CLADDING TECHNOLOGY SHANXI CO., LTDComparison and Selection of Strip Electroslag Welding Overlay Process for Duplex Stainless Steel
Literature Overview
This 2014 study by Kong Lingwei from Shenyang Special Equipment Inspection and Research Institute, together with Liu Aimin, Zheng Yan, and Xing Zhuo from Shenyang Oriental Titanium Industry Co., Ltd., presents a systematic comparison of strip electroslag welding (ESW) overlay processes for duplex stainless steel. Duplex stainless steels such as UNS S31803 (2205) and UNS S32750 (2507) are increasingly specified for pressure vessels, pipelines, and heat exchangers in the oil, gas, and chemical industries due to their excellent combination of high strength, good corrosion resistance, and resistance to chloride stress corrosion cracking. The study addresses the critical challenge of producing a high-quality duplex overlay layer with the correct microstructural balance of austenite and ferrite phases.
Technical Background and Process Parameters
Strip ESW overlay offers significant advantages for thick-section duplex stainless steel cladding due to its high deposition rate, deep penetration, and the ability to produce thick overlay layers in fewer passes. However, the process also presents unique challenges related to phase balance control, hydrogen pickup, and grain coarsening.
The study compares multiple strip ESW overlay configurations, varying parameters such as strip composition, flux type, welding current, voltage, travel speed, and number of overlay layers. The key process windows investigated are summarized below:
| Parameter | Configuration A | Configuration B | Configuration C |
|---|---|---|---|
| Strip composition | 2205 duplex | 2507 super duplex | Customized balanced duplex |
| Flux type | Sintered low-alloy | Submerged arc flux | Special ESW flux |
| Current (A) | 800-1200 | 900-1400 | 850-1300 |
| Voltage (V) | 30-38 | 32-40 | 31-39 |
| Travel speed (mm/min) | 150-250 | 120-200 | 140-220 |
| Layer thickness (mm) | 8-12 | 10-15 | 9-13 |
| Preheat (degrees C) | 100-150 | 150-200 | 120-180 |
| Interpass temperature (degrees C) | 150-250 | 200-300 | 180-250 |
Phase Balance Analysis and Microstructural Control
The critical quality criterion for duplex stainless steel overlay is the austenite-to-ferrite ratio, which should ideally fall within the range of 40-60% austenite and 40-60% ferrite by volume fraction. This balance ensures optimal mechanical properties, corrosion resistance, and weldability. The study employs the Schaeffler diagram and the DeLong diagram as predictive tools, supplemented by ferrite number (FN) measurements per ASTM E1678 and metallographic phase analysis per ASTM E399.
The researchers found that Configuration C, utilizing a customized balanced duplex strip with optimized Cr, Ni, Mo, N, and Nb content, achieved the most stable phase balance across multiple overlay layers. The addition of niobium (Nb) at 0.1-0.3 wt% proved effective in stabilizing the ferrite phase and preventing excessive austenitization during the multiple thermal cycles inherent in multi-layer ESW overlay. Nitrogen content control was identified as particularly critical, with target levels of 0.10-0.20 wt% for 2205 and 0.20-0.30 wt% for 2507 grades.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Excessive ferrite (>75 FN) | Low Ni/N content, high cooling rate | Adjust strip composition, increase Ni or N content |
| Excessive austenite (<35 FN) | High Ni content, excessive heat input | Reduce Ni, increase Cr, optimize heat input |
| Sigma phase precipitation | Slow cooling in 500-800 degrees C range | Control cooling rate, apply PWHT at 1050-1100 degrees C |
| Hot cracking | High sulfur, restricted solidification | Use low-S strip, optimize solidification rate |
| Hydrogen porosity | Moisture in flux, high hydrogen content | Dry flux thoroughly, use low-hydrogen flux |
| Bond-line cracking | CTE mismatch with substrate | Use compatible transition layer, control preheat |
Standards and Qualification Requirements
The study references several applicable standards for duplex stainless steel overlay qualification:
- NB/T 47014-2011: Chinese standard for qualification of welding procedures for pressure vessels, specifying essential variables for ESW including strip composition, flux type, current range, voltage range, and travel speed range.
- ASME Section IX, QW-451: Qualification requirements for electroslag welding processes, requiring demonstration of mechanical properties and fracture toughness of the overlay.
- API 934: Standard for welding consumables for overlay welding, providing composition and performance requirements for duplex stainless steel overlay consumables.
- ASTM A263/A264: Specifications for austenitic and austenitic-ferritic stainless steel plate for pressure vessels, applicable to the base plate specification.
- EN 10028-7: European standard for stainless steel plates for pressure purposes, including duplex grades.
Engineering Practice Implications
The findings of this study have direct implications for the fabrication of duplex stainless steel clad pressure vessels per GB/T 150 and ASME Section VIII Division 2. When specifying strip ESW overlay for duplex stainless steel, the following engineering controls should be implemented:
- Consumable qualification: Perform full qualification testing including chemical analysis, ferrite number measurement, tensile testing, impact testing (minimum 27 J at -40 degrees C per ASME Section VIII Div. 2), and intergranular corrosion testing per ASTM A263.
- Procedure qualification: Qualify the ESW overlay procedure per NB/T 47014 with documented control of all essential variables, including strip chemistry, flux type, preheat, interpass temperature, and heat input.
- In-process monitoring: Implement real-time ferrite number measurement on each overlay layer to ensure phase balance remains within specification. Corrective action should be taken if FN deviates outside the acceptable range.
- Post-weld heat treatment: For super duplex overlays, solution heat treatment at 1050-1100 degrees C with water quenching is recommended to dissolve any precipitated sigma phase and restore the optimal phase balance.
Study Insights and Conclusions
This paper provides a valuable comparative framework for selecting strip ESW overlay processes for duplex stainless steel applications. The emphasis on customized strip composition with niobium stabilization represents a practical approach to achieving stable phase balance in thick multi-layer overlays. The study reinforces the principle that duplex stainless steel overlay quality is fundamentally a composition control challenge, where even small deviations in Cr, Ni, N, and alloying element content can lead to significant phase balance shifts and subsequent property degradation. Engineers designing duplex overlay systems must integrate metallurgical knowledge with process engineering to ensure long-term service reliability in aggressive environments.