CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electrode Electroslag Cladding of Duplex Stainless Steel on Tube Sheets
Literature Overview
This 2024 study by Xing Zhuo from Shenyang Instrumentation Research Institute Co., Ltd., published in Pressure Vessels journal, addresses the specialized challenge of applying duplex stainless steel cladding to tube sheets in heat exchangers using strip electrode electroslag welding (ESW). Tube sheets are critical components in pressure vessels and heat exchangers, serving as the structural interface between the shell and tube bundle while providing leak-tight sealing. In corrosive service environments—particularly those involving chloride-containing media, sour gas, or high-temperature oxidizing conditions—tube sheets require corrosion-resistant overlay layers to ensure long-term integrity. Duplex stainless steels, with their combined austenitic-ferritic microstructure, offer superior mechanical properties and corrosion resistance compared to single-phase stainless steels, making them an attractive cladding material for tube sheet applications.
Duplex Stainless Steel Properties and Selection
Duplex stainless steels combine the advantages of austenitic and ferritic stainless steels, offering approximately twice the yield strength of austenitic grades and superior resistance to chloride stress corrosion cracking. The most common duplex grades used for cladding applications include:
| Grade | Composition (wt%) | Typical Yield Strength (MPa) | PREN | Key Application |
|---|---|---|---|---|
| UNS S31803 (2205) | 22Cr-5Ni-3Mo-0.15N | 450-550 | 34-38 | General corrosion service |
| UNS S32750 (2507) | 25Cr-7Ni-3Mo-0.35N | 550-700 | 38-42 | Severe chloride environments |
| UNS S32760 (Zeron 100) | 24Cr-7Ni-3.5Mo-0.5Cu-0.4N | 550-650 | 40-45 | Extreme corrosion conditions |
| UNS S32205 (1.4462) | 22Cr-5Ni-3Mo-0.15N | 450-550 | 34-38 | European standard equivalent |
The corrosion resistance of duplex stainless steels is quantified by the Pitting Resistance Equivalent Number (PREN), calculated as PREN = %Cr + 3.3×%Mo + 16×%N. Higher PREN values indicate superior resistance to pitting and crevice corrosion in chloride-containing environments.
Strip Electrode Electroslag Welding Process
Electroslag welding (ESW) with strip electrodes is a high-deposition-rate process particularly well-suited for thick-section cladding applications. The process involves the formation of a molten slag pool that provides both heat generation (through electrical resistance) and shielding of the weld pool. Strip electrodes, typically 50-100 mm wide and 3-8 mm thick, are fed into the slag pool along with a continuous strip of filler metal.
Process Parameters
| Parameter | Typical Range | Influence on Cladding Quality |
|---|---|---|
| Welding current | 400-1,200 A | Controls heat input and penetration |
| Arc voltage | 30-45 V | Affects slag pool stability and heat distribution |
| Travel speed | 200-600 mm/min | Determines deposition rate and cooling rate |
| Electrode stickout | 15-30 mm | Controls heat input and penetration profile |
| Slag composition | CaO-SiO2-Al2O3-FeO system | Controls slag fluidity and deoxidation |
| Preheat temperature | 150-300 °C | Reduces cracking susceptibility |
| Interpass temperature | 200-400 °C | Controls microstructure transformation |
Cladding Layer Configuration
The duplex stainless steel cladding on tube sheets typically employs a multi-layer configuration:
- Bonding layer (if required): A transition layer of compatible material to ensure metallurgical bonding between the carbon steel tube sheet base and the duplex stainless steel cladding. This layer may use an austenitic stainless steel or a specially formulated transition alloy.
- Build-up layers: Multiple passes of duplex stainless steel strip electrode ESW to achieve the required cladding thickness (typically 5-15 mm for tube sheet applications).
- Surface finishing: Mechanical machining or grinding to achieve the required surface finish and final thickness.
Microstructure Control and Quality Considerations
The microstructure of the duplex stainless steel cladding layer is critical to its corrosion resistance and mechanical performance. The target microstructure consists of approximately 40-60% ferrite and 40-60% austenite, with the austenite phase distributed as islands within the ferrite matrix or vice versa.
Phase Balance Control
The ferrite-austenite ratio in the cladding layer is influenced by several factors:
- Chemical composition: Higher nitrogen, nickel, and manganese promote austenite formation; higher chromium, molybdenum, and silicon promote ferrite formation.
- Heat input: Higher heat input increases cooling time, allowing more austenite to form upon cooling.
- Cooling rate: Slower cooling rates promote austenite formation; faster cooling rates favor ferrite.
- Interpass temperature: Higher interpass temperatures increase austenite content in subsequent passes.
The study emphasizes that maintaining the proper phase balance is essential for optimal corrosion resistance. Excessive ferrite content (>70%) can lead to 475 °C embrittlement and reduced toughness, while excessive austenite content (<30%) may compromise strength and increase susceptibility to chloride stress corrosion cracking.
Common Defects and Countermeasures
| Defect Type | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Hot cracking | High sulfur/phosphorus, excessive ferrite | Visual, MT | Control consumable chemistry, optimize heat input |
| Cold cracking | Hydrogen embrittlement, high hardness | UT, delayed cracking | Preheat, low-hydrogen consumables, post-weld heat treatment |
| Incomplete bonding | Insufficient penetration, surface contamination | UT, bond strength test | Clean base metal, optimize welding parameters |
| Excessive dilution | Too deep penetration, excessive heat input | Chemical analysis, metallography | Reduce heat input, use bonding layer |
| Phase imbalance | Incorrect composition, improper cooling | Metallography, magnetic measurement | Adjust consumable chemistry, control cooling rate |
Engineering Application and Inspection Requirements
The application of duplex stainless steel ESW cladding to tube sheets requires careful attention to several engineering considerations:
Base Metal Preparation
- Surface preparation to remove scale, rust, and contaminants to within 25 μm roughness
- Removal of any existing coatings or previous cladding layers
- Verification of base metal chemistry to ensure compatibility with the cladding material
- Machining of tube holes to ensure proper fit and sealing
Inspection Requirements
Per applicable standards (ASME VIII Div. 1, GB/T 150, NB/T 47002), the following inspections are typically required:
- Visual inspection (VT): 100% inspection of all cladding surfaces for surface defects, undercut, and porosity.
- Magnetic particle testing (MT): 100% inspection of the cladding surface and heat-affected zone for surface and near-surface cracks.
- Ultrasonic testing (UT): Inspection of the bond line and cladding layer for lack of fusion, porosity, and inclusions. TOFD or PAUT methods may be employed for improved sensitivity.
- Chemical analysis: Verification of cladding layer composition by spark emission spectroscopy or optical emission spectroscopy.
- Hardness testing: Hardness mapping of the cladding layer to verify uniformity and detect excessive dilution zones.
- Metallographic examination: Cross-sectional examination of representative samples to verify phase balance, microstructure quality, and absence of defects.
Key Reflections and Study Insights
This study addresses a highly specialized application that combines multiple technical challenges: the demanding requirements of pressure vessel tube sheets, the complex metallurgy of duplex stainless steels, and the process control requirements of electroslag welding. The integration of these elements requires deep understanding of materials science, welding engineering, and pressure vessel fabrication standards.
One of the most significant insights from this work is the recognition that strip electrode ESW offers unique advantages for thick cladding applications on large components like tube sheets. The high deposition rate (10-30 kg/h) and deep penetration characteristics make ESW particularly efficient for building up thick cladding layers, while the slag pool provides excellent shielding and deoxidation. However, the process also presents challenges in terms of heat input control, phase balance management, and potential for excessive dilution.
The study also highlights the importance of process qualification and operator training for specialized cladding applications. ESW with strip electrodes requires careful control of multiple parameters simultaneously, and the consequences of parameter deviations can be significant in terms of cladding quality and component integrity. The investment in process development, qualification testing, and operator training is essential for successful implementation.
From a broader perspective, the application of duplex stainless steel cladding to tube sheets represents an evolution in pressure vessel design philosophy. Rather than using expensive duplex stainless steel for the entire tube sheet, which would significantly increase material costs, the cladding approach allows the use of economical carbon steel or low-alloy steel base material with a corrosion-resistant duplex overlay. This approach can reduce material costs by 50-70% while maintaining the corrosion resistance required for the service environment.
The study also underscores the importance of comprehensive quality control for cladding applications on pressure components. The combination of visual inspection, magnetic particle testing, ultrasonic testing, chemical analysis, hardness testing, and metallographic examination provides a multi-layered quality assurance approach that addresses different types of potential defects and ensures the integrity of the cladding layer.
In conclusion, Xing Zhuo's study provides valuable technical guidance for the application of strip electrode ESW cladding of duplex stainless steel on tube sheets, addressing the critical technical challenges of phase balance control, quality inspection, and process optimization. The work demonstrates that with proper process development and quality control, this approach can provide an economical and reliable solution for corrosion-resistant tube sheet applications in pressure vessels and heat exchangers operating in demanding service environments.