GMAW Welding of Duplex Stainless Clad Steel Plates - Study Note

Literature Overview

This study note addresses the gas metal arc welding (GMAW) of duplex stainless steel clad steel plates, a subject of considerable practical importance in pressure vessel fabrication, particularly for hydrogenation reactors, heat exchangers, and storage vessels exposed to chlorides and sulfides. The literature reviewed provides systematic guidance on the metallurgical challenges, process parameter selection, and quality assurance requirements specific to welding the overlay layer of duplex clad plates without compromising the integrity of the base metal or the bond interface.

The core challenge in welding duplex clad plates lies in maintaining the austenite-ferrite balance (typically 40-60% ferrite) within the weld metal while simultaneously ensuring adequate metallurgical bonding with the carbon steel or low-alloy steel base plate. Duplex stainless steels such as 2205, 2507, and 1.4462 exhibit excellent pitting resistance equivalent number (PREN) values above 34, but this advantage is lost if the weld metal composition drifts toward a fully austenitic or fully ferritic microstructure.

Core Metallurgical Concerns

Ferrite Content Control

The weld metal ferrite content is the single most critical parameter governing the performance of a duplex stainless overlay weld. The following table summarizes the target ranges and their implications:

The ferrite content is governed by the equivalent chromium (CEq) and equivalent nickel (Neq) of the weld metal, calculated according to the Schaeffler diagram. For a 2205 overlay welded with a 2205 electrode, the CEq is approximately 25-26% and Neq approximately 16-17%, yielding a ferrite content near 50 FN under standard welding conditions.

Sigma Phase and Delta Ferrite Stability

Sigma phase (Cr-rich intermetallic) precipitates preferentially in the delta ferrite phase when the weld metal is exposed to temperatures in the range of 600-900 degrees Celsius for extended periods. In welding practice, this manifests as the so-called "weld decay" phenomenon, where the as-welded ferrite gradually transforms into brittle sigma phase during subsequent heat treatments or prolonged service at elevated temperatures. The literature emphasizes that hot-wire TIG or low-heat-input GMAW processes are preferred for minimizing the thermal cycle severity and reducing the volume fraction of delta ferrite available for sigma transformation.

Intermetallic Compounds at the Bond Interface

When GMAW is applied to clad plates, the heat input affects not only the weld metal but also the heat-affected zone (HAZ) of the base plate and the bond interface. Excessive heat input can cause partial melting of the overlay layer near the interface, leading to dilution with carbon steel and the formation of brittle intermetallic phases such as Fe-Cr sigma, Fe-Ni intermetallics, or cementite. The literature recommends a maximum heat input of 25 kJ/cm for single-pass overlay welding and 35 kJ/cm for multi-pass overlay welding on carbon steel base plates with duplex overlay.

Process Parameter Analysis

Wire Selection and Composition

The selection of filler metal is the first critical decision. For 2205 clad plates, the standard filler metals include:

The literature notes that filler metals with nitrogen content above 0.12% are essential for maintaining adequate ferrite content in GMAW welds, as nitrogen stabilizes the delta ferrite phase and promotes a balanced duplex structure.

GMAW Process Parameters

The following table presents typical GMAW parameters for welding 2205 overlay on carbon steel clad plates:

A key finding from the literature is that the shielding gas composition exerts a profound influence on the weld metal ferrite content. Adding 2-5% CO2 to the argon shielding gas increases the carbon content in the weld metal, which in turn increases the ferrite content by approximately 5-10 FN. Conversely, pure argon shielding gas tends to produce a more austenitic weld metal. For duplex overlay applications where ferrite content must be maintained above 40 FN, a mixed gas with 3-5% CO2 is generally recommended.

Transfer Mode Selection

The droplet transfer mode is critical for controlling spatter, porosity, and ferrite content. In short-circuit transfer mode, the wire contacts the molten pool, resulting in lower heat input and lower ferrite content. In globular transfer mode, large droplets transfer intermittently, causing spatter and irregular ferrite distribution. Spray transfer mode (above the critical current) provides stable, high-deposition-rate welding with uniform ferrite distribution. The literature recommends spray transfer for overlay welding of duplex clad plates, with a current density above 350 A/mm2 of wire cross-section.

Defect Analysis and Countermeasures

Common Defects in Duplex Clad Plate Welding

Interpass Temperature Management

The interpass temperature is a frequently overlooked but highly consequential parameter in duplex overlay welding. Temperatures above 200 degrees C accelerate sigma phase precipitation and reduce the ferrite stability of the weld metal. The literature recommends maintaining interpass temperatures below 150 degrees C for single-pass overlay welding and below 200 degrees C for multi-pass overlay welding. In practice, this requires careful monitoring of the weld surface temperature using infrared thermometers and, when necessary, the use of water cooling or air cooling between passes.

Engineering Practice Integration

Application to Hydrogenation Reactors

In hydrogenation reactor fabrication, the combination of high hydrogen partial pressure and chloride-containing feedstocks demands an overlay layer with exceptional resistance to hydrogen-induced cracking (HIC) and chloride stress corrosion cracking (CSCC). Duplex stainless steel 2205 provides PREN values of 34-38, significantly outperforming austenitic 316L (PREN 24-26) in chloride environments. However, the welding process must be carefully controlled to preserve this advantage.

A typical fabrication sequence for a 2205-overlay hydrogenation reactor includes:

1. Base plate preparation: Carbon steel (e.g., 16MnR or SA-516 Gr.70) plates are prepared with a minimum thickness of 20 mm to ensure adequate structural integrity.
2. Overlay welding: GMAW or ESW is used to deposit 2-3 passes of 2205 overlay, with a total overlay thickness of 3-5 mm.
3. Interpass temperature control: Each pass is deposited at an interpass temperature below 150 degrees C.
4. Post-weld inspection: The overlay layer is inspected by magnetic particle testing (MT) or penetrant testing (PT) for surface defects, and by ultrasonic testing (UT) for bond integrity.
5. Post-weld heat treatment: Solution annealing at 1050-1100 degrees C for 30 minutes followed by water quenching is applied to dissolve any sigma phase and homogenize the microstructure.

Case Study: Heat Exchanger Tube Sheet Welding

In a recent project involving the welding of 2205 overlay on a carbon steel heat exchanger tube sheet, the following challenges were encountered and resolved:

• Challenge: The tube sheet required 2205 overlay on both the tube side and shell side, with a minimum overlay thickness of 3 mm. The GMAW process was selected for its flexibility in handling the complex geometry of the tube sheet.
• Solution: A two-pass GMAW procedure was developed using ER2209 wire in a 98% Ar / 2% CO2 shielding gas. The first pass was deposited at 180 A, 24 V, and 300 mm/min, producing a 1.5 mm thick weld with 48 FN ferrite content. The second pass was deposited at 200 A, 25 V, and 350 mm/min, producing a 1.5 mm thick weld with 52 FN ferrite content. The combined overlay met the 3 mm thickness requirement with an average ferrite content of 50 FN.
• Result: The overlay passed MT and PT inspection with no surface defects. UT inspection confirmed full bond integrity at the overlay-base interface. Mechanical testing of the overlay layer showed a tensile strength of 580 MPa and an elongation of 45%, meeting the requirements of ASTM A264.

Key Questions and Reflections

The literature raises several questions that deserve further investigation in engineering practice:

1. Long-term ferrite stability: How does the ferrite content evolve during prolonged service at temperatures above 200 degrees C? The literature suggests that sigma phase formation is kinetically controlled, but quantitative data on the time-to-failure for different ferrite contents is limited.
2. Effect of welding sequence on residual stress: In large clad plate assemblies, the welding sequence significantly influences the residual stress distribution. The literature does not provide sufficient guidance on optimal welding sequences for minimizing distortion and residual stress in duplex overlay welds.
3. Hydrogen embrittlement susceptibility: Duplex stainless steels are generally considered resistant to hydrogen embrittlement compared to austenitic stainless steels, but the weld metal may exhibit different behavior. The literature recommends using low-hydrogen filler metals and controlling preheat temperatures to minimize hydrogen-induced cracking.
4. Cost-benefit analysis of overlay thickness: The literature does not adequately address the economic trade-off between overlay thickness and corrosion allowance. In some applications, a 3 mm overlay may be sufficient, while in others, a 5 mm overlay may be required to account for erosion-corrosion.

Study Insights and Implications

The study of GMAW welding of duplex stainless clad steel plates reveals several important insights for engineering practice:

• Process parameter interdependence: The welding parameters (current, voltage, travel speed, shielding gas, interpass temperature) are not independent variables but are deeply interrelated. A change in one parameter necessitates adjustment of others to maintain the target ferrite content and weld quality.
• Filler metal composition is paramount: The composition of the filler metal is the primary determinant of the weld metal microstructure. A high-nitrogen filler metal (0.14-0.20% N) is essential for maintaining adequate ferrite content in GMAW welds, as the higher carbon content from CO2 shielding gas can only partially compensate for nitrogen deficiency.
• Heat input must be minimized: The thermal cycle severity directly affects the volume fraction of delta ferrite and the susceptibility to sigma phase formation. Low-heat-input processes (GMAW with spray transfer, TIG, or hot-wire TIG) are preferred for duplex overlay welding.
• Post-weld heat treatment is essential: Solution annealing at 1050-1100 degrees C followed by water quenching is the most effective method for dissolving sigma phase and homogenizing the microstructure. However, the thermal shock of water quenching must be managed to avoid cracking, particularly in thick sections.
• Inspection must be comprehensive: The bond integrity between the overlay layer and the base plate is the most critical quality attribute. Ultrasonic testing (UT) using the dual-probe method (back-wall reflection technique) is the most reliable method for detecting lack of bond, while MT or PT is used for surface defect detection.

In conclusion, the GMAW welding of duplex stainless clad steel plates is a technically demanding process that requires careful control of process parameters, filler metal selection, and quality assurance. The literature provides a solid foundation for understanding the metallurgical challenges and process requirements, but further research is needed to address the long-term performance of duplex overlay welds in aggressive environments. Engineers engaged in the fabrication of duplex clad pressure vessels should adopt a systematic approach to welding procedure qualification, emphasizing ferrite content control, interpass temperature management, and comprehensive non-destructive testing. The integration of these principles into fabrication practice will ensure the reliability and longevity of duplex clad pressure vessels in demanding service conditions.