Zhuojin Pipe Fitting Co., LtdStrip Electrode Electroslag Surfacing of Duplex Stainless Steel on Tube Sheets
Research Context and Technical Challenge
This paper by Xing Zhuo from Shenyang Institute of Instrumentation Science and Technology (Pressure Vessel, 2024, Vol. 41, No. 11, pp. 74-84) addresses a critical engineering challenge in pressure vessel and heat exchanger design: enhancing the corrosion resistance of tube sheets through duplex stainless steel surfacing. Tube sheets, which serve as the structural interface between tubes and the shell in heat exchangers and reactors, are frequently exposed to corrosive process fluids on the tube-side and/or shell-side. The selection of appropriate surfacing materials and processes is therefore essential for ensuring long-term service integrity.
Duplex stainless steel (DSS), characterized by its dual-phase microstructure of austenite (γ) and ferrite (δ), offers superior mechanical properties and corrosion resistance compared to conventional austenitic stainless steels. However, the surfacing of DSS presents unique metallurgical challenges that distinguish it from austenitic stainless steel surfacing, particularly regarding heat treatment limitations and phase balance control.
Metallurgical Considerations and Process Design
The paper identifies several key metallurgical principles that govern the surfacing process:
Ferrite content control: The desired ferrite content in the overlay is typically 35-65% (per ASME standards), with the target being approximately 40-60% for optimal corrosion resistance and mechanical properties. Excessive ferrite (>65%) can lead to 475°C embrittlement and increased susceptibility to intergranular corrosion, while excessive austenite (<35%) reduces the beneficial strengthening effect of the ferrite phase and may promote sensitization.
Post-weld heat treatment limitations: This is perhaps the most critical technical constraint. Unlike austenitic stainless steels, which can be stress-relieved at 600-900°C, DSS overlays cannot undergo conventional stress-relief heat treatment in this temperature range. Exposure to temperatures above approximately 550°C for extended periods causes 475°C embrittlement, where chromium-rich sigma phase (σ) precipitates at ferrite grain boundaries, severely degrading toughness and corrosion resistance.
Transition layer strategy: The paper proposes two approaches depending on the base tube sheet material:
| Base Material Condition | Recommended Approach | Rationale |
|---|---|---|
| High hardenability base metal (e.g., high-carbon steel) requiring PWHT | Austenitic stainless steel transition layer + DSS overlay | The austenitic transition layer can accommodate PWHT without phase transformation issues; DSS overlay is applied after PWHT of the transition layer |
| Good weldability base metal (carbon steel, low-alloy steel) | Direct DSS overlay without transition layer | The linear expansion coefficient of DSS is close to that of carbon steel and low-alloy steel, minimizing thermal stress; no PWHT is required |
Process Parameters and Quality Assessment
The strip electrode electroslag surfacing (SESS) process was selected for its ability to produce thick, dense overlays with low dilution. The key process parameters and their effects are summarized below:
| Parameter | Typical Range | Effect on Overlay Quality |
|---|---|---|
| Welding current | 400-800 A | Higher current increases deposition rate but may increase dilution |
| Travel speed | 100-300 mm/min | Controls heat input and dilution; must be balanced with current |
| Electrode feed rate | 0.5-1.5 m/min | Must match travel speed to maintain arc length stability |
| Flux composition | CaF2-Al2O3-SiO2 system | Affects slag viscosity, fluidity, and overlay cleanliness |
| Interpass temperature | 100-250°C | Prevents cracking while avoiding excessive heat accumulation |
The paper reports that both approaches (with and without transition layer) successfully achieved ferrite content above 40% in the overlay, confirming good phase balance control. The overlay metal exhibited good comprehensive properties, including adequate hardness, tensile strength, and corrosion resistance.
Engineering Application and Practical Considerations
Several practical aspects of this research are particularly relevant to pressure vessel and heat exchanger engineering:
- The electroslag surfacing process is well-suited for thick overlay applications (typically 6-25 mm per pass), which is advantageous for tube sheet repair where significant material buildup may be required.
- The dilution rate in electroslag surfacing is typically 5-15%, which is lower than conventional arc welding processes. This is critical for maintaining the corrosion resistance of the DSS overlay, as excessive dilution with carbon steel base metal would introduce carbon and manganese, degrading the pitting resistance.
- The absence of PWHT for the direct DSS overlay approach simplifies the manufacturing sequence and reduces production costs. However, this also means that welding residual stresses remain in the overlay, which must be managed through careful procedure design.
- The transition layer approach adds complexity and cost but provides a safety margin for base metals with high hardenability. In such cases, the austenitic stainless steel transition layer (typically 309L or 316L) acts as a buffer zone that accommodates the differential thermal expansion between the base metal and the DSS overlay.
Study Insights and Implications
This research provides a clear and practical framework for DSS surfacing on tube sheets, addressing the most critical technical challenges: phase balance control, heat treatment limitations, and base metal compatibility. The dual-approach strategy (with and without transition layer) offers engineers flexibility in process selection based on the specific base material and service conditions. The emphasis on the 475°C embrittlement limitation is a critical reminder that DSS surfacing is not a drop-in replacement for austenitic stainless steel surfacing; the process must be designed with full awareness of the thermal constraints imposed by the ferrite phase. For engineers designing heat exchangers and pressure vessels for corrosive service, this research provides actionable guidance on selecting the appropriate surfacing strategy to ensure long-term service integrity.