base plates, the dissimilar weld joint adds another layer of complexity. The base metal dilution in the first pass can be substantial, and the resulting weld metal composition may fall outside the optimal phase balance range. This necessitates a deliberate strategy of using a high-nickel filler for the root pass and transitioning to a standard DSS filler for subsequent passes.

The evolution of welding technology offers new opportunities for DSS fabrication. Hot-wire TIG (HWT) welding, which combines GTAW arc stability with GMAW deposition rates, has shown promise for DSS overlay applications. The lower thermal input of HWT compared to SAW, combined with the ability to control wire feed rate independently of arc current, provides a more precise tool for managing the thermal budget in DSS welding.

Conclusion

The metallurgical behavior of SAW joints in duplex stainless steel S22053 is governed by the interplay between thermal input, filler metal composition, and welding sequence. The maintenance of phase balance is the primary objective, and this requires careful control of thermal input, interpass temperature, and filler metal selection. The standards provide adequate qualification frameworks, but the practical implementation requires a deep understanding of the underlying metallurgy. As DSS continues to gain acceptance in increasingly demanding applications, the development of optimized welding procedures that balance productivity with metallurgical integrity remains a critical engineering challenge. The integration of advanced welding technologies such as HWT and laser cladding offers promising avenues for further improvement, and continued research into the long-term behavior of DSS welds under service conditions is essential for ensuring the reliability of pressure vessels fabricated from these materials.