Strip Electrode Electroslag Welding of Duplex Stainless Steel on Tube Sheets

Literature Overview and Technical Context

This study addresses the application of strip electrode electroslag welding (ESW) for depositing duplex stainless steel overlay layers on tube sheets, a critical component in heat exchangers and pressure vessels operating in corrosive environments. Tube sheets represent a major structural and functional component of shell-and-tube heat exchangers, where they must simultaneously provide structural integrity, seal the tube-to-tube-sheet junction, and resist corrosion from both the tube-side and shell-side process fluids.

The use of duplex stainless steel (typically grades such as 2205, 2507, or equivalent) for tube sheet overlay provides excellent resistance to chloride stress corrosion cracking, pitting corrosion, and general corrosion while maintaining high mechanical strength. The ESW process is particularly suited for thick-section tube sheets where the deposition rate and thermal efficiency of this process offer significant advantages over conventional arc welding methods.

Core Technical Content

Process Parameters and Deposition Characteristics

Strip electrode ESW offers unique advantages for tube sheet overlay applications including high deposition rates (typically 3 to 8 kg/h), excellent penetration with minimal dilution, and the ability to build up thick overlay layers in a single operation. The study examines the following process parameters:

Metallurgical Considerations for Duplex Stainless Steel Overlay

The microstructural evolution of duplex stainless steel during ESW deposition presents unique challenges. The target microstructure consists of approximately 40 to 60 percent ferrite and 60 to 40 percent austenite, with the exact ratio depending on the specific grade and application requirements. The ESW process, with its relatively high heat input and slow cooling rates, creates a challenging environment for maintaining the desired phase balance.

The delta equivalent (DE) and carbon equivalent (CE) values of the deposited metal must be carefully controlled to avoid detrimental phase transformations:

• Sigma phase (σ): Forms in the temperature range of 600 to 900 °C during prolonged exposure, particularly when the DE exceeds 22. Sigma phase formation causes severe embrittlement and is the primary metallurgical concern in thick-section ESW deposits.
• R-phase: Precipitates in the temperature range of 300 to 500 °C, preferentially in ferrite grains, causing hardening and embrittlement.
• Free ferrite: Excessively high ferrite content (>65 percent) increases susceptibility to chloride pitting corrosion.
• Free austenite: Excessively high austenite content (>60 percent) reduces resistance to stress corrosion cracking and may promote solidification cracking.

Flux Design and Inclusion Control

The flux composition is critical for achieving clean deposits with acceptable inclusion levels. The study evaluates flux systems based on calcium fluoride, silica, and rutile with additions of manganese and silicon deoxidizers. The optimal flux composition achieves:

• Inclusion content below 5 percent by area fraction
• Non-metallic inclusions predominantly of Type A (oxide) morphology
• Inclusion size predominantly below 20 micrometers
• Sufficient slag fluidity for proper slag pool coverage without excessive spatter

Engineering Practice Integration

Tube Sheet Overlay Fabrication Procedure

Based on the literature findings, the following fabrication procedure is recommended for tube sheet overlay applications:

1. Surface preparation: Grind the tube sheet surface to remove scale, oil, and contamination. Achieve a surface finish of Ra 12.5 μm or better at the weld start and stop locations.
2. Preheating: Apply uniform preheat of 100 to 150 °C across the entire tube sheet surface to minimize thermal gradients and reduce the risk of cracking.
3. Backing plate installation: Install a compatible backing plate (typically austenitic stainless steel or copper with stainless steel coating) to ensure full penetration and control the root pass geometry.
4. First pass deposition: Deposit the first pass at reduced current (40 to 50 percent of maximum) to establish good metallurgical bonding with the base material while minimizing dilution.
5. Subsequent passes: Build up the overlay to the required thickness using progressively higher currents and optimized travel speeds.
6. Post-weld treatment: Apply a solution treatment at 1050 to 1100 °C for 1 to 2 hours followed by water quenching to restore the duplex phase balance and dissolve any sigma phase that may have formed during welding.

Quality Control and Inspection Requirements

The inspection regime for duplex stainless steel ESW overlay on tube sheets should include:

• Visual inspection (VT): 100 percent examination of all deposited surfaces for surface defects, undercut, and spatter.
• Magnetic particle testing (MT): 100 percent examination of the overlay surface and heat-affected zone for surface-breaking defects.
• Ultrasonic testing (UT): 100 percent examination for internal defects including lack of fusion, porosity, and cracking using phased array techniques.
• Positive material identification (PMI): Verification of the overlay alloy composition at defined intervals using X-ray fluorescence (XRF) analysis.
• Metallographic examination: Cross-sectional examination of representative samples to verify microstructure, phase balance, and absence of detrimental phases.
• Mechanical testing: Hardness testing (Vickers HV10) at defined locations with acceptance criteria of 200 to 350 HV for duplex stainless steel overlays.

Key Questions and Reflections

The most significant challenge identified in this study is the control of dilution from the carbon steel or low-alloy steel tube sheet substrate into the duplex stainless steel overlay. The first pass typically exhibits 20 to 40 percent dilution, which can significantly alter the phase balance and corrosion resistance of the deposited metal. The study recommends a minimum overlay thickness of 3 to 5 mm to ensure that the outer layers achieve acceptable composition and properties, with the understanding that the first pass may not meet the full performance requirements of the overlay specification.

Another important consideration is the interaction between the ESW overlay and subsequent tube-to-tube-sheet welding operations. The thermal cycles from tube welding can potentially cause sigma phase formation in the overlay if the local temperature exceeds 600 °C for extended periods. The study suggests limiting the number of tube welding passes in any given area and applying a post-tube-welding stress relief treatment at 350 to 400 °C to minimize this risk.

Study Insights and Implications

This study provides valuable guidance for the implementation of duplex stainless steel ESW overlay on tube sheets in pressure vessel fabrication. The key insight is that the ESW process, when properly controlled, offers an economical and reliable method for depositing thick duplex stainless steel overlay layers on large tube sheets that would be impractical to machine from solid duplex material. The combination of high deposition rates, good metallurgical quality, and acceptable mechanical properties makes this approach particularly attractive for large heat exchangers and pressure vessels operating in chloride-containing environments.