Zhuojin Pipe Fitting Co., LtdComparison and Selection of Duplex Stainless Steel Strip Electrode Electroslag Surfacing Processes with and without Transition Layer
Literature Overview
The paper authored by Kong Lingwei, Liu Aimin, Zheng Yan, and Xing Zhuo, published in "Pipe Technology and Equipment" (2014, Vol. 3, pp. 54-57), addresses a practically significant problem in the fabrication and repair of duplex stainless steel components. The authors conducted welding procedure qualification (WPQ) studies on strip electrode electroslag surfacing (SEES) of duplex stainless steel, comparing two process variants: one with a transition layer and one without. The research was carried out jointly by the Shenyang Special Equipment Inspection and Research Institute and Shenyang Oriental Titanium Industry Co., Ltd., combining institutional expertise in inspection with industrial manufacturing capability.
The study is particularly relevant to engineers working on corrosion-resistant overlay applications in the oil and gas, chemical, and marine industries, where duplex stainless steel overlays are frequently specified for their superior chloride stress corrosion resistance (SCCR) and higher yield strength compared to austenitic stainless steels.
Core Technical Findings
Process Configuration
The two process variants investigated are as follows:
| Parameter | With Transition Layer | Without Transition Layer |
|---|---|---|
| Base material | Carbon steel pipe (20#) | Carbon steel pipe (20#) |
| Transition layer | 304L austenitic stainless steel | Not applicable |
| Overlay material | Duplex stainless steel (2205 type) | Duplex stainless steel (2205 type) |
| Welding process | Strip electrode electroslag surfacing | Strip electrode electroslag surfacing |
| Number of overlay passes | 2-3 | 2-3 |
Test Results Comparison
The authors performed a comprehensive suite of qualification tests including side-bend testing, chemical composition analysis, non-destructive testing (NDT), and hardness testing. The key finding is that both process variants produced results within acceptable limits, with no significant differences in critical performance indicators.
| Test Item | With Transition Layer | Without Transition Layer | Acceptance Criteria |
|---|---|---|---|
| Side-bend test | Qualified | Qualified | No cracks, no delamination |
| Chemical composition | Within spec | Within spec | Fe, Cr, Ni, Mo, N within duplex range |
| NDT (RT/MT) | No defects | No defects | Per ASME/GB standards |
| Hardness (HV) | Acceptable | Acceptable | Below 350 HV for duplex |
Key Conclusion
The authors concluded that the process without a transition layer is a viable and advantageous alternative. The elimination of the transition layer offers several practical benefits:
- Reduced manufacturing cost by eliminating the need for a separate austenitic stainless steel transition layer
- Shortened production cycle time
- Reduced total overlay thickness, which is beneficial for space-constrained applications
- Simplified quality control procedures
Technical Interpretation and Engineering Analysis
Metallurgical Considerations
The success of direct duplex stainless steel surfacing onto carbon steel without a transition layer is metallurgically non-trivial. In conventional practice, a transition layer is often specified because:
- The large difference in thermal expansion coefficients between carbon steel and duplex stainless steel can induce high residual stresses at the interface.
- Dilution of the overlay by carbon steel base material can shift the microstructure away from the desired 50/50 austenite-ferrite balance toward a more ferritic structure, increasing susceptibility to 475°C embrittlement.
- The high carbon content in carbon steel can promote the formation of brittle iron carbides at the interface.
The fact that the direct surfacing process without a transition layer passed all qualification tests suggests that the strip electrode electroslag process provides sufficient thermal input and dilution control to maintain the metallurgical integrity of the duplex overlay. The electroslag process is characterized by a relatively low cooling rate and stable thermal cycle, which may contribute to the favorable results.
Process Parameters and Control
Strip electrode electroslag surfacing involves a flux-covered strip electrode that melts in a slag pool, with the molten metal and slag being deposited in a controlled manner. Key process parameters include:
- Strip electrode feed rate: Controls the deposition rate and dilution ratio
- Current and voltage: Determine the thermal input and slag pool stability
- Travel speed: Affects the bead width and overlap
- Flux composition and moisture content: Critical for slag pool stability and nitrogen pickup control
For duplex stainless steel overlays, nitrogen control is particularly critical because nitrogen is a strong austenite stabilizer and excessive nitrogen pickup can shift the microstructure toward austenite, reducing SCC resistance. The electroslag process, being conducted under a protective slag atmosphere, offers inherent nitrogen pickup control advantages over open-air arc processes.
Integration with Engineering Practice
Application Scenarios
The findings of this study have direct applicability in the following engineering scenarios:
- Pipeline repair and overlay: Duplex stainless steel overlay of carbon steel pipelines in sour service (H2S-containing environments) where API 5L X70 or X80 grade pipes require corrosion-resistant surfaces.
- Pressure vessel repair: Overlay of carbon steel pressure vessels operating in chloride-containing environments, such as desalination plant components.
- Heat exchanger tube sheets: Surfacing of carbon steel tube sheets with duplex stainless steel to improve resistance to pitting and crevice corrosion.
Practical Recommendations
Based on the study findings and engineering experience, the following recommendations are offered:
- The direct surfacing process without a transition layer should be qualified per the relevant standard (e.g., ASME Section IX, Part QW, or GB/T 19542) before production use.
- Pre-welding inspection of the base material surface is essential, including removal of rust, scale, and contaminants.
- Post-weld heat treatment may be required to relieve residual stresses, especially for thick overlay sections.
- The overlay hardness should be monitored throughout production, as excessive hardness (>350 HV) can indicate a deviation toward a fully ferritic microstructure.
Key Questions and Reflections
One important question that arises from this study is the long-term performance of the direct-surfaced duplex overlay under severe corrosion conditions. The qualification tests confirm that the weld is sound and meets mechanical and metallurgical requirements, but long-term exposure testing in chloride-containing environments is essential to validate the SCC resistance. The absence of a transition layer means that the interface between the carbon steel base and the duplex overlay is more directly exposed to the corrosive environment, and any microstructural deviations at this interface could become initiation sites for corrosion.
Another reflection concerns the reproducibility of the results. The study was conducted under laboratory qualification conditions, and the transition to production-scale surfacing may introduce variability in process parameters, flux condition, and base material preparation. A robust production qualification program should include multiple batches of test coupons to establish process capability indices (Cpk) for critical parameters such as overlay hardness, dilution ratio, and microstructure.
Study Insights and Implications
The most significant insight from this study is that conventional wisdom regarding the necessity of a transition layer in duplex stainless steel surfacing can be challenged when the appropriate welding process is selected. The strip electrode electroslag process, with its inherent thermal stability and slag protection, provides a process window in which direct surfacing onto carbon steel is feasible. This finding has direct economic implications for fabrication and repair shops, as eliminating the transition layer can reduce material costs by approximately 15-25% and shorten production lead times.
From a quality engineering perspective, this study exemplifies the value of systematic process qualification. The comprehensive testing approach, covering mechanical, metallurgical, and NDT criteria, provides the confidence needed to adopt a simplified process. Engineers should adopt a similar rigorous approach when evaluating any process innovation in overlay welding applications.
The study also highlights the importance of understanding the underlying metallurgy rather than relying solely on empirical rules. The success of direct duplex surfacing without a transition layer is rooted in the specific thermal and metallurgical characteristics of the electroslag process, and this understanding can guide the selection of appropriate processes for other overlay applications where transition layers are conventionally specified.